how calculate gears from scratch

A technical problem I face is how do I figure out how to calculate gears from scratch in a assembly system. Meaning i have a space X between two centers and i need to gears to lock in with some ration how do I figure out Dedendum Addendum working depth and all the other stuff. I can figure out how to draw the involute curve but how can I figure out the rest. I know the number of teeth and gear rations are based on diameter of gear but whats the relation of the measurements to the center distances of two gears. Anybody have any idea how should I go about this?

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4 Answers

Outer diameter = Pitch diameter + 2 x module
Inner diameter = Pitch diameter - 2.5 x module

 
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I hate to bother you Aled here is what I need basically center to center distance is 18.9 mm I need to two gear top one 32 teeth bottom one 13 teeth my problem is figuring out the size of each gear and how to make the teeth match with each other.

 
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I found solution page 14. Only thing I dont get is how did he calculate that dp=20/3 and dg = 40/3 . Can someone explain?

 
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