fewest teeth without undercut this gear tooth thickness at tip
The cutter is a straight-sided rack with 20° flanks: the simplest involute "gear" there is, with an infinite radius. Roll the blank against it, one pitch of rack travel per tooth, and whatever it sweeps through comes away. The involute flanks appear by themselves. Below 17 teeth the cutter's tips dig into the root and eat the bottom of the flank (undercut), which weakens the tooth and shortens contact. Shifting the cutter outward (positive profile shift) stops that, at the cost of a thinner, pointier tip.