for making jigs and fixtures where high levels of accuracy are needed. Accuracy checking and calibration The simplest method of checking a low grade square for accuracy is to draw a line across a board with a flat face and straight edge (illustrated). Then flip the square over 180 degrees and check if the blade aligns with the line just drawn. Any error is multiplied by two and appears as a wedge or taper. The ultimate method of mechanically calibrating a try square or similar is with a cylindrical master square sitting on a granite or cast iron surface plate that is flat. By nature of their construction a cylindrical master square cannot help but sit at a perfect 90 degree angle to the flat surface. A square to be tested can be presented to the master and feeler gauges used with a source of light behind the gap to get a reading of deviation. If done cleanly and correctly, this method can find errors down to 10 μm, which is the limit of practical accuracy since thermal expansion from contact with the fingers will distort the blade by approximately that much. Other methods of calibration utilise modern technology by using a CMM (Coordinate Measuring Machine) for example, or a vertical machining center fitted with a dial indicator. References Metalworking hand tools Metalworking measuring instruments Squares (tool)