Straightness testing of marble square ruler

Fix the marble square steadily on the marble flat plate to maintain its working surface in a horizontal state, and select the appropriate inspection bridge plate according to the length of the square's working surface (usually 8-10 steps, with a span between 50 and 500mm); Then place the bridge deck on one end of the marble ruler and fix the level on the bridge deck; The bridge deck is gradually moved from one end of the right angle ruler to the other end according to the span, and the reading at that position is taken from an electronic level with a scale of 0.001mm/m for each span moved; The difference in its value is the straightness error of the working face of the marble square ruler. The straightness error of any 200mm on the working surface can be determined by selecting 50mm or 100mm bridge slabs and inspecting them within any 200mm using the above method.