https://www.youtube.com/watch?v=uj7BnldaOSc

Modeling an Equation Driven Involute Spur Gear in Solidworks

The method given in this video works when diameter of base circle is bigger than that of the dedendum circle.
When the number of teeth is increased to big enough, the base circle diameter will become smaller than than the diameter of dedendum circle. Then the gear profile will be all involute. One can use similar approach given in this video for such case.
The tutorial file for this video can be downloaded from
https://drive.google.com/file/d/0B5meorzDrBfRUVAwQk1VeG1mVkk/view?usp=sharing