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The Burr

1. What is happening geometrically that causes a burr to rise when the beveled flat crosses the center plane of the blade?

2. If a blade has its bevel properly set, will attempts to “set” it cause a burr to quickly rise?

3. If #2 is true, why won’t burrs always rise at any stage of honing? Do finer grits avoid this? Is it alternate lapping that avoids it by constantly removing them?


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steveclarkus

Goose Poop Connoisseur
Interesting questions. I have an idea how it happens but it is only an idea. @Slash McCoy will likely have the answer. I believe the answers to #2 and #3 would be yes. I have a knife sharpener that uses various grits of sanding belts. The instruction for the coarse belt is to draw the knife through the guide repeatedly until a burr forms then do the opposite side the same number of pills then do a few alternating pulls. Then go through the rest of the grit progression using alternating pulls. It always works. Since it uses belts the edge will be a bit convex unlike a razor which is honed on a dead flat surface.
 
Steel is not like stone or glass. Steel is plastic, it deforms and bends. This is especially true when it becomes wafer thin. Steel foil will behave a lot like aluminum foil.

When you are honing, the burr is what happens when the micro thin edge is deflected up, away from the grit that keeps pushing on it.

The first thing that happens when you start honing the other side is you begin to grind off that burr that folded up until you could feel it. If you stop honing on the other side at the exact right moment, you will find no burr, but instead a micro-blunt edge.

Then, you grind a little more, and sharpen that micro blunt. Just a wee more and a foil or fin edge starts to take shape, which will then form a burr.

Technically, you can raise a burr at any grit level. However, to do so requires more and more downward pressure as the abrasive particle size gets smaller, because there is less and less forward pressure to fold the fin upwards for you.

Yes, if you take a shaving sharp razor back to bevel setting stones, you can get a detectable burr in as little as 10 swipes on one side. I've done that often enough with knives as a method to check if the bevel was set. Also yes, if you raise a burr, remove that burr and then alternate 1:1 swipe/side thereafter, you will remove the beginnings of the burr on one side as soon as you swipe the other way.

This is why Slash started incorporating his "pull stroke" close to the end of balsa stropping. It helps remove the micro tiny fin edge (nacent burr beginning).
 
Steel is not like stone or glass. Steel is plastic, it deforms and bends. This is especially true when it becomes wafer thin. Steel foil will behave a lot like aluminum foil.

When you are honing, the burr is what happens when the micro thin edge is deflected up, away from the grit that keeps pushing on it.

The first thing that happens when you start honing the other side is you begin to grind off that burr that folded up until you could feel it. If you stop honing on the other side at the exact right moment, you will find no burr, but instead a micro-blunt edge.

Then, you grind a little more, and sharpen that micro blunt. Just a wee more and a foil or fin edge starts to take shape, which will then form a burr.

Technically, you can raise a burr at any grit level. However, to do so requires more and more downward pressure as the abrasive particle size gets smaller, because there is less and less forward pressure to fold the fin upwards for you.

Yes, if you take a shaving sharp razor back to bevel setting stones, you can get a detectable burr in as little as 10 swipes on one side. I've done that often enough with knives as a method to check if the bevel was set. Also yes, if you raise a burr, remove that burr and then alternate 1:1 swipe/side thereafter, you will remove the beginnings of the burr on one side as soon as you swipe the other way.

This is why Slash started incorporating his "pull stroke" close to the end of balsa stropping. It helps remove the micro tiny fin edge (nacent burr beginning).

I guess that was my understanding. What I don’t understand is why the burr magically starts when the bevel reaches the center plane of the blade. Surely the blade is very thin before it actually reaches the center plane.

It’s not so much that a burr develops that I don’t understand. What I don’t get is that it develops at the center plane where the bevels on both sides are equal in width and the center plane bisects the edge’s included angle.


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The bevel has to meet at the center plane for the burr to start because the steel normally must be at it's thinnest before a burr will start to form and grow. At a certain point, the steel at the tip of the apex just pushes away from the hone because at that level of thinness it takes more pressure to cut the steel than it does for it to flex away. So as a bit of steel is cut away on the bottom of the "foil" or burr, the top part flexes away. When you flip the blade, the same happens on the other side. This is why, if you were to examine the edge at a very small scale, you'd see that the angle of the "foil" bit is considerably more acute than the rest of the bevel.

It helps to understand if you remember that steel at this level is extremely flexible. We're talking apex width in the range of 0.1 micron, give or take. That's 4 millionths of an inch. A shim made of steel at this thickness could very easily be blown around with your breath, and torn in half by a toddler.
 
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The bevel has to meet at the center plane for the burr to start because the steel normally must be at it's thinnest before a burr will start to form and grow. At a certain point, the steel at the tip of the apex just pushes away from the hone because at that level of thinness it takes more pressure to cut the steel than it does for it to flex away. So as a bit of steel is cut away on the bottom of the "foil" or burr, the top part flexes away. When you flip the blade, the same happens on the other side. This is why, if you were to examine the edge at a very small scale, you'd see that the angle of the "foil" bit is considerably more acute than the rest of the bevel.

It helps to understand if you remember that steel at this level is extremely flexible. We're talking apex width in the range of 0.1 micron, give or take. That's 4 millionths of an inch. A shim made of steel at this thickness could very easily be blown around with your breath, and torn in half by a toddler.

