[Exla357]

There is a simple way to do it on Solidworks 2009, but it takes a lot of time because gear mates have to be made for each "tooth"

Make a cylinder, on say the Top Plane. Now, create a new sketch on the same plane as a point for the extruded cut that will be the airspace between the gears (if that makes any sense). Now, make the extruded cut. In features, select the pattern tab and choose "Circular Pattern." Select the extruded cut, check "Equal Spacing." For Parameters, select one of the edges of the circle. Now, increase the number of features until you get what you like, than accept.

Hope that made sense.

-Alex