SF Spotface
Spotface · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “SF ⌀ 30” states a spotface requirement.
What controls
A shallow flat recess machined around a hole to create a clean bearing area perpendicular to the axis, usually on a rough or inclined surface. Unlike a counterbore for a screw head, it is not meant to hide the head but to provide a flat seat: its depth is the minimum needed to clean up the surface, and many drawings specify it exactly that way, with no fixed dimension.
How to verify
Visual inspection that the whole annulus is machined, with no areas of original surface left uncut, plus a diameter measurement with a caliper. Perpendicularity to the hole axis and flatness are checked with a dial indicator or by coordinate measurement when the drawing tolerances them. If the depth callout is «minimum required to clean up», the acceptance criterion is visual plus the remaining wall thickness, not a depth value.
Common mistakes when interpreting it
- Confusing it with a counterbore for a screw head: the point here is not to hide anything but to create a bearing surface.
- Going deeper without checking the remaining wall thickness on a casting.
- Passing a seat with as-cast skin left on part of the annulus.
How to get it in the workshop / if it comes out of tolerance
It is the classic solution on castings: instead of machining the whole face, a local seat is made where the screw or washer bears. Since the starting surface is irregular, the actual depth varies from part to part and that is normal; what you have to guarantee is that the whole annulus cleans up, with no as-cast skin left. Pilot it from the hole so it comes out perpendicular to the hole axis.
Annulus with areas not cleaned up: go deeper, checking the remaining wall thickness first. Seat not perpendicular to the hole: machine it piloted by the hole itself or in the same setup. If going deeper brings you close to the minimum wall, ask: on a casting it may be preferable to accept a partially cleaned-up annulus rather than weaken the area.
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Other symbols in Dimension indicators
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