Hello Ronald,

Yes it is, much depends on what IS inaccurate. If it is the rear plate mounting to the chuck, that is a relatively easy salvage which involves turning away the old location, bonding on a ring to replace it and turning the new register to suit the chuck.

The second fault concerns the chuck jaws that may have gone 'bell-mouthed' with being forced to grip just on the tips [a form of regular abuse].

You will need a toolpost grinder or similar to correct this fault. First turn a ring with a reasonable aperture, open the chuck jaws outwards to grip it on the outer steps and then very carefully grind new gripping faces onto the INSIDE facing jaw surfaces. Move the grinder in and out with the chuck rotating slowly to cover the whole surface until the shine is even throughout on all three jaws.

Test the results on a true bar for runout and repeat if you have to. If the body guides to the jaws themselves are badly worn and sloppy it is best to scrap the chuck and fit a new one.

Take care to remove ALL grinding dust after, cloth protection on bedways is good before you start.

You may well find in the end it is only fit for roughing work, in which case keep it for such.

Good luck Brian

Hello Ronald,

There is another, rather tedious way of correcting bell mouthed jaws, I used it many years ago to put my father's small scroll chuck right when it came to me with the Myford ML4 lathe after he died. You need a LOT of patience!!

Use a black magic marker to coat the jaw gripping surfaces first. Then, with a true and round bar in a tailstock chuck you trust, close the 3 jaw chuck down until only one jaw is touching the 'test' bar. Idle the lathe at slow revs, move the testbar in and out to rub the marker away on the jaw that touches.

Take the jaws out and with a fine oilstone, polish that jaw over the bright area, holding it carefully to grind only that bit. Rebuild the chuck and test again with remarked jaws, repeating the process until you are satisfied that all 3 jaws grip evenly both front and back. This also means checking for runout as the polishing proceeds. Look especially for any misalignment along the lathe axis where the chuck tries to grip on the back or front of a jaw. That will show up as increasing displacement of a DTI along the length of a true test bar. Axial displacement alone is parallel to the lathe axis.

As I said, you do need lots of patience; in my case I was rather fond of the lathe and this little chuck, it is now amongst the best in my collection, so it does work.

Brian

The problem with that video is the jaws are being tensioned inwards by the acrylic ring. The scroll and jaw teeth don't tend to wear so much in that mode. He should be tensioning the jaws outwards in the same way they would be when holding something. I've actually done this with an old chuck (not too knackered) and got the thing to hold amazing true throughout it's full movement. Don't forget to use the same key position each time too and mark it somehow. I turned up what was essentially a top hat with the very top of the hat missing. I took out three section of the tube on a rotary table with an end mill slightly narrower than the thickness of the jaws. I then gripped this in the chuck with the jaws in the gaps so I could then send in a small grinding wheel on a flexible drive clamped in the tool post. You only need to take the barest minimum off.

I think it depends on how 'mullered' the chuck is to begin with.The Pratt-Burnerd which came with my Hofmann dividing head had jaws which looked like someone had been beating them with a hammer.
I used all the methods outlined above to try and correct the problem ( including toolpost grinder ), but nothing worked.

Ronald,

As I understand it, the jaws should be pulled outwards as close to their ends as possible when grinding to overcome the effect of "bellmouthing".

I have constructed the contraption shown, and so far have attacked 2 worn 3 jaw chucks with very satisfactory results.

http://ebookbrowse.com/chuck-truing-scan-pdf-d379399518

Gordon.

Michael,

Thank you very much what a great tip, after stripping/cleaning the scroll licking off the burrs and skimming the backplate on my very old "The Burnerd" 5" chuck it still had very poor repeatability just tried your idea and now have repeatability to about 0.001" not bad for a chuck which must be >60 years old

One thing I will add my chuck jaws do have a little sideplay so in an attempt to counteract this I ran my Boxford both backwards and forwards whilst lapping, this did produce a visible "facet" on the ground surface of the jaws but seemingly no ill effects.

Thanks again

Paul