Date of Award

2-1975

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Supervisor

Dr. J. Tlusty

Abstract

The accuracy of work on N.C. machine tools is much more affected by weight, clamping and thermal deformations as well as by variations in accuracy throughout the entire working zone than that of machine tools using other modes of operation.

It is therefore very important to precisely define the concept of accuracy and the methods of testing and evaluating it.

In this thesis the shortcomings and even possible fallacies of the classic form of accuracy evaluation procedure are discussed, together with an appreciation of recent significant contributions to the field which utilise modern instrumentation and high frequency response transducers in sound and informative experimental procedures.

A new system is developed where the problem of formulating a comprehensive and meaningful machine tool accuracy capability statement is practically solved by directly relating the accuracy of the machine tool to the accuracy of a workpiece machined on it. This is achieved by deriving a tolerance law applicable to the evaluation of machine tool translative deviations where the law relates to the practical system of tolerancing a part dimension as well as to the usual mode of use of an N.C. machine.

An extensive of the basic tolerance law provides for a comprehensive statement of accuracy achievable throughout any defined two-or three-dimensional working zone of a machine tool. For every type of machine tool configuration there is a finite (minimum) number of translative deviation measurements that is required to be evaluated only along well defined lines located at extreme offsets of the tool with respects to each moving member of the machine.

Additionally the procedure for including in the machine tool capability statement the adverse "further" effects of weight, clamping and thermal deformations is outlined.

While it is comparatively straight-forward to formulate tests such that the effect of weight and clamping deformations may be defined, the problem of correctly specifying a thermal test cycle warranted a rather more extended study.

It is shown how the correct identification and classification of the type of heat source affecting a machine tool's structural components leads to rather precise knowledge of the particularly affected translative deviation measurements. The design of the test cycle, the general laws governing the specification of the tests and the test procedure, are developed by reference to many practically obtained measurements complemented by computational studied of simplifies and simulated machine tool structures.

Two computational procedures are developed for the calculation of steady-state and transient temperature fields together with the resulting thermal deformations: a lumped-mass/time-continuous technique the solution of which leads to an eigenvalue problem, and the finite-element approach to the problem. In each case a step by step solution technique is outlined, and a Fortran IV computer program listing is included in the Appendix for the finite-element analysis of two-dimensional structures using triangular elements.

Finally, an investigation is made of new concepts of errors of an axis of rotation. The experimental technique results in the generation of a polar trace on an oscilloscope screen, the form of which relates directly to a Talymonogram which could be obtained from a machine specimen if there were no unwanted disturbances of the cutting process. In this way, an exact picture of the influences of spindle rotation errors on the workpiece accuracy is obtained by an idle run test at normal spindle speeds without machining. The design of a comprehensive electronic instrument is included in the Appendix together with details of associated instrumentation.

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