OK. Some "lapping plates" started dripping from china and just got the diamond cup wheel from ARC.
ARC cup wheel looks rough outside, but most likely fit is good and does not cause too much problems.
I have 90W 2800 rpm extra motor I'm planning to gear down with flat belt or round rubberthane or whatever those heat splice round belts are. Since this is fabrication, I have been thinking to use one of following methods to clamp the thin lapping disc to backplate:
1: Clamping ring/boss + nut (maybe a knurled one) to clamp the centre
2: Couple of supermagnets on the back + retaining pin on the middle. Ejection? "captive" nut or something like #1 to clamp/eject?
3: Small vacuum. I have a small quiet pump, not much is needed, but needs a rotary union or other induction. A bit over engineerin.
I would love to settle on #1, will it work. If it does - KISS, but I don't want to simplify it too far.