‘ Pitch circle‘ is the diameter of a pair of plain cylinders that would have the same gear ratio and centre-to-centre distance as the intended gears.In fact, if they are different diameters, to mesh perfectly they will have different shaped teeth. This does not mean that they will have the same shaped teeth. To mesh properly, involute gears only need two things in common: modulus and pressure angle.Involute is a type of shape, very important to most gears, and easily drawn.Now for some other useful gear facts if you are to diy:

However, it does not provide a format that DesignSpark Mechanical can directly import.

Gear generator is an on-line gear generator websites by Abel that is easy to use, and you can download CAD files from it for a small payment.

The author, Matthias Wandel, will also sell you a programme to run on your Windows computer. This one handles ‘undercut’, which removes the gear-interference problem that prevents the easy creation of gears with fewer than 17 teeth.

Woodgears is an excellent on-line gear generator website which is very flexible (frankly, amazingly flexible) and has this excellent associated video.

Also, there are two that will design gears for you.

CAD package Fusion 360 will draw gears for you – so you might want to stop reading here, and download that instead.

The slides mention Shigley’s Mechanical Engineering Design, which maybe what they are based on.

The best diy gear information source, of all the many many videos, websites and documents that I read to design my gear pair, this presentation was far clearer than the rest, and had extra information.

Designspark mechanical gears