Checking For Square

With obvious exceptions such as turning and carving work, most woodworking projects rely heavily on straight lines and right angles. This is true for the individual components and the entire assembly, and accuracy starts with the simplest tools and methods.

Common layout tools for woodworking include several types of squares that are used to mark and check right angles to ensure they are a true 90 degrees. These are the try square, speed square, a combination square, and a framing square.

Even though most of these tools are manufactured to close tolerances, not all of them will have the same degree of accuracy. The simplest method for checking the square is to align the indexing edge or body/handle against a piece of plywood or MDF with a straight factory edge and mark a reference line along the blade. Then flip the tool over so the body/handle is on the other side of the line; the blade should be perfectly parallel to the line. If the square is off, repeat the test several times to make sure you’re getting a consistent result.

If the square is not true, remedies can vary. If possible, have the seller exchange it for a replacement that is accurate. If not, some tools offer options. Combination squares have a sliding removable blade; take it out and inspect the groove where it seats to make sure there’s no debris or other problem that might cause misalignment. If it’s a framing square that’s off, you can use a hammer and center punch to dimple the corner. Just scribe a diagonal line between the inside and outside corners of the square and punch along that line — toward the inside corner to open the angle, or toward the outside corner to close the angle. This adjustment can correct slight discrepancies of a degree or two if the square is not a true 90 degrees.

Because setup tools are critical to adjusting your table saw, jointer, and other woodworking machines, make sure these squares are accurate. If they are and your machining methods are sound, the lumber you build your projects with will be straight and square. (See SHOP TIP, below.)

Still, accurate parts are not enough to ensure that the overall assembly will be true and square. You can use squares to check the corner joints, but for any square or rectangular assembly, measuring diagonal distances between opposite corners is a simple and proven technique. Make sure the assembly is flat and free of twist or other distortion. Simply hook the end of your tape rule on one outside corner and record the distance to the opposite corner. Then repeat for the diagonal between the other two corners. If there is a discrepancy, push on the corners with the longer reading and re-check until the diagonals are of equal length. With the diagonals equal, the assembly is square.

This technique is based in part on the Pythagorean theorem of right angles in triangles. For shop or construction use, laying out a triangle with sides measuring in multiples of 3-4-5 respectively will yield a true 90-degree angle at the corner opposite the longest side (hypotenuse).

If you are in a pinch without a tape measure, or simply want to avoid numeric readings, you can use a long narrow strip of wood to take a direct reading on the diagonals. Cut one end to a point and fit it into an inside corner of your project, then mark the stick where it intersects the opposite corner. Try the same fit on the other diagonal and compare the readings; adjust until they are equal.

SHOP TIP—When you check the end or edge of a board for square, hold it up toward a window or other light source. A small gap toward either side will show up as a crack of light and you’ll know the cut is not square.