(1) Twist drill labeling reference system

Twist drill has a relatively complex shape and cutting part: in order to facilitate the labeling of its geometric parameters, it is based on the structural characteristics and working hours of the twist drill. For the motion characteristics, in addition to the base plane, cutting plane, and orthogonal plane, end planes, column profiles, and mid-sections are also used, which are defined as follows.

End plane: a plane perpendicular to the axis of the twist drill. This plane is also the back plane of any point on the cutting edge and is perpendicular to the base plane of the shop.

Cylinder section: The column section at any point on the main cutting edge is a cylindrical surface that passes through that point, with the radius of gyration of that point as the radius and the axis of the twist drill as the axis. It is tangent to the work plane at that point and perpendicular to the base plane at that point.

Medium section: the axial section passing through the axis of the twist drill and parallel to the two main cutting edges.

(2) Geometric angle of twist drill

1) Vertex angle 2Φ

It is the angle between the projection of the two main cutting edges in the mid-section. The smaller the vertex angle, the longer the main cutting edge, the load on the unit cutting edge is reduced, and the axial force is reduced, which is effective for the axial stability of the drill bit. And the tip angle at the outer circle is increased, which is beneficial to heat dissipation and improves the durability of the tool. However, the reduction of the vertex angle will weaken the strength of the drill tip and increase the chip deformation, resulting in an increase in torque. The standard twist drill head angle is approximately 118°. HSS high-speed steel drill bits: The vertex angle is generally 118 degrees, sometimes greater than 130 degrees. HM carbide drill bits: the vertex angle is generally 140 degrees; straight flute drills are often 130 degrees, and three-edged drills are generally 150 degrees.

Figure 2-4 The edge inclination angle, main declination angle k, rake angle, and rear foot a of the drill bit

1) Main declination angle and end edge inclination angle

Twist The leading angle of a selected point on the drill's main cutting edge is the angle between the projection of the main cutting edge on the base plane at that point and the drilling feed direction. Since the base surfaces of each point on the main cutting edge of the twist drill are different, the main declination angle of each point also changes accordingly. After the twist drill grinds the vertex angle 2Φ, the main declination angle of each point is determined. The relationship between them is

tan=tanΦcos Formula (2-2)

- ---The end face edge inclination angle of the selected point, which is the angle between the projection of the main cutting edge in the end face and the base surface of the point.

Since the absolute value of the edge inclination angle at each point on the cutting edge gradually increases from the outer edge to the drill core, the leading angle at each point on the cutting edge is also larger at the outer edge and smaller at the drill core.

2) Rake angle

The angle between the rake surface and the base surface in the orthogonal plane. The relationship between the rake angle of any selected point on the main cutting edge and the helix angle, main deflection angle, and edge inclination angle of that point is: