Countersink (⌵) — Diameter and Included Angle

The ⌵ symbol before a diameter and an included angle specifies the conical entry that lets a flat-head fastener sit flush — the only hole callout whose angle is part of the specification.

Definition

The countersink symbol ⌵ precedes, with no space, the dimensions of the countersink. For countersunk holes, the diameter and included angle of the countersink are specified: ⌵⌀10 X 90° over a ⌀5 THRU hole is a 90° conical entry, 10 across at the surface. The included angle is not optional information — it is half the specification, because the cone's slope is what a flat-head screw's seat contacts.

Alternate specification methods

The diameter-and-angle pair is the default, but the standard permits other methods. What never changes is that the cone must be fully defined — diameter and angle together, or an equivalent that fixes both:

• Diameter and included angle — the default pair, e.g. ⌵⌀10 X 90°
• A gage diameter — the diameter the cone reaches at a defined depth, verified with a plug gage
• A depth in combination with other parameters — where production measures depth more naturally than a surface diameter
• The nX prefix multiplies identical countersinks the same way it multiplies holes

Countersink vs. counterdrill

A counterdrill is the same idea stepped: a cylindrical enlargement with diameter and depth specified, its included angle optional — it typically transitions from one diameter to another or clears a fastener shank at an angle. The depth dimension of a counterdrill is the depth of its full diameter from the outer surface, the same rule a blind hole follows.

On curved surfaces, and what flush means

Where a hole is chamfered or countersunk on a curved surface, the diameter specified applies at the minor diameter of the chamfer or countersink — the true cone meets the true hole, not the outer skin. That definition matters because the purpose is flushness: the flat-head fastener's cone seats against the callout's cone, and any mismatch shows up as a head proud of or below the surface. Where flushness must be guaranteed rather than hoped for, a profile or perpendicularity control on the countersink's face takes over from the notation.