The burr

Discussion in 'Maintenance, Tinkering & Embellishment' started by Jason B., Dec 9, 2009.

  1. Jason B.

    Jason B. KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 13, 2007
    The burr is often a troublesome factor for those new and sometimes advanced sharpening enthusiast. A burr is a plastic deformation of metal at the apex of the cutting edge, think of it like a mud slide down a hill but the metal that does not get ground off forms itself to the edge extending slightly past the apex. The burr is often referred to in sharpening is two different ways, the first being a coarse sharpening burr that helps you to judge when a worn or damaged bevel has been realigned. The second and often problematic one is the one that comes in the final stages of sharpening and unless you know what your looking for can often be missed.

    Sometimes "chasing" the burr can lead to more problems than solutions, when using a coarse stone to change the angle or fix damage to the edge pressure and the want for speed usually are our biggest enemy. More often then not we use more pressure than needed on a coarse stone because when we do it cuts into the edge faster making time spent working on the knife shorter. In reality though you end up making more work for yourself in the end, using too much pressure causes a higher degree of plastic deformation and often difficulty's in holding a precise angle. When light pressure and good angle control are kept you can let the stone work the edge and not the edge working the stone. Instead of trying to form the burr form the edge, as you sharpen work on holding the proper angle throughout the process and making them meet in the center. When done like this your angles per side are usually more even because your not grinding one side until it folds over the other angle on the opposite bevel and repeating.

    Picture example 1, ZDP-189 sharpened on a DMT XX-coarse benchstone. As you can see even at 120 microns you can achieve a almost burr free edge and what burr is their is in line with the edge and not the cause of increased angle.

    This next picture is a example of a burr caused by excessive angle to one side.
    Picture example 2, 420HC using a DMT fine 600 mesh benchstone, excessive angle applied to back side. 100x

    As you can see for example 2 the burr has now overtaken the edge and when felt will feel like the edge has rolled over and is very tough. When this has happened the angle to the side with the burr must be met to the angle of the side that has created the burr. The side with the burr will take only minimal passes to reform the burr back to the side with the steeper angle but now when you go back to the other side and try to push the burr back your angle exceeds the center line of the knife even more and continues to change angles per side of blade. Watching for angle change and amount and type of burr formation is a must if you desire a even and properly formed bevel.

    Now on to the finer side.

    When sharpening with fine grit abrasives your still creating this burr but instead of being able to feel it you must find it. One of the best ways if you don't have any magnification equipment is to look under bright warm lighting. Another way to tell is if you blade will only pass the shave test on one side but sometimes a burr can be too fine to even be noticed that way. Once you equal or surpass a 8000 grit finish the burr if any and if the edge has been formed properly is usually a mute point because the first time you cut something your probably creating more damage to the actual edge than the burr is resisting. Though if not formed properly edge retention and cutting performance can be pretty bad.

    Picture example 3, S30V sharpened on a lapped spyderco UF ceramic.
    Its not the best picture but you can still see the very small burr. pic at 400x

    enhanced picture from above
    I wish I could show more but until someone sends me a SEM this is as much as I can show ya. :)

    Hopefully this has been of some help and please share your comments.
  2. So-Lo


    Jun 12, 2006
    Great post :thumbup:

    Your right a lot of people really push down on the course stone, and it only makes for more work. :cool:

    Nice pix also :)
  3. rpttrsn

    rpttrsn Gold Member Gold Member

    Nov 1, 2006
    Great photo's and very informative. Thanks for taking the time and effort to share your knowledge with us all.
  4. thombrogan


    Nov 16, 2002
    Awesome thread, knifenut1013! Very well stated.
  5. cgmblade


    Jun 4, 2009
    So, let's see if I understand this; less pressure equals less burr?

    Is a burr inevitable or can it be avoided if pressure is light enough?

    It seems you were writing primarily about using flat stones. Am I correct in thinking that it would be easier to keep edge angles even by using something like the Sharpmaker?

    What role does stropping play in burr removal?

