What does it take to break a collet?

NRDock · May 6, 2019

I bought a used mill with a set of double angle collets for a NMTB30 holder, and yesterday I noticed the 1/2" collet is broken at one place on each end. The exposed face just has a subtle crack. The small end is cracked and displaced. It seems like this would be the result of a fairly traumatic event.

Andrewdini · May 6, 2019

the actual tool holder? dropping them on concrete could do it. unlikely, but possible. in my opinion

sfriedberg · May 6, 2019

Could also be the result of over-compression from someone tightening the collet with nothing (or something too small) in it.

Orange Vise · May 6, 2019

Common problem caused by user error. A tool is only partially inserted into the collet and torqued down. This puts excessive tension on the bottom of the collet and it cracks.

Usually the minimum engagement of a tool in a collet should be 2/3 the collet length.

michiganbuck · May 6, 2019

They are hard so over opening them or pushing a part in crooked can wipe them out. Possible one could have a heat treat crack.. Likely broken would still hold fairly close.
I used to true them up with putting on a mandrel with rubber bands to grind one end, then move the rubber bands and grind the other end. Best ones mostly came with up to .0002 error. No I did not grind new collets. did take a dremel brush and a hone to de-burr new collets.
But often the holder or something would also be off so to get .0002 on a part was not always that easy.

NRDock · May 6, 2019

OK, partially inserted tool makes sense for the damage I'm seeing. I feel better not picturing some kind of horrendous crash.

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What does it take to break a collet?

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