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Thread: What to do?

  1. #111
    It may just be me, and I have had fine experiences with CKTG for minor things, but n-u-b-a-t-a-m-a sounds like NAMBLA for the 21st century.

  2. #112
    Quote Originally Posted by mzer View Post
    .....but n-u-b-a-t-a-m-a sounds like NAMBLA for the 21st century.
    NAMBLA-tama

  3. #113
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    Quote Originally Posted by Dave Martell View Post
    This link shows how thick these knives are when new (VERY thick) but it also shows us how not to grind the shoulders off of bevels. To me the idea is to thin and blend without transforming an originally symmetric wedgey knife into an asymmetrical mess. Good idea - poor execution.
    I've been trying to figure out specifically and objectively what's wrong with the geometry of both the before and after photo's of this Artifex. This is what I see, correct me if I'm wrong:

    The original blade is a fairly fat wedge, with both faces completely flat, which then has a bevel at the edge. The edge looks very slightly asymmetrical, maybe 55/45, but not apparently intentionally asymmetrical. When I compare this to my kitchen knives, a kitchen knife shouldn't be symmetrical, and the faces shouldn't be flat. For a righty, the left face should be relatively flat, and the right face should be relatively convex. i.e. A good kitchen knife isn't a wedge with an edge. This knife is. Also, this knife looks very thick at the shoulders of the edge bevel.

    Then we move on to the "fixed" blade that had the shoulders ground off. The problem that I see is that they didn't consider what the overall geometry of a knife should be. They focused on grinding off the shoulders and blending it into the faces, as a result it looks like they only hit at most 20% of the knife's faces, but more of it needed to be thinned. They also shouldn't have ground the faces all the way down to the edge. By doing that, they left it so thin that it will never hold an edge. Once they get a bevel on it with enough steel behind it that the edge won't just chip off, they will be left with a slightly shorter knife that has a slightly (if at all) improved geometry. It probably won't be quite as thick behind the edge, but it will get thick quickly.

    As far as "an asymmetrical mess" goes, it looks to me that they (unintentionally) left the right face slightly flatter than the left, but still not the ideal geometry for a lefty. I would guess that they intended to make it symmetrical, but didn't get it quite right.

    Am I on the right track here, or is there something else that's horribly wrong with this knife that I'm just not seeing?
    Last edited by CompE; 04-15-2013 at 08:03 PM. Reason: fixed a typo

  4. #114
    Senior Member labor of love's Avatar
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    As far as i can tell, in the first pic there is barely a grind at all to speak of. The knife looks like a blank. Im not sure what you mean when you say the faces shouldn't have been ground all the way to the edge. A thinner edge is a more delicate edge for sure but it shouldn't effect edge retention.

  5. #115
    Quote Originally Posted by CompE View Post
    I've been trying to figure out specifically and objectively what's wrong with the geometry of both the before and after photo's of this Artifex. This is what I see, correct me if I'm wrong:

    The original blade is a fairly fat wedge, with both faces completely flat, which then has a bevel at the edge. The edge looks very slightly asymmetrical, maybe 55/45, but not apparently intentionally asymmetrical. When I compare this to my kitchen knives, a kitchen knife shouldn't be symmetrical, and the faces shouldn't be flat. For a righty, the left face should be relatively flat, and the right face should be relatively convex. i.e. A good kitchen knife isn't a wedge with an edge. This knife is. Also, this knife looks very thick at the shoulders of the edge bevel.

    Then we move on to the "fixed" blade that had the shoulders ground off. The problem that I see is that they didn't consider what the overall geometry of a knife should be. They focused on grinding off the shoulders and blending it into the faces, as a result it looks like they only hit at most 20% of the knife's faces, but more of it needed to be thinned.......It probably won't be quite as thick behind the edge, but it will get thick quickly.

    As far as "an asymmetrical mess" goes, it looks to me that they (unintentionally) left the right face slightly flatter than the left, but still not the ideal geometry for a lefty. I would guess that they intended to make it symmetrical, but didn't get it quite right.

    Am I on the right track here, or is there something else that's horribly wrong with this knife that I'm just not seeing?

    You nailed my thoughts on this and I may have overstated the asymmetrical mess part, maybe it's not a mess, yet I do believe that a knifemaker should be able to do a lot better in thinning while keeping an symmetrical knife symmetrical. Of course it's always possible that the owner of the knife asked for an asymmetrical mess, I have no info on that.

