My stepper motor has an 8mm shaft, but I can't find any pulley's with that bore. Any of you have any ideas?
8mm drill. can't get it at Home Depot but Grainger, McMaster, any real hardware store, even stateside.
"enlarge" the shaft on the motor from 5/16" to 1/2"? Are there metal collars somewhere I can use?
It would be called a bushing. That would be hard to find in that ID/OD - probably fastest to have one cut for you at a machine shop out of brass or aluminum.
The problem with timing belts is you can't adjust the size. So literally I'll have to create the mechanism the size of the belt and then get a tension-er. With the V-Belt I've found an adjustable V-Belt
Tensioning is best achieved by having an adjustment to the motor's distance from the axle rather than trying to have an exact belt length. A powertwist usually goes on a 3/8" sheave/pully for woodworking equipment use. They're designed to slip a little rather than positive grab all the time. not recommended.
Also, trying to find a Floor Flange/Die Cast hub with a 3/4" bore is quite challenging.
It's a standard NPT size, not a Home depot item but a real plumbing/gas supplier would have a bin of them. It would be for 3/4" ID pipe, not od, though. 3/4" OD not sure there is such a thing if that's what you meant. (like a 3/4" rod used as an axle.)
I think the belts with the teeth on them would be okay
Those are usually called gilmer belts. They are hard to spec gears for but if you have any old inkjet printers they often have small ones, perhaps could be doubled up. For this application you can shorten one by cutting 45 degrees on each end (aligning the teeth) and taping it back together. Don't know the tape used but many of them are made with tape splices from the factory. EditAll tape would work if you can find some.