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Metric threads on an imperial lathe.

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  • Metric threads on an imperial lathe.

    I wish to contribute what I can to the clarification of what happens when an imperial lathe is used to cut metric threads.

    For my dissertation I will assume the lathe has a 8TPI lead screw and a QCGB with several tpi threads.

    To cut metric threads we need a means to convert the movement from the 8TPI lead screw to something more suitable for the metric world. The 120/127 gear ratio will do this and effectively converts your 8TPI lead screw to a 3 mm pitch. 3 mm pitch is finer than 8TPI so we need the conversion gears to decrease the speed of the lead screw, for example spindle pinion meshes with 127 gear and 120 gear meshes with GCGB pinion.

    With the conversion gears mounted, the spindle pinion equal to the QCGB pinion and the QCGB set to straight through the lathe will cut a 3mm pitch thread. This is exactly the same configuration as would be used to cut 8TPI if the 120/127 conversion gears were not included.

    Suppose your imperial lathe has an imperial thread table you could draw up a metric table beginning with the 8TPI position entered as 3mm. Now 3x8=24 and for all the other positions you need only divide the TPI value into 24. For example set the QCGB to 16TPI, 24/16=1.5 so enter 1.5mm in your metric thread table. Note that this only holds true so long as the spindle pinion and the QCGB pinions are equal.

    You imperial gear box will inevitably throw up a lot of metric threads that are not according to common use.

    But there are still plenty of standard metric threads that can be cut on your imperial lathe:


    The first number is the required metric thread and TPI number is the setting you would use on your imperial lathe when you add 120/127 gears.
    0.2mm, 120TPI
    0.25mm,96TPI
    0.3mm,80TPI
    0.4mm,60TPI
    0.45mm,53TPI
    0.5mm,48TPI
    0.6mm,40TPI
    0.75mm,32TPI
    0.8mm,30TPI
    1mm,24TPI
    1.5mm,16TPI
    2mm,12TPI
    3mm,8TPI
    4mm,6TPI
    6mm,4TPI

    I believe this is all the standard metric fine threads (as per ISO 261) with the exception of 1.25mm. There are no approximations in the above list.

    ...this is a good but not entirely complete range of metric threads and includes some that might be impossible on your imperial lathe (0.45mm for example).



    There is also the possibility that your imperial lathe can cut threads that are not any standard metric thread when using the 120/127 conversion gears An example is 19TPI which will cut a 1.26mm thread which might be close enough to 1.25mm depending on the application and if so would fill out the entire list of fine metric threads.


    There is a whole 'nother world of threads you could cut by tweaking the sizes of the spindle and the QCGB pinions including I expect an accurate 1.25mm thread.




    As always E&OE.

  • #2
    Quite awhile back I bought some stuff at an estate sale and in a small Craftsman machinist's tool box there was a 100/127 tooth gear that was made for a 9" SB lathe plus a bunch of gears to go with it to enable accurate metric threads to be cut. Since then I found a chart online which shows which gear to use in conjunction with the QC gearbox. It also graphically backs up The Artful Bodger's post.

    Last edited by Arcane; 11-10-2016, 08:25 PM.
    Location: Saskatoon, Saskatchewan, Canada

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    • #3
      I've got a Logan and there is a similar chart on the Logan site. Been thinking of making the 127T gear (to buy is about $300), but calculate the gear diameter at about 8 inches, seems awfully large for that gear train. It's a 16DP gear. Wondering if anybody has one, if it is in fact 8+/- inches in diameter, and if that can be fit into the gear train?

      I don't NEED the ability to cut metric threads on the imperial lathe, but would be handy for some of the larger sizes or multiples.

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      • #4
        Originally posted by eeler1 View Post
        I've got a Logan and there is a similar chart on the Logan site. Been thinking of making the 127T gear (to buy is about $300), but calculate the gear diameter at about 8 inches, seems awfully large for that gear train. It's a 16DP gear. Wondering if anybody has one, if it is in fact 8+/- inches in diameter, and if that can be fit into the gear train?

        I don't NEED the ability to cut metric threads on the imperial lathe, but would be handy for some of the larger sizes or multiples.
        Try 37T and 47T. The ration is 1.27027027, or 0.021% over exact, which is likely the smallest error contribution cutting metric threads on imperial, or vise-versa. The error over 1m is 0.21mm (8 thousandths of an inch). These gears have the additional advantages of being less expensive, available (Boston, for one) and easy enough to make yourself, if need be, on the lathe.

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        • #5
          Some lathes, including my Taiwanese 12 x 36, use a 47:37 pair in lieu of the 127,

          47/37 = 1.27027027027...
          Regards, Marv

          Home Shop Freeware - Tools for People Who Build Things
          http://www.myvirtualnetwork.com/mklotz

          Location: LA, CA, USA

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          • #6
            I make camera threads as part of a hobby, which are a mix of Whitworth, USA and Metric.

            In 2013 I contacted Mal at Australian Metalworking Hobbiest.

            I purchased a Hercus compound gear of ratio 63:64, after Mal confirmed that it was compatible with the SB9A.

            The ratios available are not quite perfect by calculation.
            However I have made various metric threads and they fit store bought fasteners etc,
            and so far each camera thread has worked.
            The pitches vary from ideal only by a maximum of 0.007 mm

            Here is a screenshot of the spreadsheet I made of the gear selections on the SB9A.
            https://app.box.com/s/wknpisxj8wcfuws5a4ko82uon391r0om

            Here is a photo of the compound gear on the SB9A
            I made a brass bush for the idler with a flange to space it correctly.
            https://app.box.com/s/zpeonmwwygh2evfj17ltdfj4wxr42azz

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            • #7
              What the op posted looks just like my harbor freight combo model 44142. Google the manual and charts are in it

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              • #8
                could you make a chart for my Rivett? I have the 50/50 gear set and For change gears I have a 127,80,75,70,65,60,55,51,45,42 tooth.





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                • #9
                  I have cut only a few metric threads in my life. They were all non-critical and not over 5 or 6 threads long. In all cases my 13x36 Jet had a regular feed setting that was close enough to the desired pitch do the job. My lathe has metric ratio gears in the headstock, but so far, I have never needed them.

                  RWO

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