So like here you can see Michael explain to Brian how to make a rack for the pinion...

http://moi3d.com/forum/lmessages.php?webtag=MOI&msg=1420.8

But really though, if you are designing for precision and manufacturing of real gears, there are things that need to be considered beyond simple modeling. Especially when translating radial motion to linear motion... things have to be engineered....

Anyway....