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First Time Gear & Keyslot Cutting

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  • First Time Gear & Keyslot Cutting

    1. How can I determine the pressure angle so I can get the correct cutter?

    2. This gear is from a friend's Sumter magneto (made @ 1912), made of a fiber nonmetallic material. What would be a good current material to use for a new one? I'm told it's made of fiber to run quieter and prevent damage to the mag if it sticks, not for electrical isolation. The owner of the mag said aluminum would be ok. The material has to be able to hold a clean edge for the key slots.

    3. ID is 1/2".

    I have a lathe, mill, and dividing head.

    Any and all advice is welcome.

  • #2
    Measurements please

    First of all we need some measurements. Looks in section 14-1 of this manual to get a start, then provide the group with the appropriate numbers for advice.


    • #3
      You can get gear profile gauges or even compare it to a profile out of a good quality gear catalog but that one looks to far gone to get any reliable comparison.
      The shortest distance between two points is a circle of infinite diameter.

      Bluewater Model Engineering Society at


      • #4
        Mating gear

        If the mating gear is intact you can probably figure out the correct pressure angle from it.


        • #5
          bear brand or similar tufnol, [pheolic resin reinforced with cotton, remarkable stinky stuff, but its good]
          the usual pressure angle was 14 1/2 degrees, diametral pitch gears [20 degreee and module are later]
          thats the extent of what i can offer!
          Last edited by boslab; 09-19-2011, 07:38 PM.


          • #6
            You could try pressure forming a plastic gear, acetal etc.

            The technique is to cut the gear blank to size and mount that in a lathe, the matching gear is mounted on the tool post so that it can rotate against the gear blank.

            Bring the gear into contact with the blank and run the lathe at high speed.

            The gear running against the blank will make a rumbling sound until temperature from friction begins to soften the blank, at that point increase the pressure, repeat, increasing the pressure until the plastic gear is formed.

            It took me longer to type this than it would to make the magneto gear.


            • #7
              Do you have a local place that sells gears? If so, go there with the mating gear (the one pictured is too worn to decipher much from it) and have them help you determine what it needs. Then just buy a plastic or phenolic gear blank from them for cheap and simply deal with the bore to fit the shaft (and maybe face the gear to the proper width).

              Yea that's not as hobbyish, but if you were me that's what you'd do.


              • #8
                The one in your picture is 14 1/2 degrees pressure angle.

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


                • #9
                  Delrin is a good material to replace fiber, although it's a little stronger. Very easy to machine.


                  • #10
                    That manual is going to cost me a lot off sleep...

                    I'll try to get the mating gear and confirmation on the date of manufacture.

                    When did 20 degree PA replace 14.5?

                    Thanks everyone for all the options.


                    • #11
                      Originally posted by John Stevenson
                      The one in your picture is 14 1/2 degrees pressure angle.

                      Good eye!
                      Location: Saskatoon, Saskatchewan, Canada


                      • #12
                        And hear I thought that 14.5 replaced 20PA as most catalogs have a wider selection of 14.5 than 20 ...???
                        If you can get a hold of a full size pattern of either/or P.A. (14.5 or 20) it will let you know what you have, as they are definitely NOT interchageable!
                        I cut it twice, and it's still too short!


                        • #13
                          IIRC what I have read, 20* has less undercutting of the tooth profile than the 14.5* which becomes important in the smaller tooth count gears.

                          I would guess 14.5* having "more" could be because there is more need for those to be replaced? [Many catalogs I have seen are more 20* than 14.5* with some about the same.]

                          Again, IIRC, 20* is more "current".


                          • #14
                            Found this elsewhere. Dose it look right?
                            This goes along with RussZHC's assessment.


                            • #15
                              I sent the above diagram to the owner of the rest of the engine that has the mating gear to determine the pressure angle. I think I've seen enough references here and other places to believe that 14.5 is correct for the time it was produced. Here's what I got out of the manual link 914 provided. This method is presented on page 14-3 using only the outside diameter and number of teeth of a sample gear, formulas follow on page 14-4.

                              My OD=1.633
                              My NT=15

                              Pitch Diameter= (NT)OD/NT+2

                              Diametral Pitch=NT/PD

                              Whole Depth=2.157/DP

                              I realize these calculated dimensions are a bit compromised due to the condition of the sample gear, although the actual whole depth looks a lot like the calculated dimension. Probably the best way to do this is to get the dimensions of the mating gear and distance between the centers of it and the position of my gear. That's a 4 hour round trip.

                              According to these numbers I need a #7 cutter, but since my calculated DP is 10.42 and 10 is the closest DP I've found is 10 the one to use?