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Steam Bend or Shape?

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Forum topic by rushbikes posted 03-14-2018 12:04 PM 336 views 0 times favorited 5 replies Add to Favorites Watch
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rushbikes

1 post in 226 days


03-14-2018 12:04 PM

I’d like to make a seatback rail out of white oak similar to the one depicted in the link below. In this particular case, it looks like they used 5 sections of wood with two shaped pieces in the corners to get the curvature. But I’ve seen continuous arm windsor chairs with rails that are 1 piece and steam bent.

How should I decide which technique to employ?

https://www.danishdesignstore.com/products/space-copenhagen-fly-chair-sc1-and-tradition


5 replies so far

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eflanders

310 posts in 1966 days


#1 posted 03-14-2018 12:58 PM

If you’ve never steamed wood for shaping, then use the other method as there is a bit of a learning curve to get wood to form. Here are some tips though if you to try steaming: Use green wood. It forms easier and will not spring back nearly as much. It is VERY hard to form dried wood as the wood cells have been changed that allow the wood to bend have been damaged. The wood needs to “cook” in the steam one hour at temperature per inch of thickness. Your bend for radius needs to be tighter than the required finished radius because the wood will spring back some after it is released from the form. Leave the wood on the forms until it has cooled completely. Thin strip will bend far easier than thick ones. Use a metal strap on the outside of the radius as support for the wood as you bend it around the form. This will be very important for wood that is more than a 1/2” thick and especially for tighter radiuses. Don’t put the wood into your steam box until the entire box is up to temperature which is the same as your local boiling point temperature. I usually cook my wood a little bit longer than recommended just to make sure the core of the wood has been heated through. NOTE: White oak is a great wood to steam bend! Most wood boats are made with white oak.

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BoardButcherer

144 posts in 210 days


#2 posted 03-14-2018 04:27 PM

I’d also add that for thicker pieces (1.5-2” it might help to bend it most of the way then place the form, if possible, back into the steam box for another 30 minutes or so before bending it the rest of the way on tight radius’. I haven’t done it personally but a guy I know used to brag while drinking that this was how he got bends in hardwoods that “no one” else could get.

Supposedly…

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Loren

10477 posts in 3763 days


#3 posted 03-14-2018 04:41 PM

That wouldn’t be practical to steam bend
in one piece. With a continuous arm windsor
it’s more or less a square cross-section so
it can be bent in more than one axis. Thicker
pieces can be bent but if you try to bend on
the narrow dimension of a piece that’s a lot
wider than thick it’s likely to twist at the very
least.

While it would be technically possible to steam
bend it, the setup to do so would be costly
to put together and require testing and redesign
before you struck on a solution that worked to
make the part correctly.

You’ll get it done faster and more predictably
by gluing up a blank and shaping it with hand
tools.

I do encourage you to try steam bending at some
point. It’s a terrific process for making some
curved parts efficiently, just not this one.

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runswithscissors

2818 posts in 2140 days


#4 posted 03-14-2018 10:25 PM

One way to approach this would be to laminate thin layers, over a form, of course. They should be a wide as the total vertical dimension of the back rail. When glue is dry, cut out the vertical shape. The form should have the curves a little tighter than the finished piece, as there will be some spring back.

But if I were doing this, I would pre-bend the laminations with heat (not steam) such as a hot air gun or strong infra-red source. This is very quick to do, and the cooling happens fast as well. I’d just do it free hand, using a bench vice and a gloved hand (applying the heat with the other hand). These don’t have to be precisely alike, as they will conform nicely to each other when glued and clamped over the form. There would be very little spring back doing it this way.

Though I always use a bending strap when heat bending, the wide pieces would make it very difficult to do so (What to use as a strap?—maybe a strip of 18 or 20 gauge steel, 4 or 5 inches wide?) Which is why the laminations should be very thin. The bending strap should be well-clamped both ends, as the point of it is to force the re-orientation of the wood fibers to occur at the inside of the bend. Otherwise, the outside-of-bend fibers will pull apart—that is, break.

Though I like epoxy for difficult glue ups, it has a reputation for not getting along very well with white oak, as I believe the tannins in the oak react with the epoxy. I can’t verify this from experience, however.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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eflanders

310 posts in 1966 days


#5 posted 03-15-2018 02:02 AM

Many boat builders use epoxy for glue up on white oak. But other adhesives will work too.

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