|Microbevels front and back.|
|Use a jig.|
|Copyright (c) 2002-15, Brent Beach|
"Fully hardened steels, cast iron and concrete, undergo little or no plastic deformation before fracture"
- Higgins - The Properties of Engineering Materials
Edge tools like plane irons and chisels are fully hardened steels. Some other tools, like saws and scrapers, are not fully hardened. You can roll a burr on a scraper, or set a saw tooth. Less than fully hardened steels can undergo plastic (permanent) deformation without accompanying fracture.
The metal in the flexible wire edge has clearly undergone plastic deformation. What does this mean? Was Higgins wrong? No. The wire edge is plastic because the steel in the wire edge is no longer fully hardened steel! The interaction of the abrasive and the steel has affected the structure of the metal.
As discussed in Grinding, abrasives cause two types of changes to the metal. First, the obvious surface scratches. With large grits, these scratches are visible to the unaided eye (for those with good eyes!). Second, the hidden changes to the structure of the metal well below the scratches. In fact, metal up to 20 times the depth of the scratches has lost much of its crystalline structure.
The very presence of a flexible wire edge indicates that grinding has indeed change the structure of the steel.
The wire edge is a flexible piece of steel. The scratch lines should be straight to the edge, but because the wire edge can plastically deform and thus be at a slightly different angle than the rest of the bevel, the scratch lines reflect light in a different way. This makes the scratch lines look different in the wire edge.
Can't see the wire edge in the above images. Well, here is an image where the wire edge is huge.
This blade originally had a 30 degree primary bevel. I ground it back to 25 degrees using the belt sander, then honed the first microbevel at 29 degrees using 40 micron Silicon Carbide paper.
The bigger the grits, the longer the wire edge possible. This wire edge is about 0.003" wide.
If you don't remove the wire edge before using the iron, the wire edge will break off in the work, leaving metal bits to foul up the iron. So, no matter how you do it, be sure there is no wire edge when you start working.
Some people remove the wire edge by using the iron like a knife, slicing into a piece of softwood. They are breaking the edge off before use, but breaking it off.
Most people lightly hone the iron on the back side. This bends the wire edge the other way. Alternating between front and back with a fine abrasive will break the wire edge off. Is this different in effect than breaking it off as in the previous example?
Most knives and all straight razors are sharpened from both sides. Some knife sharpening systems sharpen both sides of a knife at once, drawing the knife between two possibly rotating abrasive surfaces. During manufacture, the concave sides of a straight razor are produced by drawing the straight razor between two spinning abrasive wheels. Most straight razor honing systems use flat abrasives and alternate sides, applying exactly the same process to one side as the other.
Thinking of a razor then, working from one side has produced a wire edge. Flipping the razor over and honing with the same abrasive at the same angle will bend the existing wire edge but will alsocontinue creating the wire edge, from the other side. Some of the existing wire edge may break off, but wire edge formation continues.
If the wire edge was created while you were using, for example, a 15 micron abrasive, then honing one side then the other may remove some or most of the wire edge. However, the scratches right up to the edge are the same. That means that the damage to the steel and the loss of crystal structure is the same, even after the wire edge breaks off.
With straight razors, leaving any wire edge has much worse consequences than for plane irons - the wire edge will scratch.
Does this mean that a final honing with a 1 micron abrasive means we cannot get a wire edge?
Unfortunately, it does not. This is a sketchup model of the edge after both sides have been honed with 15 micron abrasive. The bluer outer layer is the steel that has been affected by the abrasive. This is called the damaged layer - the depth of steel in which the basic crystal structure has been modified by the forces resulting from the contact of the 15 micron grit particles and the steel.
All of this layer is now plastic - it can plastically deform. That is, when you bend it is retains the bent shape rather than springing back to its original shape.
Honing this metal, even with an abrasive that does not cause new damage, is still working with damaged metal. The honing process can produce a wire edge. The honing will leave an edge tool with damaged metal at the edge.
This model shows what happens if you hone both sides with a +4 degree microbevel using a 1 micron abrasive. No new structural damage, old damage almost removed at the edge.
At this point, the metal at the edge retains its non-plastic nature. It can flex elastically or it can only snap. At this point the honing process is producing NO wire edge.
