Spotface — The Minimum-Depth Flat Seat

A spotface is the minimum-depth flat seat machined onto a rough or curved surface so a fastener head or washer bears on clean, flat material — dimensioned ⌴ with the SF note where the entry needs clarifying.

Definition

A spotface is a shallow, flat-bottomed seat machined onto a surface so a fastener bears on flat, clean material. Where no depth is specified, the spotface is by definition the minimum depth necessary to clean up the surface to the specified diameter — cut just deep enough that the rough skin, scale or curvature is gone and the bearing area is flat. Its diameter is the distance across the flat and does not include any fillet that may be specified at its edge.

When depth or remaining thickness is stated

The minimum-to-clean-up default covers cast and forged faces, but two cases need more: where machining allowance must be guaranteed, a depth (⌴⌀20 ↧2) or a remaining thickness of material is specified; and where the entry is ambiguous — a through hole on a curved face, or a spotface on the far side — a notation may be necessary to indicate the surface to be spotfaced. A spotface may also be specified entirely by a note, without being shown pictorially.

Reading the symbol: ⌴ plus SF

The spotface symbol and the counterbore symbol are the same flat-bottomed glyph; the standard's own example writes the spotface as the symbol with the letters SF shown with the callout. Read them apart by intent, not by glyph: the counterbore's depth seats a fastener head and is stated; the spotface's depth is whatever minimum produces a clean flat. Where a drawing relies on the distinction, the SF note removes it — and the two pages should be read as a pair.

Where it earns its keep

• Bolt seats on cast, forged or molded faces where the as-cast skin will not carry a load flatly
• Curved entries — a hole meeting a cylinder or a fillet needs a spotface to give the head a plane to bear on
• Washer seats on rough fabrications, where the washer must not bridge scale or mismatch
• Anywhere a torque-critical joint needs flatness under the head without a functional recess depth