I just did a steam bending operation with 1/2" thick walnut. It was successful, but I was in such a connundrum over - should it be green? yes, only not. Can it be kiln dried? No, yes? I heard, soak it in water first. So I tried that. Now, the begging question is, how much water did the kiln dried wood absorb, how long should I wait to take it out of tension? I think it will spring back too much if it's too wet.
Springback (as a basis starting point) = archeight/(layers*layers)
Thickness of layers also plays a factor. More to follow on Thursday.
If the wood was already below the fiber saturation point, your brief soak added only unbound water.
You should be safe as soon as the piece feels the same temperature as other wood in the room. A day or at most two.
It won't "spring" if it's wet, but relax slowly.
2 weeks ago I attended the area Q MTCA meeting. There was a demo on steam bending by the boat school in Wilmington NC (a 1 year course on traditional boat building). According to the demo, wood needs to be in the 15% or so moisture content prior to steaming. The Hot steam drys out the wood, so if it is kiln dried to 5-6% you are more likely to get failure. They soak the wood in water (actually sink it in the river) prior to bending. They routinely bend timber up to 4 inches thick for railings on boats.
tim
MTCA= midwest tool collectors association
BUB is talking about steam bending one piece. You're talking about making laminating curves. Two different beasts.
But your formula is correct for laminated members, of course.
G
Wow. One thing I've found is that bending takes a LOT of pressure. How much of a bend are they putting into that 4" wood? Is there a Bobcat involved in the process?
It's amazing how much force you can exert with a piece of rope and a stick if you know how to do it.
Actually it takes a lot of force, and mustn't use too much pressure (force divided by the area it's applied over) . For thick bends you can easily require a force that's more than the crush strength (as a pressure) of the wood, if you let the force be concentrated in too small an area.
I've used hydraulic bottle jacks to bend thick timber (although not a Bobcat !). It's quite easy to get the force you need, but spreading it out with packing is hard - there's never enough fingers around when you pull the stack out of the steamer.
The best way is usually to use truck tie-down straps about 3"-4" wide, with wooden packing blocks loosely strapped to them. This keeps them all together where you need them.
When I was a kid I always wanted a Bobcat....
...The kind with wheels or the kind that eats small rodents?
Those skid-steer loaders that look like a bath-chair from Thunderbirds. I used to see them at agricultural shows and I was just taken with the idea of steering by sliding.
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