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Lathe work with a 4 jaw questions

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  • Lathe work with a 4 jaw questions

    First off I have very little experience with 4 jaw operations. I would be able to indicate a round piece to zero but that is about it. I have 2 round blanks that I need to bore a hole though .750" off center. They need to be as exact as possible with each other. How does one setup to get the offset and then have it the same for the other pieces? Thanks.

  • #2
    Much of what you want to do depends on exactly the diameter & length of the work piece. If you have a mill it may be easier to simply do this operation in the mill. Or, if the piece is too long then set up the parts in the mill to make a locating hole in the end of the work piece, then indicate in the hole.

    A pic or drawing sure would help.

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    • #3
      What are the dimensions of the blanks?

      And what size are the holes?

      Dave

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      • #4
        I would think that you offset the jaws to place the workpiece off-center for drilling and boring the hole. Then you call two of the jaws 'fixed' and don't move them. Each workpiece is then marked for position, then mounted and bored using only the other two jaws to secure it. They should both come out the same. Note there will be some 'wiggle room' depending on how you tighten the jaws, etc. You should use the same tightening sequence each time you mount the pieces.

        My 4 jaw makes it easy, as all the jaws can be secured independently once adjusted into position. If you can't do this with yours, then you may have to rely on the adjustment screws not turning once set to where you want the jaw. A bit of tape over the screw head might help with this.

        Another option would be to dial in the position for each workpiece. You might have to rely on the minimum and maximum readings to get the desired offset, since it could be difficult to get the indicator back into the exact position to enable you to use the exact point on the dial again. You can use the indicator to find the high or low spot on the workpiece so you can mark it, but this is not very exact.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Did exactly that today. Well, I turned part of a shaft off centre, and to be candid about it I blew it the first time. I made the offset twice what it should have been.

          So if you're prepared to take my advice after that admission, here goes. You want a hole .750" off centre ? When you have the part centred for your hole, the centre of your part will be .750" off centre. This means that one side of your part will be .750" higher than normal and the other side .750" lower than normal. All this means that if you use a dial indicator, it will need to swing to plus .750" to minus .750" as you turn the chuck. The DI will swing 1.5".

          Centre the part first. Then choose two jaws, say 1 and 3, and move those so the part ends up swinging the DI 1.5". Only adjust jaws 2 and 4 to accomodate the move, and to keep the part located, and move them equally. But at the end of the day, if the DI swings 1.5" you're spot on whatever jaws did the adjustment.

          When you go for this off centre part with the centre drill, you think it'll break the drill. But it works OK.
          Richard - SW London, UK, EU.

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          • #6
            Another option would be to use Toolmaker's Buttons. Simply mark the end of the face approximately where you want the hole, center punch, drill for tap and use appropriate tap for your Button. Install the button using a micrometer or dial caliper if that level of accuracy is acceptable to insure proper location of the button. Put the piece in the 4 jaw chuck and center off the Button. Now you are lined up for drilling and accurate boring.

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            • #7
              The blanks are roughly 2.5" dia by 1.5" thick(sorry I don't have any actual measurments handy). They are going to be eccentric chain adjusters for a motorcycle swingarm. I actually need 3 holes in each one. 2 are not that important, they are mostly for weight reduction and looks. The third will be for the axle. The back side of the hole will have to be bored for a very slight slip fit of the axle(16mm I think). There will then be a through hole for a bolt and the front side will be counterbored for a cap screw and then a groove put in for an internal snapring.

              I agree with Dr Stan that most of this will be easier done in a milling machine. I probably still have to 2nd op in a lathe for the snapring groove but once the hole is there it will be easier to indicate it with the offset. If I can get access to the mill at work on a Saturday I will go that route. If not I will have a lathe up and running soon at the house and I will have to deal with it that way.

              Here is a pic of the blanks



              They then go into here



              And the whole thing gets welded into the swingarm that I machined(it has been already welded in but no pics)

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              • #8
                Machine one piece, twice the thickness, cut it in half, proceed from there. Bob F.

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                • #9
                  Mount your piece in your three jaw chuck and then chuck that up in your four jaw.

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                  • #10
                    I've had good luck with dialing in the first piece and put the hole where you want it then just loosen two jaws 1/4 turn, take out drilled piece and insert new piece and turn the two loosened jaws back in 1/4 turn.

                    Otherwise I would just center punch the mark and drill press them.

                    Thats what I would do anyhow.
                    Andy

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                    • #11
                      I've bored offset holes with the part in a 3-jaw chuck by starting one of the jaws after the scroll has gone around a couple times. It may not be as accurate as setting it up in a 4-jaw, but it's very repeatable.

                      You can make a shim to fit around the offset jaw to get the exact offset you want.
                      Any products mentioned in my posts have been endorsed by their manufacturer.

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                      • #12
                        I'm a fan of making one extra-long piece and then parting off. Makes it so much easier.

                        If it doesn't need to be exactly .750", I've cheated on this. Face the stock, apply layout dye or Sharpie, then dial out until the tip of the cutter is to the offset you need. Just barely kiss it at the part, enough to face or polish off later but leave a mark in the dye.

                        Then put a center in your tailstock and adjust the chuck until the center meets up on that line.

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                        • #13
                          Why not mark it out, centre-punch it and drill it in a drill press?

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                          • #14
                            Off centers

                            Exactly thats how I do it, with layout and scribe marks where I want the
                            offset, then nail it with a sharp dead center, and slowly tighten the 4 jaws.

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                            • #15
                              If I was to do it on a lathe it would be set up on a face-plate.

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