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Recommendation? Ideas for sharpening some convex kitchen knives

Discussion in 'Maintenance, Tinkering & Embellishment' started by maximus83, Nov 23, 2020.

  1. maximus83

    maximus83 Basic Member Basic Member

    Nov 7, 2011
    Among our kitchen knives I have a few of the Tojiro DP series, like this petty knife. The 4 knives I have from this series all come with a convex grind. Both ChefKnivesToGo (where I got them), and a local pro sharpener I know, recommend using my freehand stones or using something like Sharpmaker, to just put a regular 15 dps (30 inclusive) beveled edge on these when I sharpen.

    Does that recommendation make sense? With my sharpening experience and tools, it's probably more maintainable that way than for me to try and maintain a convex grind on these.

    The problem I'm running into is these knives don't seem to be getting super sharp when I sharpen them that way. There's definitely something wrong, I can't even get them to easily pass most of my standard sharpness tests (cleanly slicing along full blade length, push-cutting, and filleting receipt paper, shaving arm hair, etc).

    Open to ideas from the pros. Or even non-pros with good ideas. ;)
     
  2. lonestar1979

    lonestar1979

    Mar 2, 2014
    You sharpen the way they told you,on stones,naturally youll get slight convex,sharpmaker is for maintenance,not major sharpenings.Watch some instructional videos on youtube,theres tons of them,and practice,thats key.
     
  3. HeavyHanded

    HeavyHanded

    Jun 4, 2010
    You can maintain them on regular bench stones or wet/dry sandpaper over steel, glass, or a brick.

    I would recommend practicing on a cheap convex, or manually converting a sabre grind to convex. Its not rocket science but getting a nice cosmetic outcome by hand can be a challenge initially. Do not convert them to a bevel grind unless you flatten the primary grind first.
     
    Ben Dover and JJ_Colt45 like this.
  4. maximus83

    maximus83 Basic Member Basic Member

    Nov 7, 2011
    That right there, I suspect is the crux of the issue. I wondered about the advice when they gave it to me, and if it was really a good idea. You're saying it's not.

    Unfortunately I had to find out the hard way--I took their word for it and put a 15 dps bevel on the edges of 2 of them. They are barely sharper than when I started, even though ordinarily I can get my other knives (incl say my Wusthof or Vic kitchen knives) extremely sharp. The difference is not my methods, but the convex grind.

    So you're saying I'd need to either maintain these on a bench stone or sand paper on a hard surface (not the old mousepad approach, with "give" in the surface). Or do some serious reshaping of the blade to convert to a FFG. But realize what that means: these blades are not like some convex I've seen, where up high the blade looks like FFG, and the convex part looks like it kicks in just near the edge. These knives are a full convex, from edge to spine.
     
  5. HeavyHanded

    HeavyHanded

    Jun 4, 2010
    If you put them on a belt grinder or benchstone and flatten from the spine you'll see where the flat primary grind is going to fall. You can leave them the way they are but performance will suffer and over a relatively short time you'll want to thin the convex anyway.

    If you convert them back to a convex right to the edge you'll want to thin anyway right behind the edge and back into the primary. Older Sabatiers etc all need this done at some point as well as the edge retreats back into the primary from repeated sharpening.

    If you do not have the means to flatten the primary this will be the approach anyway, I'll see if I have any old videos of hand sharpening a convex on hard surface. It is not difficult once you tackle a couple, in fact is easier than a flat bevel as long as you diligently thin the convex behind the edge when you sharpen. Cosmetically a bit of a challenge but in terms of technique is very easy - only a few light passes recreate the apex as opposed to a number of passes on the same angle.
     
    JJ_Colt45 likes this.
  6. DeadboxHero

    DeadboxHero Triple B Handmade, Custom Knives Knifemaker / Craftsman / Service Provider

    Mar 22, 2014
    It's Tojiro DP, nothing crazy exotic needed with the geometry.
    So don't go crazy and scuff up the side of the blade.

    Just sharpen freehand on a stone at your chosen angle.

    If you use your knives rougher, go a higher angle, if you use your knives with more finesse, go a lower angle. Use a sharpie to see what you're doing.

    Freehand on bench stones naturally creates a convexed bevel.


     
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  7. Eli Chaps

    Eli Chaps Gold Member Gold Member Basic Member

    Apr 20, 2018
    Am I thinking wrong or doesn't the DP line have a convex primary bevel (blade) and then an actual secondary bevel (edge) on the right side? I thought I remembered seeing that somewhere...

