Building a Timber Frame
Copyright 2008 by Morris Rosenthal
All Rights Reserved
Steam Bending White Oak
Here we are bending 1-1/8" White Oak boards, 7" wide and 104" long, for use
in a laminated arch brace. The boards were first sliced from a 7"x12" green
timber, fresh from the sawmill using a chainsaw mill. Next they were planed
to thickness, roughed with #80 grit sandpaper, and the edges were chamfered
to discourage cracks from starting. The boards were each steamed for an hour
plus six or seven minutes, or 1 hour per inch, in our multi-level steam box.
The first step, after rushing the steaming wet board from the steam box to
the bending jig, is to clamp one end. Time is crucial as the hot boards dry
quickly, as you can easily from the color contrast in this picture.
The bottom board on our bending jig serves as a template for later boards,
and also gives us something to screw the boards to so the clamps can be removed.
If you don't use screws, you'll have to come up with a strapping system to
hold the bent board in place, because it would otherwise be impossible to
bend more than one board at a time with the clamps in the way. The ends of
the board will get cut off in any case, and you should always bend stock
that's a good deal longer than the final piece you're trying for. Over bending
by several inches also helps with spring back. We found that clamping the
board to the jig right at the top of the arch is necessary to prevent it
from rising off the mold.
Finally, we clamp the far end of the board to the jig. The wood doesn't exactly
bend like spaghetti, but it can be hand bent to the mold (a 17" radius) without
resorting to the clamps, which merely hold it in place for screwing. The
whole bending process takes less than a minute when executed properly.
In previous attempts to bend stock that had been over steamed and dried out,
we had to use clamps to draw down the last 8" or so. When those boards were
released from the jig they sprang back practically flat. Trying to bend board
with knots anywhere near the center of the arch is also a complete waste
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