Monday, March 19, 2007

The Road to Bending pt.1

I recently got a question about my steamer setup and it got me to thinking about covering the topic of bending wood. As I thought about it, I realized that most of the important action comes long before the steamer gets turned on!
Bending green wood gives chairmaking its creative freedom. I can draw a line on a piece of plywood, cut it out, and bend a piece to shape within minutes. It is truly acrobatic. To get this freedom, there are a few basic techniques to follow, luckily, the bending is built into the nature of the tree.
The strength of a tree depends on its flexibility, the ability to bend in the wind will be the same characteristic that we exploit in bending with steam. To preserve this characteristic, we split the wood. Careful splitting with a froe can produce pieces very close to finish dimension and with the long fibers intact. The froe is a levering tool. The split is guided by driving the froe into a bolt of ring porous hardwood (oak, ash, hickory etc...) and pulling on the handle, which torques the blade. Splitting a bolt into equal halves normally results in a straight split. Proceeding slowly will allow the fibers time to separate, think of pulling apart a stalk of celery. If the desired piece is less than half of the bolt or the split starts running towards one of the sides, you can pull on the larger side and redirect the split. Dave Sawyer has a great way of explaining this. He says that the goal is to equalize the curve that each side has leading to the split. If the curves are the same, the split will run straight. I've found that this can be achieved not only by pulling on the heavy side but by adding more weight to the light side. I do this by placing the heavy side down, with the froe in the split, and putting my weight on the light side (see photo, with strangely disembodied arms). This has the desired effect of equalizing the curves by stressing the heavy side and restraining the light side. Often, I will actually apply a constant force to the froe, not quite enough to run the split, and then pull down on the light side as described. It takes less stress for the wood to split along the fibers, as opposed to breaking them, and when the pressure that I am applying by pulling on the wood is enough to equalize the curves, the split will run start running. This may sound complex but it is simple in practice and vital to both following the fiber line and getting the most out of my logs. Here are a few tips to using the froe
1. The wood will always split easier in the radial plane
2. Try to keep the pieces as square in cross section as possible, the more extreme the rectangle, the more difficult it becomes to direct the stresses effectively
3. Split off any wavy pith wood first, the tensions in wavy wood can throw off your judgement about what is equal mass
4. Split off the sap wood, often weakened by decay, it may be necessary to split it halfway and then flip the bolt and come in from the othe side. Adding mass to the sap wood by pulling on it may help the split run straighter. If it does run out, no big loss, it's waste anyway.
5. Let the wood do the splitting, don't bully it. Wait until the wood responds and then listen. When the noise of fibers separating stops, proceed with more pressure
6. Blame your froe! I am in the process of discovering what a good froe needs to be and the store bought one that I've been using since my antique one broke, ain't it! The blade is too wide and too fat at the top. The pressure it exerts is overwhelming and tough to control. More on this later
I'll be continuing this line of info through the bending process, I hope it's helpful and appreciate feedback


Jean-Francois said...

I noticed your kettle steam box in a previous picture (tripleback picture, in the background).

I have a big steam box with propane burner, but I have bought the same kettle as you to build a smaller box that requires less setup.

Will you be talking about your steam box?

Peter Galbert said...

I will definitely be discussing my steamer. I think that you'll find that a relatively tight box that is just large enough to accomodate your largest armbend, and no more will work great with the steamer pot.