    Sorry if that's not correct.
     
  8. maximus83

    maximus83 Basic Member Basic Member

    Nov 7, 2011
    ^Yes @eli, I think your description is actually correct. And my earlier statement (about blade being a full-blade convex) was incorrect. It's a convex-ground blade that actually has a "9 to 12 degree dps blade angle." That edge is so tiny you can barely see a bevel, it's a REALLY thin blade. To confirm that, I found a better description of the features on another site besides the place I bought them, this is actually correct and has more details about the knife than Tojiro's own site does. :) Here's a description of the Tojiro DP Gyuto:

    https://www.cutleryandmore.com/tojiro-dp/chefs-knife-p113709

    To confirm that I also emailed Mark Richmond, the sharpening guy at ChefKnivesToGo where I got these, and asked his opinion on this. He confirmed that this is correct, gave this suggestion for sharpening method:

    So thanks for all the replies, very helpful! This evening I'm gonna' try what Mark suggested on the 2 blades I tried to sharpen at 15 dps, just lower angle to more like 12. If that doesn't work, I'll circle back here and figure out what to try next.
     
    Eli Chaps likes this.
  9. maximus83

    maximus83 Basic Member Basic Member

    Nov 7, 2011
    Ok got this thing fixed. 12 dps, took a good hour and some scrubbing on the Arctic Fox. I think I had the trouble earlier because I was sharpening at too high an angle, and also, I had let this edge fall into pretty bad disrepair. Anyway that cleaned it up nicely.

    Arctic Fox (JIS 700) > Ptarmigan (JIS 4000) > 1 micron cbn on basswood strop
     
  10. Alberta Ed

    Alberta Ed

    Jun 29, 1999
    I sharpen my convex edged knives by pulling them backward (edge trailing) across the hone, rather than forward. Works for me anyway.
     
    Ben Dover and 115Italian like this.
  11. Ben Dover

    Ben Dover Gold Member Gold Member

    Aug 2, 2006
    For convex edges, I've had good luck with wet/dry sand paper (used very wet) over a firm mouse pad. 320 grit then 1000 then 2500. (I only have two convex blades so my experience is somewhat limited)

    This requires a LIGHT touch! If a higher polish is needed, CBN spray on leather will do the trick.
     
  12. Mikel_24

    Mikel_24

    Sep 19, 2007
    Instead of a mousepad I got a piece of harder thin foam from my uncle. The use it at work to provide cushioning between removable casts and arms/legs. I glued it to a piece of hardwood and then I attach pieces of sandpaper with one strip of masking tape at each end. I think I have a worn out 600 grit paper on it. I don't go any higher. Once I am happy with the sandpaper I go directly to a loaded strop (green compound, aluminum oxide I believe) and I am done.

    Works for me!
     
  13. Ben Dover

    Ben Dover Gold Member Gold Member

    Aug 2, 2006
    Green is usually Chromium Oxide.

    IME, it works quite well on softer steels, but is a bit too soft for the higher alloy steels. That's where CBN shines.
     
    Mikel_24 likes this.
  14. maximus83

    maximus83 Basic Member Basic Member

    Nov 7, 2011
    An interesting side note that all this brings up though is the different approaches to convex that seem to be out there. You see all of these if you look around:
    * Some blades are a full, true convex of the entire blade, right down to the edge. Example: some Bark River knives, some Fallkniven, also I have one folder that was ground this way by Razor Edge Knives.
    * Some are a flat grind, but then are intentionally convexed ONLY at the edge. Example: some Busse and Swamprat knives
    * Some are a convexed blade but a traditional beveled edge. Example: the Tojiro DP knives I'm talking about in this thread. To be honest, I don't totally get WHY you'd do it this way, but the knives work awesome so no complaints.

    Interesting Q to me is, what are the trade-offs among these different approaches, and is one approach clearly better than the others? Or, each of these approaches the best choices for certain knife applications, aka "use the best tool for the job"?
     
  15. garry3

    garry3

    Sep 11, 2012
    Are those sharpened 70-30 from the factory? Might explain the lack of progress but I don't know. Their ITK bread knife is a winner though.
     
  16. maximus83

    maximus83 Basic Member Basic Member

    Nov 7, 2011
    I believe the Tojiro acc to Mark Richmond from CKTG are just an ordinary bevel, e.g., 50/50.
     
    garry3 likes this.

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