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Re-threading a chuck back plate

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  • Re-threading a chuck back plate

    I've removed the back plate from a small 3-jaw chuck to re-thread for use on a small indexing head. I'm thinking about bolting it to a face plate on my lathe for the operation.

    Another idea I had that sounds a bit silly but I'll run it by you all anyhow is holding a sturdy bar in my lathe either by collet or 4-jaw then chucking this little 3-jaw chuck with it's back plate attached to it, backwards. I think this would help ensure the back plate bore is concentric to it's little old 3-jaw chuck?

    Either way a kick in the right direction would be appreciated. Thanks.

  • #2
    Originally posted by SirLesPatterson View Post
    Another idea I had that sounds a bit silly but I'll run it by you all anyhow is holding a sturdy bar in my lathe either by collet or 4-jaw then chucking this little 3-jaw chuck with it's back plate attached to it, backwards. I think this would help ensure the back plate bore is concentric to it's little old 3-jaw chuck?
    No, the backplate will only be centered (on the bar) as good as the three jaw allows, which won't be very good. You need to mount the backplate on the lathe faceplate and use a test indicator and tap it into concentricity. Center the backplate on the faceplate as you would center work in a four jaw chuck.
    Last edited by Rosco-P; 02-24-2015, 09:51 AM. Reason: Dang spell check changing meanings

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    • #3
      Les
      You don't say exactly what kind of re-threading you wish to do.
      I assume you mean a larger thread and not just a clean-up ?
      Chucking the chuck backwards on a bar is not a good Idea "if" the jaws are worn. You will have warped thread mount so to speak
      It is best to remove the backplate and mount it on your faceplate.
      There are three surfaces that need to be concentric:
      The face of the backplate where it fits to the chuck should have a shoulder, or lip.
      Bolt some small scrap pieces of material to the faceplate and then face them and turn a step to match the lip /shoulder for accurate registration
      Now mount the backplate so it will lock onto those pieces and clamp.
      Now you can re-thread the bore, but make sure you clean the face of the backplate during the process. The face will then
      be perpendicular to the threads when mounting on the dividing head

      Rich

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      • #4
        Ok, thank you both for the info. That sounds like a good plan of action. And yes, to goal is to open a bit and re-thread as the indexing head has a larger spindle than this face plate.

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        • #5
          The choice of process is sort of a pick your poison. The procedure of clamping the existing chuck onto a piece of roundstock has the best theoretical accuracy but as Rosco points out it's compromised by the accuracy of the chuck itself. The three jaw should be reasonably repeatable, even if wear has made it less accurate at different diameters. SO ... if you do a lot of work at, say, 1-1/2" you could put a slightly larger piece in the other chuck, turn it to 1.500", clamp the 3-jaw on it back to front and do the backplate.

          Alternatively, if you remove and mount the backplate separately, you can make sure the faceplate is flat, dial in the backplate, cut the new threads but even if it's absolutely perfect you're depending on the 3-jaw to also be accurate. If the back of the chuck itself isn't also perfect the previous accuracy will be lost.
          .
          "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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          • #6
            Wouldn't it make sense to first thread the backplate onto the spindle and check with an indicator that it runs true axially and radially? Then true it up
            if it doesn't?

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            • #7
              Originally posted by TGTool View Post
              Alternatively, if you remove and mount the backplate separately, you can make sure the faceplate is flat, dial in the backplate, cut the new threads but even if it's absolutely perfect you're depending on the 3-jaw to also be accurate. If the back of the chuck itself isn't also perfect the previous accuracy will be lost.
              That's been mentioned by TGTool. Should already be "flat" and running perpendicular to the existing thread, but it still should be checked. Another unknown is there enough meat on the backplate spiggot to cut a larger thread? Might have to start from square one with a raw backplate.

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              • #8
                Isn't there a step or two missing here?

                Install the 3 jaw on the lathe, chuck in a piece of stock, take a truing cut, THEN remove the 3 jaw with the stock. Install the 4 jaw, clamp on the trued part of the stock, indicate to the limits of precision then do what you were going to do.

