Scale the teeth 0.8 units in the Local Scale X & Y axis (Fig.09).
Rotate the cog 90° on the Z axis to stand it up. Delete History and Reset Transformation (Fig.10 - 11). And the first cog is complete!
It is important to remember this cog has 20 segments (10 teeth) and a radius of 10 units. It is important to remember that all future cogs will be multiples of these numbers.
This means a cog twice the size will have 40 segments, a radius of 20 units and 20 teeth. A cog half the size will have 10 segments, a radius of 5 units and 5 teeth. If you follow this simple rule, the gears will easily fit together and it will be easier to calculate the rotation values.
Now you understand how a cog is created, delete the first cog and we'll start again.
Creating the Prototype Cogs
Now that you know how to create a basic cog, create four new cogs (small, medium, large and extra large). These cogs will form the basis of your gear system by matching them together in different combinations (Fig.12).
Check they have the correct Radius and Subdivision values, and then rename them.
Select and extrude the alternate faces around the circumference to create the teeth (Fig.13).