Please expert How to calculate gears and racks?

For a standard spur gear: (standard tooth height)

Top diameter of gear = (modulus+2)*number of teeth

(short tooth height)

Top diameter of gear = (modulus+1.6)*number of teeth

Helical gears can only be calculated according to the above sectional modulus, and if you need normal modulus, you have to measure the helix angle and multiply cosine value of end modulus by helix angle. In this case, you take a gear, measure out the maximum outer diameter and then follow the formula above to deduce the modulus, get the modulus and then check the standard modulus table to find the closest value, so that is the modulus of the gear.