Let’s say you have an off center apex. The bevel on one side is wider than the other. The edge and metal would be feathery thin there to...no? Why wouldn’t a burr form on either side at that point.

I guess I cannot grasp why the burrs guarantee a centered apex. Why can’t it happen when a very thin apex is off center?


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Ahh I see your problem. You're misunderstanding what's going on. A burr does NOT mean that the bevel is centered. It only means that the apex is formed - in other words that one bevel is cut completely down to the apex, and so is the other. They are both fully apexed and meeting each other. They don't necessarily have to be in the geometrical center of the blade. A burr will form even in the case of one bevel being huge and the other being the width of a piece of paper, as long as both are fully apexed. Think of a Japanese chef's knife grind - asymmetrical - grind bevel on one side and completely flat on the other. A burr will still form when the bevel is honed all the way down to the apex.

When I mentioned center, I just meant the center of the two bevels meeting, which does not need to be in the exact center of the blade itself. Probably should have used "central" but was picking up the term "center" from your post.
 
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Ahh I see your problem. You're misunderstanding what's going on. A burr does NOT mean that the bevel is centered. It only means that the apex is formed - in other words that one bevel is cut completely down to the apex, and so is the other. They are both fully apexed and meeting each other. They don't necessarily have to be in the geometrical center of the blade. A burr will form even in the case of one bevel being huge and the other being the width of a piece of paper, as long as both are fully apexed.

Ahhhhhh...OK. That begs to ask how we know when it is centered. Do we need optical comparators? Maybe it doesn’t matter so much.

With knives, uneven bevels are easy to see. Straights are a whole different story.


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It doesn't really matter. I would lay a wager that very few blades are actually perfectly centered. Probably approaching none at all.
 
It doesn't really matter. I would lay a wager that very few blades are actually perfectly centered. Probably approaching none at all.
Yep, that's about the size of it.

If it looks under a jeweler's loupe that the bevels on both sides are pretty much equal, and the grind line along the spine is about the same width on both sides, then it's probably about as good as you're likely to find. That's assuming you even want to be that picky about it. IF you can't naked eye see a real difference, and the razor takes a good shaving edge, you'll probably (like 99%++) never encounter an issue.

From there, even if they aren't exactly equal, and the apex isn't exactly centered, so long as you don't grossly neglect one side while honing, the amount of runout shouldn't increase over time.
 
Yep, that's about the size of it.

If it looks under a jeweler's loupe that the bevels on both sides are pretty much equal, and the grind line along the spine is about the same width on both sides, then it's probably about as good as you're likely to find. That's assuming you even want to be that picky about it. IF you can't naked eye see a real difference, and the razor takes a good shaving edge, you'll probably (like 99%++) never encounter an issue.

From there, even if they aren't exactly equal, and the apex isn't exactly centered, so long as you don't grossly neglect one side while honing, the amount of runout shouldn't increase over time.

Good. I won’t obsess over perfection.


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A burr forms because the abrasive particles deflect the very thin steel at the apex up instead of cutting it off. Burr formation is different on different stones and with different pressure -- a very "sharp" stone with minimal pressure will develop only a small burr even on fairly coarse stones, while a blunter stone with more pressure can produce a quite significant one. I've done it, accidentally. Burrs will ONLY form when an apex, even a poor one, is achieved, for any other condition the steel is too stiff to bend much.

All stones produce some amount of burr if used only on one side of the razor, but polishing stones won't make a very large one -- the size of the burr is partially determined by how much the particles stick up from the surface.

Alternate side honing (laps up and down the stone) will produce the smallest burr, and help keep the apex centered on the blade. Typically one does not want a burr to form while honing a razor, or at least the very smallest burr possible, but when first learning, a burr is a handy tool for telling when you have achieved an acceptable apex. I am lucky, my stones change sound remarkably when the apex forms, so I can tell without much of a burr, be it knives, plane blades, or razors I'm working on.
 
So then by theory a leading edge lapping verses a trailing edge lapping shouldnt make a difference as long as the last lapping strokes are edge leading and across the stone to remove all traces of the burr (think X stroke).....correct?
 
That is correct. A few "pull strokes" -- moving the razor sideways only -- will help. Very light pressure and alternating strokes.
 
So then by theory a leading edge lapping verses a trailing edge lapping shouldnt make a difference as long as the last lapping strokes are edge leading and across the stone to remove all traces of the burr (think X stroke).....correct?
This has been a very helpful discussion. Thanks.

As far as removing the burr, I actually just saw a video online where a guy very lightly cut into the edge of a soft piece of wood to remove the burr. Then proceeded to honing. This was on a chef's knife however... I wonder if it would work on a razor.
 
I would not advise it. Tearing off a burr is a good way to set an edge WAY back. Better to minimize the burr to begin with by using short alternating strokes toward the end of the honing process. Slash's way of doing this will work just fine also.
 
I would not advise it. Tearing off a burr is a good way to set an edge WAY back. Better to minimize the burr to begin with by using short alternating strokes toward the end of the honing process. Slash's way of doing this will work just fine also.
Thanks for the reply. I figured that to be the case, because razors seem to have a much finer edge than your typical kitchen knife. Figured I'd ask anyway though.
 
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