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Some of my favorite tools are those that conceal many
levels of usefulness under the guise of utter simplicity. The best example
I know of is the standard steel carpenter's, or framing, square. It looks
like nothing more than a simple reference tool, one you might casually
grab to use as a ruler if you'd misplaced your tape measure. What secrets
does this plain piece of steel hold? I'm going to show you a few.
All Kinds of Angles
The framing square has a body (the wider blade) and
a tongue (the narrower one), set conveniently at 90° to one another,
but it's much more than just a 90° reference guide. When you're building
a roof, a flight of stairs or any other angled structure, it can be used
to accurately lay out any angle from 0° to 90°. But to use it,
you've got to learn to speak a different kind of angle language
than they taught you in high-school geometry class.
The way angles are expressed for use with the framing
square is based not on the division of a circle into 360° but, instead,
on a particular kind of fraction. A carpenter will say that a building
has, say, a 5/12 roof. That's framing square talk-it means that for every
12 inches of horizontal run, the roof rises 5 inches. The reason the angles
of roofs and other building features are sometimes expressed in fractions
is that this mode of expression makes angles so easy to lay out with the
framing square. (Although any combination of numbers can be used to express
an angle, it's a matter of standard building convention that angles are
expressed in relation to a 12 inch horizontal run, which forms the denominator-the
lower half-of the fraction.)
The drawing here shows how the gradations on the outside
edges of a framing square are used to determine the critical angles necessary
to lay out a typical roof rafter. For any given pitch of roof, this simple
process automatically calculates the correct angle at the peak, the correct
rafter length and an accurate angled pocket (called a bird's mouth)
at the bottom where the rafter will mesh with the top of the wall.
Another standard feature of the steel carpenter's square
is the labeled, six-line rafter table etched on its side. This amazingly
informative table gives (among other things) the correct lengths for ordinary
rafters for every pitch from 2/12 to 18/12, and also the lengths for hip
and valley rafters for all these pitches (assuming that they'll intersect
with the peak of the roof at a 45 ° angle). All the information is
based on a 12 inch unit of horizontal run.
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One carpenter's square accessory that I'm particularly
fond of is my pair of angle buttons. These are little brass clamps that
grip the outside edges of your square, to aid in building rafters, stair
stringers or any other angled objects you need to lay out accurately.
To use them, clamp the buttons so they line up with the numbers corresponding
to the angle fraction you're after, and they act as reference stops that
make it easy to hold the square at exactly the same angle during a series
of repetitive positions. Absolute precision for less than $7.00!
Tuning Up Your Square
How square is your square? It's surprisingly easy
to check and, if it's a little less than precisely accurate, you can adjust
it. Here's how: Find a 4' (or longer) straight-edged piece of wood that
you don't mind drawing on. A fresh sheet of plywood is ideal. Hang the
inside edge of the longer, 24 inch blade (the body) of the square against
the edge of the wood and use a sharp utility knife to score a line across
the face of the plywood, following the outside edge of the tongue. (Don't
use a pencil, which would make too fat a line.)
Now, flip the square over so that it's in the same relative
position, but on the other side of the scored line. (The inside edge of
the 24 inch blade should still be against the edge of the plywood.) If
your square is perfectly true, and the edge of the test board straight,
the tongue of your square will line up exactly with the scored line. If
it doesn't line up, you have a little work to do.
First, lightly incise a 45° line across the point
of the square and, depending on whether your square needs to be opened
or closed, use a center punch to make a series of dimples in the metal,
either at one end or another, using a hammer and center punch. Do this
work on a solid-steel surface, such as an anvil or bench vise.
This kind of careful checking and adjusting isn't really
necessary if all you intend to use your square for is framing. But if
you use it for laying out marks on large furniture parts, you'd be wise
to invest the time.
Seeing Clearly
Over time, the numbers etched into the surface of
a steel square get harder and harder to see. I solve this problem -for
a few years, anyway-by wiping a light-colored coat of paint onto the surface
of the square, then wiping it off again, so the paint remains only in
the etched markings and makes them easier to read.
I'm a long way from being able to use a carpenter's
square to the limit of its capabilities. (Mine has at least one table
of numbers on it that I've never used.) I've heard of old-timers who could
use a square to effortlessly draw circles of all sizes. Who knows how
much more I don't know? Part of the carpenter's square's attraction for
me are the mysteries I haven't yet plumbed.
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