Have you measured your bit to see if it is actually 3.175mm? I had a similar problem with a 1/8" bit that should have been 0.125" but when I measured it I found it to be 0.123". Same with a 1/4" bit that should be 0.250" and is actually 0.245". So I now have those defined in the tool library with their actual size and my parts come out pretty accurate.
David
David