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3 jaw chuck 101.

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  • 3 jaw chuck 101.

    Here are some tips for indicating in a part on a three jaw chuck. I keep reading about guys saying their three jaw chuck is not concentric and has too much runout. A friend of mine an old machinist showed me this I am a home shop self taught guy so I thought it was cool, so I am going to share it with you. If you are an experienced machinist this probably is a waste of your time or maybe you can add something for the rest of us.

    The first picture is a piece of seamless pipe I picked up for another project and it is pretty round or concentric so I decided to use it for this demo.



    The second photo shows me setting up the indicator on the end of the pipe. Snug up the chuck but don’t cinch it down yet. Rolling the chuck by hand to find the low point and setting zero there.



    The next photo shows the high point about .012” out some would say bad chuck or the pipe is not round.



    The third photo is me tapping the high side with a small soft hammer. Then roll the chuck and keep tapping the high side until it is concentric. Then snug up the chuck and roll it again might need a love tap or two just don’t beat it with a BFH or you might damage your chuck. If the part is not round it will never indicate in but it will be closer than just chucking it and going from there.



    I was able to get this part from .012” out to .001”. If you need better than that better dig out the 4 jaw.
    If this was any help great it has helped me.
    GD
    Last edited by gundog; 07-20-2006, 12:47 AM.

  • #2
    Gun Dog,

    That's really neat.
    How does your friend make it run true at the jaws?

    The usual procedure I learned is true it at the jaws, then tap the end true.

    It can be trued at the jaws by putting shims at the high point of the runout.
    This is a less than ideal solution being somewhat trial and error.

    I made the chuck/backplate locator rabbit .010" loose so I can true things up by tapping the chuck body to true the part at the jaws, and tightening up the chuck retainer bolts, then tap the end true like you said.

    That works like a poor mans adjust true chuck.

    Kap

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    • #3
      runout

      So what do you suggest, that everytime we mount a piece of metal we do the tap dance? It seems that this is the only way to get it right though. I think it also depends on the quality of the chuck, do you agree? Thanks. Vic

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      • #4
        Kap this works for me right at the jaws also. Before I learned this I thought my chuck was bad and was considering replacing it. I am not trying to say this is a fix all just passing some of my very limited knowledge to some guys who may not know this.
        GD

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        • #5
          Or you could re-bore your chuck jaws, theres a multitude of different ways of going about this, some better than others, try to mimick same as work conditions...

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          • #6
            One kinda related 3 jaw tidbit is that many quality 3 jaw chucks will have one of the key holes marked or only have a single key hole to begin with. This is the hole to use when tightening as it "loads up" the force on the scroll as it was when the jaws were ground at the factory. It can/will make a multi thousandths difference. Den

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            • #7
              Boring the jaws on a 3 jaw scroll chuck isn't a fix for centering. The usual source of inaccuracy is the scroll itself. You will usually find that the chuck centers better at some diameters than others. This is because of variation in the scroll.

              There is no fix for this as it is dependent on the scroll. Having to tap the work as shown to center it usually means the jaws are bellmouthed and aren't holding the work securely. In that case boring (grinding) the jaws may help but it won't improve the accuracy of the scroll.

              The 3 jaw isn't meant for accurate centering of work. It is used to hold work where the concentricity, if required, is produced by the sequence of operations.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                So-- why are't we using a 4 jaw and doing the same thing getting better concentricity and grip?

                Yeah, I know, this thread wasn't started for the curmudgeon could get in a growl but there's some points.

                Most 3 Jaw chucks after a few mild wrecks have bell mouthed jaws. I'm not at all surprised that one could knock 0.012" of TIR out of that hunk of tubing 3" out from where it's gripped.

                A three jaw chuck is not a universal solution to lathe work holding.

                Here's the rule for three jaw chucks: they are used for work where reliability of grip and second operation concentricity is not a paramount concern and where lobed diameters can be tolerated. In all other cases use soft jaws bored to suit the work, a collet, a face plate, or a four jaw chuck.

                Adjustable three jaw and six jaw chucks like the Grip-Tru can offer first rate concentricity so long as they are undamaged. It's not wise to even part off work in them for fear of a wreck. For all its mass and apparent strength a three jaw chuck is truely delicate.

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                • #9
                  My newbie ignorance surfaces again. Learning something new every day is good, but since I started my descent into darkness (aka machinist hobby) I still find something new in most every post.

