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Cutting Metric threads on a American Lathe

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  • Cutting Metric threads on a American Lathe

    Hello everyone
    I have been needing to cut metric threads more as the yrs pass.
    The problem is I have a American Lathe, a 1941 Sidney 14X30
    Is it possible to do this? What would I need to do to the lathe ?
    or is this just wishfull thinking on my behalf.
    In the past , I had a 12x37 import lathe. and on the rare times
    I needed a metric thread, I beleive I had to swap out a few gears in the LH
    end of the gear box. I memory serves me, one of the gears had to be 127 tooth. I have been getting away with useing taps & Dies for any metric threads, but would like to get this machine to do both, if I could?

    Thoughts on this?
    TIA
    Paul

  • #2
    Hello,

    I read somewhere, on the PM website, that it is possible to use a 34 and 52 tooth gear to get a close approximation of metric threads. The error was claimed to be 0.04 of 1%. The posting was in the South Bend forum; but, it is very hard to find as I spent quite some time until I found it. I also believe that it was for a South Bend 9.

    I made a 34 tooth gear and briefly tested it for a couple of threads and it seemed to work on my South Bend 13. I did have to shift the chart slightly as the gearing is different between the South Bend 9 and 13. (i.e. Set the gear box to 36 TPI for 1.00mm threads instead of the 18TPI listed.

    Anyhow, it may work or get you close enough to work. Here is the information as metric size/gear box setting:

    34 Tooth Stud Gear:

    0.25/72
    0.45/40
    0.50/36
    0.75/24
    0.90/20
    1.00/18
    1.50/12
    2.00/9
    3.00/6
    4.00/4.5
    4.50/4
    6.00/3
    8.00/2.25

    52 Tooth Stud Gear:
    1.25/22
    2.50/11
    5.00/5.5


    Regards,

    Brian
    There are only 10 types of people in the world: Those who understand binary and those who don't.

    Comment


    • #3
      The way I did it I explained in the Nov./Dec. 2007 issue of HSM. The 127 tooth gear translates Metric/inches and was easy to make.

      Edit 04072011: For size considerations the 127T and 40T gear were made at 1/2 the DP of the stock Atlas change gears. These were the only gears that were necessary, to make just about any common metric thread.
      Last edited by Jim Hubbell; 04-07-2011, 04:21 PM.
      Jim

      Comment


      • #4
        Thanks guys
        I will search around for this to get the details.
        Jim, I wish I had the artical you wrote up. Wouldnt happan to have a copy of that stashed away some where, would you? Was it just the 127 tooth gear you had to make?

        Thanks
        Paul

        Comment


        • #5
          According to my super accurate calculations, the error with a 34/52 combination is about 0.35%. The 37/47 combo gets you down to 0.02% so it is technically superior. Each of these approximate translations will require a different set of stud and screw gears as one approximates the exact ratio of 1.27 while the other goes for an approximation of 120% of that ratio (1.524 = 1.27 X 1.2). Many other approximate combinations have been proposed.

          The 127 tooth is the one gear that provides an exact translation. (10" = 254mm EXACTLY. 254/2 = 127. 127 is a prime number and can not be further divided so it is the smallest number that provides an exact translation.) It is often used with a 100 tooth, but some have used it with a 50 tooth and others with a 120 tooth. This second gear is simply one that expresses a round number distance in English units. It is choosen for the best use of the existing set of change gears or for best use with an existing quick change box.

          In most cases, you must have two gears with the proper ratio between them. They are usually combined into a single compound gear on the same shaft. I have made and sold such sets for the SB 9 and sold them along with the proper chart to set up many metric threads. I did discover that some setups were simpler with just the single 127 tooth gear mounted on the lead screw so I made my set as two pieces that were bolted together to form a compound or each could be used separately. I also found that an Excel spreadsheet was a very good way to discover all possible threads that could be made with a given set of gears. If you send me a private message with your e-mail, I can send that Excel spreadsheet (for the SB 9) to you.



          Originally posted by Rif
          Hello,

          I read somewhere, on the PM website, that it is possible to use a 34 and 52 tooth gear to get a close approximation of metric threads. The error was claimed to be 0.04 of 1%. The posting was in the South Bend forum; but, it is very hard to find as I spent quite some time until I found it. I also believe that it was for a South Bend 9.

