Understanding Twist Drill Geometry: The Chisel Edge Angle

When working with tools like a twist drill, understanding its specific geometric features is crucial for effective machining. The question asks to identify a particular angle formed on the twist drill at its point, specifically the angle located at the web between the chisel edge and the cutting lip. Let's explore the geometry of a twist drill to pinpoint this angle.

Key Angles of a Twist Drill

A twist drill has several important angles that define its cutting action and performance. Understanding these angles helps in selecting the right drill bit and setting up machining operations correctly.

• Point Angle: This is the angle formed at the very tip of the drill, typically measured in the plane of the drill's axis. It's the angle between the two primary cutting lips. A common point angle for general-purpose steel drilling is around 118 degrees, though this can vary depending on the material being drilled.
• Helix Angle: This refers to the angle of the spiral groove (flute) along the body of the drill. The helix angle influences how efficiently chips are evacuated from the cutting zone and affects the drill's cutting characteristics.
• Chisel Edge Angle: This is the angle formed at the center of the drill point, where the two cutting lips meet. It is located at the central thickened part known as the web. The chisel edge itself isn't a primary cutting surface; it often performs more of a rubbing or ploughing action.
• Lip Relief Angle (Clearance Angle): This angle is formed between the flank of the cutting lip and a plane perpendicular to the drill axis. It provides clearance to prevent the flank surface behind the cutting edge from rubbing against the workpiece, reducing friction and heat.

Defining the Chisel Edge Angle

The question specifically describes the angle formed "at the web of a twist drill between the chisel edge and the cutting lip."

Visualizing the tip of the drill, the web is the central core. The cutting lips are the edges that perform the cutting. At the very center, these two lips meet, forming the chisel edge. This edge is essentially the boundary between the two helical flutes at the apex. The angle measured in the plane perpendicular to the drill axis, which represents the angle formed by the meeting of the two cutting lips at this central point (the chisel edge), is precisely known as the chisel edge angle.

Analyzing the Options Provided

Let's examine the given options in the context of the question:

• Point angle: This is the overall angle of the drill tip between the two cutting lips, not the specific angle at the very center where the chisel edge is located relative to the lip itself.
• Helix angle: This angle relates to the spiral flutes and has nothing to do with the angle at the drill point's center.
• Chisel edge angle: This angle is formed at the center of the drill point (the web) by the two cutting lips. This perfectly matches the description in the question.
• Lip relief angle: This is the clearance angle ground behind the cutting lip to prevent rubbing, located away from the center.

Conclusion

Based on the standard definitions of twist drill geometry, the angle formed at the web between the chisel edge and the cutting lip is identified as the chisel edge angle. This specific angle influences the drill's ability to center itself and the forces required for penetration.