Twist drills – local cutting speed along the lip: Does the cutting speed remain the same from the drill centre to the periphery?

Introduction / Context:
Cutting speed in drilling refers to the surface speed at the cutting edge. Because a drill rotates, the local linear speed depends on radius. Understanding this variation explains chip formation and wear patterns (e.g., higher wear at the outer corners).

Given Data / Assumptions:

• Twist drill of diameter D rotating at N r/min.
• Uniform angular speed for the whole tool.
• Conventional geometry and feeds.

Concept / Approach:
Linear speed V at a point equals ω * r. At the centre (r = 0), V = 0, so material removal at the chisel edge involves extrusion rather than true cutting. At the periphery (r = D/2), V is maximum. Therefore, cutting speed does not remain the same along the lip.

Step-by-Step Solution:

Verification / Alternative check:
Examine chips: little true cutting at the centre, thicker chips near the corners; wear is heavier at outer edges where speed is highest.

Why Other Options Are Wrong:
“Correct” would imply uniform linear speed, which contradicts rotational kinematics.

Common Pitfalls:
Setting speed based only on material without considering diameter; larger drills require lower rpm to keep peripheral speed within limits.

Final Answer:
Incorrect