The use of milling cutters in milling straight racks

Why can we use an 8th gear disc milling cutter with the same module when milling straight racks? In which situations are the vertical displacement method and the horizontal displacement method applicable? Let's take a look together with the editor below.

Because the tooth profile of the rack is a straight tooth profile, which is equivalent to the tooth profile shape of a spur cylindrical gear when the number of teeth tends to infinity, the profile of the 8th milling cutter of various moduli is manufactured according to the straight tooth profile. Therefore, the 8th gear milling cutter of the same modulus can be used to mill the straight rack.

The longitudinal displacement method is generally used to mill long racks, while the transverse displacement method is generally used to mill short racks.