Do You Really Need to Measure Diagonals?
I was one of those kids who continually asked “why?” and I’ve never outgrown that. My mother taught me how to look things up and my dad, a card-carrying chemical engineer, taught me to weigh the results of anything I read against common sense and my own experience. Before I ever wrote a word about woodworking I spent many years learning and practicing the trade. I also read almost everything available written about the trade. When there is a disconnect between what is written and what happens in a working shop, my “why” reflex kicks in.
Yesterday I put together a medicine cabinet that I began in a recent class. It goes without saying that I want the carcase to be nice and square when the glue dries. In this piece the top and bottom fit in shallow dados in the case sides. This time around I added trim head screws that will be hidden by 1/4″ square plugs. If you read a lot about case construction you may be expecting me to say “then I measured the diagonals to check for square”. I did think about it, but in the context of “why does almost everyone recommend doing that, when I hardly ever do?” This is one of the things that well-meaning people read and repeat, but most of the time there are better alternatives, assuming your goal is to actually make something.
Measuring diagonals is a valid technique, but it is a fussy process on something the scale of a typical cabinet. I think it’s better suited to larger scale work, something that is much bigger than any of the squares I have available near my bench. I used it the other day to set the posts of a pergola with a 10′ x 10′ footprint on the patio. In the shop it’s faster and easier to simply hold the parts in place one corner at a time.
So I reached for my Besey corner clamps.The beauty of the corner clamps is that they ensure that the two parts that make up the corner stay together at a right angle. It doesn’t matter how thick the parts are (up to the limit of the clamp) or if they are the same thickness. Put the pieces together, set the clamp and move on to the next corner. There is an open area on the outside of the clamp that lets you get in there to set a clamp, drill a hole, drive a screw, or shoot in a nail.
I bought a pair of these about 25 years ago, and I think they cost about $20/each. I just checked online and the price is about the same. I can’t imagine what it might take to wear one out. The only maintenance required is to make sure there aren’t any dried bits of glue on the faces or the screw.
While the clamps are handy, they aren’t essential; what you need is something square to clamp in the corner. In several of the cabinet shops I worked in, everybody made a few sets of L-shaped blocks from two layers of 3/4″ thick particle board or MDF. If the legs of the L are about 3″ wide, the blocks won’t flex as you bump the pieces into place and set the clamps. You can also use a block of wood with a square corner. If you use blocks, it’s a good idea to knock of the corner to prevent the block from sticking if you have any glue squeezing out.
I also use my speed square for the same purpose. I don’t remember what I paid for mine, but you can get one for less than $10. It’s also handy for setting up jigs and checking corners for square. Definitely not a “designer tool” but you should have one or more.
My hope is that you don’t accept the information in this post without going out to your shop and trying my suggestions. Compare the ease of clamping something square in a corner with whatever method you may use to measure diagonals. Examine and analyze how long the task takes, your levels of confidence vs. frustration and the quality of the results.
You might prefer to measure diagonals to prove a case is square, or you might not. Almost every task in woodworking has a number of viable alternatives. What tools you have, you budget and how your brain works all factor into the equation of how to do anything. The best way to find the “best way” is to step away from the computer screen, put down the book and go out to the shop and find out.
Amen on the corner clamps Bob. I have resisted buying those little doohickeys that go on the end of a tape to hold it on a corner. I can never get closer than a 1/16th variance anyway. If it looks right and nothing rolls off it, it’s good, move on.