Cutting a useable gear for a magneto or crankshaft can be a daunting task if
you are a backshed machinist , self taught , like me.

(click on images for larger view)

When I only had a drill mill, I made this simple dividing head which uses gears as the index. Thus, number of teeth on the back gives you the same number at the front. Tooth size doesn't come into it.
I still use it in preference to the fullsize dividing head I now own as it is so much simpler to use for the one off gear that is usually needed. (I also use it to make cams for the model engines as a series of tiny flats do not matter in the running.)  

The size of the unit is not important although I have put a rule in a couple of pics to give an idea of mine. A small chuck is really the determining factor here.
The body is made out of a lump of hex bar and some square box sect and angle that was in my 'stock' at the time of building. The bore is reduced behind the bearings to give a shoulder for them to sit against.
There is a spacer between the two bearings in the main body to stop 'crush' on them when the index gear is tightened onto the shaft.
Again, use whatever you can find in the making.

The shaft section that the index gears lock onto is 5/8"dia. The gears I use are from motorbike gearboxes,etc. Anything that has the number of teeth I want. I just bush them to 5/8".



The flycutter, above, (A bit of square H/S lathe tool) is ground to the shape of
the root groove of the teeth being cut. (Usually you are making a replacement so you have a pattern). The toolholder is easily understood from the photos. A simple subtraction of the root diameter from the outside diameter gives
you the depth of cut. 


Detail of the index finger stop


I have made lots of gears out of nylon and similar which run very quietly. Alloy and brass also cut easier than steel and will surely outlast the maker in most cases!

The gear you finish up with is not as accurate in tooth form as a professional one but, it WILL do all you need and will bed in to its mate quite quickly. The money saved on the first gear will almost pay for the tool as well.