Why Sharpen? - Popular Woodworking Magazine

Why Sharpen?

 In Techniques

(Excerpted from The Perfect Edge)

First, a definition of what “sharp” means: A sharp edge results from the zero-radius intersection of two planar surfaces. In other words, where the back of a plane blade meets the bevel is the cutting edge. If that edge is honed as close to zero radius as is possible, that edge is as sharp as it can be – a perfect edge. A true zero-radius edge is a purely theoretical thing, but it represents the bull’s-eye of all sharpening techniques and practice. The reason a zero-radius goal is only a dream is because the blade you are sharpening has to be made from something – for instance, steel – and that something is made up of crystals, which are made of molecules, which are made of atoms; and all of these microscopic building blocks have size. However small these microscopic building blocks may be, there is still some “there” there and it is that little bit of size which determines how close to the goal of zero radius the laws of physics allow you to get. The smallest radius possible is the diameter of the largest discrete particle in the metal’s mixture that can’t be abraded away.

Your fresh and painstakingly-honed perfect edge begins to wear the instant you put it to use; its radius increases, becomes rounder, on its way to becoming dull. If you’re a glass-half-empty type, you might say that there are no truly sharp edges, just ones that are dull or duller. So, it seems that “sharp” needs a yet more practical definition. A blade is sharp when it cuts what it is supposed to cut according to the specifications of the person doing the cutting. Put simply: sharp is as sharp does.

I tolerate a not-so-perfect edge for certain purposes that for others would be intolerable. How sharp is sharp enough is a function of how much pressure I am willing to apply to the edge and how critical is the surface left behind. Sometimes when I cut a sandwich in half, I cut it with the table knife that I just used to spread the mayo. That knife isn’t very sharp compared to the chef’s knives in the kitchen, but the table knife is handy, will do the job adequately and I don’t have to clean and dry a chef’s knife after I’m done (no stainless steel knives in my kitchen – only old-fashioned, high-carbon steel, treated with care). I may need to push harder with that table knife to cut my sandwich than I would have with a sharper knife, but the difference in this instance is inconsequential.

The sharper the edge, the less effort required to cut with it and the cleaner and smoother the just-cut surface will be (still no biggie where the sandwich is concerned). Similarly, for certain woodworking tasks an extreme degree of sharpness would also be a waste of sharpening time. A scrub plane is used for dimensioning rough lumber quickly, where the surface finish is not the goal, just the final dimension. Yes, it needs to be sharp, but a stropped, mirror finish is unnecessary for the rough work that it is to perform. Save the strop, and your valuable time, for a blade intended for more precise work – where the surface left behind is more important than a sandwich.

This last statement assumes, however, that you don’t enjoy sharpening and that sharpening blades is a means to an end, a necessary function from which to carry on. As it turns out, many people live to sharpen – finding sharpening a satisfying endeavor in its own right. Me? I actually like to mow my lawn because of the way it looks (and smells) when I’m done. And I appreciate the time I spend doing it as a sort of outdoors meditation, away from the phone and all. Others so dislike mowing that they’ll pay a neighbor kid to do a so-so job and are happy with that. By the same token, some woodworkers will hone and polish their scrub plane blades all the way to a mirrorlike finish and all of their edged tools are carefully sharpened and maintained to the same degree. Others want tools that are sharp enough to do the work at hand and want the sharpening process to be as fast and easy as possible: “Get it done and get back to the woodworking.”

I’ve included tips to help the spectrum of sharpeners achieve their goals with both “get it done” minimal techniques to keep you productive, as well as the “gnat’s eyelash” types that seek the optical-mirror-flat-back edge-radii that can be measured only using wavelengths of light.

Sharpening is a fundamental woodworking skill – as vital to your woodworking success as any skill you apply to the wood. In his book Woodcarving, Chris Pye says, “A master woodcarver once told me that when costing a piece of work, he would allow up to one third of the allotted time for sharpening and maintaining his tools.” You weren’t born with this skill – it must be learned. To learn any skill takes practice. Give in to the learning process. The time you spend learning to sharpen will pay off later as it will become second nature to know which grit to use and when, how sharp any given tool needs to be for the task at hand, and when it is time to re-sharpen an edge.

