Accuracy of Feed Drives

The accuracy of modern machine tools is measured with an increasing number of new and revised inspection and acceptance tests. Where years ago purely geometric acceptance tests predominated, today’s routine methods include dynamic tests such as circular interpolation and free-form tests, thermal tests such as described in ISO/DIS 230-3, and for production machines, capability testing during acceptance or regular inspection. The various influences of the cutting processes, geometric and thermal accuracy, the static and dynamic rigidity and the positioning response of the feed drives on the attainable accuracy of the workpiece can be more specifically analyzed. Machine errors are becoming increasingly obvious to the user. Considering the increasing frequency of changing jobs and the concomitant reduction in batch sizes, reducing the thermal or systematic error of a machine tool through tedious optimization of individual production steps is seldom feasible. The accuracy of the first part is gaining in importance. In particular the thermal error of machine tools is drawing ever more interest. The following text shows that thermal error can be quite significant, especially for the feed axes. Unlike structural deformation, errors of the feed axes can be dramatically reduced through a choice of simple and readily available measuring techniques.

Feed-Drive System Design

An exact error analysis of position measurement via rotary encoder and feed screw begins with a consideration of prevalent mechanical feed-drive systems.

Although machine tool designs vary immensely, the mechanical configuration of their feed drive is largely standardized (Fig. 1). In almost all cases, the recirculating ball screw has established itself as the solution for converting the rotary motion of the servomotor into linear slide motion. Its bearing takes up all axial forces of the slide. The servo-motor and ball screw drive are usually directly coupled. Toothed-belt drives are also widely used to achieve a compact design and better adapt the speed. For position measurement of feed axes on NC machine tools it is possible to use either linear encoders or recirculating ball screws in conjunction with rotary encoders.

A position control loop via rotary encoder and ball screw includes only the servomotor (Fig. 1, dotted line). In other words, there is no actual position control of the slide, because only the position of the servomotor rotor is being controlled. To be able to extrapolate the slide position, the mechanical system between the servomotor and the slide must have a known and, above all, reproducible mechanical transfer behavior.

Fig. 1: Typical drive system of a numerically controlled machine tool with linear scale on the slide and rotary encoder on the motor. Unlike in position control with rotary encoder and ball screw, a linear encoder includes the feed drive mechanism in the control loop.

A position control loop with a linear encoder, on the other hand, includes the entire mechanical feed-drive system. The linear encoder on the slide detects mechanical transmission errors, which are compensated by the machine control unit.

Differing terminology

Different terms are used to distinguish between these two methods of position control. German speaking and some English-speaking communities generally refer to them somewhat inaccurately as ”direct and indirect measurement.” However, these terms are rather poorly chosen because, strictly speaking, both methods are direct. One method uses the line grating on the linear scale as the measuring standard, the other the pitch of the ball screw. The rotary encoder simply serves as an interpolating aid. Here the Japanese concepts of ”semi-closed- loop and closed-loop control” seem appropriate, since they more aptly describe the actual setup.

Trend toward digitally driven axes
As a result of the trend toward digital axes in drive technology, a large share of new servomotors feature rotary encoders, which in principle can serve together with the feed screw for position control. With such a drive configuration the decision must be made as to whether to add a linear encoder or simply to use a ball screw working in combination with the already existing motor encoder.

One should remember to consider the problems discussed in the following text regarding position measurement using a rotary encoder/ball screw system. They can quickly increase the cost of an ”economical” machine if the owner finds that its accuracy does not suffice in certain applications.

Kinematic error
Kinematic error that can be directly attributed to position measurement using feed screw and rotary encoder result from ball screw pitch error, from play in the feed elements, and from the so-called pitch loss. Ball screw pitch error directly influences the result of measurement because the pitch of the ball screw is being used as a standard for linear measurement. Play in the feed transfer elements causes backlash. The pitch loss [1] results from a shift of the balls during the positioning of ball screw drives with two-point preloading and can lead to reversal error on the order of 1 to 10 µm.

Error compensation
Most controls are capable of compensating such pitch error and reversal error. However, to determine the compensation values it is necessary to make elaborate measurements with comparative measuring devices such as interferometers and grid encoders. In addition, the reversal error is often unstable over long periods of time and must be regularly recalibrated (Fig. 2).

Fig. 2: Circular test of a machining center without linear encoders in new condition and after one year. The reversal error has significantly increased in the X axis.

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