One thing I've had floating around in my little brain for a spindle is the fact that you can get a collar to fit just about any shaft. The metric shaft collars may be harder to find but not for you guys in the big cities. Simply get a collar that fits snugly inside a pipe (the spindle material) or nipple at the end flush with the end. For non-welders, perhaps JB Weld would hold it, hard to say. Probably not. Even welding might be tricky. Or since a collar has a set screw in it, tap a hole in the spindle material and screw the set screw down thru the collar into the spindle. Or you might have to get a slightly longer set screw. Or you could just use a bolt. If I could draw, I'd make a drawing. Maybe later.
After looking at plumbing parts at the hardware store, forget slipping the inside of the pipe nipple. It could still be done, but it would probably be easier to just get a coupler that's the same O.D. as the collar. Stand the coupler up, sit the collar on top of it (centered) and weld together. Then you can screw a nipple into the coupler for your spindle. Spindles/drive rollers range from 7/8" to 1 1/2", so you may need to use another coupler/reducer/enlarger or adapter to be able to screw on the diameter drive roller that you're wanting to use. I think the smaller the drive roller, the slower the speed and vice versa. If you'll be doing any welding, it would be best to use black pipe types of coupler instead of galvanized. Black pipe is somewhat cheaper anyway. In my local hardware store, the main shelves house the galvanized fittings and n the top shelf, the same fittings but in black, non-plated types.
