In Chris Schwarz Blog, Marking and Measuring

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Inspired by Robert W. Lang’s article on making wooden try squares in the Autumn 2009 issue, I decided to make a batch of squares this weekend.

Yesterday at lunch I bought some quartersawn European steamed beech that was on sale at the local lumberyard. The clerk at the yard described it as “rustic,” which must be a local Ohio term meaning “crap.” I found one 12′ board in the whole stack that had enough straight material suitable for making layout tools.

The price was right ($1.25 a board foot). And after a lot of handsawing and bandsawing last night I squeezed out enough beech to make seven squares and two nice bonfires.

Then the fun began.

And by fun, I mean translating 18th-century French. I spent an hour poring over “Le Menuisier En Batiment,” one of Andre Roubo’s volumes on the craft. He wrote specific instructions for the dimensions of a “triangle,” which is what he calls a square. I translated those dimension to English and then to modern Imperical dimensions. A French inch (pouce) is equivalent to 1.066″ in modern imperial. Each French inch is further divided into 12 “lines.” Each line is equivalent to .088″ today. The French foot is 12.44″.

Then I checked Roubo’s account against the try squares in Benjamin Seaton’s tool chest. And surprise, Seaton’s small wooden square is almost exactly the same size as Roubo’s, though Roubo’s is fancier.

But there was one curious detail about Seaton’s three try squares. The text describing them says all three blades taper in thickness. One blade is described as tapering from 1/4″ thick to 7/32″. Because all three taper, I presume it was deliberate. But why?

To take some weight off the end of the blade?

To expose more end grain of the blade (sort of like in a coffin smoother or a traditional straightedge) to make the blade respond faster to seasonal changes in humidity?

Beats me. I drew up Roubo’s square in SketchUp and plan to make a few of the squares with tapered blades. If Roubo mentioned tapering, I missed it.


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Showing 16 comments
  • CaseMan

    Interestingly enough, check out the taper on this square.

    http://toolmonger.com/2009/11/17/flexible-japanese-square/

  • Lewis A. Saxton

    It has been my understanding for many years that a properly made carpenters square was tapered so that it did not expand out of square when it was laying in the sun while you worked outside.
    Living in the west and southwest most of my life you don’t pick up an iron tool that has been in the sun without gently touching it to see if it has reached frying temperature.

  • PAUL (But I'm Much Better Now)

    Chris,
    Actually I have no experience riving wood. However I am Gathering info on froes, and may possibly attempt to have one made.

  • Adrian

    Chris,

    at the risk of repeating myself, if you’re going by the Paris standard as it was frozen by the 1799 law then that defined the meter in terms of the foot then the french foot is exactly 144/443.296 meters. This works out to approximately 12.78895 modern international inches. (Try to make a square with that level of accuracy!) Dividing by 12 to get the pouce (inch) you get 1.06575 inches for the french inch, and dividing by 12 again the ligne (line) works out to 0.088812 inches. Actually it seems kind of odd that someone would agree on the pouce and ligne but not the foot, since they are all related, and in fact the pouce and ligne derive from the foot. In other words, as Steve noted, if you multiply your ligne by 144 you should get your foot…but you’re off by about a quarter inch.

    Of course, if you’re not in Paris, then things could be significantly different. If you’re really interested in old French units of measurement, you might consider hunting up a copy of Zupko’s book. (It’s in English.)

  • Christopher Schwarz

    Adrian,

    The French foot value is one I got from Rob Tarule (who translated part of Roubo as he built his bench).

    If that value is wrong, could you provide the right one?

    All the best,

    Chris

  • Adrian

    Yes, indeed, a complication regarding old units of measurement is that they weren’t necessarily the same everywhere. This was true in Britain as well as France. So there was a Paris foot and associated units but other cities in France may have used different measurements. (Where did Roubo live?) However, each region generally used units that were internally consistent, so the inch should divide evenly into the foot.

    The value Chris posted for the French inch is the Paris standard. The conversion factor comes from a French law in 1799 that defines the length of the meter in terms of the french foot. (See "French Weights and Measures Before the Revolution" by Zupko.) This defines the meter as 3 feet plus 11.296 lines. You can turn this around to get the french foot at 12.79 inches and then determine the size of the inch (pouce) and line.

