Best Day Essay

Have you ever felt the emotion when you are going to see your favorite band or singer alive in a concert. This happened to me when I went to Sofia last year to see one of my favorite singers Enrique Iglesias. It was fantastic experience that I will never forget. The day was near, it was ten days befor the concert, and every day I asked my father for permission and money. One day he finally said â€Å"yes†, and he gave me the money. He told me that it was the gift for my birthday. I was so happy and immediately called my friend to tell her about the news. The next day we bought our tickets and we were waiting for the big day.We couldn’t believe that we would be in the most waited concert on September 29th , we never thought that in few hours, we would sing all the song of our favorite singer. When we arrived,we sat in the chairs, and we were waiting for Enrique. The lights turned off and all the auditory screamed like they were crazy. The orchestra began to play one of my favorite songs. The best singer appeared in the scene. That moment was so awesome and exciting for me, and everyone was all of the songs. In conclusion, I will tell tha, the concert was a big experience for me and I was so happy. At the end, we bought some souvenirs from the concert, like a shirt, some posters and a cup. The concert finished at 12 p.m. and we came back to our town with an experience that I will never forget. I will remember this moment as if it had been yesterday.

Albert Camus’ novel, The Stranger Essay

Albert Camus’ novel, The Stranger, strongly resembles the concepts of absurdism, the efforts of humanity to find meaning in the universe all fail because no such meaning exists. Life just has no meaning or purpose. Camus accepts one inevitable thing, death. The idea that everyone will meet death, then everyone’s life is equally meaningless. The inability to find meaning in life is not logical impossible but rather humanly impossible. The title, The Stranger, is significant towards Meursault’s character in the book because he sees himself as a â€Å"stranger† in the world. It seems to be a nickname for him. He only shares certain things and keeps everyone at a distance. Society does not appreciate him like their own; he does not conform to their ideas and rules. The title is also a reflection of the author because when he wrote the book, no one truly knew the philosophical views of the absurd. The idea that things sometimes happen for no reason, and that occasions sometimes have no meaning to anything in life is disruptive and threatening to society. Meursault seems to be the only one who thinks he has the freedom to do whatever he wants and therefore he does not preoccupy himself with what society has set as the norm. An interesting relationship is between Maman and Meursault. In the beginning of the novel, Maman passed away and Meursault seemed to show no emotions towards her death. He accepted that she died and moved on. He knew that putting her in the Morengo Home was the best for her. Meursault tells his boss that is not his fault that Maman died. Throughout the novel, he contemplates memories of her and what he knew about her. When he is sentenced to a horrible fate, he understands his mother more than ever. Meursault knows why she wanted to be around people who cared and loved her when she died. He realizes that she understood what life was really like similar to him. At the end, he knows that she left this world just like he is going too, meaningless and purposeless. Albert Camus’ novel, The Stranger, strongly resembles the concepts of absurdism, the efforts of humanity to find meaning in the universe all fail because no such meaning exists. Life just has no meaning or purpose. Camus accepts one inevitable thing, death. The idea that everyone will meet death, then everyone’s life is equally meaningless. The inability to find meaning in life is not logical impossible but rather humanly impossible. The title, The Stranger, is significant towards Meursault’s character in the book because he sees himself as a â€Å"stranger† in the world. It seems to be a nickname for him. He only shares certain things and keeps everyone at a distance. Society does not appreciate him like their own; he does not conform to their ideas and rules. The title is also a reflection of the author because when he wrote the book, no one truly knew the philosophical views of the absurd. The idea that things sometimes happen for no reason, and that occasions sometimes have no meaning to anything in life is disruptive and threatening to society. Meursault seems to be the only one who thinks he has the freedom to do whatever he wants and therefore he does not preoccupy himself with what society has set as the norm. An interesting relationship is between Maman and Meursault. In the beginning of the novel, Maman passed away and Meursault seemed to show no emotions towards her death. He accepted that she died and moved on. He knew that putting her in the Morengo Home was the best for her. Meursault tells his boss that is not his fault that Maman died. Throughout the novel, he contemplates memories of her and what he knew about her. When he is sentenced to a horrible fate, he understands his mother more than ever. Meursault knows why she wanted to be around people who cared and loved her when she died. He realizes that she understood what life was really like similar to him. At the end, he knows that she left this world just like he is going too, meaningless and purposeless.

International markets small business in USA Essay

International markets& small business in USA Introduction   Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚   Venture capitalists invest massive of money to companies that want to expand but have no enough access to public funds. They are willing to invest in such companies because they earn massive benefit when the company succeeds. Consequently, venture capitalists look for firms with strong management staff, large markets and unique goods with high competitive advantage (Gladstone & Gladstone 2004). They are driven by the desire to own a large portion of the company so that they can influence its direction.   Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚   International markets are complex due to their vast geographical coverage in the worlds rapidly changing and growing nations. The basics for international markers ranging from diverse culture and languages, handling issues related to business ethics, selection of distribution and global marketing channels and international communication. They are the key issues that guide the operationality of worldwide markets (Omar, Abduh, & Sukmana, 2013)   Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚   This situation represent personal and professional dilemma where by the personal and professional interests that are simultaneously conflicting. I will use the principles of ethical decision making process seek supervision self mirror image and illumination of the process will also consider legal obligation that supersedes my professional and personal ethics (Wueste,1994). I will talk both the boss and supervisor to examine their feeling about my situation as that pertain my professional ethics.   Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚  Ã‚   The small businesses are important in US economy in many aspects. They employ many people, actually about the half of the total workforce in there a key in driving the economy as opposed to giant multi-billion dollar companies (White House Committee on Small Business, 1962).They are instrumental in stabilizing forces in the economy .the owners are backbone for creativity innovation production and value creation in United States. The small business is actually what stimulates the economy. References Gladstone, D., & Gladstone, L. (2004). Venture capital investing: The complete handbook for investing in private businesses for outstanding profits. Upper Saddle River, NJ: Prentice Hall/Financial Times Omar, A., Abduh, M., & Sukmana, R. (2013). Fundamentals of Islamic money and capital markets. Singapore: John Wiley White House Committee on Small Business (U.S.). (1962). Small business in the American economy: Its contributions and its problems [and] the role of the Federal Government. Washington. Wueste, D. E. (1994). Professional ethics and social responsibility. Lanham, Md: Rowman and Littlefield. Source document

Organizational Development & Human Resources Term Paper

Organizational Development & Human Resources - Term Paper Example As such, the topics which will be analyzed relate to performance evaluations, successful management, treatment of difficult situations, and issues relating to recruiting and compensation. The first concept that this student would like to discuss relates to the issue of performance evaluations. Whereas the performance evaluation is an oft dreaded part of many jobs, the key concepts that were learned within the class helped this student to approach the issue with a new mindset. The fact of the matter is that performance reviews are not in and of themselves a negative; rather, employees have been conditioned to think so due to a number of factors. The first and most prescient of these, as has been demonstrated within class, is the fact that the performance review process is often ignored up until the very last minute. In this way, the employee oftentimes has positively no indication of how their performance has been gauged for the entirety of the period in question. Accordingly, what th is affects is a situation in which dread is the primary emotion with which the employee greets the process (Muller 2009). However, as the readings and course work have demonstrated, a far superior model whereby to engage employee and/or shareholder buy in within such a process is to make the entire performance review process something that is straightforward, clearly enumerated, and transparent. In this way, the element of the unknown is removed as the employee is able to engage with the employer (and vice versa) as a means of understanding the extent to which obligations, goals, and metrics were met during the period in question. Rather than seeking to fundamentally redefine the performance review process, the best model of improvement is contingent upon all parties involved keeping clear lines of communication open throughout the period so that when the review comes up, no clarifications will be necessitated. The role of management within a given firm or organization is a topic th at has encouraged a great many professionals and writers to expound upon better and more sophisticated models. Yet, as has been learned within this particular class, there is not a definitive definition of correct management; rather, there is a litany of examples of incorrect management. Such is the case due to the fact that the field of management and managerial practice is so broad and nuanced as a result of the host of different personalities that different managers bring to the process. Moreover, the fact that there are clearly discernible incorrect management practices is more helpful in guiding and directing the practitioner towards methods and applications that would minimize these incorrect practices and steer the process back towards a more appropriate direction. By means of understanding this, the reader/researcher and/or student can and should understand that although a litany of best practices are existent within the realm of management, the best method of directing the proper application thereof within the current workplace is to ensure that the smaller subset of negative management practices are avoided at all costs. These include but are not limited to: micromanagement, hands off approaches, the development of non-professional employee-boss relationships, unhealthy power dynamics, and many, many others. As a way of being mindful and understanding

Management issue Essay Example | Topics and Well Written Essays - 1500 words - 1

Management issue - Essay Example ation can be achieved through proper communication by the management team to their employees, which raises the morale of the working team thereby achieving the targeted results. This paper looks at the role of effective communication in the motivation of the employees. It also highlights the implications of communication and motivation with the employees on the management while looking at how this could be a problem if not addressed properly. The paper further looks at the skills the managers need in order to be good motivators of their team and communicate effectively. Motivation is mainly associated with increase in pay or other better payment packages, career opportunities and also good relationships with co-workers at the workplace. However, communication acts as a pivotal aspect in employee motivation, and the management needs to understand the role of communication in motivation (Morreale et al., 2008). Effective communication entails proper information distribution in the company as all the information involving the company decisions should disseminated to the company employees (Bates, 2009). The company should ensure that they give the employees proper information involving the company such as the sale of shares, changes in the workforce and management, which eliminates incorrect rumours that the employees may form while at the same time making the employees feel respected by the company. Proper relationships are built by the employees, the management, and all departmental heads throughout the company. Communication strengthens the relation ships between the workers and the management making the employees more productive as they relate well with their supervisors and managers (Eunson, 2007). This can be achieved by holding departmental meetings weekly where the members of the department discuss the issues of the company or even employee assistance programs by the management to help the employees improve their skill set in a given aspect. In the decision

Teenagers' cool culture Essay Example | Topics and Well Written Essays - 2500 words

