Special ER Collets

I don't understand why there is a belief that you can't use ER collets for short work-pieces. I frequently do that with no problems. The design of the collets, with slots from both front and rear, ensures that the bore remains parallel as it closes down. There should be no need to put another short end in the rear.

Russell.

Just to confuse the issue, when I made my Floating Reamer Holder, ER25 collets hold the parallel shank of hand reamers. For machine reamers, I made three oddball ER 32 size collets, with 1, 2 and 3MT bores, but only slit from the small end. (Could not figure how to slit from the front end, with apparently nothing to grip)

By the same token, it ought to be possible to shop make ones with square, or hexagon, (or whatever you want) bores, for work holding. As long as you can be happy about the concentricity!

Howard

Edited By Howard Lewis on 07/04/2016 16:51:42

My 7-6mm ER32 collet has a larger diameter for the back half of its bore as do all the smaller ones in my set. This does not cause these collets to grip out of parallel. This implies that it would cause no problem to hold anything with just half the length of any ER32 collet's bore.

Martin

Agreed, I've never had a problem either.

The ratio of bore to supported length is important.

Neil

Please explain. What is the effect if the ratio is wrong?

Russell.

If I need to hold a short workpiece in an er collet, I put a short piece of the same diameter in the back of the collet as Gordon and Michael have said, works a treat.

Edited By Michael Briggs on 09/04/2016 10:16:51

Here is a useful collection of information:

**LINK**

The comment about the holding capacity of ER-MB collets is interesting.

MichaelG.

.

P.S. Here, for Lottery winners, is the Regofix catalogue.

The warning on p24 of the PDF is pertinent to this thread.

Edited By Michael Gilligan on 09/04/2016 11:14:25

Russell,

Length/bore ratio

Just think about pulling sideways on the top of a 1 metre post that is 10cm into the ground. It will wobble!

If you are lucky, the short work won't come out as you cut!

Howard

I wouldn't call a 1 m bar short! My point is that ER collets maintain parallelism as they close so there is no point in putting a supporting piece in the rear of the collet.

No luck involved, just usual care.

Russell.

I am not so sure they can maintain parallelism if there is only contact at one end. You may well be right Russell but I can't help feeling the need to support the other end.

Doug

The leverage a part in a collet has that allows it to distort the collet depends on the supported length. Smaller parts have less strength and will bend before they distort the collet so they don't need as much support.

Neil

Well spotted, Michael. Makes sense, really. (For others, it's p3 of the catalogue numbering, p24 of the pdf file). Not normally a problem for toolholding, but a definite concern for holding short workpieces. Rationally-priced ER collets are lax enough in their run-out performance without adding our own distortions.

Roger

I think a factor must be the different tapers on the front and rear of the collets. I suspect the closing force at the rear is greater as the taper is more gradual and friction between the socket and the collet less significant. The front taper is much steeper. Imagine that the taper was actually 90 degrees at the front, there could be no closing force. So a short workpiece that only enters the front of the collet may not be held so tightly.

Looking at Michael's link what always fascinates me is that the tightening torques for collets are (at the recommended 80% of maximum) comparable to what you would use for diesel engine cylinder head bolts...

Hands up anyone here who pulls over 100lbs on the end of their 8" long ER25 collet spanner?

Neil

I'm looking at ERs for my little mill, and was shocked when I saw the torque spec for an ER-32 nut!!

I'm with Neil in asking this question.....

Pete

Maybe the torque setting is the max that the threads will stand ? I have a ER32 ( best thing I've bought, so far ) And usually just hand tighten the nut, holding the spindle with a tommy bar. This is for workholding not toolholding.

I don't faff around with a mere 8 inches. I use 12" spanners on my ER20 collets. Of course I only use them for tool holding, and that I think is the point. Milling cutter shanks are hardened so the gripping force relies almost entirely on friction. To obtain a high value the nuts need to be done up very tightly.

Andrew

My ER40 collet spanner is getting on for 1' long.

I use a puny 4" tommy bar and an 8" c-spanner with as much force as I can muster, but nowhere near 130Nm on such short tools. the tommy bar is an 'interesting' shape.

Neil