A New Straightness Testing Instrument

A new straightness testing instrument of high accuracy has been developed at the National Research Laboratory of Metrology.Test pieces up to 500mm in length and up to 300mm in diameter can be tested by the instrument, when they are cylindrical. A carriage with sensitive electronic micrometers on both its sides is mounted on a bed with flat and vee guides and is driven by an electric motor.The bed has two tables for supporting a test specimen on both its sides with dimensions 600×200mm.A test specimen is set at first on one of the tables with its test surface approximately parallel to the direction of the guide rails, and the relative deviation of the surface to the moving trace of the carriage is detected and recorded.And then the surface is measured on the other side, and the deviation is recorded again. If the carriage is driven along the same trace in the repeated recordings, the error of the straightness of the movement of the carriage can be easily separated by examining the two records and only the error of the straightness of test specimen is obtained.After experimental examination of the instrument about the repeatability of the traces of the carriage, temperature fluctuation, stability of the electronic micrometer, recorder and so on, it is concluded that the instrument has an accuracy with in 0. 3μm.