Problem with 4 jaw chuck

I have an independent 4 jaw chuck sitting on my workbench not being used. It looks vintage and has no maker's name. The jaws "fit" and appear to be from a set (all marked 171) but I am unable to say if they are original.

The problem with this chuck is the jaws do not close parallel with each other. A superficial glance suggests bell-mouthing through wear but on closer inspection the inner faces of the chuck jaws show they are in excellent condition - they still show the original machining.

The "slides" in the chuck body look good as do the screws. The jaws have minimal slop in their respective slides.

I could make a ring to hold all 4 jaws in position and then grind their internal faces. I have previously done this with a 3 jaw scroll chuck but never needed nor thought I would need to do this with an independent chuck.

Any suggestions before I attempt a fix?

Another thing I should mention is the jaws are numbered 1, 2, 3 and 4. The chuck is similarly marked with centre-punched dots.

Do(es) the slotted screw(s) in the face just attach the rear adaptor plate or is the body of the chuck a 2 piece construction? If it is two piece, will it allow you to strip it out and measure the accuracy of the jaw tenons.

Looks like it has had a component held in the very end of the jaws and been over tightened (with a pipe on the chuck handle?). Common problem with unskilled operators attempting to 'true' work up.

Martin

I wonder if it has been previously used for holding short insertion-lengths very tightly

... is it possible that just the first ’step’ is distorted ?

MichaelG.

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Edit: Martin beat me to it

Edited By Michael Gilligan on 14/04/2021 10:32:39

Just because the inner face of the jaws are in god nick does not mean it has not had a hard life, could well have done a lot of work with the jaws the other way round, nicks in the face of the jaws confirm that and worn the slots.

It all looks very worn, rounded and pitted, the "machine marks" could even be where someone else has had a go at it rather than being original which would probably have been ground anyway

4 jaw independent chuck, or self centring chuck ?

The jaws, and body, could be distorted rather than worn, ie bent. To regrind using the usual clover-leaf plate could be fiddly with independent jaws, trying to get them all in exact alignment. Much easier on a self-centreing 3-jaw. So a common alternative is to regrind them square on a surface grinder with jaws clamped to a suitable angle plate etc. But could be done in the lathe with some fiddling. I suppose you don't have to do them all together like a three jaw. You could do them one at a time.

Ive done it with a couple of equally ancient and abused looking three jaws and very happy with the result.

Edited By Hopper on 14/04/2021 13:11:36

Strip it down and get a square onto those jaws, you can't fix what you can't measure.

I suspect that they will need re-machining, the best method will depend on what you have access to.

MichaelG.

Take the jaws out and lat the chuck face down on a surface plate. I bet it will rock like a boat.

More often than not bell-mouthed jaws is actually bent chuck face IMO.

I had one like that, more commonly known as a door stop.