In the previous two posts (here and here) we’ve been working on flattening the back face of a cutting tool. If you have created a consistent scratch pattern across the tool’s back, stretching from side to side and from the bevel up about an inch or so, your tool is now as flat as your shop’s reference surface. Now it’s time to polish.
Why Spend the Time?
The best arguments I can advance to explain why polishing the tool is important come from two sources. The first one is the definition of the sharp edge: the intersection of two flat polishedplanes. Logically this makes sense. Logically, the quality of a cutting edge is a product of the quality of the bevel and the quality of the back.
Visual proof of this statement can be found in micrographs of cutting edges. Fine Woodworking readers will recall an article written by Aime Ontario Fraser in the July/August 2002 issue that graphically illustrates this point. You can take my word for it, or go back and look at those photos. The point of Ms. Fraser’s article was to compare honing systems. My point is slightly different: when you look closely at the photos, understand that all the scratches you see in the tool steel transfer directly to your workpiece. So the finer the scratch pattern on the steel, the more refined a surface the steel will leave behind.
In practice, the final proof of this concept comes from comparing the surfaces obtained from well-polished steel and from a fresh tool, right out of the box. It isn’t a fair fight.
So how to get that shimmer, that silky-smooth surface right from the tool? It’s all in the polish. And here, let me interject that this concept of polishing the back of the tool is called honing when you do it to the bevel. Bevel or back, the end result should be the same – flat, scratch free steel.
If you’re using silicon carbide paper to flatten, the steps are fairly straightforward. Moving to progressively finer grits, erase the scratches left from the previous flattening operation. To make this a lot easier, change the angle at which the tool is moved across the surface of the abrasive. For example, at 220 grit, hold the tool so the flattening scratches are parallel to the tool’s cutting edge, and are at a right angle to the tool’s sides. Continue doing this until you have flattened the tool as described in the previous post. Now change abrasive, moving to 320 grit. This time, move the tool so the scratches are at a 45-degree angle to the sides. Flip the tool over after a few strokes, and you will see two very clear patterns of scratches – the 220 grit scratches with the 320 grit pattern overlaid at a 45-degree angle. Now go back to work and erase all of the 220 grit scratches. If you have really flattened the back of the tool, you’ll notice that the time you spend with the 320 paper is considerably less than the amount of time it took to flatten with 220. Once you’re done at 320, move to 400, and so on. Don’t cheat, and don’t skip grits. Get all of the coarser grit scratches polished out each time you change grits. Remember that you will only do this once for the back of each cutting tool and that the results are certainly worthwhile.
Again assuming that you’re using paper for the backs of your tools, progress through the grits up to 2000 grit if you can find it. At 2000 grit, the back of your tool will be a mirror, and if you’ve stayed true to your reference surface, it will be just as flat. This is a good time to interject a reminder: we’re after two flat, polished surfaces. Flat but not polished is better than polished but not flat. There are a lot of ways you can polish a tool without flattening it. A buffing wheel with tripoli and rouge polishing compounds is a good example; so is the leather stropping wheel on a Tormek. Either of these will give you a mirror polish, but neither will improve the tool’s performance alone. I still use a Tormek occasionally, but the stropping wheel is only used to buff carving tools.
If you’re using stones to obtain the polish, work through the grits you own. My shop setup consists of 800, 1200, 4000 and 8000 grit man-made Japanese water stones. Because these stones are fairly soft, they need to be flattened, which I described in Part II or this series. If you’re doing a lot of work with the stones during a flattening session, you may need to flatten the stones more than once. To maximize the time between flattening, use the entire surface of both faces of the stone. The idea of changing the angle at which you move the steel across the stone is the same. Because the jump between grits is larger, you will spend more time on each stone relative to the time you spend on each paper grit. So the down-side of flattening with water stones is flattening the stones themselves – and the additional time you’ll spend on each. The up-side is that one good set of water stones will last you a lifetime. Silicon carbide is marginally faster, but needs constant replacement.
To summarize: flatten until you can see yourself in the back of the tool. No errant scratches. Stay true to the flat reference surface you use; don’t rock the tool, keep the steel absolutely flat on the abrasive.
If you’ve followed these steps, your tool has one flat, polished surface. You have one more to go – but you’re way more than half-way to a keen edge.
Questions? Let me know.
Jeff Zens owns and operates Custom Built Furniture in Salem, Oregon. He is a frequent woodworking instructor and writer.