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May 26, 2004
Paradise, Ca
It's my opinion that you are pissing up a rope my friend. If the probe returns a diameter of .3000" on a .3000" master tool, then the probe is not the issue.


You are finding that machining is not automatic. You have recieved many replies from many very experienced machinsts to disregaurd the probe results (or don't probe diameter at all, which is what I ended up doing when I went down this exact same road many years ago) and make your parts correct-to-size with wear offsets. This is how it's done in any shop that isn't running a $40,000+ presetter (and they still have to tweak stuff to size). Wear offsets are used every day around the world. You should use them too.
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May 11, 2017
Minneapolis, MN, USA
I ran a .25" end mill to cut a .3750" hole and it was undersized by about .0025" using Meyer Z- gage pins.
Not a good test. The motion path for that is .125" diameter, so unless you cut it at an absolute crawl you could have motion issues; in order to get accurate hole diameters I use the high feed codes and slow down the feed a bunch, plus two skim passes.

What you described earlier also does not rule out deflection, if those cuts were made with the 2" indexable and you did not use proper skim passes. Indexables have higher deflection force than solids, large diameter cutters have more deflection force than small ones, and a skim cut is by definition zero stepover, not just a light finish pass. Take a fresh, new, sharp, material specific 1/4" endmill, and cut a width that you can measure with a mic. After the finish pass, measure it, then skim it twice and measure it again.

If the calibration tool measured accurately, then the probe is working correctly.


Hot Rolled
Jan 11, 2015
Bay Area, CA
I don't understand how someone can be so clueless. (Or just full of shit? or both)

I have decades of experience working with expert CNC operators that worked for me. We built racing motors among other automotive work. I have worked with various manual mills and lathes for decades.

But can't understand deflection. It's a fucking TM3P, made of paper-mache.

The best part is this
Wear comps are not an issue. Using them would be a work around.

Obviously-not-a-machinist, you really should do yourself a favor and forget everything you know, then enroll in a beginner course at your local community college.

Or at least try putting an indicator on your spindle nose, then pull and push on a tool holder. See how easily it moves. Your tools are deflecting under cutting forces. You're measuring the size under zero lateral forces. No fucking shit the part will be big and the holes small. The tool, machine, workpiece can all deflect away from the cut.