To burr or not to burr?

[cbwx34]

The difference is the mobility of the abrasive particles. The burr is a result of the abrasive catching onto the steel and pushing some out of whack as it scrapes material away. Grinding action is basically scraping material away (aside from CbN if properly aligned is more of a tooling action). When it scrapes, it forms a wake at the lead edge and the sides of the abrasive, whether it is leading or trailing - though trailing will make a more pronounced burr faster. This wake is what forms the burr.

As the abrasive becomes more and more loose, it catches and digs less. Some of the abrasives will slide between the workpiece and the stone, some will tumble. The burr that forms as the steel is pushed past its ability to resist, becomes smaller - with a hard fixed abrasive the abrasive wins out and the wake is going to be determined by the properties of the steel and how much pressure the abrasive has behind it. With a more mobile abrasive the furrow might generate enough resistance the abrasive shifts before the burr reaches the same size - now the wake is influenced in some part by the properties of the binder the abrasive sits in, or the texture of the surface the abrasive is resting on.

When the abrasives are more free to roll, slide and tumble the conditions that lead to burring are greatly reduced. As you get closer to lapping you might have no burr form. I also noticed on some stones intended for different industrial sharpening applications, the binder was so soft the edge would barely form a burr even with a fair amount of force on a trailing pass. Many softer waterstones fall into this category and almost have to be used with a trailing pass to get good results. Loose grit on a hardwood knifeboard is the same principle - you can grind with a scrub or a leading pass, but to clean up the edge it had to be finished with a trailing pass.

Same steel, same stone, first pic with slurry, second is no slurry. You can see in the slurry image almost no trace of the track marks stopping in mid grind, the average depth is less....

Thanks again for the reply.

Obviously a difference in your pictures (I combined to make comparison a bit easier)...

... but couldn't it just be that a stone with slurry creates a finer finish than a stone with no slurry? So, finer finish = less burr... no different than just sharpening on a finer stone.

I spent a bit of time this a.m. on a King 1K waterstone with slurry and no slurry... honestly couldn't tell much difference, other than a bit more 'misty' finish with slurry.

Anyway, thanks again... something I'll play around with a bit more, and see what happens.

 

[HeavyHanded]

Thanks again for the reply.

Obviously a difference in your pictures (I combined to make comparison a bit easier)...


... but couldn't it just be that a stone with slurry creates a finer finish than a stone with no slurry? So, finer finish = less burr... no different than just sharpening on a finer stone.

I spent a bit of time this a.m. on a King 1K waterstone with slurry and no slurry... honestly couldn't tell much difference, other than a bit more 'misty' finish with slurry.

Anyway, thanks again... something I'll play around with a bit more, and see what happens.

You're dead on, its pretty much impossible to clearly demonstrate this graphically. The differences that are visible are suggestive but far from conclusive.

You could try more stones and even tinker around with loose grit on hardwood, slurry on softer metal etc. I did some noodling with a variety of Jointering stones and found all but the hardest of them to be useless for sharpening as the binder was too soft. It was almost impossible to get a burr though I could see plenty of metal coming off.

The biggest differences will be between items based on composition like an India stone to your 1K king. Again, it means starting with an edge that is clean and seeing how large the burr gets, or more subjective comparisons - how easy is it to remove the burr on this stone vs another, and why.


In my experience it is easiest to see at lower grit 240-2k JIS.

My Norton waterstones on some steels don't even make a burr. I shoot for three finger sticky and move on.

 

[LAtraffic]

Thank you kindly, I have been considering getting a microscope of some kind. Maybe just start with something similar to see if I actually like using it. Thank you for the reply.

 

[Spartan00]

A little thing I've been doing , I work with cars all day so my fingers are pretty numb . Anyway I got tired of rubbing the skin off my fingers sharpening so started wearing gloves when I sharpen .

The low grit stones the burr that forms grabs the gloves .when you get up to 4 and 6 the burr doesn't really grab the gloves as much as the lower grit stones .

 

[MichaelMyers]

I'll add to this, I personally do not see any point in the act of "destressing" an edge unless one knows the thing is low RC steel that has been burnished many times or worked hard to a burr (usage so aggressive the edge was turned instead of being dulled). Most edges are dull - the steel that used to form the edge has been abraded away far more gently than any abrasive will manage. So unless the edge is drawn out or beaten to what appears to be an existing burr, why bother?


Otherwise the steel isn't going to be fatigued by use anymore than it would be by the initial face grinding resulting from the act of destressing in the first place.


As for sharpening without creating a burr - technically possible or one wouldn't be able to cleanly remove the burr either. Question is whether it is easier, more reliable etc - in short, is there is a good reason to do so and I don't believe this to be the case.

Sharpening to a burr is fast and thorough. One can use tactile aids throughout - three finger sticky, lateral rub etc - no need to rely so much on visual cues. The forces that caused the steel to burr are going to be present in burrless methods as well - a percentage of the subsurface steel being dislocated by the abrasive where it hasn't been cleanly sheared away. In the case of destressing with coarse or medium grit I'd expect a greater depth of this to be happening as well. And the likelihood of creating a small burr anyway at least in some spots along the edge will always be present, might as well just get on with it and get as efficient at deburring as possible.

Brent Beach has an interesting read on this topic - his take makes far more sense to me but I've never noticed a difference in longevity all else being equal.

His stuff is well researched and he qualifies his conclusions without making many blanket statements. The whole site is a good read, this is the page I'm referring to:
http://www3.telus.net/BrentBeach/Sharpen/jig faq 02.html#thebest

Thank you for your input on "de-stressing". It makes sense to me. Those are great images as well. Am I wrong to say the no slurry image has a crisper apex with less rounding? Seems more jagged though. How do they compare in cutting tests?

