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An excerpt from the Spring 2007 issue of Woodworking Magazine, available on the newsstands now.
My boss, Steve Shanesy, held up a dial caliper to make his point about precision woodworking. “If you can work to 1″, then you can work to 11?64″, or to .005″ or to whatever,” he said. “Those are all just arbitrary numbers, and you can work to any of them.”
I allowed Steve’s admonition to become ingrained in my heart and hands that day. I bought a dial caliper. And for years I used that caliper as my sixth sense in the shop and experienced every aspect of my joinery through its steel jaws.
In many ways, the caliper pushed me to become a better woodworker. It showed me how closely each of my tenons fit. It pointed out every joinery flaw and forced me to find ways to work that were more precise and repeatable.
But the dial caliper can be a cruel master.
It measures things that are difficult , sometimes impossible , to do anything about. Let’s say your boards come out of the planer and they are .004″ thicker than what you wanted. What do you do? For years I struggled to get a sensitive touch with the adjustment wheels on my heavy machines. I succeeded, but I could never live up to the expectations of my caliper.
Then one day I was at a woodworking show in Canada and there was an old-timer there who was selling old folding rulers. They were beautiful things with brass corner joints. Some of the scales were made of ivory. Most were boxwood. Naturally, I checked the price tag on one. I don’t remember the price, but I do remember what was scrawled next to it: “French inches.” French inches? What the heck are those?
Before the French invented the metric system (yes, something else to blame the French for) and then formally adopted it in 1799, there were competing systems of measurement that would vary by region. The French pouce (inch) was a little shorter than the inch we use today, about 7 percent shorter.
Until that moment, in my mind there were only the metric and imperial systems. The idea that there were other ways to measure things in the world of furniture was confusing. And so I began to realize that all measurement systems are arbitrary. I eyed my caliper warily and wondered if life might be better if I switched to the metric system, where I could divide anything by 10.
But, as it turned out, archaic measurement systems aren’t arbitrary. As I read more, I discovered the Japanese shaku, an archaic unit of measurement still used today by temple carpenters. The shaku, developed independently of our system, is 11.93″ long. The ancient Egyptian foot measures 12.25″. And many of the measurements that eventually evolved into the imperial system were based on the human body, such as the cubit , the distance between an average-sized man’s finger and elbow.
And because our furniture is supposed to fit our bodies, it makes sense that our measurement systems should spring from there.
But what about the ancients and their way of working? Would they mock the caliper? Well, it turns out that tiny units are nothing new, either. The Indus Valley civilization (2,600 B.C.) had measurement units that were less than .07″. So while we desire to have our measurement systems reflect our bodies, we also need to quantify , measure , anything we can see or feel. Hence, the caliper.
In the end, I’ve concluded that for me, calipers are like another important ancient invention: beer. Both must be used sparingly , or I’ll never get anything done.
I always shoot for tight-fitting joints instead of hitting an arbitrary number on a caliper. I strive for beauty to the eye rather than on-the-nose tenon lengths.
But how do you get there? How do you teach yourself to make furniture without someone looking over your shoulder at the critical first stages of learning the craft? You need an unyielding master who can point out the things you haven’t yet trained your eye to see. You need a master you can someday outgrow or even exceed.
In the modern home workshop that master just might be a dial caliper.
– Christopher Schwarz
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Furlong/Firkin/Fortnight (FFF) system
It is perhaps remotely notable that using the FFF system the speed of light may be expressed as being roughly 1.8 terafurlongs per fortnight.(wikipedia)
When an Air Force General was assked about the accuracy of bombers he replied, "100% accuracy, every one hits the ground".
On the farm my father always talked of land measures, chains, links, rods, sections, acres. Then there was the proverbial 10 foot pole that you wouldn’t touch something or someone with, along with the distance that a bull can be thrown by the tail. There was the "hair", the "skosh" (sp) as in "just a skosh more", the titch and the nose. Whenever close work got difficult you probably weren’t holding your mouth right (Keep this in mind when carving or dovetailing).
Printing is measured in Points and Picas, shotguns in gauge (number of balls the diameter of the barrel to make a pound), wire has gauge sizes as does sheet metal, and nails are measured by the penny (d).
Be careful when you are at those flea markets that you don’t get a shrink rule to measure with unless you are doing foundry work. I wonder if they were ever laid out in pouces?
