Garrett Hack replies: I use a good edge of a sheet of plywood and hold one leg of the square against it and mark a line along the other leg.
Flip the square and compare the new position of the leg with the marked line. If the square is slightly under 90°, give the inside corner a firm rap with a centerpunch and hammer. If the square is greater than 90°, hit to the outside of the corner.
Elliot Savitzky replies: There is an easy technique to check if your square is actually “square.” Simply take a sharp marking knife and scribe a straight line perpendicular to a square edge on a board using the square. Then flip your square on the vertical axis such that the edge of the square is now theoretically parallel to the line you just scribed. Using the marking knife, scribe another line next to the original line. Check the two lines to see if they are actually parallel (the gap between the two lines remains constant). If they are parallel, then your square is “square.” If not, there is a technique for correcting it in an article—Truing Up a Combination Square by Ben Strano in FWW #259–Jan/Feb, 2017. There is also a FWW video—finewoodworking.com/project-guides/hand-tools/truing-combination-square.
Bob LaCivita replies: If you have a high quantity square such as a Starrett , Brown and Sharp and there are many more, they are inspected and come with an inspection ticket with the tolerances that the square met.
A simple method to check a square is as follows. Get a straight board, place the head of the square on the edge of the board and draw a nice even line on the face of the board. Rotate the square 180 degrees. The head of the square is still on the edge of the board but on the other side of the line. If the blade of your square matches the pencil line, it is square.
Jon Siegel replies: You do not need to purchase an expensive reference square. Instead, use this method to test your square. Find a flat board that is as wide as the length of the blade of your square. Make one edge of the board perfectly straight. Use your square and scribe a fine line across the board. Turn the square over and compare the blade to the previously scribed line. It should coincide exactly. Because this method doubles the error it will reveal any significant deviation. This is a theoretically perfect test, but its accuracy depends on your ability to scribe an accurate line and the limits of your eyesight to detect any deviation.
Bruce Wedlock replies: I use a 6˝ engineer’s square for machine setup and checking other squares. It has a solid construction, so this is no possible error from an adjustment like in a combination square. Six inch solid squares from Starrett or Woodpecker are precise enough for checking other squares any woodworking machine adjustment.
I suspect no square is perfect, although the higher the quality the more likely it is close. With cast bodies and hardened steel blades few are adjustable either. Big framing squares are easy to adjust and could be used as a reference square.
Richard Oedel replies: Use your own square! If you extend your square out almost all the way and scribe a line using it, then turn the square over, using the same edge, and scribe a second line, you can visually see the error (actually, you are seeing twice the error—once going to the left and once going to the right, which makes any error easier to see). This is usually good enough for woodworking, and I can usually get my square “square” within about 0.002 inches over the 12˝ length.
Steve Colello replies: All my joinery work uses slip tenons. I have a horizontal boring machine (slot mortiser) and I use up cut spiral router bits because they evacuate the chips well. The up cut bits are made to remove the chips upward. However, when I rout a groove for stringing, I use a down cut spiral bit. The stringing groove is usually shallow and the down cut spiral bit will prevent chipping on the surface. This is especially important because most of my stringing work involves thin delicate veneers.
Claude Dupuis replies: Use a board that has been jointed straight. Using the jointed edge, strike a pencil line then flip the square over. If the square is true the line will be aligned with the square’s edge. Woodpeckers are making very accurate squares and you could use one with a great deal of confidence that it is square.
Steve Costain replies: I have a precision angle block that I clamp the square to and then measure the out of square with a test or dial indicator.