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in machine building, a rule for testing the flatness and parallelism of a surface. Straightedges measure the maximum distance between points on the surface’s profile and a contiguous straight line—the edge of the rule.
Two types of straightedges are distinguished: toolmakers’ straightedges (master straightedges) and straightedges with a broad working surface. The master straightedges may have two edges, three faces, or four faces. Other straightedges have a broad working surface with a right angular or bow shape or I-beam cross section. Master straightedges are used to test the flatness and parallelism of a surface by means of the gap between the surface and the edge of the straightedge applied to it. It is thus possible to detect gaps of 1–5 microns (μ). Straightedges with broad working surfaces determine flatness and parallelism by measuring linear deviations of the surface being inspected in comparison with the surface of the straightedge, which is held on supports. Checking for flatness may also be done by painting the surface to be inspected and moving the straightedge across it; the paint will then be scraped off the raised portions of the surface. Angled straightedges are used only with this method.
Master straightedges are made in various lengths from 80 to 500 mm. Straightedges with broad working surfaces have lengths of 200 to 4,000 mm, and angled straightedges have lengths of 630 to 1,000 mm, with angles of 45°, 55°, and 60°. The working surfaces of master straightedges deviate from linearity by 0.6 to 4 μ, depending on the length and the degree of accuracy. The variation of flatness in straightedges with broad working surfaces depart from flatness by 2.5 to 100 μ.
N. N. MARKOV