G0768 3 Jaw issue

T

TDuff

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Hello everyone:

I'm a noob here and need to get some advice on an interesting issue with my 3 jaw chuck. Bought a used (2 - 3 yr old ) G0768 lathe in what looks to be pretty good condition, with all the original accessories. Started working on some 2 1/8 inch diameter aluminum bar stock - no issues. Especially after going through the usual cleaning, oiling, and adjustments.
it runs smooth! Next I put a pc. of 1 inch diameter aluminum bar stock in and it was spinning with a major amount of run out right at the jaws. Put the 2 1/8 inch pc. back in, which uses the next larger set of clamping faces on the jaws and no noticeable run out. So apparently the inner-most clamp points of the jaws were not machined accurately from the factory. Fortunately the alternate jaws - the inside diameter jaws - look/work great on the 1 inch diameter stock, so I'm back in business but, my question is - can the O.D. jaws be corrected somehow or do I need to buy new jaws if I want then to be "correct".

Hope this gibberish makes some sense, and thanks for any suggestions you might have.

Terry
 
The scroll could be bent due to the former owner having a wreck, if this is so you will have a tight spot and it will throw off one of the jaws.
 
Take the chuck apart and give it a good look over, clean it up well. For a small lathe/chuck, a run out of 0.015" would be a huge amount. Sounds like there is something not set up correctly. Let us know what you find.
 
I would suspect the scroll has chips in it creating the runout on the jaws.
 
loosen the jaws just enough to turn the 1" stock several different locations ----does the runout improve--maybe the 1" stock isn't totally straight or round.
 
I assume that you are seeing acceptable runout using one set of steps and unacceptable runout with another set. If that is true, it can be verified by a simple set of measurements. Place a short dowel pin against the surface of the jaw step and measure the distance between the pin and the outside surface of the jaw. Record the measurement. Move to the next step and repeat. The difference between the two measurements should be the same for all three jaws. If not within an acceptable amount of runout, the jaw grind is defective. I tried this for my 602 jaws and the numbers were ..7929, .7922, and .7911 which I deem acceptable for a three jaw chuck,
 
Thanks for the replies. Not able to spend as much time digging into this as I would like - work keeps getting in the way!
However, this is what I have found thus far: As RJS states above, there is virtually no run out on the second set of jaw faces, so I am assuming bad grind on the smallest faces. With this thought, and the jaws removed, I tried to measure the innermost point to the second set of faces with the depth finder of digital calipers. Certainly a lesson in frustration! If I was not working from home I could take them into work and use a better set up. Measured three times on each one and the avg is: Jaw 1 - .5355. Jaw 2 - .533. Jaw 3 - .545. So I think that may be the culprit, But I need to try RJS's method outlined above. Also, I think removing and thoroughly cleaning the chuck is a good suggestion. Once I do that I will report back. But it appears reworking the jaws will require a precision grinding operation, which I do not have access to.
Thanks everyone!
Terry
 
Regrinding jaws isn't that difficult. It requires a tool post grinder of some sort. I reground the jaws on my four jaw chuck and greatly improved the runout. https://www.hobby-machinist.com/threads/grinding-my-jaws.50747/

Before doing so though, you will want to be absolutely certain that the jaws are defective. "Correcting" the jaw geometry when there is another cause for your problem will just make things worse.
 
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