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Re: Gear Cutting

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Posted by Dianne/">Dianne on December 22, 1998 13:41:25 UTC

: I have recently hobbed a gear using a 7/16-14 tap hoping for 240 teeth on a 5.460" dia. wheel. Unfortunately, the cut produced 244 teeth. Does anyone know the proper formula to use to find the correct outside dimension of the wheel? Any help will be greatly appreciated.

Gear diameters are measured at the "Pitch Circle", which is the point of contact between the teeth of mating gears. The O.D. (outside diameter) is not relevent but is dictated by tooth profile. The Pitch Diameter can be approximated as the circle which passes through the tooth at the shoulder. Unfortunately, cutting teeth with a tap will not give you the "coniflex" tooth shape used on spur gears. You would find that a mating gear will not turn - the pair will jam because the teeh interfere with each other as they try to move apart.

I built a 1/2 scale steam engine 3 years ago with bandsaw-cut teeth and, as long as the proper tooth shape is used, they work pretty good; though they will never run as smooth as professionally cut gears.

If you need gears and can afford a few $$, check out Boston Gear - they are on the Internet

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