It’s been said that a dull knife is more dangerous than a sharp one. A tool shouldn’t have to be forced to do its job, or there is the risk of that force moving in an undesirable direction, not to mention the wasted energy required to complete the task. Try cutting carrots with a dull kitchen knife, and you’ll get my drift. Understanding the geometry of a cutting edge allows you to sharpen just about any tool, and also to recognize when it’s dull.
With that in mind, let’s examine the conditions that give rise to a keen cut, and the options for achieving it.
The cutting edge
Here’s the salient fact about a cutting edge: it’s the intersection of two faces. So picture that-two surfaces tapering together to the extent they create a leading line of sharpness. When you look at a cutting edge, say on a pocketknife, if the edge is sharp, you actually won’t be able to see it. In other words, there’s no third surface. If you can see the edge, it’s dull.
So when sharpening an edge, we need to consider which face we’ll be removing metal from. Sometimes it’s from both faces, but often it’s just one. On some tools, depending on how they’re designed to confront the material to be cut, there’s a flat face leading to the edge, and the other face changes direction before the edge. This is known as a bevel. The bevel makes the cutting edge less vulnerable to forces by increasing the angle of the intersection. Examine edges of the tools you use in your life, and you’ll begin to get a sense of the geometry at play.
Pocketknives and kitchen knives often have a double bevel. This means that as the two faces converge, they change direction slightly to give a sturdier lead cutting edge. When you sharpen these tools, you’re maintaining this relationship by removing material from only these smaller faces near the edge.
These edges usually approach dullness gradually, so they require modest methods of material removal. Your options here include a variety of hones, from honing rods, to flat honing stones. Each of these has evolved over time to include stone, steel, and diamond variations on a theme.
With a rod, you’re pulling it across the cutting edge on each face until you no longer have the third face. Using a flat hone requires that you slide the knife perpendicularly across the surface, while keeping the bevel or face in contact.
What you need to pay attention to is the “wire” that forms parallel to the edge as metal is abraded. Diminishing the force with each pass will lead to a finer wire at the edge, until it disappears and you finally get back to sharp. This will take some practice, but it’s a skill worth the investment. A note of caution: not just because the edge is sharp, but also because you don’t want to drive any remaining wire into your skin, never slide your fingers parallel to the edge! A simple test for sharpness is to rest the edge on a fingernail. If it’s sharp, it’ll slightly “dig in,” and not skate across the nail. A magnifying lens can be of real use here to help see what’s truly happening.
Axes and lawnmower blades
Implements like axes and lawnmower blades tend to take far greater abuse, and thus need to have more material removed at a time. While they can be honed, a preferred method is to use a single-cut/bastard file. Files are generally more aggressive at removing metal, and thus leave a coarser edge.
You need to consider how a tool is to be used, and decide just how fine an edge you need. Cutting trees and the lawn are rougher tasks than cutting carrots and whittling, so the edge produced by filing is OK. A note here: lawnmower blades, which should be removed and held in a vice for sharpening, have a bevel on one side only. This is the only face you should be touching with the file, whereas an axe is double beveled, so both surfaces should be dressed. Aim to keep the filing perpendicular to the edge. This way the striations at the cutting edge resist breaking in use. You can certainly refine these edges with a honing stone, should you deem the filed edge too coarse.
This is really just a primer on sharpening to get you thinking about cutting edges, and what their amending entails. Further investigation into methods and approaches would be prudent and advisable. Once you get a sense of cutting edges, you’ll be empowered to tackle scissors, chisels, and all manner of cutting tools that make life at the cottage run smoothly.
Sean Ledoux is a designer/master fine craftsman working with wood and other found media to create unique furniture. Restoring antiques helps support studio life in North Bay, ON. www.seanledouxfurniture.com