Three Jaw Chuck (Pratt and Burnard)

I have recently acquired a Myford Super 7 B lathe and discovered that when I did a test turn on a piece of round stock once removed from the chuck it was very difficult to maintain accuracy when replaced back in the chuck ,it became a laborious process moving the stock around in the chuck in small increment until I found the sweat spot again.

I don't think the chuck has been abused but something is wrong. Is it possible to refurbish it or am I looking at a new one.

are th echuck jaws in the right slots both slots and jaws should be numbered

No three-jaw chuck is dead accurate. The act of removing the work and replacing it means that it will be impossible to accurately (to precision limits that is) replace the work.

The chuck itself is made to tolerances, and whilst a Pratt-Burnard may well be one of the better chucks, it will still be subject to these tolerances. This means that although you may turn a work piece, thepart you have turned will not be concentric with the part within the jaws hence it will be, as you have found out, almost impossible to get it back in the "sweet" spot.

Furthermore, the act of slackening and retightening will change the forces within the chuck, hence the amount of flex in the chuck and the work will be different.

My suggestion is to not remove the work until you have finished turning.

FWIW, using a four-jaw independant chuck with an indicator of some description although superficially more messy to use, will eventually become second nature to use and it will then become easier to replace work reasonably accurately.

If, as Roy says, the jaws are correctly located, then there is no need to replace the chuck, simply accept it's limitations.

regards,

Peter G. Shaw

If you are going top tap the work to true it, I recommend you use a copper/hide mallet - it's much less likely to mark the work.

If you can afford it*, a Griptru chuck can be adjusted to make the piece concentric.

If the work is small enough, a good quality collet chuck will give a much better concentricity, and with far better consistency between workpieces, or on replacing the same piece.

BTW, the correct name of the chuck maker is Burnerd, not Burnard. Might help if you are searching for one, though perhaps best to try both, as so many people get it wrong.

David

*If you need to ask the price, you can't afford it!

Edited By David Littlewood on 06/11/2012 12:05:21

Apart from all the above, the starting stock material may itself not be "round". The concentricity of a chuck will also vary with diameter held.

With a three jaw the trick is to plan your ops to do as many as you can without removing the work from the chuck. If you do remove it, accuracy is usually lost. At that point if you need to re-centre the work for further ops you use a 4 jaw and re-centre the work in that. With practice it takes at most a few minutes to get work in a 4 jaw running perfectly (if the lathe bearings are set up correctly and not worn out)

Your self centreing 3 jaw may gain some improvement in smoothness and repeatability if you dismantle it, clean the scroll and jaws thoroughly, and re-grease it.

JD