MPCNC - Issue with curves

Hello All,
I recently dusted off my MPCNC and installed a new computer to run it. Everything seemed fine with the initial test (just did some straight line cuts). I then ran my go to test cut that does two passes, one with an 1/8 bit for a rough cut out and then a second pass with a V carve bit to clean up the edges. The rough cut allows for 1mm finishing tolerance. When it’s complete, the bottom left 1/3 of the finished piece looks pretty good. But the further up you go, the more the V carve cut looks to be drifting to the upper right. I’ve attached photos. I’m not sure if this is a problem with the machine, my cut settings, or my starting position on the 2nd pass just being slightly off.

Any input would be appreciated.

This is going to be a tricky problem.

Here are some thoughts:

  1. Is the workpiece moving? I have had a job that looks a lot like this, but I decided the wood bowed after I removed a binch of material with the roughing pass. The perimeters were deeper in my case, as the wood lifted after it was weakened.

  2. Is the Z height the same across the piece to begin with? The flat bit is pretty forgiving. If you cut 0.5mm deeper with a flat bit over 25cm, you need calipers to tell. With a V bit, it is immediately obvious. I would expect the v bit to match the depth of the flat bit everywhere though.

  3. Remember that the V bit gets wider as it goes deeper and smaller when it is higher. I expect the XY to be nearly perfect and the Z to always be a bit off. If you are definitely seeing problems in XY, in addition to Z, then something moved between runs. The Z is hard to make right across the whole area. The XY is easy to get right.


If you have one motor that’s good, and another motor that’s loose on the same axis, then it can produce results that are good on one side of the workspace and sloppy on the other side. Check the grub screws (on all five motors while you’re at it) and see if any have become loose.

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I’m not sure if this is a credible lead, but what about the grain structure? If the gantry has wobble and the grain structure has hard strains - could it influence the cut? It almost looks like it from the photo, but I’m quite sure this is a dead end…

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Wow, thanks for the quick responses everyone. I’m going to give the rest of the machine a good once over and tighten everything up. After I do another test I’ll update if there are any changes in the result. I’m hoping it’s either that or the piece just moving slightly between passes.


After giving the machine a once over to tighten everything up and better securing the piece down, the cuts are much better now. Thanks again for the input everyone.

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Oh great. I was worried this was going to be a long living gremlin for you.