All good points.
I have a Mitutoyo digital micrometer which measures to .00005" but, I have difficulty in getting repeatable measurements to less than .0002–3" accuracy. The motor shaft seems to measure .1248" sometimes and .1250" at other times. It is only about .33" long but as near as I can tell there is no taper or out of roundness. I did not think to measure the drill shanks as they are standard 1/8" carbide PCB drills.
Although I used a .124" reamer followed by a .125" reamer the hole is somewhat larger than .125" , a .1250" gage pin fits in, a .1255" pin does not (both are + or "go" types). The coupling was made of brass, I am thinking of trying to make another out of steel and lapping it to size.
Set screws can be used to hold something in very a precise position as evidenced by end mill holders, but possibly not with a coupling of this diameter / wall thickness.
Craig
---In Homebrew_PCBs@yahoogroups.com, <amvweb@...> wrote :
I wonder if your added runout is due to the hole being not EXACTLY 0.125”, or one or both of the shafts being a smidge small. Also, if you use set screws only on one side, you may distort the coupler enough for 0.004” runout. Collets, after all, go to a lot of trouble to grip the drill shaft on all sides equally.
Andy