Thanks for the link to that eBay seller. I've already ordered bearings, but the Lovejoy couplings they stock look to be a good deal, so I've ordered some. The 050 size should be OK for my steppers and the 035 size look like they'll work OK for the spindle couplings, as they are rated to 31,000 rpm.

The next challenge is to work out an easy way to connect the chuck shaft to the coupling.

Jeremy