          I made a 34 tooth gear and briefly tested it for a couple of threads and it seemed to work on my South Bend 13. I did have to shift the chart slightly as the gearing is different between the South Bend 9 and 13. (i.e. Set the gear box to 36 TPI for 1.00mm threads instead of the 18TPI listed.

          Anyhow, it may work or get you close enough to work. Here is the information as metric size/gear box setting:

          34 Tooth Stud Gear:

          0.25/72
          0.45/40
          0.50/36
          0.75/24
          0.90/20
          1.00/18
          1.50/12
          2.00/9
          3.00/6
          4.00/4.5
          4.50/4
          6.00/3
          8.00/2.25

          52 Tooth Stud Gear:
          1.25/22
          2.50/11
          5.00/5.5


          Regards,

          Brian
          Last edited by Paul Alciatore; 04-05-2011, 03:07 AM.
          Paul A.
          SE Texas

          Make it fit.
          You can't win and there IS a penalty for trying!

          Comment


          • #6
            Logan Lathe recommends 127/100 transposing gears for exact conversion to metric, they also have 37 T. and 47 T. Transposing Gears (2/100 of 1% Inaccuracy). The smaller gears are what I use and the work well while still fitting under the gear cover. Atlas uses a 54/42 combination to approximate most metric threads.

            Comment


            • #7
              Monarch changes the DP for metric threading gears, mainly due to the OD of the 127 T gear, a 16 DP 127 T gear is 8-1/16" OD, and to get them under the gear cover on the left end. On my 12" CK, 16" CY and 16" Series 60 the metric gears are 16 DP. Both 16's use a 127/50 compound gear, the 12" uses an 80/63, the 50 and 63 tooth gears will be the same DP as the gearbox. The only other gears used replace the spindle stud gear, and there 6, or so, gears.
              Your lathe is very similar to Monarch, and I suspect the metric changes gearing will be similar.
              Harry

              Comment


              • #8
                There is a lot of good info in these posts. Unless the factory or someone else has worked out a scheme for your particular lathe or one with the same gear train, you will have to work it out yourself. There is no generic solution but there is a lot of information in books, in forums, and elsewhere on the internet. Martin Cleeves 'Screwcutting on the Lathe' has some real good information.

                You will need a pair of gears for altering the pitches to more nearly the metric value. Whether you use the 127T gear for an exact ratio is up to you. Nothing else about making threads is 'exact'. Even if you have a Quick Change Gear Box you will also need some change gears as the English box ratios only work for a few metric threads.

                If your lathe has a gearbox, a handy thing to know is that if you set it up to cut a 1MM thread with the gearbox set to 24tpi, then 24 divided by any other setting gives you the MM pitch for that setting. If you set the gearbox to 16tpi you will cut a 1.5MM thread, etc.. If your gearing gives a 1MM thread at 20tpi, then 20 divided by your gearbox setting gives you your MM pitch. If you set your gearbox to 16tpi you will then cut a 1.25MM thread. In other words, for any gearing setup, the gearbox setting times the metric thread cut at that setting is always the same number. This can save you some calculating.

                The gearbox columns containing 16, 20, 24, and 30 are usually the only ones providing ratios useful for metric threads. Others give odd fractional pitches.
                Don Young

                Comment


                • #9
                  One issue that I have not seen addressed yet is the issue of thread dials.

                  Unless you are lucky enough to have an apron controlled reverse to the lead screw, it will be difficult to cut the metric leads as it is difficult to use the standard trick of leaving the half nuts engaged and reversing the spindle to return to the beginning of the thread for the next pass. I believe your older Sidney will need to reverse the spindle electrically, which will be even more difficult if it is powered by a single phase motor.

                  You can buy an accessory thread dial that will allow you to pick up the thread again for subsequent passes when disengaging the half nuts. Google Ainjest for more info. I think that name is right, if not I could look for sure.

                  Dave

                  Comment


                  • #10
                    The reason why the 127 works is 5 x 25.40000000000000 = 127.0000000000000 and the international inch is defined at being exactly 25.4mm.

