Some common engineering drawing symbols you should know include counterbore, countersink, spotface, depth, radius and diameter. These basic symbols help engineers, draftsmen and fabricators read and interpret technical drawings.
One frequently used symbol in engineering drawing is counterbore, represented by a line drawing of a square with no top and often labeled C’BORE or CBORE, a cylindrical flat-bottomed hole that enlarges another hole. In mechanical drawings, a counterbore symbol indicates that a fastener, such as screw head, must be flush or below a surface.
Like counterbores, countersinks, represented by a downward pointing arrow (labels CSK or CSINK), show where holes are to be drilled, except a countersink is conical. Countersink symbols usually mark locations of countersink screws (for flush finishes) and can also specify the degree of chamfer of the hole.
Spotface – symbol SF, S/F, SFACE or S/FACE – indicates where a smooth-machined surface is required, such as around a bolt hole where a shallow counterbore is needed, while the symbol for depth, a downward-pointing arrow below and attached to a straight horizontal line, indicates the depth specified of a feature. Two other common symbols in engineering drawings include R, for the radius of an arc or a circle (sometimes rendered as CR for “controlled radius”) and D, diameter of a circle, sometimes also abbreviated DIA or ø.