Making a collet chuck

[franco]

Jordan,

I was surprised to see your suggestion that the threads on some lathe spindles might not be concentric with the spindle centre line. To me this would seem to be a basic manufacturing fault which should not happen. However, I suppose that in the modern world of relatively cheap imported machine tools, anything is possible.

I agree with you that, in my experience, the centering action on the backplate is carried out by the thread rather than the register. This is more or less backed up by dicussion in a thread by Jeff_G in the Practical Machinist Antique Machinery and History Forum in 2007 headed "Threaded Spindle Questions" which will show up in a search of that forum if you are interested.

However, even if the spindle thread was slightly out of concentricity with the spindle taper, in Bryan's design the area of the collet in the chuck taper is much larger than the area contacted by the collet nut, so I doubt if the collet nut would be able to pull the collet out of true, though it might possibly lead to slightly uneven tightening of the collet on the workpiece. It would be interesting to make the chuck and see whether (a) there was any lack of concentricity, and (b) whether this had any practical effect when using the chuck.

Your suggestion of the two piece nut and cone would certainly get around this, and should be truly concentric with the spindle. However, you would then lose the safety aspect of Bryan's design, where the collet nut also holds the chuck firmly (maybe too firmly!) in the spindle. This probably would not matter in use because when cutting on a lathe there is almost always some axial load towards the spindle (unless cutting towards the tailstock for some reason), which would tend to keep the chuck in place. If it was mine, I would feel more secure if I knew the chuck was positively held in place though.

Frank.

[nadroj]

Hi Frank,

I think it's always been the case that one can't assume absolute accuracy of the spindle threads.
Pratt-Burnerd operating instructions state that when fitting their chucks, "It is recommended that the chuck mounting spigot be machined with the adaptor plate mounted on the machine spindle to which it is intended to be used, to ensure optimum accuracy."

In the case of very springy collets like the ER type, I can envisage a misaligned front taper upsetting things. It's just my guess though! Compare the Myford collet system that uses a big nut on the spindle thread to tighten - it pushes not on a taper but on a flat surface in a grooved section of the collets, so no problem of it influencing alignment.

I'm not sure how a 2 piece nut wouldn't be as safe? It would still apply pressure to the collet/chuck. It wouldn't be as solid as a one piece nut, but at least it'd compensate for any misalignment.

Jordan

[Bryan]

Jordan, is this the sort of thing you mean? Y'know, it might even be easier to make. Bit fiddly to use though.

[franco]

Hi Jordan, Bryan,

Ah! OK, now I see what you mean. Yes, the arrangement in Bryan's latest drawing should cover any of the concerns raised previously - thanks for that.

Frank.

[nadroj]

Jordan, is this the sort of thing you mean? Y'know, it might even be easier to make. Bit fiddly to use though.

Yes Bryan - separate threaded and tapered parts, with flat surfaces at interface. A circlip might also be added, to keep it all in one assembly?

Jordan

[Bryan]

Jordan, I'm trying to think of a way to include a collet ejector system that would also retain the ring. Maybe some spring loaded pins? But my simple idea is starting to get complicated! And there's still a spanner....

Thing is, if I'm going to buy collets, it's only a little bit more to get a chuck, so I may as well get one. Then I'll have something that works and there's less pressure on my little idea. But whether I get a MT or backplate mount, I'll still have to make an adaptor, so once again, nothing's straightforward with this lathe.

Thanks guys, I really appreciate your input.

[nadroj]

It's all good fun. Keep the ideas coming.

Jordan