If you have a lathe and drill press, you can make a very positive coupler. Make the 8mm to 1/2 inch coupler of mild steel, 1 1/2 to 2 inches long. Cross drill with a 1/8" diameter drill, and press in a 1/8 hardened pin. Make the pin a little shorter than the coupler diameter, (recommend a minimum of 7/8 stock) so you can peen over the top of the hole above the pin to ensure pin retention. We have used this on 5/16 CIM motor shaft to DeWalt transmission adapter with no failure. This should see less load than that.