                    Now this is where even imperial users and defenders such as myself have to blush although its largely in the past now...there is more than one inch! There was a British, Canadian and American inch, mostly replaced by the 25.4 however the American survey inch (used only for surveying) survives which is 2 millionths of an inch longer than the int inch. The inch you regularly encounter is the same, and our gauge blocks are the same....however that may not be the case if they're earlier than 1958. This of course doesn't affect use in a practical sense, but is interesting.

                    so 127 teeth lets you get to metric with 0.000000000000000% error
                    in Toronto Ontario - where are you?

                    Comment


                    • #11
                      Originally posted by Mcgyver
                      The reason why the 127 works is 5 x 25.40000000000000 = 127.0000000000000 and the international inch is defined at being exactly 25.4mm.
                      Unless you are on a Pentium, than it is 127.000000000001

                      Comment


                      • #12
                        Hi Guys
                        Well I have been doing a ton of reading since posting this topic. I am far from getting a handle on this metric threading. Im still lost on alot of this stuff but doing my best to learn.

                        I did get a copy of the manual for my lathe from a PM member that has the same machine. In the manual, there is alot of infro on the end gears for metric threading, so what Im try to do, isnt impossible, but very confusing to me.

                        The manual shows the gear train set up for what they call STD End Gearing.
                        There is one thing that stumps me with this, the idler gear is claimed to be 72 tooth, and mine is 60.
                        Hoping I have this right, the gears are as follows
                        Top , Stud gear 32 tooth
                        2nd, idler gear 60 tooth
                        3rd tumbler gear 36 tooth
                        4th gear box 48 tooth
                        The manual says these are 12P, and 20 o PA is stamped on the face of the gears
                        Question: does the idler gear tooth not count part of all the ratio,s?
                        Im supposed to have a 72T and I have a 60 T
                        I have cut many different TPI threads on this lathe, and for anything I have done, its allways worked out fine.

                        The drawing for metric end gearing shows the 127T comp gear with the 60T behind it. Also has a chart for stud gears ranging from 32 to 56 . It also shows the idler as a 40t gear. So I get the feeling Im going to need a 127-T, and a 40-T and a few different tooth counts for the stud gear to get common pitches in millimeters.

                        Is there any threading guru,s here that would look at these charts, and try to make some sense out of all this for me?

                        One of the posters had mentioned about having a threading dial, leadscreww to reverse, that would be yes on both counts, the lathe is equipped with both. The the lathe is 5hp 3 phase so the carriage can be reversed one way or the other with the half nuts closed.

                        A few questions before I go.
                        The manual says the gears are 12-P , is that the dia pitch?
                        and on the face of the end gears is stamped 20o PA , is this the pressure angle? Reason Im asking is , if I was to order a gear,or gears. I would think tooth count, pitch & pressure angle would be the information they would need to know.
                        Im hoping to learn enough about this before my hair gets any grayer, or I rip it all out in the process,lol.

                        Thanks for the input guys
                        Paul

                        Comment


                        • #13
                          Idler gears don't count in the gear ratios.
                          Apparently you have 12 diametrical pitch and 20* pressure angle gears.
                          Harry

                          Comment


                          • #14
                            This should become a sticky.

                            Comment


                            • #15
                              The drawing for metric end gearing shows the 127T comp gear with the 60T behind it. Also has a chart for stud gears ranging from 32 to 56 . It also shows the idler as a 40t gear. So I get the feeling Im going to need a 127-T, and a 40-T and a few different tooth counts for the stud gear to get common pitches in millimeters
                              You don't need as many as you think ..a lot of them double for double mms pitch..so a 1mm pitch set up will also do a 2mm and 4mm pitch, if i remember right ..so on and on ..you just change the stud gear and use the chart for the gearbox settings.
                              the full set that is recommended ..covers some obscure pitches that you will never use in a life time. like 0.9 pitch

                              here's the metric chart for a 9 inch south bend with screw cutting gearbox

                              if you haven't got the screw cutting gearbox ..then good luck to you ..i don't think i would bother.



                              here's the gear set for South bend 9 to cut all "common" metric threads if lathe has screw cutting gearbox.



                              all the best.markj

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