Here is the Tutorial.
Make 2 circles.
Make the profile of the gear tooth spaces.
Trim the circles side ways.
Now enter "bo" command. It will show the boundary creation dialogue. Click Pick point. This command is going to be used for creating the polyline. We can also use PEDIT command. But let's try something new.
Click inside the region and then press enter.
Now we have the polyline created. Remember bo command doesn't join the lines but creates a polyline bounded by a region. Therefore the lines and arcs are also there.
Make a circle of 50mm radius.
Extrude it by 15mm.
Now extrude the tooth polyline by 25mm.
Array>>polar array>> and select the tooth profile extruded to array and the center point be origin and no. of items let be 12. Click OK
Now subtract the tooth profile arrayed from the cylinderical disc and we have the gear body.