Understanding Drill Bit Angles: Helical Angle and Rake Angle

Drill bits are essential cutting tools, and their geometry plays a critical role in how effectively they cut various materials. Several angles define the cutting performance of a drill bit, including the helical angle, rake angle, lip angle, and web angle. This discussion focuses on the relationship between the helical angle and the rake angle.

What is the Helical Angle?

The helical angle, also known as the spiral angle, is the angle of the chisel edge relative to the axis of the drill bit. Imagine the flutes of the drill bit as threads; the helical angle is essentially the lead angle of these threads. It determines how quickly chips are guided up the flutes and away from the cutting zone.

• A larger helical angle means the flutes spiral more steeply. This is good for softer materials like aluminum, allowing chips to evacuate quickly.
• A smaller helical angle means the flutes spiral less steeply. This is preferred for harder materials like steel or cast iron, providing a stronger cutting edge.
• A zero helical angle would mean straight flutes, often used for specific applications like drilling brass.

What is the Rake Angle?

The rake angle is a primary cutting angle on the drill bit. It is the angle between the face of the cutting lip (the flute surface) and a plane perpendicular to the axis of the drill. The rake angle significantly influences the cutting action and the force required for drilling.

• A positive rake angle helps shear the material away efficiently.
• A zero rake angle scrapes the material.
• A negative rake angle pushes the material away.

For standard twist drills, the flute surface behind the cutting lip forms the rake face, and the helical shape of the flute directly determines this angle.

How Helical Angle Determines Rake Angle

The helical angle of the drill bit directly determines the effective rake angle at the cutting edge. The inclination of the helical flute surface relative to the drill axis dictates how the material is sheared as the cutting lip engages. Specifically, the helix angle is essentially the rake angle measured along the circumference of the drill.

Think of it this way: As the drill rotates and feeds into the material, the cutting edge, formed by the intersection of the flank and the helical flute, encounters the workpiece. The angle at which the helical surface (the rake face) meets the workpiece relative to the direction of cut is the rake angle. Because the flute spirals at the helical angle, this spiral angle directly establishes the rake angle at the periphery of the drill.

Analyzing Other Options

Let's consider why the other options are not directly determined by the helical angle:

• Cutting Angle: This term can be ambiguous but often refers to the point angle (the angle between the two cutting lips) or includes the clearance angle and rake angle in combination. The helical angle influences the rake angle, which is part of the overall cutting geometry, but the point angle itself is set separately and isn't determined solely by the helix angle.
• Lip Angle: This typically refers to the point angle, the angle between the two cutting lips at the tip of the drill (usually $118^{\circ}$ for general purpose drills). This angle is determined by the grinding of the point, not the helical angle of the flutes.
• Web Angle: The web is the thin central section of the drill bit that separates the flutes. The web angle refers to the thickness of the web as it approaches the point. While related to the drill's strength, the web angle is primarily determined by the manufacturing process of the web tapering, not the helical angle of the flutes.

Therefore, the helical angle's primary geometric consequence is on the rake angle of the cutting edge.

Conclusion

Based on the geometry of a twist drill, the helical angle is directly responsible for establishing the rake angle at the cutting edge along the periphery of the drill. This relationship is fundamental to understanding how drill bits function and how different helical angles are chosen for different materials.

Revision Table: Drill Bit Terminology

Additional Information: Helical Angle Variation and Applications

Different materials require different helical angles for optimal performance. Drill bits are manufactured with varying helix angles to suit specific applications:

• Slow Helix (Small Helical Angle, e.g., $10^{\circ} - 19^{\circ}$): These drills have a smaller rake angle, providing a stronger cutting edge. They are suitable for hard and tough materials like high-strength steel, cast iron, and some plastics, where chip formation is segmented.
• Standard Helix (Medium Helical Angle, e.g., $20^{\circ} - 35^{\circ}$): This is the most common type for general-purpose drilling in a wide range of materials, including mild steel, aluminum, and brass. It offers a good balance between chip evacuation and cutting edge strength.
• Fast Helix (Large Helical Angle, e.g., $35^{\circ} - 48^{\circ}$): These drills have a larger rake angle and promote rapid chip evacuation. They are excellent for soft, ductile materials like aluminum, copper, and magnesium, where chips are continuous and need to flow away quickly.

Selecting the correct helical angle for the material being drilled improves cutting efficiency, reduces heat buildup, prolongs tool life, and results in a better hole finish.