I'm making up a small set of split collets; 49mm long with an OD of 35mm and IDs in the range of 23-25mm and 27-29mm, with a single split along the length. These will be compressed in a square block, suitably bored out and cut in half – and the purpose of the collets is to round out and reduce the diameter of short, thin brass tubes.

I've knocked up some prototype collets in EN8, mainly to see how well they work and whether or not they have enough 'spring' in them. They don't need to move very far…0.1mm is fine, anything else is a bonus – just as long as they spring back to size (more or less).

EN8 works fine, but getting a decent smooth finish on the bore is a tricky straight off the lathe – and this is a particularly important requirement, as I don't want to go down the route of honing. Or hardening.

So – any suggestions as to the best steel to use? I make a lot of workshop tools in 304 stainless, and get very good surface finishes straight off the lathe…but I'm not sure whether it'll be as tough and springy as EN8.

Just a l'il update..

Big thanks to Mark for the advice – got some EN16T in and I'm very pleased with the results.
Sure, it's a proper sod to cut with a hacksaw but it turns really well and takes a nice finish (with a bit of emery). Better still, it seems to have more spring than the other steels I've been using.

The downside is that it's relatively hard to come by (in short lengths) and somewhat pricey – but for my purposes I doubt it'll ever wear out.