One of the easiest ways to create a piece that curves across its face is kerf bending. As the name implies and the photo above shows, this is a method of bending wood that utilizes the reliefs created by a series of kerfs on one face.
Unlike more complex processes of creating curved workpieces, such as steam bending or constructing laminated panels, kerf bending takes little time or effort. The only tools you need are your table saw and a miter gauge, along with a bit of planning beforehand to properly incorporate the curved workpiece.
RIBS & WEBS.
The idea behind kerf bending is that the workpiece will bend across the thin, connective pieces (the webs) while still benefitting from the support of the thicker parts between the kerfs (the ribs). As the piece is bent, the webs allow for the movement while the kerfs close and the ribs touch again to form a solid connection. While a layer of veneer can easily cover a curve and provide the same look, it doesn’t have the strength to be the curved piece itself (not to mention warping). A kerfed board can do both, as the ribs still provide it with strength.
PLYWOOD OR SOLID WOOD?
Although most of the photos feature plywood, this technique can be used on nearly any material. Plywood, softwood, hardwood, and even hardboard are all contenders, with varying degrees of success. For plywood, I find I have the most success with hardwood plywoods (here you’ll see me using Baltic birch).
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| In the router table project, we use a special kind of bending plywood to construct curved panels. However, a board cut with this kerfing technique can be used instead, kept rigid by being epoxied to a laminate backer. |
For solid wood, yew and cedar are good softwoods, while oak, walnut, and ash are good hardwoods. Avoid anything too hard, like hickory, or a wood prone to splitting, like pine. Hardboard is a unique case, as it’s rarely something we show off in fine woodwork. In terms of kerf bending though, it’s one of the few materials that can be bent concavely or convexly without cracking, so it is something to consider if you’re making an S-shaped curve.
KERF APPLICATIONS.
There are a number of ways to utilize kerf bent pieces. Arched tops, such as one might find on a clock or a bookcase, are common as the kerfs can be easily disguised by adding a panel in front. Similarly, curved fronts also work well when supported by a backer. A rarer case is the panels in the router cabinet. Since these don’t have anything to back them, a sheet of paperback veneer or plastic laminate can instead be attached across the back. By connecting to the ribs, this restores strength to the kerfed piece.
CUTTING THE KERFS.
In practice, kerf bending is really quite simple — all you have to do is right there in the name. To get the job done, you just need a way to ensure even spacing between the kerfs.
A KEYED FENCE.
To do the actual cutting, the table saw is the tool of choice. A standard combination blade works well for plywood, but a crosscutting blade will be a good choice when kerfing hardwood (more on that later). To take care of the spacing, I attached an auxiliary fence to my miter gauge with a simple key.
The idea here is similar to what you may have seen before on finger joint jigs. I begin by cutting a kerf in the auxiliary fence to show me where the blade is. I follow that by marking out the distance I want my kerfs to be apart and installing a key there. (We’ll get into how to determine that spacing shortly.)
Unlike a finger joint jig, I don’t use a hardwood key. The precision of one is nice, but ultimately unnecessary — you’ll never be seeing the kerfs themselves, only the curve they create. A simpler (if less accurate) method of indexing can be used instead, so I opt for a #4 screw. After driving the screw in, I cut the head off with a hacksaw. This leaves just the shaft of the screw, which is an excellent fit for the kerfs left by a table saw blade.
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| All you need to start making kerfed boards is a long auxiliary fence on your miter gauge with a key. After installing the fence, first make a cut in it to establish the kerf. | A #4 screw works as an easy key. After driving it in beside the kerf, I cut off the head, as the shaft of the screw fits neatly into an 1⁄8" kerf. |
MAKING THE CUTS.
Once you’ve got a key set up, the actual cutting is easy. First, set the blade to about 1⁄8" below the full thickness of the workpiece. From there, you’ll begin as in Step 3, with the workpiece butted against the key. Make the initial cut, then reset for the second one, slotting the key into the freshly cut kerf (Step 5). Repeat this across the workpiece. In the end, you’ll be left with a rather floppy board that can be bent to your desired radius.
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| To make the initial kerf, butt the workpiece in question against the key and make a cut about 1⁄8" shy of the workpiece’s full thickness. | The spacing between kerfs determines the maximum radius the resulting curve can have. More kerfs mean more flexibility, though less stability. | Continue to make subsequent cuts across the board. The key allows you to quickly register the board for each cut as you work. |
THE DETAILS.
That covers the basic process. But, the devil is in the details. How deep does the blade need to cut, precisely? How do I determine proper spacing? How do I incorporate a curved workpiece into a project? We’ll cover these specifics on the next pages.
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MATERIAL CONSIDERATIONS.
With the general concept of kerf bending addressed, let’s get into the specifics. First off: material. As mentioned before, both plywood and solid wood can be kerf bent, though there are some unique considerations for each.
KERFING PLYWOOD.
Between the two options, plywood is the more easily workable choice. This is due to its composite nature; a couple plies are much easier to bend than even a thin layer of solid wood. Of course, how you cut those plies makes a big difference.
