[Ben Piecuch]

To answer this question you really need to know what sized shaft you have. Yes yes, I know you said 3/8", but not all 3/8" shaft is created equal. You need to use a micrometer or caliper to determine the actual diameter of your shaft. Most shaft will be +/- .002" while others are oversized (+.0002") or undersized (-.0002").

If you're looking for a precision fit in the slot, you'll need to use a smaller endmill (<3/8") and run a few passes to create a slot .0005" to .002" wider than your measured shaft diameter.

If you just want something to keep the shaft from wandering all over the place, you could use a 3/8" endmill to make the slot. Then, make a 2nd pass with .005" to .015" offset.

Another note... the material you use will change the fit of the shaft over time. Aluminum pieces should wear much more quickly than similar steel pieces. And, as always, check the fit before you bolt the whole thing together. You can always go back and open up the slot again.

Good luck.

BEN