Here are two eight-tooth sprockets for standard 1/2-1/8 single-speed bicycle chain. One has a D-shaped cutout for the axle of a NEMA23 stepper motor. The other has a hexagonal cutout to take a 1/4-20 hex nut, which has a wrench size of 7/16". The OpenSCAD source code to generate these sprockets also appears here.
I am intending to use these in a homebrew stereolithographic printer. Fingers crossed.
DISCLAIMER: I originally posted these files on August 13 2014 and when I had them printed (Shapeways, Metallic Plastic) I discovered that the friction fit I'd planned for the stepper sprocket wasn't going to work, so I've added a hole for a 4-40 set screw. There should be enough room to use a machine screw for that. And as long as I was doing a re-spin anyway, I put a cap over the hex nut so it's just a little more captive. More recently I've reshaped the teeth for a more forgiving design. I've learned that you need to watch out for stiff chain links (see http://bicycletutor.com/stiff-chain-link/ for advice). I think the design uploaded 6 Sept 2014 is going to be final.