Teenagers' cool culture - Essay Example Rushkoff describes in vivid detail this loop, as the media studies the kids in order to acquire popular images and then sells them the images of themselves. The youth and teens are instantly drawn to these images thereby desiring them. Following this replication, the media observes the interaction among the adolescents and then create new images to fit with the latest trend, and so on. The important question here is the authenticity of the images of the prevalent teen culture ardently pursued by the youth, which is shaped by the advertisement conglomerates, whose sole purpose is the sale and profits of their businesses, regardless of the protection and protection of the true teen American culture and identity. The ‘cool’ youth culture is actually known to have been initiated in the 1980s when parents began to spend more and more on the needs and desires of their children, as a result of the onset of the nuclear family systems coupled with the double income from both working parents. This also happens to be the period when conglomerates began their cold wars with each other in order to hook the teens into buying their brands and maintain loyalty towards their brands. Marketing and selling acquired a major role in the process, where marketing companies began to hire spies, to inculcate the ‘cool’ teen behavior into their advertisements, in order to lure the youth into buying their products. The impressionable adolescents were obviously drawn towards these images of themselves like a pierced nose or eyebrow, or cuffed leg or sleeve. The cycle of pursuit of ‘cool hunting’ was a vicious and never-ending one because the moment a ‘cool’ behavio r was identified and adopted it ceased to be â€Å"cool†! The intelligent kids obviously became aware of this cycle and the 1990s saw an absolutely new defensive to this mechanism, a rebellious defensive by the youth culture. The smart teens refused to accept or adopt anything that was publicized as

Security Practitioners Essay Example | Topics and Well Written Essays - 750 words

Security Practitioners - Essay Example The whole procedure of software/application development is usually guided by a systematic software/application developing method that includes the following phases. The first step is evaluating the weaknesses of the existing software by interviewing the users; the second phase is where the new system is defined which takes care of the deficiencies in the old model. The third phase involves designing of the proposed system, in this phase, strategies are put in place regarding the physical development, hardware, operating system, programming, communication and security matters, the fourth phase is the development of the new software, in this phase, the users are also trained on its use. In addition its performance capabilities are also tested, if need be, adjustments are made at this stage. The next phase is putting the software into use where it can be phased in by gradually removing the old system or removing the old system at once and replacing it with the new system. ... This forms the basis of handling future projects whereby the background knowledge endowed to the students (what they learnt at school) is fundamental in enhancing their skills. A major challenge through this form of learning is that the security part, which is very critical in any software, is usually left to the professional to handle it therefore denying software development learners a chance to have an intensive hands on experience in software security. Software developer training and learning involves certification in SAS certified base programmer and SAS certified advanced programmer which are very important especially to programmers who deal with business applications and software. The other relevant certifications include Siebel, Oracle and Microsoft (McGraw, 82). Due to the rise in cybercrimes, it is vital for security training to be undertaken to ensure that all the information is safeguarded. To enhance security, IP filtering is necessary to ensure that specific information is only available at restricted areas; the students should b taught on the same though practical lesson by working alongside professionals while undertaking such assignments. Software developers learning has not been much concerned about the security of the applications that they make since their training involves creating applications with little attention to security issues of the software. This can be explained by the fact that the curriculum that is used in formal training institutions was made when there were few security threats therefore it did not put into account training in security. However, with increased security concerns, there has been a change in the tread with most software

Business plan Essay Example | Topics and Well Written Essays - 1500 words - 3

Business plan - Essay Example Definitely different boutique had clients who had attended the ceremony and whose designs were outstanding. The journalist who reported this on the news paper termed the designs as definitely different. Initially, the owner carried out designing at his home. He could take customer requests and design their outfits. This lasted for seven years till when loyal and impressed customers urged him to direct his talent toward a retail operation with the promise that they will continue to support the boutique. The customers helped in the establishment of the boutique by providing up to 30 percent of the startup costs. Definitely different boutique has been in this business for a while now and people’s responses have been similar. Definitely different boutique’s designs have impressed many people and been receiving positive responses from our clients and prospective clients. Consideration of a client’s request and demands come first in execution of the boutique’s operations. Most of the fashion trends always focus on the youth. Our target market will be the youth mid twenties professional men and women, and mature women with more disposable income who are looking for definitely different outfits for special occasions and home or business wear. Definitely different company has six competent employees, 3 sells assistants and 3 seamstresses. The seamstresses are graduates from the London School of Fashion. In addition, they also have attained needlework degrees from the London School of Needlework. They are highly trained and competent and experienced. They trained for three years and gained experience in dress making and broidery at the Paris Fashion House. Moreover, they also gained experience while training in Italy. The three sales assistants are highly competent and experienced. They previously had been employed on a part time basis as in-house models for a TV show on air fashion

What does a geomatic engineer do in his career and what are the future Essay

What does a geomatic engineer do in his career and what are the future prospectus of this career - Essay Example Most of the tasks and activities of the geomatic engineers are integrally linked with civil engineering. Such tasks include but are not limited to mapping for the construction projects, control surveys, and preparation of design and layout of the urban subdivisions and public infrastructure. The graduates of geomatics engineering make use of a diverse array of technologically advanced tools such as the equipment of Global Positioning System (GPS), digital total stations, computer-based geographic information systems (GIS), and digital aerial imagery done with the help of satellites. The level of education required to serve as a geomatic engineer varies from one organization and country to another. However, in general, highly qualified and educated geomatic engineers get more and better opportunities of employment like positions in academic and research organizations. Generally, a four year degree is required to be able to survey the oil and gas sites. Geomatic engineers design system s in order to gather locational data. Design and use of these systems is critical for a wide range of applications including scientific surveys and navigation systems in cars. In the field of geomatic engineering, advanced degrees are also available as geomatic engineering integrates a variety of disciplines like mathematics, geology, and surveying. Locational data includes physical maps’ underpinning so that navigational information and different types of data based on map can be obtained. These maps provide the details of geology, meteorology, and demography of different areas. Using this information, geomatic engineers design systems to obtain and record the information. Many of these systems interact with the systems of GPS as well as other points of reference that are used for accurate recording of the data. Utilization of these systems is often part of the job of a geomatic engineer. Employment opportunities for the geomatic engineers exist in the private companies, gov ernment agencies, and other non-profit organizations which are involved in such activities as spatial design and surveying. A geomatic engineer may be required to work as a team member in the operation of exploration of oil and gas. In such tasks, the geomatic engineer is asked to collect the location information, mark the data that is relevant, and make sure that he prepares a map recording all information, materials, and features of interest required to prepared the background on the survey location. These survey teams often get the help and participation of other engineers in the tasks of preparation of the home sites, collection of data for research, and involvement in research and development based on location. In order to achieve this, geomatic engineers require an in-depth understanding of the location data and spatial relations as well as the discipline in which the data would be utilized so that reliable and useful knowledge can be created. As a geomatic engineer, an indivi dual can serve in different sorts of technical positions that include but are not limited to photographic image processing, global positioning, satellite image processing, remote sensing, three-dimensional computer visualization, management of land and natural resources, spatial data infrastructure, surveying, and mapping. There is a variety of career pathways for a geomatic engineer to select from. They can choose to specialize in cadastral or land surveying, engineering surveying, positioning, mining, remote sensing, and topography. In addition to them, a variety of other areas of specialization are available. A cadastral or land

Reliance on National Test Scores in Admissions Essay

Reliance on National Test Scores in Admissions - Essay Example It is perfectly clear that people differ in the way they perceive and process information, and for some students, it may be difficult to deal with SAT and ACT just because they are presented in the form of tests. Also, depending on a person's temperament, time of information processing changes as well. What is more, while some people are able to deal with pressure they face while completing any important task, such as testing, others are more exposed to stress and cannot complete tests properly even though they have enough knowledge to do it. External factors depend on the character of the tests, on those who compose them, and on the society as a whole. For instance, it is believed that the results of these tests highly depend on a class a student belongs to: the richer the students' parents are, the higher are his / her scores. In addition to this, taking into account high stakes, people try gaming standardized tests, and quite often, they do it criminally. Therefore, there is no gu arantee that a student is as good at studying as his / her scores show. The second argument that explains irrelevance of standardized tests is that these tests lack content validity. Content validity is a very important characteristic feature standardized tests should have. It is defined as the extent to which a test represents the content of an area that is measured. SAT and ACT do lack this characteristic feature. The matter is that they leave out a significant portion of material covered while studying.

Requirement Analysis Assignment Example | Topics and Well Written Essays - 2000 words

Requirement Analysis - Assignment Example The task at hand is to present a comprehensive requirement analysis and select the most appropriate paradigm(s) for preparing the requirement analysis. The task also aims to be an effective exercise in information gathering. This section targets at presenting various requirement analysis models present in literature. Various models will be scrutinized and the most relevant strategy chosen to proceed towards the analysis development 2. The basic idea of the behavior-oriented approach is to study the behavior, the decision-making style, and the data used by the pertinent personnel to extract information for the development of a critical information framework for the generation of requirement analysis. Background analysis is necessary to understand a particular problem, only then the designer can forward. Backward analysis helps to determine if the personnel were faced with a similar situation earlier in time. This helps to gain an insight into how similar problems had been solved in the past. The top executives are then interviewed to determine the main causes of the new problem prior to gathering of detailed information about the problem and summarizing the problem. This sort of model finds best application while developing an executive information systems or top-level decision-support system. ... The top executives are then interviewed to determine the main causes of the new problem prior to gathering of detailed information about the problem and summarizing the problem. This sort of model finds best application while developing an executive information systems or top-level decision-support system. The prime hurdles impeding in the efficiency of this model is the inability to quantify executive behavior and overlooking the needs of over-looking the needs or middle managers, supervisors, and operational personnel because they are usually the primary users of an information system. Information-Oriented Model The information-oriented model approach of specifying the requirement analysis focuses primarily on the information system products that will actually be used by the supervisory and the middle level managers. Investigating the information needs of the management gives a bottom-line compared to which the requirements might be given priority or their association with a new problem or opportunity assessed. The requirement analysis generated as a result of employing the information oriented model helps the designers to come with a comparatively more functionally useful system. The reason, as discussed earlier, is the focus on how the information technology will be utilized by the major users of the system. However, special attention to the middle management and supervisory ignores the needs of operational personnel. Industry Analysis Model The industry analysis model is based on one basic principle depicting that survival in the marketplace demands the development of an information system that is at least comparable in terms

