Simple Options for Squaring Up Boxes
The final act of gluing up any frame or case assembly is to be sure that it is square. Most other checks for square are made using a try square — not so when the work is in clamps, the reason being that the pressure exerted to close up the joints and shoulder lines also bends the various parts of the assembly. Although it is only slight, the distortions which occur render a try square ineffective. What we do instead is to check that the diagonals are equal in length. If they are, then simple geometry tells us that the assembly is, in woodworking terms, "square." If the diagonals are not equal, the geometry is adjusted by moving the clamping blocks, which redirects the clamp pressure.
These days, most diagonal checks are made using a tape. In the hand tool era though, diagonals were checked with a device called pinch rods. Like so many things of that time, they were simple, shop-made items consisting of two thin strips of wood shaped at end to a "chisel edge," with the tips beveled to about 60 degrees. You can still use them today to great benefit, and the process is simple: hold the strips face to face, chisel edges out. Slide them apart, advancing the chiseled edges into the corners of your glue-up so that they are a friction fit. Next, and without shifting your grip on the rods, transfer them to the other diagonal. If the assembly is square, the rods will fit as they did in the first diagonal. This reading is tactile, whereas a tape measure is visual. In both cases, accuracy is crucial. With the pinch rods, it is important to have the same amount of chisel edge pressed against the workpiece at both ends — otherwise, you are measuring a dihedral. The feel of the friction fit on both readings is something that you need to experience to truly understand.
With a tape measure, the same is true in a different way. If you are working alone, then how and where you place the lip end of the tape has to be exactly the same on both diagonals. The tape must be as straight as it can be — no sags, no twists. In the best and simplest of situations, two people measuring a case has one person at the "holding end" of the tape out to two or three inches. The "measured" end can be seen very clearly. Not all assemblies are this simple. Sometimes, complexities and many clamps in the way make a tape impossible, but pinch rods can often come to the rescue.
When the diagonals are not equal, it's almost always the result of the clamping blocks not being positioned symmetrically, or they are sized differently in different corners. Clamps are only as effective as the clamping blocks you use. Move one set of blocks and check the diagonals. If they are worse, you went the wrong way. Reverse the procedure until you get the measurements "spot on."