Burnerd 3 jaw chuck

[exairman]

Well fellows you have certainly given me plenty of ideas of how to go about solving the problem. I suppose the most sensible one is the regrinding after all the chuck is pretty useless as is. My problem there is l don,t have tool post grinder, and have never used one, still could try Chronos! Will keep you dated on any if any results. Many thanks to all, exairman

 

[BaronJ]

The soft jaws sold by Cromwell fit MY P-B chuck on MY Myford ML7 , I posted this link because it was the only place I could find ANY jaws for imperial sized Burnerd chucks , if they haven't got your's that's bad luck.

I agree its my bad luck ! The sales guy that I spoke to said they won't fit a 3.5" Myford P&B chuck. However they do stock the part number that you quoted and will quite happily sell me a set.

Please explain how to fit an aluminium set over existing jaws , I can't see how you could fix them in place. The whole point of soft jaws is that they can be machined for a particular job , taken out and put back in the chuck without losing the initial accuracy. I have several sets , each for a particular production job that is too large for a collet.

Aluminium bar drilled to go over the jaw. Drill and thread 2BA from the top and just use a grub screw to bear against the back of the jaw. Crude maybe, but it works well particularly for small parts.

You don't need to bore out a steel disc , the outer race of a large roller bearing is easy to obtain , very accurate and will do the job admirably , however the reground jaws will only be truly accurate at the ground diameter as the forces on the jaw teeth and scroll will be reversed from those applied when in actual use. Even so badly bell mouthed jaws can be improved using the method.

The only problem I see with using a ball race outer is that it will flex and distort as you apply pressure from the jaws. I do agree that after grinding there will be a sweet spot in terms of holding diameter. However all that is asked for is that the jaw faces are parallel and not bell mouthed.

 

[goldstar31]

The only problem I see with using a ball race outer is that it will flex and distort as you apply pressure from the jaws. I do agree that after grinding there will be a sweet spot in terms of holding diameter. However all that is asked for is that the jaw faces are parallel and not bell mouthed.

There is a wee bit of crooked thinking here.

Initially, a Griptru isn't a normal 3 jaw SC chuck. It has built in adjustment over a limited 'throw' So adjustment and comment about 3 jaw Myford chucks needs some clarification. Again, if the 3 jaw is a standard Myford one, it hasn't got a back plate. With a back plate, one can make it act as a Griptru. All that is needed is to widen the fit and elongate the adjusting screws. It's not new thought but merely that it is not known or has been forgotten. The two pundits that I recall were Martin Cleeve and Professor Dennis Chaddock. The latter was his Model Engineer article on the Quorn construction in the early part of 1974.

Let's move back to lathe jaws again. This has been flogged and flogged but the real answer is that unless it is only the jaws that are worn and the rest of the chuck is OK, you CANNOT renovate a worn chuck- without a lot of gear and even more experience. All that one can do is get ONE dimension. You can put on soft jaws and cut them but you return to my statement of ONE dimension.

However, if it is only the jaws that are bell mouthed, they can be re-ground to virtual normality. I've done it with nothing more than a router with a stone in it. But , and it is a big BUT Baron is right about the jig. If you tighten the chuck, you will be probably unlucky and here you OPEN the jaws against three pressure wedges of exact dimensions such as precision ground bar.

Let me take this a little further. The real experts( and that does not include me) always used ONE adjusting hole in a three jaw which gave the most accurate result. Again, the authority on this was the late Tom Walshaw who built his and described his engines under the name of Tubal Cain.

If this is a lot to digest, the correspondence is of far greater length and depth. This, humbly, is a very condensed but hopefully accurate précis.

Regards

Norman