Sharpening Theory - Standard Double Bevel Blades
Many people struggle with sharpening and have difficulty attaining a satisfactory edge. Others can sharpen anything and don't understand what the big deal is. Most likely, the difference is that the ones who can sharpen well have a good understanding of sharpening theory and therefore understand, and can visualize mentally, what is happening to the blade on a microscopic level at each successive stage of sharpening. The sharpening "pro" has a good understanding of what he's trying to accomplish and is able to recognize the signs that tell him he's accomplished what needed to be done at a particular stage and can now move on to the next. When he runs his thumb over the blade he knows what he's feeling for. When he looks closely at the blade, he knows what he's looking for. When he tests sharpness by shaving a little hair off his arm, he knows how to do it without cutting himself. Somewhere along the line, he was educated in basic sharpening theory and then, became a "pro" through practice, experience, and acquired knowledge. Without the basic education in sharpening theory, no amount of practice would have ever helped him.
Sharpening is really not all that difficult, and while there are some basic rules that need to be adhered to, there is some room for variations in personal technique. However, before we can sharpen anything, we must have a thorough understanding of what is actually happening to the blade, way down on an almost microscopic level, at each progressive stage in the sharpening process.
BEVEL GRINDING, SHARPENING, HONING
The following describes the worst case scenario and assumes that we are starting with a very dull blade or a blade blank that has no bevel grind at all. Throughout the process the grits change but the rules don't. If we're just doing a touch-up on an already fairly sharp blade, we certainly wouldn't start with a file, but with a coarse or medium grit stone instead.
Okay, we've all heard this a thousand times: To sharpen anything, there are three simple rules of procedure:
1) Maintain a consistent angle
2) Raise a burr
3) Flip the blade, remove the burr.
Often, when we hear the same thing repeated over, and over, and over again, its meaning eventually gets lost or at least diminished. We tend to say, "Oh yea, I know that part, now let's get to the good part." We have to try hard not to let ourselves fall into that old trap. Just like anything else, the finer points of sharpening are of little value if we don't fully understand and follow the basics.
So, that's simple enough, right? Just three basic steps to successful sharpening. That's all there is to it... so easy anyone can do it. Well, apparently it's not quite so simple for a lot people. It really should be every bit that simple though, because if we do follow these three basic rules to a "T", then the blade will be sharp when we're done. Whenever we fail in our attempts at sharpening anything, it's invariably because we didn't follow one or more of the rules thoroughly and completely.
1) Maintaining a consistent angle:
Regardless of the abrasive we're using, (files, stones, sandpaper, whatever) we need to work the blade at a consistent angle at all times. This is critical at all stages of the sharpening process, be it coarse bevel grinding, sharpening, honing, or even stropping. That's certainly no surprise to anyone. And it is critical. In fact, it's the single most important rule of sharpening. If the file, or stone, or blade wobbles even a degree or two as we work the bevel ... well, we can keep that up until the cows come home but the blade will never get sharp.
So what angle? I believe that far too much emphasis has been put on the precise numerical sharpening angle. With a few exceptions, and within the bounds of reason, the specific angle is of little consequence. Do you really care if your knife is beveled at exactly 22-1/2 degrees or do you just want it to be sharp when you're done? If we're free-hand sharpening, it's going to be the angle that feels right to each individual, and it will vary from person to person. For those who have difficulty maintaining a consistent angle when free-hand sharpening, guided systems can greatly reduce possible mistakes, but there are no magic wands.
2) Raise a burr:
Work the blade at a consistent angle until you raise a burr on the opposite side of the blade. Everyone's heard that before right? But hold on a second: What exactly is a "burr"? And why is it so important?
Look at the pyramid on the back of a one dollar bill. The pyramid is a good visual aid in understanding the concept of sharpening. You'll notice the flat top on the pyramid. Imagine the pyramid as a cross sectional view of a dull cutting edge. It's dull because it has a flat on the top. It has a flat on top because the bevels have not been ground sufficiently to form a true intersection. They have been ground at a consistent angle, but not far enough to intersect and raise a burr.
Now think of your blade in cross section ... like the pyramid, it has two bevels that intersect to form a V shape. It is very important that we get those bevels to completely intersect and form a true point on the tip of the V at the very beginning stage of sharpening. That means don't leave your coarsest stone until the blade is very sharp.
