Countersink
Glossary · Part Features
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A countersink is a conical enlargement at the top of a hole that lets a flat-head screw sit flush with the surface.
In practice
Angled rather than flat-bottomed — usually 82° or 100° in inch practice, matched to the screw head. It is also used lightly as a way to break the edge of a hole.
Why it matters
The angle has to match the fastener. An 82° screw in a 100° countersink contacts on a narrow ring instead of the whole cone and will pull itself in crookedly.
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