There are a couple ways you could do it that I can think of offhand:

1). Create a NURBS circle with 50 sections. Select a cv, then select every other CV (select - skip - select - etc) until you have even selections around the whole circle. Uniformly scale the cvs out to create the cog shape. Lastly, run bevelPlus on it to create the cog geometry with the correct bevels, distance, etc. If you output your geometry to polygons when you bevelPlus, you can easily do booleans and whatever else you need on the inside of the cog.

2) Another way would be to create a poly cylinder, then create the trapezoidish tooth shape separately. Place it on top of the cylinder and scale it properly. Now, hit the insert key and move the tooth's pivot point to the middle of the cylinder. Hit "insert" again to get out of the edit pivot mode. Now just open up your duplicate options box, figure out the math of 360 divided by how many teeth you want....that's your # of duplications. Make sure it's set to duplicate along the proper axis, and duplicate. Now just boolean>union one piece at a time until you have all the teeth and the cylinder as one piece of geometry. Edit what you need from there.