If we don't get the bevels to truly intersect at the coarsest stone stage, and the tiniest, microscopic flat is till present at the tip of the V, then we can progress through finer and finer grits and what we'll end up with is beautiful, mirror-polished bevels... and a blade that's dull as a stump. Why? Because finer grit stones don't really make a blade sharper, they only refine the sharp edge we formed at the beginning stages of sharpening.
At the beginning stages of sharpening, as we grind away at the bevel, the filings are cut cleanly from the entire bevel. Now, as we continue to grind away, eventually we get to the point where enough metal has been removed and the bevels will come to a true intersection. There is now no flat spot left at the cutting edge.
At this point there is nothing left to support the structure of the cutting edge itself. The metal has become so thin at the edge that rather than being cut cleanly, it begins to bend or deflect away from the stone ... and that is the burr. If you lightly drag your fingertips straight out from under the cutting edge, you'll feel a coarse, microscopic ribbon of steel along the underside of the cutting edge. As thin as the burr may be, finger tips are very sensitive and you'll be able to feel it. If you can't feel it, that means it's not there and you need to keep working the blade with the coarse stone until you can.
The more we work the bevel, the heavier and more pronounced the burr becomes. Raising the burr is critical because the formation of a good heavy burr is the sign that tells us beyond any doubt, we have ground the bevels to a true intersection, there is no longer a flat spot on the cutting edge, and we can now move on to the opposite side of the blade.
Jackpot! This is the signal that we've been waiting for. When you can see and feel the burr running continuously from the tip all the way to the heel of the blade, you've got it licked. If you can't see or feel a continuous burr, stay with the coarse stone until you do. Moving to a finer grit stone before the burr is raised will only lead to frustration.
Flip the blade and remove the burr:
So now we have ground the bevel sufficiently on one side of the blade to raise a burr on the opposite side. Next we flip the blade and repeat the exact same process on the other side. The first few strokes over the stone will remove the burr and now we just need to give equal treatment to this side of the blade so that the bevels will be uniform on both sides and keep the cutting edge centered on the blade. Work the second side of the blade for the same amount of time or number of strokes you used on the first side. Now you will have a burr raised again, but on the opposite side of the blade. At this point it’s time to get rid of the burr and expose the sharp edge hidden beneath it. The way to do this is with alternating strokes on each side of the blade. Move the blade into the stone as though you were trying to shave a thin slice off the stone. If you’re using a rod-guided system, it’s the same except that you move the stone instead of the blade. Five or six alternating passes are about all that’s needed to finish up.
At this point you will have a very sharp blade, but only "coarse sharp." Next, you need to refine the edge by repeating the process with successively finer grit stones until you’ve reached the level of polish and sharpness you want.
Sharpening is really not all that difficult, and while there are some basic rules that need to be adhered to, there is some room for variations in personal technique. However, before we can sharpen anything, we must have a thorough understanding of what is actually happening to the blade, way down on an almost microscopic level, at each progressive stage in the sharpening process.
BEVEL GRINDING, SHARPENING, HONING
The following describes the worst case scenario and assumes that we are starting with a very dull blade or a blade blank that has no bevel grind at all. Throughout the process the grits change but the rules don't. If we're just doing a touch-up on an already fairly sharp blade, we certainly wouldn't start with a file, but with a coarse or medium grit stone instead.
Okay, we've all heard this a thousand times: To sharpen anything, there are three simple rules of procedure:
1) Maintain a consistent angle
2) Raise a burr
3) Flip the blade, remove the burr.
Often, when we hear the same thing repeated over, and over, and over again, its meaning eventually gets lost or at least diminished. We tend to say, "Oh yea, I know that part, now let's get to the good part." We have to try hard not to let ourselves fall into that old trap. Just like anything else, the finer points of sharpening are of little value if we don't fully understand and follow the basics.
So, that's simple enough, right? Just three basic steps to successful sharpening. That's all there is to it... so easy anyone can do it. Well, apparently it's not quite so simple for a lot people. It really should be every bit that simple though, because if we do follow these three basic rules to a "T", then the blade will be sharp when we're done. Whenever we fail in our attempts at sharpening anything, it's invariably because we didn't follow one or more of the rules thoroughly and completely.
