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Thread: Acceptable runout?

  1. #11
    Join Date
    Oct 2015
    Location
    Wyoming
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    120

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    0.002" runout on a foreign 3-jaw is actually pretty good - for what it is.

    If you want to get a three-jaw chuck dialed in to dead-nuts-on, then you need something like an "Adjust-Tru" chuck. If you want to get your chuck to be dead-nuts-on, then you might need to grind your jaws when you have them clamped down on a precision-ground ring.

  2. #12
    Join Date
    Jan 2004
    Location
    Missouri
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    31,542

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    DO NOT BOTHER to grind jaws if you are trying to improve accuracy.

    The only reason to grind the jaws would be that the jaws are bell-mouthed, or otherwise damaged.

    Grinding is of no particular use for accuracy, unless you are working with a 2 part soft jaw, and are doing a setup for a particular part. Then you will be always working at the same point on the scroll.

    A 3 jaw scroll chuck is a compromise, and is never going to be especially accurate. Maybe there is a difference between 0.002" and 0.004" as an error, but all I see is "an error", the argument is just about how huge it is. Just never expect a scroll chuck to be accurate, and you will be OK.
    1601

    Keep eye on ball.
    Hashim Khan

  3. #13
    Join Date
    Nov 2006
    Location
    durban s africa
    Posts
    1,429

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    I think you got a german made chuck that is rebranded by the Indian reseller. The comments of "not good enough for me "I think refer to the end product,not the performance of the chuck run out.

  4. #14
    Join Date
    May 2011
    Posts
    859

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    There are various work-arounds for your problem. Below is a quick and dirty one but it will work.

    Take a piece of expendable round stock larger in diameter than your workpiece and drill and tap a couple of holes at 90 degrees to each other and 90 degrees to the centerline of the stock. Put it in the 3 jaw and do not remove it for any reason till the job is done. Next drill and precision bore the round stock to the exact diameter of the O.D. of the part you need to work on. Your part should be a very close slip fit into the bored hole. Tighten the screws and you should be running dead true to the bearings of the lathe which is as accurate as you are going to get on that particular machine. For screws use No-mar set screws (plastic tipped) or brass to protect the surface of your workpiece.

  5. #15
    Join Date
    Mar 2018
    Location
    Finland
    Posts
    365

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    I agree that .002" runout is very good, as is .004" runout on a three jaw, I have like .010" runout a lot of the time on my worn out chuck. Have got to work around it. I am planning to buy an adjustable 3-jaw chuck so I can dial in the last thousandths. I've also shimmed the 3-jaw from time to time.

  6. #16
    Join Date
    Dec 2018
    Posts
    403

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    .002 runout on a 3jaw is good, and for building muzzle loading ramrods, that would beautiful.
    If you particularly want the jag and tip to match to the point the joint line goes away, build the parts a bit oversize, then assemble them, then turn jag+tip all together as one.
    Easier yet, just use collets in your spindle.
    Last edited by Ringo; 07-12-2019 at 08:26 AM.

  7. #17
    Join Date
    Feb 2005
    Location
    gettysburg pa.
    Posts
    912

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    it is a 3 jaw .002 is good. you want to do work like you describe use a indicator and a 4 jaw or a collect.

  8. #18
    Join Date
    Dec 2015
    Location
    Chilliwack, BC, Canada
    Posts
    5,455

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    If this is a really old chuck of the same vintage as the lathe then odds are pretty good that the jaws are a bit bell mouthed. And some of your runout could easily be due to that.

    You can check this for yourself. Lightly close the jaws on a piece of cold rolled or a shaft of some sort with a good parallel shape. So something other than hot rolled steel. If the jaws are belled they will have touched down deep at the back of the jaws and the near ends won't be in actual contact. You can feel this by wobbling the test shaft around. Needless to say I really do mean "lightly touching" so you can feel this in action.

    If it's not actually bell mouthed and is simply not centering when was the last time you took the chuck apart and cleaned out the scroll and ways for the jaws? 3 jaw chucks are terrible for "eating" swarf and getting it stuck in the teeth so they don't run cleanly on the scroll. So often a teardown and cleaning will bring back some of the accuracy.

    But I'm with the others in saying that your chuck is not bad at all. In fact if you have .002 total runout that's amazing. If it's a total of .004 that's still darn good. Three jaw chucks are not intended for ideal centering accuracy. They are intended for holding rough stock.

    My roughly 30 year old 3 jaw which has only ever seen my hobby use currently has .005 to .009 total runout depending on the diameter of the test piece. I just went and tried it with three different size diameters for giggles.

    Now you MIGHT try setting it up and grinding the jaws. But due to all the various issues with 3 jaw chucks in general you might or might not get a lower runout. But if done correctly it'll fix any bell mouthing you have.

    Still.... relying on a three jaw for multiple holds on any item where concentricity is important is not a great option. A lot of your need for concentricity would be better handled by altering how you make the parts so that all the steps that really require spot on centering are done in one hold of the rough stock. And if there is an additional step that demands spot on centering then make all the parts you need but skip that one final step until the end. At that point chuck up some stock and machine a fixture that will hold the parts for that final step. By turning the fixture then using it directly without removing it from the chuck it will be 110% centered. If you want to use it again then make the fixture so it has a step that rests against the nose of the jaws and also mark the position of jaw #1. There's still no guarantee of perfect centering but it'll generally be good to a couple of thou or less

    If you really need concentricity on things like this the more proper options are four jaw chucks and collet chucks. It's a slow process to center parts on a four jaw when you start but it's surprising how fast you get with a bit of practice.

    For small parts a really handy accessory I've got is a 4" four jaw that is mounted to a stub arbor that I hold in my three jaw. I came with a 3/4" or 7/8" direct threaded hole to fit the arbor of some manner of lathe. For centering up small parts without needing to swap chucks it's proven to be really handy. That might be an option for yourself too.

  9. #19
    Join Date
    Sep 2009
    Location
    NW Illinois USA
    Posts
    752

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    Quote Originally Posted by Bented View Post
    I made 10 new ones using an adjustable 3 jaw chuck, then drilled and reamed a block of steel held in a QCTP holder with tools held in the chuck then did not move anything, placed the center drill in the reamed hole in the holder held by set screws. Drilled 20 ends after adjusting the chuck for each end and they passed inspection.
    This was all very very time consuming as in 6 hours to drill centers in the ends of ten 14MM X 350MM long shafts.
    Seems a long way to get it done. Would not a steady rest have accomplished the task?

  10. #20
    Join Date
    Feb 2013
    Location
    NW PA
    Posts
    49

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    One joint in the center of the ramrod has to be turned and machined to match the other end. I have a joint that is acceptable to me right now, but you can still feel a difference if you rub your finger over it.... but I have to polish yet, so it will get better.

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