Products : Machine Tool/CNC : CNC Linear Scale Upgrades : Accuracy of Feed Axes

Accuracy of Feed Axes by HEIDENHAIN

Is thermal expansion relevant.q. In the total error budget of a machine tool the positioning error values of the feed axes play a critical role.

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The discussion as to whether linear encoders or rotary encoders are the best solution for measurement on NC machine tools is experiencing quite a comeback. This is due, in part, to the post recession idea that machine tool manufacturer`s need to cut costs to maintain machine sales. Could it be that too much attention has been put on cost, and too much reliance on new technology.q. Under the pressure of such opinion, one can easily come upon the idea of cutting costs by leaving linear encoders off the lowestpriced models, and then offer them as an accuracy-enhancing optional feature.

This line of thought is reinforced by the drive industry where a large portion of the new servo motors now feature built-in rotary encoders. With this trend, there is now a decision of choosing between a ballscrew/rotary encoder system, or a linear encoder solution for position control. In the heat of such decisions, one should not forget to consider the problems known to be involved with position measurement using a rotary encoder/ballscrew system. In order to make an educated decision, you must first have a general understanding of mechanical feed drive systems.

Although machine tool designs vary, their feed drive systems remain largely standardized (See Picture). In almost all cases, the recirculating ballscrew has established itself as the solution for converting the rotary motion of the servo motor into linear slide motion. The ballscrew is normally fixed at only one end with preloaded ball bearings, which take up the axial forces of the slide.

A position control loop using a rotary encoder and ballscrew includes only the servomotor (Fig. 2). In other words, there is no actual position control of the slide, because only the position of the servomotor is being controlled. To be able to predict the slide position, the mechanical system between the servomotor and the slide must have a known and, above all, repeatable behavior.

On the other hand, a position control loop with a linear encoder, embraces the entire mechanical feed-drive system. This takes into account all native errors associated with rotary encoder feedback. Plainly put, the linear encoder on the slide gives the control unit a direct indication of it`s position. When using a rotary encoder for linear position of the slide, there are numerous mechanical effects that will cause errors in the positioning accuracy, thus increasing the potential of scrapped parts.

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