Hole finishing: The operation that produces a cone-shaped enlargement at the mouth of a drilled hole is called

Introduction / Context:
After drilling, holes are often modified to seat fasteners or improve fit. Recognizing the correct secondary operation is important for proper assembly and surface quality.

Given Data / Assumptions:

• A drilled hole requires a conical mouth to seat a flat-head screw or remove burrs.
• Standard shop operations are considered.

Concept / Approach:
Counter-sinking cuts a cone at the hole entrance, typically matching screw head angles. Counter-boring creates a cylindrical recess for socket heads; spot facing produces a flat, smooth seating surface around a hole; trepanning removes a circular disk rather than producing a conventional hole.

Step-by-Step Solution:

Verification / Alternative check:
A countersink tool has a conical cutting form (commonly 60°, 82°, 90°, or 120° included angle) corresponding to standard screw heads.

Why Other Options Are Wrong:

• Counter-boring: cylindrical recess, not conical.
• Trepanning: core removal method.
• Spot facing: flat circular seat, not a cone.

Common Pitfalls:
Confusing counter-sinking with counter-boring; always visualize the required seat shape.

Final Answer:
counter-sinking