Picture of How to square a combination square
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.
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Step 2: Adjusting the square

Adjusting the square is simple. Inside the channel the blade of the square sits in are two tiny islands of metal that sit proud of the bottom. It's these two islands, one on either side of the screw that holds the blade tight, that the blade itself sits on.

To adjust the squareness we adjust the height of one of these islands by wrapping the blade of the square in sand paper and grinding it down. Which island you target depends on which direction the blade of the square needs to move.

Only grind a little bit at a time and test for squareness frequently. A little grinding goes a long way and it's really hard to put the material back on once you've removed it.

Dr.Bill1 year ago
Thank You for opening my eyes to this defect.
I kinda get tired of producing cockeyed electric panels and fixing them using ah sheet of paper.
The next square I get will be a good one.
ironsmiter1 year ago
There is "construction square" and there is actually square.
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.
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.

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" blade.
It's all a matter of how close you need to get.
Phil B1 year ago
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.

I did write to the manufacturer. The representative sounded shocked that their squares would not be square.