3 jaw chuck internal clamping alignment issue

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Strtspdlx

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It's a 10x54 atlas lathe th54 to be exact. If I clamp externally say to the Id of a pipe I can somewhat align it (within 3-5 thousandths) no problem. I tried ccentering a piece of 1" round stock in the center of the jaws slightly inserted into the bore in the center of the chuck and for the life of me cannot get it to indicate anywhere near zero. It's usually .250 +\- .050. I'm assuming the chuck is shot or is it possible I'm going about this incorrectly? I'm very new to setting up a lathe. I have run them. Setup however was always left to a more experienced person.


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Strtspdlx (please sign posts, or set up your Signature),

It is possible, depending upon softness and diameter of the workpiece and shape of the inner surface of the jaws, by putting one jaw at 12:00, to pinch the work between the bottom two jaws so that it is not even touching the top jaw. However, there would be an eighth inch air gap between the workpiece and the top jaw. For this reason, you should always put one jaw at about 06"00.

Second possibility would be that one of the jaws was meshed with the scroll out of sequence (2, 3, 1 for example) or jaws are in the wrong slots. But the runout that would induce would be pretty consistent. And you wouldn't be getting 3 to 5 TIR chucking on the ID.

Third possibility is that one or more of the jaws wasn't made for that chuck. Each jaw should have a number on it between 1 and 3. If you remove the jaws from the chuck and stack them, the only difference should be that the threads on the back of each jaw are each offset from the next by one-third the pitch. Jaws from two chucks with the same model number are not really interchangable. Although you should be able to regrind them to work. Jaws from chucks from two different makers are definitely not interchangable.

Try a radically different diameter workpiece.

Disassemble the chuck (it probably needs a good cleaning anyway) and examine the scroll.
 
If he is getting only 3-5 Thou with clamping on the ID could he cut a short RING and clamp that, then grind the internal face of the jaws.
Using the force of the clamped ring to keep the jaws from any movement?

Or at least use that and then take an indicator to the inside jaws faces to see if they have the same .250 =/- .050 that he sees when he clamps a part against them.

If he gets the same run out using an indicator when the jaws are fixed clamping a part on the ID, I would think he could grind them to get very close.
 
I had the chuck apart 6 months ago. Cleaned very well as it was packed with brass shavings and such. Reoiled and reassembled the jaws in 123 order. I definitely remember the jaws being number I cannot remember if the chuck was also. I did notice they only line up close in one order. I tried a few different all 123 but clockwise from the adjustment in the chuck tried putting number 1 in different locations and this was the only way the lined up enough to clamp in the Id of anything.


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Thanks. Strtspdlx


Atlas th54.
Miller aead-200le
 
Make sure all the jaws are matched. Make sure all the jaws fit the track in the chuck head nicely. Make sure none of the jaws have any nonfactory grinding on them. Mike each jaws overall length and see if they all have the same number? The jaws should each have a proprietary number ie: 1, 2, 3. And the chuck head should have proprietary numbers as well for each track. You can start the number 1 jaw in any track, just as long as you keep all the jaws in assembled order. But, it’s best to put the jaws in their original track for optimum accuracy as from the factory…Good Luck, Dave.
 
It seems I'm in need of a new chuck. I disassembled cleaned and reassembled this chuck. In this process noticed on jaw #3 the guide in the chuck itself isissing half way down on both sides. It is a little better after I cleaned it up and filed down some high spots on the scroll. But I think it's time to take the dive and buy a nice 6" 4 jaw chuck
 
Sounds like an accurate diagnosis. If you can only have one chuck, a 4-jaw should normally be the choice. However, if you can afford two good ones, you will never wear the 4-jaw out as it won't see much use in the normal course of events.
 
I bought a 6" 4 jaw chuck on eBay. It's a craftsmen chuck with key and it seems brand new. Everything is super tight compared to my 3 jaw and I Ike being able to adjust the wrk on all clamps. My only concern is this chuck sticks out about a half in further then my 3 jaw as it has a collar on the back about half inch tall. Is that supposed to be machines or can I just screw it on and go play? I've already screwed it on and have been able to indicate in everything within reason ( I'm impatient sometimes). If I took more time I'm sure I could get it perfect.


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