(1) Helix angle β Helix angle is the angle between the outermost helix on the spiral groove of the drill bit and the axis of the drill bit after unfolding into a straight line. Because the lead of each point on the spiral groove is the same, the helix angle at different diameters of the drill is different, the helix angle is the largest at the outer diameter, and the helix angle is smaller as it is closer to the center. Increasing the helix angle increases the rake angle, which is conducive to chip removal, but the rigidity of the drill bit decreases. The helix angle of standard twist drill is 18°～38°. For drills with smaller diameters, the helix angle should be smaller to ensure the rigidity of the drill.
(2) Rake angle γOm [Because the rake surface of the twist drill is a spiral surface, the rake angle of each point on the main cutting edge is different. From the outer circle to the center, the rake angle gradually decreases. The rake angle at the tip of the knife is about 30°, and it is about -30° near the chisel edge. The rake angle on the chisel edge is -50°～-60°.
(3) Clearance angle αOm [The relief angle of the selected point on the main cutting edge of the twist drill is expressed by the feed relief angle αOm in the point column profile. The column profile is a straight line parallel to the axis of the drill bit through the selected point m of the main cutting edge, and a cylindrical surface formed by the rotation of the straight line around the axis of the drill bit. αOm also varies along the main cutting edge, the closer to the center the αOm increases. The relief angle α at the outer circle of the twist drill is usually 8°～10°, and the relief angle at the chisel edge is 20°～25°. In this way, it can compensate for the effect of the reduction of the actual working clearance angle of each point on the main cutting edge due to the axial feed movement of the drill bit, and it can adapt to the change of the rake angle.
(4) Entering angle κrm The entering angle is the angle between the projection of the tangent to the selected point m of the main cutting edge on the base plane and the feed direction. The base surface of the twist drill is the plane that passes through the selected point of the main cutting edge and contains the axis of the drill bit. Since the main cutting edge of the drill does not pass through the axis line, the base surface of each point on the main cutting edge is different, and the entering angle of each point is also different. When the vertex angle is ground, the entering angle of each point is also determined. Entering angle and vertex angle are two different concepts.
(5) Frontal angle 2φ The frontal angle is the projected angle of the two main cutting edges on a plane parallel to it. A small front angle is easy to cut into the workpiece, the axial resistance is small, and the working length of the cutting edge is increased, and the nominal thickness of the cutting layer is reduced, which is conducive to heat dissipation and improved tool durability; if the front angle is too small, the strength of the drill will be weakened. The deformation increases, the torque increases, and the drill bit is easy to break. Therefore, a reasonable front angle should be sharpened according to the strength and hardness of the workpiece material. The front angle 2φ of the standard twist drill is 118°
(6) Chisel edge bevel angle ψ Chisel edge bevel angle is the angle between the main cutting edge and the chisel edge projected on a plane perpendicular to the axis of the drill bit. When the flank of the twist drill is ground, ψ is formed naturally. It can be seen from Figure 3-5 that the chisel edge angle ψ increases, the chisel edge length and axial resistance decrease. The bevel angle of the chisel edge of a standard twist drill is about 50°～55°.