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  • Advice needed on gears

    The Smart & Brown model A at the museum is sadly lacking any extra changewheels. It can produce 27 imperial threads from 8 to 76 tpi. Ok, there are some metric threads which are near in pitch to imperial, but I would like a few of the common metric pitches, particularly 1, 1.25, 1.5 and 1.75mm.
    The change wheels are 24DP, 20PA.
    The 120 standard gear is approximately 5 3/32" diameter.
    I have just seen on eBay, a 125/127 gear, 5 1/16" diameter over the 127 which is from a Chester 920 lathe, most likely some type of metric tooth form.
    They are quite close in pitch, do you think I could get away with the mismatch?
    The wheel is cheap, and I would expect to carry out modifications to make it fit.
    Last edited by old mart; 08-20-2017, 10:26 AM.

  • #2
    If the pitch is close enough it would probably work out just fine. Not much load on them.

    Comment


    • #3
      Originally posted by old mart View Post
      The change wheels are 24DP, 20PA.
      The 120 standard gear is approximately 5 3/32" diameter.
      I have just seen on eBay, a 125/127 gear, 5 1/16" diameter over the 127 which is from a Chester 920 lathe, most likely some type of metric tooth form.
      They are quite close in pitch, do you think I could get away with the mismatch?
      The wheel is cheap, and I would expect to carry out modifications to make it fit.
      I tried looking for that gear but only found a 1mod 127t gear. it appears to be a 20 degree pressure angle and your gears are most likely 14.5 degrees.

      you might be able to get away with running them together if one of them was plastic.. it would sort of wear in..

      Comment


      • #4
        As the meshing is adjustable, I just might get away with it.

        Doing a bit of maths and having enough space to move the gears axially and still get the cover on, I find with 120,125 and 127, I could obtain pretty good approximations of the following metric threads:

        12tpi with 127/120= 1.9999mm pitch

        15tpi with 127/125= 1.72mm pitch (not so good)

        16tpi with 127/120= 1.4999mm pitch

        20tpi with 127/125= 1.2499mm pitch

        24tpi with 127/120= 0.9999mm pitch

        36tpi with 127/125= 0.694mm pitch

        These common pitches without changing any other wheels. The wheels are not necessarily in that order, ie, 120/127 might be 127/120 etc.

        The original S&B gears are 20PA.
        Last edited by old mart; 08-20-2017, 01:23 PM.

        Comment


        • #5
          3d print one in nylon. It should work fine for cutting threads.

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          • #6
            I missed out the result

            32tpi with 127/125= 0.8065mm pitch

            32tpi with 127/120= 0.7499mm pitch.

            14tpi with 125/120= 0.17417mm pitch. (a better result than my first effort)

            That's more or less all the metric threads that I'd ever want.
            Last edited by old mart; 08-20-2017, 04:15 PM.

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            • #7
              Thread pitches vary in requirements. It's nice when internal and external thread have exactly the same pitch but in fasteners and connection threads the length of engagement is relatively short and there is some accommodation from the elastic and plastic properties of the threaded materials.

              Take for example, a steel nut on a steel bolt torqued to say 30,000 PSI. The Young's modulus of steel is roughly 30,000,000 PSI. Therefore if the relative lead error of the nut and bolt is less that .001" per inch of engagement the engaged thread flanks will load share. This is an over-simplification that doesn't consider nut dilation, pressure angle error, axis alignment, drunkness of lead and a host or other small factors each contributing or cancelling an effect on the fastener strength.

              What I take away from these technological peregrinations is: you can't make threads perfect - and - close enough is close enough. How much is close enough for thread lead error? From the above example, I suggested 0.1% relative lead error. 0.1% is a proportion and means the same in Metric or Imperial provided you don't change units in mid-calculation.

              If your desired lead is 1.8 mm (got your decimal point and 127 tooth counts right, old mart?) and the best you can do with the available gearing is 1.7417: 1.8 / 1.7417 = 1.033+ The desired error I proposed would be in the range of 1.001 and 0.999. Too much in theory but for non-critical low-stressed fasteners and connection. It will loosey-goosey probably be OK. There is narrative in the relevant standards addressing lead accuracy in far more authoritative detail.

              1.8 Metric is not closely proportionate to any preferred Imperial thread pitch when run through the usual 127/120 translation gearing. You don't state your lead screw pitch or the machine ratio so there is insufficient data to calculate a ratio for you. If you need to cit 1.8 Metric pitch on your lathe the solution can't be far. I worked out a single pair for my lathe but that probably won't work for yours.

              OTH, if both threads are cut on the same machine and there is no concern for interchangeability with other threaded components (for example, a bearing retainer tread fitted to a housing) lead errors are equal and compensating and therefore of no significant consequence.
              Last edited by Forrest Addy; 08-20-2017, 08:21 PM.

