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Thread: true up a 3-jaw chuck

  1. #1
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    Default true up a 3-jaw chuck

    hi again happy thanksgiving to all.i have a question on my atlas lathe,got it put back together but the chuck has a lot of runout.i put a piece of ground shaft in it and it wont run true.it has a 5'' chuck and the 3 jaws dont quite come together tight when closed all the way ,is this something that could be bored out ?or is this in the scroll and not worth fixing?and if junk what would you recomend as a replacement? its a 10x36 thanks for any help.

  2. #2
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    Somebody here has a picture of the process. They cinched a disk in the chuck behind the working surface of the jaws and which ensures the jaws are in tension. He then used a Dremel with a grinder on it to grind the jaws as the chuck spun. As with any 3-jaw this is the only place in the jaw movement range where the jaws are true, but if the jaw scroll is not too badly worn it can be good enough.

  3. #3
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    Unfortunately 3 jaw chucks do not take well to abuse. Overtightening the chuck will cause damage to the scroll beyond what the normal wear and tear of the stuff that gets into the chuck will do. The scrolls get distorted and even the chuck its self can letting the jaws clamp up bell mouthed with short material. One thing you can do after tearing the chuck down and cleaning it is to chuck up a short piece of material and then measure from the front of the jaw to the chuck body. Unchuck the piece and remeasure. If you see a big difference then you definitely have wear issues. On the smaller chucks it should be possible to turn them into adjust true style chucks even if you have to make an adaptor plate. Also when you remove the chuck from the spindle make sure that there is no foreign matter between the mating faces and the area that locates the radially. Indicate the spindle to see how true it runs. I am assuming a threaded chuck here but you should have a short diameter that the chuck locates on IIRC

    And now for the soapbox. I really don't like 3 jaw chucks for the most part. If I need to put something in a lathe chuck I would just as soon use a 4 jaw. First, they hold better. It takes a serious crash to knock the piece around in the chuck. Second, indicating the piece in is not rocket science. All it takes is a little practice and you would be surprised to find just how close you can eyeball it to start with. And third, most small lathes have far too much overhang from the bearings for my taste. Plus the bearings and shaft are too light to start with. As this is an Atlas I am assuming that they are sleeve bearings so I wonder how much internal clearance you have not that it would cause run-out
    The optimist says the glass is half full, the pessimist says it's half empty. The paranoid in me says somebody put a hole in it.

    Remember pessimists are at heart opptomists. They know things can and will get worse.

  4. #4
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    My advise, born of experience, is to dismantle the chuck and thoroughly clean it first. Reassemble and recheck the runout. Make sure everything is clean, sometimes that's all it takes. If there's still significant runout (significant is a relative term, how much is up to you), then grinding is in order.

    Joe
    "Against stupidity, the gods themselves contend in vain"

    Friedrich Schiller, 1801

  5. #5
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    Agreed, get it completely clean before even thinking of "tuning". While you're in there look for damage, wear, or distortion. If all looks well, put it back together.

    Next, check your spindle, just to cover all the bases. Should be near zero run out.

    Then mount the chuck. Check it's machined surfaces. Should also be very close to zero. If not, then it could be a problem with the register and/or back plate. Address this first.

    Finally check the jaws. Use a couple of precision rounds of various sizes. Ground pins on smaller end work well. Use a Sharpie or the like to mark high points and values on the chuck. Try each size several times, opening and closing the jaws a bit each time, always using the same scroll key/socket. Does the high point move around each time you reclamp on the same size? If so, give up now; there is no point in continuing. If a pattern is developing, repeat with each scroll key/socket and record results. I marked high points with a small A, B, C, and so on along with a log to reference what each indicates. While doing this it is also useful to look at variation in run out several inches from the chuck face. Variations there from clamp to clamp generally indicate bell mouthing.

    If you found a distinct pattern, choose a spacer to support the back of the jaws at a diameter that is roughly average of the pattern you observe. Then use a tool post grinder to touch up the jaws.

    I recently went through all of this on my 10" 3 jaw that came with my new lathe. Due to variations from clamp to clamp, no real pattern developed. Grinding might have helped a little, and would have corrected the slight bell mouth I detected, but ultimately I decided it just wasn't worth it as "fixing" the run out at any given point would not even have yielded consistent results AT that dimension, much less at anything else. I'm currently looking for a new chuck, and this one will likely wind up as a mill work holding device...

  6. #6
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  7. #7
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    Good advice given by the others here.

    If it seems like the jaws need truing, read this:
    http://www.homemetalshopclub.org/new...04.html#truing

    It's an excellent write-up on the subject.


    .
    Thomas

    Problems worthy of attack prove their worth by hitting back
    - Piet Hein


  8. #8
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    Aug 2002
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    Quote Originally Posted by shoprat
    the chuck has a lot of runout.i put a piece of ground shaft in it and it wont run true.
    How much runout does the chuck actually have? "A lot" is qualitative and will have different meanings for everyone on this board. I think that a quantitative measurement would give everyone a better idea of what to recommend. The recommended actions would be very different for 0.005" and 0.050"

  9. #9
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    It may have been one of my posts you saw.

    http://bbs.homeshopmachinist.net/sho...chuck+grinding

    As I said there, the good thing is it worked. But the bad thing was, it didn't. The grinding technique was fine. Bit my chuck was too far gone for this to help.

    I suspect that many problems are due to wear on the scroll, not on the jaws. The scroll can become damaged in local areas so the runout will be different when different diameters of work are chucked. I suspected this in my case, but I tried it anyway. If you want to check for this, get some test bars of different diameters and check the runout with each of them. Record the high and low spots in terms of the dial reading AND in terms of where on the chuck they occur. If they are different and occur at different angles around the chuck, then the scroll is probably damaged and no amount of jaw grinding will help. I would estimate the chances of jaw grinding actually improving a chuck's runout as perhaps only 25%.

    One other thing, someone above said that you put something at the rear of the jaws where they are not used and put them in tension. That is not correct. You want them in compression or tightened in the same way that they will be stressed when in use. And you need all of the jaw accessable for grinding as some work is longer and will go back past the rear of the chuck. What I did was to drill (carbide bit) three small holes (1/8") in the top of the highest step and install pins in the holes. Then the pins are tightened against a metal ring that has a large enough hole to allow your grinding point to pass through. They don't have to be super tight, but firmly tightened so nothing slips or moves around. This isn't perfect, but it works.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  10. #10
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    Quote Originally Posted by Paul Alciatore
    It may have been one of my posts you saw.
    That's the one I saw.

    One other thing, someone above said that you put something at the rear of the jaws where they are not used and put them in tension. That is not correct. You want them in compression or tightened in the same way that they will be stressed when in use.
    That was me, too. I phrased it wrong. And I'd already had my morning coffee! Thanks for the clarification.

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