  6. #116
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    Quote Originally Posted by labor of love View Post
    As far as i can tell, in the first pic there is barely a grind at all to speak of. The knife looks like a blank. Im not sure what you mean when you say the faces shouldn't have been ground all the way to the edge. A thinner edge is a more delicate edge for sure but it shouldn't effect edge retention.
    What I meant was that I believe that when thinning the faces of the knife, they should have left some of the original bevel intact, even 1 mm probably would have been enough. The angle of the edge could then be lowered if desired without appreciably reducing the height of the knife. By grinding the faces all the way down to a very acute edge, when an edge bevel is added it will shorten the knife more than it would have if they left just 1mm of the original bevel. Even a 15 degree included angle put on that edge will shorten the height of that blade by 2-3mm at least. Maybe it's just me, but I find the lower the bevel, the larger my micro-bevel needs to be or else the entire micro bevel will chip off.

  7. #117
    Senior Member Benuser's Avatar
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    Excuse my ignorance, but could you explain how building a 15 degree inclusive edge may cost 2-3mm blade width?

  8. #118
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    Quote Originally Posted by Benuser View Post
    Excuse my ignorance, but could you explain how building a 15 degree inclusive edge may cost 2-3mm blade width?
    The knife that we were looking at had an extremely low angle, under 5 degrees I'd say at the very tip, but let's stick with 5 degrees for the estimate. Say that you want to put a 15 degree micro bevel on it, and make the thickness at the point where you transition from 15 to 5 degrees 1mm thick (this is to make the math easier, I know that's thicker than necessary). Imagine that your tip is an isosceles triangle with the top point 5 degrees. tangent (5/2 degrees) ~= .0437, so your triangle with a 1mm base is 1/.0437 * (1mm/2) ~= 11.5mm tall. Now you want to draw a triangle with a 15 degree tip but a 1mm base on top of that. tangent (15/2) degress ~= .131, so that triangle would have a height of only 3.8mm tall. So, to put a 15 degree bevel on a 5 degree wedge with a 1mm base, you'd have to shave 11.5-3.8mm off your knife or ~7.7mm.

    The above were just estimates to make the point. Sure, you could make the steel behind the edge less than 0.5mm thick, but maybe that bevel is less than 5 degrees. You'd have to leave only 0.26mm thickness behind the bevel if you want to take off only 2mm from the blade's height. I think that the estimate that 2-3mm will come off that knife isn't way out there. That could have been avoided if, when thinning, just a little bit (1mm or less) of the original 30 degree edge was left of the blade when it was thinned.
    Last edited by CompE; 04-16-2013 at 05:53 PM. Reason: another typo

  9. #119
    Senior Member Benuser's Avatar
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    Thank you, CompE, after some playing with pencil and calculator I got your point!

  10. #120
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    Quote Originally Posted by CompE View Post
    The knife that we were looking at had an extremely low angle, under 5 degrees I'd say at the very tip, but let's stick with 5 degrees for the estimate. Say that you want to put a 15 degree micro bevel on it, and make the thickness at the point where you transition from 15 to 5 degrees 1mm thick (this is to make the math easier, I know that's thicker than necessary). Imagine that your tip is an isosceles triangle with the top point 5 degrees. tangent (5/2 degrees) ~= .0437, so your triangle with a 1mm base is 1/.0437 * (1mm/2) ~= 11.5mm tall. Now you want to draw a triangle with a 15 degree tip but a 1mm base on top of that. tangent (15/2) degress ~= .131, so that triangle would have a height of only 3.8mm tall. So, to put a 15 degree bevel on a 5 degree wedge with a 1mm base, you'd have to shave 11.5-3.8mm off your knife or ~7.7mm.

    The above were just estimates to make the point. Sure, you could make the steel behind the edge less than 0.5mm thick, but maybe that bevel is less than 5 degrees. You'd have to leave only 0.26mm thickness behind the bevel if you want to take off only 2mm from the blade's height. I think that the estimate that 2-3mm will come off that knife isn't way out there. That could have been avoided if, when thinning, just a little bit (1mm or less) of the original 30 degree edge was left of the blade when it was thinned.
    I don't know why you would be wanting anything near a 1mm base. It's fine to grind the face of the knife to a 'zero edge' at 5 degrees per side. In fact I have done that and then made a pass or two at 15 degrees on an 8k stone. The base of that bevel is 20 microns, and is strong enough to cut wood without plastic deformation. There is barely any loss of blade height, maybe 100 microns at the most, which won't be missed.

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