Twist drill geometry: The angle formed by the leading edge of the land (cutting lip direction) with a plane containing the drill axis is called what?

Introduction / Context:
Twist drills have several key angles that govern chip flow, cutting forces, and heat generation: helix (rake) angle, point angle, chisel edge angle, and lip clearance angle. Correctly identifying each angle ensures proper selection for different materials and hole qualities.

Given Data / Assumptions:

• The leading edge of the land follows the helical flute path.
• The angle is measured to a plane that contains the drill axis (a reference longitudinal plane).
• Standard right-hand twist drill assumed.

Concept / Approach:
The helix (or rake) angle is defined as the angle between the direction of the cutting lip/flute and the drill axis reference plane. It controls chip evacuation and effective rake at the cutting edge. Common helix angles range from about 20° to 35°, with lower values for hard/brittle materials and higher for soft/ductile materials to promote chip flow.

Step-by-Step Solution:

Verification / Alternative check:
Compare with other angles: point angle is the included angle between the two lips; chisel edge angle is the obtuse angle between chisel edge and a lip in end view; lip clearance angle measures relief behind the cutting lip. None match the given definition.

Why Other Options Are Wrong:

• Point angle: different geometric feature (tip included angle).
• Chisel edge angle: end-view angle at the web.
• Lip clearance angle: measured on the flank to prevent rubbing.

Common Pitfalls:
Assuming a larger helix always improves drilling; excessive helix can weaken the web or cause chip packing in certain materials.

Final Answer:
Helix (rake) angle