Combination squares are useful inexpensive tools. Unfortunately if you picked one up at your local big box store chances are good it's not actually square. The method for testing a combination square for squareness is as simple as the fix if it's not.

Step 1: Test for squareness

To test any square for squareness all you need is a surface with a known straight edge, a sharp pencil, and your square.

Butt the square up against your straight edge and strike a line. Flip the square over and strike another line starting in the same place as the first. If your two lines diverge then your square is not square. 
<p>thank you, very good method and instruction</p>
Thank You for opening my eyes to this defect. <br>I kinda get tired of producing cockeyed electric panels and fixing them using ah sheet of paper. <br>The next square I get will be a good one. <br>
There is &quot;construction square&quot; and there is actually square. <br>These combo rulers are definitely for woodworking. If you ever check the squareness of a framing square, you will probably be utterly shocked. But for rough framing, which is the job it's designed and made for, it's 'close enough'. More accurate would likely entail more cost, and for most jobs it's just not worth it. The version of your tool for metal working will be a lot straighter out of the box, but could still use fine adjustment for maximum accuracy. Just make sure it's actually the ruler that isn't held straight. You may find that adjusting for a square 90 deg. measure makes the 45 deg. angle no longer acceptable. <br>If that's the case, only careful filing or grinding will correct the matter, THEN you have to re-fix the blade holding cam. I only really mention this because in step 2, I can clearly see the rough machining marks. Since they are machined surfaces, they SHOULD be close enough for any but the finest work... but the machine operator is a variable no-one can account for. <br> <br>On the other hand, even high precision machinist squares are actually press fit blades, so that they can be adjusted to be truly square. And by truly square, I mean seconds of a degree off. Less than a thousandth of an inch off square, in a 12&quot; blade. <br>It's all a matter of how close you need to get.
Thank you for this. I bought one of these squares at a big box store, and it was off of square a bit more than 1/32 of an inch over 12 inches. I did something similar to what you did, but did not believe I had space for sandpaper. At first I tried a stiff saw blade, like the kind used by a Sawzall. It worked. Since I have tweaked it a little more with a thin file. Now I am quite pleased with the accuracy of my combination square. <br> <br>I did write to the manufacturer. The representative sounded shocked that their squares would not be square.

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