Burnt edges?

Discussion in 'Maintenance, Tinkering & Embellishment' started by Shotgun, Mar 29, 2019.

  1. Shotgun


    Feb 3, 2006
    There was a thread here that I read recently arguing against the idea that you can’t ruin the edge of a blade through machine/hi-rev sharpening.

    I’ve experienced this. I had a fiskars made, gerber branded axe that had this problem. The consensus, whether true or not, was that the final sharpening ruined the temper at the edge through overheating. Like others I experienced chipping and came here all “WTF” and saw the advice of;

    “The final sharpening burns the edge. Hand sharpen past the “burnt” part and it will go away.” -synopses. Not verbatim.

    I did that and the blade singed with sharpness. It did not chip. My brother in law needed a root grubbing tool so I gave it to him knowing it will do the job with full confidence after chopping with it.

    Is there any data on how much heat a small piece of steel(like an edge) experiences through sharpening? Any kind of sharpening. By machine or by hand.
    [email protected] likes this.
  2. pinnah


    Jul 28, 2011
    Have you posted this in the Maintenance forum. I'll wager that those guys would know.

    Following as I'm interested too.
  3. Bill3152


    Nov 27, 2018
    You can blow a temper for sure. But if you use common sense and dip the blade after each pass you will not. Keep the blade moving and you should have no issues. The critical temperature is whatever the last heat treat was done on that particular steel. Since machine grinding is so much faster than using hand tools. I figure if I do it slowly and carefully I am still way ahead of the game.
    GABaus and marrenmiller like this.
  4. Mo2


    Apr 8, 2016
    Most steel will find its maximum potential after about 3 sharpenings give or take.
    miso2 and John_0917 like this.
  5. John_0917

    John_0917 Gold Member Gold Member

    Apr 15, 2014
    What he said!

    Also to add to that, I like to use a course diamond for the first stage of sharpening the first 1-3 times I sharpen a knife, this helps speed up the process of getting to the “good steel”.

    My D2 knives are just hitting their stride now after about 5 sharpenings.
    Mo2 likes this.
  6. tomhosang

    tomhosang Gold Member Gold Member

    Feb 17, 2017
    Both @Mo2 and @John_0917 are correct. Most knives come from the factory being sharpened on some sort of powered belt. While you may not end up with a "burnt" edge (which is easy to see. The steel will have turned a brown/purple color usually), it definitely produces fatigued steel. That fatigued steel may not chip, but it 100% has lower wear resistance. Check out YouTube and take a look at some of the testing Outpost76 has done. He does a lot of tests on brand new knives to find out how many sharpenings it takes to get to the "good" steel. It generally falls between 3-6 sharpenings to reach its maximum potential. I've done a couple tests myself; one with a Bradford Guardian 3 in M390 and one with an Ontario Rat 1 in AUS-8. Both fall in line with Outpost76's results. You don't really generate a lot of heat by hand sharpening. If you're really trying to, you could probably feel it get luke warm. Belt sharpening is a different story though. If you aren't careful, the entire blade can get very hot in a short amount of time with the hottest part being the cutting edge.
  7. FortyTwoBlades

    FortyTwoBlades Baryonyx walkeri Dealer / Materials Provider

    Mar 8, 2008
    The key with sharpening on a grinders is fresh belts, rapid long passes, and a light touch. The apex of an edge is foil thin, and can flash-heat before the material behind it even has a chance to become warm to the touch. I retire my coarsest belts from service after only 1-3 regrinds because doing full flat grinds or other heavy beveling work with large contact surfaces makes so much friction if you don't. For edge bevel grinding they last much longer, but as soon as the surface finish produced stops looking "sparkly" from how crisp the scratches are I retire the belt to doing spine-grinding and wood sanding.
  8. mattmanyam


