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3 Jaw Chuck Runout

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  • 3 Jaw Chuck Runout

    I just purchased a Jet 13" x 40" bench lathe new. My impressions on the machine so far are positive, but I do have an issue with runout when chucking barstock in the 3 jaw chuck, to me it seems excessive and looking for thoughts on what I might do. With the chuck removed I indicated the spindle. Radial runout is .0002" and axial runout is .0002" With the chuck mounted and holding a piece of round cold rolled material (I know this is not the most accurate but for my purposes now it is round enough)I get more runout than I think I should. The chuck is a 6" dia 3 jaw with a D1-4 mount. I have mounted the chuck in the 3 different possible positions and get the following runout when indicating the stock, .034, .038 & .040. I am a home/hobby machinist and this is my first experience with a new chuck, am I expecting too much from an inexpensive chuck? I have not contacted Jet yet, looking for comments from people with more experience on something like this than I have. Any constructive thoughts will be appreciated.
    Regards, 34 Ford.

  • #2

    How close to the chuck are you getting .034? Try some different sizes of stock. If you obtain similar results close to the chuck after re-seating the stock several times and with different sizes of stock you have a beef. I would not expect to see more than .004 at most. As has been said before, make sure the jaws are in the right place.
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    • #3
      Yikes! That is some BIG runout. You need to check runout using ground rod or drill rod. Cold rolled won't be very accurate. Get something or turn something and measure the diameter at 90* spots. (Quarters) Then, do the rest below.

      First thing is to check to make sure there's no dings or burs on mating surfaces. Next, check to make sure the cam pins are a little loose. These should NOT be tight. They should jiggle a little. (Cap screws at the base of each pin keep them in place.)
      If the chuck is a plain back and mounted to a D1-4 adapter, check for loose cap screws holding the chuck to the adapter. You might try mounding just the adaptor and check its runout. There could be stuff between these.
      If all else fails, call Jet for a replacement chuck.


      • #4
        First, check if the jaws all come together OK. They should form a small triangle which should have each jaw touching each other one evenly.
        Check if that small triangle seems to be true on center, by rotating slowly with the smallest pin it will hold in it.

        Then see if the chuck body is rotating true. Any decent chuck will be very close to perfectly true on the body.

        If the body is true, but the jaw center is not even, and the center is not true, it's the chuck.

        If the body isn't true, the jaws come together OK, but the center of the jaws is not true, it is likely the mounting, either chuck side or crud on spindle, etc.....

        If the jaws are OK, the center is true, and the body is true, you did something wrong somewhere. Be sure you are using a solid reference, preferably off the headstock casting.


        • #5
          I use ground carbide reamer blanks for this or high grade pins. What ever you use, it should have a mirror finish if possible.


          • #6
            Just wanted to say thanks to the 4 gentlemen that responded to my question. I performed their recommendations and was able to reduce the runout somewhat. With all the facts at hand I called Jet and they willingly agreed to send me a new chuck.
            Thanks for the help.