Graham T. Smith (auth.) - CNC Machining Technology_ Volume I_ Design, Development and CIM Strategies

  • No School
  • AA 1
  • 186

This preview shows page 1 out of 186 pages.

You've reached the end of your free preview.

Want to read all 186 pages?

Unformatted text preview: CNC Machining Technology Volume I Design, Development and elM Strategies Graham T. Smith CNC Machining Technology Volume I Design, Development and elM Strategies With 83 Figures Springer-Verlag London Berlin Heidelberg New York Paris Tokyo Hong Kong Barcelona Budapest Graham T. Smith Technology Research Centre, Southampton Institute, City Campus, East Park Terrace, Southampton S09 4WW, UK Cover illustration: Ch.l, Fig.46. An application of a rotary inductosyn. ISBN-13:978-3-540-19828-4 e-ISBN-13:978-1-4471-2051-3 DOl: 10.1007/978-1-4471-2051-3 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. © Springer-Verlag London Limited 1993 The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Typeset by Best-set Typesetter Ltd., Hong Kong 69/3830-543210 Printed on acid-free paper To my grandfather Mr T. W. Chandler who encouraged me to take an interest in all things ro<por ANHP 0 ES /,MAr JIE!PAr ,d!,dArKOMENOr rO<POTATOr ,dE 0 EK THr TQN AAAQN Translation: A wise man learns from experience and an even wiser man from the experience of others PLATO 428-348 Be Contents 1 1.1 1.2 1.3 1.4 1.5 1.6 2 The Development and Design of CNC Machine Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Historical Perspective - the Early Development of Numerically Controlled Machine Tools. . . . . . . . . . . The Economics of CNC . . . . . . . . . . . . . . . . . . . .. The Design and Construction of CNC Machine Tools. CNC Principles of Control . . . . . . . . . . . . . . . . . .. Measuring Systems for Machine Tool Path Determination . . . . . . . . . . . . . . . . . . . . . . . . . .. A Review of Typical CNC Machine Tool Configurations. . . . . . . . . . . . . . . . . . . . . . . . . .. Current Developments in Flexible Manufacturing Cells and Systems, Leading to Complete Computer Integrated Manufacture . . . . . . . . . . . . . . . . . . . .. 1 1 3 8 32 43 55 65 2.1 Introduction............................. 65 2.2 The Importance of "Logistics" in a Flexible Manufacturing Environment, its Feasibility and Simulation during the Development . . . . . . . . . . . . . . . . . . . . . . . . . . .. 66 2.3 Flexible Manufacturing Cell and System Configurations. . . . . . . . . . . . . . . . . . . . . . . . . .. 80 2.4 Condition Monitorings of Intended Plant: FMc/S and CIM Installations. . . . . . . . . . . . . . . . . . . . . . . . .. 97 2.5 The Monitoring Systems Necessary for High Part Quality During Untended Machining . . . . . . . . . . .. 100 2.6 Automated Auxiliary Equipment to Ensure Accurate Quality Assurance in an FMc/S Facility . . . . . . . . .. 108 2.7 Computer Integrated Manufacture (CIM) in the Automated Factory - a Case Study. . . . . . . . . . . . .. 110 2.8 Present and Future Trends in Turning and Machining Centre Development . . . . . . . . . . . . . . . . . . . . . .. 123 viii Contents Appendix National and International Machine Tool Standards . . . . . . . . . . . . . . . . . . . . . . . . . 141 Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . . . . . .. 142 Selected Bibliography ........ . . . . . . . . . . . . . . . .. 165 Company Addresses. . . . . . . . . . . . . . . . . . . . . . . . . .. 167 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 Preface Each volume in this series of three attempts to explain the design of turning and machining centres and how they are operated through part programming languages. Furthermore, a discussion about how such stand-alone machine tools can be networked into flexible manufacturing systems is given along with the problems relating to such interfacing. These volumes were written as a companion book to the successful Advanced Machining - The Handbook of Cutting Technology published jointly by IFS and Springer Verlag in 1989. The individual volumes look at interrelated aspects of using turning and machining centres: Volume I considers the design, construction and building of turning and machining centres, then goes on to consider how these individual machine tools can be networked together providing the desired communication protocols for flexible manufacturing systems, leading to a complete Computerised Integrated Manufacturing system. This latter philosophy is discussed in terms of a case study on the most automated factory in Europe, ironically manufacturing turning and machining centres. Finally, mention is made of the efforts given to ensure significant advances in both ultra high-speed machining design and submicron operation, which is sure to have a major impact on general turning and machining centres in the future. Volume IT discusses the crucial point of ancillary activities associated with these machine tools, such as the cutting tool technology decisions that must be made in order to ensure that each machine is fully tooled-up and optimised efficiently. A brief review is also given on cutting tool materials development and tooling geometry considerations. Modular quick-change tooling is reviewed together with both tool and workpiece monitoring systems. A discussion follows on tool management, which becomes a major activity when a considerable tooling inventory exists within a manufacturing facility. Cutting fluids are an x Preface important complement to cutting tools, as they not only extend tool life but additionally enhance the workpiece machined surfaces; therefore it is important to choose the correct cutting fluid and handle it in the approriate manner to obtain maximum benefit from its usage. Workholding technology is an expensive burden that requires careful consideration to achieve an economic optimisation, particularly in a larger scale automated facility such as in an F.M.S. environment and a range of workholding strategies and techniques are reviewed. Volume III is a highly focused text that discusses how a part program is generated - after a general discussion about controllers. Consideration is given to the fundamentals of CNC programming and this becomes a major part of the volume with a structured development of how to build programs and where and when the "word address", "blueprint/conversational" and "parametric" programs are utilised. High speed machining fundamentals are considered along with the problems of