Much has been written about sharpening. I’ve contributed to this mass of sometimes-confusing, often-contradictory literature. As a frequent instructor on the subject, I have some modest experience with the successes and failures of those learning to tune up their edges.
If any part of sharpening can be called fun, flattening is the least among them. It is certainly the most time-consuming, and it can be one of the most frustrating aspects, as well.
In earlier posts on my blog, I have recommended that students begin their flattening with a known quantity: a true, flat reference surface. Some students listen and take the plunge, purchasing a granite surface (test) plate, or one of the more expensive diamond lapping plates, also known to be flat. Others, for a myriad of reasons, choose to use a piece of glass, or a broken counter-top. Surface plates are precision instruments. Glass and polished granite are not.
Many folks believe that these surfaces are fine to use for flattening. Float glass, in particular, is called the “flattest” glass. As often as I have looked, I can’t find a definition of “flattest”. Flatter than what? The old rippled glass from the late 1700s? Flatter than a stained glass window? I just don’t know. Same for a piece of countertop. I haven’t found a standard that defines when granite is flat enough to be called a countertop, and if that standard implies true “flatness” the way we need it for sharpening. I do know this: glass does not need to be flat to do its job, only clear and somewhat resistant to breakage. Countertops are polished to a point of high gloss, but not tested for flatness. That characteristic is not needed to hold up a pot of spaghetti.
I’ve seen folks try to flatten their stones using a small DMT plate, with the long dimension of the DMT held at right angles to their water stones. If ever there was a sure bet to dish out (or round over) the water stone, that’s it. I’ve had students use all kinds of surfaces as their reference, trusting that these are indeed flat. Many of these same students are dismayed at the condition of the backs of their tools during the flattening process.
Why does this matter?
It matters because flattening is (at least) one-third of the sharpening process. The steel from the back of the tool makes up half of the cutting edge. And creating a true, flat surface on the back of a tool is the only action we can perform to guarantee a tool works the same way, sharpening after sharpening.
It matters even more because of the time you spend with your stones and your tools. Even a well-manufactured tool takes a while to polish. A less-expensive tool might take hours. How frustrating must it be to work on the back of a tool, obtain a uniform scratch pattern (one of the telltales that our back is as flat as the abrasive), switch to a finer abrasive, and then discover that you’ve still got a hump or a hollow on the back? If you make this scary discovery, you are left with few options. You must make the correction at this point. All of your previous effort must be corrected, and if you don’t make the correction at this point, all your subsequent work will be wasted. So you go back, flatten your coarse stone on your reference surface, and try again.
But what it the reference surface itself is the culprit? What if there is a hollow in it, or a hump? Then you’re creating the opposite sort of defect in the stone, and your tool will never, ever be flat.
So, back to the beginning of this post. If you’re using glass, or a counter top, or any other “pretty flat” surface, you’re taking your chances. It matters not that float glass is “the flattest.” What matters is how flat your piece of glass, or granite, or steel, or whatever, really is. Unless you know the answer to this question, you take the chance that all of your efforts at obtaining a flat back will need to be redone.
This is why those of us who sharpen a lot, and teach others to do so – make the recommendations we do. It isn’t so our students run out and buy something else to clutter the shop. It’s a lesson most often learned through the painful experience of having to re-flatten tools because we didn’t listen when we should have.
Jeff Zens owns and operates Custom Built Furniture in Salem, Oregon. He is a frequent woodworking instructor and writer.