[scottandme]

We used a CIM-SIM, but same concept. CIM-SIM > Coupling > 1/2" Keyed Shaft with wheels on 1/2" keyed bore AM hubs.

Just search "shaft couplings" on McMaster and you can drill down to what you need. We used a helical beam coupler that allows for a little bit of angular/axial/parallel misalignment, but a standard coupling would work fine as well.

It's part number 9061K163, 5/16" is close enough to 8mm (0.0025") that it worked fine. They're made by Ruland, who provides the shaft collars in the KoP. Pickier people could use the metric to metric versions and turn down their output shaft for a closer fit, but the clamp style makes it unnecessary in my eyes.