Understanding the Key Features of Twist Drills and Their Impact on Machining From BMR TOOL GROUP

Understanding the Key Features of Twist Drills and Their Impact on Machining From BMR TOOL GROUP

In machining, understanding the critical characteristics of twist drills is essential for selecting the right tool for a given material. Different materials have varying requirements for the cutting tools used, making it important to choose the right twist drill for optimal drilling performance. The following sections will answer some key questions, including the definitions of secondary cutting edge and helix angle, and how they affect the machining process.

Twist Drill Key Features:

The eight key features of a twist drill include:

· Point angle

· Main cutting edge

· Web

· Clearance face (large and small)

· Flute shape

· Flute profile

· Core diameter

· Secondary cutting edge

· Helix angle

For optimal cutting performance across various materials, these features must work in harmony. We will compare three different types of twist drills to illustrate this point:

· HSS-G (High-Speed Steel Ground)

· HSS-Co5 (High-Speed Steel with 5% Cobalt)

· HSS-Co8 (High-Speed Steel with 8% Cobalt)

· Carbide Drill Bits(Solid Carbide with 10% Cobalt)

1. Point Angle (Top Angle)

The point angle is located at the tip of the twist drill and is the angle measured between the two main cutting edges at the top. A sharp point is necessary for the drill to stay centered in the material. A smaller point angle allows the material to center more easily and reduces the risk of the drill slipping on curved surfaces. A larger point angle reduces drilling time but requires higher contact pressure and makes positioning more difficult. From a geometric perspective, a small point angle requires longer main cutting edges, while a large point angle results in shorter main cutting edges.

2. Main Cutting Edge

The main cutting edge is responsible for the entire drilling process. Twist drills typically have two main cutting edges, which are connected by the web. A longer cutting edge generally offers better cutting performance, even though the difference in performance may be minimal compared to shorter cutting edges.

3. Web

The web, located in the center of the drill, has no cutting function but is an essential structural component that connects the two main cutting edges. The web helps to apply pressure and friction to the material, which, in turn, leads to increased heat generation and energy consumption, negatively impacting the drilling process.

4. Clearance Face (Large and Small)

The clearance face helps to reduce the web thickness at the top of the drill, which, in turn, decreases the friction within the material. This reduction in friction lowers the necessary feed force, which speeds up the drilling process. The most common forms of clearance faces are spiral points (Type N) and split points (Type C).

5. Flute Shape

Flutes are designed to provide a channel for chip removal. A wider flute profile facilitates better chip collection and removal. Poor chip removal leads to increased heat, which can cause workpiece annealing and eventually lead to drill breakage. Flute profiles come in flat or deep forms, with the depth of the flute influencing the thickness of the core. A flatter flute profile corresponds to a larger core diameter, while a deeper flute profile corresponds to a smaller core diameter.

6. Core Diameter

The core diameter of a twist drill is a critical factor in determining the tool's stability. Drills with larger core diameters offer greater stability, making them suitable for higher torque and harder materials. These drills are also more effective in handheld operations as they resist vibrations and lateral forces better. The core diameter increases from the tip to the shank to facilitate the efficient evacuation of chips.

7. Guide and Secondary Cutting Edges

The two guide edges are located within the flute grooves. When sharpened, these edges play an additional role in guiding the drill through the material. The quality of the hole wall depends significantly on the performance of the guide edges.

The secondary cutting edge forms the transition from the guide edge to the flute profile. It helps loosen and cut any chips that may adhere to the material. Both the guide and secondary cutting edge lengths are largely dependent on the helix angle.

8. Helix Angle

The helix angle is one of the most fundamental characteristics of a twist drill. It determines the formation of the chips during the cutting process. A larger helix angle (27°-45°) is more effective for removing softer materials, which tend to form long chips. On the other hand, smaller helix angles (10°-19°) are better suited for machining harder materials that produce short chips.
Twist drills with a very small helix angle (10°-19°) have a long spiral shape, while those with a larger helix angle (27°-45°) feature a more compact spiral. The most common helix angle for general-purpose twist drills ranges from 19° to 40°.

Summary

Now that you are familiar with the features of twist drills that influence the drilling process, you can more easily identify the right drill for your material. By comparing the unique features and functions of different twist drills, you will be able to make an informed decision about which tool to use for specific machining tasks.

The table below summarizes the most important characteristics for particular functions. Following the guidance provided will help ensure you select the optimal twist drill for your needs.

If you have any other questions about the twist drill bits, please feel free contact BMR TOOL GROUP.