I seem to be having the exact same issues, though mine keep changing. I’ve been carving stars to make American flags. Yesterday the stars on the lower part of the union were shallower than the stars on the upper part. Today, I made sure the pulley set screws and all of the belts were tightened properly, then checked all four corners, keeping the bit at the same level to determine if there were any height discrepancies on the Z axis. I used my Z probe plate and a business card as my guide. I had to raise the z axis .007" when I moved along the X axis to the next corner. I then moved it up the Y axis to the next corner and had to raise the bit .029". On the last corner, I had to lower it .024". I made some adjustments to the rails and everything somewhat equaled out. It wasn’t perfect but it was close. I then skimmed my waste board. My first carve after that was done was a reverse of my issue yesterday. The stars at the top of the union were now shallower than the bottom. I’m using birch plywood and it is sitting completely flush against the surface of the waste board. The thickness variation when testing all four edges of the wood was no more than .002". Not enough to cause this much of a discrepancy in depth.
I masked off the bottom several rows of stars and tried to carve the top three rows again when the machine went a little haywire and just started carving downward into the corner (home position). I stopped it before it went down into the waste board. I then flipped the wood over and tried carving text. Again, the text on the bottom was deeper than the top, but the spindle was also very choppy in the way it carved. I tried carving again, this time at 1/4" deep (first time was 1/8") and while the machine was moving, nothing was actually happening. It wouldn’t carve any deeper. Once it returned to the home position, I set the carve depth to 3/8". Again, the same problem occurred. It wouldn’t go any deeper than the original depth, and it was very choppy with the movements. Obviously there’s some sort of issue with the Z axis being off, but I don’t know what that issue is.
Also, the Y axis is dead on with travel. The X axis is ever so slightly off. When skimming the waste board, it skimmed deeper towards the front of the washboard than the back, but maybe only by 1/32". That shouldn’t attribute to a 7" piece of wood having that much of a difference in carving depth, should it?
As for the choppiness and not carving deeper, could the problem be my slow internet connection? It’s never been an issue before, but I’m starting to wonder if both kids streaming and playing Xbox while I’m 150 feet away from the router in my workshop is possibly causing the machine to not communicate as well with my laptop.
I apologize for the long-windedness of this. It’s been a very long and wasted day of trying to fix this machine and I’ve got nothing but more problems to show for it.