The following equation can be obtained from the geometric knowledge of involute gear:
Where, α T is the end face pressure angle of dividing circle; α N is the normal pressure angle of dividing circle; β Is the helix angle of dividing circle; MT is the end module; Mn is the normal modulus; χ T is the modification coefficient of the end face; χ N is the normal displacement coefficient; DB is the diameter of base circle; Z is the number of teeth; H * at and h * an are the height coefficients of end face and normal addendum respectively; C * t and c * n are the end face and normal top clearance coefficients respectively.
The parameter MT necessary for diagramming the measuring ball diameter DP of helical gear can be obtained by the formula, α t， χ t，db，hat，ct。