Z axis zero changing during run

I have always had an iffy z axis. So as a test, I just cut a square 0.1" deep. After the cut, I measured the depth of cut and the starting point was indeed 0.1. But the ending point was a depth of 0.15 and the surface of the material was no longer 0. Then I reran the gcode so it was actually cutting air. This time it appeared to cut at the proper depth and zero was still at the surface of the material. I have thought my table was not flat was why my cutouts went through on one end but not the other. However, I zeroed z at the surface, then raised z and jogged x & y about 12 inches away and lowered z back to zero and it was within 3 thou. So the table seems flat.

I know I am not bottoming out, the backlash is set so I can still turn the z by hand (with effort). The two pulleys are tight. I read I should tension the z axis drive belt, but I can’t figure that one out.

Any ideas?

Could be the bit sliding in the collet.

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@Bruce Welcome to the group, Bruce. You will find this place an excellent resource, I am sure.

You say that you can’t figure out how to adjust the Z gantry drive belt. Take a look here:

Part way down, you will see a reference to belt tension.

With reference to your problem, if I understood your post, everything is fine when you run in air. The problem arises when you are cutting. I would agree with @JHahn that the bit may be slipping. Are you using a router or a spindle ? Does the problem arise with all bits? If it is present when using 1/8" shank bits, are you using a 1/4" to 1/8" adapter or the 1/8" collet.

The one I was currently using is a 1/4 and I had cleaned out all of the sawdust before inserting the bit. I reviewed and reinstalled the z axis pully belt ( it did seem as though it may have had a little slop). I accidently broke the bit I was using, so I will be now using a new one. I measured the shaft and it came out to 0.2495, so it was within 5/10,000. so if the problem now goes away i will be happy, but not know if it the bit slipping or the lose belt on the pully.

Thanks for all the answers.

i have changed my cutout bit from a 3/16 to a 1/4 and so far it has not happened again.

@Bruce Bruce: If you are confident that you have resolved your issue, I will close this topic.

Im 90% sure its fixed. thanks

As the original issue has been addressed, I’m closing this topic. If anyone else has a similar concern, feel free to start a new topic, which will ensure that it is addressed promptly.