Search the Community

The Edge. The business end of a knife or cutting tool. The perfect edge occurs at the intersection of two planes, which are the sides of the knife that intersect each other; the perfect edge formed by the two planes would a one dimensional line of zero radius. This is just not attainable, because the planes are made of something whose molecule has a radius, and where they intersect an edge is formed by at least a single molecule that has a radius. In practical terms, this ain't a going to happen. Maybe in a lab, maybe. But there is a downside, it is a fragile edge that won't hold up and will round over to the point that it is less sharp but more durable. That is the scientific claptrap. Our best edge is going to be rounded to the point that you and I can see it with a loupe. I have four loupes, a 5x, a 10x, a 20x, and a 30x, and some comparator scopes that go way beyond that. If I take a brand new Irwin Utility knife blade out of the box and look at the edge under 5x, I can see the edge. At 10x the edge is obvious, and a 30x it looks like a landing strip. Quality scalpels are somewhat better, but still a highway at 30x. Quality of the plane grinds (facets, bevels) and polish are an important in attaining a small radius, but included angle of the edge (plane 1 degrees from centerline + plane 2 degrees from centerline) is as important a factor. The size of the radius will increase with the size of the included angle. Unfortunately, the durability (how much strength or metal is behind the edge) decreases with the decrease in included angle. We are always fighting the metal to get the best balance between edge radius and durability. Utility knives, razors, scalpel,s and anything of that ilk sacrifice durability for sharpness. Sharpen often or put in a new disposable blade. So edge angles are going to be minimal, but these things will really cut; for a while. So what is the proper angle for a knife edge? I feel a range of 20° to 50° is about right, but covers a lot of territory. I guess some examples might be appropriate. Bench Knife or skiver -- 20° to 25°, maybe even 30° Shoe Utility Knife -- 25° to 30° Pocket Knife -- 25° to 30° Kitchen Knife -- 25° to 35° Head Knife -- 20° to 25° Wood Chisel -- 25° to 35° depending on use Planer Blade -- 40° Axe -- 40° to 50° Lawnmower Blade -- 90° or a bit less Art