gear cutting
There are probably a lot of better ways to cut gears, but this how I make mine and so far they have worked just fine for every cirkut I have ever used.
You need a metal lathe and a milling machine fitted with a dividing head and tailstock. As well you need the correct gear cutter installed on the mill and centered to the tailstock.
You will need to make a special arbor shaft that allows a rough gear blank with a ¼ reamed hole to be added. I have a 5/8 shaft turned down to ¼ inch and then threaded on the end with ¼ 28 thread. After I made it, I hardened the steel. The arbor shaft is center drilled on both ends so it can be set up with a face plate and lathe dog to turn it.
I rough cut my brass gear blank with a band saw and drill an undersized ¼ hole which I ream to exactly ¼ inch. Then the blank is put on the arbor and turned down on the lathe to the right outside diameter. Then it is moved to the dividing head on the mill and the teeth are cut. After the gear is cut and before it is removed from the arbor, it is put back in the lathe and lapped with very fine grinding compound using a spare prime number gear with a steel shaft. I simply hold the lapping gear shaft in my hands and mesh it into the fresh gear. My lathe can run in reverse so I lap in both directions. This removes edge burrs from the gear cutter and just evens thing out a bit all over. Final polish is with tripoli.
My arbor shaft allows for five blanks to be cut at once, or with smaller spacers I can cut just one gear as well.
So that covers the basics, but there is some great math needed to calculate the outside diameter of the gear blank, the depth to set the gear cutter, and which dividing plate to use for the right number of divisions. It isnt difficult, but you have to think about it all the time. I made more than a few gears that came out goofy and had to be re-cut to make a gear with fewer teeth before I figured it out. Mostly, as with anything, it just takes patience.
If you are curious about the math, it goes something like this:
If you need a gear with a specific number of teeth say 37, add one to the number needed (this gives 38 in our example). Then divide by the pitch (cirkuts normally use 32 pitch except for #5 and #6 cameras, they use 48 pitch) That will give you the outside diameter of the gear blank. 2 divided by the pitch number gives the depth to run the cutter, but you also need to go slightly deeper for clearance. (I go 5% 7 %, and sometimes less, this is where the practice comes in because of the range of teeth each cutter can make causes some fudging to be required) There are plenty of books telling you how to use a dividing head, I am always amazed at what plate and number of holes end up with the division you really need, it just never seems right but it works.
Hope this helps.
Ron
