incomplete cutting path? DOC/Z axis issue?

[ForumShop]

Hello! I have more beginner questions:

I am learning to use the mill we have in our shop and have a small project that I'm working on. I am cutting rounded slots in a straight path across mild steel. Using a 5/16 ball end mill and cutting half of the diameter of the bit on two halves of plate steel. This way I can sandwich a 5/16 round heating rod in between the two plates. I am still learning the mill and how to use it, and realize that steel is high bar for a beginner.
The issue at the moment is that the tool is not cutting to the depth I expected and also that it seems to cut again on additional passes with the same programming or DOC (according to my DRO).

I had planned on cutting each half of the plate steel to a .156 to start with and did so in four passes programmed into CAM/ICN. (.395, .790, 1.185 and .1560. Completed these cuts and measured about .138-145 in the finished slots. I tried a few different things and re-measured many times. Also tried re-running the same path and depth (or same program) and noticed I got a bunch of chips again on my .1560 path, and again a third pass but less. I'm not getting any chatter and each pass leaves a nice finish. After the additional passes (re-run to same depth), it is much closer to my spec.

Where am I goofing up or what expectation do I have wrong? Should I be inspecting the tolerances of my Z axis? I can get repeating and seemingly accurate DRO numbers while setting up parts and tools, etc.

Shizuoka ST-N Knee mill with CNC12
RPM ~1200
10 IPM
.002
.3125 Ball end Mill

Thank you!
-Nick

[tblough]

Without photos of your machine and part setup, this seems like classic tool/machine deflection. Your feedrate seems very high for a ballnose EM. Try something more like 1200RPM and 1.5IPM. Instead of evenly spacing your depth passes, try 50%, 75%, 90%, and 100%.