Thursday, January 7, 2010

Boy, That Looks Familiar

Last night as I drifted off to sleep, this image stuck in my head. It reminded me of something, and then it hit me, boy am I a dope. It's the marks that the bandsaw made on the bending form!

Here's the culprit.

I've been known to make some pretty rough and ready bending forms, it's actually one of my favorite parts of chairmaking that I can cut a shape on the bandsaw at a moments notice and bend to it. The white oak that I normally work with never softens to the point that the tiny surface variation creates a problem.
But the walnut is a different story. It gets so soft when steamed that the tiny ridges left by the bandsaw cause a slight crease that is the beginning of the compression failure. It's important to note that the ridges aren't just embossed into the surface of the walnut, which would steam out, but that the failure they start goes deep into the wood.

Smoothing the form and fairing the curve is no problem, I simply run the spokeshave across the surface while holding it skewed, which cuts the high spots.

Here is the finished surface, I even sanded it.

With the air dried wood, I've been using an overbend form to push the wood beyond the final curve before I relax it and kiln it into final shape. The bend went beautifully on the smoothed form which also had less extreme of a bend than the previous attempt. After bending, I let it sit in the form for a few hours and then take it out to sit overnight. Below, you can see the springback after I unclamp the bend. One of the beauties of using air dried wood is that the bends set almost instantly.

You can see in the image below that the bend, when placed against the final form is actually a tighter curve, just what I want.

My friend Andy Jack wrote me with some sage advice about bending. He recommended that if I was having no trouble on the expanding surface yet compression failure, that I might be steaming too long. This makes great sense as a means to developing the steaming times of any wood, the point being that oversteamed wood gets too soft and fails on compression and that there is no reason to steam beyond the point that the expanding surface can stretch to the bend.

So for now, I'll console my feeling like a dope for not refining my form properly with the satisfaction of having learned something.


Anonymous said...

Well done for working that one out , I did not think of it, I thought this was going to be a long drawn out trial and error ordeal , glad it was not and glad to know somebody else falls asleep with chair making in mind , does it ever stop ? , my wife , well lets say this conversion would humour her . MiM .

Christopher said...


Great info. Just a quick question about overbending. How long do you keep it in the overbend form before moving it to the final form, and then how long in the final form? Thanks.


Peter Galbert said...

I wish I could say that it stops sometime, but I think that there is some small part of my reptilian brain that is always thinking chairs!

I leave the bend in the overbend form for a night or so. You can leave it in the overbend as long as you like because the curve doesn't really "set" until it's put in the kiln. For the bend I just made, I'll kiln it in the final form on day 2. Best bets on this stuff are always to wait longer, as tough as this may be!

Christopher said...

Thanks, Pete.

Scott said...

For what it's worth, steam bent ash displays this same proclivity towards softness and absorbing the imperfections from the bending form...