Introduction

Customer complaints, as well as our own experience with our machines has shown a consistent problem with “Z-banding”. This is a periodic waviness in vertical walls with a regular 5mm pitch.

This issue arises from “Wobble error” in the Z ballscrews. Currently we use C7 ballscrews in all 3 axes, but the Z axis is much more sensitive to this wobble error than the X and Y axes. This is coupled with the fact that there is no specified wobble error for C7 screws, as shown below:

Judging by the step up in travel error from C5 to C7, we would expect roughly 24μm of wobble error in out screws

Investigation

We built an automated test jig to find the actual wobble error of our screws. By rotating the screw with a precision-encoder clear path servo we eliminate stepper-related errors. The true travel of the screw can be verified over several rotations to find the screw’s true wobble error. By comparing the screw’s expected position for a given angle of rotation against its actual position, error can be calculated.

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Results

Below is a summary plot of 15 screws with 2 test runs each, as well as the error plots of the best and worst screws in the set:

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The max error over the full dataset was 123μm, the RMS error over the whole dataset was 28μm. Given a nominal layer height of 400μm, assuming 2 123μm ballscrews with their peak errors 180º out of phase with each other, it would be possible to see a layer thickness error of 62% (!!!).

Given the range of performance we see it’s reasonable to assume that machines that have low-to-no Z-banding have gotten lucky in the ballscrew lottery, whereas poorly performing machines have not. There also exists the possibility of a machine having different performance on the home end of the Y axis vs the Far end of the Y-axis.

Resolution

We have a sample lot of C3 and C5 ballscrews en route from our supplier, which we will test internally to verify the Z-banding issue is resolved. We expect replacement screws and service toolkits to ship out to the most affected customers at the beginning of April.