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How Long Does Shaving Sharp Last?

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by scott kozub, Aug 6, 2019.

  1. John mc c

    John mc c

    304
    Aug 23, 2018
    I stop at 1000 grit waterstone
    I think guys can overthink sharpening
    Having said that I really want a Belgian coticule that's supposed to leave a unique toothy edge that lasts and lasts
     
  2. scott kozub

    scott kozub Gold Member Gold Member

    312
    Jan 1, 2018
    Good point. I have got to hair shuddering
    I've never heard of that. Just looked it and appeared to look like a regular stone. What's different about them from a Japanese stone?
     
  3. Storm W

    Storm W

    273
    Feb 19, 2019
    Very true. I still haven't got my razor as sharp as it should be. With some knives I will take them to scraping facial hair but I would never want to shave with one. I have tried a patch and it's not nice.
     
  4. milkbaby

    milkbaby

    581
    Aug 1, 2016
    Belgian coticules are natural whetstones. I think they are most well-known to straight razor sharpening enthusiasts. I think people feel they can give similar edges that Japanese natural stones do. From what I understand, the variety of sizes of particles in these natural stones can also give a high polish with toothiness as well as various nice cosmetic finishes. Everything is just a tool though, depends on what you're looking to accomplish.
     
  5. John mc c

    John mc c

    304
    Aug 23, 2018
    A coticule has regular grit sized particles then about 30% garnets
    So you get a mirror polished edge and the garnets cut irregular teeth in to it
    Ends up with much more polished edge between teeth as opposed to getting a toothy edge with a pass or two on a coarse j stone
    Only need 1 stone too,with a heavy slurry it's equivalent to 1000 grit,water down slurry for finer all way to just water on it for honing
    They work slower than j stones tho and getting the slurry right is a lot of work
     
    milkbaby likes this.
  6. ckdexterhaven

    ckdexterhaven

    40
    Feb 16, 2019
    Very extensive tests in the German kochmalscharf forum have shown that the following factors together will lead to the longest lasting edge:

    - A guided sharpening system with pressure control. On the very fine stones (beginning with 12K) the blade should hardly touch the stone (a few grams), otherwise the very fine edge will be damaged.
    - An inclusive angle of 36 degrees. This is a good compromise, lower lasts shorter, higher cuts worse.
    - Finishing on a very fine stone (Shapton 30K or Gokumyo 20K) and then a natural stone.

    I sharpened a kitchen knife made of 1.3505 (comparable to 52100) this way and the knife was easily slicing ripe tomatoes after more than 6 months with daily usage.
    Of course, this can be considered overkill and it's not necessary in any way but these findings were crosschecked by many in the forum.

    If you think about it, it makes sense. A highly polished edge is normally very short lasting because it gets damaged while sharpening by hand or without pressure control. The very fine tip of the edge is very thin and the normal pressure of the blade on the stone easily causes micro damage.
     
  7. ckdexterhaven

    ckdexterhaven

    40
    Feb 16, 2019
    Coticules (due to the round garnet) make a very smooth edge which is great for shaving but less so for cutting tomatoes.
     
  8. Storm W

    Storm W

    273
    Feb 19, 2019
    Just to put this out there. I am going to guess that the fine edge is not what is doing your cutting but rather its is the fine apex. If you are using simple steels regularly in the kitchen your knife may be sharp but its will not have anything close to a razor edge on it. There is cutting board of course but a single cut of a high acid food such as citrus or tomatoes with strip of the super aggressive "razor " edge and you are left with a "working " edge.
     
  9. Rsq

    Rsq Gold Member Gold Member

    161
    Aug 7, 2011
    Something I don't see discussed (ever) is asymmetry. I use my knives for wood, so my use is a little different, but I sharpen them a little like they're chisels. I hollow and polish the back (EDIT: back = the side in contact with the work surface), and then sharpen from only one side, but with a coarser grit. I have found that asymmetric sharpening provides a wonderful balance of properties.

    This is very important to me because I like whittling aggressively hard woods like african blackwood, which murders edges. I rough with custom knives because they last orders of magnitude longer than the expensive hand tools, which I can then save for when they're needed. Cutting with the polished side down leaves a very nicely finished surface, almost as good as full polished, but it will last a good bit longer, and is much easier to revive with a strop. I believe that the steel has a lot to do with how long this lasts, but geometry is more important, and within a single steel type and heat treat, a shocking variety of results are possible. For wood, I mirror polish the back (to 1200 on diamond lapping plates, then 1k, 2k, 5k stones and a strop), but only take the edge somewhere between 400 and 1200. Coarser lasts a little longer, but starts leaving a finish that I wouldn't quite call polished. I like to go as polished as the steel will allow, but this is case by case. I find 1200 to be the upper limit of useful for even fine grained steels, but for something like k390, there's no reason to go over ~600. Realistically, 1200 on V4e is really just because I like the finish from the stone. 800 would probably be a better target, but I don't have an 800 grit stone. I also never leave a diamond finish. Diamonds set bevels, but waterstones finish edges. Convexing the coarse side also has a noticeable positive effect. The angles and grits I use reflect the steels I like (Vanadis 4e, cpm 4v, and k390), so

    While my criterion for sharp is leaving a polished cut and yours is shaving, my sharpening technique (one polished flat, one coarser convex side) has made a bigger difference to my use than any other single factor. There's a bigger difference between the same k390 with good vs bad geometry than between k390 and 1095 with the same, excellent geometry.

