Experiments in kerf bending with a router

Experiments in kerf bending with a router

There are various well-known methods to bend wood: steam bending, bent lamination and kerf bending. Kerf bending might be the most accessible of these techniques but it usually doesn’t look too good. When done with a table saw, unsightly cuts in the wood remain visible in the side view. With a tapered router bit, however, the cuts that allow the wood to bend almost disappear.

I wanted to experiment with this technique for a while and finally got around to it. For a start, I use an off-cut of 24mm plywood and make a simple shelf that bends twice, once over itself and a second time around a corner in a wall. This is a combination I have not seen yet: kerf bending combined with a 3 dimensional bend. Plywood is certainly not the most beautiful material for a piece of furniture but for a test it is perfect: it allows to test the concept and the layers of the plywood highlight potential misalignements and other defects.

The router bit I use is a tapered single flute bit with a very small tip radius (0.25mm) and shallow angle (5 degrees). A shallow taper angle is needed to get less bend per cut and therefore more cuts and an overall smoother curve. The pointier the tip, the smaller the missing section in the bend when the slots are folded up. I do not want to invest money in a good bit when I don’t know if the test will succeed or not, so I went with a cheap bit from Amazon. It is surprisingly good quality and resulted in clean cuts.

A jig for the simple bend

Simple jig to guide the router perpendicular to the shelf.

In order to bend over onto itself, the piece of wood must be cut perpendicular to the bending direction. A simple jig ensures straight cuts with a consistent spacing.

The guide jig in action. The router is guided on both sides so the bit cannot wander and screw up the cut.

The depth of cut must be deep enough so the remaining material can bend but not too deep that it might crack. The exact values depends strongly on the wood used and the target radius of the bend. For plywood it is a good idea to cut everything but the last layer of veneer.

The guide jig in action.

It is necessary to clean up the routed slots. Remaining fuzzies can prevent a slot from closing properly. A soft score on the inside also allows for smoother bending without cracking.

Glue up

The glue up is easy but messy with so many glue joints. Depending on the angle, it can be a challenge to clamp and a clamping jig might be needed. I just need a spacer piece that clamps the folded over section parallel to the bottom.

A knife helps to spread the glue down into the very narrow slots. The bottom of the slots are just half a millimeter wide.

Attempt a consistent bend in all joints at the same time for good glue squeeze out. Wetting the outside with a damp towels makes the bends less likely to crack.

I went a bit overboard with the glue but better too much than too little. Excess glue should be cleaned up as much as possible while it is still wet. The inside radius is a pain to clean and sand later. At least if you don’t have a spindle sander.

The outside of the bend is faceted but can be sanded smooth later. With plywood I actually like the faceted look but with solid wood a smooth curve would be nicer.

The little bit of fuzz seen here are partly broken wood fibers from bending and partly fibers standing up from applying water to soften the wood. These can be sanded off and are not visible in the finished shelf.

Routing jig for the 3D bend

Varying the angle of the cuts allows bends in almost any direction. For a 90 degree bend around a corner, I need cuts at 45 degrees. The spacing is much tighter than in the other bend to get a feeling of how small of a bending radius I can reasonably achieve.

The jig for the simple bend can be altered and rescrewed to achieve a 3D bend. Palette wood ripped with a clean, straight edge work well.

Cutting the slots for the 3D bend is the same as for the simple bend. Each slot is just a bit longer, so it takes quite a bit longer routing all slots. Also the single flute bit is not great at removing material but the shallow taper angle is more important than a second flute.

Clean up of the routed slots and glueing them up is the same as before. This time, it is just a bit more difficult to fix the piece in exactly the right orientation until the glue is dry. Since each joint has a bit of play, the overall angles might not come out as 180 degrees (folded over) and 90 degrees (around the edge) and need some persuasion by additional clamps.

Experiment successful

Finishing such a piece with tight curves and non-planar surfaces is always annoying and requires much hand sanding. Especially the 3D bend takes time because a bend in two directions creates staggered edges, half of which are poorly accessible. Since it’s only a test, I keep sanding to a minimum. For an acutal furniture piece, a spindle sander might be a good investment or at least a handheld, drill-powered sanding drum.

The raw shelf after glueing but before shaping, sanding and finish.

The finished shelf comes out quite nice considering that this is only an experiment. Here are a few impressions from different angles.

The grain is continuous along the 3 dimensional bend which has a really nice effect. Imagine how this would like with a nice piece of walnut or oak!

The layer lines of the plywood enhance the effect of a continuously bent piece of wood. The cuts are barely visible and could be hidden even better with more experience.