Spontaneous Recovery and Extinction Essay Example for Free

Spontaneous Recovery and Extinction Essay Spontaneous recovery from extinction is one of the most basic phenomena of Pavlovian conditioning. Although it can be studied by using a variety of designs, some procedures are better than others for identifying the involvement of underlying learning processes. A wide range of different learning mechanisms has been suggested as being engaged by extinction, most of which have implications for the nature of spontaneous recovery. However, despite the centrality of the notion of spontaneous recovery to the understanding of extinction, the empirical literature on its determinants is relatively sparse and quite mixed. Its very ubiquity suggests that spontaneous recovery has multiple sources. Previous SectionNext Section Experimental extinction is one of the fundamental observations of Pavlovian conditioning. Just as the arranging of a positive relation between a conditioned stimulus (CS) and an unconditioned stimulus (US) produces acquisition of conditioned responding, breaking that relation produces extinction of that responding. However, similar to many terms in the behavioral sciences, the word â€Å"extinction† is used in at least three different senses: as a procedure, as a result, and as an explanation. If we are to understand extinction experiments, it is extremely important that we keep these senses distinct from each other. One use of the term is as an experimental procedure or independent variable under the control of the experimenter, as when one says, â€Å"Following learning, we subjected the animal to an extinction procedure.† Most frequently, this is meant to refer to a procedure in which the original conditions of learning are disrupted. The most common extinction proce dure consists of presenting a stimulus alone, so that it now fails to signal the outcome. However, other procedures, such as retaining the US but arranging for it to be independent of the CS are also available and of interest (see Rescorla 2001a). Another use of the term is as an experimental result or dependent variable under the control of the animal, as when one says, â€Å"When the stimulus was presented alone, the behavior extinguished.† The prototypical example is one in which responding that was established by training deteriorates, often to a level such as that prior to learning. A third use of the term extinction is as a process or  intervening variable that is intended to provide an explanation, as when one says, â€Å"When we arranged for the stimulus to be presented alone, the behavior deteriorated because of extinction.† Normally, it is this process that is of interest. We would like to understand the basis of the change in behavior resulting from the change in procedure, whether that understanding is achieved at a conceptual or a neural level . Consequently, throughout this article the term extinction will refer to the learning process inferred when the procedure produces a particular result. When there is the possibility of misunderstanding, the phrase â€Å"extinction process† will be used. Because interest primarily centers on the learning process that occurs as a result of an extinction procedure, it is important to separate that learning from a wide variety of other effects that govern performance. The issue here is analogous to that of understanding the learning that occurs during an acquisition procedure. Elsewhere we have argued that the measurement of learning demands attention to two points in time: t1, during which the opportunity to learn is given, and then a separate t2, during which an assessment is made of that learning (see Rescorla and Holland 1976; Rescorla 1988). The comparison that indicates that learning has occurred is that between two animals (or two stimuli or responses) given a common t2 test following different opportunities for learning at t1. This comparison is superior to the common alternative of examining responding during t1, at which the animals are receiving different learning treatments. Data taken during t1 necessarily confound the differences in the current circumstances under which learning is assessed with differences in the learning that prior treatments might have produced. We have argued that for this reason acquisition curves are in fact deeply flawed as a way to measure learning. A similar point applies to the learning that occurs in extinction. In this case, we need to administer a common test for stimuli or animals given different extinction experiences, as indicated in the first portion of Figure 1. In the simplest case, we can compare responding to two stimuli (S1 and S2) at a common t2 after both have had the same initial acquisition but then differ in whether or not they were given extinction at t1. Differences in t2 test performance would then index the differences in learning that occurred at t1. Clearly, comparisons between stimuli during the t1 extinction experience are of limited value because any differences might be  the product of the current conditions of testing rather than of the memory for the learning that has occurred. That is, extinction curves are of very limited use in understanding the underlying process. Figure 1 View larger version: In this page In a new window Download as PowerPoint Slide Figure 1 Experimental designs for the study of extinction and spontaneous recovery. (A) A recommended procedure for studying extinction, in which the critical comparison is responding to S1 and S2 at a common test time when the two stimuli differ in their extinction history. (B) The design in which spontaneous recovery is sometimes inferred from the greater responding to S1 during test than during extinction. (C) A better spontaneous recovery design in which S1 and S2 are both trained and extinction, but then tested for recovery after different time periods. (D) An alternative design in which S1 and S2 are tested in a common test session, despite different times between extinction and test. In this context, the phenomenon of spontaneous recovery has a complex role. That phenomenon suggests that the results that we obtain in a t2 assessment may be quite different depending on the length of time that intervenes between the t1 extinction experience and the t2 test. It has been known since Pavlovs (1927) early experiments that the loss of behavior that results from presenting the stimulus alone at t1 is not entirely permanent. Rather, with the passage of time following nonreinforcement, there is some â€Å"spontaneous recovery† of the initially learned behavior. Introducing greater time delays between t1 extinction treatment and t2 test provides the opportunity for greater spontaneous recovery. At minimum, the phenomenon of spontaneous recovery provides some information about what process fails to account for the loss of behavior when an extinction procedure is conducted. It suggests that the loss does not simply involve the removal of what was learned in acquisition. As Pavlov noted, if an extinction procedure had erased the acquisition learning, then there would be no basis for behavior to return with time. It suggests that instead something happens during the extinction procedure that temporarily  suppresses performance while leaving some of the initial learning in place. Of course, the finding of spontaneous recovery does not imply that there is no removal of the initial learning or even that there was any learning during extinction. It only implies that some of the initial learning survives an extinction procedure. The finding of spontaneous recovery may also be taken as providing some information on the nature of the processes that suppress behavior. If one believes, as Pavlov did, that the return of behavior after nonreinforcement represents the loss of a learning process that occurred during the extinction procedure, it suggests that this learning is malleable, changing with time. That is, it suggests that one property of the extinction mechanism is its relatively lower stability with time. For both of these reasons, many have hoped to learn something about the processes underlying performance loss in extinction by an investigation of spontaneous recovery. Although there have been substantial advances in the neurobiological study of extinction in the last few years (for reviews, see Myers and Davis 2002; Delamater 2004), almost all studies of spontaneous recovery have been at the purely behavioral level. Consequently, the discussion that follows will focus exclusively on such behavioral studies. Previous SectionNext Section Designs for Studying Spontaneous RecoveryThe standard description of spontaneous recovery is that the responding that was depressed during an extinction session is partially restored in a test session that is administered after a delay. As illustrated at the second panel of Figure 1, spontaneous recovery is commonly inferred from a comparison between responding at the end of an extinction session and at the beginning of a test. Greater response in the test is taken to mean that some portion of the initial learning survived the extinction. Implicit in this interpretation is the assumption (1) that the behavior that appears on the early trials of the test session is attributable to the original learning rather than to something else, and (2) that the increase between the extinction and the test represents a reduction in the effect of the learning that occurred in extinction. To justify the first assumption, comparison really should be made between responding to an extinguished stimulus and responding to one that has never been trained prior to extinction but is still given the same  interval between extinction and test sessions in which to â€Å"recover.† Otherwise, it is possible that the increase in responding represents a general tendency to increase responding with time independently of the original acquisition; that is, it might not reflect recovery of the initial learning at all. In fact, there are only a few studies (see Robbins 1990) that have deliberately made such a comparison. But any detailed investigation of spontaneous recovery should be sensitive to this possibility. To justify the second assumption (that the increase in responding represents dissipation of the extinction learning), comparison should be made with a stimulus that has trained, but not been extinguished, and that evokes responding at a level comparable to that of the extinguished stimulus before the delay interval. For instance, one might compare, for the same animal, changes with time in a trained and extinguished stimulus and a stimulus that is only partially trained, so as to attain the same response level. It is only if the former stimulus shows greate r growth with time that one would conclude that there is a loss of the learning that occurred during extinction, as distinct from a general change in performance for any previously trained stimulus showing behavior at that level. Aside from the need to occasionally include these comparisons, the spontaneous recovery design shown in Figure 1B has the drawback of repeated testing with the same stimulus, with the consequence that different numbers of extinction trials have necessarily preceded the trials being compared. A better design, which is also frequently used, is shown in Figure 1C. In that design two stimuli are both trained and extinguished but then given different amounts of time to recovery prior to the test. This design avoids repeated testing on the same stimulus and has the advantage of an explicit test session. But unfortunately, the tests of the two stimuli differ not only in the time since extinction but also in the time since original training and in the overall test context and age of the animal. To avoid these confoundings, we have frequently adopted a somewhat different design for studying spontaneous recovery, as illustrated in Figure 1D. In this comparison, two groups of animals receive co nditioning followed by extinction and a test. The groups differ in the placement of the extinction with regard to the test. For one group (S1) the extinction occurs at a temporal distance from the test, so as to allow spontaneous recovery. For the other group (S2), extinction occurs  immediately prior to test, minimizing recovery. The evidence for recovery is then the difference in responding at the time of the common test. That allows comparison of responding during the same test session, after the same number of extinction trials, to stimuli that share the time since original training but differ in the time since their extinction. The design can be further strengthened if the two stimuli are both trained in the same animal. An especially important advantage of such a within-subject comparison between S1 and S2 is that it involves a common test session in the same animal. This means that any recovery cannot be attributed to general changes in the animals state or to differential similarity of the test conditions to those of original training. One difficulty with the standard procedure for assessing spontaneous recovery is that one part of the comparison comes from a session in which the animal is undergoing an extin ction procedure and is therefore likely to be