Jaw grinding

I think in many cases this is a last gasp procedure especially in cases of extreme bell mouthing it makes the difference between a terrible chuck and one that's a lot better than it was even with the concave profile of the jaw tips
Also too with 4 jaw independent chucks the jaws could be removed and ground on a surface grinder, they don't have to be done in situ
mark
 
I suppose that once you got the 3 jaws ground to diameter, then you could take them out of the chuck and then grind the tips square, stopping when the curved depression almost goes away equally on all three jaws... Good enough for a 3 jaw.
 
Yes, that would work if you can do it accurately enough.
 
From your description of the need to flatten the surface of the jaws it sounds like it would work best if the chuck was mounted to a rotary table on a mill.
Then index the jaws and use the X-axis to flatten the surface rotating 120 deg to access each jaw surface.
Actually I suppose the indexing for position would be the first step.
 
Well, it would probably be easier, assuming that the mill can turn up to a suitable RPM for grinding.
 
In my case the jaws were so worn that even if they were ground on a surface grinder they would still bell mouth, the jaws and the chuck body have slop in them so a square ground jaw could tip out at the bottom when it was tightened. The first thing I tried was truing them up on a disk grinder, I got them really close to square on the combination square I was using but they still wouldn't hold the work piece, still rocked after tightening. After grinding them in place they hold the work piece much better. I just took a jaw out and took a photo of the angle that the jaw ground to when tightened down.
IMG_20180924_175212369.jpg

Now granted the proper thing to do would have been to replace the chuck but the rest of the machine shows the same kind of wear so is it worth it to spend a bunch of money on a 108 year old heavily worn machine? not to me, I made it work and it can do some pretty good work now but I don't use it much anymore since I got the JET 1440, except when I need 4 jaw work, I really need a 4 jaw for the JET.
 
bpimm said:
In my case the jaws were so worn that even if they were ground on a surface grinder they would still bell mouth, the jaws and the chuck body have slop in them so a square ground jaw could tip out at the bottom when it was tightened. The first thing I tried was truing them up on a disk grinder, I got them really close to square on the combination square I was using but they still wouldn't hold the work piece, still rocked after tightening. After grinding them in place they hold the work piece much better. I just took a jaw out and took a photo of the angle that the jaw ground to when tightened down.
View attachment 276254

Now granted the proper thing to do would have been to replace the chuck but the rest of the machine shows the same kind of wear so is it worth it to spend a bunch of money on a 108 year old heavily worn machine? not to me, I made it work and it can do some pretty good work now but I don't use it much anymore since I got the JET 1440, except when I need 4 jaw work, I really need a 4 jaw for the JET.
Click to expand...
The jaw in the picture actually shows the opposite of bell mouth. Normal wear on chuck jaws makes them have a bigger diameter at the upper tips of the jaws. Yours has a smaller diameter there. Actually, with the wear that likely exists on the rest of the chuck and jaws, those will probably be close to matching the center line of the lathe when tightened onto workpieces.
 
Exactly, this is after grinding them with the tightening jig shown in the earlier post #6, the 4 jaw, it was bell mouthed and now it clamps down good. Just posted this pic to show how much wear was in the chuck preventing it from being ground on a surface grinder and the benefits of this procedure on an old worn chuck.
 
I suspect that even when new, chuck jaws would exhibit a slight clearance at the heel. Because they must have some clearance between the jaw and the slots in the chuck in order for them to move.
 
What I meant by that was that to make a satisfactory chuck, the maker must somehow be able to compensate for the fact that the jaw will cock slightly when tightened, One way would be to grind all jaws square to one end, and then to assign three (or 6) jaws to a chuck body, install and load them in that body, and finish grind the noses. However, that would take four setups as each set of jaws is usable on both ends. Someone who happens to live close to a chuck manufacturer could probably find out.
 
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