Bending hardwood with kerf cuts (down with living hinges!)

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#1


Maple/wenge 1/8". No veneer was harmed in the making of this box. Entire box is sanded to 600 grit and waxed/oiled with Feed-n-wax. Overall dimensions: 5 x 2-5/8 x 1-1/8" (127 x 67 x 25mm)

I wanted to make a curved box, but I don’t like the look for living hinges, and I don’t want to fool with using veneers to cover them up.

Enter kerf cutting. The general idea is that you remove material from the inside of your curve, and the wood can then be successfully bent. I’ll do this a bit differently this time and provide pictures with details inline, followed by technique and notes.


Maple/wenge 1/8"

I intentionally kept the edges of the lid (maple) brown, just as they came off the laser. I liked the contrast, and know from previous experiments that end grain looks different from side, so it would have had an inconsistent color on the edges, which I didn’t like.
I chose to cut the box in two pieces and join them with decorative butt joints. This allowed me to use less material… if you choose to do it as one long band, the resulting strips are quite long.

How long, you ask?

Here’s a strip, essentially one half of the box.


You can see it’s already 7" plus long, and this is a small box. It adds up quickly.


Maple/Wenge
Lid detail. The finger holes are .75"(19mm) diameter. They are two stacked rings of wenge that pass through the lid, which is maple on top, with a baltic birch insert (not pictured) to seat into the box.

What, you wanted to see the inside of the lid? Ok...

Here you go, the inside of the lid.


Maple/Wenge/Birch
I mean, it’s not super exciting, but I do have some mighty laserfingers.


Maple/Wenge
Detail of the exterior, it’s truly a round-cornered hardwood box.


Maple/Wenge
Here you can see details on the bottom. I elected to keep it “open” like this, and didn’t clad the bottom with a layer of maple. I like how it shows the kerfbend so clearly.


Maple/Wenge
Full look at the bottom and lid together.


Maple/Wenge/Rice paper
Rice paper? Are you crazy?
Well, yes. Rice paper is not an ideal box liner, it’s not tough enough. That’s why this is only temporary, and is only applied with scrapbooking tape. You might be able to get it to last by firmly gluing it down and sealing it somehow, but I like the idea of a replaceable liner when you want a different look. This was a leftover piece of wrapping paper that I just had laying around, thought it would look great.


Maple/Wenge/Rice paper
A little overhead view, shows some detail of the corners and lid design.


Maple/Wenge/Rice paper
Eagle-eyed viewers will see that the back of the box is where an owl lives. This led to an interesting #makerfail, each half of the box needed one end to be ready for the raven insert and the other end to be ready for the owl. Guess how I figured that out?

The hard way.


Don’t be me. These would work great if you had a double-owl box, but for my raven/owl box, not so much.


Maple/Wenge/Rice paper
Detail of corner interior. As you can see from the open gaps, I could have bent the corner even tighter, but you start to have some problems at that point. I’ll get into that at the end of the post.


Maple/birch/clamps
Assembly required a little custom tooling to keep everything aligned properly. In boatmaking this is called a strongback, I’ll let a woodworker comment and tell me what it’s called here, but essentially I needed a way to clamp the woods in their final shape as glue set. It involved lots of clamps and some scraps of wood to keep everything flat and aligned on the butt joints. The clamping frame had the proper dimensions and radii on the corners to keep everything in place.


Baltic Birch
Here’s a detail pic of my clamping rig. The cutouts along the edges allowed me to get the clamps in place to hold the side butt joints in place as the glue set.


**Build notes and whatnot:**

I’m not sure anyone has gone so far as to actually make a curved hardwood piece like this yet. It’s been discussed in theory, and kerf bending is a well known technique, but I was unable to find anyone else who had gone this far with it. If anyone knows of a prior post, please let me know.

I started thinking this would be pretty easy. I made a “sawtooth” engrave to remove the kerf cuts, and got my settings dialed in, the end result would be a deep v-shaped engrave slot every 0.05" or so. This seemed like a good place to start.
Pro: Engraving the kerf relief worked.
Con: Settings proved really hard to get right. It seemed like it really wanted to just barely overburn and show through on the final outside surface. I had to do several test pieces to get it dialed in. Engraving is tricky.

To get the bends to be consistent and not crack, you have to soften the wood before you attempt to wrap it around your strongback. I used water. I wetted the surface of the maple over the entire kerfcut area and let it soak in. Once it was thoroughly waterlogged, bending went really well.
Pro: it was very successful
Con: it led to really messy laserfingers (water dissolved a bunch of the residue from the cuts/engraves) that wanted to put brown fingerprints on the wood. Don’t get it dirty, because you don’t have a lot of wood to sand away to clean it up later. It also added time to the process, because you have to let it dry thoroughly before moving onto final sanding/finishing. I left my masking on as long as possible to prevent as much mess as I could.

If you’re going to try it:

Give yourself ample time to work out your proper engrave depth, or perhaps use the defocusing technique that @paulw fooled with here. This was by far the trickiest part. Unfortunately (?) for me, I hadn’t seen Paulw’s post before I built this, so I never tried his defocusing method.

Test your bend radius before you commit to a fullsize piece. Bending a lot tighter is possible, but I found that when I tried to go too tight, the wood fibers started to split unpredictably. You just can’t go past a certain limit with maple, even when it’s softened. Think like a popsicle stick (from a popsicle you just finished). They break but sort of “softly”, kind of peeling apart before they go to pieces. This will no doubt vary by species or wood involved.