    Apr 28, 2010
    This is the key to grasping this phenomenon.
  9. marrenmiller

    marrenmiller Basic Member Basic Member

    Apr 6, 2017
    I'm sure it can easily happen, especially on a thin blade, but I don't think it's a guarantee that it'll burn an edge if you use a machine. I think it's more likely that a factory using a belt grinder/sander produces a poor edge for other reasons, like poor angle control, obtuse angles, improper apexing, and overall lower grit finish. I'm not sure how much "edge fatigue" occurs when belt sharpening that isn't present during normal sharpening, or why that would be much of a factor if you're not building up a huge burr.
    GABaus and Bill3152 like this.
  10. Bill3152


    Nov 27, 2018
    I've sharpened many many knives on a belt and or paper wheel. I've also free-handed many many knives. I don't see a difference between the 2 methods in edge retention. Again it can happen. But if you are careful and do it knowing that it could happen it's not likely. Now a factory in a third world country with a 1000 knife sharpening daily quota? You bet!
    HeavyHanded and marrenmiller like this.
  11. brownshoe

    brownshoe I support this site with my MIND

    Sep 6, 2002
    I had a BM that was new with a discolored tip from to much heat in the final sharpening. The dealer sold it to me at deep discount and BM replaced the blade for free.
    FortyTwoBlades and Mo2 like this.
  12. ronnie hood

    ronnie hood

    Mar 12, 2018
    Would leaving the edge too thin pre heat treat to reduce time spent during final sharpening have a similar result?
    Or conversely if it’s too thick resulting in more time on the belt during final sharpening?

    What I’m asking is if it’s too thin pre heat treat could it be mistook for being burnt in final sharpening because that’s the final part of the process?
    Last edited: Apr 1, 2019
    jpm2 likes this.
  13. Mr.Wizard


    Feb 28, 2015
  14. Bill3152


    Nov 27, 2018
  15. HeavyHanded


    Jun 4, 2010
    I've done regrinds with brand new Norton Blaze 60 grit belts, making steel wool without heating the blade up. Another thought re how thin the edge is and so more likely to heat up, it is also more likely to conduct heat away.

    There is no doubt it happens but I too have seen no consistent difference between intelligently sharpened blades done on a belt and ones done by hand. I have seen plenty of poor quality factory sharpened edges, and a few good ones.
    Eli Chaps and Bill3152 like this.
  16. jpm2


    Nov 19, 2014
    I’ve burnt edges both intentionally and not while grinding blades post ht. In every case, it softened the steel where it would easily roll/bend.

    In the cases where the edge crumbles, my theory is either the edge was work hardened brittle during grinding, or was possibly too thin during ht (heated too fast and too long a soak at high temp, damaging the structure).
    ronnie hood likes this.
  17. wootzblade


    Feb 24, 2014
    Production shaving razors tolerate heating to 660 F without changes to the edge properties.

    At 3:40 they mention that they bake the coating onto the razors at 660 F (350 C) for 20 min. I'd say 20 min is very significant for a production, where the blank 3-step heat treatment takes only 60 sec.
    So, heating the razor edge for 660 F (350 C) is safe in sense of effecting the edge temper. Obviously, this regimen was chosen not to de-/over-temper the blades; if they could bake it quicker at a higher temperature, they would.

    It's a pity that Cyrano's cool experiment with the thermal lacquer was limited to 400 F.
    Last edited: Apr 6, 2019
  18. Mr.Wizard


    Feb 28, 2015
    @wootzblade Presumably the razor steel and heat-treatment was chosen to allow for a >350°C temper, right? I have seen temper protocols for cold work tool steel as low as 150°C; one would have to exercise significantly more care with this material I think.
  19. Joker66

    Joker66 Gold Member Gold Member

    Apr 20, 2014
    Larrin just addressed this topic and posted this short video.

  20. Bill3152


    Nov 27, 2018
    Interesting video. But my opinion hasn't changed. You CAN damage an edge with power tools but you don't have to. And that's based on my own observations and use. I'm convinced. But I'm not going to try to convince anyone else. Lol..

Share This Page