When to grind the jaws in a chuck

[todd281]

I watched a Tubal Cain video a while back showing how to set up, and grind, the jaws in a three jaw lathe chuck. I am wondering, how far off does the chuck need to be before one says its time to do this?

 

[benmychree]

When accuracy is effected, and especially if the jaws get worn in a bell mouth fashion.

 

[todd281]

I guess what I'm asking is , most of us ,when we want the part to be "right on" we switch switch to our 4 jaw and dial it in. That assumes that the three jaw is off to some degree. So how far do we let it go before it time to make a jig and grind the jaws?

 

[Joeman77]

When accuracy is effected, and especially if the jaws get worn in a bell mouth fashion.

IMHO, 3 jaw chucks by nature are almost always off by a little at some point due to slight variations in the scroll or the mounting or times the moon... they aren't usually spot-on at different points as far as repeatability.
If the jaws aren't badly worn or belled, I wouldn't worry too much. If I need to pull parts & put them back more than once I either use a 4 jaw and an indicator or a collet chuck that I trust.

 

[JRaut]

I re-ground my 3-jaw chuck a few months back.

I didn’t do it because of accuracy concerns (as suggested, I usually switch to a 4-jaw if that’s critical), I did it because my jaws were horribly bell-mouthed. They were only gripping parts near the back, leading to all sorts of problems.

If the weren’t bell-mouthed so bad, I’d probably just have left them alone.

 

[JRaut]

And I guess I should say, it was super easy. I had been putting it off for a good while because I didn’t want to ‘open a can of worms’. But really, it was quite easy.

1) I turned a ring from some junk that could slip over the outside of the jaws when they were open about 1.5 or 2 inches.
2) I slipped the ring over the jaws and grabbed it by opening the chuck. No need to go very tight.
3) I’ve got a shop-made mount that allows me to put a Dremel in the tool post (I made a post about it a while back), so I tossed that in the tool post.
4) Using whatever ‘stone’ I had in the dremel box, I just ground the jaws about a thou at a time until they were clean, working the carriage back and forth.

I didn’t even dress the stone or anything like that. Just dumb and dirty, and it worked great.

 

[mickri]

I watched a Winky's Workshop video the other day about truing up jaws. Instead of grinding the jaws he used a carbide boring bar and took light .001 cuts until the jaws were trued up. Does it matter what you use to true up the jaws?

 

[Flyinfool]

I would think there is a good chance of shattering your nice carbide boring bar with the interrupted cut into hard steel.

 

[Joeman77]

And I guess I should say, it was super easy. I had been putting it off for a good while because I didn’t want to ‘open a can of worms’. But really, it was quite easy.

1) I turned a ring from some junk that could slip over the outside of the jaws when they were open about 1.5 or 2 inches.
2) I slipped the ring over the jaws and grabbed it by opening the chuck. No need to go very tight.
3) I’ve got a shop-made mount that allows me to put a Dremel in the tool post (I made a post about it a while back), so I tossed that in the tool post.
4) Using whatever ‘stone’ I had in the dremel box, I just ground the jaws about a thou at a time until they were clean, working the carriage back and forth.

I didn’t even dress the stone or anything like that. Just dumb and dirty, and it worked great.

If you grabbed it by opening the jaws are they still square when they close? Sounds like your method should work great with the Dremel, I've got something like that on my build-list too!
I had thought about doing this with an older Chinese chuck I've got laying around, but decided it was a little too sloppy for anything but a weld positioner, which I have yet to build.

 

[bill70j]

I guess what I'm asking is , most of us ,when we want the part to be "right on" we switch switch to our 4 jaw and dial it in. That assumes that the three jaw is off to some degree. So how far do we let it go before it time to make a jig and grind the jaws?

I guess I'll add one quantified data point.

I got an old 3-jaw Atlas chuck along with a lathe I purchased off Craig's List. The best TIR I could get was 0.008", and that was not repeatable. One could clearly see daylight between the chucked up part and the front of the jaws.

After in-place grinding, that TIR is down to 0.002 - 0.004"