The underlined square box is mainly used for the inspection of parallelism and perpendicularity of parts, as well as marking. The square box is made of cast iron or steel.6A hollow cube with one working faceVMold slots. Environmental conditions for square box inspection: room temperature, high humidity. 

The requirements are that all working surfaces of the underlined box must be free of rust, scratches, cracks, dents, and any other defects that may affect the measuring performance. Non-working surfaces should be sandblasted and painted, with the edges rounded off. 

Underlined box accuracy: For square boxes with scraping, in addition to checking the flatness, the contact spots must be colored for legal inspection. 

In a square with sides of25mmNumber of spots within any square:1Grade2Grade not less than25Dot3Grade not less than20Dot.