LINEAR SCALES

Problems often encountered when using Digital Linear Scales

Digital linear scales are indispensable to any milling machine, plane grinder or EDM. The benefits of fitting digital linear scales to a machine include an increase in machining accuracy, and the elimination of the drudgery of calculating and recording measurements. In addition, many aspects of machine errors can be corrected. Most of the digital linear scales have an accuracy ranging from 0.005mm to 0.01mm. Unfortunately, most of the additional functions of digital linear scales are often misunderstood or even neglected. This means that after a digital linear scale is fitted into a machine, the working accuracy of the scale drops to 0.02mm to 0.05mm. In this article, we outline some simple methods to correct common errors that occur when using digital linear scales. Hopefully, these tips will help all users of digital linear scales to rely less on the suppliers of the scales for support. In turn, this will enable users to make significant savings on the cost of calibrations and adjustments.

1. Excessive position error (e.g. above 0.02mm)

Possible causes include:

a) The weight of the workpiece causes tilting of the table, i.e. the table traverse direction is not parallel to the
    digital linear scale.



b) The adjustment of the guide-way is incorrect, resulting in the workpiece not being parallel to the X-Y plane.

c) The digital linear scale and the machine guideway are not parallel.

The solution is simple (please refer to figure 3). A workpiece of known length and a dial tester are used to determine the error of the digital linear scale. A compensation value will be set to the system to complete the test.

2. Errors due to unwanted lateral displacement, especially in the Y-direction (please refer to
    figure 4)

Such errors are caused mainly by play in the guideway and the digital linear scale being offset from the table's centre-line. Here are some possible solutions:

a) Use the "compensation parameter" (Lost Motion) of the scale counter unit. Most counter units of milling
    machines and EDMs have this function.

b) Reduce the play of the guideway.

c) Mount the digital linear scale as close as possible to the table centre-line (Abbey error).

3. Parallelism between the linear scale and the machine guideway

Most users believe that their linear scale will be precise as long as it is mounted to the machine. But they neglect the fact that parallelism between the linear scale and the machine guideway is a major cause of error. When mounting a linear scale, a dial test indicator should be used to check the parallelism between the scale and face A and face B. Taking a 700mm-long linear scale as an example, the parallelism with face A for full travel should be under 0.1mm, while that with face B should be under 0.2mm. If this is not the case, the scale cannot give the desired accuracy, even with compensation setting.

4. The error is not stable

This problem is caused mainly by the movement of the scale reading head (such movement is often not smooth) or by obstruction from cables. The scale reading head should be installed to the machine bed and the scale body should be installed to the traversing table. If this mounting configuration is observed, such problems can be avoided.

Summary

The above-mentioned problems are often encountered during the operation of digital linear scales. If you exercise care while installing and adjusting digital linear scales, there should be no need to call your suppliers for support. If you do encounter serious problems, the supplier is always the first one you should ask for help.