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Thread: Why does my bending fail?

  1. #1
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    Why does my bending fail?

    I want to bend some 5/8"x7/8" walnut strip into a 8" radius quarter circle.
    I got some fresh walnut and milled it to size, as I read that kiln dried won't bend.
    I made a steaming box out of 1.5" pvc pipe, with dowels to keep the wood out of the condensate on the bottom.
    I used a wall paper stripper for steam; it worked pretty well as steam came out the condensate drain on the far side of the steam box.

    I steamed my first piece for an hour. As soon I bent it, it cracked. i figured I was too slow, so I practiced so I could do it instantly; and steamed for 90 minutes.
    Second piece cracked immediately also.
    It was much easier to crack than unsteamed wood would have been, but that doesn't help much.

    The grain isn't perfect, but it is pretty straight.
    Any ideas what I could be doing wrong?

    I should mention that the wood came from a branch the electric company trimmed, rather than from a trunk; but can't see why that would matter.

  2. #2
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    That's a pretty tight radius for something that thick. I prefer to use glued laminations rather than trying to steam bend a thick piece, but it's possible. You need really straight grain to start with. AD is better than KD. Walnut doesn't bend too well though, from what I've read. Anyway, you need to use a metal strap on the outside of your wood to keep it from cracking around a radius that small. The strap needs to have two blocks attached to it on either end of your piece of wood, too, so that the wood is completely captured. It's also best to put your piece of wood into nearly boiling water with a little Downy fabric softener for any hour or so, then into the steam bath for another hour. That has always worked better for me than just steam.

    Soften in hot water, steam, remove and insert into the metal strap, bend quickly and clamp to your form.

    John

  3. #3
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    Maybe I am expecting too much; I thought 5/8" was fairly thin and 8" was okay. I can't do much about the 8", but I can plane the wood down to 3/8".

    I am not sure what a metal strap is. A piece of aluminum flashing maybe?

  4. #4
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  5. #5
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    That's a lot of bend for a relatively thick material. Try bent lamination; works much better and less chance of spring back.
    Walnut will change color when gets wet/damp (probably you have noticed already) and makes a mess that doesn't go away with sanding.
    I've done quite a bit of bent lamination of walnut and if you use darker glues you can't tell the glue lines.

  6. #6
    Yeah, you're trying to do too much. Too much material at too small a radius. As everyone else has suggested, go for thinner material laminated together and you should have better luck.

  7. #7
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    I disagree that 8" radius is too aggressive for 5/8" stock. I've bent Black Walnut to half that radius successfully. But you absolutely need a bending strap to succeed at that tight of radius. Also you need straight grain, with as little runout of the grain in the long dimension as possible.

    If you really want steam bent, rather than using bent lamination as others have suggested, you also need to be prepared for quite a bit of relaxation of the bend. Since you don't have the experience to have a feel for your bend, getting a precise 8" radius is going to be hard. You'll have to over bend and probably experiment a couple of times before you get the right degree of bend to match relaxation to your target radius. Rather than that, to get a precise curvature from steam bending I generally use stock 1/8" to 1/4" thicker than my target and then after over bending to an approximate match, template rout down to the final dimensions. If it's long stock with longer radii, this isn't necessary, because you can generally pull those into their final radii with other parts of a structure.

    So here's how I would do this:

    1. Create a bending form with a 7.5" radius over 100 - 110 degrees of curvature and 6" of straight at either end of the curve.

    2. Cut straight grained walnut 7/8" X 7/8" X 24" long. This should cover the entire length of the form.

    3. Steam for about an hour (once you have full steam)

    4. Bend with the bending strap (making certain that the strap is tight on the outside of your curve), with slow even pressure - you should spend at least 15 seconds on the actual curve - which is longer than you think.

    5. Clamp for an hour.

    6. Remove from the form, put a clamp across the ends to prevent further relaxation, and let it set for a day.

    7. Joint flat and dimension in the width.

    8. Rout the final curve in the depth dimension.

    9. Cut at the ends of your quarter circle.

    There is plenty of margin of error in what I suggested above that you should come out fine.