              Comment


              • #8
                33 and 34 tooth gears are introduced into the gear train before the gearbox on the Myford Super 7 to give (very close to) metric pitches.
                The point of a 127 tooth gear is that with change wheels (not a gear box) and an imperial lead screw it is possible to get exact metric pitches with absolutely no error at all.
                If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

                Comment


                • #9
                  Originally posted by Magicniner View Post
                  33 and 34 tooth gears are introduced into the gear train before the gearbox on the Myford Super 7 to give (very close to) metric pitches.
                  The point of a 127 tooth gear is that with change wheels (not a gear box) and an imperial lead screw it is possible to get exact metric pitches with absolutely no error at all.
                  Yep - that's so as 127 = 25.4 x 5 which is a pretty popular and metric to imperial and metric to imperial screw thread pitch for lathe change wheels.

                  Comment


                  • #10
                    There is no such thing as no error at all. First, any lead screw will have some error.

                    The thing that a 127 tooth gear does is it performs a theoretically accurate conversion between English and metric because one inch has been DEFINED as exactly 25.4mm with only zeros beyond that first decimal place. It is supposed to be an exact conversion. The 127 tooth gear works because 25.4 / 2 = 12.7, exactly and since 12.7 teeth is not possible we multiply that by a factor of 10 to get 127. That factor of 10 is taken back out by other gears in the train, usually a 100 tooth or a 120 tooth or sometimes even a 50 tooth and another one with an even number of teeth (2 X 50 = 100). But the lead screw error and all the other errors in the lathe will still be there.

                    Also, most lathes with quick change gear boxes will also use a 127 tooth gear, combined with one of the others I mentioned above, and also with some other changes to the gear train leading to the quick change box to allow you to cut metric threads with the quick change box. The math becomes more complicated and often you will need to use different additional gears for different metric pitches because the ratios between standard thread sizes are different in English and metric threads.

                    I have a 127/100 gear for my manual change gear SB9 and I don't think that any of the combinations of the standard change gears that are used for cutting metric threads will cut an English thread if used without it. Different math. But I was able to find gear combinations for all common metric threads.



                    Originally posted by Magicniner View Post
                    33 and 34 tooth gears are introduced into the gear train before the gearbox on the Myford Super 7 to give (very close to) metric pitches.
                    The point of a 127 tooth gear is that with change wheels (not a gear box) and an imperial lead screw it is possible to get exact metric pitches with absolutely no error at all.
                    Paul A.

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

                    Comment


                    • #11
                      At present, the S & B has only 48-120-48, the 120 used as an idler. I don't have any others.
                      I have toyed with the idea of producing the gears myself, on the mill with the rotary table.
                      The calculated difference in pitches between the S & B gear teeth and the Chinese ones is 0.008".

                      Comment


                      • #12
                        If those are the only gears you have got you will lose nothing from looking about for a complete set of gears off another lathe. So long as they can be made to fit your shafts (or the shafts adapted to fit the gears) it might be the cheapest and simplest route.
                        Peter - novice home machinist, modern motorcycle enthusiast.

                        Denford Viceroy 280 Synchro (11 x 24)
                        Herbert 0V adapted to R8 by 'Sir John'.
                        Monarch 10EE 1942

                        Comment


                        • #13
                          I have considered different gears, Myford ones are plentiful, but their 127 is 5/8" greater radius than the S & B equivalent. Some estimation of the available space will be carried out on Wednesday.
                          I have also thought about getting some aluminium gear blanks.

                          This is my fourth attempt to post this reply, despite being logged in each time.
                          Last edited by old mart; 08-21-2017, 04:27 PM.

                          Comment


                          • #14
                            I think that your math is showing "exact" conversions for most of these. The ones that end in all 9s come out as exact metric pitches on my calculator: 2mm, 1.5mm, 1.25mm, and 1mm. I am sure you can get "exact" conversions for all the metric pitches if you use just one compound (127:120) and add some additional gears on the stud or on the quick change box shaft.



                            Originally posted by old mart View Post
                            As the meshing is adjustable, I just might get away with it.

                            Doing a bit of maths and having enough space to move the gears axially and still get the cover on, I find with 120,125 and 127, I could obtain pretty good approximations of the following metric threads:

                            12tpi with 127/120= 1.9999mm pitch

                            15tpi with 127/125= 1.72mm pitch (not so good)

                            16tpi with 127/120= 1.4999mm pitch

                            20tpi with 127/125= 1.2499mm pitch

                            24tpi with 127/120= 0.9999mm pitch

                            36tpi with 127/125= 0.694mm pitch

                            These common pitches without changing any other wheels. The wheels are not necessarily in that order, ie, 120/127 might be 127/120 etc.

                            The original S&B gears are 20PA.
                            Paul A.

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

                            Comment


                            • #15
                              What I find most interesting is the availability of 10 of the commonest metric threads with either exact or passable pitches using only 120, 125 and 127gears. This would work with many lathes assuming that the same inch pitches were available.

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