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Development of methods for the numerical error correction of machine tools

Objective


In order to describe the geometric errors of a machine tool a general model has been derived. This general model, relates the errors in the location of the tool, with respect to the workpiece, to errors in the location of coordinate frames attached to successive components of the machine. With this general model it is not only possible to model a specific machine tool, like the investigated Maho milling machine, but it can be applied to any multi axis machine, composed of rotary and linear elements in an arbitrary serial configuration. The general model has been elaborated to the individual model, which describes the geometric error structure of the Maho milling machine. Therefore all geometric error components have been measured, using the developed direct measuring techniques.

The finite stiffness errors have been investigated. The finite stiffness errors due to cutting forces turned out to be negligible when the workpiece is finished with a low feed and small cutting depths, which is necessary to achieve the desired surface roughness. The additional error to be investigated was the bending of machine parts due to a workpiece load on the machine tool table, which showed that only in the X-axis a significant finite stiffness errors occurs. The finite stiffness errors are modelled as a change in the geometric errors dependent on the workpiece load. Thus, the individual geometric error model can be applied to describe these finite stiffness errors.

In order to describe thethermomechanical behaviour of a machine tool, 2 different approaches have been investigated: an empirical and an analytical approach. A new method has been developed to calibrate a machine tool by milling test workpieces. With these workpieces both the geometric and the thermally induced errors of a machine tool can be determined. Four of these workpieces are sufficient to accomplish a complete calibration. A software package has been developed to support the evaluation of these workpieces.
% LThe test workpieces were also applied to verify both thermomechanical models in the most frequently used positions in the working volume (centre position). The verification has been carried out by milling 4 workpieces in 2 different planes, with different process conditions (which results in stationary and transient states). During these experiments, similar efficiencies have been found for both models. As the empirical and analytical model have their own advantages, each model has been implemented in the real time software error correction.

An external personal computer (PC) determines the machine's status, recalculates the parameters and sends the compensation tables to the machine tool controller. This sequence is repeated every 60 seconds. This open architecture has been developed to enable the implementation of both thermomechanical models. The software correction system itself runs in a real time environment. Hence, the errors can be compensated during every machine movement.
Inaccuracies in the use of machine tools are mainly caused by errors of form geometry, changes of the geometry due to finite stiffness and, most important, changes of the geometry due to thermal effects. The aim of this project is the on-line improvement of the geometrical accuracies of machine tools by the adaption and extension of the correction methods used in the field of coordinate measuring machines. Although establishing general principles, the project will mainly deal with a five axis milling machine for which an error model describing these errors will be developed and its effectiveness demonstrated.
A bibliographical study on currently available error models, methods of temperature corrections and methods of parameter extraction from measurements of work pieces will facilitate the establishment of concepts for an error model which will take into account the finite stiffness of the machine as well as the influence of a given temperature distribution on the geometry. The error model will be based upon a combined analytical and empirical approach. In order to develop and verify the concepts and model, measurements of objects will be made on the milling machine and on a 3-coordinate measuring machine.
Important aspects of the project will be the implementation of the hardware and software needed to demonstrate the feasibility of on-line error compensation on this machine and the assessment of the actual improvement of the accuracy brought about by the project.

Status:
In progress.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Eindhoven University of Technology

Participants (3)

MAHO AG
Germany
Address

8962 Pfronten
NV Philips Gloeilampen Fabrieken
Netherlands
PHYSIKALISCH-TECHNISCHE BUNDESANSTALT
Germany
Address
Bundesallee 100
Braunschweig