Chair Making & Stem Bending
I am teaching myself to make Shaker style ladderback chairs. I will want to bend the 1-1/4″ Diameter rear posts about 2-3″ out of straight over the bottom 20 inches (this is about a 20″ radius if my math is correct).
1. Can I just steam the part of the posts that I want to bend, i.e., the bottom 20″ or so?
2. Do I need a bending strap for such a bend?
3. I know that springback is unpredictable, but would 10% sound like a good guess?
Yes, You can make a steam chamber which is a simple CDX plywood box, with holes cut into the sides where the part to bent enters and or exits.
This is a good way to cycle parts through, without having to open a larger chamber, loosing steam and heat.
I think you will get better results by using a backing / compression straps. You can get around having to buy the straps with the threaded adjustment, by just getting a sheet-metal shop to clip you some metal, that you fasten blocks wooden onto yourself.
You can get a really tight fit by flexing the strap out to get the end started, then driving it on in, This keeps the whole leg in compression, making for better memory, and less likelihood of failure which is most likely to happen on the tension side.
Thanks Keith. I'll probably "spring" (pun intended)for the Lee Valley strap, but I was balking at buying the clamp because it comes with too much other stuff (e.g.,socket wrench).Steaming part of the leg would allow me to solve my steam generator problem a bit more cheaply. However. Lataxe makes a good point about steaming actually drying the wood, which might reduce the risk of splitting. I thought that I wanted the post (mortise) to shrink around the stretcher (tenon)for a tight joint. I can see that I have a lot to learn. But that's one reason why I'm doing this.
It may be better (and easier) to steam the whole leg. Paradoxically, steaming can actually begins to dry out the green timber - maybe down to about 20% after it's cooled down. I don't know for certain why - perhaps the steam is swelling/breaking cell walls to let moisture out quickly; or maybe the sugars leaching out of the wood means it holds less water (sugar attracts 2.5X it's own weight in water when put into solution).
This reduction in leg moisture content means that when you fit the dry round tenons of the rungs (so the round mortises you drill in the legs can shrink around them) there is less chance of the legs splitting as they dry out around the tenons. When I make a ladderback chair (similar to your Shaker style) I steam the front legs too, even though they won't be getting bent, for that reason - to reduce their moisture content before drilling the round mortises.
The easiest way to bend the legs is to make a single softwood former with the required curve (actual curve plus another few degrees of bend). Then put two steaming legs parallel on the former and clamp both ends with double arched pieces of wood and F-clamps to keep them in place. You have to clamp them quick-like. Leave for 24 - 48 hours and they should have dried into shape.
You can make the wooden double arch things by drilling two 1.3 - 1.5" diameter forstner bit holes in a square of wood then cutting it in half. An F-clamp will grip the (flat) bottom of your former and the flat top of the arch-block, which are not quite parallel to each other but the woggly foot of the F-clamp will automtically compensate. The arches need to be just slightly greater in diameter than your legs.
Edited 9/15/2008 12:33 pm ET by Lataxe
Thank you, Lataxe. Very interesting. My thinking was that a 20" long(more or less) box would be easier to supply with steam than a 48" box.I take it you don't feel that I need to worry about a compression strap for this large a radius?Doug
C,I suppose the degree of force needed to bend chair legs might depend on the type of timber you use. Oak and Ash certainly bend easily without a compression strap, at least to the degree necessary for rear chair legs. Maple and cherry I don't have experience of but hear they're harder to bend.My leg former does have a handy Big Lever on it to bend the legs down so that the little arches can be clamped in place at either end. This is sort of a compression strap, I suppose; but is just a temporary helper and not a strap. :-)My steam box is around 5 feet long with an end dimension of around 10" X 10". It has a 1" diameter hole at one end and a loose-fit lid with chamfered edges at the other that just jams in place. It has a couple of feet on the box-bottom to keep the steam-pipe end up and the small drain holes (at the other end) down. The parts to be steamed sit on a few floor-battens glued in at an angle to allow any condensation to run off them. It was made of old 3/4 ply, screws and a bit of bathroom sealant.A small wallpaper stripper machine (like a weak electric kettle) blows more than enough steam into this volume to keep things hot. The steamer has a larger tank than a kettle so will steam for an hour or more on one fill. But it's wattage is pretty low. It was also cheap (under £20 as I remember) and came with all sorts of hoses and gubbins for carpet cleaning etc... Incidentally, have you made a former with the right amount and shape of bend in it for your legs? I imagine there will be various patterns for different chair types on the web.....? Or do you have a handy book? You'll also want a former for steam bending the ladders in the back, will you not? I use a two part "male/female" softwood former for this and squidge it up with 3 or four steamed ladders in it at a time, using those ubiquitous F-clamps. I have a ready-made bent-ladder supply lurking in the shed now, all dried out and ready to be shaped for the next chair or three. Similarly with various lengths of rung. It's only the legs that need to be shaped, steamed, mortised and fitted all at (more or less) the same time, as they must be semi-green during the chair-build.Photos of these devices would probably help but all the stuff is in a shed elsewhere just now and I'm also having problems posting photos to this site (again) after "upgrading" the browser to IE V 8 (beta).Lataxe, a victim of Mr Gates
Hello, againYour information is most helpful, especially about your steam supply and the size of the box. I'm happy to have a testimonial for a wallpaper steamer. That's one of the alternatives that I am considering. I wondered whether the electric ones would be able to produce enough steam. The box you describe is approximately the size I had in mind. I'll probably make mine a bit smaller -- large enough to hold two legs and four back slats.Interesting that you use a sort of press for bending the slats. It makes good sense. I have yet to make my bending forms, but I intend to follow your advice about the "U"-shaped cauls.You ask whether I have books. Oh, my, do I have books, and wonderful reprints of past FWW articles, and various web pages. But of course they can't tell me everything. I shall just have to jump in and start swimming. It would just be such a bi*ch to run out of water, clamps, etc. etc. in the event.I aim to make two chairs that will harmonize well with the ones we already have in our dining room, but a bit wider and deeper, since we and our friends seem a bit wider now than when we bought them 40 years ago (the chairs, I mean). Maybe we are deeper, as well. Depends on whom you ask. I have made a child-sized ladder-back chair for practice, with straight legs, straight slats, seat woven of cotton Shaker tape. I made a few mistakes, but it came out OK. I shall make another, this time with curved slats, to practice that. Then it will be show-time.Like you, I am an amateur (I insist, in the original, positive sense of the word). As always, my dilemma is that I like "inexpensive" and hate "cheap." It takes a lot of studying to walk the line between them. BTW I spent a little time in your neck of the woods (roughly) in Lymm, near Warrington, while I worked in Manchester. Beautiful country, nice people. I miss it.All the best.
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