    The last thing I can add is that every knife is different. There are just too many microscopic variables, even between 2 identical models from the same company. The best protocol I have found is to set up your edge the way you like it (for me, as above) and use it as you plan to, but a little worse, and see how it fails. Shaving sharp is easier with a razor because the geometry is purpose designed for it. I don't test woodworking edges by shaving (at least not for any compelling, empirical reason). I have a piece of african blackwood that is loaded with unpredictable ultra hard inclusions and is the current champion for the hardest on edges of any piece of wood I have ever seen. I sharpen a knife, press it into the side to see how far it will go (this tells me the bulk geometry is good), then take a few torquing cuts into the figure in a few different spots and examine the edge. If there is any damage, the apex was too acute and I'll go a little steeper on the convex side and try again. The best edge for your use is always going to be the one you find empirically like this.
     
    Last edited: Aug 8, 2019
  10. DeadboxHero

    DeadboxHero

    Mar 22, 2014
    I feel you're drawing conclusions from idiosyncrasies from how you Sharpen and what you can execute with in practice that doesn't apply if your process was ruled out more.

    There may be two bevels but there is only one point, so a duel finish doesn't make sense. Also, it is not as effective as just using the same finish on both bevels blended between polish and toothy if that is the goal. It also makes proper apex formation and burr removal easier.

    I find being more on the polished side works better for wood.

    Wood is cleaved apart through displacement, with push cutting. I find it is not sliced with a toothy edge and a light draw cut. A toothy edge meets more resistance with push cutting, This is why higher polish slides through the wood and makes cleaner curls better.



     
    Last edited: Aug 8, 2019
    Storm W likes this.
  11. Rsq

    Rsq Gold Member Gold Member

    161
    Aug 7, 2011
    Polished is absolutely important for wood. It could well be that stropping means my bevel is in fact highly polished, and the grits I use are incidental, but just reduce the amount on the surface by the right amount to make stropping faster. I don't mean to draw conclusions in this regard. I can tell you that it works for me, I can tell you why I think it works, but I can't prove it.

    I don't know if the same finish on both bevels would help, but I do feel that the contact side of a knife should be polished for 3 reasons. Firstly, the polish left by the edge is both from the initial cut and the burnishing that trails the cut. The smaller and more polished the bevel, the more pronounced the burnishing effect (a shallow hollow grind reduces this surface area, and also increases the contact pressure and therefore the burnishing as well). This polished side is also the side where friction is dominant, so polishing it can reduce the force required to make the cut. Secondly, if re-honing isn't going to be performed often (which it shouldnt on this side of the asymmetric grind I use), polishing is necessary to inhibit corrosion. Thirdly, a flat lapped working surface is an important reference for indexing cuts as you make them. It gives you greater control when the cutting path is guided relative to a flat.

    It might well be that the burnishing dominates in what I see, and my sharpening conclusions aren't applicable to food or shaving. Regarding the type of cut that works on wood: horses for courses. In my hands, a puukko often slices wood, a detail knife always push cuts. For the most part, I agree, though. You would never want to try to slice with a chisel, and the more polished the cleaner the cut. I still think a (somewhat) less polished bevel cuts longer, and a polished back is still important.

    I've never even thought about a "blended" edge on both sides. It might be best for something, but I wouldn't want to use it for my purpose for the reasons I already gave.

    I couldnt say if my way is the best way, only that it works for me because it makes it easy for me to refine an edge until it performs well at the task it will be used for.
     
    Last edited: Aug 8, 2019
  12. Hengelo_77

    Hengelo_77 Basic Member Basic Member

    Mar 2, 2006
    Also shaving sharp doesn't automaticaly mean it cuts well.
    The edge only has to cut the thickness of a hair. You can sharpen a way to thick edge to hair shaving sharp but it won't cut well
     
    allenkey likes this.
  13. Dragons Breath Knives

    Dragons Breath Knives

    753
    Aug 24, 2014
    I must agree with the “over-sharpening” statement. I have had customers ask to be able to reflect the print of newspaper in the edge. I replied, do you want me to make you glasses..... I sharpen all my knives for the purpose each one is designed for. I like a little “bite”⚔️
     
  14. Cushing H.

    Cushing H. Gold Member Gold Member

    465
    Jun 3, 2019
    Has anyone ever obtained SEM (scanning electron micrograph) images of different edges? “A picture is worth a thousand words” ... might be helpful here...
     
  15. Cushing H.

    Cushing H. Gold Member Gold Member

    465
    Jun 3, 2019
    this may have been discussed on the forum before ... but with a quick search and interesting discussion, with pictures, came up. It can be had HERE ... (if the link does not work, here is the address.. https://www.razoredgeknives.com/2014/12/05/coarse-apexed-vs-fine-apexed-edge/ ). Interesting photos of edge and bevel geometry versus grit ... and also some discussion of trade-offs between edge and usage ...)
     
  16. DeadboxHero

    DeadboxHero

    Mar 22, 2014
    Sure, $250 per hour including sample prep is the cost for SEM pictures at the Metallurgical lab by me. My wallet seems to be a little too light for that on a knife maker salary :D
     
  17. DeadboxHero

    DeadboxHero

    Mar 22, 2014
    https://scienceofsharp.wordpress.com/

    Keep in mind, Todd is talking about mostly straight razors not knives. The apex radius on a knife is a much thicker geometry. Also different use. A straight razor for those that don't know is so thin that it can loose it's sharpness cutting paper.
     
    Storm W likes this.
  18. Cushing H.

    Cushing H. Gold Member Gold Member

    465
    Jun 3, 2019
    Hmmm. No promises ... but if i were able to lay my hands on some small (1” square or so) samples sharpened to varying degrees, i might well be able to get some images....
     
    DeadboxHero likes this.

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