  8. #8
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    I assumed I could use a series of clamps to gradually make the bend, but that isn't going to happen in 15 seconds.
    $80 for a strap is making me seriously reconsider the project.
    Maybe I will just saw them out of a board. The end grain shouldn't be a problem.

  9. You could probably make your own strap clamp with some metal pipe hanging strap from home depot. It's probably not the right kind of steel compared to the Veritas strap clamp, but i doubt that matters if you're using it for one job.

    Make a loop in each end and use a bolt or something through two of the holes in the strap, so you have something to put your clamp in.

    Here's a 10ft roll for $3.24: http://www.homedepot.com/p/Oatey-3-4...9232/100167964

  10. #10
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    Bending works best with riven pieces of wood and green lumber. Lumber that has been cut with a saw has grain ends exiting the side and those places usually fail. Try splitting off a piece of stock from a wider board and then use a plane to smooth the sides and reducing the thickness. It should bend better.
    Lee Schierer
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  11. #11
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    Quote Originally Posted by Lee Schierer View Post
    Bending works best with riven pieces of wood and green lumber. Lumber that has been cut with a saw has grain ends exiting the side and those places usually fail. Try splitting off a piece of stock from a wider board and then use a plane to smooth the sides and reducing the thickness. It should bend better.
    Wouldn't it warp badly while drying?

    I've got green wood and a splitting maul...

  12. #12
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    You don't need to spend money on a bending strap. I've made several from the steel banding straps that loads of dimension lumber used to be banded together with, they use plastic now, but steel coils are still banded with steel and the bands are thrown out. Find a steel warehouse near you or perhaps a small manufacturer of steel products.

  13. #13
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    Wade,

    First, you can't just use clamps. But you can easily make the bending strap for a few bucks. For what I wrote above, you just need a strip of reasonably heavy gauge sheet metal about 3' long and 2" wide. 12 or 16 gauge should work. Bolt a 4" X 2" X 1" block of wood on one end, and an 18" X 2" X 1" handle on the other, with 24" between the blocks. I drew a picture.

    The key is after you've made this jig to cut the wood to be bent so it fits snugly in the gap between the blocks. When it comes out of the steam, bang it between the blocks. If necessary, put a wedge at the end so that it fits so snugly that the metal strap is tight along the length of the wood to be bent when the strap is straight. Clamp the 4" block to your form, and use the handle to bend the wood around with the strap to the outside. Then clamp strap and all all the way 'round to the handle end. As you wrap the wood and strap around the form, the metal strap will apply a steady heavy pressure to the outside of the bend, preventing the wood from separating. Go fairly slowly, and you'll be find.

    Jig.jpg

  14. #14
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    Here's some general guidelines from online regarding the thickness of each ply:

    • 2″ to 4″ radius — 3/32″ thick
    • 4″ to 8″ radius — 1/8″ thick
    • 8″ to 12″ radius — 3/16″ thick
    • 12″ radius or larger — 1/4″ thick

    The article goes on to say, wood type, grain, etc., could require some variance.

  15. #15
    Certainly no expert on steam bending, but I know cracking like that is probably from grain fibers breaking out because they are not continuous fibers, but rather they'e been cut obliquely through the milling process. Normally you don't care, but when steam bending it can matter. I suspect this is probably species dependent to some degree. You also need a board with straight grain.

    Check out Curtis Buchanan on YouTube he makes Windsor chairs. Lots of bending info there. He uses mainly white oak, but I think the principle would apply to any straight grained piece of wood.

    One thing I noticed, he rives all his bending wood out of green logs rather than using dimensional lumber. This produces a board with all the long grain fibers running continuous and parallel from one end to the other. Then there is a much better chance of bending without the convex surface fracturing.
    Last edited by Robert Engel; 02-24-2017 at 8:11 AM.

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