1. Design and manufacture of four-blade twist drills, as shown in Figure 1. In addition to adding two spiral cutting edges on the drill back, the other is exactly the same as the general twist drill, only need to choose a suitable thickness. A concave arc milling cutter that guarantees the width of the two front and rear belts, mills out the drill back?, leaving four spiral cutting edges, and then milling the chamfer with a 45° milling cutter.