Straightness measurement for marble straightedges

Support the marble straightedge with a marble parallel gauge at the standard support marks located 2L/9 from each end of the straightedge. Select an appropriate bridge plate based on the length of the straightedge's working surface (typically 8 to 10 spans with a span of 50 to 500mm). Place the bridge plate at one end of the straightedge and secure the level on the bridge plate. Gradually move the bridge plate across the straightedge from one end to the other, reading the position at each span from an electronic level with a graduation of 0.001mm/m. The difference between the highest and lowest readings is the straightness error of the straightedge's working surface. For a straightness error of any 200mm on the working surface, choose a bridge plate of 50mm or 100mm and inspect within any 200mm range using the aforementioned method.

Natural Deflection Testing of Marble Flat Rule The natural deflection testing of the marble flat rule involves supporting the flat rule with a marble parallel gauge at the standard support marks located at 2L/9 from the ends of the flat rule, placing it on a datum plane. A micrometer or comparator with a graduation of 0.001mm is then used to measure readings at the midpoint of the working surface length; subsequently, the marble parallel gauge is moved to the large support distance position (at the ends of the flat rule), and after stabilizing for 10 minutes, readings are taken by the micrometer or comparator; the difference between the two readings is the natural deflection of the marble flat rule.