Predicting Steam Bent Spring-back

Jeff McCampbell writes, “I’ve asked about seat comfort, but I was wondering about the curves and steam bending,” and let’s see, this is Jeff. Man, you’ve been asking all the questions. He wants it all. “My own experience of snowshoes and bending wood in a canoe in high school where both curves were glued up. Bending legs in seat and backs would require more precision in where the final piece will end.”

So, yes, it does require precision, but steam bending is not necessarily an imprecise activity, and what I discovered years ago was, in trying to rein in my precision in consistency with the company, so that the comfort of all my steam bent chairs was the same, I would steam bend oversize parts and try to route them to shape. But what I found is that if they weren’t the right curve before I routed them, I’d be taking material off the back of this one, the front over here, the back over here, in order to correct that. And a steam bend has a lot of surface tension in it because you’ve compressed the back of a curve, you’ve stretched the front of a curve, and when you start releasing, or relieving, or cutting away those tense fibers, then the piece will change its shape right in front of you. So it’s important to get the process of steam bending into a much more controllable thing.

I’m staring a series now with Fine Woodworking. They’re supposed to be shooting here pretty soon for the first step of that, but it’s How to Get Your Curve On. It’s comparing steam bending to lamination to sawing a curve, why you would do each one. I’ll also be covering that some more in future classes, but steam bending is a great option for chair work. It is the least expensive once you’re tooled up for it. It’s the most efficient, and it allows you to sculpt pieces like this after you’ve bent them. If I was to cut away and start shaping this, if it was a bent lamination, then I’d be exposing low angle glue lines, which would be unattractive. So in this case, bending was the best option.

Now I’m laminating these back slats right now because I want to make them as thin and strong as I can. Lamination, done well, is actually going to be stronger in its volume, for its volume rather, than a steam bend is, but what I’ve got here is, to your flexibility thing, is I’ve eliminated the arm to back attachment, so that this is quite flexible here, so when you lean on this, the entire back is free to flex, and that is from experimenting here with what I can get away with.

Knowing how big and strong a part needs to be and how thin and flexible it can be, requires some testing to build a sense of structural integrity, and you’ve just got to make some stuff and destroy it. There’s no other way to gain a sense of what wood can withstand or what a chair needs to have for strength. Beyond guiding you in your chair design work, having that sense of structural integrity is a really critical piece in mastering woodworking.

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