Steam Box Construction Ideas
Here's a handful of ideas for building a simple steam box for steaming and bending wood. December 9, 2010
Question
I have decided to experiment with steam bending .25" flitches before I glue them up into the final radiused chair back. I plan to steam five each of .25" flitches that measure 10"x 28". I think steaming the individual flitches first and then letting them set before I glue them up in the final press will help make the glue-up process go much smoother eliminating a lot of the spring back. I’m looking for some ideas about steam box design and construction. Aside from using an electric heat coil versus an open flame to generate the heat I’m open to whatever ideas you can offer.
Forum Responses
(Furniture Making Forum)
From contributor D:
I use a five gallon steel bucket (the kind my lacquer comes in) with a 3/4" pipe flange mounted on the lid to boil the water, using a propane camp stove for the heat source. The steam goes through a few feet of flexible vinyl tubing from the pipe flange to the steam box. I use a piece of PVC pipe for my steam box. It sounds like you'll need a bigger box, could be made of plywood. Don't forget to put in some small holes to vent the steam. You don't want your box to blow up!
From contributor H:
My boxes are always for short term one of a kind bends. As such I always need a different size and shape box. Years ago I started making them out of 1" Polyisocyanurate foam board. That is the 4x8 sheets of yellow foam board with foil on both faces. It's easy to rip on the table saw (don't forget to use bypass mode on a SawStop) and you can assemble the box with 16d galv. box nails by just pushing them in by hand. Fast, easy and insulated. A meat thermometer pushed in through the side of the box lets you know the box temperature.
From contributor W:
Bang up any old plywood box with a lid. Go rent a wall paper steamer and stick the hose through a hole in the box. Insert your sticks and cook until soft and tender. Serve up hot over the form and enjoy! After you've figured out that steam works for you (or not) you can build something a little more permanent.
I've also used a capped black PVC sewer pipe to cook my wood in. The portable wall paper steamer goes to the job-site or the shop and then back to the rental yard. I could buy one but steaming wood is always a last resort for me. Years ago I built a stainless steel vacuum chamber system along with (deadly) Anhydrous Ammonia gas. The whole thing cost quite a bit of money. It really worked too, but not quite as well as the black PVC pipe. After that, I learned to cut my curves instead of torturing the wood into submission.
From contributor L:
First of all, I could recommend a great book on bending which will show you lots of cool steaming set-ups, but it's awfully self-serving to do this so I never would. Just about anything that'll boil water will work to generate steam. Just make sure to check the temp in the box. If it isn't at least 200 degrees in the box, you need more steam. For larger boxes a turkey fryer is a great way to go and for smaller boxes like yours, an electric tea kettle (or two) will work fine. It doesn't much matter what you make the box out of as long as it's big enough for the parts. More importantly, it needs shelving. I use dowels to keep the part(s) suspended in the box. As said, drill lots of vent holes in the box. All you'd have to do is make your layers thinner and you could avoid all this steaming stuff. The springback would go away also. A third option is to whittle the part out of a solid block; no springback, no steam, no glue.
From contributor O:
I bought a commercial wallpaper steamer from a rental center for $75 and used PVC pipe found laying on a jobsite. It works great.
From contributor J:
I assume your bend is with the long way, not the short? In any case I agree that your laminations are a bit on the thick side. The steam should help some, but it’s just an added step. If you use 1/10" lams, with plastic resin glue, your springback woes will be history. Steaming is kind of fun though.
From contributor V:
I made mine from a wall paper steamer. $45 at the "box store" and $20 of pvc parts. A ten foot eight inch PVC pipe, cap on one end screw end on other. Four 12 x 14 plywood scraps with 8.5 inch holes in center, each .5 in closer to bottom for slope. Slip supports onto pipe, evenly spaced (PVC will sag if not) with a drill hole in the cap to release water. Mine is portable. I trimmed a commercial job with a ton of radius walls. Steamed base to fit and it worked well.
From contributor K:
If you are going to the trouble to steam, I wonder why you don't just steam it full thickness. I guess you have never faced the problem of trying to glue an arc of different radii which are cupped due to changing MC. You can bolt through some sheet-metal to make end stops for compression straps. Once you have done this easy bend, you have less work and time in the parts than trying to steam and laminate.
From the original questioner:
My original plan was to steam the full thickness piece of 1" into the form. If that didn’t work (due to the fact that the wood is kiln dried) I was going to try making the piece from five laminations. I am able to bend up the laminations in the form without steam bending the wood but I wanted to eliminate some of the internal stresses in the final piece. I did a sample bend and after the final shaping of the piece the glue lines are quite visible. The shape of the piece requires that the edges be rounded over and this cuts across the glue lines at a 45 deg angle making the glue lines look wider and darker. My final option is to cut the piece from a single solid block but finding 24/4 quarter sawn mahogany might be difficult.
From contributor K:
I didn't recall that you were using mahogany. I have never tried steaming mahogany, but I would be thinking about substituting sapele or andiroba if you have a problem with the mahogany.
From contributor L:
You commented that you had trouble bending the mahogany "because it was kiln dried". There are a host of reasons it didn't bend well, the least of which, in my opinion, that it was KD. I bend KD all the time.
Mahogany is one of the most difficult species to steam-bend. That said, I'd try the following approach if I were doing this job: 1/10th thickness veneers, matched in a flitch. I'd use medium color Unibond 800 for glue.
You didn't say how many of these you're making, but even if it was a handful, setting up to bend is pretty easy once you get the drill. You mentioned visible glue lines because they're cut at an angle. Yes, that's a challenge, but I wonder if you're getting sufficient and even clamp pressure. If you do a good job clamping, the glue lines will be almost invisible. Take a close look at a Thos Moser continuous arm chair up close some time for an example of bent lamination done really well.
From the original questioner:
Actually I didn’t have any problems bending the flitches. Perhaps my statement was a bit confusing but my point was that if the full thickness (1") mahogany started to give me problems during the steam bend (due to the KD) then I would try another method, like steaming flitches first than glue up. I like the idea if 1/10 veneers, Unibond 800 and a vacuum bag because I can use my compressor and a venturi valve to generate the vacuum for the bag which will eliminate any voids in the glue-up. There are six chairs and two tables that I have to make.
From contributor A:
My steam box is actually 4" PVC pipe from any home fix-it store. Be sure and put some dowel rods through the middle (side to side) to lay the wood on. This keeps the wood from laying in the condensation on the bottom of the pipe. I then use radiator hoses from tea kettles on a camper stove for the steam. One end of the pipe should be higher than the other to allow the condensation to escape (through a hole in the bottom). There should also be a hole in the top (higher end) for the steam to escape. PVC caps on either end keep the steam in the pipe. With this set up, you can also lengthen the main pipe at any time be using a coupler (for just a few bucks).