experiencing a variety of new stimulus events that may have various emotional consequences. The presence of those new events could, in themselves, artificially depress responding to a lower level than that which is warranted by the learning that is occurring. The likely absence of those stimuli at the beginning of the test session could allow greater responding for reasons that are not central to the learning that is occurring in extinction. But the final design avoids such differences in the context of performance for the stimuli being compared. By using within-subject versions of this last procedure, Rescorla (1997a,b) has shown evidence of substantial spontaneous recovery in a variety of conditioning preparations. An example is shown in Figure 2 for a Pavlovian magazine approach situation with rats. In this preparation, S1 and S2 are counterbalanced as a 30-sec light and noise, each paired with the delivery of food and then given nonreinforcement. The result is an increase and decrease in anticipatory responding involving investigation of the food delivery site, the maga zine. A test then takes place either immediately after the last extinction session or after a delay. The comparison of interest is that between S1 and S2 in the common test session. Figure 2 clearly indicates greater recovery for the stimulus extinguished first, S1. One may note that S1 also shows more responding in the test than it did at the end of extinction; but as noted above, this comparison is flawed. Figure 2 View larger version: In this page In a new window Download as PowerPoint Slide Figure 2 An illustration of spontaneous recovery using the design shown in Figure 1D. Rat subjects were given Pavlovian magazine-approach training and extinction with two stimuli, S1 and S2, and then tested in a common session. The superior responding in S1, compared with S2, is used to infer spontaneous recovery. Of course, no procedure is without its shortcomings. In this alternative procedure for assessing spontaneous recovery, one of the stimuli must necessarily be extinguished before the other. Consequently, the order in which the stimuli are extinguished, and the interval between training and test, are both confounded with the interval of interest, between extinction and test. Although application of the design has not revealed any resulting differences in the course of extinction itself (Fig. 2), one must still be cautious about the implications of such a confounding. Consequently, it may be most wise to use both procedures C and D in any research program investigating spontaneous reco very. Previous SectionNext Section Bases for Spontaneous RecoverySpontaneous recovery is such a widespread phenomenon, both in terms of the variety of learning paradigms in which it occurs and the frequency with which it is reported, that it would be surprising if it had only one source. Indeed, many different sources have been suggested, most of which have received some empirical support. But not all of these are of equal interest in helping us understand the learning involved in extinction. Here, I attempt a rough categorization of those sources. Local Performance EffectsAs noted above, when one makes the standard comparison of responding on the final trials from extinction with that on the initial trials in a test session, one can expect that they will differ in a number of ways unrelated to the learning that occurs in extinction. For instance, the repeated evocation of a conditioned response might lead to temporary fatigue that adds to the loss of behavior. It is not uncommon to see behavior decline over the course of a conditioning session  even when reinforcement continues (see McSweeney and Swindell 1999). Similarly, the surprising deletion of anticipated reinforcement may well lead to emotional responses that have a disruptive effect on performance. For instance, the surprising omission of food can be expected to lead to frustration, which changes the stimulus environment and may evoke responses of its own (see Amsel 1958). Effects such as these might well be expected to dissipate before the beginning of the test session, hence revealing any incompleteness of the extinction process that they masked at the end of the extinction session. Although such effects may contribute to the deterioration of performance during the administration of an extinction procedure, they may have little to do with any underlying learning process. If spontaneous recovery could be accounted for solely on the basis of such effects, it would be of substanti ally less interest. Contributors of this sort to response suppression seem especially likely to affect the standard spontaneous recovery designs that compare responding to the same stimulus in two different sessions. They are less likely to contribute to recovery when it is measured as the difference in responding to two stimuli during the same test session. Indeed, part of the motivation for developing the alternative assessment of spontaneous recovery (D) was to reduce the various general differences that are confounded when responding in two different sessions is compared. Spontaneous Recovery Despite Loss of Acquisition LearningDespite the widespread agreement that spontaneous recovery indicates that the primary extinction process does not involve the erasure of the original learning, there have been some attempts to preserve an erasure theory and still explain recovery. Perhaps the most well known attempt is the suggestion by Skinner (1938) that an extended extinction session leaves the stimuli fro m the beginning of the session incompletely extinguished. Consequently, those stimuli still have some strength that can be exhibited at the beginning of the next session. Skinner seems to have believed that extinction eventually removes all of the original learning but initially leaves the session-beginning stimuli partially effective, thereby explaining spontaneous recovery. Although there is some evidence that stimuli that are explicitly presented by the experimenter at the start of a session can retain some strength when extinction takes place in their absence (see Burstein and Moeser 1971), it seems unlikely that this will fully account  for spontaneous recovery. Several experiments have found evidence of strong spontaneous recovery even when the session starting stimuli are well controlled (see Thomas and Sherman 1986) or when an extinguished stimulus is tested in the middle of a session only after another stimulus has completely lost its spontaneous recovery (see Robbins 1990). Moreover, it is hard to see why this account would anti cipate one of the basic findings of spontaneous recovery, that it increases with the passage of time. A somewhat more successful version of such an account was offered by Estes (1955) as a component of his influential stimulus sampling theory. Estes argued that what experimenters identify as stimuli can actually be viewed as constructed of many elements with occurrence that fluctuates in time. Acquisition and extinction produce changes in the associations between the outcomes and those elements over the course of trials. According to that theory, when an element is paired with a reinforcer, it immediately becomes fully conditioned; when it is presented without reinforcement, it immediately reverts to its unconditioned state. It is only the random fluctuation in the selection of elements over trials that gives the normally observed gradualness to changes in behavior. At heart, this is a theory in which extinction involves the actual erasure of learning. The important point to notice is that when trials are given in close proximity, they tend to share more elements; when trials are separated, then they sample different elements from the pool that constitutes a given stimulus. Consequently, with repeated extinction trials, performance can decrease even though some unsampled elements remain conditioned. With the passage of time, the likelihood of sampling those nonextinguished elements can increase, generating spontaneous recovery. Like the account offered by Skinner, this account correctly predicts that repeated extinction will diminish spontaneous recovery. Unlike that account, it gives no special role to session-initiating stimuli, and it can account for the growth in recovery with time. The stimulus sampling theory also correctly anticipates the occurrence of spontaneous recovery with all of the various assessment procedures. These successes demonstrate that one can account for at least some of the basic facts about spontaneous recovery even on the assumption that the extinction process involves (partial) removal of the original learning. They also highlight the fact that the observation of spontaneous  recovery does not imply that extinction must leave all of original acquisition in place. Spontaneous Recovery as an Indication That Extinction Involves Inhibition If one envisions an extinction procedure as leaving intact even a portion of the original learning, then it is natural to conclude that some new learning is occurring that is superimposed on acquisition and prevents performance. The classic candidate for such new learning has been â€Å"inhibition.† A wide variety of different kinds of inhibition have been proposed, suggesting quite different ideas about the nature of extinction. But in each case, spontaneous recovery is accounted for by the dissipation of that inhibition with the passag e of time. CS-Based InhibitionPavlov (1927) was the first to suggest that extinction involves inhibition and that spontaneous recovery is an indication that this was so. He viewed the learned inhibition that he supposed to occur during extinction as more vulnerable than the excitation that develops in acquisition to such intrusions as the presentation of a new stimulus (i.e., disinhibition) and the passage of time (i.e., spontaneous recovery). Although it is not widely appreciated, Pavlovs notion of inhibition was highly focused on the CS and was envisioned to develop independently of whether or not the reinforcer occurred. Pavlov believed that every CS presentation leads to the building up of a kind of fatigue in the neural cells stimulated by that CS, whether or not the reinforcer followed. This leads to a reduced ability of the CS to stimulate its neural targets, which would in turn lead to reduced behavior. That is, the natural consequence of repeatedly presenting the CS is a deterioration of behavior. However, during acquisition, this process is overwhelmed by the growth of an association which the CS develops with the reinforcer. With rest, the CS-based inhibition fades and responding can recover. Robbins (1990) proposed a related notion of inhibition, which he described in terms of reduced attention to the CS. He suggested that during acquisition, attention to the CS grows whereas during extinction it falls. Part of the reason for the decline of behavior in extinction is then reduced attention to the CS with the attendant loss in its ability to evoke responding. With rest, that attention partially returns. Unlike Pavlov, Robbins envisioned the effectiveness of the CS as dependent on the trial consequence, growing with reinforcement and declining with nonreinforcement. In support of this view,  Robbins (1990) found evidence, in a sign-tracking preparation with pigeons, that a CS los t its ability to serve a variety of different functions during an extinction procedure but then recovered them with time. He trained a CS simultaneously to have an excitatory association with a US and also to serve as a conditional signal that another CS would not be followed by a US. When he extinguished the excitatory association, the CS lost both properties and then regained them both with the passage of time. That result is consistent with the view that one contributor to the loss of behavior with extinction and its restoration with time may be changes in the processing of the CS per se. On the other hand, both Bouton and Peck (1992) and Rescorla (1997a) have found evidence for spontaneous recovery following counter-conditioning in which one US is replaced with another. Because counter-conditioning yields a CS that continues to produce some behavior, those cases of spontaneous recovery seem unlikely to be attributable to loss and recovery of the processing of the CS. Of course, the processes underlying spontaneous recovery after such procedures may differ from those underlying the changes after extinction. Response-Based InhibitionShortly after Pavlovs work became available in this country, Hull (1943) suggested a similar account of extinction and spontaneous recovery that focused on the response rather than on the stimulus. As part of a multiprocess account of extinction, Hull argued that each occurrence of a response leads to the building up of a fatigue-like process that is specific to that response and that reduces its subsequent likelihood of occurrence. This process contributes to the decrease in performance during extinction but dissipates with time, permitting spontaneous recovery. Similar to Pavlov, Hull seems to have believed that the changes in this response-fatigue are independent of reinforcement contingencies; indeed, he believed it left no permanent learning impact at all. That is, this particular inhibitory notion is more akin to the performance effects discussed above. It has proven difficult to confirm the most obvious implication of such an approach, that the effortfulness of the response should influence the course of extinction and spontaneous recovery (see Mackintosh 1974). But, one advantage that can be claimed for the alternative test of spontaneous recovery described above is tha t it provides a common test session in which all stimuli should share any general fatigue processes. Outcome-Based InhibitionOne way of viewing  extinction is that a CS that was once followed by an effective outcome is now followed by an ineffective one. Indeed, there is evidence that if the outcome potency is deliberately partially reduced, then behavior partially deteriorates (see Wagner and Rescorla 1972; Kehoe and White 2002). This suggests the possibility that one change that occurs in extinction involves a reduction in the memory of the outcome itself. Based on this kind of thinking, Rescorla and his collaborators (see Rescorla and Heth 1975; Rescorla and Cunningham 1978) suggested that one process that occurs in an extinction procedure is a depression in what they called the â€Å"US representation.