At this point, I think the important thing is to talk to support. 0.175mm of error on a Nomad, in plastic is not okay.
That said, if I were to try to handle it myself without support, my ideas are:
- If you’re okay hooking up a 24V power supply to something, buy an optical sensor like this and hold it on your machine (e.g. in a vise) and move the endmill towards it to get the diameter optically. This is the method I used in another thread. The sensor costs about $20 and had repeatability of ~2um as far as I could tell. I didn’t quite figure out how to calibrate it though.
- Otherwise, buy an endmill that actually has specifications. PreciseBits has some suitable plastic endmills and their diameters are specified to +/- 5 tenths accuracy. Harvey Tool has them at around 2 thou but you need to be careful - the 1/8" endmills are less accurate.
On that last point by the way, if you’re after accuracy, you usually want to avoid endmills with a cutting diameter that matches the shank diameter. One of the guys from PreciseBits elaborated on that a bit here.
Anyhow, those are ways to get an endmill with a known diameter. If you still get weird results, it’s probably not the endmill.