gcode for cutting 12 pitch spur gears?

Hi guys:

I have the need for cutting these
Imperial 12 pitch spur gears for a
gear-reduction CNC project:

- normal exterior spur gears:
1.5" dia = 18 teeth
4" dia = 48 teeth
5.25" dia = 63 teeth

- interior "ring gear"
8" dia = 96 teeth
12" dia = 144 teeth

I thought I'd ask if anyone has a methodology to
write the gcode to mill these, before I go off and
start another mini-project to figure it out...

best regards
Doug Fortune