† This led them to explore manipulations that might affect the state of the US representation independently of its association with a particular CS. By using a fear conditioning preparation in rats, they documented the phenomenon of â€Å"reinstatement† first report ed by Pavlov, that responding could be restored to an extinguished CS by separate presentations of the US alone. Although others (see Bouton 1984) have given different accounts, Rescorla and Cunningham described this manipulation as restoring a portion of the extinction-depressed US representation. They also argued that there might be a recovery in the US representation with time, leading to the phenomenon of spontaneous recovery of responding to the CS. In support of that possibility they found that, under some circumstances, recovery could be undermined by the nonreinforcement of another CS immediately prior to testing, a manipulation presumed to reduce the US representation. However, various other implications of this notion have not received support. For instance, Robbins (1990) found evidence of independence in spontaneous recovery for two stimuli that had been conditioned with the same US. Moreover, Tomie et al. (1980) reported spontaneous recovery after an extinction procedure in which the US was continued but made independent of the CS, a procedure that should maintain the US representa tion. Nevertheless, it is possible that changes in the memory for the US may make a contribution to spontaneous recovery in some preparations. Associative InhibitionMost contemporary views of Pavlovian inhibition involve not individual events but rather associations between events. For instance, Konorski (1948) argued that a variety of conditioning paradigms, including extinction, lead to the development of an inhibitory association between the CS and US that is parallel to, but the opposite of, the excitatory  associations set up during acquisition. In later writings, Konorski (1967) offered a somewhat different account, according to which inhibitory learning involves associations between the CS and a consequence that is the opposite of the US, the â€Å"no US.† Most contemporary discussions of Pavlovian inhibition implicitly accept one or the other of these views. The standard ways of detecting such associative inhibition are to ask whether an inhibitory stimulus can reduce responding to an otherwise effective excitor (a summation test) or is slow to acquire excitation if it now receives an excitatory training treatment (a retardation test; Rescorla 1969). Most contemporary evidence suggests that an extinguished stimulus does not in fact demonstrate net inhibition with either of these tests. Presumably this is because the inhibition that builds up during nonreinforcement is only just sufficient to counteract the original excitation, but does not develop beyond that level so as to generate a net inhibitory stimulus. However, there is some evidence that an originally neutral stimulus that accompanies an excitatory CS during an extinction procedure does capture inhibition, as assessed by these tests (see Rescorla 1979, 1999). Moreover, there a re some reports (see Henderson 1978) that a net inhibitory stimulus does lose some of its inhibitory properties with the passage of time, in the way required if this were to contribute to spontaneous recovery. An alternative associative basis for extinction has been proposed by Colwill (1991) and explored by Rescorla (1993). According to that account, the inhibition that develops during extinction involves associations between the stimulus and a response. These inhibitory S-R associations are envisioned as existing side by side with excitatory associations that involve the US. Consistent with this view (but not with the notion of inhibition between the CS and US), Rescorla (2001b) used outcome-specific transfer procedures to reveal that the CS-US associations maintained their full net strength after extinction, despite the reduction in responding. Moreover, he found no evidence that those CS-US associations changed in the way expected with the passage of time if they were to be the basis of spontaneous recovery. He proposed instead that extinction can involve an associative response-specific process that depresses responding but deteriorates with time. An alternative view of Pavlovian conditioned inhibition in terms of modulation has received substantial recent attention. Several investigators (see Holland 1983,  Rescorla 1985) have suggested that under some circumstances an inhibitory stimulus does not develop an association with a separate event, such as US or a response, but rather acquires the ability to control the strength of an excitatory CS-US association. This type of inhibition is not the opposite of an excitatory association but rather plays more of a hierarchical role, modulating a CS-US association. Bouton (1991) has suggested that both contextual and temporal stimuli can serve this modulatory role for a stimulus subjected to an extinction procedure. During the extinction procedure, the excitatory association is seen as remaining intact but being disabled by the presence of contextual or temporal stimuli that had accompanied nonreinforcement. With a change in either the context or the temporal stimuli, this inhibitory process is attenuated, allowing the return of performance. The phenomenon of â€Å"renewal† provides evidence for this view. If, following extinction, the stimulus is tested in a different context, there can be a substantial recovery of responding (see Bouton 1991). If one views the passage of time as analogous to changing the context, this view provides a way of conceptualizing spontaneous recovery. Differential RetrievalThe inhibitory accounts of extinction and spontaneous recovery all envision the strength of original learning as remaining unchanged throughout extinction and spontaneous recovery. They see the decrease in responding during extinction as attributable to the growth of the inhibitory process, and the increase in responding with spontaneous recovery as attributable to the dissipation of that inhibitory process. In effect, the memory for the extinction process loses strength with the pas sage of time. An alternative framework for understanding extinction and spontaneous recovery involves differential retrieval of the acquisition and extinction experiences, both of which remain fully intact. For instance, Spear (1971) and Bouton (1993) have both noted the formal analogy between interference paradigms as studied in humans and the acquisition/extinction sequence of Pavlovian conditioning. In both cases, the organism is exposed to two competing pieces of information that might be expected to interfere with each other. This led both investigators to the proposal that differential performance might reflect differential likelihood of retrieving the two experiences, rather than a weakening of stored information about either experience. According to this view, manipulations that make the  retrieval of one experience or the other more likely will result in changes in overall performance. For instance, a stimulus that accompanies extinction might serve as a retrieval cue, presentation of which would promote the retrieval of the memory for extinction. Evidence for such a process can be found in the ability of some such stimuli to diminish the magnitude of spontaneous recovery (se e Brooks and Bouton 1993). A related view, focusing specifically on spontaneous recovery, is incorporated in Devenports (1998) temporal weighting rule. According to that rule, when an animal has multiple experiences with a stimulus prior to a test, it weights those experiences according to the relative time that has passed between each and the test. In the case of acquisition followed by extinction, this means that performance will deteriorate under the currently experienced nonreinforced treatment. However, as time passes, the relative temporal advantage enjoyed by the recent nonreinforcement experience will diminish, leading to spontaneous recovery. On views such as these, there is a permanence for both the acquisition and extinction experiences; what changes with time is their relative likelihood of retrieval. As this discussion indicates, a broad range of different processes have been envisioned as contributing to the decrement in performance resulting from an extinction procedure and to the recovery from that decrement with the passage of time. Moreover, one can identify evidence supporting each idea. It seems likely that each of these may contribute to the changes in various situations. However, none of them seems sufficient to provide an account on its own. One challenge that each contributor faces is to provide an account of the demonstrated empirical determinants of spontaneous recovery. For this reason it is worth reviewing some of those determinants. Previous SectionNext Section Basic Empirical PropertiesDespite the centrality of spontaneous recovery to the phenomenon of extinction, there is actually surprisingly little well-documented information on its detailed properties. However, four features of spontaneous recovery seem to be widely accepted. Spontaneous Recovery Increases in a Negatively Accelerated Fashion Over Time Almost every description of spontaneous recovery includes the claim that recovery is greater the greater the delay between extinction and test. Indeed, there is a sense in which greater recovery with more time is a defining property.  Moreover, the form of that increase appears to be negatively accelerated. For instance, in recent years, negatively accelerated recovery has been found with eyelid conditioning in rabbits (Haberlandt et al.1978), sign-tracking in pigeons (Robbins 1990), and fear-conditioning in rats (Quirk 2002). Almost all of the potential contributors to recovery listed above appear to be consistent with such a pattern o f change. Although the various inhibitory theories make no specific predictions about how that inhibition fades with time, the negatively accelerated nature of many biological processes makes the finding unsurprising. The stimulus sampling mechanism described by Estes (1955) and the differential weighting rule proposed by Devenport (1998) also yield this expectation. Perhaps the only potential contributor that does not anticipate this is Skinners (1938) suggestion that at the beginning of the session stimuli retain their ability to evoke a response. Spontaneous Recovery Is IncompleteAlthough Pavlov claimed to have observed full recovery from extinction, most other investigators have reported only partial recovery. Even when recovery of responding appears to be complete on the first test trial, the rapid loss of responding over the course of testing suggests that recovery was actually less than full (notice, for instance, the rapid loss during testing shown in Fig. 2). The incompleteness of recovery appears to be mandated by some of the mechanisms described above. For instance, the stimulus sampling account of Estes and the weighting rule of Devenport appear incapable of allowing the impact of extinction to disappear altogether. The other mechanisms are less well-specified in this regard. Spontaneous Recovery Declines With Repeated ExtinctionIt is widely agreed that the greater the amount of extinction, the less the magnitude of spontaneous recovery after any fixed waiting time. This most frequently shows up as the reduced amount of recovery from day to day over multiple days of extinction. An illustration of that decline is shown in Figure 3, which displays results from a recent magazine-approach study with rat subjects done in our laboratory. That figure shows responding during repeated daily extinction sessions in which a 30-sec noise was presented eight times without its food pellet. Another stimulus, a light, received intervening reinforced trials. It is clear that there is repeated recovery of responding that gets smaller over the course of extinction. Figure 3 View larger version: In this page In a new window Download as PowerPoint Slide Figure 3 The decline in the magnitude of spontaneous recovery with repeated extinction. Rat subjects were given Pavlovian magazine-approach training with a noise and then repeated extinction sessions. Actually, in the absence of some better understanding of scaling issues, it is difficult to compare quantitatively the amounts of recovery for stimuli that have undergone different amounts of extinction. Clearly, if minimal extinction has occurred, there is less decrease in behavior and hence less opportunity for recovery to occur. Similarly, with massive extinction, performance may be at a floor, and hence, even substantial recovery may be difficult to detect. But most mechanisms of recovery appear to anticipate that the deeper the extinction the less the recovery. That prediction is obligatory for accounts of recovery such as that offered by Estes and by Devenport, who see repeated extinction as accumulating a relatively permanent depressive process. Data such as those shown in Figure 3 are commonly taken as consistent with this prediction. Spontaneous Recovery Can Be Brought Under Stimulus ControlThere is good evidence that the learning process that occurs during extinction can be brought under stimulus control. For instance, Rescorla (1979) reported that a stimulus present during the nonreinforcement of a previously trained excitor took on the properties of a conditioned inhibitor, suppressing responding to other excitors. Indeed, the so-called conditioned inhibition paradigm consists of little more than intermixing reinforced and nonreinforced presentations of a stimulus while signaling the nonreinforced presentations by a second stimulus. Similarly, Bouton (1991) has reported that if the context present during extinction is removed by testing in another context, then the suppressive effects of extinction are reduced and behavior is â€Å"renewed.