    The discrepancy that Steve noted between line and inch comes from the way Chris rounded off the values. The line should actually be 0.0888 inches. It does appear, however, that Chris got the length of the French foot wrong.

  • Sean

    I’m interested in calling the square a "triangle." Could this be the origin of "try plane"? Maybe this is what you were saying in the "Names for Planes" post. When you flatten a board, you aren’t testing it, or trying out how flat it is. You’re making it square. In Roubo’s French, are you making it "triangle"?

    "Try plane" sounds like it might be an effect of translation.

    Jus’ wondering.

  • Thick end at the hub. Thin end at the end, I meant.

    I wish my flaws would turn into features. Heh.

  • Solutions to these kinds of puzzles might be found in the construction methods?

    Much like the progression of laminated plane blades. Back when steel was expensive, but iron was cheap, it made financial sense to spend your money welding the two together. Later when sheet steel is plentiful, your doubloon goes farther if you just cut the whole blade out at once.

    Anything in the construction methods that give us clues?

    I know in my shop it’s way easier to plane or forge something tapered than it is to get it dead on parallel sided. Maybe it was just easier to take the (inevitably formed) thin end and at least put it at the hub instead of the end?

    flaw (in modern eyes) turned feature??

  • Christopher Schwarz

    Paul,

    I should have mentioned that the squares were made from mahogany….

    Ever tried to rive mahogany? Good luck.

    Chris

  • PAUL (But I'm Much Better Now)

    Perhaps the taper is from "Riven" stock! Methinks this would produce more stable grain.

  • Andy

    The old French units of measure seem to vary from one time and place to another.

    Templeton’s Millwright and Engineer’s Companion of 1854 lists the following conversions:

    1 Ligne = .091 inches
    1 Pouce = 1.090 inches
    1 Pied = 13.110 inches

    It would be alot easier if J-A Roubo would let you borrow one of his rulers to lay out the dimensions.

  • roger savatteri

    Chris,
    As to a reason for the tapering of the blade……
    My guess is that it sorta works like the clamping action of a Bowclamp.
    I’ll explain, When you have an especially long blade and you want to draw a line, after firmly placing the handle against the stock, you would then take your left forefinger (if your right handed) and hold down the tip of the blade. With your right hand you take your pencil and draw your line from handle to tip of blade.
    So ……. the taper helps the "clamping action" of the handle/blade hold itself to the stock being marked
    (while pushing down on the tip) and therefore helps to prevent it from slipping.
    (the longer the blade the more this comes into play)
    …..just like a Bowclamp!
    I’m sure if you were to make one with a taper and one without, you would prove my point.

    cheers, Roger

  • Bob Passaro

    @ Steve: Ah, "moment of inertia"! Thanks for reminding me of the proper term. That’s what I was trying to describe above.

  • Steve

    I have a Japanese square, and it tapers abruptly along each arm over a distance of about 1-1/2" from the right angle. Both arms are of uniform thickness beyond that point. I assume that the taper is both for balance (reduces moment of inertia) and strength (puts the most metal where it’s needed the most to keep the square square).

    By the way, your French numbers don’t quite add up. At 0.088 modern inches each, 12 lines would add up to 1.056 modern inch, not 1.066. Neither jives with the French foot, but that could be correct, since the inch and foot have separate historical derivations. At least you don’t have to deal with ciceros and didot points.

  • Bob Passaro

    I have an old Stanley framing square from probably the ’20s — metal, of course. The blade and the tongue both taper in thickness, getting thinner as they extend away from where they meet. I was puzzled by this too. Must of been a lot of extra expense to make them that way — they certainly don’t do that any more. So, why did they do it?

    I posed the question on the Old Tools list and the thinking is that the taper means less weight at the ends, which means it is much easier to maneuver and flop over back and forth as you need to. If you remember back to physics class, weight near the ends would have a lot more effect on this motion than weight near the center. I’m don’t know that’s the reason for sure, but it makes sense. And whether that applies to wooden squares is anybody’s guess.

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