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An Online Extra to Fine Woodworking magazine
Squaring Up a Square
An edged needle file is all you need for tuning up a combination square
by Steve Latta
To check a square for accuracy, insert the blade so that it is sticking out as far as possible. Place the square against a jointed surface and, using a very sharp pencil, draw a fine line the length of the blade. Flip the head over and trace another line on top of the first. If the head is accurate, the lines should fall directly on top of each other. If the lines trail off, the head is out of square. Manufacturers of the more expensive models adhere to rigorous standards. Starrett squares are accurate within .002 in. over a span of 12 inches, referencing from the 90° face, and within .004 in. referencing from the 45° face.
Most heads that are out of square can be adjusted by filing the raised center ridge that the blade slides on. The angle of the blade can be altered by using careful strokes and an edged needle file. Proceed cautiously, or it may turn into a "leveling a table by sawing the legs" kind of experience. Removing too much material from the ridge can render the square useless.
For tips on what to look for when you're in the market for a combination square, see Steve's article "Combination Squares" in the November/December 2002 issue of Fine Woodworking (#159).
Steve Latta teaches woodworking at the Thaddeus Stevens College of Technology in Lancaster, Pa.
Photos: William Duckworth
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