                  I knew about tapping the work piece to help align/true, I knew about the 3 jaw limitations, different TIR at differing diameters due to scroll, and boring soft jaws for specific diameters (BTW, to true up hard jaws, bell mouth or staggered, would require a tool post grinder or similar), but I stumbled on the last part in Forrest's post.
                  It's not wise to even part off work in them for fear of a wreck. For all its mass and apparent strength a three jaw chuck is truely delicate.
                  I was under the impression that "wreck" or "crash" indicated chuck or work piece (spinning stuff) impact into tool, tool holder, cross/top, or carriage (non-spinning stuff). How does this intersect with "not wise to part off for fear of a wreck"? I understand you can stall or get insane chatter, or even break a tool parting, but how would that hurt the chuck? Or is it the potential for a Bouncing Betty getting into the chuck?
                  Russ
                  Master Floor Sweeper

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                  • #10
                    If parting something sturdy like a 2" dia steel bar and the tool digs in something is going to give. First the tool most likely but the forces on the chuck jaws are high and not balanced. Imagine putting a foot long bar in the chuck and giving it a good whack on the far end with a hammer. Not good for the jaws.
                    Free software for calculating bolt circles and similar: Click Here

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                    • #11
                      4-Jaw.....

                      THE chuck to use most of the time, unless some reason not to. Totally agree with Forrest.

                      Easy enough to set up, even by eye, that there is NO reason not to. If you find it hard, force yourself to use the chuck for a week. After that you will hardle notice it.

                      By the time you mince around shimming or tapping your part in a 3 jaw until the part is in straight, you could have set up the 4 jaw and be done with the job.

                      Then also, CONTRARY TO EVAN's STATEMENT.... MOST 3 jaw problems I have seen that were significant in terms of concentricity, were due to bell-mouthing, or worn slots, etc, not the scroll. And, I have used quite a number of 3 jaw chucks of various vintages (no new ones though!).

                      I think the scroll, although admittedly a weak point, is unfairly blamed for problems that may be elsewhere. If the scroll causes 0.012 out-of-position, you could see the scroll problem by eye, a bend, gouge, etc.

                      That being the case, tapping a part into alinement can be flat silly..... because......... what if you DO get it straight? It still may not be held by a solid jaw.... might just be "nipped" at one spot near jaw base if the jaw is belled or cocked.

                      Anything but the lightest cut risks pushing the part off-concentricity, causing a "dig-in" (the "wreck" Forrest referred to) as the part moves in the chuck.

                      Who wants to suddenly be taking a cut 0.025 (or more) deeper than you expected when the part isn't securely held?

                      With most "hobby" machines, that will jam the drive, maybe bending things. It sure may chop off the cutter short if it is a threading tool, etc, etc. If the compound is stuck out too far, it may get broken off short (Atlas, mostly)
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #12
                        CONTRARY TO EVAN's STATEMENT.... MOST 3 jaw problems I have seen that were significant in terms of concentricity, were due to bell-mouthing, or worn slots, etc, not the scroll.
                        I said it was due to bellmouth, not the scroll. What I also said is that centering inaccuracy is due to the scroll. If the chuck is in good shape, mounted properly and the jaws are not bellmouthed then the scroll is the only remaining source of centering inaccuracy. Scroll inaccuracy is shown by variation in centering accuracy at different diameters. That is easy to test. Grinding the jaws won't fix this except at the specific diameter they were ground at.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Originally posted by Evan
                          If parting something sturdy like a 2" dia steel bar and the tool digs in something is going to give. First the tool most likely but the forces on the chuck jaws are high and not balanced. Imagine putting a foot long bar in the chuck and giving it a good whack on the far end with a hammer. Not good for the jaws.
                          That I can see, but I've always parted off next to the jaws, so it's generally just a stall or broken parting tool. I guess it's mainly the term "wreck" in a context that does not mesh with my intuitive understanding/definition.
                          Russ
                          Master Floor Sweeper

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                          • #14
                            Unless I have no other option I part off with the bandsaw.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              As do I. Well, maybe I wouldn't go quite so far as to say "no other option", as I sometimes part off to reduce scrap, but in general I agree. However, I'm hoping the new lathe (with much greater rigidity and it's option to part in reverse due to L00) will make parting off less of an "experience".

                              Sorry for the short side trip. Now back to our regularly scheduled topic...
                              Russ
                              Master Floor Sweeper

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