A voidance of undercutting or pointing in the toe and heel of the face-gear drives with helical pinion was mainly discussed in the paper.The expressions of inner radius and outer radius able to prevent undercutting or pointing under arbitrary shaft angle that formed by shaper and face-gear were also presented.Anew method was developed for solution of critical point of undercutting on the shaper tooth surface.Three unitless coefficients of inner radius

outside radius and tooth width were defined.The results show that these three coefficients depend on the magnitude of helix angle of the shaper and the number of teeth of spur helical pinion.