servolag and gain for both milling and turning operations. A section is devoted to "Reverse Engineering" using digitising/scanning techniques - allowing replicas to be used to generate part programs. as these techniques are becoming popular of late. Finally a discussion ensues on the design of CAD/CAM systems and how they might be used for multiple-axis machining, through a direct numerical control link. Graham T. Smith West End Southampton January 1993 Chapter 1 The Development and Design of CNC Machine Tools 1.1 Historical Perspective - the Early Development of Numerically Controlled Machine Tools The highly sophisticated CNC machine tools of today, in the vast and diverse range found throughout the field of manufacturing processing, started from very humble beginnings in a number of the major industrialized countries. Some of the earliest research and development work in this field was completed in the USA and a mention will be made of the UK's contribution to this numerical control development. A major problem occurred just after the Second World War, in that progress in all areas of military and commercial development had been so rapid that the levels of automation and accuracy required by the modern industrialized world could not be attained from the labour intensive machines in use at that time. The question was how to overcome the disadvantages of conventional plant and current manning levels. It is generally acknowledged that the earliest work into numerical control was the study commissioned in 1947 by the US government. The study's conclusion was that the metal cutting industry throughout the entire country could not cope with the demands of the American Air Force, let alone the rest of industry! As a direct result of the survey, the US Air Force contracted the Parsons Corporation to see if they could develop a flexible, dynamic, manufacturing system which would maximise productivity. The Massachusetts Institute of Technology (MIT) was sub-contracted into this research and development by the Parsons Corporation, during the period 1949-1951, and jointly they developed the first control system which could be adapted to a wide range of machine tools. The Cincinnati Machine Tool Company converted one of their standard 28 inch "Hydro-Tel" milling machines to a three-axis "automatic" milling machine for this contract, having removed the contouring equipment. This machine made use of a servo-mechanism for the drive system on the axes, which controlled the table positioning, cross-slide and spindle head. The machine can be classified as the first truly three axis continuous path machine tool and it was able to generate a required shape, or curve, by simultaneous slideway motions, if necessary. At about the same time as these American advances in machine tool control were taking place, Alfred Herbert Limited in the United Kingdom had their first NC 2 CNC Machining Technology machine tool operating, although Ferranti Limited produced a more reliable continuous path control system which became available in 1956. Over the next few years in both the USA and Europe, further development work occurred. These early numerical control developments were principally for the aerospace industry, where it was necessary to cut complex geometric shapes such as airframe components and turbine blades. In parallel with this development of sophisticated control systems for aerospace requirements, a point-to-point controller was developed for more general machining applications. These less sophisticated point-to-point machines were considerably cheaper than their more complex continuous path cousins and were used when only positional accuracy was necessary. As an example of point-to-point motion on a machine tool for drilling operations, the typical movement might be: fast traverse of the workpiece under the drill's spindle and after drilling the hole, another rapid move takes place to the next hole's position - after retraction of the drill, of course. The rapid motion of the slideways could be achieved by each axis in a sequential and independent manner, or simultaneously, if a separate control was utilised for each axis. The former method of table travel was less costly, whereas the latter was faster in operation. With these early point-to-point machines the path taken between two points was generally unimportant, but it was essential to avoid any backlash in the system to obtain the required degree of positional accuracy and so it was necessary that the approach direction to the next point was always the same. The earliest examples of these cheaper point-to-point machines usually did not use recirculating ball screws; this meant that the motions would be sluggish, and slideways would inevitably suffer from backlash, but more will be said about this topic later in the chapter. The early NC machines were, in the main, based upon a modified milling machine, with this concept of control being utilised on turning, punching, grinding and a whole host of other machine tools later. Towards the end of the 1950s, hydrostatic slideways were often incorporated for machine tools of higher precision, which to some extent overcame the stiction problem associated with conventional slideway response, whilst the technique of averaging-out slideway inaccuracy brought about a much increased precision in the machine tool and improved their control characteristics. The concept of the "machining centre" was the product of this early work, as it allowed the machine to manufacture a range of components using a wide variety of machining processes at a single set-up, without transfer of workpieces to other machine tools. A machining centre differed conceptually in its design from that of a milling machine, in that the cutting tools could be changed automatically by the transfer mechanism, or selector, from the magazine to spindle, or vice versa. In this manner, the automatic tool changing feature enabled the machining centre to productively and efficiently machine a range of components, by replacing old tools for new, or preselecting the next cutter whilst the current machining process is in cycle. In the mid 1960s, a UK company, Molins, introduced their unique "System 24" which was meant to represent the ability of a system to machine for 24 hours per day. It could be thought of as a "machining complex" which allowed a series of NC singlepurpose machine tools to be linked by a computerised