Twist drill geometry: The lip clearance (relief) angle of a drill is defined as the angle between which two elements?

Introduction / Context:
Lip clearance (also called relief) on a twist drill ensures that only the cutting lips contact the work, while the flank behind the edge clears the fresh surface. Correct understanding of the angle's definition supports proper grinding and troubleshooting of drill performance (e.g., rubbing, overheating).

Given Data / Assumptions:

• Standard two-flute twist drill.
• Conventional point geometry and helix.
• Measurement near the periphery unless otherwise specified.

Concept / Approach:
The lip clearance angle is measured between the flank surface behind the cutting lip and a plane normal to the drill axis (i.e., a plane perpendicular to the axis). This geometry provides clearance to prevent rubbing while maintaining edge strength. Other angles—rake, point, chisel—are important but are defined differently.

Step-by-Step Solution:

Verification / Alternative check:
Drill point gauges and standards specify lip clearance seen in end view and side view; correct grinding shows a relief that increases from chisel toward periphery and is referenced to a plane normal to the axis.

Why Other Options Are Wrong:

• (a) Land leading edge reference is not the standard definition.
• (c) Chisel edge relation defines chisel angle, not clearance.
• (d) Rake face vs. axis corresponds to helix/rake relationships.
• (e) Helix vs. centerline defines helix angle, not lip clearance.

Common Pitfalls:
Unequal lip clearance on the two lips causes one lip to do most of the cutting, leading to oversize holes and rapid wear; verify symmetry during sharpening.

Final Answer:
Flank (relief) surface and a plane at right angles to the drill axis