Gear Making Information

[Joe Johnson]

Okay, here we go...

As to the Drill -- it is a standard full depth 20 degree pressure angle, 12 tooth, 0.7 module gear. It mates wonderfully with standard gear, but I put a few thousandths of an inch extra in the the center distance because typically the gear make (and mount to a shaft) is not often as concentric and true running as I would like -- add even MORE if you are going to heat treat the gears or weld them to the shafts.

As to the Fisher Price -- it is essentially a standard full depth, 20 degree pressure angle 16 tooth 32 Diametral Pitch gear. The tooth from is not quite standard because it is made to the plastic tooth profile standard, but as a practical matter it mates will with standard gears.

As to heat treat, I have given up the practice if there is any way to help it. I also have tried to limit welding the shafts to the gears unless I can get a reasonable shaft diameter. It is a bother that can be avoided with some fore thought.

As to materials -- almost all fall under the 1/4 inch thick plate rule. Essentially if it is made from 1/4 inch plate you need not use SPI stock, nor do you have to bother with getting SPI stock to the supplier so that they can begin cutting.

You need to find a Wire EDM source. In my opinion, the are the only way to go. We have some sources here in Detroit that will make a gear for about 150% of what SPI costs -- add that included getting them in about 2-3 days! Find local source if you can. If you cannot, contact me & I will try to give you sources that you can more or less send an e-mail to with the gear data and your Master Card Number and they will send you a gear in a few days. If you have CAD files (typically IGES files) with custom gear profiles, you can get those burned in to steel or aluminum as well.

As to gear tooth strength, I use a VERY simple approximation. I calculate the max stress by modeling the tooth as cantilevered beam with a load at the end. The load is the tooth loading. The height of the beam is the height of the tooth. The cross-section of the beam is the thinnest section of the tooth. There are pretty simple formulas for max stress for this loading case.

For steel, you want to stay below 60-70,000 psi (assuming you and get a fairly good steel stock that has about 90,000 psi in its unheat-treated state).

The Lewis formulas are VERY conservative for our typical applications.

I have forgotten the rest of your questions...

All for now.

Joe J.