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Metric conversion gear options

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  • Metric conversion gear options

    Anyone know of any conversion gear options for cutting metric threads for the Hendey Lathes? I purchased a metric conversion gear for my old SB Heavy 9 and that worked great. It was 3D printed and I know it wouldn't work for lots of production but it definitely fit the bill for small projects.

  • #2
    Since the inch is now defined as 25.4mm, a set of gears that embodies that ratio is what is used. 254 teeth on one gear and 100 teeth on the other one will do it. But 254 is a large number and we can divide it by 2: 100 also divides by the same factor. So 127 on one gear and 50 on the other is just one possibility.

    In this process it is the prime numbers that count and 127 is prime so we can not go any lower for an exact conversion. But we can use several different gears for the 100/50 tooth side. Another that is sometimes used is 120. So the three most commonly used conversion pairs are: 127-50, 127-100, and 127-120. All of these have the needed prime number, 127. Also some metric threads can just use the 127 tooth gear with other, existing gears supplied with the lathe.

    You do not say if your Henley has a quick change gear box or if it uses manual change gears. Manual change gears are a lot more versatile while a quick change box for English/inch threads will have steps that are not metric friendly. In either case someone needs to do the calculations. And, while the 127 tooth will remain necessary, there will be different lists of possible metric threads depending on the second gear chosen or the other gears originally supplied with the lathe. This is why one manufacturer may recommend one metric conversion set and another manufacturer would recommend another. Hopefully you can find a metric chart for your particular lathe as that can answer a lot of questions.

    But the 127-xxx is for an exact conversion. There are other gear pairs, and these are ALWAYS PAIRS, that can provide a very good approximation. One such set is 47-37 which provides a conversion factor of 1.27027... which differs by only 0.02%. It is good enough for most threading purposes. This is the smallest, commonly known ratio. I do not believe any set with fewer teeth will work as accurately. Another, somewhat common set is the 80-63 providing a conversion ratio of 1.26984.... But these have more teeth and may not fit in all cases.

    One disadvantage of using any approximate conversion SET of gears is that it must be used as a set of the two gears. That means that the 127 tooth gear or one that approximates that number, can not be used by itself. I have a 127-100 gear set for my SB which has manual change gears. In spite of having a full set of the OEM change gears plus a couple of additional ones and three compounds that provide many times the original ratios, there are some metric threads that I can only cut with the 127 tooth gear in use without the 100 tooth one.

    Just a quick internet search:

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    More information on your lathe and which change gears you have for it would help here.
    Last edited by Paul Alciatore; 08-08-2022, 10:18 PM.
    Paul A.
    SE Texas

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


    • #3
      Go to the Internet Archive, download a copy of 'Screwcutting in the lathe' by Martin Cleeve. On pages 48-49 it lists thirty approximate ratios. What will suit you depends on whether the machine is change gear or gearbox (gearbox limits your options considerably), the pitch of the leadscrew, which has an influence and the physical space available (e.g. many lathes cannot use 127 as it will not allow the cover to close).


      • #4
        My lathe has a quick change gear box. Thanks for the explanation. I was hoping that making a couple of gears might work for metric threading.

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        • #5
          My lathe and a few other imported lathes use a 91- 86 joined gear along with 45 and 36 teeth gears for the metric conversion gear. Then there's a table given with the lathe on which gear box settings are needed to find the typical metric thread sizes. These being scattered throughout the range of possible pitches. It's somewhat more compact than the 100-127 middle gear setup.
          Chilliwack BC, Canada


          • #6
            Depending on how your gear train is set up you may be able to change a stud gear for one with a different tooth count that would get to pretty close to metric pitches using your QCGB. I worked it out for my SB heavy 9 and it will only take a couple of gears and I'll be able to hit all the regular metric pitches.


            • #7
              Originally posted by skipd1 View Post
              My lathe has a quick change gear box.
              There is a good collection of Hendey literature at That will tell you exactly what the manufacturer did for your machine model. It wll also show the screwcutting plate for the machine.