More on Gears - Checking

When measuring a gear, there's only a couple things the average hobbyist can measure. One is the tooth count and the other is the overall OD of the gear. It's easy to get the tooth count wrong, so if you're looking to replace a gear here's a way to double check:

Calculate a probable tooth size (use a calculator or look up the formula.) Use the tooth size to calculate the pitch circle diameter (PCD) of the gear.

Using a CAD/3D modeling program or anything that will draw a gear and a circle, draw a circle of the overall diameter. Draw a gear of the tooth size and PCD and center it on the circle. The tips of the gear teeth should touch the circle.