Recommended drill point angle for bakelite and fibrous plastics In practical drilling of brittle thermosets (e.g., bakelite) and fibrous plastics, which point angle is commonly used to reduce cracking and delamination while ensuring a clean entry?

Introduction / Context:
Bakelite (a thermoset phenolic) and many fiber-reinforced plastics are prone to edge chipping and delamination during drilling. Choosing a suitable point angle helps control thrust and cutting action to protect the material.

Given Data / Assumptions:

• Brittle or fibrous non-metallic workpiece.
• Standard twist drill geometry is being considered (no special brad-point, step, or core drills).
• Conventional manual or machine drilling with appropriate fixturing.

Concept / Approach:
A smaller point angle (sharper point) like about 90° produces a keener cutting action and reduces the pushing (extrusion-like) effect that promotes cracking. Typical “general-purpose” 118° or 135° points suit metals but can induce higher thrust in brittle plastics. For bakelite and laminates, 90° and specialized geometries (e.g., spade or brad points) are commonly recommended.

Step-by-Step Solution:

Verification / Alternative check:
Shop handbooks and plastics drilling guidelines list 90° as a go-to for brittle plastics, often with reduced feed and higher spindle speeds plus backing support.

Why Other Options Are Wrong:
118° and 135°: common for steels; tend to increase thrust on plastics.150°: excessively blunt for these materials; likely to crack the surface.60°: too acute; weak chisel region and potential wandering.

Common Pitfalls:
Neglecting backup boards under thin laminates; not clearing chips (melting/smearing); using feeds/speeds meant for metals.

Final Answer:

90°