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)
Photo 3 – The resulting curve in 4×2 pine
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.
Filed under: Uncategorized | Tagged: Kerfing, SCMS, Tablesaw
excellent stuff mate, it’s great
Thanks for your explanation. I was getting a few boards to bend while other were cracking. I guess I needed to be more aggressive with the depth of the cut and the number. I was using a speed square and a circular saw though, so it took me a lot more time.
Thank you, that’s the most amazing trick I’ve seen!
Any reference for where to find a bit more of the math on how many, how thick, etc…?
Wish I could give you an easy answer, and on the face of it, it shouldn’t be that hard – a combination of timber thickness, kerf width, kerf depth, finger width, total number of kerfs to give the angle that would be achieved. In fact the maths should be relatively easy.
However, strangely, it doesn’t seem to really be out there, because as much as you can specify a formula, the real world steps in and causes issues. What works for one length causes another to fail, even if it is from the same tree. Finish also plays a significant part on whether a kerf bend will succeed or fail (not the type of polish etc, but whether the surface is straight off the saw, or has been sanded smooth, the presence of knots, grain direction etc.
However, don’t let that dissuade you from trying the technique – it is pretty easy, and isn’t that time consuming. And it is pretty cool!!
Of all the ones I have attempted, I’ve probably had a 2 in 3 success rate (this can be improved by trying an attempt or two in scrap to check the setup).
In the end, a really interesting technique, especially when applied to large panels – curving the front of the kitchen island bench for example (or making the entire island oval – no need to stick with the rectangular!)
Thanks Stu,
I guess the length of the curve should be pretty easy to figure out (1/4 circumference of a circle w/ the right radius). I was more curious about the ratio of kerf taken to the “kerf left” (a more proper term escapes me right now).
How does the wood feel when you bend it? Like a wet noodle or is there some resistance like a thin stave?
Ratio of kerf to remaining material – I typically use a 1:1 ratio, which results in a pretty smooth curve, and the remaining feathers of material close up easily on each other, supporting the job significantly (when used in compression!) If you leave more material between kerfs, the curve will become rougher and rougher on the outside face.
When the wood bends, it is definitely wet noodle territory! At least it is in the softwoods I have tried. It feels pretty solid when the kerfs all close up (at the maximum bend), but you know it can’t take significant load because of the thinness of the material. Strong enough for light work / decorative features.
Thanks! I’ll try the 1:1 ratio when I finally get a new home for my workshop. Can’t wait to see how strong it is too. I know you said non supportive but I’m thinking arm rests!
[...] and the bend is made and clamped in place until the glue dries. Stuart Lees of Stu’s Shed has a nice piece on the [...]