This FAQ on bending wood is provided courtesy of Gregg Germain. Any comments would be welcome. Comments should be directed to firstname.lastname@example.org.
I've been in the business of steambending wood for about 11 years now. I've built a variety of steamboxes and tried a number of boiler systems. What you see written here is a combination of reading and actual experience. Mostly experience.
All of my steam bending has been with either Oak or Mahogany. I've never tried any other wood as I do this work in my boatbuilding/restoration. So I cannot comment authoritatively on bending other woods like cedar, pine, poplar etc.
And if I haven't actually DONE it, I will not comment on it. I will not state anything here that I have ONLY read out of a book and not tried.
With that in mind, let's fire up the boiler....
To start with there are several rules of thumb which work quite well.
What you are doing when you are steaming wood for bending, is softening the hemicelluloses. The celluloses are polymers that behave the same as thermoplastic resins. [My thanks to John McKenzie for the last two sentences].
And you need BOTH heat and steam for this. I realize that some people in Asia "fire" bend their wood but invariably, that wood is quite wet - typically quite green. The Norse boatbuilders used to get their planks out for shipbuilding and sink them into a salt water bog to keep them limber until the time came to use them.
However, we are not always so lucky as to get green wood for our bending and you can have great success with kiln dried wood. It's useful if you have the ability to soak your wood for a few days so that the moisture content rises - those Vikings knew what they were doing.
You need heat and you need moisture.
The primary rule is the one for steam time:
One hour of steaming per inch thickness of wood.
I have found that you can OVERSTEAM as well as understeam. If you steam an inch of wood for an hour, try to bend it, and it cracks, DO NOT assume that you haven't steamed it enough. There are several factors involved which could explain the result - but we'll get to those later.
It is smart, however, to have a piece of stock in the steam box that is the same thickness as the piece you wish to bend, and that is expendable. PREFERABLY a piece taken from the stock itself. Steam that with the target piece, and after the requisite steaming time, take the test piece out and try to bend that to the mold. If it snaps, then give your piece MAYBE 10 minutes more. But no more.
Generally it is best if you can get green wood. I know that this makes the cabinetmakers among us shudder. But the plain fact is that green wood bends easier than dried wood. I can take a 6 foot long, one inch thick piece of white oak; clamp one end to the bench and hand bend the piece to the curvature I need - green wood is THAT limber. However it won't stay bent, of course, so I steam it anyways. In boatbuilding, rot is the main evil.
For those of us that have to worry about rot, the act of steaming green wood removes the tendency of green wood to rot. So no worries there - boat ribs are typically made from steam bent oak and will not rot in a well cared for boat.
But I've done a lot of steaming of kiln dried oak and it works fine too.
One thing you want to try to avoid in your selection of wood for bending is grain runout. This will promote cracking when you bend.
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 electric burners.
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 boilerAs 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.
Assume you have the wood cooking (it makes a nice smell) and the jig is ready. Take pains to place everything so that the operation of removing a piece from the box and bending it is a FAST SMOOTH operation. Time is CRITICAL.
You have only seconds.
When the wood is ready take it QUICKLY out of the box and bend it. GET CURVATURE ON THE WOOD!!!!!!!!!!! As fast as humanly possible. If inserting the wood on the jig is complicated, bend it with your hands (if possible).
On ribs for my boat - where there is a curve in 2 directions - I take it out of the box, slip one end into a brace and bend that end then bend the other end with my hands. Try to bend it MORE than the amount you need in the jig. But not too much more. Then slap the wood on the jig.
But I repeat you MUST get curvature on the wood immediately - like within the first 5 seconds. Every second the wood cools it becomes less flexible.
Length of wood and curvature at the ends:
There is practically NO WAY you can cut a piece to exact length and expect to get curvature near the ends. You simply don't have the strength and you will be thwarted by springback.
By the same token, if all you need is a 3 foot length, and the wood is greater than, say, 1/4 inch thick, you had better cut the piece 6 feet long and bend THAT. You can trim the wood to fit later. I am assuming the lack of some sort of hydraulic press in your shop - I know I don't have one. Cut the stick overlong remembering that the shorter the stick the harder it is to bend.
And if you cut it overlong, you'll have more curvature near the final finished end - the last 6 inches of a 1 inch thick piece of oak will be dead straight. Depending upon the curvature you need, you may have to resort to carving the curvature out of the end of the wood and should size it with that in mind.
When you steam bend apiece of wood, and clamp it to a shape, you wait 24 hours for it to cool thoroughly. When you take it off the jig, that wood will spring back somewhat. How much depends upon the grain and the type of wood - it's hard to say. If your stock has a natural curvature in the required direction to start with (I try to take advantage of this whenever possible), you will get less springback.
So if you have to get a certain curvature to the final product, make your jig with greater curvature.
Tis is the realm of black magick and I can't personally give you a figure. One thing I DO know is this:
It's infinitely easier to unbend some wood that was overbent, than it is to put MORE bend in a cool piece of wood (assuming you don't have incredible leverage).
Once caveat: if you are bending pieces that will be glued together to form a laminate, be sure that the jig is the exact shape you need at glue time - I rarely get much springback from well bent, glued wood.
There are an infinite variety of jigs you can build. No matter what type you choose, you can't go wrong if you own a clamp making factory - you can never have too many clamps. If you are bending wood greater than 1/2 inch thick you must see to it that the jig is built extremely strongly: the amount of stress on it is quite high.
Quite often people will use a metal strap along the outside of the wood as they bend. This helps to distribute the stresses along the length of the wood and helps to prevent cracking. This is especially true if you get grain runout at the outside edges.
Well that's all I can think of now. If I think of more I'll add it to the FAQ.
"I don't want to die, baby. email@example.com but if I gotta die...... Smithsonian Center for Astrophysics I'm gonna die last." Phone: (617) 496-7237 Robert Mitchum
August 3, 1998