RE: STEAM BENDING LUMBER
I need some advise on steam bending lumber.
I have seen several other people’s home shop operations in various publications and my old text books from woodworking school in the 1960’s. But here’s what I am using and I need to know if I am getting the maximum efficiency to steam bend 1″ thick lumber 3″ wide.
PROBLEM IS, I AM BREAKING ABOUT 50% OF THE STICKS I STEAM BEND AND IT’S VERY FRUSTRATING.
20 lb. propane bottle heating a 75,000 BTU stove;
2-gallon gas can (never used for gas as it was new when purchased) with a 1/2″ radiator hose connecting to a 10 foot long x 4″ daimeter ABS drain pipe w/ two 1/4″ drain holes. I usually heat a piece of wood for a solid hour before removing to clamp to a curved fixture.
IS THE RADIATOR HOSE LARGE ENOUGH DIAMETER TO EFFICIENTLY HEAT THE LUMBER?
AND, IS THE 1 HOUR PER INCH THICKNESS STILL A RULE OF THUMB?
ANY SUGGESTIONS?
UM
Replies
Chuck,
Let me be the first to ask:
1. What type of wood, and how is it sawn?
2. What shape are you trying to create and what radius for the bends?
3. How are you bending the wood in the jig, are you using a strap with end blocks?
John W.
1. I'm using basswood and maghogany milled to 3/4" x 3" x 6'-0" to 10'-0".
2. Three feet on one end is to be heated and bent in a smooth progressive radius to a 9" lift on the end (envision the tip of a water ski). This is a special project where several pieces will be steamed and clamped in a pre-built press. Later all pieces will be edge glued together. I think this process is a common process for boat builders. I hate wasting wood so when I break a piece in the press I have to think I'm doing something wrong in the steaming process. I think the waste is about 50%.
I used a rented steam powered wallpaper remover and it worked fine but then I built my own boiler. Steam is steam so what the hell, why can't I just boil water and pipe it to a steam chamber?
Why is it I always start out, "Well, I'm no expert..." LOL. I've done very little wood torturing, but I've done some. Are you breaking more basswood? I'm looking at a table of woods suitable for bending and basswood isn't on it. If the wood is cut such that there is grain run-out, or if it has been kiln dried or flat sawn, the odds of bending it with success are poor.
Steaming is an art. The one thing that has been impressed upon me is that you need LOTS of HOT, MOIST steam. As was noted above, if the wood is sitting on the bottom of the steam box, it might be moist, but it ain't hot, and it ain't steam. Using a strap on the outside of the bend, as has been mentioned, might help. But you must be quick going from steam box to bending jig.
You could, perhaps, do similar to what was done here. This set-up keeps the steam flowing even as the frame is bent into place. Note the steam exhausting out the top.
View Image
Mahogany is very short-grained and notoriously difficult to steam bend: when is comes to mahogany, I have learned that glue lamination is the best way to bend. When bending short-grained woods, or pieces in which the grain runs off the edge of the board, it sometimes helps to support the outside surface of the piece being bent with a stainless steel strap or other form of containement.
Good luck,Paul
Whether you think you can, or you think you can't, you're right.
Best to use straight-grained air-dried timber and bend it with, not across the grain.
Kilned wood in some species can have huge failure rates. Same with any kind of grain runout...the closer to riven the better....and if you have grain runout, place it ont he inside of the caul. You can't just bend anything....species, grain, M/C all play a huge part.
An inch an hour sounds fine.
Edited 12/1/2003 1:34:13 PM ET by Bob
I have to agree with John. Your problem is more related to your wood and probably your technique and set up than the radiator hose diameter. Go back and read those books some more. Air dried wood is the preferred wood and certain species work better for bending. Woodcraft has an excellent book on bending that goes into what species bend best and some info on attainable radiuses. Also FWW has a reprint that covers the bending strap in detail. Plus there is springback to consider so you have to over bend. Laminate bending is whole lot easier. There's an old technique of frame sawing that has been modernized. They can gang slice multiple boards so they come out with a surface that is suitable for gluing up without needing another operation like sanding the surface and kerf loss is minimal.
