burr on 'both sides'?

hardheart

hardheart

Joined
Sep 19, 2001
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Why is this done? If you form a burr, that means one bevel has met the other at a thin enough edge radius to form the burr. I can only see burr raising twice if the angles on each side are uneven. The edge radius itself is already thin enough for a burr, so making it all over again at the same angle isn't telling you that the edge itself is any sharper.

It can take a lot of strokes to form the burr the first time, when the blade is dull or you are changing edge angles. Then, it just takes a few to remove it. After that, it only takes a few to reform it, because you don't have to reduce the radius anymore for burr formation.
 
Yeah, but if you don't sharpen the other side, the edge will become asymmetrical?
Even if you start with 50/50 edge angles, if you keep sharpening just one side the sharpened side wil gradually move towards the other side and upwards, thus increasing the angle of the unsharpened side.

If I understood what you were asking though :)
 
Yeah, I see asymmetry as being an issue down the road, but that would depend on how the burr is raised. Personally, I don't alternate strokes 1:1 at the start, but generally do 10:10 or 20:20, etc. So, I can raise a burr somewhere in the middle of 20 strokes, but that only puts me off parity by a handful of strokes.

Now, if I did 100 strokes or something on one side before going to the other, or just worked one side til there was a burr, then there is a much larger difference. I will sometimes do 50:50 in heavy reduction, but I still do the equal number of passes after the burr is raised. I don't need it to make the edge sharper, just to even it up. This wastes metal and thickens the edge on flat grinds, so I try to avoid it on anything other than a workhorse beater.

I don't see how the angle would change, it will be the same off vertical, the bevel just gets narower on one side and wider on the other (depending on primary grind)
 
The whole burr-forming principle assumes that your edge is sufficiently worn down that you need to reestablish the bevels on both sides. If you consider the ideal edge cross-section a V then your worn edge was a U . You can raise a burr by only working on one side of the U, but it won't look like a V. When I do the first side I don't try and get a burr along the full blade length, maybe more like 80% of the edge, but make sure that I get burrs near the tip and heel of the edge. Then I work the other side and make sure I get a light burr on 95% of the edge. Then I move to working left-right/left-right side alternate strokes to remove the burr and establish a symmetrical V-edge.
 
What I'm trying to see is what kind of U shape we're dealing with, in varying circumstances. The sharpest edge is still U-shaped, as the bevels of steel cannot meet at an infinitely small point. When the blade dulls by wear, the radius of the U increases. When we subsequently decrease the radius again through sharpening, the radius will reach a size where folded over metal can be seen/felt on the side opposite the abrasion. If you switch sides and raise a burr again, all you know is that the radius is still thin enough for you to see/feel that folded bit of steel. It is not an indicator that the edge radius is any smaller.

If the bevel has a bit of convexity, and you are resharpening to a different level of it, even 0 curvature, then raising a burr from that side would be an indicator that any metal along the bevel face is not preventing the abrasive from working the edge at that angle. But you are still sharpening to the same level of keenness, because you are still sharpening to achieve the same result, a burr at the given grit. It doesn't tell you that the U shape of the edge is smaller, just that curvature above it has been altered/removed.
 
You raise an interesting point or perhaps lead me to an interesting question.

In Juranitch's book, he shows an illustration of a blade that is very rounded off and dull. As you said, very U shaped. He then shows grinding off one side until an edge is formed so the resulting blade is flat on one side and U shaped on the other. This would lead me to believe that you'd have an edge face that is not as thin as it should be, and probably wouldn't cut as well as a true V shaped edge.

But this begs the question: If your blade is well and truly dull and very rounded off, can grinding on one side *only* ever produce a burr? My gut says that the only way this can happen is if you grind so far through that all of the roundness is gone and you have a very asymmetrical bevel. In the extreme case, your edge would be slanted WAY off to one side (the side you didn't grind on).

So I think Juranitch's illustration is misleading. I don't think you can get a burr if one side of the edge is "very round". So maybe symmetry is the only reason we do this.

I only know one thing: My sharpening reached a new level once I learned to feel for the burr, all the way up the edge, and on both sides. I wonder if the "both sides" part was even necessary. Hmm.

Brian.
 
Unless the knife is beat, i alternate every stroke and dont worry about burrs. However if the edge steel is weaken, then i will raise the burr.
 
For me, the double-burring is so that I know that both sides are at the angle I'm working at, ie when I'm rebeveling (which seems to be all the time, but I guess I just don't use my knives enough to have to *just* sharpen them very often, so end up tinkering.)
 
I deburr by elevating my honing angle slightly and doing a few more edge-forwards strokes alternating sides. This cuts through the base of the burr to remove it and does not reduce the final edge angle as much as you would think. When you hone to a very low angle the flexibility of the edge causes it to flex away from the hone. Your final edge profile looks a little like a Y instead of a V. When you superelevate briefly you knock down the base of the Y to be more V-like and hence stronger.

Anyway a burr will start to form as soon as you remove material at the edge. It is both the remnant of material at the apex of your old edge that remains when you cut down your edge length and also some metal pushed aside by the honing process. Your hone particles are not sharp like mill cutter teeth and plow material aside somewhat as they work. This can displace a residue around the hone contact area even on the edge side of your stroke. This is more conspicuous if you use edge-trailing honing strokes. The flexibility of this material and of the edge under it tends to make it build up when your edge gets thin, even before you reach the bottom of your U.
 
If I read correctly your talking about a dual burr when sharpening?

A burr from what I have studied and what I have seen under magnification at a point seems to be more related to the plastic flow of the metal. Sharpening medium seem to also be closely related to the amount of burr produced in the sharpening process. What I see looks more like little balls of metal stuck to the edge.


If it is a dual burr that you are speaking of to me this is a sign of your angles coming together perfectly and in the next few strokes if the angle is held closely the edge can be made almost burr free. The burr or flow of metal that is equal to me means that the edge is almost at its finest point, something I've seen a lot of with my finer diamond hones and even moving from 1 to 0.5 micron compound on a strop.
 
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