There are lots of ways to confirm that a carcase or sub-assembly is square, but my favorite method is to use so-called “pinch rods.”
Pinch rods are simply two pieces of wood that have pointy end bits. You put the pointy ends into two diagonal corners of your carcase. If that measurement matches the measurement of the other diagonal, your case is square.
There are lots of ways to hold these two sticks together, from splines to blue tape to a commercial product from Veritas. The best way, in my opinion, is to make a metal and wood assembly yourself. I first saw this sort of arrangement at Roy Underhill’s “The Woodwright’s School.” I was so enamored with them, that I decided to make some for myself and friends today while taking a break from an editing project.
I went to the home center and bought some 3/4” steel square tube (with 1/16”-thick walls) and some 1/4” x 20 thumbscrews. For about $12, I got enough metal supplies to build a dozen sets of these jigs.
Here’s how a friend and I made them:
Cut the steel tube to 7/8” lengths – you’ll need two of these bits to make a single jig. Deburr and dress all the raw edges. Drill a pilot hole in one of the pieces and tap it for 1/4 x 20 hardware (a very common tap size).
Mill up some wood. You’ll need something stable with straight grain. I used old heart pine flooring I had sitting in the racks. The interior dimension of the channel is 5/8” x 5/8”, so I milled up two sticks that were 5/16” x 5/8” x 30”. Two 30”-long sticks will handle most assemblies, but your mileage may vary.
Point the ends of the sticks and you are ready to assemble.
Some of the hardware I bought was covered in zinc, so I stripped that off with a citric acid solution. This greatly improves the appearance of the metal bits.
That’s about it. Sleeve the two metal bits onto the wood and engage the thumbscrew (make sure the screw is pressing into the face grain of one of the sticks instead of pressing into the seam between two sticks). You are done.
You can finish the sticks if you like, but heart pine makes its own resinous finish.
In my opinion, pinch rods trump all the other systems I’ve used for checking the squareness of an assembly, from measuring diagonally with a tape to using oversized try squares.
— Christopher Schwarz