Because the tooth groove of a helical cylindrical gear is a spiral groove, when milling with a gear disc milling cutter, the workbench must turn a helix angle to ensure that the spiral plane of the milling cutter is aligned with the direction of the coexisting tooth groove, that is, the tooth shape of the milling cutter should be the same as the normal tooth shape of the gear, that is, the tooth shape of the equivalent gear of a helical cylindrical gear. So when milling helical cylindrical gears, the tool number should be selected based on the equivalent number of teeth (i.e. equivalent number of teeth) of the gear.