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kerf_bending_2x4.jpg

Here’s a traditional wood-bending technique that seems ready-made for CNC millers, and yet I can’t find much online evidence that it’s being done.

To make a kerf-bend, the wood is first corrugated on the inside of the intended radius. The width, depth, number, and spacing of the kerfs all affect the qualities of the finished bend. The open kerfs are flooded with glue, 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 subject.

I imagine it’s tedious work, cutting all those kerfs, at least if you’re doing it manually with traditional tools, like a table saw or a fence router. And perhaps more intimidating, for some, is the mathematics of figuring out just how many kerfs you need, and just how thick, deep, and widely spaced they ought to be to achieve a particular radius. Yet both functions can be performed automatically by a CNC system–the cutting by the hardware and the calculating by the software.

In fact, it seems like such a natural fit that I’m having a hard time believing it’s not already out there. So sound off, CNC-millers: Who’s doing this today, and where can I learn more about it?

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


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Comments

  1. wikityler says:

    Maybe it’s because most people using traditional wood working techniques don’t have a CNC machine, but do have a table saw. Despite what might be assumed when reading this blog, things like CNC mills and laser cutters are not common household tools.

    1. Mig says:

      I have never yet had to bend wood but if I did my last resort might be to spend several hundred or thousand pounds on a CNC mill or laser cutter.

      It it a good idea but not many people actually have CNC mills. I would bet 75% of the population of the UK at least does not know what one is.

      1. RocketGuy says:

        Given the odd looks I get and explanations I subsequently have to make when I mention cnc, I’d say it’s not exactly a household word here either. Heck, Milling isn’t either. Myself, I plan to get a rig in a few years post-house-remodel.

        That said, this is a great application. Seems like some experimentation could yield interesting results, like a multi-axis bend if you kerfed in two dimensions. Not sure if that would work, but I encourage experimentation.

        “If you don’t feel stupid, you’re not trying hard enough to push back the boundary of knowledge”

        Route on!

  2. papagun says:

    Kerfing is actually pretty easy. I use a radial arm saw or sliding miter chop saw. I could use a table saw but the other tools are easier to set up. I doubt I would use a CNC machine to do this, but then I don’t have one big enough.

    Figuring the number of kerfs is rather simple. The difference in the circumference of the outer bend (1/4 (in the case of a 90 degree bend) or portion therof) from the inner bend (same portion) divided by the thickness of the blade (kerf) will give you the number of kerfs you need. Find the depth by experimentation as it will vary with different materials.

    One does not need a computer to figure out “everything”… one sometimes needs to simply “think” about the project at hand.

    1. Well, I’m not saying kerfing is all that difficult as far as manual carpentry goes, especially for a skilled craftsperson. My point is more about how well it fits the CNC toolset: if you had a mill and the right software, generating the bend would be as simple as selecting the area to be bent in your CAD program, entering the desired radius and bend angle, and punching “Go.” The kerf-bend seems a natural complement to the slotted panel half-lap joint that is ubiquitous on CNC milled furniture, and yet I can’t find a single piece of CNC milling that includes it, which means there’s an opportunity for some enterprising Shopbot-ter to stand out by incorporating kerf-bends to achieve a look that isn’t out there right now.

  3. brk says:

    This is one of those applications where if you do any woodworking to the level where you are doing kerf-bending, you can do this in 5 minutes on a table or radial arm saw. It would take more time to program and setup in a CNC machine than to just zip this out in the “old fashioned” way.

    Not to mention that CNC machines are very rare in most woodworking shops. I also wonder for the typical CNC bit if you wouldn’t have to program this as a multi-pass, as the depth of those cuts are a little much for a single pass.

    I think the reason you don’t see more mentions of this with CNC machines is because it’s not really a logical use…

  4. John Cabrer says:

    Some stepper/servo couplings use a similar technique to achieve flexibility. Actually, the spiral cut employed in those seems more versatile to me.

    http://www.automotsys.com.au/huco.html

  5. Nick says:

    Most CNC woodworking is done on a CNC router, not a CNC milling machine (also a “CNC miller” is the machine, not the person, the person would be a woodworker or a machinist or both) . This means you are limited to using a router spindle perpendicular to the work. So your kerf would be limited to the depth and diameter of an router bit. For deep, thin slots, you would be severely limited in options. A 2″ deep x 1/8″ wide kerf would be problematic and likely involve breaking bits and burning the wood. Router bits are not as rigid as you would think, deflection is also a real issue. You could do it in thin wood I suppose, but then the kerf thickness would be wider than is aesthetically pleasing, at least to my eye.
    You could in theory make a horizontal saw arbor of some sort but as many have already pointed out, for such a simple operation it is likely done faster and easier manually.

