Quote Originally Posted by routercnc View Post

I also thought about whether I was missing steps on change of direction, maybe somehow the step active high/low setting had changed? Next chance I get in the workshop I'll check that and I'll run some G-code moving the Y axis back and forth various distances and at various feedrates (without load), stopping back against a DTI, and see what happens. Then I'll take it from there, repeating with the spindle on (no load), then some edge cuts, etc. and see if it returns to DTI zero.
That was my first thought when I saw your post. I found the same problem on my current machine on the Z axis - didn't show up on simple profiling jobs but when I did some heavy-duty 3D carving, it rapidly showed up. It's probably only a single microstep per reversal so a bit of gcode that moves, say, 50 times in each direction and comes back to a dial gauge will show it up. The clue in my case was that the error was pretty consistently number of reverses times distance moved per microstep. At least it's a quick one to check and eliminate!