When working with wood, it is very common to start off only really thinking every joint must be 90 degrees and each member must be either horizontal or vertical. After a while, as experience builds, you start to venture into having (shock horror) some items at 45 degrees, and all the extra problems this causes!
Even then, the concept of working with curves is left to those that can justify the expense of buying a bandsaw, or get into the very specialised area of steam bending and even then, significant curves are often avoided. (Don’t get me wrong though, a bandsaw is an excellent investment, and there are a number of ways of creating curved work.)
It doesn’t have to be that way! If you have a tablesaw, or a SCMS (Sliding Compound Mitre Saw), there is another way you can include curves in your project. Think of a Dress Mirror with a curved top, a round box, or…well let your imagination run. Did you know that every guitar is full of kerfing, reinforcing the interior corners?
So what really is kerfing?
Kerfing is in simple terms the act of cutting a series of kerfs (cuts) in a piece of wood in close proximity, so the wood can be curved. It is important not to make the cuts too deep, resulting in the wood cracking completely through, or not deep enough so instead of bending, it snaps (and therefore weakens the wood…….). The wood needs to be cut to the point that the remaining fibres are free to bend. You can only kerf by crosscutting- you cannot kerf with the grain as the likelyhood of the workpiece splitting is huge. This doesn’t have to be solid stock either – you can kerf whole sheets and bend entire panels.
Photo 1 shows a series of kerfs cut, and the depth of cut. I did try a slightly shallower cut, but the wood snapped when I bent it. It just goes to show that test cuts are imperative with this technique. It is very dependent on the type of wood, the moisture content, the relative humidity, the width of the blade, which way you hold your tongue…..
Photo 1 The kerfs and depth of cut
I find the easiest way of getting a consistent distance between cuts, is to line the edge of the previous cut up with the channel in the table. You can set up a jig for more accuracy, but this seems to be pretty successful. (Photo 2)
Photo 2 – Getting consistent kerf placement
Finally, when you have cut enough slots, you can bend the wood, and hope for the best! There are ways of calculating how many cuts are required for a given bend, but personally, I go by trial and error (more trials, less errors)
The result is pretty spectacular.
To fix the kerfing, I tend to use lots of glue! You can fill it, and if you want to disguise the kerfing, either mix sawdust in with the glue (well, so I’ve heard, but when I tried it, it looked pretty crap), or use an appropriate wood filler. Of course, you can also accentuate the effect by using a contrasting wood filler.
It is a great technique, and is worth persevering with until you get one that is successful. If you are getting consistent failures, the chances are you are being too conservative on the depth of cut, and the outside of the curve is resisting the bend and fracturing.
Whatever you do, don’t bend the kerf the other way (with the fins on the outside). Not that the wood doesn’t bend that way, but it looks pretty silly, and makes for an incredibly weak curve. Bent in, the spines all end up impacting on each other, and therefore support each other. They also give you something to glue together. A kerfed curve is never going to be a structural member, but where absolute strength is not required and the curve is important for aesthetics, then this technique may be worth considering.