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  • Baffled by metric threads

    The rear end of my Chinese lathe's spindle is threaded internally. I want to make a fixture that will screw into these threads. This will require single pointing the thread, but I have no clue as to what the sizes or pitch is. The id over the thread measures 40.15mm, and I counted 11 threads over a space of 20mm, measured with a scale. I couldn't find a match using either my metric or SAE pitch gage. It looks close to 1.5 or 1.75 but no cigar. I need some education on metric threads before attempting to cut them.
    “I know lots of people who are educated far beyond their intelligence”

    Lewis Grizzard

  • #2
    You might want to lightly oil the threads and pour some wax on to the bottom of the spindle once hardened remove, it ought to be a lot easier to check, guess you could take the impression to a hardware store and find a bolt that has the same T.P.I.
    Might want to check with a supplier or ebay a cheap set of T.P.I. gages are reasonable and handy.

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    • #3
      Are you sure your presumably North America market lathe has metric threads?

      My Chinese 12x36 lathe has 1.5mm thread in the spindle but yours is much coarser than that. 11 threads over 20mm would be 2mm pitch if you included the thread at the start and at the end.

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      • #4
        This is why I spent a couple bucks to buy a metric thread pitch gauge.

        I've never seen a metric thread that wasn't a standard pitch, though I have seen all kinds of intermediate diameters. If it's not a metric thread, 11 threads in 20mm would be really close to 14 threads per inch.

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        • #5
          Have a look here <http://thread-chart.ueuo.com/ISO-Metric-Thread-Specification.html>
          What I found was a minimum dimension for female threads of 40.129- 40.659 = M45 with a pitch of 4.5mm
          Using the numbers you gave for the threads I get a pitch of 1.8181 which is much finer than the standard given above.
          I would use both imperial and metric pitch gauges in the thread in the spindle to see what is the closest.
          Then try and determine the OD dimension as close as you can of the thread in the spindle.
          Then just cut the thread that most closely matches what you have.
          I think you are going to find that it is a bastard thread for that diameter.
          I hope some of this makes a little sense.
          Larry - west coast of Canada

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          • #6
            Dave, it is more than likely 2mm pitch, buy a bolt from the hardware store and try for a pitch match, a 16mm bolt for example.

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            • #7
              Originally posted by Big Rack View Post
              You might want to lightly oil the threads and pour some wax on to the bottom of the spindle once hardened remove, it ought to be a lot easier to check, guess you could take the impression to a hardware store and find a bolt that has the same T.P.I.
              Might want to check with a supplier or ebay a cheap set of T.P.I. gages are reasonable and handy.
              I like the wax impression idea, and I did use SAE and metric TPI gages, but neither had a leaf that matched the spindle thread
              “I know lots of people who are educated far beyond their intelligence”

              Lewis Grizzard

              Comment


              • #8
                [QUOTE=The Artful Bodger;991213]Are you sure your presumably North America market lathe has metric threads?

                Not sure of anything except that it's a Harbor Freight lathe, and all screws and bolts encountered to date have been metric.
                “I know lots of people who are educated far beyond their intelligence”

                Lewis Grizzard

                Comment


                • #9
                  Originally posted by Cuttings View Post
                  Have a look here <http://thread-chart.ueuo.com/ISO-Metric-Thread-Specification.html>
                  What I found was a minimum dimension for female threads of 40.129- 40.659 = M45 with a pitch of 4.5mm
                  Using the numbers you gave for the threads I get a pitch of 1.8181 which is much finer than the standard given above.
                  I would use both imperial and metric pitch gauges in the thread in the spindle to see what is the closest.
                  Then try and determine the OD dimension as close as you can of the thread in the spindle.
                  Then just cut the thread that most closely matches what you have.
                  I think you are going to find that it is a bastard thread for that diameter.
                  I hope some of this makes a little sense.
                  Thanks for that link Larry. I'm afraid that the spindle thread is not the only bastard feature of this machine.
                  “I know lots of people who are educated far beyond their intelligence”

                  Lewis Grizzard

                  Comment


                  • #10
                    You said it's a harbor freight lathe. Why not call them and talk to tech support? You'd be surprised what they can look up.

                    Comment


                    • #11
                      It is unlikely to be a bastard pitch (but possible). More likely a bastard combination, a pitch not usual for the diameter.

                      Better find the pitch, and then see what major diameter may be from that and ID.
                      3751 6193 2700 3517

                      Keep eye on ball.
                      Hashim Khan

                      If you look closely at a digital signal, you find out it is really analog......

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                      • #12
                        What model lathe is it?

                        The very common Chinese CQ6230... 12x36 lathe with the 38mm spindle bore has a 1.5mm thread in a slightly enlarged bore at the outer end.
                        Last edited by The Artful Bodger; 07-06-2015, 01:22 AM.

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                        • #13
                          Eleven threads in 20mm is real close to 14 TPI. Eleven threads at 14 TPI will equal 19.957mm. Real close. Did you try a 14 TPI gauge?
                          Paul A.
                          SE Texas

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

                          Comment


                          • #14
                            it probably will be 43 x 2.
                            to find out the pitch take a piece of soft wood about 41 mm wide and screw it into the thread, unscrew and measure the marks the thread has left behind.
                            if it turns out to be 2 mm your outside diameter would be 42.8
                            bigger metric threads, other than fasteners, usually have a 1.5mm or 2mm thread.

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                            • #15
                              Originally posted by Dave C View Post
                              I need some education on metric threads before attempting to cut them.
                              Dave,
                              With all due respect it's not a lesson in metric threads you need but a lesson in measuring threads - period.

                              If this thread were imperial you would have the same problems.
                              .

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



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