1) Maintaining a consistent angle:
Regardless of the abrasive we're using, (files, stones, sandpaper, whatever) we need to work the blade at a consistent angle at all times. This is critical at all stages of the sharpening process, be it coarse bevel grinding, sharpening, honing, or even stropping. That's certainly no surprise to anyone. And it is critical. In fact, it's the single most important rule of sharpening. If the file, or stone, or blade wobbles even a degree or two as we work the bevel ... well, we can keep that up until the cows come home but the blade will never get sharp.
So what angle? I believe that far too much emphasis has been put on the precise numerical sharpening angle. With a few exceptions, and within the bounds of reason, the specific angle is of little consequence. Do you really care if your knife is beveled at exactly 22-1/2 degrees or do you just want it to be sharp when you're done? If we're free-hand sharpening, it's going to be the angle that feels right to each individual, and it will vary from person to person. For those who have difficulty maintaining a consistent angle when free-hand sharpening, guided systems can greatly reduce possible mistakes, but there are no magic wands.
2) Raise a burr:
Work the blade at a consistent angle until you raise a burr on the opposite side of the blade. Everyone's heard that before right? But hold on a second: What exactly is a "burr"? And why is it so important?
Look at the pyramid on the back of a one dollar bill. The pyramid is a good visual aid in understanding the concept of sharpening. You'll notice the flat top on the pyramid. Imagine the pyramid as a cross sectional view of a dull cutting edge. It's dull because it has a flat on the top. It has a flat on top because the bevels have not been ground sufficiently to form a true intersection. They have been ground at a consistent angle, but not far enough to intersect and raise a burr.
Now think of your blade in cross section ... like the pyramid, it has two bevels that intersect to form a V shape. It is very important that we get those bevels to completely intersect and form a true point on the tip of the V at the very beginning stage of sharpening. That means don't leave your coarsest stone until the blade is very sharp.
If we don't get the bevels to truly intersect at the coarsest stone stage, and the tiniest, microscopic flat is till present at the tip of the V, then we can progress through finer and finer grits and what we'll end up with is beautiful, mirror-polished bevels... and a blade that's dull as a stump. Why? Because finer grit stones don't really make a blade sharper, they only refine the sharp edge we formed at the beginning stages of sharpening.
At the beginning stages of sharpening, as we grind away at the bevel, the filings are cut cleanly from the entire bevel. Now, as we continue to grind away, eventually we get to the point where enough metal has been removed and the bevels will come to a true intersection. There is now no flat spot left at the cutting edge.
At this point there is nothing left to support the structure of the cutting edge itself. The metal has become so thin at the edge that rather than being cut cleanly, it begins to bend or deflect away from the stone ... and that is the burr. If you lightly drag your fingertips straight out from under the cutting edge, you'll feel a coarse, microscopic ribbon of steel along the underside of the cutting edge. As thin as the burr may be, finger tips are very sensitive and you'll be able to feel it. If you can't feel it, that means it's not there and you need to keep working the blade with the coarse stone until you can.
The more we work the bevel, the heavier and more pronounced the burr becomes. Raising the burr is critical because the formation of a good heavy burr is the sign that tells us beyond any doubt, we have ground the bevels to a true intersection, there is no longer a flat spot on the cutting edge, and we can now move on to the opposite side of the blade.
Jackpot! This is the signal that we've been waiting for. When you can see and feel the burr running continuously from the tip all the way to the heel of the blade, you've got it licked. If you can't see or feel a continuous burr, stay with the coarse stone until you do. Moving to a finer grit stone before the burr is raised will only lead to frustration.
Flip the blade and remove the burr:
So now we have ground the bevel sufficiently on one side of the blade to raise a burr on the opposite side. Next we flip the blade and repeat the exact same process on the other side. The first few strokes over the stone will remove the burr and now we just need to give equal treatment to this side of the blade so that the bevels will be uniform on both sides and keep the cutting edge centered on the blade. Work the second side of the blade for the same amount of time or number of strokes you used on the first side. Now you will have a burr raised again, but on the opposite side of the blade. At this point it’s time to get rid of the burr and expose the sharp edge hidden beneath it. The way to do this is with alternating strokes on each side of the blade. Move the blade into the stone as though you were trying to shave a thin slice off the stone. If you’re using a rod-guided system, it’s the same except that you move the stone instead of the blade. Five or six alternating passes are about all that’s needed to finish up.
At this point you will have a very sharp blade, but only "coarse sharp." Next, you need to refine the edge by repeating the process with successively finer grit stones until you’ve reached the level of polish and sharpness you want.