I'm cutting and 1/8 wide groove around a boss and the 1/8 groove dia is at the correct size in x but is under size in the y! The boss is correct in size and roundness, so i dont think the ball screws are the problem. If the cutter is dull could it do this? The cutter is 3/8 dia x 1/8 wide with a 5/32 shank. what is the proper depth of cut on 1018 crs?

Not the cutter.

You milled the boss as well your saying?

Better look at your code.



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Think Snow Eh!
Ox

How much difference are we talking here? Is there possibly a feature that the shank is rubbing on that's oriented in Y?

Check code... but

Dull cutter could do it..
spinning cutter in wrong direction could do it...
If boss isn't machined at the same time, bad offset could do it...
Taking up cutter comp on an arc could it....
Excessive cutting feed could do it....
Excessive tool hang out could do it...

You didn't say how deep the groove is....

Is the undersize at the spot where you arc into or out of the contour? Just a little dwell with a springy setup could over-cut locally.