† Brooks and Bouton (1993) have extended these observations to the responding observed in spontaneous recovery. They found (see also, Brooks 2000) that if an explicit stimulus is present during extinction of an excitor, then that stimulus has the ability to diminish  spontaneous recovery if it is presented at the time of the test. Although few theories of extinction are challenged by the observation that whatever is occurring in extinction can be brought under the control of a stimulus, retrieval theories seem like the most natural account. For instance, Bouton has argued that a stimulus present during extinction is especially good at retrieving a memory for a CS-US association. There is now substantial evidence that one stimulus can be learned as a signal of the relation between another stimulus and the US (see Schmajuk and Holland 1998). Such modulation or â€Å"occasion setting† could be the mechanism by which explicit stimuli, contexts, or even time, might activate the mem ory of a CS-US association (see Bouton 1991, 1993). Previous SectionNext Section Determinants Needing Further InvestigationIn addition to these four well-established findings, there are a variety of other manipulations that have been claimed to affect spontaneous recovery but for which there is substantially less or even contradictory evidence. This is unfortunate because the effects of many of these manipulations might be informative in identifying the contributions of particular mechanisms of recovery. The Symmetry of Spontaneous Recovery and RegressionAccounts of spontaneous recovery differ in the degree to which they treat extinction as engaging a special learning process with distinctive properties, such as the likelihood of its memory fading in time. Beginning with Pavlovs, the various proposals of inhibitory processes have tended to see them as different from excitatory process precisely in their greater instability with the passage of time. This is clearly true for the fatigue-like processes mentioned by Pavlov, Robbins, and Hull, but it also seems true o f some associative inhibition accounts, such as those described by Rescorla and Bouton. By contrast, the stimulus sampling theory of Estes and accounts that appeal to retrieval or relative weighting seem to make little distinction between acquisition and extinction processes in their inherent vulnerability to time. They see the animal as integrating two experiences that it receives sequentially in time in a similar way regardless of the identity of those processes. This means that the latter accounts anticipate that one should observe a companion phenomenon to spontaneous recovery from extinction if one were to interchange the order in which extinction and acquisition were administered.  That is, they expect to see regression of responding after acquisition if that acquisition were preceded by some sort of nonrein-forced training. The evidence for such regression is highly mixed. Notice that the simple deterioration of performance from day to day during acquisition is not sufficient to identify regression that is the opposite of spontaneous recovery. The critical obse rvation is that there is a deterioration in performance that is attributable to a prior history of nonreinforcement, just as the critical observation for spontaneous recovery is that there is an improvement with time that is specific to stimuli that have a history of reinforcement. When animals are given in sequence two reinforcement experiences that differ in reinforcer valence or frequency, there is evidence that regression in the direction of the first performance can be observed with time (see Bouton and Peck 1992; Mazur 1996). But the results are less clear when nonreinforcement of a stimulus precedes reinforcement prior to the waiting period. Some early experiments reported positive results (see Spear et al. 1965; Konorski 1967). But some more recent studies have found no evidence for regression or the opposite results (see Kraemer et al. 1991; Rescorla 2001b). Clearly it would be valuable to understand the circumstances under which one obtains either regression or its opposite if one is to evaluate the contributions of various mechanisms to spontaneous recovery. It will surely be important to distinguish among different kinds of nonreinforcement experience that might precede reinforcement. The simple exposure to a stimulus prior to any reinforcement is certain to endow it with properties different from those of a stimulus that signals nonreinforcement explicitly, as in the case of conditioned inhibition training or even extinction. But there are not sufficient data to indicate whether or not this distinction matters for the production of regression. Recovery Following Massed or Spaced Extinction TrialsThere is reason to anticipate that conducting extinction with short intertrial intervals may encourage more rapid response decrement followed by more substantial recovery. Certainly this is the expectation of accounts such as that of Hull, which emphasizes short-term fatigue-like effects, and of Estes, which emphasizes that massing of trials would yield repeated sampling of the same stimulus elements but neglect of others. Indeed, one might argue that there is a logical sense in which spaced trials should lead to slower decrement  and less recovery. Presumably widely spaced trials would allow for any recovery between individual trials, resulting in slower behavioral loss over the course of an extinction procedure but more substantial change by the time that a test for recovery is imposed. Despite the appeal of these arguments, the evidence on the impact of massing or spacing extinction trials is quite mixed. A number of investigators (see Rescorla and Durlach 1987; Cain et al. 2003) have reported that massing produces rapid loss of performance. However, Rescorla and Durlach reported no difference in the magnitude of responding in a subsequent test for spontaneous recovery and Cain et al. (2003) reported continued less responding after massed extinction even with the passage of time. To complicate matters further, Stanley (1952) reported that for an instrumental training task, massing slowed extinction on one measure and speeded it on another in an instrumental choice situation. Interval Between Learning and ExtinctionAlthough most attention has focused on the interval between extinction and the recovery test, it is also of interest to ask about the impact of the interval between the original training and extinction, as a determinant of spontaneous recovery. The retrieval theory proposed by Spear and the weighting rule described by Devenport both suggest that spontaneous recovery should be maximal when the interval between acquisition and extinction is minimized. In both cases, the intuition is that when training and extinction are close in time, it should be more difficult for the animal to recall which is the more recent. Immediately after extinction, the relative temporal recency of the nonreinforced experience should be maximal. However, as time passes, and the two experiences are more equally distant in time, they should become more equivalent in their likelihood of being retrieved. The increase in the relative likelihood of retrieving the original acquisition experience would then produce spontaneous recovery. A similar reasoning would lead to the relatively greater impact of the acquisition experience according to the weighting rule. Mechanisms of recovery that appeal to the loss of the extinction experience have no natural way to predict that the interval between training and extinction should matter. Unfortunately, there are very few attempts to examine this possibility. There is some supportive evidence from studies of proactive inhibition in humans (Underwood and Freund 1968) and from  counter-conditioning in rats (Gordon and Spear 1973), but little for the case of extinction. Recently, Rescorla (2004) has reported that a longer time interval between training and test diminishes spontaneous recovery in magazine approach and instrumental responding in rats and in sign-tracking in pigeons. One illustration is shown in Figure 4. That figure displays the results of extinction and testing with two stimuli given acquisition, extinction, and a test for spontaneous recovery in a magazine-approach procedure using rat subjects. The stimuli shared the same 48-h recovery interval after extinction but differed in that a greater interval (8 d versus 1 d) intervened between training and extinction for S1 than for S2. The two stimuli showed virtually identical behavior over the course of extinction. However, the results of a test for spontaneous recovery given 48 h after extinction show greater spontaneous recovery for the stimulus with the shorter training/extinction interval (S2). Figure 4 View larger version: In this page In a new window Download as PowerPoint Slide Figure 4 Evidence for greater spontaneous recovery with a greater interval between training and extinction. Rat subjects were given Pavlovian magazine-approach training, extinction, and a common test for spontaneous recovery with two stimuli, S1 and S2. The stimuli differed in the interval between their original training and extinction. Results such as these suggest that, consistent with some retrieval theories, spontaneous recovery may be a decreasing function of the acquisition/extinction interval. But clearly more work needs to be done on this potentially informative parameter. Previous SectionNext Section Conclusion The picture that emerges from this discussion of spontaneous recovery is one of a process that is greatly in need of further empirical investigation. The available evidence fails to identify any one proposed process as the sole basis for spontaneous recovery. However, there is also evidence in support of all of the suggestions so far offered. This, together  with the ubiquity of spontaneous recovery, encourages the belief that it is a result that is multiply determined. Perhaps this is not surprising because it seems almost certain that the response decrement that is observed in extinction itself has multiple contributors. The fact that spontaneous recovery is likely to have multiple sources limits our ability to use it to identify the learning underlying extinction. The inference that extinction does not fully remove all of original acquisition seems secure. Spontaneous recovery is joined by a variety of other phenomena, such as disinhibition, renewal, reinstatement, and augmented summation (see Reberg 1972) as a basis for that inference. But the simple observation of spontaneous recovery does not force the inference that all of original learning remains nor even that the learning that occurred during extinction fades in time. In the light of this conclusion, it is unfortunate that we do not have a clearer picture of how some of the parameters of most potential interest affect spontaneous recovery. But it suggests that if one is to use spontaneous recovery as a tool to understand the nature of the processes occurring in extinction, one cannot simply celebrate its occurrence or its failure to occur. We will have to do much more analytic experiments determining the circumstances under which it occurs in the particular extinction situation under study.

