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Collet question

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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?
 

Comments

#2
Someone with WAY more savvy than I will be along shortly. It ain't much, but I would offer MY experience- I have never seen a machine running R8 collets that had a key or pin that engaged the slot in the collet. Absent, never there, not sure. I do KNOW that my Millrite mill and the two Taiwanese mill-drills I have time on have absolutely nothing standing proud in the bore of the spindle where the R8 resides. They rely purely and solely on the interference fit of the tapers, and the pull of the drawbar. Therefore, the fit of the tapers would seem to be critical.
 
#3
My Bridgeport has a key. .050 on a diameter and relying on the top angle to tighten it up seems a stretch.
 
#4
The key inside the taper is an alignment pin. It really doesn't stop the collet from turning in use - the drawbar locks the taper in position solidly. Many of us remove the pin and have had no issues without it.
 
#5
Just finished the new drawbar assembly to retrofit R8 collets to my P&W mill. Works great!
 

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