Kerfing – Bending wood on the tablesaw

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…..

 

1-setting-blade-height.jpg

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)

2-aligning-successive-k.jpg

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)

3-the-resulting-curve.jpg
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.

 

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29 Responses

  1. excellent stuff mate, it’s great

  2. 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.

  3. 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!

    • KERF-CUT FORMULA
      1. Take the Outside Perimeter of the
      radius and subtract the Inside
      Perimeter. This gives the amount of
      frame to be removed.
      2. Divide this amount by the thickness
      of the saw blade. This gives the
      number of saw cuts.
      3. Last, divide the Outside Perimeter of
      the radius by the number of saw cuts.
      This gives the distance between
      saw cuts.

  4. [...] 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 [...]

  5. [...] [...]

  6. I’ve had an idea based on your technique that might be worth trying. Instead of cutting channels, cut a V shape at regular intervals. The wood could then be bent to close the V’s and form a curve, If glued, this could be reasonably strong.
    I’ve calculated that with a 1.5″ thick length of wood, cuts 0.4″ wide, separated by 0.4″, would each provide 15 degrees of bend. (That’s if my Maths is okay!)
    I haven’t tried it yet, so it might split. Worth a go, though.

    • It is an interesting idea, and logical. I have a very fine point, solid-carbide router bit that would be excellent to test out the concept.

      It would certainly result in a significantly stronger joint than one produced with saw kerfs.

      You would use different router bit with a different angle for different radiuses. Also the distance between cuts has significant impact.

      Interested to try the concept out.

  7. Sorry! My Maths IS bad – divided the wrong thing. In fact its very close to just ONE degree per cut. Possibly best for shallow curves.

  8. Very good article. I used kerfs to create two curved risers for my stairs earlier this year (curved riser project. I have two comments:

    1) It is important to select quartersawn wood (grain parallel to length of board) to cut down on cracking.
    2) I found it difficult to cut across the boad with a table saw, so I actually cut my kerfs with a circular saw and it worked just fine.

  9. This is a technique that I’m very interested in learning. Thank you for sharing this.

    Question, do you find that you get any wood splintering on the bent edge?

  10. I had a question. Can you soak the piece of timber you are kerfing to decrease the chances of the timber cracking or snapping? also what are the best timbers to use for kerfing?

  11. I meant soaking in water by the way, i heard its meant to make the fibres of the timber more flexible.

    • You can, but then you would have to hold the item in that position for the moisture to leave before applying glue, and then the glue would not be able to get into the areas where the kerfs have now closed and are touching.

      Steaming the item may be a better compromise.

      As to what timbers, experimentation! And smooth surfaces – I found may bends failed not because the timbers were not flexible enough, but because of a rough outer surface, a crack formed very easily.

  12. Interesting..your page helped me to understand better the kerfing methot!..Thx

  13. Want to make my daughter some shelves for her bedroom. Idea was to make the shelves form the letters of her name. By using this technique, I can make the letters more appealing. And I’m the prowd owner of my late dad’s radial arm saw – perfect for trying this!

    Thanks for the idea!

  14. Very instructional! I’ve tried filling wood with a mixture of sawdust and linseed oil. After about 24 hrs. of drying, the color matched well (slightly lighter), but the fill doesn’t stick, so I coated the surface with polyurethane and that did the trick. The glue + sawdust combo didn’t work for me.

  15. I”ve also used the kerfing method in the past with hollow square aluminum tubing to bend it. Filled it with builders bog and then spray painted.
    Works well!

  16. I have done this in the past. I filled the voids with Bondo. You can tint it before adding the hardner. I chose a contrasting color.

  17. [...] laser cuts along the desired radius, the plywood can be bent by hand. The technique is called kerf bending and is perfect for putting an organic touch on the usual 90° angle project boxes we [...]

  18. [...] laser cuts along the desired radius, the plywood can be bent by hand. The technique is called kerf bending and is perfect for putting an organic touch on the usual 90° angle project boxes we [...]

  19. [...] laser cuts along the desired radius, the plywood can be bent by hand. The technique is called kerf bending and is perfect for putting an organic touch on the usual 90° angle project boxes we [...]

  20. [...] laser cuts along the desired radius, the plywood can be bent by hand. The technique is called kerf bending and is perfect for putting an organic touch on the usual 90° angle project boxes we [...]

  21. This does certainly work, had a few lighthouse type jobs where had to make complete 360* circles, kerfing works well but better with timber with above 12% moisture content, green oak works every time, iroko too! But when workin with curved frames I tend to strengthen the inside unseen corner with thin ply glued on with a second piece on that, glued to 1st piece!, fill up with sawdust and glue from same material, leave proud and sand back and bobs your uncle, fannys your aunt….curved wood!

  22. Hello,

    I am making a porch with a curved corner. Can this technique work with a 2 X 8 X 10 pressure treated timber?

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