Steam Bending Tight Turns
I think you have to go with bent lamination, cutting your stock into thin strips and then gluing it back together, clamped to a form slightly tighter than the 18" radius. This would allow for some springback. At that small a radius, I think you would need to cut the strips to around 1/16" thickness. A lot of work, but worth it if it's needed.
You could use strips of 1/8" on that radius. I'd glue it up at 17" radius to allow for spring back.
I have steam bent white oak (much better than those woods) for chair backs, but they slowly sprang back 50% over a year. Very disappointing. I should not have used kiln dried. Green or air dried is the way to steam bend, but it was too time consuming to learn and set up. Unpredictable, too. Make some extras.
I've done bentwood laminations with oak and cherry, using strips from 1/16" to about 3/16" thick. I make a positive and negative form to hold the pieces in place while the glue dries, and once it does dry, I remove the piece from the cauls and have never had any springback whatsoever. If there is no glue creep, then the pieces cannot move relative to each other, thereby locking the piece into the shape imparted by the forms.
Ditto the strips. I steam bend them first to get about half the way there. 1/8" sounds about right, but test them.
Make a form: I use an inner form the exact size and a segmented outer form, one or two clamps per segment. Notch the other form for the clamps as well. Cover the forms with packing tape to prevent glue adhesion.
Steam the strips and let them dry a day in the form. Glue and clamp. If you don't let the strips dry first, they will dry, get thinner, and the change in geometry will cause the curve to tighten up on you. Otherwise, you should have no appreciable change in radius. It is much, much faster to segment and bandsaw, but this looks prettier.
I have steam bent 2" x 2" European beech to a 48" radius that settled at around 52". The key was using the spring steel strap with handles and lots of muscle. I also left it in the form for 1 week and then on a peg board for 2 more. Beech is a great wood to bend.
The bending you would like to do can be done with pre-compressed lumber. Precompressed lumber is available from Bethel Furniture Stock.
From Gene Wengert, forum technical advisor:
There is a lot of technique to proper bending. First, the wood should be about 25% MC for severe bends and no drier than 16% MC for gentle bends. You may need to add end pressure in order to keep the entire piece under compression while bending. If you develop much tension, it will break. Then after bending, hold the piece bent and dry it to 7% MC.
The key to successful bending is the wood itself. In addition to correct MC, you should have the grain running parallel to the sides of the piece. Any grain angle will reduce the wood strength by 20% or more. Grain angle and moisture overshadow all other bending factors. Also, you will have better quality when bending the radial face rather than the flat grain face.
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