In an effort to reduce machining surface errors due to tool deflection in the end milling process, methods regulating cutting forces have been implemented with on-line feed rate controls. Such schemes are able to improve the parts dimensional accuracy, but unfortunately they can exhibit undesirable aspects in which the alleviation of the cutting conditions deteriorates the productivity. In addition the frequent changes of the feed rate would spoil the surface quality. As a new approach to achieve the precision machining efficiently, this paper introduces a tool deflection compensation system. This compensation system is a computer controlled special tool adapter which is capable of measuring the cutting forces and minutely adjusting the position of the tool without interfacing with the NC controller of the milling machine. Such a system allows for on-line estimation of the tool deflections and reduction of the surface errors. Experimental investigations for typical shaped workpieces representing various end milling situations are performed to verify the ability of the system to suppress the surface errors due to tool deflections in more productive machining condition.

A Tool Deflection Compensation System for End Milling Accuracy Improvement

A mathematical model for predicting dynamics between the feedrate and cutting force in the lathe cutting process is derived by combinations of a parameter identification technique and geometrical consideration for chip thickness time variation. The model prediction of transient responses is in good agreement with the experimental results. The model is used for tuning a PI controller for regulating the cutting force as well as to show performance limitations of the conventional PI controller. A simple adaptive control scheme which makes use of the model structure is proposed. Experimental results from laboratory evaluation of the proposed controllers are presented.

Modelling and Conventional/Adaptive PI Control of a Lathe Cutting Process

The potential for improving the performance of machine tools is considerable. However, for this to be achieved without tool failure or product damage, the process must be sufficiently well understood to enable real-time monitoring and control to be applied. A unique sophisticated measurement platform has been developed and applied to two different machining centres, particularly for high-speed machining up to 24 000 rpm. Characterization and on-line monitoring of the dynamic behaviour of the machining processes has been carried out using both contact-based methods (accelerometer, force sensor) and non-contact methods (laser Doppler vibrometry and magnetic shaker) and numerical simulation (finite element based modal analysis). The platform was applied both pre-process and on-line for studying an aluminium testpiece based on a thin-walled aerospace component. Stability lobe diagrams for this specific machine/component combination were generated allowing selection of optimal process parameters giving stable cutting and metal removal rates some 8-10 times higher than those possible in unstable machining. Based on dynamic characterization and monitoring, a concept for an adaptive control with constraints based machine tool controller has been developed. The developed platform can be applied in manifold machining situations. It offers a reliable way of achieving significant process improvement.

A sophisticated platform for characterization, monitoring and control of machining

Illustration is given here of the different development, stages required in the retrofitting of an existing numerical control machine, giving appropriate details of the interface link Some software developments are indicated that enhance the range of the machine tool when connected on-line to a minicomputer, and mention is made of the production of Novikov gears

D.A. Milner

Papers

Adaptive control of feedrate in the milling process

Controller system design for feedrate control by deflection sensing of a machining process