    And last question; is it possible to get a knife too sharp e.g is a hair whittling edge the best for cutting rope or whittling wood, or skinning a rabbit?

    Thank you very much for your post. I've been attempting to sharpen knives since I was a little kid. Sometimes I get it right and sometimes I don't. I figure it's about time I really learned what I was doing.
  6. Morrow

    Morrow Don't make this weird

    Apr 11, 2007
    good stuff. thanks for taking the time to post this.
  7. Jason B.

    Jason B. KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 13, 2007

    The burr in a sense is always there but once you get to a point like 0.5 micron the scratch pattern is so fine (this is my opinion) that the metal becomes blended together to the point where if there is a burr you would need to view it at 5000x to find it. Less pressure is causing less flow in the metal and less build up on the edge, so yes, by using less pressure you form less of a burr and have better angle control. You don't want to make the knife follow a angle you want the knife to follow its own angle.

    Flat stones, round stones, by hand, with a sharpmaker, convex, aligner, edgepro, or on a strop, if you use a abrasive to grind metal you can create a burr and all for the same reasons. The sharpmaker may have set angles but your still doing it by hand and no offense to anyone but adding to bad technique. Sharpening is a lot about feel and when you feel the bevel come into proper contact with the stone and let the blade follow its own angle with proper control on the sharpeners part the bevel always looks proper for said knife.

    Stropping with a compound is just another form of sharpening, your using a fine abrasive to refine steel.

    And for the last question, I think most knives become too sharp around 0.25 microns and sometimes 0.5 is a bit too much but it also depends on the steel. When a knife is sharp its sharp at any level and with that level of sharpness comes a unique cutting technique. If you don't know how to use the applied edge your results at the cutting task could or could not be in your favor. I recently loaned out my spyderco mule team in 52100 steel, the edge angle was around 15-18 degrees and polished to 0.5 microns. When the knife was returned from the hunting trip the user stated how he needed one of those and asked where he could get one, when I asked how it cut his response was with the greatest of ease. If you know how to use it and its sharp then you will not have any problems but sharper never hurts ;)
  8. shecky


    May 3, 2006
    While it's common to advise burr fomation in the sharpening process, I find it much preferable to avoid forming a burr in the first place. This is done using a light touch on a good, clean, sharp abrasive medium. Burr raising consumes more time to eliminate in the long run, and usually causes more problems by causing excessive fatigue in the very area you don't want it. The result is an edge that is fundamentally weak and prone to rolling of breaking in use. Loaded stones, and worn out sand paper/belts often require more pressure, which only increases the chances of deformation and heat generation, which cause metal fatigue and minute amounts of annealing at the thinnest portions of the edge.

    It can be a pretty difficult goal to chase. After all, weak, burr prone steel is bound to be found if you look close enough. The ideal is to keep your burr raising so small that it is undetectable by normal means. Once burrs reach such a minute scale, you can pretty much call them sharp.
    Last edited: Dec 10, 2009
  9. Jason B.

    Jason B. KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 13, 2007
    Very well put.

    On that note take a good look at this picture and lets see if everyone can see what I see :) (its a factory edge)

  10. cgmblade


    Jun 4, 2009
    Thanks for the response to my questions above, an NO I doubt I see what you see. Is that a very thin burr on the edge? To my untrained eyes it looks pretty good.
  11. gunmike1


    Dec 9, 2005
    Great thread and pics. To me the burr is the enemy. On rebevels I just try to form the tiniest burr I can with my DMT XX Coarse, which can be tough, but I always finish with light strokes on alternating sides to get as burr free of an edge as possible with that stone that evenly shaves arm hair. After that I just pay as much attention as possible as I go through the grits to get my scratch pattern to the edge evenly without going past it. Of course I am not a machine and end up with a tiny burr, but with every grit I de burr to keep my edges as clean as possible. At 8000 grit it is really hard to try to even detect a burr just like you said, and under 1 micron (especially at .05 micron) it is pretty much impossible to see a burr even under high magnification. I think understanding the burr and knowing how to minimize and remove burrs is pretty much the key to sharpening, and one reason why I advocate using lighted microscopes to look at your edge closely and understand what is going on with it.