Milling machines: An introduction

Milling machines: An introduction Chapter I 1.1 Introduction The ability of any nation to manufacture is essential but to deploy that ability in a certain nation is the trick as every nation defers from one another, so in order to enhance Arab nation ability to manufacture we will provide a better cheaper design for a CNC milling machine. The Primary Objective of This Project is to present and develop a prototype for a substitution of the expensive hard to acquire machining facility for the Middle East industrial society; these machining facilities include the regular Turning, Drilling, Shaping, Milling, and Grinding Machines. Our primary survey concluded that there always must be a turning machine beside a milling machine, but a milling machine (CNC milling machine) can include the rest of processes required within its operation to a good degree and the turning operation to an acceptable degree. So we chose the milling machine as our initial model to undergo our optimization plan. Therefore our Primary objective of choice is to Design a Basic Three Axis CNC milling machine. Also to manufacture a working Prototype for this machine fabricated all with nationally available components in order to minimize its cost but maintains an acceptable level of accuracy to suite a Computer Numerical Control Machine. Another Thing that we must point out is that in order to accomplish this project we need more than one trial to find the suitable combination of components and specs to pursue in order that this machine comes to light. 1.2 Proposed Approach A new approach is needed that approach is based upon the 2010-2015 World Outlook for Computer Numerical Controls [1]. The results of that survey were mainly based upon word wide research includes Middle East and North Africa. These results showed that any machine tool required for the Egyptian industrial society could be one of the following: Turning, Drilling, Shaping, Milling, and Grinding. So the first step was to pick up the operation over which this thesis will focus on. Milling can include in its operation: Drilling, Shaping, and Grinding. And might contain Turning under certain conditions. So milling was chosen. And this result might be verified by the fact that 90% of the working machine tools in the field of traditional CNC machines are either Milling or Turning Centers. Now we will compare different configuration of milling machines Case 1: A Universal Milling Machine. Case 2: A regular CNC Milling Machine. Case 3: Gantry style CNC Milling Machine. Case 1: A Universal Milling Machine From Cost point of view: Cost is high because of the usage of large material to build the bulky parts of the machine tool might be as high as a CNC Milling Machine From Design point of view: Design level might be considered low compared with the design required in both the regular CNC machine and our machine. From Accuracy Point of view: Accuracy is very low compared to the other two cases but could be difficulty approached with the use of very skilled labor and with high cost. Case 2: A regular CNC machine From Cost Point of view: Cost is much higher because of the usage of both large material amounts with high machining grade as will as the cost of the motion control equipments. From Design Point of view: Design is better than the first case for every part is designed to enhance the entire machining operation. From Accuracy Point of view: Attained accuracy is very high due to perfect guide ways assisted by ball screws along with feed back control. Case 3: A gantry CNC milling machine From Cost Point of view: Cost is mush less than the later case and might be less that the first case as the entire machine is only build from a metal sheet and some Bars. From Design Point of view: Design is much sophisticated than the first and the second case as the design is optimized to hold the predetermined forces as a light weight structure. From Accuracy Point of view: Accuracy is for sure less than the second case but higher than the first case as there is no feedback control technique employed. Discussion of results: For case 1: Universal Milling Machine Our results dont imply that the use of that machine is not appropriate but we only mean that each application requires a certain machine and also each goal. The cost is very high in EGYPT as almost all of them is imported but not made in EGYPT but there maintenance and overhauling is available domestically. For Case 2: three axis CNC milling machine Vertical Column type. The usage of this type of machines in any workshop requires a great deal of money so most of the manufacturers comprises between money and accuracy. The cost is definitely much higher as there is almost no one building or even providing maintenance for these machines in EGYPT. For Case 3: Gantry style CNC milling machine. The usage of this machine is aimed to workshops who would like to get both cheap machining with good accuracy. The cost is very low as of the innovative design of putting the entire axis moving over each other rather than independent from each other as in the regular design of any CNC machine Chapter II Literature Review 2.1 Milling Machine Milling is the process of machining flat, curved, or irregular surfaces by feeding the work piece against a rotating cutter containing a number of cutting edges. The milling machine consists basically of a motor driven spindle, which mounts and revolves the milling cutter, and a reciprocating adjustable worktable, which mounts and feeds the work piece. 2.1.2 Milling Machine History Unlike lathes, which have been known for thousands of years, milling machines are less than two hundred years old. Because they require much more power than hand-driven lathes, their introduction had to wait for the invention of industrial water and steam power. Also, all their mechanical components had to first be made available, such as accurately fitted slides, large castings to resist cutting forces, calibrated lead screws, and hardened steel cutting tools. Eli Whitney is credited with inventing the first milling machine about 1818, but the knee-and-column support arrangement of the universal milling machine of Joseph A. Brown (later of Brown and Sharpe) dates from 1862 and marks an important step in the machines development. During the last half of the nineteenth Century, milling machines gradually replaced shapers and planers which have lathe-type, single-point tool bits that move over the work in a straight line and scrape off metal one stroke at a time. Milling machines, with their continuous cutting action, not only remove metal faster than shapers and planers, they perform additional operations like cutting helices for gears and twist drills. Today, milling machines greatly outnumber shaping and planning machines. Americans in New England and later the Midwest continuously added features leading to the modern milling machine. Another important development came in the 1930s when Rudolph Bannow and Magnus Wahlstrom brought out the Bridgeport-style vertical milling machine. This design offers versatility and economy in place of the higher metal removal rates of traditional horizontal milling machines. Because of this versatility, there are more Bridgeport-style mills in existence today than any other milling machine design. Horizontal mills are now usually reserved for production applications where high metal removal rates on identical parts are needed, not prototyping and short runs. Bridgeport-style machines are also called knee-and-column machines and turret mills. For production applications, there are large, expensive milling machines with three or more axes under computer control. Some machines perform all operations including automatic tool changing. However, today there is an intermediate step between a manual mill and a fully automated one. Adding a computer, digital readouts, and actuators to the X- and Y-axes of a Bridgeport-style mill does this. Not only can this enhanced machine tirelessly perform all its existing repetitive functions, it also has added new capabilities. Now the mill can engrave (drive the tool to cut numbers and letters in various sizes and fonts), cut radii and angles without a rotary table, make islands, pockets, and cut ellipses, and frames. Entering the position, diameter and number of holes, automates cutting a bolt-hole pattern; the system does the math. The computer can automatically compensate for the reduced diameter of re sharp end milling cutters, saving time and money. The system can be manually programmed through its control panel, use stored programs, â€Å"learn† new tasks by memorizing a series of manual operations as the operator makes the first part, or accept files from CAD programs. 2.1.1 Type of MILLING MACHINES Milling machine are classified on the base of the position of their spindle. The spindle operates in either a vertical or horizontal position or on base of construction. In a following section we represent the different type of milling machine. Column Knee-type Milling Machine Fig 2 machines are characterized by a vertically adjustable worktable resting on a saddle which is supported by a knee. The knee is a massive casting that rides vertically on the milling machine column and can be clamped rigidly to have column in a position where the milling head and milling machine spindle are properly adjusted for operation. The plain vertical column knee type machines are characterized by a spindle located vertically, parallel to the column face, and mounted in a sliding head that can be fed up and down by hand or power. Modern vertical milling machines are designed so the entire head can also swivel to permit working on angular surfaces. The plain horizontal milling machines column contains the drive motor and gearing and a fixed position horizontal milling machine spindle. An adjustable overhead arm containing one or more arbor supports projects forward from the top of the column. The arm and arbor supports are used to stabilize long arbors. Supports can be moved along the overhead arm to support the arbor where support is desired depending on the position of the milling cutter or cutters. UNIVERSAL MILLING MACHINE The basic difference between a universal milling machine and a plain column knee type milling machine is the addition of table swivel housing between the table and the saddle of the universal machine. This permits the table to swing up to 45 ° in either direction for angular and helical milling operations. The universal machine can be fitted with various attachments such as the indexing fixture, rotary table, slotting and rack cutting attachments, and various special fixtures. RAM-TYPE MILLING MACHINEThe ram-type milling machine is characterized by a spindle mounted to a movable housing on the column to permit positioning the milling cutter forward or rearward in a horizontal plane. Two popular ram-type milling machines are the universal milling machine and the swivel cutter head ram-type milling machine. Gantry Milling Machinefigure 5is simplest construction of milling machine is usually use in CNC machine for cutting wood plastic and light metal Rotary-Table Milling MachinesThese are also called continuous milling machines, as the work picas are set up without stopping the operation. Rotary-table machines are highly productive; consequently, they are frequently used for both batch and mass production. The Work prices being machined are clamped in fixtures installed on the rotating table (2) figure 6. The machines may be equipped with one or two spindle heads (1).When several surfaces are to be machined; the Work pieces are indexed in the fixtures after each complete revolution of the table. The machining cycle provides as many table revolutions as the number of surfaces to be machined. 2.1.2 MILLING CUTTERS The milling cutters are selected for each specified machining duty. The milling cutter may be provided with a hole to be mounted on the arbor of the horizontal milling machines, or provided with a straight or tapered shank for mounting on the vertical or horizontal milling machine. Figure 7 visualizes commonly used milling cutters during their operation. These include the following: Plain milling cutters are either straight or helical ones. Helical milling cutters are preferred for large cutting widths to provide smooth cutting and improved surface quality (Figure 7 a). Plain milling cutters are mainly used on horizontal milling machines. Face milling cutters are used for the production of horizontal (Figure 7 b), vertical (Figure 7 c), or inclined (Figure 7 d) at surfaces. They are used on vertical milling machines, planer type milling machines, and vertical milling machines with the spindle swiveled to the required angle. Side milling cutters are clamped on the arbor of the horizontal milling machine and are used for machining of the vertical surface of a shoulder (Figure 7 e) or cutting a keyway (Figure 7 f). Interlocking (staggered) side mills (Figure 7 g) mounted on the arbor of the horizontal milling machines are intended to cut wide keyways and cavities. Slitting saws (Figure 7 h) are used on horizontal milling machines. Angle milling cutters, used on horizontal milling machines, for the production of longitudinal grooves (Figure 7 I) or for edge chamfering. End mills are tools of a shank type, which can be mounted on vertical milling machines (or directly in the spindle nose of horizontal milling machines). End mills may be employed in machining keyways (Figure 7 j) or vertical surfaces (Figure 7 k). Key-cutters are also of the shank type that can be used on vertical milling machines. They may be used for single-pass milling or multi pass milling operations (Figures 7 l and 7 m). Form-milling cutters are mounted on horizontal milling machines. Form cutters may be either concave as shown in (Figure 7 n) or convex as in Figure (7 o). T-slot cutters are used for milling T-slots and are available in different sizes. The T-slot is machined on a vertical milling machine in two steps: 1-Slotting with end mill (Figure 7 j) 2-Cutting with T-slot cutter (Figure 7 p). Compound milling cutters are mainly used to produce compound surfaces. These cutters realize high productivity and accuracy (Figure 7 q). Inserted tool milling cutters have a main body that is fabricated from tough and less expensive steel. The teeth are made of alloy tool steel, HSS, carbides, ceramics, or cubic boron nitride (CBN) and mechanically attached to the body using set screws and in some cases are brazed. Cutters of this type are confined usually to large-diameter face milling cutters or horizontal milling cutters (Figure 7 q). Gear milling cutters are used for the production of spur and helical gears on vertical or horizontal milling machines (Figures 7 r and 7 s). Gear cutters are form-relieved cutters, which are used to mill contoured surfaces. They are sharpened at the tooth face. Hobbing machines and gear shapers are used to cut gears for mass production and high-accuracy demands. 