A measuring tool you didn’t know you needed
I’m always amazed at how many woodworkers don’t know about calipers. These little metal instruments of hyperaccurate joyous delight are an absolute staple for machinists; they wouldn’t be caught dead without one. It was the only tool I was required to buy in order to complete my engineering degree. Yet many woodworkers don’t have one and wouldn’t know what to do with it if they did.
A caliper is a brilliant tool designed to measure hyper-super-ultra-accurately in three ways. First, it measures thickness by pinching. This is the best known use for calipers. You can pinch anything in their metal jaws, and the readout will tell you how thick it is to an accuracy of a few mil. Mil, by the way, is the oddly-named abbreviation for thousandth of an inch. My hair, for example, is four mils thick. You would know how thick your hair was too if you had a caliper. Doesn’t that just make you want to go out and buy one right now?
Anyway. There’s a second wicked-looking set of backwards-point-jaw-things on the backside of a caliper. You can use those instead of the main pinchers with exactly the same readout. These measure width–just stick them in a hole and open the calipers until they won’t open any more. You can use it to measure the width of a dado, the interior diameter of a pipe, the width of a kerf, and all sorts of other things. As with all measurements using a caliper be sure not to push too hard, as you can compress the wood fibers and make it look bigger than it really is.
Finally, the one most people don’t know about–depth. As you open the caliper, a metal protrusion slides out of the bottom of it. You can set the caliper on the edge of a hole and extend this protrusion to measure the hole’s depth, again to mil-level accuracy. Again, perfect for dado depths and the like, but I often repurpose it for length measurements as well–just lean the shoulder of the caliper against one end and extend the protrusion (damn I love that word) to the other end and you’re good to go.
So which caliper to buy? First off, stay away from slide calipers and vernier calipers, unless your goal is to exercise your eyeballs and improve your slide-rule math skills, respectively. There’s just no reason to abuse yourself like that when the alternatives are so cheap.
So that leaves you three good choices: a digital caliper, a fractional dial caliper, and a decimal dial caliper. All three are excellent choices.
I personally use a decimal dial caliper for nearly everything. I find it fastest to read, but that’s probably because it was my first one and I’m used to it. I also have a digital caliper, I use from time to time–it’s faster to read sometimes, but the numbers tend to jitter a bit when the thing you’re measuring is a little soft and/or has some give to it (like, say, wood). The one I haven’t used, but would probably recommend for beginners by reputation alone, is a fractional dial caliper. This mechanically indicates the measurement on a dial, but instead of reading off in mils, it reads off in fractions. The big downside is that it’s less accurate than a decimal dial or a digital caliper, because one revolution around the circle is one inch. On a decimal dial caliper, TEN revolutions are one inch, meaning you can read about 10 times as accurately. For this reason, you might want one of each–a fractional caliper for standard measurements, and a digital or decimal dial one for when you’re trying to shave a hairsbreadth (literally) off a tenon.
Oh, and length–I use 6″. A 12″ can be handy sometimes, although it’s a little less accurate due to flex over the length of the instrument. 4″ is nice because it fits in your pocket.
Get yourself a caliper, or two. You’ll thank me for it.
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