Machinist Square


Two typical engineer’s squares

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°. In the accompanying image, there is evidence of pinning at the intersection of the blade and body: the heads of three pins are visible as dark circles. A small notch has been added to the inside corner of the square to prevent small particles from accumulating and affecting the square’s reading. This notch is typical of machinist squares and is occasionally found on carpentry squares as well.


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 either check for squareness or to mark out the body of the workpiece.


Square check.GIF

Machinist squares should have a linear error of no greater than 0.0002 in/in.[1]

Squares must be occasionally checked for accuracy. The four disk method is one way to verify overall squareness. However, it cannot detect bent blades.




This information originally retrieved from
on Monday 1st August 2011 4:56 pm EDT
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