Check the perpendicularity of the side of the ruler to the working surface

Place the ruler on a flat surface and align the gauge holder with a dial indicator with a division value of 0.001mm to zero on a standard square ruler using a standard round rod. Then, in the same way, the dial indicator is placed against one side of the ruler, and the reading of the dial indicator is the perpendicularity error of that side. Similarly, detect the perpendicularity error on the other side and take the error value.