The spotface symbol is used on a drawing to indicate that a spotface hole feature is required. A spotface is a very shallow flat-bottomed cylindrical hole that is larger than and coaxial to another cylindrical hole. Its purpose is to provide a flat mounting surface for mating parts, such as washers or pan head screws. Though a spotface and a counterbore are very similar, a spotface only provides a flat mounting surface, where a counterbore provides a flat and recessed mounting surface. A spotface symbol is similar to the counterbore symbol, with the addition of the letters “SF.”
A spotface dimension is structured in the same way as a counterbore. The spotface dimensions are listed directly below the dimensions of the smaller coaxial hole, as shown in the model below. The spotface dimension contains the spotface symbol, the diameter of the spotface, and the depth of the spotface, as applicable. While a spotface is not required to have a depth callout, it must be machined until the minimum perfect diameter (the diameter listed in the spotface dimension) is reached.
In the example below, there are three identical thru holes with spotfaces. The diameter of the thru holes is indicated to be 5.5 +/-0.2, and the spotface dimension is listed directly below it. The spotface callout indicates a spotface having a diameter of 12.24+/-0.25 and a depth of 0.75+/-0.25.
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