2.1.3 Milling operation Milling is the removal of metal by feeding the work past a rotating multi toothed cutter. In this operation the material removal rate (MRR) is enhanced as the cutter rotates at a high cutting speed. The surface quality is also improved due to the multi cutting edges of the milling cutter. The action of the milling cutter is totally different from that of a drill or a turning tool. In turning and drilling, the tools are kept continuously in contact with the material to be cut, whereas milling is an intermittent process, as each tooth produces a chip of variable thickness. Milling operations may be classified as peripheral (plain) milling or face (end) milling (Figure 8). In peripheral milling, the cutting occurs by the teeth arranged on the periphery of the milling cutter, and the generated surface is a plane parallel to the cutter axis. Peripheral milling is usually performed on a horizontal milling machine. For this reason, it is sometimes called horizontal milling. The appearance of the surface and also the type of chip formation are affected by the direction of cutter rotation with respect to the movement of the WP. In this regard, two types of peripheral milling are differentiable, namely, up-milling and down-milling. Up-milling is accomplished by rotating the cutter against the direction of the feed of the WP (Figure 9 a). The tooth picks up from the material gradually; that is, the chip starts with no thickness and increases in size as the teeth progress through the cut. This means that the cycle of operation to remove the chip is first a sliding action at the beginning and then a crushing action takes place, which is followed by the actual cutting action. In some metals, up-milling leads to strain hardening of the machined surface, and also to chattering and excessive teeth blunting. Advantages of up-milling include the following: It does not require a backlash eliminator. It is safer in operation (the cutter does not climb on the work). Loads on teeth are acting gradually. Built-up edge (BUE) fragments are absent from the machined surface. The milling cutter is not affected by the sandy or scaly surfaces of the work. Down-Milling is accomplished by rotating the cutter in the direction of the work feed, as shown in Figure 9b. In climb milling, as implied by the name, the milling cutter attempts to climb the WP. Chips are cut to maximum thickness at initial engagement of cutter teeth with the work, and decrease to zero at the end of its engagement. The cutting forces in down milling are directed downward. Down-milling should not be attempted if machines do not have enough rigidity and are not provided with backlash eliminators. Under such circumstances, the cutter climbs up on the WP and the arbor and spindle may be damaged. Advantages of down-milling include the following: Fixtures are simpler and less costly, as cutting forces are acting downward. Flat WPs or plates that cannot be held can be machined by down-milling. Cutter with higher rake angles can be used, which decreases the power requirements. Tool blunting is less likely. Down-milling is characterized by fewer tendencies of chattering and vibration, which leads to improved surface finish. up milling down milling In face milling, the generated surface is at a right angle to the cutter axis. When using cutters of large diameters, it is a good practice to tilt the spindle head slightly at an angle of 13 ° to provide some clearance, which leads to an improved surface finish and eliminate tool blunting. Face milling is usually performed on vertical milling machines; for this reason, the process is called vertical milling, which is more productive than plain milling. 2.2 Computer Numerical Control The abbreviation CNC stands for computer numerical control, and refers specifically to a computer controller that reads G-code instructions and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via a software load program. 2.2.1 Historical overview CNC was preceded by NC (Numerically Controlled) machines, which were hard wired and their operating parameters could not be changed. NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing. Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades after 1950, until it was eventually superseded by RS232 cables, floppy disks, and now is commonly tied directly into plant networks. The files containing the G-codes to be interpreted by the controller are usually saved under the .NC extension. Most shops have their own saving format that matches their ISO certification requirements. The introduction of CNC machines radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action has been dramatically reduced. With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality have been achieved with no strain on the operator. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required changing the machine to produce different components. 2.2.2 Production environment A series of CNC machines may be combined into one station, commonly called a cell, to progressively machine a part requiring several operations. CNC machines today are controlled directly from files created by CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the manufactured component. In a sense, the CNC machines represent a special segment of industrial robot systems, as they are programmable to perform many kinds of machining operations (within their designed physical limits, like other robotic systems). CNC machines can run over night and over weekends without operator intervention. Error detection features have been developed, giving CNC machines the ability to call the operators mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts it is already loaded with up to that tool and wait for the operator. T he ever changing intelligence of CNC controllers has dramatically increased job shop cell production. Some machines might even make 1000 parts on a weekend with no operator, checking each part with lasers and sensors. 2.2.3 CNC Components and Control System The two main components of a CNC machine are the machine tool and the machine control unit (MCU). The MCU is the computer that operates the machine tool. There are many manufacturers of these control units, so program codes may vary somewhat between machines. Figure 10 shows three types of control systems for NC and CNC machines. The least expensive type is called a point-to-point (PTP) control system. A PTP control system can only move a tool in a straight line and is limited to hole operations (drilling, boring, reaming, etc.) and spot welding. The tool path between points is not controllable in a PTP system. A straight-cut control system is capable of moving the cutting tools in a straight line between points. However, only one axis drive motor can move at one time, so arcs and angles are not possible. The contouring control system is the most expensive and flexible of the three systems. With the recent advances in electronics, the price difference between these systems has narrowed so much that every CNC machine is now manufactured with a contouring control system. A contouring system is capable of doing what the other systems can do while also being able to cut arcs and angles. 2.2.3 Positioning Formats A CNC program is written using the Cartesian coordinate system. The CNC tool can be moved inside the rectangular coordinates in either two or three dimensions. Lathes are programmed two-dimensionally using only the x- and y-axes, while milling machines are programmed three-dimensionally using the x-, y-, and z-axes. The z-axis is always the axis that determines the depth of cut. The x-axis is parallel to the longest length of the machine table, and the y-axis is parallel to the shortest length of the machine table. CNC programs can be written in one quadrant or four quadrants. CNC machines can be positioned in two ways with respect to the Cartesian coordinate system. They can be positioned in an absolute format or an incremental format. In the absolute format, all machine locations are taken from starting point (0, 0), which is referred to as the home position. In incremental format there are no assigned or home coordinates. The cutter tool bit moves in spaces (increments) relative to the coordinates previously listed. The absolute format is typically used for the main body of a CNC program. This allows the cutting tool to be given a home position to start and end its job. The incremental format is used in the middle of a program when a repetitious cycle (commonly referred to as a canned cycle) is needed. A canned cycle is a cutting pattern that can be programmed to repeat as many times as desired. It can reduce the size of a program tremendously, depending on the number of machining operations involved. The size of a program is reduced by a canned cycle because a programmer does not have to give the fixed coordinates of each move of the cutting tool. A programmer must only give the distance from each previous move of the cutter for one cycle. After that, a programmer merely has to list the number of times the cycle must be repeated. A programmer can also call up the canned cycle and use it again to do a similar machining operation in another part of the work piece. 2.2.4 CNC Programming Because of the added feature of a computers memory, CNC programming is a lot easier and simpler than conventional NC programming. Most CNC machines can be programmed using a standardized list of codes that were developed by the Electronics Industries Association (EIA), as shown in Table 1. These codes include preparatory functions or G codes. Some of these functions include cutting an arc in a clockwise direction (G02 circular interpolation) and telling the machine that the dimensions will be given in millimeters (G71-metric programming). 2.2.5 Task Flow in CNC The task flow that is needed for producing apart using a CNC machine can be summarizedas Fig.1.2.The tasks can be classified as the following three types: Offline tasks: CAD, CAPP, CAM. Online tasks: CNC machining, monitoring and On Machine Measurement. Post-line tasks: Computer Aided Inspection (CAI), post-operation. Offline tasks are the tasks that are needed to generate apart program for controlling a CNC machine. In the offline stage, after the shape of apart has been decided, a geometry model of this part is created by 2D or 3D CAD. In general, CAD means Computer Aided Design After finishing geometric modeling, Computer Aided Process Planning, CAPP, is carried out where necessary information for machining is generated. In this stage, the Selection of machine tools, tools, jig and fixture, decisions about cutting conditions, Scheduling and machining sequences are created. Because process planning is very Complicated and CAPP is immature with respect to technology, process planning generally depends on the know how of a process planner. CAM (Computer Aided Manufacturing) is executed in the final stage for generating apart program. In this stage, tool paths are generated based on geometry information from CAD and machining information from CAPP. During tool path generation, interferences between tool and work piece, minimization of machining time and tool change, and machine performance are considered .In particular; CAM is an essential tool to generate 2.5D or 3D tool paths for machine tools with more than three axes. Online tasks are those that are needed to machine parts using CNC machines. A part program, being the machine understandable instructions, can be generated in the above mentioned offline stage and part programs for a simple part can be directly edited in CNC by the user. In this stage, the CNC system reads and interprets part programs from memory and controls the movement of axes. The CNC system generates instructions for position and velocity control based on the part program and servo motors are controlled based on the instructions generated. As the rotation of a servo motor is transformed into linear movement via ball screw mechanisms, the work piece or tool is moved and finally, the part is machined by these movements. In the online stage, the status of the machine and machining process may be monitored during machining. Actually, tool breakage detection, compensation of thermal deformation, adaptive control, and compensation of tool defection based on monitoring of cutting force, heat, and electric current are applied during machining. On Machine Measurement is also used to calculate machining error by inspecting the finished part on the machine, returning machining errors to controller to carryout compensation. The post-line task is to carryout CAI (Computer Aided Inspection), inspecting the finished part. In this stage, inspection using a CMM (Coordinate Measurement Machine) is used to make a comparison between the result and the geometry model in order to perform compensation. The compensation is executed by modifying tool Compensation or by doing post operations such as re machining and grinding. Reverse engineering, meaning that the shape of the part is measured and a geometric model based on the measured data is generated, is included in this stage. As mentioned above, through three stages, it is possible for machine tools not only to satisfy high accuracy and productivity but also to machine parts with complex shape as well as simple shapes. Because CNC machines can machine a variety of parts by changing the part program and repetitively machine the same part shape by storing part programs, CNC machines can be used for general purposes.