79 Woodworking Steambending wood: Steamboxes:
Description
This article is from the Woodworking FAQ Collection 1, by multiple authors.
79 Woodworking Steambending wood: Steamboxes:
It is not necessary - and is in fact detrimental to the bending
process - to have a steambox that is absolutely airtight. You WANT
steam to be emanating from the box. If you don't get a flow through of
steam you will not be able to bend the wood - it will crack as if you
steamed it for only 5 minutes.
I know - I've created a lot of kindling in this manner.
Steamboxes can come in many shapes and sizes. You want one big enough
so that you can suspend the wood off the surface, and get a good flow
of steam around most of the wood surface. A box made of 2 x 8 pine
boards will work. One suspension method is to drill a hole through the
sides and run a hardwood dowel through. The dowel holds your wood up
and minimizes the amount of wood touching a surface. You don't want
the box to be SO big, however, such that the amount of steam your rig
generates is too small to fill up the box. You want a wet, steamy box
BILLOWING steam. So the box has to be sized to the boiler (or the
boiler sized to the box ;^) ).
I have 2 boxes:
For small stuff like 1 1/2 x 5/8 by 6 foot long oak for ribs, I use a
2 inch diameter piece of PVC. I have it resting on a 2x4 so that it
won't deform under the heat. I've also nailed sides to the 2x4 so that the
tube doesn't flatten. For a boiler i use a whistling tea kettle
with the whistle and top taken off. A length of radiator hose connects
the kettle to a suitable reduction on the end of the PVC. For a heat
source i use one of those counter top electic burners.
Works great.
When I had to steam bend 17 foot long, 7 inch wide, 3/4 inch thick
mahogany for the new cabin trunk of my boat, I used a steambox built
with 2 x 12 inch pine. For a boiler i had a 20 gallon steel boiler.
Heat source was a propane burner I bought at Ace Hardware Store. This
burner is GREAT because it's convenient and mobile. It generates
45,000 BTU of heat. It's an aluminum
bowl on 3 legs with one burner about 8" in diameter.
Lately, I noticed a 160,000 BTU propane burner in the West Marine
Catalog for $50. I bought it. Now I'll be able to generate enough
steam to bend ribs for the Constitution.
Now when I say "one hour of steaming per one inch of wood" I mean one
hour of SERIOUS steam with NO interruptions. Therefore you have to
pick a boiler whose capacity will be sufficient for the steam time you
are looking for. I have used a 5 gallon UNUSED gasoline can for this
purpose.
NEVER put the wood in the steambox unless you have full steam and the
box is completely filled. Be ABSOLUTELY certain that you don't run out
of water BEFORE the necessary steam time. If you do, and are forced to
add more water give it up...you'll generate kindling.
One way of maximizing the water use is to have the box tilted at an
angle so that any condensation within the box runs BACK towards the
boiler.
Another way is to set up a siphon system so that the boiler is
constantly being refilled at the rate at which water is boiling off.
A crude ascii picture of this follows:
hhhhhhh s Where: b's are the boiler
h h s s's are the steam outgo
| h | h bbbsbbbb l's are a level indicator:
| h | l b b metal tube from side
|wwwwwhw| l bwwwwwwb of boiler with clear
| h | l b b plastic vertical tube
| | llllllllb b h's is water hose from
| | b b auxilliary tank
--------- bbbbbbbb w's are water levels
Aux tank boiler
As the water in the boiler evaporates, the siphon brings more water
from the auxiliary tank. the level gauge is a simple metal tube
extending from the side of the boiler with an elbow pointing up. Over
the elbow you slip a piece of clear plastic. This way you can
observe the level of the water in the boiler. The feeder hose from the
aux tank fits inside the clear plastic level hose so that you can get
inflow and still see the water level.
One important point:
If you find you have to add water to the auxilliary tank, be
sure to add water a LITTLE BIT AT A TIME. Otherwise the flow of cool
water into the boiler will inhibit the boiling and you will get an
interruption in steam generation: not good.
It's also best to begin with a full aux tank to start with so
that you minimize the need to add cool water to the aux tank. I like
to leave a little air space in the boiler when I begin.
Many steam boxes have a door at one end to allow you to slide in
pieces when you want to - and take them out when needed. For example,
in ribbing out a boat - something you'd like to do in a day if you
can, you crank up the boiler and (when steam is up) you put in your
first piece of wood. 15 minutes later you put in the second. Fifteen
minutes later the third and so on. Then, when the first piece is
ready, you yank that out and bend it. This is all supposing that the
process to bend and install the rib takes less than 15 minutes. When
the first rib is in, the second piece of wood is ready..and so on.
This allows you to do a great deal of work while avoiding
oversteaming.
The door serves another important function. And the door doesn't have
to be solid either - on my small steam box i LOOSELY stuff in a rag. I
say loosely because you want steam to be able to come out of the end
(remember you need steam flowthrough). The secondary purpose is to
preclude cool air from entering the steambox underneath the suspended
wood.
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