A study of the conversion of machine tools to computerised numerical control

Abstract

The particular application of the study is the conversion of the Elliott Laboratory Lathe to CNC. Since the subject spans a large number of machine types, an attempt has been made to take a general approach to it as much as, possible without going into excessive theoretical detail. In the meantime, the items discussed are applied to the Elliott Lathe. Hence the analysis always ends with a practical application. Chapter I starts by presenting an overview of the conversion of machine tools to CNC. After describing the major differences between NC and CNC, a short history of control leads to retrofitting and to the reasons why a retrofit is usually performed. Then a set of design criteria are set up and these are discussed in the chapters which follow. In Chapter II, the mechanical design requirements are taken into consideration, both with respect to elimination of components and the introduction of new items. Obviously, not all the aspects dealt with gave the desired results (e.g. there is little one can change in the vibration characteristics of the old machine). Once the mechanical situation of the machine was assessed, and found to be suitable for conversion, the control system and can now be chosen. This is discussed in Chapter III, and although Control Theory was studied in some depth only the most relevant aspects were introduced. Since Chapters II and III are quite long, a resumee is given at the end of each. Chapter IV then discusses the actual implementation. The alignment tests are presented and where possible, particular British Standards quoted. Since the aim of the work is to give a general outlook on the subject, working drawings were avoided. Having considered all the necessary items, the computerized control facilities available were studied. This chapter, Chapter V, serves to outline the capabilities of this powerful tool, the computer, which is in the hands of the engineer today. Finally examples of part programming are also given, some exercises of which were performed on the CNC lathe in the laboratory. Chapter VI deals with the cost analysis, citing actual examples of conversions and also comparing the cost of a new lathe with a retrofit of the laboratory lathe. Finally, a general conclusion is drawn in Chapter VII.