                Assuming the 3 jaw isn't trashed to the point that the stock wiggles no matter how much you tighten, the truing cut compensates for jaw/scroll wear and ensures that the stock is true to the spindle.

                bob

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                • #9
                  Originally posted by rowbare View Post
                  Isn't there a step or two missing here?

                  bob
                  Without a doubt. No one is posting a "How to...", just questions answered and some considerations.

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                  • #10
                    Originally posted by Rosco-P View Post
                    ...Another unknown is there enough meat on the backplate spiggot to cut a larger thread? ...
                    Yep, seems to be plenty of material to work with.

                    Originally posted by RichR View Post
                    Wouldn't it make sense to first thread the backplate onto the spindle and check with an indicator that it runs true axially and radially?
                    This little 3-jaw and back plate doesn't fit any of my lathes. It's just a rogue item I caught scurrying around the workshop.

                    Overall my thinking with the whole cutting the threads with it attached the the chuck was that if the chuck jaws were out of spec that this might be a way to align the bore to the chuck jaws. Once it's re-worked to fit the indexing head it won't be spinning at high RPM's anymore, whether that is relevant anything or not I don't know.

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                    • #11
                      Originally posted by Rosco-P View Post
                      Without a doubt. No one is posting a "How to...", just questions answered and some considerations.
                      My point was that the "It will not work" post was incomplete and gave the mistaken impression that OP's "idea that sounds silly" was unworkable when, with a simple modification it is quite sound. I totally agree that simply chucking a bar in a 3 jaw chuck and chucking the other end into a 4 jaw will likely not give the desired result. However by adding "a step or two" i.e. turning the bar in the 3 jaw before chucking it in the 4 jaw, it is a perfectly usable solution and doesn't sound silly at all...


                      bob

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                      • #12
                        Originally posted by rowbare View Post
                        My point was that the "It will not work" post was incomplete and gave the mistaken impression that OP's "idea that sounds silly" was unworkable when, with a simple modification it is quite sound. I totally agree that simply chucking a bar in a 3 jaw chuck and chucking the other end into a 4 jaw will likely not give the desired result. However by adding "a step or two" i.e. turning the bar in the 3 jaw before chucking it in the 4 jaw, it is a perfectly usable solution and doesn't sound silly at all...


                        bob
                        Much as hate to see this in posts...OMG! Is today, overly sensitive Tuesday?

                        I think everyone who contributed to the thread helped make the OP think about what the end result should be, what he's thinking of doing and why it will or won't work the way he desired. Machining, welding, woodworking, any craft....it's a thinking man's (or woman's) game. If a person has the basic skills, a little nudge is usually all they need to get them to succeed. Personally, I wouldn't trust the jaws and jaw slides unless the chuck was new.

                        It's doesn't have to be a tutorial, do steps A, B, C, but remember...don't do step E until you've completed step D.

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                        • #13
                          Just a thought, but since you say it is a small chuck, will the whole chuck fit in your 4 jaw? If so, I suggest a two step:
                          First chuck a piece of stock in the small chuck. Then center by the chuck body in the 4 jaw. Does the stock run true to your satisfaction? If so, consider the chuck body itself for the job.
                          Step 2, remove small chuck jaws, chuck the body and backplate in 4 jaw, re-center, and go to town.

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                          • #14
                            Turn a shaft true in your big lathe chuck, tighten the small chuck onto the trued shaft so it's facing the big chuck and then recut the threads in the backplate.

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                            • #15
                              Originally posted by Gary Paine View Post
                              Just a thought, but since you say it is a small chuck, will the whole chuck fit in your 4 jaw? If so, I suggest a two step:
                              First chuck a piece of stock in the small chuck. Then center by the chuck body in the 4 jaw. Does the stock run true to your satisfaction? If so, consider the chuck body itself for the job.
                              Step 2, remove small chuck jaws, chuck the body and backplate in 4 jaw, re-center, and go to town.
                              That suggests a 1 Step alternative. Chuck a piece of ground round stock in the three jaw. Mount the chuck body in the four jaw making sure that the chuck wrench can access it between the larger jaws. Indicate the portion of the ground rod that's now protruding through the back of the chuck and adjusting to make sure it's concentric and doesn't slope in any direction. In other words straight with the ways. Now loosen the jaws of the three jaw, remove the stock and the back of the three jaw is ready to machine.
                              .
                              "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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