Let’s take a look at the eight basic features of a twist drill: point angle, main cutting edge, chisel edge, major and minor relief faces, flute, flute contour, core, secondary cutting edges, and helix angle.

To achieve the best cutting performance in different materials, these eight features must work in harmony.

To illustrate, we will compare the following three twist drills:

Twist Drill HSS-G

Twist Drill HSS-Co8

Twist Drill HSS-Co5

1. Point Angle
The point angle is located at the tip of the twist drill and is measured between the two main cutting edges at the top. A sharp angle is necessary to center the drill in the material.

A smaller point angle makes it easier to center the drill, even on curved surfaces, reducing the risk of slipping. A larger point angle shortens drilling time. However, a smaller angle requires higher contact pressure and is harder to position. Geometrically, a smaller point angle requires a longer main cutting edge, while a larger point angle allows a shorter main cutting edge.

2. Main Cutting Edge
The main cutting edges are responsible for the entire hole-making process. Compared with shorter edges, longer cutting edges provide higher cutting performance, even if the difference is slight. A twist drill always has two main cutting edges connected by the chisel edge.

3. Chisel Edge
The chisel edge is located at the center of the drill and does not contribute directly to cutting. However, it is an essential structure because it connects the two main cutting edges.

The chisel edge penetrates the material and applies pressure and friction. These characteristics can be unfavorable during drilling, increasing heat generation and energy consumption.

4. Major and Minor Relief Faces
Relief thinning reduces the chisel edge at the top of the twist drill. This thinning significantly reduces friction in the material, thereby reducing the required feed force. Relief thinning is decisive for material centering and improves drilling speed.

The most common shapes are split point (C-shape) and standard point (N-shape).

5. Flute Contour
The function of the flute is to provide a channel for chip removal. The flute contour facilitates chip collection and discharge.

The wider the flute, the better the chip collection and removal. Poor chip evacuation generates high heat, which can cause annealing and ultimately drill breakage.

A wide flute has a shallow contour, while a narrow flute is deep. The flute depth determines the core diameter. A shallow flute results in a larger core diameter; a deep flute results in a smaller core diameter.

6. Core
The core thickness is decisive for drill stability.

Twist drills with a larger core diameter are more stable, making them suitable for higher torque and harder materials. They are also ideal for handheld drills because they resist vibration and lateral forces.

To facilitate chip evacuation through the flute, the core size should gradually increase from the tip to the shank.

7. Secondary Cutting Edges and Guiding Edges
Two guiding edges are located at the flutes. Sharpened edges provide additional cutting on the hole sides and guide the drill during operation. The quality of the hole wall depends largely on the guiding edges’ performance.

Secondary cutting edges form the transition from the guiding edges to the flute contour. They help loosen and cut chips sticking to the material.

The length of the guiding edges and secondary cutting edges largely depends on the helix angle.

8. Helix Angle
The helix angle is a fundamental characteristic of a twist drill. It determines the chip formation process.

A larger helix angle effectively removes soft materials, which tend to form long chips. Conversely, a smaller helix angle is used for hard materials that produce short chips.

Twist drills with a very small helix angle (10°–19°) have a long spiral. Drills with a large helix angle (27°–45°) have a short (stamped) spiral. Ordinary twist drills have a helix angle of 19°–40°.