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Split collet steel recommendation

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Stephen Howard21/08/2020 10:08:57
4 forum posts

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.

Mark Rand21/08/2020 21:18:34
927 forum posts
6 photos

Any steel will be pretty much identical in 'springyness' and recovery back to original diameter for those sorts of movement and diameters.

Without hardening most steels are fairly similar although the 'free machining' steels seem to have more of a resemblance to warm butter if you are unfortunate enough to drop a finished part...

If you want a longer wearing/more robust collet without having to harden them, you could look for EN16T or EN24T. The T stands for the pre-heat treatment condition and ends up about 30-34hRC. Also machines better than the annealed versions, being less grabby.

Sorry if I'm teaching you to suck eggs!

Stephen Howard22/08/2020 11:04:39
4 forum posts

Thanks for that, Mark - much appreciated.

I mostly work with non-ferrous metals and silver steel, so some of the more esoteric/harder steels are a bit off my radar - and as most of my workshop-made tools tend to be mandrels of one kind of another, I find that 404 stainless is easy to work with and keeps the rust at bay.

I like the sound of EN16T, and from having a look through the forum I reckon my Emco Maximat should be up to the task - but I think I'd be sensible to find a stockist who'll saw it into 50mm lengths...otherwise it's just me and a hacksaw.
I think I'll also need to treat myself to a couple of morse taper drills!

Stephen Howard08/09/2020 19:37:12
4 forum posts

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.

thumbs up

Mark Rand08/09/2020 21:03:08
927 forum posts
6 photos

The trick with a lot of the alloy steels, stainless, manganese bearing etc. is to use a coarse hacksaw blade, don't cut at more than about one stroke per second (even that is about 100 feet/second!) and lean on the blade fairly hard. That way, there's less chance of work hardening and you compensate for the fact that the steel's a bit harder to start with.

14tpi hacksaw blades should be present in every workshop!!!

DC31k09/09/2020 07:41:03
282 forum posts
1 photos
Posted by Mark Rand on 08/09/2020 21:03:08:

... that is about 100 feet/second!

The only time I have managed to hacksaw at this speed is as a passenger in a van on the motorway.

Stephen Howard09/09/2020 10:31:25
4 forum posts

I tried it with a 14tpi blade - picked up a bunch of NOS Keraunos blades at the tip a few years ago - but it didn't seem to want to know. Ended up using a bi-metal 24tpi and slogging away.

I'll try the next one with a new blade and a bit more welly - and perhaps partly part the bar in the late beforehand.

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