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Cleaning up threads

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  • Cleaning up threads

    So I cut a bunch of internal threads on the lathe. 96% of the time they turned out great, but on one piece, the outside facing edge of the threads on one side of the piece had this galled appearance. When I say that "one side" has a galled appearance, I mean one half of the circumference of the piece. Sorry I don't have a picture, but I hope you know what I mean.
    Anyway, I'm wondering what happened here. Only thing I can think of is that the piece shifted slightly during the cutting operation.

    To clarify, I cut the threads from the inside to the outside (they were bottoming threads) on the back side of the piece (the side furthest from me). To cut from inside out, the tool faces upwards and the piece rotates clockwise as viewed from the tailstock. The compound is set at -29 degrees (handle tilted towards the headstock). It cuts the side of the thread that you can't see, as viewed from outside the piece. Hope that is clear as mud.

    Next question is how to clean up this edge? Problem here is that if you are cutting inside to outside, the carriage and leadscrew is backlashed in such a way that it is stable to being pushed towards the headstock, but unstable if pushed towards the tailstock. Cleaning up the outside facing side of the threads requires a carriage that is stable towards being pushed towards the tailstock. Only way I can think of doing this is to get a tool with the opposite orientation and cut from outside in. In this case, it is kind of dangerous since these are bottoming threads.

    Any other good way to clean up this galling (besides using sandpaper)?

    Oh and another thing. This 29.5 degrees thing is supposed to take a chunk off of one side of the thread and a sliver off the other side. However, I've never seen any sliver of material being removed.
    Last edited by beanbag; 10-21-2009, 03:46 AM.

  • #2
    Clear as mud is an understatment. Carefully read and study the link.

    http://www.metalartspress.com/PDFs/6...ee_threads.pdf
    Non, je ne regrette rien.

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    • #3
      Perhaps what you need is a spindle crank for easy control of threading in the inward direction.



      This allows you to thread up to a shoulder or the bottom of a blind hole with no sweat. The shaft is a close fit for the inside bore of the spindle and goes into it for about 6 inches or so for stability. The aluminum block you see tightened on the spindle OD transmitts the rotational force. The hand grip is a bicycle hand grip.

      I was cutting a 1mm thread here and it needed to go right up to the shoulder.
      Last edited by Paul Alciatore; 10-21-2009, 11:59 AM.
      Paul A.
      SE Texas

      And if you look REAL close at an analog signal,
      You will find that it has discrete steps.

      Comment


      • #4
        Neat idea Paul. Thanks.

        That's an interesting toolpost too. What make, or is it homemade?

        Gary

        Comment


        • #5
          The 'sliver' off the backslide of the thread isent so much a sliver as dust

          Try adding some blue layout fluid before your cut and see if its removed off the backside, if its not removed, your actualy feeding at 30 degrees or over, you CRITIALY need to be <30 degrees. how far under doesnt really matter, but the closer to 30 degrees the better for your cutting tool forces.
          thats why 29.5 degrees is considered 'ideal'

          PS: I wonder what happens when you try threading a material that work hardens with 29.5 degree infeed, Would'nt the skiming cuts work harden the material?
          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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          • #6
            Instead of a spindle crank, I've used a chuck key in the 3 jaw

            As for the work hardening problem, I read that the CNC folks (with no backlash on the carriage) use things like radial infeed (straight in) or alternating flanks infeed (alternate between 29 and - 29)

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            • #7
              Try a small wire wheel on a drill motor to smooth out the galled surface. But improve your cutting process first per above suggestions.

              Same deal with shortening screws: Cut with whatever, grind square, grind chamfer away from threads, hit with wire wheel to roll over the burrs, and you are good to go. I can do this in seconds. Even if you cut off cleanly with a lathe and a file you will probably still need to hit with the wire wheel to smooth the burr left in the thread.

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              • #8
                I assume the ID is too large to be economical to buy a bottoming tap to clean up with?

                Upside-down and backwards makes perfect sense to me

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                • #9
                  just cut the tread oversize the first lap and call it good. it wont hurt a thing.

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                  • #10
                    Cutting oil

                    From my experience, a good portion of the poor thread finishes could be cured by the liberal use of a good quality cutting fluid. Jmho
                    I spent most of my money on women and booze, the rest I just wasted.

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                    • #11
                      I have used a tap of the same thread pitch as the thread I cut internally to clean the thread. Run the lathe about 50 rpm and hold the tap against the thread and it will chase the threads. It's about the only way to clean an internal thread. Use a bottom tap if you have one.
                      It's only ink and paper

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                      • #12
                        Originally posted by chief
                        Clear as mud is an understatment. Carefully read and study the link.

                        http://www.metalartspress.com/PDFs/6...ee_threads.pdf
                        IMO, Figure 7-150 has the compound set at the wrong angle
                        Last edited by beanbag; 10-22-2009, 12:34 AM.

                        Comment


                        • #13
                          Originally posted by gfphoto
                          Neat idea Paul. Thanks.

                          That's an interesting toolpost too. What make, or is it homemade?

                          Gary

                          The toolpost is shop made. I was trying to avoid dovetails and still have a rock solid, and repeatable design. It has a broad flat for indexing and has worked very well. I have an article in to George and he keeps saying is is in line for publication sometime.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

                          Comment


                          • #14
                            Originally posted by Paul Alciatore
                            It has a broad flat for indexing and has worked very well. I have an article in to George and he keeps saying is is in line for publication sometime.
                            Thanks, I hope the article gets published soon.

                            If it isn't giving away too much, why is the center post(?) so high, and why is it (or is it) squared off on top?

                            Thanks,

                            Gary

                            Comment


                            • #15
                              Originally posted by Paul Alciatore
                              I have an article in to George and he keeps saying it is in line for publication sometime.

                              Funny you should mention it Paul; Craig and I were just looking at it yesterday. We are hoping to run it in the F/M 10 issue of Machinist's Workshop. Craig was going to contact you to see about getting some higher res photos.

                              George
                              George
                              Traverse City, MI

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