Tuesday, February 14, 2017

Centering in 4-Jaw Chuck

I am trying to center a rod in a 4-jaw chuck.  There are a number of descriptions such as this.  But when I try them, the step of loosening to jaw on top and tightening the jaw on the bottom fails because the other two jaws ale preventing the rod from moving.  Loosen them and the rod falls out.  Suggestions?


  1. I recall watching our *very* experienced machinist use two "chuck keys" at the same time on opposite ends to make those adjustments. I never could master it, so if/when that needed to be done I called on him to do it. He made it look easy, but I'm sure there's more to it.

    Except for what he called "building torpdeos" level of precision, he always used the lathe with the three jaw chuck.

    Watched him do some pretty amazing stuff with that setup.

    Of course, he had the experience of actually having to build "torpedo" level stuff under his belt...

    Me, not so much.

    Good luck. I'm confident you'll get this down pat.

  2. I haven't done any machining in years, but I do remember that this just takes time and gradually tightening down. You can get the needle pretty steady with patience.

  3. Position the jaws like an (X), instead of a(+) so the part is more stable while you move the jaws. You're still moving individual, opposing jaws, but they are angled 10-4, and 2-8.

  4. This is one of those things that take time to learn how.
    The jaws have to be tight enough to hold and loose enough to adjust.
    Eventually you will get the touch.

    It is like the old story about a bill for fixing something, "$1 to tap it with a hammer and $99 to know where and how to tap it".

  5. Here's the easy method, and yes, 2 chuck keys helps a bit:

    Clamp work in chuck, using the engraved lines on the face to center it as well as possible. Place indicator against center of work. Many of us with QCTPs have an indicator permanently mounted on a toolholder.

    Rotate chuck with work and note extremes of indicator travel. Rotate work so indicator reading is half of extreme. Set indicator dial to "Zero".

    Rotate work so that a chuck jaw is directly in line with indicator plunger. Adjust that jaw and opposite jaw so indicator reads "Zero". Note that work must be clamped "just so", tight enough to hold, but not so tight that you can't perform adjustments.

    Rotate work 90 degrees until another jaw is in line with indicator plunger. Adjust that and opposite jaws until you get "Zero" on the indicator.

    Rotate chuck/work to check centering, you should be VERY close.

    See: http://bbs.homeshopmachinist.net/threads/72783-Centering-stock-in-4-jaw for a recent discussion of the subject.