I have a little bit of a enigma and the answer is eluding so I'm asking for advice on this problem.

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I don't know what is causing the ribbing on the final pass on recess carving I have change speed from 480 mm/min up to 900 mm/min and with different step-overs and different spindle speeds, I've even tried different material and ballnose cutters but still the problem remains.I've also been using a extractor and a air line to keep the cutting path clear.But I think looking at the the ribbing it would seem that the z-axis is dropping, Is this a software problem.James