I mentioned that some people like sharpening simply because the activity of sharpening can be so satisfying, like the activity of waxing the car or, in my case, mowing the lawn. We resonate with the results on a fundamental, aesthetic level. But in addition to being aesthetically satisfying, a sharp, polished edge will last longer in use. At some microscopic level, every edge appears as a row of “teeth.” The size of a tooth is in direct proportion to the size of the abrasive particle that scraped away the steel beside it. The finer the abrasive used, the smaller the teeth that comprise the edge. Coarse-grit sharpening abrasives make relatively large, deep scratches on the steel’s surface that translate to large, saw-like teeth at the edge. Those teeth will cut aggressively at first, but their sharp points are subject to the entire cutting force and will become dull sooner than smaller teeth. If the teeth are large enough, they can leave visible striations on the surface of the wood you’re cutting. Continued honing with subsequently finer grit sizes reduces the size of the teeth along the edge and, as the size of the teeth decreases, the number of them along the edge increases. The concentration of force described above is in effect here: smaller teeth require less force to cut the wood and, with a greater number of them to share the overall cutting force, the teeth will tend to stay sharp longer, leaving a smoother surface behind.

Another reason sharper is better: a polished blade is smoother and slides through wood fibers with less effort, which translates to more control, resulting in a precise, satisfying cut. I have encountered a number of novice woodworkers who have never used a well-tuned plane that has been fitted with a sharp blade. Their only experience with hand planes was typified by the frustration of making a shaving with the neglected and dull bench plane from junior high woodshop. That plane hopped and chattered and instilled a sense that hand planes are horrible tools to use or that the student was not competent to use them. Both sad conclusions, to be sure. But hand these former shop students a tuned-up, plain-Jane #5 with a properly sharpened blade and they go slack-jawed with amazement. How easy to push, how thin the shaving, how smooth the surface just planed, and how very satisfying the experience. It can change lives.

A properly sharpened edge eliminates one important variable when you’re learning a new woodworking procedure. To flatten a board with hand planes is a task that incorporates a number of skills, tricks and metrics. If you haven’t done it, it’s not as easy as it seems, but there’s plenty of help, including many excellent instructional media. Before you can begin to flatten a board, you must be confident that your plane is set up to function properly. It is not an exaggeration to say that all woodcutting operations start at the sharpening station and you cannot be sure of a plane’s performance unless its blade is flat and sharp.

Though not as life-altering an observation as the first-time feel of a properly sharpened plane blade pushing against a wooden surface, polished steel is less inclined to rust than rough steel is. Those water droplets and oxygen pests in the air look for surface imperfections in the steel to cling to and oxidize (rust!). Polishing your blade is by no means rust-proofing, but the shinier the steel surface, the less inclined it will be to rust.

Though it’s my purely subjective opinion, I feel it important to mention at this point that sharp tools are better tools. There is nothing like the simple pleasure of using a properly sharpened tool. A chisel with a polished, properly shaped edge is more likely to cut exactly where you want it to cut. Hand planing is a delightfully sensuous experience when all aspects of the plane are working properly. The planing action is smooth and easy, there’s a pleasing “shisss” as gossamer shavings are released to float to the floor, and the surface left behind has a sheen that begs to be touched. The same satisfaction applies to paring with a chisel, bucking firewood with your chain saw, ripping on the table saw or carving your family’s Thanksgiving turkey. Although a thorough discussion of sharpening can be a bewildering mix of physics, geometry and metallurgy, with a dizzying array of gadgets and methods, once you’ve achieved a basic understanding and mastered a few techniques, a perfect edge is easy and quick to create. The process itself is quite satisfying.
The small investment of time spent sharpening your tools makes a huge difference in how they perform – and that’s really what it’s all about for me. I truly enjoy using a sharp knife, chisel or saw and enjoy the sense that it’s just me and the work – the tool acting as an extension of my arms and hands, a willing agent at its design best, collaborating with me on the work to be done.

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