The gear addendum is created by the tool dedendum for a topping tool. Since topping tools which are also cutting the tip diameter are usual only for very small gears the dedendum of the tool is often bigger than the addendum of the basic rack haP *. So the basic rack addendum h aP * is defining the outside diameter of the gear. The outside diameter of the gear is
Alternatively, the input values for the basic rack addendum can be calculated
from the outside diameters by pressing the convert button 
beside the
field.
The sizing button 
will set the values of the basic rack addendum to the
values needed for constant tip clearance (see 11.1.2).
