Three Jaw Chuck (Pratt and Burnard)

This topic has 7 replies, 7 voices, and was last updated 6 November 2012 at 12:45 by Jeff Dayman.

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3 November 2012 at 16:31 #102652
Martin Len
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@martinlen44626
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.
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3 November 2012 at 16:31 #22302
Martin Len
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@martinlen44626
6 November 2012 at 10:30 #102956
roy entwistle
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@royentwistle24699
are th echuck jaws in the right slots both slots and jaws should be numbered
6 November 2012 at 10:57 #102957
Peter G. Shaw
Participant
@peterg-shaw75338
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
6 November 2012 at 11:14 #102958
MICHAEL WILLIAMS
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@michaelwilliams41215
Hi Martin ,

As Peter says all three jaw self centering chucks of the more ordinary types have limitations on their accuracy .

Some things that you can do to minimise problems are :

(1) Mark the workpiece with felt tip pen to show where it was before removal – I always draw a box around no1 jaw but there are other ways . Put work back in exactly the same place when rechucking .

(2) Use same tightening hole in chuck for each chucking and tighten up to same feel .

(3) Keep chuck well lubricated and free of debris in working parts . Difference in accuracy between dry and dirty and clean and oiled can be as much as ten thou for the same chuck .

Regards ,

Michael Williams .

PS: This is not exactly scientific but if work piece has been successfully rechucked nominally true but still with slight wobble you can often bring it to exactly true by tapping it gently away from the high side of the wobble while giving jaws final tightening . Tapping work true is a definite art but it can work very well with practice .

Edited By MICHAEL WILLIAMS on 06/11/2012 11:27:32
6 November 2012 at 12:03 #102964
David Littlewood
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@davidlittlewood51847
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
6 November 2012 at 12:12 #102966
KWIL
Participant
@kwil
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.
6 November 2012 at 12:45 #102970
Jeff Dayman
Participant
@jeffdayman43397
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
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