My "new" Pratt &Whitney 3C horizontal bench mill uses unobtainium 4pn collets that have external threads and use a draw tube. The collets have no key way like R8 collets and just use friction to turn with the spindle (is that the right term?). I am going to try using an R8 collet because it is 0.05" smaller in diameter but slightly longer and I'll make a draw bar and rely on the same friction fit to turn the collets. Is there a reason that won't work? If you need a draw bar for R8 collets, why does it need the key and key-way? Am I missing something?