CNC lathe takes 16c collets, but we don’t have let’s say 7/8” needed for aluminum shaft to be threaded. We do have 1.25” 16c collet and 7/8” 5c collet that is 1.25” od and slips right into the 16c, to be held by friction when hydraulic collet closer activates. The 7/8 aluminum rod would be inside the 5c for that collet’s full length. Already tried the multi-jaw chuck and it doesn’t work well on this shaft so installed the collet chuck. Haven’t tried this yet and there’s no emergency, if this is definitely a bad idea we won’t do it. Slippage between collets I doubt will occur, the collet closer exerts a huge squeeze, forgot the number.