  6. stunmonkey says:

    You may be correct when describing the population as a whole.
    However, for readers of this particular magazine I would hazard a guess that is actually a fairly common household tool.

    I have a couple of machines, and those few of my friends that don’t have their own CNC knows at least several people who do. Failing that there are two art co-ops nearby where you can borrow or rent time on one.

    CNC machines are not only becoming a common tool, they are pretty rapidly becoming a standard appliance for anyone who is considered at all a serious maker.

    1. Mig says:

      I’m not sure where you live mate but art co-ops are also not common where I live.

      I also find it a strange thing to say that they are becoming “standard”. It really depends on what you’re making, how often you make etc…

      I would say that the people who write this magazine have access to a lot of skills, knowledge, people and tools that many “lone” makers do not. This gives the impression that this whole thing is in some way a “movement” when I think most people reading this blog are the only people out of all the people we know who have ever heard of it.

  7. bob says:

    It’s rather inefficient to mill cuts like these compared to a table saw or the like.

  8. Joel says:

    I bet CNC kerf bending would work better with two minor modifications:

    1. a paste of sawdust and woodglue, rather than glue alone, and

    2. drill rather than saw. As one person pointed out, a CNC router may need several passes to get the necessary depth. A better tactic might be to honeycomb it with drill holes. The web left between holes would have to remain as narrow as the spacing between kerfs, but it would maintain a lot more of the wood’s integrity.

    Also, you forgot to mention one important aspect of some kerf bending: moisture & temperature can temporarily soften the wood.

  9. Sean Michael Ragan says:

    I really should’ve included an explanation of the connection between this post and my other one about the old Popular Science plywood project contest. That book, 67 Prizewinning Plywood Projects, includes several pieces that incorporate kerf bends, most notably a lounge chair with high arched arms. And all of them are in 3/4″ thick material.

    I would also emphasize, out of respect for all the authentic craftspeople that’ve sounded off here, that I’m really not talking craftsmanship. I’m talking about dumbing down wood-bending to the point that you can make point-and-click bends in panels using the same CAD software you use to design the part profiles. There’s always going to be a place for quality hand workmanship and I don’t think CNC is ever going to supplant that.

    Had some great comments here, people. Thank you all for chiming in. Thanks also for pointing out the important distinction between a CNC mill and a CNC router, which I will be sure to observe in the future. Cheers!

  10. gear head says:

    If you were seriously considering using a CNC mill for this geometry then you’d just mill the radius on the outside of the work from solid stock. The only way that you could realistically use a mill to create those kerfs is either ‘gang’ mill them using several thin cutters (much like a stacked and spaced dado set) in a single pass, or multiple passes with a single blade, step and repeat. Either way, your talking about a horizontal mill and not a vertical spindle machine. The ratio of diameter over depth of cut precludes using ‘router’ style bits here.

  11. Jim says:

    Gearhead, in some instance, e.g., where you want the grain pattern to continue and wrap from the top to the side, “simply” milling the end shape won’t work. A kerf bent approach would still give you the desired appearance.

    While CNC might take longer and require more passes, it would enable those who might otherwise be too intimidated to try kerf bending.

    I’d be very interested to know if anyone has tried the honeycomb idea suggested above. I’m having trouble envisioning the honeycombs actually allowing the kind of crushing that would be necessary to create the desired curve. Or at least, allowing the crushing without just simply being ruined.

  12. aihyah says:

    its a lot of work to keep the veneer of a continuous piece. glue filling those channels? maybe it needs some filler as well…glue might shrink a bit more and be even weaker. just seems functionally inferior to the regular solutions.

  13. Unto This Last uses this method in a lot of their pieces, all CNC cut (can\’t find the pic that shows it, but it\’s there somewhere.) The kerf cut corners in the sides lock into a single piece CNC face frame. http://www.untothislast.co.uk/Storage/Sideboard%20Ply%20Feet.html

    Also, this is similar, but requires a tapered end mill. The varying widths allow a variable radius. http://www.designtoproduction.ch/content/view/14/44/

    You don\’t need to cut your kerfs as thin as those in the pic. Respectfully, all the commenters telling you to just cut it on the table saw aren\’t really seeing the point here. This technique is ideal for curving sheet material in a production setting with minimal labor input.

  14. Jon S says:

    I did a table using kerf bending on a CNC after I learned what I was doing on the tablesaw through some trial and error.

    http://cargocollective.com/jonsandoval#Kerf-Table