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Ideas on a 3/8" hex holder to fit a Mill


Active Member
Whenever I contemplate holding tooling in the mill, my go-to is the R8 ER-32 collet chuck. Problem is that you can't buy any hex or square collets for the ER series (seen them only in 5C). You could use a drill chuck but I find that it adds too much length to the quill and I don't have clearance to have the 3/8" shank tool and workpiece fit in the same work envelope.

Saw a recommendation on another site that said to take a 6 or 12-sided 3/8" socket and cut a slit in it, then cut off the drive end and hold that in a collet. I understand the approach but I wouldn't think that a single slit would be proper (would need 3...1 for each intersect) and I doubt I could make them to near-collet accuracy. I've also seen slit sleeves for holding taps but those are 4-sided...not 6.

Anyone aware of any other pre-made product that would readily adapt hex to round and provide sufficient grip/concentricity?
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Ultra Member
Premium Member
When you say 'holding something in the mill' - do you mean a hex/square tool in the spindle? Or hex/square stock held in a base fixture?

If spindle, you can get R8 based hex & square collets & very low a profile. Maybe not built for heavy machining loads but some guys do light milling this way. But R8 will also have a max tool insertion depth dictated by the collet/drawbar end if that's an issue. Also I think these special collet shapes probably have a tolerance erring to holding undersize stock. Like if you have 0.375 nominal square collet, that might be the max but optimal grip might be 10 thou under. You would have to confirm. But a lot of plain hex/square stock like CRS & HRS are not to such specs & may have scale tec. You don't want to grip oversize stock in these collets, even a couple thou or they can be damaged. Tool steels are closer to spec,again you would have to check tolreance specs.

I don't think the slit a socket idea will work too well, or at least not very accurately. A good socket is forged so very tough, hard to clamp on that which clamps on a tool. A collet is a different principle. A tapered collet like R8 has slits but its being drawn up on its tapered nose & therefore closing. Because of that it ssomewhat limited in range. An ER style collet has slits originating alternating from one end to another. This gives it more range & less clamping force to close. Even so, the tightening torque is quite high to get them on concentric spec.

I'm trying to visualize what kind of hex or square tool you are spinning in a mill?


Active Member
R8 spindle; who sells hex and square collets in that format for a reasonable price: Hardinge which are $100USD/ea....
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Active Member
Weird that many have become more scarce.
Yeah...hens teeth is coming to mind. At even 50usd/each, just too costly. Ironically you can get them much cheaper from Alibaba but min order is 10 and I doubt I could get 9 other folks interested in buying a range of hex and square collets

Tom Kitta

Active Member
Check out MT3 hex / square collets - if I remember correctly there is MT3 to R8 adapter available everywhere so if you could get square MT3 collets you would be there.

Out of sheer curiosity why would you want to hold square stuff in the spindle? If custom tooling you can get round HSS blanks. Are you using your mill as a short part lathe?

Tom Kitta

Active Member
It was just a thought to check for yet another system.

I only saw C5 collets in square and hex & I don't even own any of these.

I guess you could also just buy an emergency R8 collet and drill / broach it square through that would be expensive unless you could borrow square broach from someone.
Indeed there is a gap between what 'could be' and 'what is' which is why I started the thread initially; I found nearly nothing except the $50~100 R8 collets.

For you 3DP guys to ponder...could a plastic collet be designed hex inside/round outside w/3 slits and hold that in an ER-32 collet with enough force that it wouldn't spin easily? Filament choice probably would play a important role in that approach...
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Well-Known Member
Shear force of even 100% fill isn't strong enough: in the order of 400 PSI; a little more for ABS, but not much. You could do it, and take a teeny cut... my guess is not for long

BUT it would be worth a try!

What is the tool you are holding and how much of a chip do you wnat to take. there are a lot of ways to skin this cat - If I knew what you were trying to achieve.
I was concerned about spin/shearing using a plastic-based sleeve. It's for infrequent use so I don't need to go to the effort of broaching a steel collar or spending $$ on this. I'll skip trying to grab on the hex section of the shaft and slide further down to the round and see if I can get enough bite...holding back on speed/feed will influence the outcome and taking a smaller chip is just fine.