Originally Posted by
JIMinPHX
It's a matter of getting it to move freely without introducing too much error into the screw.
First you do a bunch of gently & careful Lee-menting type work, sometimes to the screw, sometimes to the cylinder body, & then you check the mic against standards of known size at several different places in the screw's travel. You also measure cylinders at several different angles to see if the faces are flat & parallel. If you don't have actual standards, then Jo-blocks are a close second. If you don't have a set of (even "B" grade) Jo-blocks laying around, then dowel pins will get you within a few tenths. If you have nothing else, then buy a set of good quality drill bits & measure them when they are brand new. If all the readings come out good, then you should be all set. If you see variations of 3 or 4 tenths from what you expected, then you can probably only trust the mic to .0005" or so. A professsional Mic calibration will cost you more than a cheap mic. If you want something that will meet cal. spec., then buy a good one.