Sunday, August 23, 2009

Gear Cutting

I decided to be cheap and make ALL of the transmission's gears myself. I'd machined a few gears in the past, but I figured I had more patience than before so it wouldn't be too bad. I started off by turning a "blank" for the gear. This was just a cylinder of the gear's outside diameter. Then I had to machine the teeth into the blank. This is the nasty part. There are a set of cutters meant for involute spur gears. There are 8 cutters required to cut any number of teeth for a particular tooth size. Each cutter has a range of teeth that it can cut. This is because of the gear's tooth profile. A standard gear has what is called an involute curved tooth. This curve is essentially the path taken by the end of a string that is unwound from a spool of thread. Gear teeth require this shape for their teeth in order to have a smooth and constant rotation rate.
Cutting the teeth requires a rotary table or spin indexer and one of the involute gear cutters.

The gears came out looking pretty nice. I'm a bit concerned for the 13 tooth gear. The involute gear cutters don't cut prefect profiles for all the numbers of teeth. 13 tooth gears have the worst profile. It should work in the transmission, but it will probably sound terrible.

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