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Brian Nystrom

wrote:
On Jun 25, 10:36 am, Todd wrote:
A piece of 1" red oak can be bent to radius of 1" if supported by a
metal strap and 11.5" unsupported according to "wood Bending Handbook"
published by Woodcraft Supply.

I think you meant that one inch oak can be bent to a radius of one
foot! I am sure most caught that though.

No the original post is correct.

That sounds about right to me as long
as the strap is fixed at both ends so that the neutral axis is pushed
to the outer radius. Also, remember that it is primarily the heat that
plasticizes wood and that you do not gain any additional softening by
heating the wood beyond the boiling point of water.

Exactly, if you heat it to much it either crushes, or tears. Like I
said, a general rule is 15 min/quarter inch of thickness.

Nope. If you get it too WET, it will crush and/or tear. If you heat it
too much, it drives off the moisture in the wood and it becomes brittle.
The typical failure mode it a sharp break.

Luthiers use dry
heat as did barrel makers. I have never had very good results from
simply getting the work piece wet, it must get hot.

The whole point of using steam is that it heats the wood quickly and
evenly to a consistent temperature, the boiling point of water
(100C/212F). The moisture helps to carry and transfer heat more
efficiently than dry air does.

If you need to do further testing for your application before building
a steam box you can try boiling the piece of wood or wrapping the
section that you need to bend in rags and pour lots and lots of
boiling water over the rags.

A note. Once wood has been steamed once, the cells will not soften
again with heat.

Not exactly. As long as it has been steamed, it can be re-steamed. If
you heat it any higher, such as is done during kiln drying, it alters
the lignin in the wood and make bending it more difficult. However, I've
seen good bending results with kiln-dried oak that was subsequently
steamed, but you can't achieve as small a bend radius with it.

Also, when you remove the part from the steam, you
have literally seconds, maybe 7-10 to get the part bent or it will
cool and lose it's ability to bend, it will most likely crack.

This is a common myth that is pure bunk. It takes ~1 hour to heat 1"
thick wood sufficiently in a steam box, so there's no way that it can
cool enough in a few seconds to make any difference in how it bends. In
fact, one of the biggest causes for failures/breakage during steam
bending is rushing it. It takes a little time for the fibers in the wood
to slip and the thicker the wood, the more time it takes. Bending
should be done smoothly, deliberately and as quickly as the wood will
allow, but you have a LOT more than a few seconds to bend it.

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