Although the geometry of twist drill is more reasonable than flat drill, it still has the following shortcomings:
(1) The rake angle value at each point on the main cutting edge of a standard twist drill is too different from the inside and outside. The rake angle of the main cutting edge at the outer edge of the bit is about +30°; and near the core, the rake angle is about -30°. The rake angle near the core is too small, resulting in large chip deformation and high cutting resistance; and near the outer edge When the rake angle is too large, the cutting edge strength is often insufficient when machining hard materials.
(2) The chisel edge is too long, and the rake angle of the chisel edge is very negative, reaching -54°～-60°, which will generate a large axial force.
(3) Compared with other types of cutting tools, the main cutting edge of standard twist drills is very long, which is not conducive to chip separation and chip breaking.
(4) The secondary clearance angle of the secondary cutting edge at the land area is zero, which causes the friction between the secondary flank face and the hole wall to increase, the cutting temperature rises, the wear at the corners of the outer edge of the drill bit is greater, and the surface roughness of the processed surface deteriorates .
The above defects often cause the twist drill to wear quickly, which seriously affects the efficiency of drilling and the improvement of the quality of the processed surface.