Issues with Steam Bending Wood

[Alan Lightstone]

I need to bend 3/4" cypress to a hoop of 24" diameter. I've bent two pieces, but the problem I'm running into is large amounts of springback (approximately 6 inches), as well as the wood splitting at the end of the bend.

I'm thinking both are caused by the wood cooling too much before I have completed clamping it to its form.

I'm clamping it to 1.5" thick plywood, with holes drilled for clamps. I'm using the Veritas compression strap setup:

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The wood was steamed to 212 degrees F for 1.5 hours in a custom built steam box with an Earlex steam generator.

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As far as I've read, that should be more than sufficient for 3/4" wood.

Now, to my questions:

Is that much springback normal (it seems like 6-8 inches in a 24 inch diameter is huge springback)?

The wood is expanding dramatically towards the end, causing the strap to buckle. This wastes time as I'm trying to complete the bend. What am I doing wrong, there?

Is there a way to reheat the wood while it's on the form to continue the proper bending? (Putting the hole form with clamps in a makeshift steam box? Directing steam directly at the ends of the wood being bent? Using a torch and some water? Pouring boiling water on the ends?)

I've seen the Rough Cut show where Tommy MacDonald bends the wood for a reclining outdoor chair. I'm can't imagine getting that bent quickly enough around the forms. Does he have 6 guys helping him put on clamps? No way I could do that quickly enough.

[Sam Murdoch]

Hi Alan, I have not steamed cypress so I do not know how much spring back is normal but 6" certainly seems dramatic. Maybe you need to add more moisture. Just a guess. Your set up, and that you are allowing for plenty of wood past your form, otherwise looks great, though you might try adding a few more inches to the form with straight sides off the curve. Is it right at the ends of the form that the splits are occurring? Of course you could try 1/2" pieces. One more lamination might alleviate lots of aggravation.

You do need to work fast and that usually means lots of clamp helpers or setting one end into a hold/stop then wrapping to the other end as quickly and as tightly as you can to fix the last clamp. Then add clamps in between. The band is essential. Don't know how to soften the wood once it starts to cool. That is why I suggest adding more moisture (if possible with your set up). Someone with experience bending cypress might be able to offer more useful advice. I stick with ash or white oak when steam bending but even then you run into an uncooperative piece. Not unusual to bend 3 times to get one success.

[Dell Littlefield]

If your cypress is air dried, it will bend easier. However, canoe builders use thinner strips to form the inner and outer stems for the bow and stern of the canoe. The strips are clamped around the form while still wet and hot and left for a day or so. They are then separated, glued and clamped around the form until the glue dries. If you use a plastic resin glue like Weldwood, you will get less creep than using Titebond or Elmer's. You may still get a little spring back but it will be greatly reduced.

[Jim Mackell]

Just saw that show and he was steaming for two hours and using green wood to boot.! Try a longer run in the steamer.

[Mac McQuinn]

Are you getting good steam? Had an issue with Ash before and ended up adding salt to the steamer's water, which according to my Chemist friend lowered the water's boiling point significantly. Also I think you would be more successful using a longer piece and anchoring it further down on your bench on both sides and adding additional pull-in to compensate for spring back. I feel the wood fibers towards the ends are not being forcibly slid past each other, therefore springing back. I feel doing it this way would not require as many clamps and allow you to get things into position much quicker before cooling takes place. Then trim off the waste.
Good luck,
Mac

[Alan Lightstone]

I think the hint of attaching the first and last clamps first, then clamping in between is a great one.

I'll try the next piece for two hours instead of 90 minutes. The thermometer I'm using says 212 degrees, so I think the heat is adequate. I also think that 6" of springback is excessive. I'm thinking that the ends are not being bent, just the middle section.

For kicks, I put a plastic tarp over the whole wood/clamp assembly, and ran the steamer under it for an hour The end near the steamer did seem to bend more, and easily. It will be interesting to see if the bend lasts. I also ran a butane torch near the ends before I tightened the clamps, so I had the combination of heat and moisture. Hopefully that will help. I'll see in a few days when I release the clamps.

