Counterbore (⌴) — The Fastener's Flat-Bottomed Recess

The ⌴ symbol before a diameter (and optional ↧ depth) specifies a flat-bottomed enlarged hole — the recess that lets a fastener head sit flush below the surface.

Definition

The counterbore symbol ⌴ precedes, with no space, the dimension of the counterbore: ⌴⌀15 ↧8 over a ⌀8 THRU hole specifies a flat-bottomed cylindrical enlargement, 15 across, 8 deep, coaxial with the through hole. The recess is what lets a socket-head cap screw sit below the surface — its function defines everything about it: flat bottom for the washer or head face, generous diameter for the tool, depth for flush or sub-flush seating.

Depth, or remaining material instead

The counterbore's depth follows the depth symbol after the diameter. Where the thickness of the material remaining under the counterbore has significance — wall strength, thread engagement, a seal land — that thickness is dimensioned in place of the depth. One or the other governs; dimensioning both over-constrains the same wall. A fillet radius at the counterbore's base may be specified where the transition needs one.

Location: the coaxiality question

A counterbore is useless if its hole wanders out from under it, so the drawing must state how the two relate. Three patterns cover the practice — the standard's own figures use all three:

• One positional tolerance governing both the hole and the counterbore — the tightest, simplest statement when they serve one fastener
• Separate positional tolerances with the same datum references — when the two features need different tolerances but share a location basis
• Separate positional tolerances with different datum references — when the counterbore serves a different fit or a different mating part than the hole

Counterbore vs. spotface

Both are flat-bottomed enlargements cut by similar tools, and the symbols are near twins. The difference is depth of intent: a counterbore has a functional depth that seats the fastener; a spotface has only whatever minimal depth clears the surface to a clean flat seat. See the spotface page for the shallow case and its [SF]-marked symbol.