Friday, March 23, 2007

Bending


On to the act of bending. There are a few concepts that might help you when considering bending a piece. It is the heat that allows wood to bend, not the water. The moisture in the wood and the steam act to transfer the heat, try taking a heat gun to a spindle and gently heat an area (be careful not to scorch it!) As you heat, it apply pressure to bend it and you will be able to permanently set a bend (or straighten it!) It takes longer and risks degrading the wood through overdrying, but it works. I like to turn on my steamer and let it heat up for 10 minutes or so before putting in a workpiece (with a center mark for aligning with the form and a string to fish it out). I have found that steaming a green piece in my particular steamer works fine after about 30 minutes in the steamer. Air dried wood may take and hour and I don't recommend steaming for longer than this as you risk degrading the all important outer fiber, and if they go, it all goes.
There are two actions going on in the wood as you bend it. The outer fibers go into expansion and the inner go into compression. You will probably find that wood (even though it will most likely fail on expansion more than compression) likes to stretch more than compress. Somewhere near the middle of the bend is a neutral axis that stays the same. As the bend gets tighter, the wood will reach a point at which it won't compress any more and this neutral point shifts, causing extra pressure on the outer fibers and POP! Any scientists out there should for give my laymans understanding of the actual physic, but it suits for my understanding. Most distortion of the bend will happen on the inner face of the bend and if you look closely, you'll see that the inner side actually becomes thicker. Think of the inner face as a loaded spring looking for a place to release. The outer face, barring any fiber breakage is generally well behaved and can be easy to shave clean later. This brings up a good point. Any wood cut with metal, which means all of it, will reveal this interaction by staining in the steamer. All pieces should be oversized enough to shave clean later. The inner side may act hornery because of the waviness of compressed grain and may scrape easier.
Now the wood is in the steamer. I take this time to set up my form and practice the movements I will use. This walk through will ensure that all of my bending tools are in place. When I take the workpiece out of the steamer, I count on having about 1 minute to get the major bends done (adjustments can happen for 5 or more minutes). I try to move with purpose, not panic.
Practice it until you feel comfortable.
Once I remove the bend from the steamer, I place it in the form and lock it in the center with a peg and wedge. Finally the bending! Pull in a fluid motion, no quick jerks. Your speed shouldn't be too fast for the fibers or to slow for the heat! Only experience will tell. I pull the bend against the form and place a peg to hold it in place, I don't worry about the wedge yet, it is far more important to bend the other half before it cools than to fine tune the first half. This is an easy mistake to make, you'll have plenty of time to noodle around later, get on with it!
So it starts to break! Have a bunch of tacks on hand and at the first sign of a fiber peeling off of the out face, pin it back. Any breaks that run across the workpiece are likely to lead to a total fracture. There are as many different problems to encounter as bends to make. But experience will show that through consistent work, success becomes more likely. I rarely lose a bend. That said, some logs just don't like it! I've had hickory that I could tie in a knot and hickory that breaks when I look at it. Try to be consistent so you can make the right jugdements as to the problem you may be having.
When I am bending a complex curve, like the continuous arm you see in the photo, I will remove the steam pot from the steamer and keep it next to me. When I get to the secondary bends, which have been cooling as I bent the first ones, I will pour the boiling water over the wood. Then as I bend it, I keep a well gloved hand applying pressure on the moment of the bend. I cannot think of a single bend that I've lost since doing it this way.
One last dirty little secret. If the fibers do start to peel away, tack it and then finish the bend. Grab your Gorilla Glue, or any other moisture activated polyglue and smear it into the area as best you can. Wrap the area tightly with celophane tape and behold the miracle! The glue will expand into the fissure and heal it. But of course, this will never happen to you!

5 comments:

Anonymous said...

I seem to get more fibers pulling up in the secondary bend( c-arm) only with white oak. Red oak seems to bend easier but I have been told white oak is superior to red in strength. Is this why you pour boiling water over the secondary bend or could this just be specific to this species of white oak? I bend using 5" PVC with kettle similar to yours for 45 minutes.

greg said...

Gorilla Glue! Why wouldn't hide glue be a good candidate for fixing bending delaminations on the fly?

Dave Sawyer fixes delaminations after the fact(after the workpiece is taken off the form)- with methyl-2-cyanoacrylate (Superglue). The water-like viscosity glue goes up into the cracks with capillary action & he holds it in place with ~12" long X 1/4" strips of inner tube wrapped around and around the effected area. After it is wrapped he taps the area with a hammer to bring the glued pieces closer together. In an hour or so you're good to go.

Peter Galbert said...

I use white oak because of it is denser and stronger. I don't know about it bend any worse than red oak. I have found the hot water is helpful, just like introducing more steam right where I need it most.

Peter Galbert said...

Yep Gorilla Glue, as far as hide glue goes, this is one place that it shouldn't be used. A warm wet environment is perfect for dissolving hide glue, not curing it. Of course, if you restrain the break and wait until the piece dries, almost any glue will work. CA glue has the advantage of capillary action and I'm not about to start arguing with Dave!

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