This project evolved from a coffee table. I picked up a 14’ cedar beam from a neighbor that was replacing their deck. I thought that I’d use this free wood to experiment with kerf bending by making a 4” thick coffee table. After measuring up the available wood, I realized I needed to re-think my project. About that time, my wife mentioned it would be nice to have a place to store the pillows etc. from the deck, when it rains. So I went with that. Instead of 4 inches thick, I decided to cut the beam in half. After jointing and planing, I ended up with 1.6” thick wood for the kerf bending.
To calculate where to place the kerfs, start by measuring the thickness of the cut with your saw blade. Next decide on the outside radius you want the bend to be. Using the formula C=2πr, figure the circumference and take 1/4th of that, since you’re only making 1/4th of a 360° bend. Next subtract the thickness of the wood from the outside radius, to find the inside radius. Use C=2πr x .25 to find the inside circumference. Now subtract the inside circumference from the outside circumference. This is how much wood you want to lose to bend the arch. Dividing this number by the thickness of the blade will tell you how many cuts you’ll have to make. The last step is to divide the outside ¼ radius by the number of cuts you need to make. This total is the center to center measurement between cuts. Beware, when you get close to the number of cuts that you calculated, stop cutting a couple of cuts ahead to check the bend. You may find that your calculations may be off by one or two cuts. As far as the depth of the kerfs go, you’ll have to experiment, because every different type of wood is going to flex differently. Keep the thickness at the base of the kerf as thick as you can, keeping in mind that you will be removing some of the thickness at the final sanding. Especially if you are gluing two or more sections together, as I did. You may have to remove a little more material in order to even out the joints.
On the patio box I wanted the front door and back panel to be recessed. So I cut a rabbet on the edges before cutting the kerfs.
Make a form to hold the wood at the angle you want when you glue it up. Don’t just pour glue in the kerfs, because it will just run to the bottom, open part of the kerf. Use an old credit card to smear the glue on the portion of the kerf that will be bent together. I use the waterproof wood glue for this, even if it is for indoors, because the waterproof glue has a longer set up time for assembly. On the patio box that I made, I purposely went five cuts more than it took for a 90° bend. Then I flipped the board over to reverse the kerf with five cuts again. After the first kerf glue up was done, I set the form to 90° and glued up the short kerf set.
The patio storage container that I made, I bent the wood in 3 pieces and then glued them all together.
When doing that, each section was just a hair off from the others. To open up the section that has too tight of a bend, you can cut one of the kerfs open with a hacksaw or a rotary tool. Then use epoxy to re-glue that kerf.
To cover up the kerfs, and also to strengthen the bend, I made a rough cut on some 3/8” thick wood, glued it on and ran a router with a pattern in it to finish shaping the cut.
I added the 3/8” wood on both sides of each section because I was afraid of the glued kerfs not being strong enough during assembly. Later on, I tried to break one apart, and I found that the kerfs were very strong without the added support. So in the future I don’t think I will do that step.
My wood had plenty of knots in it, so I filled them before doing the final sanding.
I used car siding for the rest of the sides.
To cut the front and back panels, I laid the bent wood on top of the panels, and traced out the shape for cutting.
After some paint, here’s the finished product.