Instructables
Picture of Making a Square Square
This is a good brand name square my father bought. When I used it I discovered it was not actually square. The rivet and brass system for attaching the blade to the handle is very good. No amount of tapping would change the alignment.
 
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Step 1: Checking a square

Picture of Checking a square
If you want to check a square, you could hold it up to another square, but that one could be inaccurate, too.

The dark brown piece of wood in the photo is the end section of a piece of veneer plywood I have been carrying around for several decades. The white paper arrow points to the factory cut edge of the sheet. This makes a trustworthy straightedge. The white chipboard piece of shelving raises my work surface for this Instructable to the same height as the plywood straightedge.

Step 2: Lay down a piece of paper and tape it.

Picture of Lay down a piece of paper and tape it.
This is a piece of clean scrap paper. I taped it so it does not move.

Step 3: Score a line.

Picture of Score a line.
Hold the leg of the square firmly against the straightedge. With a fine point pen, mark a line along the other leg of the square for its entire length.

Step 4: Flip the square

Picture of Flip the square
In geometry it is true that two lines perpendicular to the same line are parallel to each other. Flip the square over and score a second line. Make it a tiny fraction of an inch away from the first line. If the square is truly square, the lines should not diverge from each other in the least.

Step 5: These lines are parallel

Picture of These lines are parallel
A close examination of the lines drawn reveals they are parallel. The square is not out of square.

But, earlier I said this square was not square when it was new. I made it square.
mowdish6 years ago
You might want to point out that this only corrects the top (outside) edge of the square. The inside angle still won't be square. I like the parallel line trick though.
Phil B (author)  mowdish6 years ago
You are very much correct. Thank you. The inside of the leg would still be in error, assuming it was parallel to the outside edge in the beginning.
aeray Phil B6 years ago
I am a professional carpenter, and I often "tune" my framing square using the parallel line method to check it, often off of the side of table saw table. I then tap gently on the inside or outside of the heel of the square using a smooth faced hammer.
Phil B (author)  aeray6 years ago
Thanks for your comment. I had never actually heard from someone who uses a hammer on the heel of the square. What would you say causes your square to go out of alignment?
aeray Phil B6 years ago
The tapping part is just as you described, but without the punch. Although I try to keep it out of the hands of my helpers and crew, it sometimes gets dropped or otherwise abused or gets in a bind in my toolbox. Another possible cause is that I often use a marking knife rather than a pencil, particularly when laying out stair carriages, which may cause excessive wear on the blade and tongue, but I think this effect is minimal.
Phil B (author)  aeray6 years ago
The kind of bumps to your square that you describe are very understandable in your situation. I have a framing square that rests against a wall a lot and has not been dropped in a very long time, if ever. I have not had to realign it since I first tweaked it shortly after I got it. I may be a babe in the woods for naivete', but I always assumed a square off the rack from a store would be square. I have since learned that is not a good assumption to make.
aeray Phil B6 years ago
The first rule of carpentry is that wood always moves and there is no way to stop it. The second rule is never to assume that anything is square, level, or plumb. This goes for houses and tools fresh out of the box.
Ah, yes. Lesson #3 of working with a construction crew. Lesson #1 was how to push a broom and Lesson #2 was how to carry lumber.
smokehill aeray6 years ago
I've seen old-time cabinetmakers (back in the 50s) readjusting large metal framing squares this way, but I suspect the handle has to come off this type, and it might not even be possible with this type of square. As they explained it, to widen the angle of the square, tap close to the inside of the "elbow." To tighten up the angle (such as if it's 91 degrees), tap on the outside of that elbow. It's important (they said) to do this on a very hard, flat surface, like a saw table or anvil, and to go very, very slowly until you reach exactly 90 degrees.
But, Phil, that's only half the fun. Don't forget to do the inside edge of the blade as well.
Lextone6 years ago
Old trick for on the fly checking for squareness is the 3-4-5 method. From either the inside or outside corner of the square, measure 3 inches down and scribe a mark. Then 4 inches across the top and scribe another another mark. When a measurement is taken diagonally between the two the marks, it should always be exactly 5 inches. By permanently scribing the marks you will always have 2 datum points to measure at a moments notice.
Phil B (author)  Lextone6 years ago
I remember learning this in grade school. I think they said at the time that it goes back to the Egyptians centuries before Christ. I used it once to lay out reference lines for laying floor tile in a basement. In order to get maximum accuracy I used the double of 3, 4, and 5 feet. It worked great in that application. Another easy possibility would be to make a straight line, the longer the better. A chalk line would work fine. Then bisect it with a large divider or a string pulled tight with a pencil to scribe intersecting arcs above and below the line. Then connect the arc intersections. The resulting line is exactly perpendicular to the first line.
Lextone Phil B6 years ago
It is a very useful formula when building deck frames or laying out a patio. Just to be clear for everyone else... This formula only verifies the corner angle is 90 degrees. If the edge on your square is not true or warped this is a moot exercise. I had a square that only the inside edge was truly square. The outside edge was almost a full degree off.
mbtria6 years ago
You can never assume that any measuring device is correct. All of them should be tested, and adjusted by reference to known standards, geometrical constructs, or other methods.

Accuracy requirements in woodworking can be surprising. While wood moves quite a bit with changes in internal stresses, temp and humidity, it can be worth working to 0.001-2 thousandths at times, and completing the joinery before the wood moves. An example of tight tolerances is in a mitered frame, where a 44.9 degree cut can show an all too visible gap when compounded by four corners. It makes you wonder how the 18th century cabinet makers managed their outstanding craftsmanship.

Reading about how to make standards from scratch can be interesting. Here is one on how to make a straight edge:

http://home.comcast.net/~jaswensen/machines/straight_edge/straight_edge.html
orksecurity6 years ago
A cheap square is good enough for framing carpentry. Not for cabinetmaking or other precision work.

If that matters to you, skip the cheap stuff and go up to one of the brands/models which are designed for folks who need accuracy. Among other things, those often come with a lifetime promise that they will will be restored to their original accuracy at no charge beyond shipping cost.

"Quality, Service, Price. Pick any two."

Though there *are* some squares which are both reasonably priced and surprisingly accurate. Lee Valley has a small one which they promise comes from the factory as accurate as most machinist's squares.

Part of the key there is "small"; the larger the square, the harder it is to maintain accuracy. Which is part of the reason framing squares are often the least accurate square in the shop... the other reason, as I suggested at the start, is that framing just doesn't need that much precision.
Phil B (author)  orksecurity6 years ago
What amazed me is that the square in the photos with the wooden handle is a very famous brand name square and it was out of square from day one.
While I do use an expensive Incra set-up square in the shop for equipment and tools, I must take exception to the idea that accuracy when framing isn't important. I try to frame to 1/16" and will settle for 1/8" if the situation allows. BUT- a 1/32" inaccuracy in a framing square, repeated 16 or 17 times when laying out a stair carriage produces an unacceptable and unusable result. As Phil B has shown, accuracy with a framing square is easy to achieve.
djmachet6 years ago
a) if the legs are too far closed I'm guessing you'll do the same thing just on the opposite side i.e. flip the square with the handle in line with the red line and follow the same filing action b) don't make the mistake of marking from the inside edge - it still isn't parallel to the outside edge or perpendicular to the reference face
Phil B (author)  djmachet6 years ago
You are correct. If the legs are too closed and the angle is less than 90 degrees, file the same pattern from the handle side. It is good to check the inside edge independently to see if it is square.
ah I see you've already discussed about the inside edge - my bad