How do you thin a Hollow Ground ?

Discussion in 'Cooking Knife Reviews' started by mostadonte2, Jul 17, 2013.

  1. mostadonte2

    mostadonte2

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    Well pretty much subj.

    I have several hollow ground knifes on their secondary bevel and I was thinking how do you thin them behind the edge when the time comes? Do I just start grinding the secondary bevel and convert it in to the V shape?

    It there any trick?

    I use freehand and water stones...
     
  2. boar_d_laze

    boar_d_laze

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    "Thinning behind the edge" is another way of saying "double bevel."  It may or may not be important.  It's mostly a question of whether the knife wedges.  

    Most people who freehand, establish their bevel angles by "clicking in" to the old one, whether intentionally or unintentionally; and that means the angle becomes increasingly obtuse with successive sharpening sessions.  Thus, edges need occasional thinning as part of routine maintenance.  As a practical matter that means taking the knife to a coarse stone every fourth or fifth sharpening and carefully establishing the desired angle, while fighting a natural tendency to allow the knife to set it. 

    People use the term "hollow ground" to mean a lot of different things.  The least ambiguous meaning is that the edge (or face) geometry was created by holding the knife against the outside of a wheel -- and is concave to some degree.  Hollow ground edges are very weak and should be re-profiled to flat or convex at the earliest opportunity. 

    On the other hand, hollow ground face geometry is -- in general -- neither particularly desirable nor undesirable.  If it works for you on your specific knife, leave it alone as much as possible.  If not, the fixes are complicated and usually involve a belt grinder.

    On the other, other hand:  If you have a flat bevel edges transitioning to a hollow ground face, and (a) the transition is a sharp shoulder; and (b) the knife tends to wedge, you may want to soften the transition.  You can do that with a belt sander, loaded buffing wheel, finger stones, sanding pads and regular bench stones.  Belt sanders and regular bench stones coarse enough to move metal quickly leave marks -- so you'll want some way of buffing the marks out if you're going to use either one of those.  The other methods take more time.

    But on the other, other, other hand, if you're using "hollow ground" to refer to dimples (aka kullens), it's an entirely different thing.    

    There's no trick to establishing a flat bevel.  Just hold the desired angle as consistently as you can.  However, no one's perfect and edges which are created freehand ALWAYS have some convexity.  FWIW, that little bit of convexity is a good thing, making the edge stronger. 

    BDL
     
    Last edited: Jul 17, 2013
  3. mostadonte2

    mostadonte2

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    The best example of what I meant by hollow ground secondary bevel would be well known Buck 110 knife.

    http://farm1.staticflickr.com/86/232999458_8df52ca125.jpg

    I can establish the primary bevel (edge) without any trouble, but if I want to think behind the edge I can not think on how to achieve it on a the bench stones. From what you are saying, BDL, I understand that one should have a belt sander or similar....

    Also, is there really a need to thin such a geometry?
     
  4. boar_d_laze

    boar_d_laze

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    Almost always:  

    No need to thin that geometry. 

    Besides:  

    Very difficult geometry to thin without really messing with the knife.  There's not much you can't do in terms of moving metal quickly with a belt sander, but thinning that knife would definitely fall in the category of "not much."  Speculatively, the only good way to do it would be on the outside edge of a wheel; but I'm not a knife maker, nor enough of a sharpener to have a meaningful opinion.     

    BDL
     
  5. rick alan

    rick alan

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    Cheap stamped kitchen knive almost always have the "both sides" concave grind.  Actually, so did the vintage high-end stainless knives like Deluxe Personna and Imperial (the latter not so good as the DP's which are of significantly harder temper), which have blades starting out very thin to begin with, like not much more than 1mm.  But with most of the cheaper knives the hollowing seems to be done more for apeaances sake and are typically very thick behind the edge.

    I believe virtually all knives with this sort of hollow grind will show the same thickness behind the edge for at least 1/8", so having to actually attack the hollow grind itself should really never be necesarry unless it was done poorly to begin with.  Still, even on better grinds you can usually make some improvements.

    I've had a bit of fun thinning out a variety of the cheaper kind, as well as DP and Imperial (the latter 2 require very little thinning).  To proceed:
    • Lay the blade on the stone so it contacts at the edge and upper end of the concave line.
    • Alternate putting pressure on the 2 contact points.
    • If you are starting with a very course stone take care not to bring the edge too close to zero thickness.
    • You'll know you've done the job when your edge appears noticeably wider.
    • On knives that are ground too thick to begin with, a steady hand with a Dremel will do the trick.  Use one of their thin, fiber reinforced disks, and make sure direction of rotation will pull you away from the edge if you slip.  You can grind back those pesky bolsters this way also.  Note that loading the disk with thin Ca will make it last about 50x longer.
    This will get you a thinner edge and improvent in perceived sharpness.

    Rick
     
    Last edited: Jul 19, 2013
  6. phatch

    phatch Moderator Staff Member

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    Rick, I think you mean concave, not convex. Convex will only contact the stone at one point, concave at two points.
     
  7. rick alan

    rick alan

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    Dohhh! Fixed it.

    Rick
     
  8. mostadonte2

    mostadonte2

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    Thanks for the information Rick.
     
  9. chrisbelgium

    chrisbelgium

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     I

    I thinned this Hiro in Gin-1 steel completely, on both sides of course. It took me numerous hours divided in a number of sessions to completely flatten both sides. I used a coarse stone to start with. It's constantly checking where you're going. Best to do both sides alternatively at small steps at a time.

    You will notice that the bevel will get wider towards the spine of the knife. This is obviously inevitable. You simply need to give more pressure towards the upper part of the bevel. Just make sure both sides of the knife are more or less the same, which they almost never are.

    I thinned this 5mm thick Hiro in Gin-1 steel completely on both sides, using whetstones only. It took me numerous hours in multiple sessions to get it all flattened out perfectly!

    I used a coarse 200-ish King stone to get the bulk of the job done. You need to watch what you're doing constantly and work on both sides at the same time. The bevel will go wider and stretch out in the only possible direction; towards the spine. On the other hand, if you're grinding too strong on the edge side, you will destroy the edge completely (read the knife). So it's constantly check and check and check where you're going.

    It is virtually impossible to reach the exact same bevel height on both sides, there will always be a minimal difference.

    Anyway, good luck, it's a hell of a job, believe me.

     
    Last edited: Jul 21, 2013
    mostadonte2 likes this.
  10. rick alan

    rick alan

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    5mm...., This owner of a Randall 8" Bowie didn't even consider the OP might be referring to something like a camping/hunting/fighting/heavy utility knife.  Oh and did I say it's a good idea for beginners to practice on cheap knives?

    Rick