# Illustration: Basic knowledge of twist drills used in precision parts processing on lathes

Twist drills are the most commonly used drilling tools in precision parts processing on lathes, and drill bits are generally made of high-speed steel. The working part of the twist drill is composed of a cutting part and a guiding part. Sharpening the cutting edge is the most basic technology for drilling, and it is also a difficult technology for machining various materials.

Drilling with a twist drill requires sharpening the twist drill. The geometric shape of the components of the twist drill is relatively complex, and limited by the shape of various types of holes in the workpiece, various angles need to be ground on the main cutting edge.

Understanding and understanding the geometric shape of twist drills has a great effect on the sharpening of twist drills and the correct use of twist drills. The top angle, rake angle, back angle, chisel edge, edge, etc. of the twist drill will also be appropriately corrected. And use.

1. Rake angle

The rake angle of any point on the main cutting edge is the angle between the base surface passing through that point and the rake surface. In the processing of precision parts, the rake angle of twist drills is related to factors such as helix angle, apex angle and core diameter, among which the helix angle is the most influential. Because the helix angle changes with the size of the diameter, the rake angle of each point on the main cutting edge also changes. The rake angle is the largest near the outer edge, and it gradually decreases from the outer edge to the center, about within 1/3 of the drill diameter. It starts with a negative rake angle and the variation range of the rake angle is +30°~-30°.

2. Back corner

The relief angle of any point on the main cutting edge is the angle between the cutting plane passing through that point and the main relief surface. The back angle also changes, the smallest near the outer edge, the largest near the center, and the range of change is 8°~14°.

3. Chisel

Refers to the intersection line of the two main flank surfaces, that is, the connecting line of the two main cutting edges. If the chisel edge is too short, it will affect the strength of the twist drill’s drill point; if the chisel edge is too long, the axial force will increase and the drilling will be weak.

4. Edge

The edge is also called the ligament, which is the guiding part of the twist drill. During the cutting process, the drilling direction can be saved, the hole wall can be modified, and the back part as the cutting part can be saved. The damage of the edge will lead to ineffective drilling, and in severe cases, the drill bit may be broken during the processing of precision parts.