    Great pic of the factory burr. They are so common it is crazy. I must say though my ZDP-189 Mule came burr free and way sharp from Spyderco, then burr free and hair splitting sharp on a 120 grit edge on it's trip back from Tom Krein to get handles and Kydex. Spyderco does a good job with it's factory edges for sharpness, but even they have sent me some butted ones. A Kershaw Cyclone I got a while back had one of the biggest burrs I've seen in my life, and a couple CRKT's had really bad burrs as well. Benchmade sent my M4 Rift really dull with a burr, and my 760 was similar. Since I put my own edge on knives the burrs don't bother me much, I mainly look for thin and acute angles from the factory.

  12. L. Richard

    L. Richard Gold Member Gold Member

    Dec 20, 2006
    Thanks for the thread. I never deliberately try to raise a burr. Now I don't feel so out of place in this forum.
  13. David Martin

    David Martin Moderator Moderator Gold Member

    Apr 7, 2008
    Many noteworthy authors have written on this subject and they agree that a coarse edge sharpened on a 100-150 grit stone is better for the cutting you inquired . DM :thumbup:
  14. cgmblade


    Jun 4, 2009
    Thanks. I had read that some sailors used a file to sharpen their knives so that the mini serrations would cut rope better.
  15. Jason B.

    Jason B. KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 13, 2007
    While a coarser edge may cut things like rope better its more about the cutting style than the edge. A coarse edge will increase the slicing/sawing cutting action but its not going to work very well for a push cut. Heavy rope, meat and wood can all be cut in a effective manor by a polished edge but the technique you use will play the biggest part in how effective the knife is for a given task. When using a polished edge a draw slice or push cut style will be needed for effective cutting and if you try and saw with this type of edge you will get nowhere fast.

    Just because a knife is polished sharp does not mean it won't be a good cutting blade it just means you need to know how to properly use it.

    I like to try and believe those that that say a coarse edge is better but in actual use I just don't see it, for me a sharper knife is always sharper.

    I've never heard of anyone saying a utility razor doesn't cut well and those have a polished edge too.
    fullrighturnedknife likes this.
  16. David Martin

    David Martin Moderator Moderator Gold Member

    Apr 7, 2008
    Thats exactly what were talking about and they say the edge cuts longer . DM
  17. David Martin

    David Martin Moderator Moderator Gold Member

    Apr 7, 2008
    Nut, You know the utility razor example is not as accurate because there are other factors which make those cut well, not just a polished blade .
    Because you just love sharpening, its your thing . Its not a bad thing to know how to obtain that level . Its just not needed for most types of cutting . DM
  18. cgmblade


    Jun 4, 2009
    That makes sense.
  19. Jason B.

    Jason B. KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 13, 2007
    True DM, but would they still cut as good with a 320 grit edge ;)

    Also if you don't have a backing (table, block of wood etc) push cuts can be a bit tough. I don't polish everything, it works for most cutting tasks but some things just require a toothy edge. Rope and to filet a fish are the first things that come to mind that I know need a coarse edge to be cut effectively. It can go both ways with rope but unless your doing freehanging cuts or have a backing to do push cuts the polished edge is not going to be the ideal choice.

    Each type of edge finish will cut differently and like said before technique will be a big factor in the performance for said cutting task.
  20. David Martin

    David Martin Moderator Moderator Gold Member

    Apr 7, 2008
    I know where your coming from and where on the spectrum you stand . I'm just making sure the middle ground gets its due with many authors in my corner . DM
    They do indeed cut very well with a 3-400 grit edge, even according to Joe Talmadge . Even though that edge does not shave much, it bites well . For most of my general cutting I'll take the edge to 550-600 a good shaving/slicing compromise . DM
    Last edited: Dec 11, 2009

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