Stop right there Ewan!...Put down that disc and motor and step away!!
If, after all I have said to you in the past, I catch you hanging a disc on a drill motor shaft, without using any extra suport bearings and a substantialy thicker shaft, I will personaly kick your back-side around the pits...If you think Im joking ...try it!!

Seriously for a moment, Christian has it about right! I couldnt put it much better my self. Think about a shaft dia. of no less than 12mm. We use 20mm on Scorpion Jr and even that is a little thinner than I would like, but testing has proved it to be safe.

Geoff.