Chuck,
Providing that you have a steady stream of steam escaping from your steam box, it's hot enough.
As others have said, wood species has an influence. Some species just don't steam well. I've bent kiln dried wood before, but Lee Valley say it can't be done, so I'd give wht they say some credence.
Surface condition has an effect - large nicks, etc... give the crack somewhere to start from
What nobody has said yet is that wood doesn't work well under tension - I'd wager that your articles are breaking on the outside of the bend, where the wood is in tension.
Lee Valley have a booklet on steam bending available on their website.
http://www.leevalley.com/wood/page.asp?page=43626&category=1,42172&ccurrency=3&SID=
Shows how to bend large radii.
Cheers,
eddie
Chuck...
Forgive me if I'm outa line... but I thought.... would kerf cutting make the bend any easier...?
Mike Wallace
Stay safe....Have fun
"would kerf cutting make the bend any easier...?"
Sure...and I do it as a last resort on boatwood that's too dry or too thick.
Fill the kerfs later with thickened, dyed boatbuilder's epoxy.
But I wouldn't do it on furniture where it shows.
Chuck,
As my first steam bending project I agreed to construct some curved benches out of ipe for a custom deck project. What can I say; I'm an idiot. Ipe is on of the most difficult woods to bend. I finally got good enough to only have about 25% failure rate beding 12' 1 x 3's to a 4' radius. Here's what I learned:
1) Your 1/2" feed line is too small. There's too much surface area relative to the volumn of steam and you're losing too much heat. I used a 2" radiator hose.
2) You need to get the wood up in the air comletely surrounded by steam. If you let the wood rest on the bottom of the pipe it won't see enough steam. I used a 6" pipe with wooded dowel supports to hold it up in the top half of the pipe.
3) Use lots of vent holes near the end of the box. You should see steam billowing from the end.
4) Use a thermometer to monitor your box. An oven thermometer is perfect; just place it in one of the vent holes at the end of the pipe. It should read ~200 deg. F when it's in operation.
5) Choose your lumber carefully to get the straightest grain possible.
6) You can over cook the wood. Don't assume that longer steam time is necesarily a good thing. With the ipe I found that 30 min. per inch worked best, probably due to the density of the wood. You may have to experiment a little.
Click on the link to see my steam bending saga.
http://www.exoticoutdoors.com/Project5.html
Chuck,
Ditto what Steve suggested.
Additionally, prior to steaming, the moisture content of the wood should be approximately 20% or higher. Although green wood is preferred, pre-soaking wood by submerging it in water for a week or more will elevate its moisture content; soaking in a 10:1 mixture of water and Downey fabric softener will reduce the surface tension inherent in the wood, allowing the water molecules to achieve better penetration than water alone.
Elevating the wood on slatted or perforated "duck boards" promotes complete steam circulation and prevents the wood from sitting in pools of cooled condensate.
When the wood is removed from the steam chamber, it is moved immediately to the bending forms where it is slowly and carefully subjected to even pressure until it conforms to the form; bending the wood too slowly can allow the wood to cool before bending is achieved, while bending too quickly can cause the wood the crack, split or rupture. Gently pre-bending parts like chair slats across their length and width before attempting the final bend allows the wood fibers to bend to occur across a broader surface area, resulting in a fairer curve and less likelihood of breakage.
Hope this helps,PaulWhether you think you can, or you think you can't, you're right.
Chuck,
You didn't indicate what length the pieces were or what the radius you are trying to bend.
Several years ago I steam bent a piece of hard maple 1" X 3" X 5" in a gentle curve. I used a wallpaper steamer and a piece of 4" X 5" metal drain leader. Raised the piece in 3/4" pieces, blocked up the ends of the leader, drilled a hole in the bottom for the hose and steamed for about an hour. Worked fine except for the springback when I took it off the form. Tried it again with a new form and it worked perfectly.
ASK
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