I am planning on making a collet chuck also. I plan to make 2, one for my Atlas mill and one for my Craftsman lathe. After reading mattthemuffet's build I plan to make one with a piece of 2" square steel I have and one with some hex stock. That way I can use them for a cheap indexer. I have er32 collets and the nut is just under 2". Not sure of the nut size for er40. It is most likely metric thread and that is what is slowing me down now. I have a qcgb and need to get additional gears for that and also add reverse to my lathe to do metric threading.