I doubt there is for practical reasons.

Can you imagine centering up a part in one?

With a 4-jaw independent, the jaw pairs act more or less independently, whereas on a 3-jaw they will all interact with one another.

Jon

I stand corrected although it seems like a hybrid…

**LINK**

Edited By Jon Gibbs on 01/12/2016 14:07:42

I can't think of anything that you could put in a 3 jaw independent that could not also go in a 4 jaw independent so it could be a redundant piece of metal that may never be used.

Martin

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I would rather not

… But, curiously enough, the tribrach has become the standard way of levelling theodolite, etc. [whereas the early ones had four-screw levelling]. Logically, one might think that the difficulties were comparable.

MichaelG.

I was once permitted to use a large lathe to turn some 14 inch diameter traction engine wheels. It had a three jaw S/C chuck, the jaws of which could be indepentantly adjusted; and when I was let loose on it, it was not adjusted. It was complete nightmare to get it right. It was not just one jaw that needed a bit of a nudge. I did manage to do the job in the end, but it was quite protracted.

Maurice

I have put a 3 jaw chuck with soft jaws on a mill and milled the jaws to a specific shape to allow production machining of water pump castings.

It's really not difficult to adjust. You have a setting disc with a centre dimple that you can position with your tailstock centre and you just run the jaws in to touch it (or just centre on the tailstock barrel. You then have a starting point. You can also position each jaw on a dial indicator.

Use: A typical use might be for machining the end of the crankcase of a model aeroplane engine. This has a round base to the crankcase and the cylinder sticking out. There are mounting lugs each side of the crankcase so difficult to hold in the 4 jaw.
Centre all jaws then open out to position 2 jaws on the case body, then independently move the 3rd jaw to hold the top of the cylinder. The 2 matching jaws position it so the centre is on a line between these jaws but may be off set a bit along the line. So measure the offset, move the 3rd jaw by that amount and finally move all together to grip it

The principle is that a three-point contact in two dimensions is always stable, four is not.

The same arrangement is used for aligning telescope finders, Where the adjustment range is small it is possible to fit the adjusters so their effect is, to all intents and purposes, at right angles over small adjustments and some finderscope arrangements do this.

This means adjusters at 0, 90 and 225 degrees. You could make a scrolled + independent chuck with jaws at these angles that would be very easy to adjust but would look very wrong…

Neil

I have seen a chuck where 2 jaws can be set, and then locked, and the 3rd one is the only one that moves. The 3rd jaw was moved like a 4 jaw, ie on it's own scroll for that jaw.

It may have been a purpose made, as I do not recall a brand name on it. With the other 2 jaws, there were various shaped jaws that were used with it. They were made in such away that the part could not be accidently placed in the wrong way. The other 2 shoes as I would call them were often referred to as grips.

Neil

OT but useful. If you put a 4 legged table on uneven ground one leg is in the air. So you move it and now a different leg is in the air. DOH! but that means there is a position between the two places you tried that is the transition between one leg in the air and the other. Careful movement will always find this spot and hence a 4 leg table can be just as stable as a 3 leg.