How To Sharpening m390 with sandpaper?

Discussion in 'Maintenance, Tinkering & Embellishment' started by Max Brosi, Dec 22, 2019.

  1. Max Brosi

    Max Brosi

    Dec 22, 2019
    ok, I realise it’s a newbie question, but is it possible to sharpen m390 well using wet and dry sandpaper on a glass back, followed by a leather strop, or is the stuff just too hard. I’m used to sharpening chisels, planes, and carving gouges, but these are straight forward, I have little experience with knife blades, especially these new steels. I’ve just ordered a Lionsteel Barlow made from m390 and I’m planning on building a homemade sharpening jig to keep a consistent angle. I’ve heard rumours that you need diamond to sharpen m390??? I only have the small DMT credit card size diamond hones. Other than that I have king water stones, and a Smiths Trihone system. Any advice would be appreciated.
  2. now


    Dec 27, 2017
    Sandpaper will do fine.
  3. If the edge geometry is decent and you're just looking to keep it as sharp as possible at that geometry, you're better off using those DMT hones, which can work in a minimum of passes. They'll do a better job with the vanadium carbides in the steel, once the apex is thinned and you start refining it.

    Sandpaper (SiC) will do OK at coarser grit levels up to 400 or so, but it won't be much help in refining the edge past about 600-800 grit. At that point, the size of the carbides will start to limit how thin the apex can be made. The sandpaper will struggle to thin the carbides while eroding supporting steel around them (makes the edge less durable), and the edge will then begin to round off. By contrast, the diamond hone at 600-1200 grit will make a much easier job of it, cutting & thinning the carbides easily to a much better refinement and leaving them better supported.

    It's not that sandpaper can't be used for a passable utility-sharp edge on such a steel. It's just that a diamond hone will do it much better (sharper, more refined, more durable), with less effort and used with a very light touch. The sandpaper and other relatively coarse synthetic stones can work to shape & thin the steel behind the edge at a coarser stage, like 320 or lower. But diamond will always do much better when the apex itself is ready for refinement. This will even be true if refining at relatively coarse grit, because the diamond is still doing a more efficient job with the carbides.

    A significant part of the limitation, with sandpaper, comes just from the relatively soft paper backing for the abrasive as compared to stones or the hard steel & nickel backing used on plated diamond hones. Even if it's firmly attached or glued to a hard backing like glass, the thin layer of paper still has enough give, it'll make it harder for the abrasive to dig or cut as cleanly into the steel and it's carbides. That little bit of give will also contribute to some rounding of the apex, even against the hard backing.
    Last edited: Dec 23, 2019
    mycough, Ace Rimmer, Danketch and 2 others like this.
  4. McSharpmeisterJ


    Dec 6, 2015
    Would that also apply to belt sander sharpening using SiC belts vs diamond for creating a stronger edge?
    Last edited: Dec 25, 2019
  5. McSharpmeisterJ


    Dec 6, 2015
    I mean, would the machine power of the sander help a SiC belt cut into the carbides better than by hand on a hard backing, or will diamond always create a stronger edge as far as cutting into the carbides?

    Thank you!
  6. The limited hardness of the SiC abrasive is the main thing. A SiC belt would still be limited to that characteristic. Because the abrasive is still somewhat less-hard than the vanadium carbides, it would still struggle. At powered speeds, that would also risk overheating issues as well. Abrasives struggling to cut a material will generate a lot more friction-generated heat at powered speeds, and the abrasive itself will break down & wear a lot faster.

    Again, this isn't to say the SiC belts can't be used to hog off metal. But the refinement of the carbides at the apex would still be limited, and the belt-driven SiC would simply plow them out of the edge, as opposed to thinning or shaping them in place.

    Diamond works better because it's about 2.5X as hard as the vanadium carbides themselves, and roughly 10X as hard as the surrounding matrix steel itself. So, even at a light touch, it'll penetrate and cut the steel and it's carbides with ease.
    Last edited: Dec 26, 2019
    jpm2 likes this.
  7. Mr.Wizard


    Feb 28, 2015
    @McSharpmeisterJ Have you seen this?

    You can clearly see from ToddS's images that diamond cleanly cuts the carbides, while alumina does not. Interestingly there is disagreement on the desirability of this, with some stating high vanadium steel should always be finished on diamond, yet Todd writes:
    kreisler likes this.
  8. McSharpmeisterJ


    Dec 6, 2015
    Thank you. I love it when I can learn something new. I have never invested in diamond sanding belts, but I think I will now.
  9. McSharpmeisterJ


    Dec 6, 2015
    I did a quick google search and am not finding much on 1x42 diamond belts. I am finding zirconia. Any suggestions?

  10. jpm2


    Nov 19, 2014
    I'm not sure what's "unimpressive" about the diamond lapped edge. It seems to me you'd want the carbides shaped with the matrix to get a keen apex.
    The carbide boulder strewn apex after using the ceramic looks like what you might have after miles of cutting with the keen apex, except they are also poking out from the sides.
    If you want a wide coarse biting edge to start with, maybe so. But If you want a keen clean shaving apex, why accelerate the matrix erosion with ceramics?

    Sharpening m390 with sandpaper is fine, just finish with diamond if you want a clean keen refined apex.
    tomhosang likes this.
  11. Mr.Wizard


    Feb 28, 2015
    I have little experience with high vanadium steel (I don't count S30V), so I don't have an informed opinion. However I am grateful for and impressed by the work of @ToddS and I do not dismiss his comments lightly. I imagine that for push-cutting the refined apex is going to be superior, but is high vanadium steel the right choice for that application? I thought that simpler alloys with fewer and smaller carbides had superior edge stability appropriate for highly refined edges, e.g. razor blades?

    I would love to know if in use the diamond-polished apex wears into the rasp-edge produced by finishing on the Shapton and natural stones, or just gets blunt; maybe Todd has or will test that.
  12. jpm2


    Nov 19, 2014
    I'm also thankful for Todd's work, as with all who put in the time/effort/finances to provide information we would otherwise not have.
  13. samuraistuart

    samuraistuart KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Dec 21, 2006
    "I would love to know if in use the diamond-polished apex wears into the rasp-edge produced by finishing on the Shapton and natural stones, or just gets blunt; maybe Todd has or will test that."

    I think this will all depend on HOW the blade is used/what is being cut. I would sort of think that in push cutting applications, the edge is likely to dull by chipping or rolling or possibly both. But that in slicing applications, especially something like rope or CATRA tests where silicon is embedded in the cutting material, the apex may show the characteristic of carbides standing proud due to the steel matrix wearing away. Good question. Maybe Todd will jump in here...

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