I'm working on an old shaper. The driveshaft had been bent, probably by a forklift. (Seems like you can really easily screw up machine tools with a forklift!) I got a nice piece of precision shafting to machine. The gear had been pinned to the shaft using a press-fit pin. I am not going to try to do that again. I'm going to machine keyseats in the pinion gear and the shaft, and use a set screw. This is a helical pinion gear, so there is side loading on the gear; in other words it will want to walk up the shaft. Actually, maybe I'll use 2 set screws 90 degrees apart - I've seen those before.
What size set screw should I use? The shaft is 7/8", and the hub is somewhere between 1-1/2" and 2" (haven't measured it or looked at it recently). I looked in Machinery's Handbook but without knowing what I was looking for (a table of set screw sizes) I failed.
Realistically, the choices are No. 10, 1/4", or 5/16" (Sorry, Brits, I'm still not metricated ..).