Sharpening Class, Part I

“When we build, let us think that we build for ever.”

English artist and writer John Ruskin wrote that in The Seven Lamps of Architecture. It’s a quotation that inspires us to do our best work, because that work will reflect on us for a long time.

Sometimes we take that quotation and apply it, mistakenly, to our sharpening practices. We either sharpen too infrequently, or not at all. We think that if we sharpen once, we sharpen forever. We fool ourselves. We might operate with the belief that when we purchase a tool it comes to us sharp and ready to use. I think for some tools the truth is a long way from our belief; for  other tools it’s a bit closer. But the sad fact is no matter where we purchase our edge tools, there is always some work to be done.

In my sharpening classes, I’ve bumped up against all kinds of reasons woodworkers use dull tools. Of course the one most often expressed is lack of knowledge. People just don’t know how. Well, no one is born knowing how to sharpen, any more than they understand at birth how to cut a dovetail. That’s why they come to class. Another reason – surprisingly – is that folks are afraid to mess up their tools. Better to use them dull. Here are a few secrets about your tools. First, they are yours, so you can use them as you see fit. You can grind them, change bevel angles, and so on. You can also choose to use them as they are (dull), but why? Sharpening can be practiced and mastered. Second, there aren’t too many sharpening mistakes that cannot be fixed. Probably the worst thing that can happen is that a tool gets blued – you draw out the temper – by overheating when grinding. Even that one can be fixed, but more on that in a subsequent post.

Understanding how to get sharp tools is essentially a two-stage process, with a few sub-steps thrown in for good measure.

The Theory

Step one involves a deep understanding of what makes up a sharp edge. It’s the most important part of this whole deal. Skipping this lesson during sharpening class will cause a person to skip vital steps while sharpening a tool. It’s the part of sharpening that seems to trip up most people who have trouble obtaining a keen edge. The key is to understand this definition of theoretical sharpness: a sharp edge is the intersection of two flat, polished planes at a point approaching zero thickness. Translating that theory into practice becomes a matter of personal choice based on individual experience, but this is the end we’re trying to achieve. There are a lot of roads we can follow to arrive at this destination. The one you choose should be the one that works best for you – not the one that involves spending the most money.

Flattening the Back

Logically, obtaining two flat, polished planes involves working both surfaces of a cutting tool: both the front and the back. For this discussion, the front of the tool will be the beveled face, and the back is the opposite surface. Most folks who are new to sharpening either fail to work the back of the tool at all, or they don’t do it enough. We’re going to start with “flat”. Understanding “polished” will come later. Here’s why flat is important.

Think about the role the back of a chisel plays in your work. It is the part of the tool that registers against the workpiece, and “aims” that sharpened cutting edge for you. If you’re familiar with the concept of a reference surface, the back of the chisel plays that role.  When you pare the face of a small tenon, it’s the back of the tool that rides along the surface of the wood as the edge removes high spots. So if your chisel is to do what you want it to, the sharpness of the bevel must only shave off the high spots and leave the low spots alone. The only way you can reliably cause that to happen is with the back of the tool held dead flat against the workpiece.

Now visualize the back of that chisel. Can it be held dead flat if it isn’t dead flat itself? What if there is a slight swelling of the steel – say a few thousandths of an inch – just behind the cutting edge? If that swelling is present, you’d have to raise up the handle a bit to engage the sharp edge in the workpiece. That swelling would act like a fulcrum – a pivot point. Drop the handle down, and the edge doesn’t engage the high spots you’re trying to pare away. Raise the handle up too much and you’re taking away too much wood.

Not significant, you say? Well, think back to Ruskin. We’re building forever. So when we make that mortise and tenon joint, the fit has to be just right. Some call it a piston fit. Not loose, not sloppy. Just right. Eventually all glues can fail. When that happens, will your joints still have structural integrity, or will they simply fall apart? So yes, it matters.

There are two more equally important reason we strive for absolute flatness for the tool’s backs and bevels. First, it’s repeatable, and repeatable makes for consistency, and that is what we want from our tools.  We want them to work the same way every time we pick them up.  If they don’t, we need to learn how to use them over and over – and who has time for that?  If, every time you hone a plane iron you put a slightly different shape on it, it’s going to work slightly differently.  The surface it leaves behind won’t be the same as last time.  If your stones are cupped, or the edges are lower than the center, your steel will take that shape.  But because the stones wear, the shape will change from one honing session to the next.  Flatness is the one constant we can depend on.  The second reason is just as important, but a little obscure.  The surface of the back of the chisel or plane iron forms one-half of the cutting edge.  Consider that for a moment, and then ask yourself why you would ignore the back if you really wanted a sharp edge!

Start with a Flat Reference Surface

How do we get this flatness? We need a flat surface with which to start. If the work done by our flattened chisel back is the desired destination, then our shop’s flat “reference surface” is the place we start this trek. In my shop, the reference surface is a granite test plate. I know its flatness. It’s certified flat to 0.001″ diagonally across its 12″ x 18″ face. A lot of people use a piece of float glass, since that’s the “flattest glass available.” Problem is, I don’t know what that means. I can’t find a standard for glass flatness, and I wouldn’t know if my piece of glass met that standard. Don’t know if there are waves, dips, depressions, or any other deviation from flat. So rather than mess with the unknown, I started with the test plate. They’re not terribly expensive if you purchase a “second”, known as tool-room grade. Mine has a small chip from the edge of the face. Perhaps one square inch of the top is blemished, and this doesn’t affect the flatness one bit. The big drawback? These test plates are very heavy, especially in the larger sizes. So they’re a load to lug around, and the shipping costs can be a bit steep. Because this is where it all begins, it’s worth it to spend a little money.  Is this obsessive?  Perhaps.  Would glass work just as well?  Maybe;  I don’t know.   I know this, though: I’m entirely capable of introducing plenty of variations to precise flatness with my own imperfect technique.  I don’t need additional help, so I try to eliminate as many sources of error as I can.  And I know one more thing: it works.

Next:  Sharpening Class Part II – Am I There Yet?


Jeff Zens owns and operates Custom Built Furniture in Salem, Oregon.  He is a frequent woodworking instructor and writer.

Categories: Sharpening, Woodworking | Tags: , , , | Leave a comment

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