Thoughts on short-stroke burr removal

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Twindog

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Probably one of the most common failures in sharpening an edge is to leave a burr or partial burr. The burr is necessary to ensure that you’ve actually created a true apex, but it can be hard to remove because the action of moving a stone over an apex metal scrapes up a new burr — a wire edge that leans away from the side of the edge you’re sharpening. If you don’t remove that burr — that wire edge — it will quickly break off with use and leave you with an edge that is no longer acutely sharp.


The usual advice for removing a burr, once it has been formed, is to gradually reduce the stone’s pressure on the edge and migrate to ever finer stones.


Coarse stones, because they remove metal faster, are strong burr formers.


High pressure also removes metal faster, making it also a strong burr former.


There is another factor, too, that I never hear people talking about: stroke length.


The longer the stroke of the stone over the apex, the more metal is removed, making it too a strong burr former.


Ideally, you not only migrate to finer stones and reduce the stone pressure, but you also shorten the sharpening stroke. By keeping the stroke short — just long enough to remove the old burr — you are less likely to create a new burr.


I’ve found that removing a burr is very easy when I shorten the sharpening strokes. By shortening the strokes, a new burr is not formed. When I finish with stropping, little or no burr is left and the strop is free to refine the fully apexed edge.


I know that most people think that the burr is constantly flipped from one side to the other, but actually, when you sharpen with edge-leading strokes, the burr is quickly cut off when you switch from sharpening one side to the other and the formation of a new burr is started. If you’re using a coarse stone, heavy pressure and long strokes, the new burr is very quickly formed, leaving the impression that the burr has flipped to the other side. It’s actually a new burr.
 
A burr is never truly gone but minimized to acceptable levels. Truthfully, if the burr is small enough that cutting paper does not fold it on the edge then it should not cause any edge retention issues. Sometimes a little burr can be a good thing and can even make bringing the edge back to life after a little use much easier. The burr breaking off theory I don't agree with because the type of failure mode does not coincide with the typical actions of edge wear. For it to break off there would need to be some sort of fracture over the length of the edge which I am not sure could happen. We are basically talking about chipping at this point which is a completely different failure mode typically localized to a small area.

I think the stroke length can help but pressure always seems to prove itself as a more important factor. Maintaining a complete stroke from heel to tip while decreasing pressure to almost nothing offers far greater consistency in the results. A small stroke means multiple points of contact per side of the blade during the sharpening stroke which will result in inconsistent contact to the apex over the length of the edge. At the end of the sharpening this will make burr removal much harder because the edge now has high and low spots over its length making contact from the stone irregular.
 
The thing about a burr, which, when large enough, I call a wire edge, is that the wire edge is unstable. It is vulnerable to all kinds of damage. It can roll. It can chip. It can break off in longer pieces. If the burr can be removed completely -- at least for all relevant scales -- then the edge will stay sharper longer and be less vulnerable to damage.

Long strokes are important to refine the edge by removing most of the scratches from the previous stone and making the edge apex less toothy (jagged). And you can minimize the burr by using extremely light pressure, as you said. But long strokes create a new burr. The last stroke(s) have to be short or you'll leave a new burr.

But I've been experimenting with the last stroke(s) of the finishing stone by keeping them short, just enough to remove the remaining burr and not too long to create a new burr. If your last stroke is long, you will leave a burr.

I've found stropping is more effective if done on a clean edge, rather than use stropping to remove a burr. But I don't have enough magnification to really see what is happening, and that's why I was interested in what people thought.
 
I have really bad OCD when it comes to sharpening and use a 60x magnification and I can get rid of almost all the wire edge without using the strop just finishing on a really fine stone.
You can use whatever stroke works for you as long as you cover the entire lenght of the blade and use light pressure, I guess it really comes down to personal skill and technique.
 
I have a few thoughts on this.

First, I believe that most of the time when a burr appears to be flipping it is in fact flipping. Sometimes on the finishing stones if it is very very quiet, one can actually hear them do so.

Consider the burr is nothing more than the incomplete tailings left behind by the plowing action of the abrasive mineral, very much like the dirt piled up on either side and in front of a plow head, but imagine another dimension as it works into open air off the cutting edge. Is unsupported steel outside the geometry of the two bevel sides. There may very well be some other residuals in the low points on a rougher edge, but it is almost impossible to remove them in detail and largely unnecessary anyway. The attachment point is like a hinge on a door - initially the hinge is sticky, but the more times the burr flips, the more readily it will continue to do so.

Pressure and mineral presentation (density of the backing the mineral rests on, how much mobility in any direction the mineral has) are the two biggest factors governing burr formation IMHO.

