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More on Steam Bending12/17
In the thread below, I think we all agreed that it would not be worth the time and effort to steam that part, but I regret not getting a free meal out of David Sochar, although I was wanting to change it to a quantity of beverage. And he expressed that he would like to se the setup, so here is how I would bend a piece that big or on up.
That being said, he mentioned that he would like to see how I would manage a bend that thick and tight, so I thought I would share my method.
I already had a cheap pipe bender, which I got from Harbor Freight. It was not hard to adapt a new surface onto the top of the dies which fit onto the end of the hydraulic ram, for the inside radius.
Then, on the two pins which had coved rollers, I simply put a piece of pipe over them, to give them a straight roller.
I still use end compression straps, and even have a wood stiffener outside of the straps, which aids in preventing reverse bending from the substantial amount of wood on the inside of the radius looking for any relief.
Other tips. This time of year, if it is cool or cold out, I would lay out a length of foil to quickly wrap around the part to be bent. Not only is this good to cut heat loss, it would prevent evaporation, which would carry the heat away faster than if it was dry.
You need to make sure the wood is up to about 210║ F all the way to the core, and it needs to stay that high all the way through the duration of the bend, so anything that can be done to keep it there helps ensure success.
Pumping this jack handle is slow enough to allow the cells to creep. You can't push it too fast.
After you reach the full bend, the foil can be knifed off, allowing the air to start the cooling and setting the new memory.
There is always some discussion concerning compensation for springback. When it has set, I have measured across the hypotenuse, and written it on the side, only to find that there was contraction, due to the shrinkage across the grain through the bend as the wood dries on down. So always try to plan for some means of dealing with those issues toward the end.
I don't get that many jobs which require steam bending, but I always enjoy the challenge, and feel good when I succeed.
I already measure the temp in the chamber, but on my next thick bend, I'll have one of those internal thermometer fitting in a close fitting hole into the middle of the thickest part, so I know exactly when the middle is right, not having to depend on the one hour per inch shot in the dark.
Well, I'll keep 6 bottles of a good quality malted beverage on hand for you Keith, even though I don't see any pictures. It's worth it just to know that there are people that can do that kind of work. A tip of the hat.
What I have a hard time with is after always working with kiln dried lumber, is how will the green or high MC wood dry out? Will it move around on you or do things that are more unpredictable than the stable stuff I am used to? How long does it take to - air - dry? Or is there some other drying method you use?
It certainly adds to the display of craft and skill to bend wood for something like a Jenny Lind bed.
Me? I'd just laminate about 5 or 7 plies of wood with every other ply at about 30 degrees from the other to eliminate the tendency towards breaking short grain.
Remember the slew of 3 ply Oak toilet seats from a few years ago? A center ply of 1/8" Oak at 90 degrees to the faces was supposed to keep them from splitting, but it also seemed to break the rules.....
I'll try to come back with some pics after I teach the class next month.
I believe that I have read that the heat in kiln drying causes the lignin to set up, and not perform the same, but I'm not sure about that, one way or the other.
Even without that, taking 6 ~ 8% MC, KD wood , then suddenly wetting it on the outside like the steam will do, would likely swell the outside enough to cause internal splitting, which may not show up until the part is much further down the production line, like when a shape is being more developed. At that point you really have a lot invested in time, to be finding that you made that mistake.
So if dry wood is going to come out around 18%, going in around that range lowers the risk, by not changing that much.
I have put some dry wood in the chamber, just to see what MC it would come out at, and although I don't recall what species it was, it was 18% by my meter. I would imagine a very absorbent wood, would gain more than a more dense closed cell type, and maybe a fair amount of variation within species.
As for drying, it will loose a lot of that surface moisture, just giving up it's heat, but then drying on down from there may depend on the nature of the piece and type of wood. I'll take it plenty slow at first, but then may put it in a chamber that I heat with a small space heater to finish it off.
Dr Wengert has stated on wood web before, air drying will only get you down to 12%, but that just heating the ambient air almost anywhere 25║ higher will dry the wood on down into the 6~8% range. If I'm wrong on any of this, I hope he will jump in here and correct me. If there is anything I hate worse than getting bad information, is being guilty of spreading it on.
I didn't read the "other" thread your referring to, however I have seen some fairly sizable timbers bent that surprised me! I believe it may have been an episode of Nova where they re-created Roman chariots. They used an old school wood shop inů..Egypt maybe?, anyway they bent some thick timbers, like 12/4 or 16/4 and I was amazed watching it. Very low tech and they did run into trouble with some of the tight bends. Overall though it was a very informative few minutes on what can be done with the right knowledge;>)