Helix Angle β: The helix angle is the angle between the straight line formed by unfolding the outermost helix of the drill's helical groove and the drill axis. Since the lead of the helix is the same at all points on the helical groove, the helix angle varies with the diameter of the drill; the helix angle is largest at the outer diameter and decreases as it approaches the center. Increasing the helix angle increases the rake angle, which is beneficial for chip removal, but reduces the drill's rigidity. The helix angle of a standard twist drill is 18°–38°. For smaller diameter drills, a smaller helix angle should be used to ensure the drill's rigidity.
Rake Angle γOm: Because the rake face of a twist drill is a helical surface, the rake angle varies at different points on the main cutting edge. From the outer diameter to the center, the rake angle gradually decreases. The rake angle at the tip is approximately 30°, while near the chisel edge it is around -30°. The rake angle on the chisel edge is -50° to -60°.
Clearance Angle αOm: The clearance angle at a selected point on the main cutting edge of a twist drill is represented by the feed clearance angle αOm in the cylindrical section through that point. The cylindrical section is the cylindrical surface formed by a line parallel to the drill axis passing through the selected point m on the main cutting edge, rotating around the drill axis. αOm also varies along the main cutting edge, increasing as it approaches the center. The clearance angle α at the outer diameter of a twist drill is usually 8°–10°, and the clearance angle at the chisel edge is 20°–25°. This compensates for the reduction in the actual working clearance angle at each point on the main cutting edge due to the axial feed motion of the drill, and is also adapted to the change in the rake angle.
Principal Cutting Edge Angle κrm: The principal cutting edge angle is the angle between the tangent to the selected point m on the main cutting edge in the base plane projection and the feed direction. The base plane of a twist drill is the plane containing the drill axis and passing through the selected point on the main cutting edge. Since the main cutting edge of the drill does not pass through the axis, the base plane is different at each point on the main cutting edge, and therefore the principal cutting edge angle is also different at each point.
Once the point angle is ground, the principal cutting edge angle at each point is also determined. The principal cutting edge angle and the point angle are two different concepts. Point Angle 2φ: The point angle is the angle between the projections of the two main cutting edges on a plane parallel to them. A smaller point angle facilitates penetration into the workpiece, reduces axial resistance, increases the working length of the cutting edge, and reduces the nominal thickness of the cutting layer, which is beneficial for heat dissipation and improving tool durability. However, if the point angle is too small, the drill bit strength is weakened, deformation increases, torque increases, and the drill bit is prone to breakage. Therefore, a reasonable point angle should be ground according to the strength and hardness of the workpiece material. The standard twist drill has a point angle 2φ of 118°.
Chisel Edge Angle ψ: The chisel edge angle is the angle between the projections of the main cutting edge and the chisel edge on a plane perpendicular to the drill bit axis. When the back face of the twist drill is ground, ψ is naturally formed. As shown in Figure 3-5, increasing the chisel edge angle ψ reduces the length of the chisel edge and the axial resistance. The standard twist drill has a chisel edge angle of approximately 50° to 55°.
