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Threading question for the folks with metric machines

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  • Threading question for the folks with metric machines

    Are the various metric thread settings on a metric lathe with a QCGB all actually mathematically exact, or are some of them just very very close to correct?

    I have not calculated all of the more common ones, but it appears that at least some may have to be just very close. Correct enough that normal use would never be an issue, but not strictly mathematically correct.

    1601

    Keep eye on ball.
    Hashim Khan

  • #2
    I don't know, but I can make a model for you🤣

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    • #3
      Model not required.😉
      1601

      Keep eye on ball.
      Hashim Khan

      Comment


      • #4
        As far as I know the threads, metric and imperial, are all without error on my CQ6230a-1 lathe

        https://metalmaster.en.made-in-china...A-1-910mm.html

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        • #5
          Afaik they are exact, on some lathes you need change gears even with QCGB and some others do it with bit more clever gearbox. Classic Norton gearbox Style doesn’t match well with the metric pitches.

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          • #6
            They are exact. Even an imperial lathe with a proper conversion gear gives exact threads.

            With the conversion gears, most setting aren't useful in metric (not much use for a 0.152 mm pitch), but all the standard mm pitches can be had.

            Here's a chart for some of the desired pitches:

            Click image for larger version

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            • #7
              Originally posted by J Tiers View Post
              Are the various metric thread settings on a metric lathe with a QCGB all actually mathematically exact, or are some of them just very very close to correct?
              ....
              By that term "metric machine," are we talking about a lead screw of metric pitch? ...or simply proper gearing for metric ratios between spindle and an imperial lead screw?
              Lynn (Huntsville, AL)

              Comment


              • #8
                A quite reasonable leadscrew conversion can be made on an 8 tpi leadscrew with a 17 and 54 tooth gear, which converts the leadscrew to look like a 1mm pitch in terms of basic movement per turn of the 17 tooth gear. The actual advance is .99953 mm instead of 1.0mm.

                23 and 73 is even better if you can fit it. 1.00034mm /mm error

                Originally posted by lynnl View Post

                By that term "metric machine," are we talking about a lead screw of metric pitch? ...or simply proper gearing for metric ratios between spindle and an imperial lead screw?
                I was referring to one made entirely metric, I know that conversions are often not exact.
                Last edited by J Tiers; 02-29-2020, 12:39 PM.
                1601

                Keep eye on ball.
                Hashim Khan

                Comment


                • #9
                  "Classic Norton gearbox Style doesn’t match well with the metric pitches."

                  why is that?

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                  • #10
                    Originally posted by J Tiers View Post
                    Are the various metric thread settings on a metric lathe with a QCGB all actually mathematically exact, or are some of them just very very close to correct?

                    I have not calculated all of the more common ones, but it appears that at least some may have to be just very close. Correct enough that normal use would never be an issue, but not strictly mathematically correct.
                    By "metric lathe" are you referring to one with a mm pitch lead screw? If so, then yes, the QCGB settings will give exact pitches.

                    If cutting metric on a lathe with an inch pitch lead screw then exact metric threads can be cut with the introduction of a 127 tooth gear in the train. (assuming 1 inch to equal exactly 25.4mm) where 127 is the smallest prime number for 25.4, we multiply 25.4 x 5 to get 127. This allows gears downstream to factor evenly for metric pitches when cutting with the inch pitch lead screw. On a lathe with a 20 tooth spindle gear we can multiply 20 x 5 and introduce a 100 tooth gear too, which helps offset the large ratio change when adding the 127 gear.

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                    • #11
                      I'm curious, why if it was a metric lathe, you would think it might be anything but exact?
                      .

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                      • #12
                        Originally posted by Mcgyver View Post
                        I am curious why, if it was a metric lathe would you think it would be anything but exact?
                        The point was mostly because the decimal relations required do not necessarily appear to come out to nice (or available) single pair gear ratios. The answer seems to be shown in post #5: some pitches require another gear change outside of a Norton style gearbox.

                        It is clearly one place where metric is pretty much a total bodge. In almost every other way, metric is superior. Threads are an exception.

                        There would have been no problem with threads being a different ratio-based system, but that was not done, presumably out of a need to be different and to be noticeably different (in fact, incompatible...even better). I do not see that it conveys any particular advantage in use, on the contrary, it is a disadvantage.

                        But it certainly fulfills the basic requirement that all things metric be different and incompatible with all other systems. (sometimes even incompatible with other metric things)
                        Last edited by J Tiers; 02-29-2020, 03:35 PM.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          What about 1.5 mm thread pitch? It is very common. Also 0.4, 0.45, 0.7, 0.8, 1.25, and 2.5?

                          Click image for larger version  Name:	Screenshot_2020-02-29 Bolt Depot - Common Metric Thread Pitches - Standard, Fine, Super Fine, JIS.png Views:	0 Size:	21.1 KB ID:	1858804

                          More standard metric threads:

                          Click image for larger version

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                          Attached Files
                          Last edited by PStechPaul; 02-29-2020, 04:06 PM.
                          http://pauleschoen.com/pix/PM08_P76_P54.png
                          Paul , P S Technology, Inc. and MrTibbs
                          USA Maryland 21030

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                          • #14
                            Some metric threads available on my HF 9x20 lathe, and some close equivalents:

                            Click image for larger version  Name:	9x20_Lathe_Threads_Metric.jpg Views:	0 Size:	135.9 KB ID:	1858809

                            Click image for larger version  Name:	9x20ThreadChart_Equiv_mm.jpg Views:	0 Size:	426.2 KB ID:	1858810

                            Here is a more complete chart for metric threads from M1 to M100, with coarse and up to three fine threads:
                            Last edited by PStechPaul; 02-29-2020, 04:34 PM.
                            http://pauleschoen.com/pix/PM08_P76_P54.png
                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030

                            Comment


                            • #15
                              Originally posted by J Tiers View Post
                              But it certainly fulfills the basic requirement that all things metric be different and incompatible with all other systems. (sometimes even incompatible with other metric things)
                              Do you really think metric is different? Countries that are different to metric include Liberia, Myanmar and of course the USA, most every other country uses the metric system.

                              Seems everyone else is out of step?

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