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Help offsetting stock in 4-jaw

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  • #16
    Originally posted by old mart View Post
    You don't have to have the offset in line with the jaws when using round stock. ...
    Yeah ... BCRider just beat you to it (by 7 minutes). I had a case of being locked into a perspective & didn't see the alternative.

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    • #17
      Originally posted by danlb View Post
      Use the centering rod as suggested by BCRider, and set the indicator against the rod. ...
      Would work if only I had a punched mark to set the rod in.

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      • #18
        Originally posted by Bented View Post
        Rock the spindle, this will tell you where the high spot is on each side.
        That will tell me where the up & down spots are but not the side spots that I need.

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        • #19
          Originally posted by David Powell View Post
          ...However, I do make things a bit easier for myself by using a " Pusher" from the tailstock, usually a drill chuck with jaws retracted into the body and a piece of wood faced with rubber or soft plastic.
          By gently pushing the work against the chuck face I can stop it from falling down when I am trying to get the offset on jaws 2 and 4 and then , once I am near GENTLY tighten 1 and 3. Then I can go round tighten all chuck jaws and check the offsets.all this without the annoyance of the job merrily falling into the swarf tray ! ...
          That would help - thanks.

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          • #20
            Originally posted by old mart View Post
            Bob, did you want to bore from both ends, or just from one end?
            Just one. The turning-around problem was Brian's

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            • #21
              If we're only concerned with one end, AND we're not concerned with insuring the axis of offset is relative to any particular feature, then I don't see why it really matters about the side jaws, other than that they have enough pressure to hold the work adequately.

              Here's what I see. Let's say you set out to achieve the offset between the north and south jaws (i.e. N-S). And you achieve that. Now, in the process of tinkering and tightening the E-W jaws you shift it a little so that N-S offset is now more like NNNE-SSSW ( i.e. say from 185 deg to 005 deg.) What difference does it really make, as long as you go back and verifiy that you do still have the desired needle deflections along the intended (now approximate) axis?
              ...this kinda goes along with Bented's "...rock the spindle..." comment.
              Last edited by lynnl; 01-13-2020, 08:01 PM.
              Lynn (Huntsville, AL)

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              • #22
                Toolmaker's button, v-block and clamp, height gage, and indicator.
                "The truth is incontrovertible, malice may attack it, ignorance may deride it, but in the end; there it is." Winston Churchill

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                • #23
                  Can you bolt a 3 jaw chuck on a faceplate, offset by the specified amount
                  (like a crankshaft grinder???)?

                  -D
                  DZER

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                  • #24
                    Originally posted by Bob Engelhardt View Post

                    That will tell me where the up & down spots are but not the side spots that I need.
                    Rock the even sides to find center in that plane then rock the offset sides until you reach the required dimensions, there is no simpler way short of building workholding that will do it for you.

                    Place a dial indicator in a tool holder on the tool post like so, write on a pad the offset dimensions and have at it until you are where you need to be.

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                    • #25
                      Originally posted by Bob Engelhardt View Post

                      That will tell me where the up & down spots are but not the side spots that I need.
                      That's the thing, you don't really care about the side spots. Just tweak it until you have twice the displacement on the dial. If you're only concerned about the offset from center it makes no nevermind how you get it. You could get the offset with evenly spaced side jaws or you could get the same offset with all four jaws at totally different spots. It really doesn't matter at all as long as you get your proper min and max in SOME direction.

                      (the two times thing someone posted above was a "wha?..... OH, RIGHT!" moment for me
                      Chilliwack BC, Canada

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                      • #26
                        Originally posted by Bented View Post
                        Rock the even sides to find center in that plane then rock the offset sides until you reach the required dimensions ...
                        Click image for larger version

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                        To find the high spot on the sides, the indicator would have to be placed as shown. But when the chuck is turned 180 for the other side, the high spot is below the center & the indicator won't show it. Won't even touch the stock. To read the right & left side distances, the indicator has to be placed on the spindle center & rocking there doesn't show a high spot - just continuous change.



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                        • #27
                          Originally posted by Bob Engelhardt View Post
                          That will tell me where the up & down spots are but not the side spots that I need.
                          Originally posted by BCRider View Post
                          That's the thing, you don't really care about the side spots. Just tweak it until you have twice the displacement on the dial. ...
                          Yeah, I realized that from your first reply (thanks again). I replied to Bented as if the problem required getting the sides the same, addressing his suggestion for doing that.

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                          • #28
                            If I were going to want to clock the ends to match I think I would put the shaft in a V block and machine a tiny flat across center at both ends in the same setup so they are perfectly parallel. Then you can use a indicator on the tool post to exactly clock the position when you flip end for end.

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                            • #29
                              how big is the part, and how much offset?

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                              • #30
                                I mentioned finding the exact same offset in the exact same plane at each end, because model one piece crankshafts are turned between centers with a lathe dog. If you get the offset perfect in each end of the round stock but they aren't in exactly the same plane, weird things happen.
                                Brian Rupnow

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