Should we grind 3 jaw lathe chuck hard jaws?

Parlo

Parlo

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There seems to be a lot of videos and advice on grinding lathe chuck hard jaws.
Considering they are the hardest parts on a chuck surely they are the least susceptible to wear, and they will wear evenly anyhow.
It's well known that the concentricity is due mostly to wear of the scroll. Grinding hard jaws at a certain diameter only helps at that diameter. If you are going to grind hard jaws to suit a certain diameter, why not bore out soft jaws?
I appreciate that they can flare out on older chucks but that problem lays with the jaw guides, not the jaws.
 
The whole reason they resort to grinding, is exactly because the jaws are hard. It's the only reasonable way.

Indeed, all sorts of setups have worked, from full blown spindie grinders, to a Dremel type tool strapped to a piece of wood with cable ties.
Do binge on the videos. I did, and I suppose my main conclusion is that I would not use the "expand into a ring" method, as shown of by Keith Rucker. Good fellow though he is, I just don't agree with him on this

Thinking about "are we grinding to suit only a certain diameter"? I think not. A chuck inherently is designed to contact a part over only a very small fraction of the circumference, at three points. This is as opposed to a collet, which is an extreme form of chuck where the fraction amounts to nearly the whole way around. Even collets have a (narrow) range of accommodation.
 
graham-xrf said:
The whole reason they resort to grinding, is exactly because the jaws are hard. It's the only reasonable way.

Indeed, all sorts of setups have worked, from full blown spindie grinders, to a Dremel type tool strapped to a piece of wood with cable ties.
Do binge on the videos. I did, and I suppose my main conclusion is that I would not use the "expand into a ring" method, as shown of by Keith Rucker. Good fellow though he is, I just don't agree with him on this

Thinking about "are we grinding to suit only a certain diameter"? I think not. A chuck inherently is designed to contact a part over only a very small fraction of the circumference, at three points. This is as opposed to a collet, which is an extreme form of chuck where the fraction amounts to nearly the whole way around. Even collets have a (narrow) range of accommodation.
Click to expand...
Yes, absolutely correct, "expanding to a ring" is completely the wrong approach, the chuck jaws need to be stressed in the same direction as when in use, I used three short lengths of cut off pieces of Allen wrench on the inner Allen bolts that hold the top jaws on and then use a ring of appropriate dimeter to pinch down on to restrain the jaws for regrinding. A high speed grinding spindle is not a prerequisite for the purpose, I used a WW-2 era Navy tool post grinder that ran at rather low speed, 2750 RPM, I think; it had a long internal spindle that simply screwed on the threaded external wheel mount, I found that it worked better for the job than the screaming loud high speed spindles, and instead of lots of sparks and loud grinding noise, it simply just did its work quietly and due to the lower speed, normal grades of internal wheels acted softer and did not glaze over as is more or less normal. KR does many things that I do not agree with, he is a bit of a putz, in my opinion.
 
This is my admittedly ignorant take on grinding jaws on a 3 jaw chuck. Why bother. I doubt that I would ever eliminate the runout no matter how hard I tried. Besides even if the there is runout in the part held in a 3 jaw chuck when you start once you turn the part it will be concentric with the lathe. No runout. Do everything you need to do without removing it from the chuck and everything will be concentric. I am probably missing something here. John and others will try to educate me.
 
The main reason that I would be regrinding chuck jaws is that with use and wear, the jaws tend to get bell mouthed, that is they get worn to the point that they bear tightly at the rear of the jaws and loose at the front of the jaw and the workpiece gyrates and moves around at the front, effecting concentricity and finish.; if one has an adjust tru type chuck concentricity is not a problem, but non parallelism of the jaw's grip is.
 
To check to see if the jaw is bell mouthed would you run a DI the length of the jaw?
 
More to the point is observing that the workpiece is not stable in the chuck, or lightly chucking a straight rod and seeing if it is held on its total length, not just at the back of the jaws. It takes a lot of daily use for that wear to happen, but with heavy daily use, I would expect to regrind the jaws perhaps yearly.
 
For some the pursuit of maximum precision is a passion in itself. For others it’s a requirement of the job.

For my little hobby shop I’ll probably replace a chuck before I’ll grind it to try and make it better. I have collets if I need less runout….

John
 
benmychree said:
More to the point is observing that the workpiece is not stable in the chuck, or lightly chucking a straight rod and seeing if it is held on its total length, not just at the back of the jaws. It takes a lot of daily use for that wear to happen, but with heavy daily use, I would expect to regrind the jaws perhaps yearly.
Click to expand...
Always nice to have a pro here :cool:
 
I have two chucks that need the jaws ground... one is worn 'bellmouthed', as John was saying... the other has had one jaw replaced by a former owner... the one jaw is longer by an estimated .020" or .030"...

I don't yet have an adequate grinding setup for my lathe... but haven't needed the chucks, so it hasn't been a priority.

-Bear
 
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