Square box function: Cast iron square box is mainly used for the testing and marking of the parallel and vertical degree of each part. Square box is a kind of cavity cube with 6 working surface wich are made of cast iron or steel, one of the working  face has a v-slot. 

Square box test method: For the square box, which side length of 315mm and below this size, put 1mm gage block at each side of the tested surface of square box, and put knife block lip ruler on the working surface, then test the plug with gage block at each tested point. The difference between the size of gage block  group which is just pluged and both side of the gage block is regarded as deviation of the tested point and the both siedes This test should be processed at several sections of the square boxes working surface in vertical, horizontal and diagonal direction. When the various locations of the measured error which in the same direction are same that means it is maximum value, when the error direction is inconsistent, take the absolute value of the max. positive and negative difference as the flatness of inspected surface.