When setting a new bevel I generally work initially in sections so the burr is raised nice and even. Once it begins to form along the entire edge, I'll start to work the entire thing for a few passes. This method especially so if the tool in question still has the factory edge and is heavily battered. I maintain this strategy on the opposite side as well, raise burr in stretches to a uniform size and then work it down with overall passes. Once it starts to disappear, I'll kick back to a more sectional approach so I don't form a fresh burr where one was removed, while attempting to remove a burr on a nearby stretch of edge. If I had much better mechanics than I do, I could see where full sweeping passes might be more beneficial, but I gave up on that a long time ago - most of the time I can deal better with working overlapping sections than holding the angle for the longer mechanics need to execute the full-edge pass.

In my experience, on a clean edge one has (with good pressure and angle control) a few passes where the edge can be worked fine without much if any burr formation. Go too long even with light pressure and a burr will begin to form if working with a hard, fixed abrasive. So the burr then needs to be removed down to the intended bevel face without forming a new one on the other side. This is why I use a sectional approach for the last bit of clean-up. Once the burr is good and gone, I'll usually make a few light full-edge length passes, leading if the stone is hard, trailing if the stone is soft, and then at least a few passes on plain paper or on to a loaded strop etc.

I agree with Jason re residual burrs - in most cases they do not really hamper a utility edge. In any event one is better off leaving a bit of burring rather than blunt the edge trying too hard to remove every last trace.

In general, I think of it as being shorter passes, but in reality I will occasionally use longer passes, but confined to a targeted stretch of the edge - whatever is called for - above all I try not to work where it isn't needed.
 
magicblade said:
I have really bad OCD when it comes to sharpening and use a 60x magnification and I can get rid of almost all the wire edge without using the strop just finishing on a really fine stone.
You can use whatever stroke works for you as long as you cover the entire lenght of the blade and use light pressure, I guess it really comes down to personal skill and technique.
Click to expand...

^^That's it, in a nutshell, for me.

My own mantra for burr removal, or minimizing them to start with, is 'less pressure, less pressure, less pressure...'. I'm convinced it's all about pressure exerted laterally against the edge, and less about length or method of stroke (as long as pressure is minimized). Lately, I've been tinkering with a couple of diamond rod-type sharpeners, one of which is a pocket hone about 4" long, the other is an oval diamond 'steel' about 10" long. I've gotten a better handle on pressure used with them, literally relying on the weight of the blade and no more, during each stroke. On either one, I've been noticing both much-improved sharpening and a lack of perceptible burr formation, even on burr-prone steels like 420HC or VG-10 (Spyderco), which is a new change for me. Every time I've noticed an improvement regarding burrs, it's always come as a result of finding a better way to minimize lateral pressure against the edge. All other factors of grit size, method of stroke, etc. can still be varied all over the place, so long as pressure is more consistently minimized. I don't worry about pressure during heavy grinding done during new bevel-setting, as shaping the geometry is the only thing concerning me at that stage; I look for the burr, even a heavy one, as a positive indication of a full apex. Once it gets down to refining the apex, dialing the pressure WAY DOWN is what it's all about, for me.


David
 
Reducing pressure is important and it used to be all that I needed to remove the full burr. But one day I reprofiled a Microtech DOC in Elmax to 30 degrees inclusive. That knife has a really thick grind, especially at the belly and tip. I could not get rid of the burr, no matter how light of pressure I used. I think that the bevel was so wide that I was scraping up a lot of metal with each stone pass. The wider the bevel, the more steel the stone is picking up and smearing it into a burr. The reprofiled bevel was about 4 mm at the widest (about 0.16 inches). And Elmax is a tough steel, which seemed to make the burr especially resilient.

What finally worked was to take very short, light deburring strokes. The burr was at an angle leaning away from the apex, leaning off to the opposite of the side that I had just sharpened. Because of that burr angle, a short, light stroke on the burr side cut it off cleanly. It couldn't flip in this case because the stone was pushing it away from the other side of the edge.

If I kept the deburring stroke long, I'd just recreate a new burr in a never-ending chase. But the short stroke worked.

As a side benefit, when I finished with a stropping (using the Wicked Edge), the edge responded better than it usually does. My guess is that the strop didn't have to finish off the last bit of burr and was able to work purely on refining the clean apex.
 
It is my belief that the material making up the burr all comes from the very edge. Everything further back that might contribute is being completely ground to dust by the field of abrasive as it goes by. This is why even if you stop in mid stoke and examine the scratch pattern (under magnification) you will see little or no evidence of burring anywhere further up the bevel face, though you might make out some spots where a grind trough stops cold. If using a softer waterstone you might observe the frosted surface typical of lapping operations as well, but still no real burr. Scandi, for example, do not make larger burrs per for the increase in bevel face.

