Weird depth of cut issue with CC

Not sure if this is a software issue with CC or CM (I suspect so with CC) but I will post it on the Shapeoko area since it is affecting depth of cut on my machine. Here’s the scenario:

I use my machine to cut a lot of precision parts for guitar making. In this case, I am trying to cut pick-guards with pockets to support electric guitar pickups and slots and other perimeter features to go around the neck joint and other electronics. The stock material is 0.1" thick. I set my depth of cut for the pockets, etc. to “use the bottom” of the stock, which is set to 0.1" in the stock set-up dimension area when I lay out my toolpaths. The problem is that when I set the toolpaths, generate the g-code, run the g-code in CM, the depth of cuts do not stop at 0.1". They always over-run the stock bottom by a large amount (not sure how much because I typically hit the pause button before the machine cuts deeply into my waste-board). I can overcome the issue by watching the job and hitting pause, but this makes me have to set up a separate toolpath for each pocket rather than running several similar pocket operations off of one set of codes. I also note that when I load the file into CM, that the Z toolpath extents looks way out of wack–like 45.somthing MMs on the z axis.

Now for the really crazy part. If I convert the stock dimensions to mm by selecting mms rather than inches in the setup section, the toolpaths run fine and stop at the bottom of the stock (in this case 2.54 mms). Again, this is a workaround, but all my SVG files are patterned on inches, so it makes for some conversion when I move everything into CC for creating toolpaths.

It seems to me that CC is having some difficulty dealing with very thin stock thickness. I do not have this issue when cutting into thicker stock (say .25"), and I am not sure why it seems to do fine if I convert the CC setup from inches to mm and run it. Any ideas?

Can you explain this process a bit more? There may be something here.

When I open CC, the first thing I do is set my stock dimensions. In this case I am using material that is 9" W X 8"H X 0.1" thick. Typically, when I set my toolpaths, I select the vectors I want cut, set the software to do an inside cut (or outside, if I’m cutting out the entire part from the stock), then (if I want to “cut out” a hole, for instance) I set the depth of cut to be “bottom of stock.” With thicker stock, I’ve had no issues.
That is, the mill runs the perimeter of the vector, steps down by the amount set in speed & feed, and stops stepping and cutting when it reaches the stock bottom depth. This is not the case with this thin material. In this instance, the mill just keeps stepping deeper until it plunges into the wasteboard.

Incidentally, I have also tried manually setting the depth of cut to something just less than the stock thickness (say, 0.09), but the results are the same.

Everett, I was specifically interested in your Z-zero technique since that seems to be the problem.

Interesting. I’ve not heard of this issue before. Paging @WillAdams

I zero as always directly off the stock surface, typically at the center of the stock, by lowering the bit till it just barely touches then zeroing Z. Hope that’s what you’re asking.

I will try to upload some files tonight. I think this has to a problem with how the software is generating g-code for the thin stock setting when using inch settings. I’ve retraced my steps and have actually created two separate CC files from scratch to see if the performance is consistent, which it seems to be. For some reason, it just does not seem to recognize a - Z value that is 0.1" although it will if I change to MM and set the stock bottom at 2.54 MM.