Good question. A high-quality square is only a mouse-click away, so why build your own? For one thing, building is cheap. Really good squares from a company like Starrett are breathtakingly expensive. More affordable squares are often a bit out of square and once you get involved in a draw-filing and calibrating your brand new square, you might as well just build the damn thing.
Also, all-wood squares are a joy in the hand. They’re weightless. Seriously; you might forget you’re holding one. This is hard to believe if you’re used to clunky commercial squares, especially ones with cast metal stocks, but these are heavy, clumsy tools covered in sharp corners and angles that will ding your work. A wooden square will give you no such trouble.
But wait, can something made of wood really be accurate enough? Sure. I wouldn’t try to do machining with wooden layout tools, but they’re great for woodwork. Do they drift out of true over time? Yeah, but only a tiny bit and they’re very easy to square up again. And if you’re still not sold, knocking together a few squares is a fine little skill-builder, and if you use scraps it’s pretty much free.
This part matters, but it’s nothing to obsess over. Pick some straight-grained, dry hardwood; something that’s been in your shop for a long time. The traditional wood is mahogany, but I’ve got better things to do with such expensive timber. I recently made a large square from poplar and a smaller one from walnut and maple. The long part of your square (often called the “blade,” “rule,” or “tongue”) is ideally made from quarter-sawn stock, but I wouldn’t obsess over that, either.
The rule is generally about ¼ inch thick. Thinner is also fine, but anything thicker is likely to be clumsy. I usually find something a bit thicker than the width of my ¼” chisel and then plane it down until the two match exactly. The stock should be about three times as thick as the blade. For length, the ratio of blade to stock is usually 3:2. So a 15 inch blade would be paired with a 10 inch stock.
Like everything else in woodworking, there are many ways to skin this cat. To join the stock and the blade, you might use a half-lap, a tiny mortise-and-tenon, or the joint Chris Schwarz demonstrated on The Woodwright’s Shop: an elegant little mortise combined with an open bridal joint. This last one is surely the most secure option, but it’s also difficult to execute. For my own work, I prefer a simple bridal joint stabilized with pins. For my larger square, I cut a small notch into the blade that could register into the stock below the bridal joint. This notch adds bearing surface and stability while also giving you a surface that you can easily trim while tweaking the square for…squareness.
Cutting the joint is straightforward. Set a mortise gauge to the width of your chisel, center it on your stock, and run your lines. Saw down close to the baseline with a back saw and then chop out the waste with the chisel. Stay away from your baseline during the chopping phase and then trim down to it carefully. Depending on how good your sawing is, you may need to clean up the inside walls with a paring chisel. While making my last square, I did quite a bit of chiseling. It still came out nicely.
If you’d like an even more basic approach, you can take a large piece of thin stock, cut it into four pieces, and then laminate them together to mimic the hand-cut bridal joint. If you don’t feel skilled enough to accurately cut such a small joint and you prefer to go the laminated approach, don’t feel bad. This is a totally legitimate way to make a square.
However you make your square, I suggest pinning the joint for stability. For my larger, poplar square, I used bits of bamboo chopstick, which were free, perfectly round, and extremely strong. For my smaller and fancier square, I used some ¼” brass rod I had laying around. Brass and walnut go extraordinary well together, but any old wooden dowel will get the job done.
If you already own a reasonably accurate square (even a cheap speed-square), then I suggest gluing up your shop-made squares around this reference surface. If you’ve already got a good 90° angle sitting around, then there’s no reason to reinvent the wheel. A nice shelf-corner or bit of sheet-goods with factory edges will work just as well.
Once your square is out of the clamps, place the beam against a known, true edge and strike a line along one side of the beam. Flip the square 180°, line it up with the mark you just made, and strike another line. These two lines should be exactly parallel to one another. If you’re very lucky, they will look like one line. If not, then you’ll easily be able to see which direction your beam is leaning. You can generally correct the offending edge by planing or chiseling away a tiny bit of material at one end and then blending the whole edge together with a file or sandpaper on a block. Make sure to repeat this process with both the edges of your beam.
Once you’ve finished, scrape or sand the whole thing and apply a film finish. I used shellac, but polyurethane and lacquer are also good choices. A hard finish will keep for square looking clean and will limit seasonal movement.
Will my square stay true forever?
Nope. But neither will your commercial squares. I stopped using my vintage try-squares when I tested them using the above method and found that all five of them were off (and not by a little bit, either). All squares move over time. At least the ones you make yourself are quick and easy to fix.
For more information on shop-made squares, check out my Youtube video and the free Tip Sheet. I also have plans for two shop-made squares available on my website.