 

[HeavyHanded]

Thank you for your input on "de-stressing". It makes sense to me. Those are great images as well. Am I wrong to say the no slurry image has a crisper apex with less rounding? Seems more jagged though. How do they compare in cutting tests?

In all reality, they perform similar unless one really takes some time to eke out the best possible result. Used with a slurry and finished backhone I found I could (off the king 1200) get a finish and performance close to the 4k.
Also took more time, switching to the 4k worked faster and more reliably.

The best things a thin mud does is keep the stone surface from loading up and keep burr formation to a minimum. If the mud gets too thick the stone stops cutting well. Running it at a reasonable amount you won't notice a ton of difference. Finishing with some mud definitely makes a less irregular edge on all but the hardest of stones, where this relationship begins to reverse. This would imply there is a density and durability level where a stone might act very similar with and without mud (up to a point) and I believe this is true, but is also highly subjective (as is this entire topic!).

I'm not clear on the apex factor because the differences in stone composition really do make one to one comparisons tough, and its difficult to maintain a uniform mud film on the stone in any event. In practice I don't often make extensive use of the mud unless for cosmetic reasons. I note it as I work the initial grind at whatever level, and once I begin to apex I'll rinse the stone and finish off on a relatively clean stone. At that stage its done what I like best - keep the stone from loading and keep burr formation to a minimum.

If the stone is too hard to effectively backhone I am not going to introduce a bunch of mud to make it work that way - is not worth the tinkering and I get more reliable results moving to a finer stone. Not to discourage anyone from trying this out especially if your stone selection is limited - it is worth experimenting with, at the least you gain more understanding of how abrasive mobility functions under some circumstances.

 

[Windwalker9649]

Old thread I know, but as of a couple years ago I went from doing a traditional sharpening job of creating a burr, removing it, then sharpening it with progressively finer grit, then stropping it. Now I'm adherent of both methods. I don't know technical terms or definitions, but personally I differentiate between "making/putting on a new edge," and "sharpening the edge" Sharpening a knife means doing both.
What I do depends on the particular knife: the steel, geometry and condition of the edge. If it's a good steel that hold edges longer then a typical budget steel and I put an edge that's either polished or with a finer grit (M390, M398, Magnacut, etc)the apex has just been worn but isn't damaged in any way (micro chips for instance), I will do a full sharpening initially, like when I first get it and remove the factory edges. After that however, I'll strop it for as long as it will go back to the sharp edge. After that I might hone it, and lastly "sharpen the edge." Once I notice a drop in performance I'll do a full sharpening again. Steels that I put a toothier edge on, but are hard wearing like CPM M4 I'll do a full sharpening, but produce the smallest burr I can that still results in a sharp edge.
I'm not the most finicky person, most things I'm not too precious about, with two exceptions: my guitars and equipment (amps, cabinets, effects, cables…right down to changing the strings every week to 10 days whether they need it or not), and the second being always having an almost irresponsibly sharp edge on my knives at all time. It's for this reason that while I only own six or seven knives (beyond kitchen or specialty), they're all the best ones I could afford of the type I needed at the time, it's also why I approach maintaining the edge in this manner. I went with a knife with 1095 CroVan that just didn't hold an edge well, and within 6 months I had sharpened it so many times that it went from a relatively wide drop point (fixed blade) to looking like a fisherman's fillet knife.
This way my knife is always ready to flay me to knuckle bone with one small slip, and doesn't become a flimsy ice pick within a couple years.

 

[lonestar1979]

When the blade is really dull,its best to form a burr and take it off!

 

[Father & Son Traditional Sharpening]

I don't think any human alive could sharpen so precisely that a burr would not form. You would need to be watching the edge with an Electron microscope and controlling the grinding pressure with electronic pressure gauges and CNC mills for accuracy.

Burrs are the result of an abrasive that has passed over the "edge" of a grind plane. All of the debris being pushed in front of the abrasive does not get cut cleanly off of the steel because the steel, though seemingly Hard by our standards, will act more like modeling clay as it's interacting with the abrasive. This Plastic flow of metal results in the burr you find at the edge of two intersecting planes.

So... if you have created a sharp edge then you have at some point created a burr. If not then you have not reached the Apex of two intersecting planes.

I believe the real argument is not so much whether or not to form a burr (as I do believe this always happens when fully sharpened) but rather to sharpen to either a visible and/or palpable burr before ending up on the finishing grit level stone. It is indeed not a requirement to sharpen to a burr which can easily be detected and simply stop grinding once light stops reflecting off the apex region. Then you simply switch to a finishing stone of higher grit and raise your edge angle enough to apply a microbevel and you're finished. Deburring can be quite difficult and there is always the potential for introducing fatigued steel at or behind the very apex so it's generally better to avoid this if you have the right stones/approach.

I always hear of people intentionally trying to raise a burr using waterstones, which is really not how I would use waterstones. Sure you can raise a burr on them if they are quite reluctant to release and abrasive mud but I feel they are best suited to simply grinding prior to actually forming the apex. The muddy debris simply dulls the edge and deburrs all at the same time if using scrubbing motion or edge forward passes. Edge trailing cause a lot of problems with holding a steady angle in many cases so that isn't a great solution either because you really do want to have excellent angle control when setting the actual apex. Simple grinding prior to apexing is not so crucial as long as you end up with a low enough angle behind apex.