Check the parallelism of the cast iron ruler

Place the ruler on a flat plate with the working surface as the base, and use a dial gauge with a division value of 0.001mm mounted on the gauge holder to measure the height difference at no less than three points on its working surface, which is the parallelism error.

When a suitable flat plate is not available, it is allowed to place the side of the ruler on a supporting surface and measure the height difference of the ruler using a lever micrometer with a division value of 0.002mm or a gauge with a division value of 0.002mm.