How to true a square

[Tom Walz]

Truing The Square
After obtaining a steel square, the first and most essential thing is to test or prove it to see that it is accurate, forming the angle of a perfect square.

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Take a board planed on one side and straighten one edge of it perfectly as described under 'Making a Straight Edge'. Make a mark across this board with the square, as shown then reverse the square to Position B. If the square is true it should exactly fit the mark made. It is necessary to work very accurately, making the mark with the point of knife and having the edge of the board absolutely straight.

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If the square is found to be out or inaccurate, it is not necessary to throw it away; it can be made true by a simple method by any handy mechanic. If you do not possess an anvil, make a substitute by sticking the ax into the chopping block, lay the square on the head on the head of the ax so that bearing will come from the throat or inside angle to the heel or outside of the square. To close up the angle, strike with a hammer a sharp blow at a point near the heel; to open the angle, strike near the throat at a point indicated in fig. 7. Don’t strike too hard. Use a bell face nail hammer and the dent will not be noticed

More at: http://blog.carbideprocessors.com/ April 12th post

[Myk Rian]

I've used a center punch to adjust them. Either way works.

[Michael Mayo]

How would you go about squaring a square that has a thick body? Not a framing square but a woodworking type square with a hefty body on it?

[Chris E Smith]

That's a great Tip Tom, I'm a blacksmith and never thought of doing this.