In terms of the tooth thickness, should we use the formulation with
respect to normal or transverse coordinate system? When normalizing
this thickness in order to normalize the backlash (backlash parameter),
we should divide by the circular pitch. Thus, when normalizing, should
this circular pitch be defined in the normal or traverse coordinate
system, depending on which formulation has been used? Is the backlash
parameter always defined with respect to the tangential plane or
normal plane for helical gears?