Friday, August 6, 2010

Wood Bending Experiment 1

Last weekend we experimented with bending some wood for the dining table that we are building. We have wanted to try bending wood for a long time, and however nerdy this may sound; it was very exciting. The legs of the table we are building have three different bent profiles, with the center profile the one that we are trying to make first.

We built a form allowing for over-bending (to account for spring back), a drying form to hold the wood in position as it dries overnight, and a small steam box. The box was made to only steam the center of the wood slats as that is the only part that is bent and we wanted to keep the ends as straight as possible.

The Steam-Box Setup

The steam box sat directly on top of the kettle which worked quite well except for when we had to refill the kettle with water. A simpler solution will have to be found for this to stop the box from losing heat when the kettle is taken out. The box is "sealed" with damp cloths to help keep the steam in.

The Bend-Form

The bending form is kept close to the steam box so the wood does not cool off before it can be bent into place. The setup used a male form with a compression strap to stop the wood from failing in tension as it is bent.

Freshly Bent Wood

This piece of wood has just been bent. The wood is clamped onto the form at the bottom and then bent up around the first corner. The second clamp is then put on to ensure the straight bit between the two corners. The wood is then bent around the second corner and the final clamp is put on. This process needs to be done in around one minute or the wood will have cooled off too much to make it around the second bend. It takes quite a lot of force to make the wood bend to this degree, making it feel like the wood is going to snap at any moment.

Tension Failure!

The compression strap we used did not cover the whole piece of wood, allowing a small crack where it wasn't supported. We think that the bend would have been successful using the a better strap and are currently in the process of making this addition.

Cooling Down

The wood cools in the form for 20-30 minutes. This allows the bonds in the wood to re-form resulting in bent wood that is almost as strong as the original straight piece despite the deformation.

The Drying Form

The bent wood then sits on a drying form for a day or two to fully cure. The drying form is closer to the intended shape of the wood as the spring back after this stage will be less.

The Result

After reading many books and articles on steam bending, the main thing to take away is that there are no clear cut rules for the process, and that trial and error are the best ways to get familiar with it.

With what we have learned we are already building a new form and compression strap for the next round. There was much less spring back than we expected, and our compression strap needs to be more robust. We have also given the form a larger radius in the corners so that the wood has an easier time bending without failure.