Quote Originally Posted by booski View Post
The issue is how to mount them to the shafts as I have limited tooling For example, the spindle itself has got a 10mm shaft so do I
1. Drill the pulley to 9.9mm using my mini lathe and hope that the runout is enough to make the hole 9.95ish for interference fit
2. Ream the pulley to 10mm and use a grub screw
Neither as since you have a mini lathe you should be able to bore the pulley to the correct size. Take small cuts towards the end and try to insert the shaft after each and you'll get the proper fit. The drill will be nowhere near and reamers are a bit hit and miss.

Don't use two opposing screws as that wont support the pulley properly. It's normal to use two at 90 degrees, but maybe 3 equally spaced since you're concerned about balance.
20,000rpm at 500W is 0.24Nm, so I'd expect grubscrews to hold. It would be better to machine the pulley such that it clamps on to the shaft, however then you would almost certainly have to balance it. Have you got an oscilloscope, or access to one? If so then it shouldn't be too difficult to dynamically balance the spindle shaft with pulleys yourself.