I mentioned before that the blade depth should generally be 1⁄8" less than the full thickness of the workpiece. While this is fine in general, it’s better to be precise when it comes to plywood. The goal is to cut partially through the second to last ply (as shown above). That second ply becomes the flexible, connective web to join the board, while the last ply serves as a supporting backer when the workpiece is bent convexly to help keep it from cracking.
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| Kerfed boards require a backer. In the end table shown above, kerfed boards are glued along the edges of the plywood top and bottom to create the large radius on the front. Support blocks behind the curved pieces add rigidity to the assembly as well. |
On that topic, the downfall of kerf bending with plywood is its weakness. You’ll find that these pieces are relatively prone to cracking, especially as you work with longer boards or reduce your spacing for a tighter radius. Sometimes cracks are minute enough that they’ll never show in the final product, but sometimes they’ll create a slight line that you’ll notice when the light hits it right. A bit of care goes a long ways in limiting potential cracks, but getting one every now and again is inevitable.
KERFING SOLID WOOD.
By contrast, solid wood is a far stronger option. With solid wood, the 1⁄8" rule for determining blade depth is golden. Even after being cut, that small amount of webbing has surprising strength — so much so that it can be difficult to bend some panels even after being cut depending on the grain pattern.
Speaking of grain, that is one vital consideration. It’s important to cut your kerfs perpendicular to the grain, as you see in the photo above. This means all of the webbing still has continuous wood fibers running through them (which is what gives kerfed solid wood its strength). If kerfed parallel to the grain, then the board risks splitting when bent.
DETERMINING SPACING.
Naturally, the distance between kerfs will determine how much of a curve your workpiece can have. But how exactly do you figure out your kerf spacing based on the radius?
A good rule to go by is that 1⁄4" spacing will result in a 6" radius at its maximum. You can extrapolate what spacing you’ll need from there to start. However, there are a few other considerations.
RIGIDITY VS FLEXIBILITY.
The main ques-tion is how flexible a workpiece needs to be. The more flexible one is, the less strength it has. This train of thought might lead to using the widest spacing possible for a desired radius.
For larger radii, that would mean less cutting and a theoretically stronger piece, but it also means a more rigid workpiece since there are fewer points it can bend at. Since the force of bending is less dispersed over multiple points, it actually increases the chances of splintering as it’s bent, despite having a shallower curve and being a stronger workpiece in other respects.
On the opposite side, a workpiece with too many kerfs will be flimsy to the point of fragility. In my opinion, going below 1⁄4" in kerf spacing creates too weak of a board (plus there are easier ways to achieve a radius under 6").
SANDING & SMOOTHING.
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A natural result of kerf bending is that it creates a series of facets (or flats) across the curved face of the board between the kerfs. The larger the distance between kerfs, the more pronounced these facets will be. However, even with narrow kerf spacing you’ll likely still notice the facets in the right light after applying a finish.
Sanding can help to minimize these facets, and even eliminate them in some cases. When working with solid wood, you’ve got a good deal of freedom here; you’ll only have problems if you sand all the way through to the kerfs. Plywood is a different story, however.
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| This desk lamp from Woodmsith #236 takes kerfing to its extreme, wrapping two blanks around the lamp. |
This photo shows how the plies bend with large spaces between the kerfs versus small spaces. With larger spaces between the kerfs, the crests between facets are too high to sand off without blowing through the face veneer. The more even curve created by narrow spacing leaves far less pronounced crests, which can be sanded slightly to knock them down without going through the face veneer.
Regardless, I recommend sanding with a hardwood sanding block, using gradual strokes to follow the curve and being careful not to go too far. Don’t use a power sander, as it’ll be far too easy to take off too much.
INCORPORATING KERF BENDS.
Including a kerf-bent piece in a project takes a bit of planning. On their own, even a solid wood piece with wide kerf spacing is less than ideal in terms of strength due to the kerfs. Aside from that, these boards need something to help them hold their shape in the first place.
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| The number of kerfs in a workpiece determines its maximum potential radius. More kerfs allow for a tighter curve with less noticeable facets, but will also result in a weaker workpiece, more prone to cracking. |
In most cases, the kerfed board is going to be attached to a larger assem-bly. You can see an example of this in the lower left photo on the previous page. On this end table, the radiused top and bottom serve as backers, supporting the ribs and providing structure to keep the kerfed boards in shape. By making contact with the ribs, the top and bottom pieces remove the weakness of the kerfed piece.
On pieces that can’t be supported this way (such as the panels on the router table), I use a sheet of plasltic laminate to provide that stability. I use epoxy or contact cement to secure the laminate and a bending form to hold it at the right curve until it cures.
To a large extent, the limit of kerf bending is your imagination. The lamp shown at the lower left is certainly a variation (with kerfs cut in the top and bottom rather than one face), but it illustrates the extreme. Kerf bending is a simple and easy technique, and it could just be the je ne sais quoi you need to complete your next project.