But I ramble.
I learned to mark a piece directly whenever possible rather than measure and transfer the measurement.
I was working on an 18th century American desk, I had the gallery apart and was interested to see that the slots that held the dividers were of varying widths and that the sides of the grooves were not parallel. Rather than plane all of the dividers to an exact width the maker just got them close. He squared a line in the case for one side and then scribed the other side to fit. Every divider was individually fit and they looked fine when it was all assembled. This saved the maker the price of a caliper and a lot of fussing around.
Mike
I do a lot of on-site repair work. Last fall, I was on a call and needed to make a small piece of wood to fix a slipping shelf on an easel. Normally, I’d run out to the van, pull out a Japanese saw, hold the wood on the bumper or toolbox and fashion one away. The homeowner invited me to his shop to do it there. I walked into a bright, spacious shop with every conceivable tool, enough to make me jealous.
He pulls out a dial caliper and measures the tenon on the existing piece to the nearest .001", then uses this to set his rip fence and cuts me a piece to use. [eyes rolling, he’s a retired engineer] I thank him, run back upstairs to install it and find there’s at least 1/8" slack because the old piece was just a loose fit anyway, since it merely kept the piece from rotating. [eyes roll again]
Accuracy is highly overrated. If it looks fair, it is fair (old boatmaker’s saying.)
Oh! What tangled webs we weave… or, to paraphrase myself, which came first, the dial caliper or the tenon? When we decide to set our standards of measurement to the specificity of a dial caliper, or any other precision instrument, we bind ourselves to an arbitrary standard that dictates all further actions. Which becomes more important: the mortise and tenon dimensions in relation to each other or their relation to the available settings of the dial caliper? The availability of hollow mortise machines, CNC machinery and even laser guided woodworking machinery alters our perception of what is more important.. to work to the standards of the machine or to the requirements of the wood.
If I choose a British pattern mortise chisel, I set my marking gauge points to match the width of the chisel. Likewise I score my tenon lines from the same standard. A standard that is variable… as I sharpen the chisel, there will be minor changes as time passes (unless it is chisel made to milling machine tolerances). The mortise and tenon dinna ken this relationship, they just want to be snug according to the structure of that particular piece of wood as it exists in the climatic environment of that day.
My only caliper is a plastic-nylon body thing that serves to transfer dimensions from one thing to another. I rarely if ever look at the scale on it.
Another view of the question is: The older you get, the less you care about the minutiea of measurement. Unless you decide to invest in a good magnifying lens…
Gary
Ian,
Good, good points. In the world outside of science, the terms "accuracy" and "precision" are virtual synonyms. And that’s too bad because their definitions in the world of science make them much more useful words.
Here’s a link that helps explain (with pictures!):
http://honolulu.hawaii.edu/distance/sci122/SciLab/L5/accprec.html
I’m going to revise the text on the blog to use more "precise" language. Thanks for the comments. It’s very much appreciated.
Forgive me for contradicting you, Chris, but you DO strive for precision.
As a chemist, it is far more important to me that I can do a measurement the same way 50 times, and get the same result. I don’t really care if the result is accurate. If I am precise (reproducible) but always 13 units low (whatever those units might be), then I can get a good sense of the true value by adding 13 units to my lab measurement, and end up with an accurate answer. This sort of correction is done every time you have a blood test done.
As a woodworker, it does not matter whether my half-inch tenon is 65/128" or 7/16", so long as I can reproducibly (precisely) fit a mortice to that tenon.
Your caliper was giving you fits, because you were striving for Accuracy (close to "truth"). The ancients were happy with precision (tight joints). I would submit (as I believe you are saying) that accuracy should never have been your target.
I believe you would have been far better served with a feeler gauge, to measure how close one piece was to the other. Guess that’s why striking knives are so popular – make one from the other. (And a hand plane helps too, to deal with that 4-thou misfit you refer to.)
I switched to using the 1 shaku = 10 sun = 100bu decimal japanese unit system about 15 years ago and have not looked back. (4×8 foot plywood sheets are a bit annoying, but they are metric thicknesses anyway which are fine) You might not know it but japanese tools aren’t actually made in millimeter widths and lengths, they are based on shaku, sun and bu. Since 1bu = 3.03mm most folks never need to know that their 15mm chisel is actually just 5bu.