A simple sharpening technique for unbelievably sharp edges that are tough and long-lasting.
By David Weaver, Winston Chang, and William Tindal
Nothing slices, chops or pares through wood like a freshly sharpened chisel, but sharp edges don’t last forever. Frequent sharpening is a must, particularly for tools that are subjected to punishing tasks like chopping and aggressive paring, as in the preparation of dovetails. Ideally, we’d all like to be able to produce long-lasting, super-sharp edges quickly and easily. Now we can.
On the WoodCentral.com Hand Tools forum, several members have been developing and refining a sharpening technique that David Weaver, the originator, has since nicknamed “Unicorning,” since the odds of his discovering a superior and nearly effortless technique seemed about as likely as stumbling upon a Unicorn, a mythical creature symbolic of things that are too good to be true. The name has stuck.
By selectively buffing just the very tip of a nominally sharpened blade on an abrasive-loaded buffing wheel for a few seconds, Weaver produced a tiny convex bevel that transformed the edge dramatically, making it exceedingly sharp and more resistant to failure from hard use, without changing how easily the edge penetrates the wood.
With this simple buffing step, he found that the edges of less expensive softer steel chisels can be made to stay as sharp as harder premium chisels. Plane blades and many other edge tools, including kitchen knives, can be made sharper and more durable with this technique.
How Edges Dull
All edge tools eventually lose their sharpness to the abrasive action of wood. Chisels used for chopping and cross-grain paring are subjected to even greater stresses. The best chisels optimize hardness and toughness to resist chipping or deforming in hard use. Most moderately-priced chisels are tempered to yield a softer steel that is more easily sharpened. These tools will deform or fold at the tip, while the harder Japanese chisels are more prone to chipping.
Cutting Edge Geometry
Traditional cutting edges are formed by the intersection of two flat planes —the bevel and the back of the chisel or plane iron. When sharpening, the back is usually polished, and the bevel is honed on some sequence of sharpening stones of progressively finer grit. Sharpness is determined by the last and finest grit in the sequence. For most chisels and plane irons, the primary bevel is typically around 25° to 30°, occasionally with a secondary bevel that adds a few degrees at the edge.
Excessive preliminary honing isn’t necessary if you plan to Unicorn the edge; we’ve found that a 1000 grit stone is adequate to produce excellent results. Finer grits yield only fractionally better results. And, since the convex profile increases the cutting angle at the tip, it reinforces the edge; so you can begin with a more acute primary bevel—as low as 20°—even on chisels made of mediocre steel. This sharper primary bevel enables noticeably better chisel penetration when chopping and paring, while the convex bevel at the tip helps resist edge damage.
Resharpening a Unicorned edge is also fast and easy. Because the convex bevel is only a few thousandths of an inch long, it takes very little time to hone it flat again and re-buff to create the Unicorned edge. Re-honing assures that the convex bevel doesn’t grow over time.
Buffing the Edge
Once you’ve finished preliminary sharpening and honing to at least 1000 grit, it’s time for the actual buffing—the “Unicorning”—of the edge.
This can be accomplished in a number of ways, and in our tests, they all produced excellent results.
The fastest buffing setup we found was a 6“ diameter, medium-density cloth buffing wheel on a 3,600-rpm stationary bench grinder or buffer; but we also had success with 4“ diameter wheels mounted on drill presses, powered hand drills and even a lathe. The density of the buffing wheel is important; loose wheels aren’t stiff enough and felt wheels are too hard. The ideal wheel for optimum stiffness is at least 1/2“ thick and stitched to within 1/2“ to 3/4“ of the circumference.
The buffing/honing compounds we used included several commonly available white, green or yellow varieties, although any alumina buffing compound under 5 microns should work well (see above).
The basic Unicorn buffing technique is to present the tip of the tool to the buffing wheel at roughly a 45° angle. As the tool edge is pushed into the wheel for several seconds, the wheel deforms around the edge. The shape of this deformation seems ideal for establishing the desired profile at the tool tip. You will want to experiment to find the pressure and timing that works best for your particular tools and buffing gear.
As an alternative to buffing, one of our team members produced acceptable results by rocking the tip of the chisel while pulling it across a fine sharpening stone and following up with a few rocking strokes on a leather strop charged with honing compound. Whichever technique you use, if you’ve done it right you will see a thin, bright band of reflected light along the very tip of the chisel.
Once you’ve learned the Unicorn technique, you may never again return to the tedium of sharpening your edge tools on traditional stones. The difference in sharpness is immediately noticeable and the edge life is dramatically improved by the addition of this tiny convex bevel – of all of this from a process that takes only seconds and is much easier and less expensive than sharpening with traditional stones. Not all Unicorns are too good to be true.
The Proof is in the Pounding
We could detect how much sharper a Unicorned edge is by comparing photomicrographs and subjectively evaluating paring performance and plane shavings, but how much tougher is it than a traditionally sharpened edge? We conducted some real world trials, including repeatedly chopping 1/16” slices off the end of a 3/4” thick strip of soft maple with a chisel and mallet, to simulate chopping dovetail waste.
For our tests, we chose an inexpensive Buck Bros. chisel from the big box store and a premium Lee Valley PM-V11 chisel. We tested them with both traditional and Unicorn sharpening methods. Both chisels were first sharpened to a 30° bevel using a honing guide and finished on a 12,000 grit Shapton stone. In both chisels, we found that the Unicorn edge treatment provided a remarkable improvement in edge durability. We concluded that the convex profile not only strengthens the tip but also it aids the wedging action to reduce the stress that can damage the edge, especially in hard use.
For a more detailed discussion and more photos, visit Winston Chang’s website: https://chisel-test.netlify.app/