It all comes from the last bit of edge and is proportional (roughly) to the size of the abrasive making the grind trough and how deep its cutting (compounded by the tensile properties of the steel etc). Again, think of like a tightly packed crowd of plowheads tearing along a field (maybe a clay field), but one that ends in open space. Factors such as how the steel reacts to being hooked by the abrasive and how far it deforms before failing (and the speed at which this happens), the shape of the abrasive as it gouges into the steel, how mobile or frangible the mineral is etc etc will all come into play. If the applied force is too great for the thickness of the steel, the last bit of edge may flex away from the source of pressure and the outermost (sub)microns of steel from the opposite side will add to the burr in that way - no longer falling within the boundaries of the bevel but not left over remnants of grinding action - a true wire edge as opposed to a burr.

I do totally agree with the short strokes for stubborn burrs. The mechanics needed to maintain a dead-on angle control, especially when working around a pronounced belly, are very challenging (for me). If the edge in question has a gentle uniform curve, I can do a pretty good job of hitting it with full passes. When they start getting recurves, aggressive belly curves etc, I find I get a much better outcome - less overall error - by segmenting the edge. Inadvertent grinding where the edge was clean will, as you mention, just make another burr on the opposite side even as the burr disappears from the target area.

Also, any time I'm changing the inclusive I assume there will be some trouble with portions of the burr being larger than their neighbors and larger overall than what I'd get from an edge that just needed a touch-up. The practice of forcing the burr to stand more proud by backdragging the edge across some hardwood at a high angle is very helpful in this situation. The burr is at a better angle to be caught and ground off rather than collapse and flip back to the other side. I will sometimes elevate the spine for a few passes as well and beat down a larger burr initially at a more advantageous angle, then drop back down to the original angle to remove the last bit of it.

Is a topic I never tire of pondering. In another thread I realized my entire sharpening philosophy at the grinding stage had shifted to where I no longer think of creating an edge, but rather removing the burr after shaping the cutting bevel. At the final polishing stage I think of it as almost creating a new burr but stopping just short.

Martin
 
I have a question: When moving from one stone to the next stone through the various stages, do you have to remove the burr that was formed from the previous stone first on THAT stone? Or can you just wait until the last stone before focusing on removing the burr figuring the next stone will remove the last burr and create its own burr? This is assuming you have completely apexed the bevel at the first stone and completely scratch marked the bevel from the previous stone.
 
I also have a second question. I started practicing sharpening on my new set up with DMT stones with a set of cheap Faberware steak knives. I wasn't aiming to create a burr (because I'm a noob and thought I know better than any of you and I thought burr means wasted metal that formed past forming the apex). After I reprofiles the bevel to 15 degrees on the XX coarse, I didn't see any burr. But when I did the X coarse, I was surprised to actually FEEL the burr that up to then have only read.

My question is: would softer steel such as the one used on my practice knife more easily create bigger burrs than hardened steel? I would assume now that a burr would still be formed but I'm wondering if it would be as pronounced.
 
Rey HRH said:
I have a question: When moving from one stone to the next stone through the various stages, do you have to remove the burr that was formed from the previous stone first on THAT stone? Or can you just wait until the last stone before focusing on removing the burr figuring the next stone will remove the last burr and create its own burr? This is assuming you have completely apexed the bevel at the first stone and completely scratch marked the bevel from the previous stone.
Click to expand...

In general it is a good idea to at least reduce to a very small level before moving to a finer stone etc. The burr can just as easily flip as be ground off, and the only way to make certain the burr you're forming at any given level is actually from the stone you're currently working, is to start with a pretty clean edge.

You will find a variety of thought on this, the above is my personal philosophy. I always try to completely eliminate it from one step to the next, but generally settle for no longer being able to feel it or see it with the naked eye under strong lighting. Depending on application, I use a range of edge finishes, so am pretty comfortable with removing the burr at any grit fairly rapidly. To start out, I'd be inclined to do your best at every level - will help in a number of ways and only downside is it will take a bit more time initially.



Rey HRH said:
I also have a second question. I started practicing sharpening on my new set up with DMT stones with a set of cheap Faberware steak knives. I wasn't aiming to create a burr (because I'm a noob and thought I know better than any of you and I thought burr means wasted metal that formed past forming the apex). After I reprofiles the bevel to 15 degrees on the XX coarse, I didn't see any burr. But when I did the X coarse, I was surprised to actually FEEL the burr that up to then have only read.

My question is: would softer steel such as the one used on my practice knife more easily create bigger burrs than hardened steel? I would assume now that a burr would still be formed but I'm wondering if it would be as pronounced.
Click to expand...

In many cases the softer (lower Rockwell hardness) will form larger burrs, or at least will do so very quickly, especially when being sharpened on tougher abrasives like diamonds. The upside is that they are also easier to grind off. Do not fear the burr, or try too hard to avoid it - it is a good sign, and learning to create and eliminate it with control is an important component of the sharpening process.
 
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