BTW,
this guy published VERY useful parametric designs for all sorts of gears.. I used one of them for my dobson encoder (large helicoidal sector).
Also, I used
this design for my timing pulley (for encoder).. However, there was an error (IMO) in the design, the timing belt thickness was not treated in calculations as it should have, resulting in wrong teeth pitch (2.4% smaller.. I notified the author about this, and it is easy to correct the design to the pulley turns out OK).