Twist drill geometry for soft materials For drilling softer materials (e.g., aluminium, brass, plastics), how should the drill point angle be chosen relative to the common 118° point used for steels?

Introduction / Context:
Drill point angle affects chisel edge length, thrust force, centering, and chip evacuation. Different materials call for different geometry to minimize burrs and improve finish.

Given Data / Assumptions:

• “Softer” materials imply lower strength and often gummy behavior.
• Standard steel drills commonly use about 118° point angle.

Concept / Approach:
Reducing the point angle (making it sharper) decreases the chisel edge length and cutting load, improving penetration and reducing thrust for soft metals and plastics. It also promotes more efficient shearing at the lips.

Step-by-Step Solution:
Target: Lower thrust and cleaner entry in soft materials.Choose a point angle less than 118° (e.g., 90–110° depending on material) to sharpen the point and ease cutting.

Verification / Alternative check:
Machining handbooks list smaller point angles for soft, non-ferrous materials; harder alloys often use 130–140° to strengthen the point.

Why Other Options Are Wrong:

• =118° is acceptable but not optimal for many soft materials.
• >118° suits harder materials; increases thrust on soft stocks.
• “Any of these” ignores material-specific optimization.
• “Exactly 135° only” is too blunt for many soft materials.

Common Pitfalls:
Using the same drill geometry for all materials; ignoring lip clearance and helix angle which also affect performance.

Final Answer:
Less than 118° (sharper point)