Seeking opinions on chuck jaw repair

[Video_man]

My youngest is restoring a 1947 Logan with a Bernhard 3-jaw. Chuck runs concentric and items held in the (apparently never-used) outside jaws run concentric, but work held in the regular inside jaws show runout. Apparently they are worn or sprung somewhat. MY QUESTION IS: would it work to gang the jaws on my surface grinder and lightly grind them all to the same height and flatness on the ends of jaws? I have heard about constraining the jaws in a hoop and boring them, but looking for an alternative.

 

[mmcmdl]

Need to grind jaws relative to the scroll so it would be like making soft jaws in a lathe .

 

[benmychree]

Grinding in place on the lathe is the only way to get concentricity, the jaws need to be constrained in the direction of clamping to achieve this goal, clamping a ring in the outward direction will not work, on one small chuck, I drilled holes in the face of the jaw on the smallest ring, inserted rol pins and clamped down on a small ring, leaving just enough room inside the jaws to fit the grinding wheel with a small Dumore grinder.

 

[Video_man]

Thanks for the replies, but nobody has answered the question I pose. I know the methods mentioned are correct, and I accept your information with gratitude, but still asking if anyone can answer my question about grinding the faces to match.

 

[OTmachine]

Thanks for the replies, but nobody has answered the question I pose. I know the methods mentioned are correct, and I accept your information with gratitude, but still asking if anyone can answer my question about grinding the faces to match.

See post #3. This is the way.
or clamp a thin ring at the innermost section of the jaws to preload, then grind true with tool post grinder. Remove jaws and use a die grinder to remove the remaining unground surfaces.

 

[Chipper5783]

Thanks for the replies, but nobody has answered the question I pose. I know the methods mentioned are correct, and I accept your information with gratitude, but still asking if anyone can answer my question about grinding the faces to match.

Okay, I’ll bite. I trued the jaws on my 8” 3J, they are 2 piece jaws, so I made a loading ring that tied into the bolt holes that secure the top jaws. I then cut the jaws with a TBC insert - with excellent results. However, to your point “could you surface grind them?” Yes, of course you could. If the chuck is that bad, nothing to lose. I don’t know that gang grinding would be the way to go. The jaw position references the load side of the scroll teeth - not the aft end of the jaw. Still, that isn’t a big deal, grind the jaw, reassemble, map the run out and which jaws are ”tall”, disassemble, grind again, keep going until satisfied.

Two features that will be a touch trickier are that strictly speaking you don’t want the jaws cut straight. When a chuck is worn, it will be bell mouthed, part of the reason for the loading ring is to load the face of chuck which cones outwards, if you then cut the jaws it will be slightly “anti-bell mouthed” - ie straight when loaded. Robin Renzetti has a great video on cutting chuck jaws and the deflection of the chuck body. The second issue is that jaw face contact is concave to improve the grip on cylindrical parts - you could manage that by dressing the SG wheel to a bit of curvature.

An alternative method (that works well) is to put a plug really deep in the jaws and crank down hard, then go ahead and cut the jaws. You’ll still need to remove the small untouched land deep in the jaws (either with a ring out near the front of the jaws, work through the hole), or remove the jaws and grind off that little nub. You don’t get as good a “reverse cone” effect, and lose a little of that very in board end, but it will achieve the majority of the benefits. To cut the jaws, whether you grind ‘em or simply cut with a boring bar probably depends on the equipment you have. I used a TBC insert because I don’t have a decent little grinder arrangement (and the TBC boring bar insert cost a whole $8 delivered Ali Express - on a bundled order). TBC was good enough for Robin, so good enough for me.

Let us know how you make out.

 

[Video_man]

Thanks everyone!

 

[woodchucker]

Thanks for the replies, but nobody has answered the question I pose. I know the methods mentioned are correct, and I accept your information with gratitude, but still asking if anyone can answer my question about grinding the faces to match.

yes they have answered. Your not listening.
You cannot do what you want.

 

[benmychree]

You could do what you want, but it would not be likely that it would give you the accurate result that you want.

 

[durableoreo]

It is unconventional but I don't see why you shouldn't give it a try. In your setup, see if you can incorporate a small angular adjustment. The jaws are cantilever beams that bend most at the tip of the jaw. You may need to grind them at a very slight angle so that the tip of the jaw (nearest the operator) touches the work first. It won't be visible but when you clamp the work tightly, the pressure exerted by the jaw should be even down the length of the jaw.

Are you opposed to grinding on your lathe? Just curious why you've ruled that out. I intend to touch up my chucks and I'll use a CBN tool because it's less mess.

Let us know how it turns out.