Drill geometry for brass: The preferred helix angle of a twist drill for drilling brass should be

Introduction / Context:
Free-cutting brasses are prone to grabbing if aggressive rake/helix geometry is used. Drill design for brass typically differs from that for steels or aluminum to prevent self-feeding and chatter.

Given Data / Assumptions:

• Work: brass or brass-like copper alloy.
• Tool: standard twist drill with modifiable helix and lip geometry.
• Goal: stable cutting without grabbing.

Concept / Approach:
To minimize grabbing in brass, use a lower helix angle and smaller effective rake (even zero/negative rake at the cutting lips by dub-off). A helix less than the common 30° (e.g., 20–25°) reduces the tendency of the tool to pull itself into the work and improves control of chip flow for short, brittle chips typical of free-machining brasses.

Step-by-Step Solution:

Verification / Alternative check:
Tool catalogs and shop practices recommend low-helix, low-rake drills for brass to avoid sudden dig-in.

Why Other Options Are Wrong:
30° or more than 30° increase the rake/helix and the risk of grabbing; the question seeks the specific adaptation for brass.

Common Pitfalls:
Applying high-helix aluminum drills to brass; the geometry suited for long ductile chips can be hazardous on short-chipping brass.

Final Answer:
Less than 30°