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Cutting Metric Theads on a PM1236 lathe

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  • Cutting Metric Theads on a PM1236 lathe

    I have a PM1236 lathe. I believe it was made in China. Nevertheless, I have never cut a metric thread using a lathe. My lathe is setup, new from the company, for cutting American threads. Though I am likely to never use the lathe to cut metric threads, I would still like to have the knowledge as to what is required to cut such threads on my lathe ……. should I ever have a need to do so.

    1. I assume there would be a need to change gear(s) in my thread-chaser (or what ever it’s called). Would I also need to change the dial on the indicator?

    2. Since I am running an imperial lead screw, will I need to change the lead screw as well and if so, to what?

    3. I have heard troubling stories regarding “problems” associated with cutting metric threads on a lathe. Is this *only* because the lathe being used was never originally setup for metric and has since been “half-heartedly” converted either from imperial to metric or “jerry-rigged” to a point that a metric thread can be cut?

    4. Lastly, those having been successful or partially successful in cutting metric threads have made comments leading me to believe that once metric threading has begun, the cutting tool CANNOT be “backed out”. I have been given the impression that the tool must remain physically engaged with the work piece until the thread is finalized. This would mean, I think, that the lathe would have to be stopped without disengaging the half-nut, the motor then reversed, and then the tool would travel back to it’s original position without losing physical contact with the piece being threaded. Is that correct?

    Harold
    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    The best solution is to get a 127 tooth change gear, this will allow you to cut metric threads with an imperial leadscrew. Do a search on it and you will find the info.

    Comment


    • #3
      [1] No because you can't use it at all [ see 4 ]

      [2] No screw and half nuts stay the same.

      [3] Problems are without a gear with 127 in it precise metric threads can't be cut but you can get very close approximations. if you are making lead screws you need a 127 gear. If you are making general threads for fastners then there is always a very close approximate.

      [4] Correct that's why [1] is irrelevant.

      If you post a picture of your screw cutting chart and a list of what spare gears came with the machine we can answer in more detail.

      [EDIT] having had a decent amount of experience with these mid sized Chinese lathes they tend not to cut as many corners as the cheaper bench top models and it's rare for them not to have the 127 in the train or supplied as standard.
      Where the main problems occur is when they are second hand the extras tend to disappear. But even that is not a problem as they use usually common across a range of machines so spares are available.
      .

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



      Comment


      • #4
        Originally posted by macona View Post
        The best solution is to get a 127 tooth change gear ..........
        Are you saying that I need to get a 127 tooth gear to be installed in my current lathe or are you implying that I need a different lathe with a 127 tooth "change gear"? Please keep in mind that I am a hobby machinist and I quickly get confused with terminology.

        Thanks,

        Harold
        For those having fought for it, Freedom has a flavor the protected will never know.
        Freedom is only one generation away from extinction.

        Comment


        • #5
          Originally posted by John Stevenson View Post
          [1] No because you can't use it at all [ see 4 ]

          [2] No screw and half nuts stay the same.

          [3] Problems are without a gear with 127 in it precise metric threads can't be cut but you can get very close approximations. if you are making lead screws you need a 127 gear. If you are making general threads for fastners then there is always a very close approximate.

          [4] Correct that's why [1] is irrelevant.

          If you post a picture of your screw cutting chart and a list of what spare gears came with the machine we can answer in more detail.

          [EDIT] having had a decent amount of experience with these mid sized Chinese lathes they tend not to cut as many corners as the cheaper bench top models and it's rare for them not to have the 127 in the train or supplied as standard.
          Where the main problems occur is when they are second hand the extras tend to disappear. But even that is not a problem as they use usually common across a range of machines so spares are available.
          Good Morning John,

          In the next hour or so I will capture an image of my screw cutting chart and provide a list of all spare gears that came new with the machine. If I am not too badly mistaken, it *does* have a 127 tooth gear in the train. But before replying, let me make certain by providing that which you requested.

          Thanks,
          Harold
          For those having fought for it, Freedom has a flavor the protected will never know.
          Freedom is only one generation away from extinction.

          Comment


          • #6
            I have avoided metric threads on a manual lathe not equipped for such festivities, but my understanding is that the tool can be backed out, but the half nuts MUST remain engaged.

            You reach the end of the thread, STOP the spindle, back out the tool, not much, just for relief, (mostly in case your gear train has a little lash) and reverse the spindle to the starting point. Adjust the tool in for the next cut and lather, rinse and repeat as necessary.
            Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
            ~Practitioner of the Electromechanical Arts~

            Comment


            • #7
              Originally posted by hwingo View Post
              4. ...the cutting tool CANNOT be “backed out”. I have been given the impression that the tool must remain physically engaged with the work piece until the thread is finalized... Is that correct?
              Originally posted by John Stevenson View Post
              [4] Correct that's why [1] is irrelevant.
              Originally posted by Weston Bye View Post
              ...the tool can be backed out, but the half nuts MUST remain engaged... You reach the end of the thread, STOP the spindle, back out the tool, not much, just for relief, (mostly in case your gear train has a little lash) and reverse the spindle to the starting point. Adjust the tool in for the next cut and lather, rinse and repeat as necessary.
              Just to be clear, Weston is correct. The tool itself IS backed out from the workpiece after each cutting pass, but the half-nuts are NOT disengaged until the thread is finalized.

              Comment


              • #8
                I have this lathe as well. It does have a 127 tooth gear. Mine had it installed already.

                Comment


                • #9
                  Originally posted by Arthur.Marks View Post
                  Just to be clear, Weston is correct. The tool itself IS backed out from the workpiece after each cutting pass, but the half-nuts are NOT disengaged until the thread is finalized.
                  Yes I agree, sorry typing to fast for a clear answer.
                  .

                  Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



                  Comment


                  • #10
                    Harold, I believe your PM1236 is a version of the very popular Chinese CQ6230A, one difference being that yours is (may be?) fitted with an imperial lead screw. An easy way to tell is if the chart on the machine shows inch or mm pitches in the 'Threading Indicator' table.

                    Whatever that table shows that is the family of pitches you can use the threading indicator for otherwise you must use some other method when restarting the threading cut such as the procedure Weston described.

                    If your threading tables show metric pitches and inch pitches it will almost certainly have a 120/127 compound gear, you can easily check by taking the cover off the end of the lathe (two knurled knobs) where you will see two big gears joined together as one. [later]I see Mike has already covered this point![/]

                    John

                    Comment


                    • #11
                      I have the Grizzly version (4003G) and it came with the extra gear to swap out for metric threading. Download the manual from here and read up on it. Someone above already mentioned that the PM1236 comes with the gear so you should be able to contact them and buy one if yours doesn't have one. First step is to remove the cover from the left end and check what gears you already have - it may be installed.

                      Comment


                      • #12
                        Pictures of the lathe threading tables will answer most of the questions being asked here.

                        Comment


                        • #13
                          For example:-



                          This is a picture of the PM1236 threading chart for imperial threads.

                          We can see from the left hand schematic that compound gearing is not being used which indicates the lathe has an imperial lead screw which in turn infers that the threading indicator can be used for imperial threads but certainly not for metric threads.

                          Comment


                          • #14
                            Sounds like a good excuse to buy another lathe to me!
                            I found a 126 tooth in a box of crap and provided your not aiming to make lead screws it works fine over a couple of inches, so she says, it's an approximation but not bad
                            Have fun and remember even if you cut a thread on a lathe there's nothing wrong with running a die down it!
                            Regards
                            Mark

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                            • #15
                              He has a PM1236 and there is no need for any approximations.

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