Speaking of which, how long do I need to let the clamps stay on / the wood to dry?

[Sam Murdoch]

I think the hint of attaching the first and last clamps first, then clamping in between is a great one.


Don't even clamp at the beginning but rather set the piece into a made up holder that will "grab and locate" ( top, bottom and faces) as you wrap to the other side.
The longer the piece can dry in the form the more memory it will have of the intended shape.

[Ryan Mooney]

Had an issue with Ash before and ended up adding salt to the steamer's water, which according to my Chemist friend lowered the water's boiling point significantly.

Nope (I had to look that one up to make sure), adding salt actually raises the boiling point, maybe up to 0.3-0.4C or so; so not a lot. Having a higher boiling point wouldn't be bad though because the steam would be hotter so it might help from that alone. However.. there are some other possible advantages, adding salt (and some other compounds but salt is easy) adds more nucleation points so you get more steam formation which also seems like a good thing.

Short Source: http://van.physics.illinois.edu/qa/listing.php?id=1457
but if you google search "salt water boiling point" (without quotes) you'll get lots of nerdy hits.

To reiterate - I'm not claiming that adding salt wouldn't help, just that it wouldn't be because it lowers the boiling point cause it doesn't do that ;)

[James Baker SD]

The longer the piece can dry in the form the more memory it will have of the intended shape.

I did not see how long you left it in the forms in your original post. I steamed some green (easier than air dry wood) oak for a chair, I left it in the forms for two weeks (I'm a hobbyist not a production, so I had the time). I think the wood needs some time to let the lignum completely harden in the new shape for it to hold. If the wood is truly dry, you might want to soak it in water for while to bring its internal moisture back up some. I would not even try kiln dried wood.

As for putting clamps on fast, I used some home made fixtures (bending form covered both sides of the wood, one end of the wood trapped by the form to hold it in place) that used U shaped bolts with cross pieces of iron to act as clamps. I tightened the nuts down with a pneumatic impact wrench to do it fast. Two people help, one holding the wood in the bent shape, while the other zaps the nuts down.

[Mac McQuinn]

Ryan,
This might very well be. I'm working off a 16 year memory although we were having trouble getting much steam and he added a spoonful of table salt and within 15 minutes we had plenty of steam. Entirely possible I misunderstood his (Chemist) explanation, sure made a difference though.
Mac



Nope (I had to look that one up to make sure), adding salt actually raises the boiling point, maybe up to 0.3-0.4C or so; so not a lot. Having a higher boiling point wouldn't be bad though because the steam would be hotter so it might help from that alone. However.. there are some other possible advantages, adding salt (and some other compounds but salt is easy) adds more nucleation points so you get more steam formation which also seems like a good thing.

Short Source: http://van.physics.illinois.edu/qa/listing.php?id=1457
but if you google search "salt water boiling point" (without quotes) you'll get lots of nerdy hits.

To reiterate - I'm not claiming that adding salt wouldn't help, just that it wouldn't be because it lowers the boiling point cause it doesn't do that ;)

[Alan Lightstone]

I'm thinking that my best bet might be to take the piece off the frame in another 24 hours, then place one end at a time in boiling water for about 30-45 minutes. Then immediately put it back in the frame, and the trailing end that has sprung back too much, will be bent correctly. Any problems with this approach?

It looks like only the ends have significant springback. The middle looks fine.

[Lee Schierer]

I've done a couple of pieces of steam bent wood. I found that 3/4" thickness was extremely hard to bend and that you had literally seconds to get it bent and clamped. I ended up making my bends with two pieces 3/8" thick and glued them together after bending.

I've read that soaking the piece of wood for several days in water helps saturate the wood and makes it bend easier. The water that has soaked in turns to steam as the wood gets hot really penetrating the piece.

[Alan Lightstone]

I was just reading Lon Schleining's book on Wood Bending. He mentioned heat blankets that luthiers use to make guitars. Pricey, but I wonder if a combination of steam and a blanket if the ends spring back too much (apply the blankets, rewarm the ends, and complete the bend) would work.

Of course, I could just get quicker at doing this. Or could switch to bent laminations.