Determining the perfect inset size is difficult as well. You can get pretty close with math and material thicknesses, but there’s a lot of complexity happening with the wood here, so getting it perfect the first time is probably not going to happen. Cut your first attempt at a bottom out of cheap materials to test the size, and only once you get it just right go for your premium material. I decided to embed the bottom in mine, so I went with baltic birch ply, as no one will see the edges. (and the color is close enough to maple to be ok)

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#2

Very nice!

And yeah, be ready to produce a bunch of scrap while refining your process. Even if you’re not trying the wet version, different amounts of humidity can affect things. And you may want to try (removable) tape on the outside of the joint to constrain while bending.

Engrave is simpler to dial in. If you decide to go with scoring instead, you will need lines that go past your piece boundaries to account for startup and slowdown.

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#3

I engraved slightly longer than my piece as well, to prevent inconsistency across the bend area.

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#4

In theory that shouldn’t happen (that’s why the no-go borders etc) but in practice it still seems to.

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#5

This looks amazing. I love seeing clean projects like this. well done

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#6

Dang, dude, I so admire people who can actually come up with new ideas. I’m more of the “steal other people’s ideas and improve on them” variety. (Which, of course, makes you a valuable resource for me! :wink:) (Edit: Except for the part where you generally have already done them so well that I can’t think of how to make them any better…!)

This would work for curves in either direction, I’m thinking…so you could get REALLY fancy and do a two-layered box with outside curves on the outside, and inside curves on the inside, and have the kerf cuts all hidden between layers…couldn’t you? :thinking:

One thing for sure, when I get back on my feet again, I’m going to need a lot more clamps!

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#7

Also, look at you gettin’ all fancy with the expanding post sections!!! I always forget those are there. :stuck_out_tongue_winking_eye:

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#8

Ooooh! Nice to see it tested. (I tried once, had less than stellar results, and had to move on to other testing at the time.) Gorgeous effect though. :sunglasses::+1:

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#9

Have done testing for a project using the same techniques. Your notes will help me hone it in a little more.

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#10

Awesome post, detailed and inspiring. Thanks!!

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#11

Yup! In theory…

In practice a veneer inside might be simpler but you could probably pull it off with two fullscale cuts.

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#12

I’d still call that a living hinge though. :wink:

Fantastic work! Thanks for sharing so much of the process.

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#13

Yeah if you used it as a hinge it’d eventually break so technically you’re correct. :slight_smile:

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#14

That is how it had been done since wood was invented. Interesting to see the results using the laser.

Truth is, a small hand saw adjusted right would probably be more efficient.
Made awesome curved wood projects with my Dad forever ago using this technique and dampened wood in shaping jigs was a proven winner.

But when you got a laser everything burns…

I think the living hinge popularity is more the newness of it than the functionality.
Just plain bowed wood has a grace that living hinges lack.

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#15

When you have a hammer… :sunglasses:

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#16

Beautiful job and great explanation. Thanks for sharing!!!

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#17

I was thinking hard about this before the Glowforge even arrived. When I first discussed it Jules made note of someone who did use that technique but was not very positive of how well it worked. The conversation was last nov? or so but could not find it,

When the Glowforge arrived my very first cut was a trial of that concept and I still have that piece hanging about. my hope was to shape the sawtooth and pull the wood into a compound “saddle curve”. That saddle concept was a fail but it bent to about 190 degrees and remains so to this day.

My next wild experiment was to make the wood bend 360 degrees and this was the post about that…

After a lot more experimentation I found the engrave to be superfluous, as long as the wood was very wet and you were very careful not to “surprise” the wood but to bend it slowly. I also found that alcohol both got the wood wet faster and dry faster, even replacing water when I bleached the wood.

That design for a core base is very interesting and I use it a lot but as I am always looking for round, I have a collection of cans and jars of the range of diameters I need. Of some interest to your project, I have found that both inner and outer support are a good thing and will hold the object tight while it dries. Once fully dry it will keep the shape well. However, a point I missed for a long time is that the wet wood grows and it shrinks a lot as it dries (it is that differential drying that seems to be a major cause of warping) I found that an 8-inch piece of wood can be 8.5 inches when wet and dry back to 8-inches. that will mean that you need to allow for that difference when setting up and drying that the shrinkage can break the wood if there is no give. also I was driving myself crazy measuring to size only to find that the piece was smaller when I tried to assemble it.

edit:I ran across that first ever piece:

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#18

@jules ? Know what he’s talking about? I’m just curious if anyone had “finished” and posted anything like this yet, it would be fun to compare notes.

@rbtdanforth did some flexible cuts of hexagon patterns, but not this sort of pure bend, except maybe the aforementioned saddle which sounds like an experiment more than a finished piece.

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#19

Yeah, that was @paulw - he responded earlier in the thread, and you already linked his tests. :slightly_smiling_face:

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#20

There was a hex pattern for the light to show through but the back side was a 35LPI engrave that was using the kerf/line thickness to provide the give I had thought was needed. In the end I discovered that the soaking was key and the engraved bit effect was minor. the hex pattern played no part as a “Living hinge” and even with very thin parts that are not perpendicular to the bend
the results usually unbalance the forces and makes the bends more difficult.

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