The only way to remove the wire edge is to hone with abrasives that do not damage the metal until you have removed all the already damaged metal at the edge.
With straight razors, the honing angle is almost fixed. We can make slight changes by adding a piece of tape (electrical tape works well) to both sides when honing with the finer grits. However, we cannot make large honing angle changes like those shown in the previous diagram.
Solving the straight razor problem will have to wait for another day. Fortunately for straight razor honers, the blade has a much smaller underlying included angle and much narrower bevels.
As a result, you will continue to get a wire edge as long as you hone only from the front side, since the damaged metal will remain on the back face.
This model is in fact a simplified explanation of the problem faced when sharpening plane irons. To fully explain the problem, we have to look at what happens to the back of the iron during use.
During planing the iron wears on both the upper and lower surface. The upper surface is worn through contact with the shaving. The lower surface though contact with the work.
This micrograph shows the back of a Stanley (bevel down) laminated iron, before (right) and after (left) use. This iron has three microbevels on the back. The area shown by the blue circle is the bevel produced by the 15 micron abrasive paper, yellow shows the microbevel produced by the 5 micron abrasive, green by the 0.5 micron abrasive. See the page on jigs and sharpening for the details of how these bevels are produced.
The back wear bevel is indicated by the red circle on the left.
This back wear bevel is about 0.002" wide.
So, does a light honing to remove the wire edge restore this back wear bevel area of the iron? Not if the iron is flat on the abrasive. You have to put a small back bevel on the iron to remove this wear bevel.
You can see that the flatness of the back as delivered from the blade maker is no longer a factor in what is going on at the edge. If you try to remove the wire edge by alternating sides, but hone flat when honing the back, you will never hone the wire edge away. It may break off, but not through progressive honing as described above.
In fact, it is clear from this model that honing with the blade flat on the abrasive will never have any effect on the bevel at the back edge.
Some people deliberately put a small back bevel on at this stage. They first put a thin ruler along one side of their fine stone. They lay the blade fully across the ruler and stone, with the edge just on the stone. They slide the iron back and forth, the blade moving across the stone, not along the stone. Depending on the thickness of the ruler and the size of the stone used, this produces a small back bevel of around 0.5 degrees. It produces a rounded back bevel, not a flat back bevel. The new back bevel may or may not actually reach the edge, depending on the thickness of the ruler, the width of the stone, and the amount of back wear.
If you use my jig and the slips, in fact there are three back bevels on the back. The first, using 15 micron abrasive, flattens the wear bevel but does damage the subsurface metal. The second, using 5 micron abrasive, starts at the edge because of the iron is tipped up on the slip, and creates a second flat microbevel extending back from the edge. A 5 micron abrasive creates much less subsurface damage than a 15 micron abrasive. The third, using 0.5 micron abrasive, starts at the edge because the iron is tipped us slightly more on the thicker slip, and creates a third flat microbevel extending back from the edge. This third microbevel removes the damaged metal created during the earlier steps and creates no new damaged metal.
About half of this time is spent on the 15 micron paper. I want to make sure the old wear bevel is completely gone. Of this minute, most is spent on the front bevel.
When I flip the jig and work the back, I usually see little glints appear in the oil on the finer abrasives. I think this is the wire edge being honed off. There is also a dark powdery material floating in the oil. I think that this is the metal that is scratched off the bevel - the filings.
A reasonable estimate is that of the 2 minutes I spend actually honing, about 30 seconds is spent on the back bevel.
A US Patent 6,863,600 discusses wire edge in knives. It includes a couple of drawings of knives with wire edges, and of a machine for improving the knife edge by reconditioning the wire edge. The device uses a spinning hard metal shape, usually at a slightly larger angle, to burnish (my guess of the nearest well known metal operation) the wire edge into the edge itself. I have a number of doubts about this invention. First, the drawings of the improved edge show no microbevel created at the edge during this operation (not possible in my view). Second, the inventor argues that a metal surface with roughness less than 10 microns is not an abrasive. Third, the patent mentions plane blades, but I suspect the inventor works almost exclusively with knives. These and other concerns lead me to doubt that this machine would improve a plane blade edge. It might improve some knife edges (depending on how badly they were ground initially).
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