A machinist square or engineer's square is the metalworkers' equivalent of a try square. It consists of a steel blade inserted and either welded or pinned into a heavier body at an angle of 90°. Usually a small notch is present at the inside corner of the square. This prevents small particles from accumulating at the juncture and affecting the square's reading. Use In use, the body is aligned against the one edge of the object and the blade is presented to the end or body of the object. If the end is being checked, then a strong light source behind the square will show any mismatch between the blade of the square and the end of the object. The purpose of this action is to check for squareness or to mark out the body of the workpiece. Accuracy Machinist squares can have a linear error of no greater than 0.0002 in/in. Squares must be occasionally checked for accuracy. The four disk method is one way to verify overall squareness. However, it cannot detect bent blades. Accuracy grades and standards There are several different International or National standards that Engineer's Squares are manufactured to, two main ones being: - BS 939. (British Standards). - DIN 875-1 (German/Euro standards). Under both these standards there are basic dimensions set out for blade/body length ratio, and cross sections of both, as well as different types of squares such as those with a solid body and an inserted blade, or those with a single piece blade and body being precision ground with another plate attached to the base of this to provide a square 'butt' to refer to. Also under these standards there are different grades of accuracy to which a square can be made. It usually relates to a deviation of 'X' amount of blade lean vs. height of measurement usually done at the top of the blade. This also assumes that the blade and body widths are both perfectly parallel, making for a true reading along the blade length and inside and outside the square. Typically there are A or B grade squares made and sold, B grade being perfectly serviceable for most general use where accuracy is required in a workshop situation. A grade squares are typically used in applications where a higher grade of accuracy is required, in a toolroom for example, or for checking other squares for square, or