I’m venturing into steam bending in order to build a couple of oak rocking chairs that are replicas of a 1920’s rocker.
My question is: Is it possible to over-steam the oak?
My success so far is acceptable–I have bent the upper and lower rails that are curved to conform to the sitter’s back, but the reason I say “acceptable” is that even though I got quite a bit of springback, the curve will still be enough to be comfortable. But, my results would not be acceptable if I were trying to exactly match my intended radius. The radius I aimed for 19-1/4, and built my form to a radius of 18-1/2. The final radius is about 21.
The pieces were 5/8″ thick x 5″ wide, and I steamed them for about 2 hours or more, but the temperature in the steam box reached 200F for only about the last 30 minutes.
Some of the reading I’ve done warns against steaming too long, saying that the wood can become brittle, but others describe steaming that goes over 3 hrs.
I’m wondering if there’s any less springback if the wood is steamed longer. BTW, I did not get any splitting or tension breaks on the outside of the bend, and did not use a bending strap.
Replies
rdesigns,
A couple thoughts:
the drier the wood when you start, the more difficult it is to bend. That is to say, Green wood that you just busted out of a freshly felled log will bend very successfully, air-dried lumber somewhat less so, and kiln-dried lumber will bend poorly.
The faster you can get the wood from the steam box into the form, the better. Every minute the wood spends cooling off before it is pressed into shape makes it less bendable. you want the forms beside the box, and all clamps wedges, etc right at hand.
Sufficient quantity of steam is important. You want to see steam pouring out the end of the box opposite the inlet. The bigger the steam box, the more volume of steam that you need. Insulating the box to reduce heat loss doesn't hurt, either. Rule of thumb for time in the box is about 1 hr/inch of thickness.
ray
Yes oversteaming is entirely possible......
......and is usually attended with bad results if the bend is anything but small.
To prevent springback, you need to overbend slightly. Sometimes you can feel the wood "give".
But if you oversteam, then the wood will most likely crack. It didn't seem to do that with your attempts but depending upon the curvature you are going after, oversteaming risks a split.
The rule of thumb is 1 hour of steaming per inch of thickness - smallest dimension. So for your application, you would steam 5/8 of an hour.
Here are the general rules of thumb to follow for steambending:
1) You get best results with freshly felled wood - the greener/wetter the better.
Air dried is next best, kiln dried is last best. The latter two can be helped if you can soak the wood for a while, and get it's moisture content up, prior to steaming.
-fear not about using green wood to then build with - the act of steaming prevents rot. This is done in boatbuilding all the time.
2) Straight grain - no runouts
3) Get the steam box up to temp FIRST prior to inserting the wood. That way you control the steaming time more exactly. You want a full head of steam blasting out of the box before you put the wood in.
4) Be sure you have enough fuel to keep the steam going the full length of steam time. If steaming ends before it's supposed to you CANNOT re-steam that peice of wood some other time.
5) Cut your pieces much longer than they need to be - this makes the bending easier.
6) Overbend slightly
7) Clamp and let cool overnight
I have a write up on my web page:
http://home.comcast.net/~saville/Steambend.htm
My web page regarding my restoration of my Herreshoff S-Boat, wherein I have steam bent 60 oak ribs (see attached photo) and have more to go:
http://home.comcast.net/~saville/SBOATrestore.htm
Now that I've read your piece in the link you gave, I am surprised I've had the limited success I did with the number of things I did (and didn't) do that made it harder.
Cut wood to final length before steaming/bending
Added water to my boiler (a 1-gallon kettle that could only be filled with about 2 qts) during the process
Did not bend the pieces quickly enough
I'm not sure if I over-steamed or under-steamed, because when I inserted the wood in the box, the steam was pouring in alright, but the temperature inside the box had reached only about 140. It took about 45 minutes before it got up to 200. The total time in the box was about 2 to 2-1/2 hrs with the 7 pieces I've done so far.
Improvements I'm planning: Get a bigger capacity boiler so I don't have to add water. Insert wood only after the temp in the box reaches 200. Rig up a way to bend more quickly. Insulate my box with 1" foam board.
One feature of my steambox that seems to work well is using the one-pipe design that was done in old steam heated buildings. The idea is that you use a steam supply pipe that is large enough to also act as a return for the condensate; the steam flows up thru the center of the pipe, and the condensate runs back clinging to the wall of the pipe. This only worked with low-pressure steam systems, which a steam box obviously is.
I added V-shaped fins along the bottom of the box to channel condensate toward the center of the low end of the box, where the steam tube enters the box. All this does not mean that all the steam condenses and returns to the boiler--plenty escapes at the door on the upper end of the box. It just means that whatever condensation is produced finds its way back into the boiler, but not enough, as I mentioned above, to alleviate the need to add water partway thru the process.
P.S. Your page on rebuilding your S-boat is absolutely fascinating, even though I've been landlocked all my life. I am consumed with jealousy ;-)
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