conveyor system. This conveyor allowed the workpieces to be palletised and then directed to each machine tool as necessary. This was an early, but admirable, attempt at a form of Flexible Manufacturing System concept, but was unfortunately doomed to failure., Its principal weakness was that only a small proportion of component varieties could be machined at any instant and that even fewer workpieces required the same operations to be performed on them. These factors meant that the utilisation level was low, coupled to the fact that the machine tools were expensive and allowed frequent production The Development and Design of CNC Machine Tools 3 "bottlenecks" of work-in-progress to arise, which further slowed down the whole operation. The early to mid-1970s was a time of revolutionary advancement in the area of machine tool controller development, when the term computerised numerical control (CNC) became a reality. This "new" breed of controllers gave a company the ability to change workpiece geometries, together with programs, easily with the minimum of development and lead time, allowing it to be economically viable to machine small batches, or even one-offs successfully. The dream of allowing a computerised numerical controller the flexibility and ease of program editing in a production environment became a reality when two related factors occurred. These were: the development of integrated circuits, which reduced electronic circuit size, giving better maintenance and allowing more standardisation of design; that general purpose computers were reduced in size coupled to the fact that their cost of production had fallen considerably. The multiple benefits of cheaper electronics with greater reliability have resulted in the CNC fitted to the machine tools of today, with their power and sophistication progressing considerably in the last few years, allowing an almost artificial intelligence (AI) to the latest systems. Over the years, the machine tool builders have produced a large diversity in the range of applications of CNC and just some of these developments will be reviewed in Volume III. With any capital cost item, such as a CNC machine tool, it is necessary for a company to undergo a feasibility study in order to ascertain whether the purchase of new plant is necessary and can be justified over a relatively short pay-back period. These thoughts and other crucial decisions will be the subject of the next section which is concerned with the economic justification for CNC. 1.2 The Economics of CNC 1.2.1 The Importance of a Feasibility Study It is normal for a company to embark on a feasibility study prior to the purchase of any capital equipment such as a CNC machine tool. This study fulfils many functions, such as determining the capacity and power required together with its configuration horizontal/vertical spindle for a machining centre, or flat, or slant bed for a turning centre. Many other features must also be detailed in the study, encompassing such factors as the number of axes required and whether the machine tool should be loaded manually, by robot, or using pallets. An exhaustive list is drawn up of all the relevant points to be noted and others that at first glance seem rather esoteric, but will affect the ability of the company to manufacture its products. It has been shown time and again that many mistakes have been made in the past when companies rush into the purchase of new equipment without considering all of the problems, not only of the machine tool itself, but of the manning and training requirements together with its effect on the rest of the shop's productive capability. Often the fact that an advanced, highly productive machine is now present in the shop could affect the harmonious flow of production, causing bottlenecks later, when the purpose of purchaSing the machine was to overcome those problems at an earlier production stage. Machine tools have even been purchased in the past without due regard for the components they must manufacture, or without correct assessment of future work. This latter 4 CNC Machining Technology point is not often considered, as many companies are all too concerned with today's production problems rather than those of the future. Taking this theme a little further, in a volatile market a feasibility study should perceive not only the short and medium term productivity goals, but also the long term ones, as it is often the long term trends of productive capability which are the most important if a company is to amortise their costs. When highly sophisticated plant such as an FMS is required, it can be several years from its original conception before this is a reality on the shop floor, and a company's production demands may have changed considerably in the mean time. If, for any reason, the wrong machine/s has/have been purchased, or more likely, something has been overlooked during the feasibility study, then the "knock-on effect" of this poor judgement is that it will have cost the company dearly and, at the very least, any future study will be looked on by the upper management with disdain and scepticism. A company should plan and discuss their products and systems to be implemented in the future with an eye on the production equipment of the present. This is very relevant, as any responsible production engineering company will invest in manufacturing equipment which has reached a reliable level of maturity, yet at the same time allow for further growth over a foreseeable time, and in such a manner, maintain and strengthen the competitiveness of the enterprise. Fig. 1.1 graphically illustrates the relationship between product maturity and level of utilisation of production technology today. In recent years, the labour overheads have reached almost the same level as the direct labour costs and this has meant that methods employed using conventional production have clearly slipped into an "ageing phase". This is also true, to a certain extent, for NC technology, as this has shifted from maturity to a particular level of ageing and in the medium term, will offer no further competitive opportunities. Obviously, planned investments must embrace the growth area technologies ~1 GROWTH. MATURITY MATURITY OBSOLESCENCE. .I...EYf..l.... Fig. 1.1. The degree of maturity and utilisation of manufacturing techniques curren...
View Full Document

  • Fall '19

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes