I’d suggest ER16 as it can hold up to 10mm. If you’re going to cut the taper on the Taig, something free cutting would be best.

You have to think about how much use they will get and if you keep it clean and lubed it should be ok.

Hardening doesn’t automatically mean glass hard, and steel like EN24 isn’t intended for such applications. What you can do is to rough it out, then harden and temper to a still machinable state before finishing.

From what I can tell EN24T is around 850MPa which is a pretty decent all round material. I haven’t worked with it, but it should be pretty much the same as 34CrNiMo6. With heat treatment you can probably go as high as 11-1200MPa and still have something that cuts well. But you’ll want to rough it out before you do that.

Out of curiosity, How are  you going to be holding your ER11 collect chuck on your Taig lathe? When I made mine, I made them finished, so the thread is concentric to the 8 deg per side taper of the ER  taper. I have found that if the thread for the nut is not concentric to the inner 8 deg taper, it can cause runnout issues with the parts etc.  So I hold mine in the 4 jaw chuck on the 16mm shank, and indicate true the inner 8 deg taper.  Sometimes I will indicate the part diameter true , but usually I indicate the inner taper. I made mine with the 13mm fine thread, M13x0.75 mm pitch. Buying very good collets long term saves time and money. I have made a few of my own for specific projects and they have worked very well.

I’d consider making the whole item on the Boxford. After machining the 3/4″ Taig thread, 2nd op. work, ie taper and nut thread, can be done on by mounting on a threaded mandrel that has been machined on, and not removed from, the Boxford. This assumes that the Boxford has both metric and imperial screwcutting capabilitiy.

Just out of interest, I’ve just ordered a C8 ER11A 100L Chuck on Amazon for £7.29.

I’ve made various ER collet holders from mild steel, and so far they show no signs of wear.

After all the only parts likely to see much motion are the clamp nut (Which I buy) and the matching thread, Relative speed and distance for the collets in the tapers are slow and short, so wear is unlikely.

For hobby use, they are not going to be used for 40 or more hours a week.

With regard to the tapers, to minimise the risk of eccentricity, I would finish them in the Taig, since this is the machine on which they are going to be used.

The release groove in the collets, needs to be slightly proud of the end of the holder, to ensure maximum clamping and ease of release when the clamp nut is unscrewed.

Howard

I am under the impression that the thread size, for a given collet range is the number of the next up.

i.e. 25 x 1,5  for ER20, 32 x 1.5 for ER25, 40 x 1.5 for ER32.

Occasionally, I do have a bit of trouble engaging or disengaging the release slot from the eccentric lip in the clamp nut, but that seems to caused by minute variations between the various nuts.

With regard to material, have made at least eight bodies for ER20, ER25, and ER32 collets from mild steel, and so far, having bought the clamp nuts, have had no problems due to to wear.

But my screwcutting is not likely to be high precision!

Howard