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| Wire Edge |
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| Finest abrasives. | ||
| Microbevels front and back. | ||
| Use a jig. | ||
| Copyright (c) 2002-10, Brent Beach |
The Wire Edge - What is it?
Grinding or honing up to the edge of an iron gradually thins the metal at the edge until the metal is so thin it bends rather than abrades. At this point it becomes a "wire edge".In this picture, both irons have been sharpened with 15 micron abrasive paper. The one on the left has only been sharpened on the front side. The one on the right has been sharpened on both sides repeatedly. The one on the right shows less evidence of a wire edge. [The wire edge is the narrow bit right at the edge where the scratch lines change direction. This one is about 0.0005" wide.]
The wire edge is a flexible piece of steel. The scratch lines should be straight to the edge, but because the wire edge can bend 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.
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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. |
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Why is it important?
It is usually thought that the exact way you remove this wire edge has an affect on the quality of the edge. The thought is that if you "break" this wire edge off in use you leave small cracks in the edge that lead to more rapid wear.Some people remove the wire edge by using the iron like a knife, slicing into a piece of softwood.
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 without harming the edge itself.
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 to remove the wire edge.
What does this light back honing do?
Alternately lightly honing front and back will remove the wire edge. But what else does it do? First, lets back up and think about the entire sharpening process.The edge is the intersection of two planes - the front bevel and the back bevel. In order to have a good edge, both planes must be flat. The usual approach is to flatten the back once, when you first get the iron. This flattening is considered a rite of passage. People often spend considerable time and great care ensuring that they have produced a perfectly flat back.
After that, removal of the wire edge is all the attention the back gets.
If you want to completely avoid a back bevel, you can lay the blade fully across the stone, then slide it back and forth. This will remove the wire edge with little chance of a back bevel.
Some people deliberately put a small back bevel on at this stage. They first put a thin ruler along one side of the stone, then again laying the blade fully across the ruler and stone, slide it back and forth. 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.
Is there a preferred back bevel angle? Light honing with the blade flat on the abrasive puts a 0 degree back bevel on the iron. Adding the ruler raises that to 0.5 degrees. Is this enough or too much? We have to consider what happens when we resharpen a worn blade to discover the correct answer.
How does the iron back wear?
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. The wear patterns are quite different.This picture 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.
Back Bevels
Rather than leave the quality of the back face of the iron to chance, I always put a back bevel (actually 3) on the iron as a part of every sharpening. This adds a little work during each sharpening, but it also means that I do not have to spend ANY time on the back when I first get the iron. Essentially I flatten the back at the edge on every sharpening.My sharpening jig is designed to allow honing of both front and back bevels. After working the front bevel on a grit, I flip the jig and iron over and work the back. Working the front bevel takes longer than the back because the front bevel is wider. So, I do the front until I can see the grit has reached the edge, flip to the back for a few passes, flip to the front for a couple more light passes, then the back again, then on to the next grit.
The front and back microbevels have exactly the same surface quality.
How long does it take
It takes me 4 to 5 minutes to sharpen 1 iron, based on the hour it took to sharpen 14 irons. About half the time is putting the iron in the jig. So, about 2 minutes actually pushing the jig around on the glass, wiping off the bevels, peering at the bevels to make sure they go across the full width of the iron.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.
Do they help?
For some time I have been wondering if using a set of 3 microbevels on the front and back makes a difference in the quality of the edge. It could produce a sharper edge, it could produce a more durable edge, it could do both, or it could do neither.It seemed to me it should do both, but I had no evidence that it did. The only certain advantage is that I never spend any time flattening the back of the iron.
As you can see in the above picture, use leaves the back roughened. It is hard to know how much roughened, but much rougher than that left by 0.5 micron abrasive. It also leaves the back face rounded - the microbevel was flat, the wear bevel is curved. So, the worn blade is both rough and dull (has a greater included angle).
Simply honing the back to remove the wire edge probably will not restore the back face to a flat, smooth surface. Honing the front bevel creates a wire edge that is in the plane of the front bevel. That is, 25 to 30 degrees above the plane of the back. Any honing on the back will tilt the wire edge into the plane of the blade back. This flexing of the wire edge causes it to break off. So you can remove the wire edge without smoothing or flattening the back.
Freehand honing to remove the wire edge is unlikely to smooth this area significantly. The problem is that the area is very narrow and, after use, not flat. This means that you cannot rock the blade to find the facet you need to flatten. You simply have to guess.
So, if freehand honing to restore the area of the back wear bevel is error prone, that suggests the need for a jig. The need to alternate front and back honing to remove the wire edge adds another crucial requirement for the jig: it has to work for both the front and back bevel without repositioning the iron in the jig!
After considerable experimentation, I arrived at a simple jig design that meets these needs.
Links
While it may not appear that polishing gemstones has much to do with sharpening plane blades, this Gemstone polishing article has a section on what happens to a crystal surface when it is scratched by a sharp abrasive. Some heavy lifting here, but very worthwhile. I discuss the implications of this article in more detail in the bevels page.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 they were ground initially).
Page History
This page was written in 2002 and 2003 and then left for several years. The material in this page does not reflect all of ideas that arose during testing and through conversations with other testers since then.In particular, in 2006 I started to do some detailed work on the geometry of sharp and dull edges. That work will eventually appear in a web page and then result in updates to this page.
The ideas in the just mentioned article on Gemstone polishing will also be incorporated into these pages some time soon.
Navigation
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