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.
Most diagonal checks are made using a tape. Hard to imagine, but there was life and methods to check diagonals before the advent of the yellow tape, when they were checked with a device called pinch rods. I’ll come back to the tape, but first, pinch rods. Like so many things of the hand tool era, they are simple, shop-made items. First, I’ll describe them and their use, then look at some refinements.
Pinch Rods to the Rescue
They consist of two thin strips of wood shaped at end to a “chisel edge.” The strips are held together face to face, chisel edges out. Holding the rods across a diagonal, the ends are advanced into the corners so that they are a friction fit. Next, transfer the “pinched together rods” 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 is visual. In both cases, there is a need for one to ensure accuracy. With the pinch rods, it is important to have the same amount of chisel edge in 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, 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 do the work.
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 them “spot on.”
A Diagonal Tale
In a small shop, an old hand was gluing up a large, complex assembly with his apprentice, who was soon to graduate as a journeyman.
The glue-up complete, they were checking the diagonals. As a sign of his confidence, the old guy was holding the end of the tape with the apprentice checking the measurement. “How is it?” asked the master. “Near enough” came the answer. To the chagrin of the apprentice came the directive, “We’ll check it again.” After the moving around and fuss of the check, “How is it?” came the question. “Near enough,” the reply. A pause, then, “We’ll check it again.”
With buttons pushed and short fuses ignited, when the question was put for the third time, “How is it?” “Absolutely spot-expletive-on,” was the reply.
“Right,” said the old man, “then that’s near enough.”