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Gear cutting

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  • Gear cutting

    I have a small TOS instrument lathe (3" x 12"). It is a metric machine with metric leadscrew. It uses change gears and I have a set of original change gears to allow me to cut metric threads from minute up to 3mm pitch. The change gears are MOD 1 and are made of steel. I would like to be able to single point cut the occasional imperial threads and for this I will require some additional change gears in the range of 56 to 72 teeth. Buying original TOS gears is difficult and expensive.

    I was thinking that I could cut the gears out of delrin. I have the opportunity to buy cheaply a MOD 1 involute gear cutter which is for "135 and rack". I have the required dividing equipmet with the lathe. So my questions

    1) Is delrin suitable?

    2) I know that that this is not strictly correct, but if I use this cutter, will the gear teeth run together satisfactorily given that I can adjust the spacing on the quadrant and that it will be only for occasional use and I can take repeated light cuts?

    You advice and opinions would be much appreciated.


  • #2
    "135 to a rack" means that the teeth flanks will be straight (as a rack is) and will be meshing with involute teeth.

    Its your call and your risk.


    • #3
      Whilst not correct to use that cutter it will work, especially in Delrin.
      Gear hobbing machines use a train of gears just like screwcutting with one exception, if you want to cut a prime number you need that prime in the gear train.
      A bit of a chicken and egg situation, I have seen it where the operator has fly cut a gear using a triangular carbide insert in alloy giving him pointed 60 degree teeth on the *gear*
      This is then fitted to the machine so a new prime gear can be hobbed before the gear that is required.

      In your case have you looked to see what is available on the C3 lathe from places such as ARC? These are Delrin gears in MOD1 and they sell two sets for imperial or metric screwcutting, just saying they may have some of what you need for not much money.

      EDIT to Tiffies post.
      The 135 to a rack cutter will not have straight sides, they will have an involute shape to them that matches a 135 gear.
      If you want straight sides you have to buy a rack cutter, rare but they are made.

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


      • #4
        Thanks for the pick up John.

        But as an appreciation of scale, 1 MOD gear with 135 teeth will have a pitch diameter of 135mm (~5.3"), and an outside diameter of 137mm (~ 5.48") and a tooth working depth of 2mm (~ 0.08").

        As the tooth thickness at the pitch lines/circle will be the same for all 1 MOD gears, there is a risk of the top of the gears cut with the "135" not slotting in easily as the crown of the 135 gears may be "too wide".

        Its not so long ago that I had a Sieg 6" (C3?) lathe that was complete with all its Delrin gears. I tossed it out - gears and all - as the speed controller failed.


        • #5

          I haven't checked, but probably have closer MOD1 cutters. If you're only cutting delrin, I could lend you one for a week or so.
          I'm near Warrington.

          John's suggestion might be easier, though.



          • #6
            Hello all,

            Many thanks for the replies.

            John: I was hoping that you would confirm that the 135 cutter should work I will go ahead and buy it (it's only £ 8 !) and give it a try.

            Tim: Many thanks for your kind offer. I have never cut a gear in my life. I would be very wary of using someone else's cutter given my state of ignorance. A preliminary look at the possible set-up on my lathe reveals that it will be a tight squeeze and I would hate to run your cutter into something that it's not supposed to cut!

            I am still ploughing through the Gears and Gear cutting book. This is a pass time, I have the delrin stock, so I will take my time and see how it goes.

            Thanks and best regards,



            • #7
              Ian -

              I'm sure you know that there are UK suppliers who sell stock MOD 1 gears in a variety of different materials that would probably work for you with a bit of modification to the hub/bore. Having said that, it is interesting to cut your own gears and in this case, even including the cost of the cutter it would be cheaper to make your own.


              • #8
                Hello Bill,

                Yes, I have actually bought one 80 tooth steel gear from HPC Gears. This was to let me cut a 4mm pitch thread, so I thought steel would be appropriate as the standard set only goes to 3mm and I suspect that the gear train will be under a bit of strain with a 4mm pitch. HPC were nice people and very fast delivery, but a bit pricey if you need a few gears, even if in cheaper Delrin. I have some Delrin and would like to give gear cutting a try. We'll see how it goes.