Pete Boorum replies: Here are a couple of tips – During my years working at Chicago Cutlery, I learned from some old hands in the “red meat” industry to judge sharpness by shaving hair off the forearm. If it shaves like a razor, it is sharp.
On the finish line at the manufacturing plant, the last step was to draw a blade though brown wrapping paper. If the knife cut cleanly with little resistance, it was sharp. This quality step helped the company build a reputation of making the sharpest knives in the industry.
Jon Siegel replies: For something like a plane blade, straight chisel, or skew chisel, use the blade to shave some hair off your arm. This test will tell you if the edge is sharp, but not if the tool has the correct geometry. For example, if the bevel has been rounded over by improper use of a stone or too much buffing, it will shave hair, but will not behave properly is use.
Bill Thomas replies: Telling when a tool is sharp is about the most fundamental subject in woodworking, and therefore probably the most discussed. I remember when “Fine Woodworking” magazine first published micrograph photos of sharp tool edges. In the micrographs, very sharp tools appeared to be terribly irregular and rough. Suddenly everyone was questioning their sharpening habits.
Well, those micrographs were interesting, but don’t really contribute much to the question of sharpness. It turns out (to nobody’s surprise) that sharpness is a continuum. What is “truly sharp”, anyway? That depends on who you talk to, and a Japanese master furniture maker is going to have a different answer than I.
We can debate the merits of different sharpening systems endlessly, but instead, I would like to say that how sharp a tool should be depends on practicality, speed, repeatability, and most fundamentally on making the tool in question cut effectively.
Being able to sharpen a tool requires knowing the way a particular tool works, and understanding its geometry thoroughly. It requires understanding how sharpening stones work as well. It also requires experience, because you don’t need to sharpen a tool once, but thousands of times – many times a day – and you need to understand when it is in need of sharpening.
All that said, a practical approach is as follows…
A dull tool has a rounded edge. The wearing effect of cutting has made it less acute. A rounded edge will reflect light. If you stand facing a light source and hold your tool with the edge up, you can rock it back and forth and look for reflected light from the edge. If you can see a bright shiny edge, you probably need to hone the tool.
After honing it, feel for the burr which will be raised on the edge. If there is a burr, you know that you have honed to the actual edge. Carefully hone to remove the burr, and then check again for reflected light. If you don’t see any, then chances are good that the tool is sharp.
I remember going to a Guild meeting on sharpening where the prevailing wisdom appeared to be that you tested your tool by shaving the hairs on the back of your hand. With the number of times I sharpen my tools in the course of a day, I know I would soon be hairless!
Dave Emerson replies: A truly sharp blade will show no reflected light from the edge, and the edge will catch instantly on your thumbnail.
Garrett Hack replies: The best way to test an edge is to pare some end grain white pine. How much effort is required is part of what I look for, but the quality of the cut surface tells even more. Only a truly sharp edge will cut the soft end grain fibers leaving a bright and glassy smooth surface. One not so sharp breaks over the fibers or tears them from the surface. The surface appears dull, and every little nick in the edge makes as a distinct white track.