I saw a write up on this somewhere a while back and gave it a shot on an old 14" square that has taken a few bumps and probably wasn't all that "square" to begin with. Here's how I did it:
1. Prior to begining this I drew a pencil line with the square, then flipped the square over, lined up the starting point (referencing the same edge)and drew another line. The lines started off at the same point but finished ~1/16" apart after traveling 14". Here's another line I marked which caused me to question this square in the begining compared to the finished "Square" square:
2. I have an 18" drafting triangle which is very accurate. It makes it easy to compare against as you straighten out the spring in the metal blade. If you don't have one of these you can use a board with a fairly straight edge. Just use the same reference edge of the board, draw a line, flip the square over and draw another line and compare.
3. Using a spring loaded center punch (you could use any hardened point...ie:nail set) I punched a few dimples on the short side of the square then compared against the triangle. The short side is the side pulling away from square. Punched, compared, etc until it was dead on. Took ~15 punches on this side and 5 more on the other before it straightened out.
Now I have a Square square. Very easy, no excuses to use non square metal squares!
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