I bet you don't know what kerf is. Well, today
let's find out. The University of Houston's College
of Engineering presents this series about the
machines that make our civilization run, and the
people whose ingenuity created them.
Think about the labor of
sawing a log into lumber. Then it'll be clearer why
pioneers built log cabins. Think about the labor of
just sawing off a tree limb. Then compare that with
slicing a log lengthwise, again and again, to make
boards.
As soon as medieval millwrights harnessed water
power to grind grain, they turned around and made
water-powered saws to cut wood. They had to make
mills to cut wood, to make more mills to grind
grain. (Have you got that? There will be a test.)
Now, if you cut off a tree limb, you don't chew
much wood into sawdust. But slice a log into
boards, and the loss adds up fast. The width of a
saw cut is called kerf. That's an Anglo-Saxon word
related to our word carve. The kerf of a lumber saw
-- the width of its cut -- might be 3/8 of an inch.
Make 50 lengthwise cuts in a 4-foot diameter log,
and you waste a lot of wood.
Wood was dirt cheap in mid-19th-century America. So
we made fast saws with a wide kerf -- like half an
inch. Then we choked on the resulting sawdust. We
used sawdust to fuel the donkey engines that drove
the saws. We burned it to keep log ponds from
freezing in winter. We burned it just to get rid of
it. And still sawdust fires destroyed logging mills
with tedious regularity.
So kerf was a huge problem. After all, a saw is a
metal plate. It has to be wide enough to keep from
buckling, and the teeth must be wider still if
they're to cut.
Now, another problem: saw teeth work efficiently in
only one direction. You don't cut on the return
stroke. In 1777 an Englishman patented a circular
saw. Circular saws are fast, and their kerf is
reasonable. Of course they use a lot of power.
An American legend says that a Shaker lady, Sister
Tabitha, had a vision. The action of a saw and a
spinning wheel merged. She realized that a rotating
disk would make the sawing motion continuous. That
story could be true. America didn't take up
circular saws until 1814. By then, our Shaker
communities were very active.
Finally, the matter of veneer. We used to make
veneer by sawing millimeter-thick sheets. Kerf made
that very costly. In 1840 we find an American
patent for a knifelike device that peels a log like
an apple and yields a continuous thin sheet of
wood. No kerf at all! It was 1870 before practical
veneer cutters were in use, and it was WW-I before
we layered veneer to make plywood.
I moved to the lumber town of Roseburg, Oregon, in
1946. It was a land of huge conical iron slash and
sawdust burners. Particle board was still a
material of the future. Everywhere, sawdust went up
in smoke. We heated our house with it. The sweet
smell permeated the county. I didn't know the word
kerf yet. I lived only for a season in all the
sawdust it produced.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
