Thanks for that info C.G!! Sorry if I am jumping in on the conversation late. But it's my understanding, and I am no expert, that a hollow ground knife is great for skinning but not recommended for wood work because it has a similar profile. But in theory knowing this shouldn't be a big deal because 3v is 2 to 4x stronger than 1095 and thus Guy can go thinner. Is that a correct statement/assumption? And is the new 4.7 going to be a convex, flat or hollow grind? And finally will you please make me a grilled cheese sandwich, cut it in quarters and take the crust off?
Thanks!!
Yeah, i need to stop in and get some more American Process "cheese" as I like that best on mine and am running short, thanks for the reminder
I think it was already mentioned that all of Guy's knives (except the BRKT-produced ones) have a flat-saber grind for the primary, the edge's are sharpened on a belt-grinder that might produce a slight convexity but are generally also a flat V-edge. Users that prefer a convex profile can easily knock down the edge-shoulders and round the apex on a strop if desired, but the primary is going to be flat.
Regarding the geometry of convex/flat/concave grinds, it is very important to remember two key principles:
1) A knife is a wedge/slope, it functions under the principle of "mechanical advantage" regarding inclined planes:
https://en.wikipedia.org/wiki/Inclined_plane the key to which is "slope" described by the bevel/grind angle
- thinner = easier penetration
2) "Angle" is a means of describing the space between two planes, i.e.
thickness over a distance/length.
Thickness is the primary determinant of the
strength (i.e. resistance to bending/stretch/compression) of the material.
- thicker = stronger
When discussing a knife bevel, there is an edge
apex and also a "shoulder" that define the primary bevel - two points between which there lies a straight, flat plane. Above this plane, a bevel is "convex" (curving outward) and beneath it is "concave" (curving inward). Defined thus by the slope of the flat-bevel between shoulder and apex, the convex bevel produces the thickest, strongest blade, concave produces the thinnest/weakest. How much stronger or weaker for a curved variation depends upon the degree to which it deviates from the flat plane/bevel - a slight convexity/concavity will not evince much difference in strength. Also if the bevel is excessively thick to begin with, then even a thin concave (hollow) grind may be more than strong enough to endure the intended use.
Most hollow-ground knives are only hollow
behind the edge-bevel, i.e. in the primary.
As an example, take a straight-razor.
Usually given a hollow-primary on a grinding wheel or belt-platen, but then given an edge-bevel on a flat hone and a final convex micro-bevel to the apex on a strop. The result is an edge that puts strength (thickness) foremost to endure the rigors of the initial cut but at the price of lost mechanical advantage, so the final bevel must be thin enough to penetrate the intended material with ease (we're talking micron-thin).
Behind this edge, a flat bevel produces a lower average angle to improve the mechanical advantage (i.e. higher slicing ability, less wedging) while maintaining strength (thickness) to support that fine edge against bending/flexing/rolling.
Behind this flat section, the blade is strong enough to support the edge but requires a sufficiently thick spine to prevent excessive flexing of the blade and give the user something to control - a hollow grind allows the blade to remain relatively thin & light with good clearance from the surface, and then more quickly increase in thickness to accommodate a strong spine. The disadvantage to give the knife such a strong spine with such a thin bevel is that it can give the user a false sense of the strength of the entire blade, and he might use it in a way that the spine can endure but the rest of the knife cannot.
It's this last point that goes to the question about hollow-ground knives for woodwork - you can certainly use a hollow-ground knife for woodwork, but only if the grind is
thick enough to endure the stress of such use, and the baseline for "thick enough" is a flat-grind - for the hollow to be just as thick as the sustainable flat-grind, the knife itself will have a thicker/heavier spine than is necessary, so why bother? And that thicker/heavier spine might encourage use that the rest of the blade cannot handle. As an example, I own a Buck Paklite skinner, a hollow-ground knife that came with an edge >0.030" thick - that is thicker than the flat-grid GSO-4.1 I recently got for testing. I could certainly use that hollow-grind blade for wood-work since it is so thick. But I would not use my straight-razor (which has a similar spine-thickness to the PakLite), nor would I use a flat-ground box-cutter blade, because of the
thickness of either in the region where strength is required for carving wood.
Take another example of a commonly hollow-ground blade -
the splitting axe. Isn't it curious to consider that the delicate straight-razor and the powerful splitting axe might be given a similar grind?
A splitting axe, like a razor blade, requires a strong apex to endure the rigors of the initial cut (exponentially more severe) - as such, a convex final bevel is a good idea, however the edge needs to be MUCH thicker to endure the impact and twisting forces of the grains of wood being split, so this final bevel may be quite large, however it should not be made too large or the blade will lose too much penetration ability and be more like a stone than an axe, losing all the force of the strike on impact - penetration is key to cutting - it just needs to be thick enough not to fold/compress/crack upon impact, retaining the ability to slip into the wood and separate it.
Behind this edge may be a flat grind that balances support and mechanical advantage to allow the edge to penetrate sufficiently deep so as not to bounce out of or glance off the target upon impact - retain the force of the strike against the wood's resistance.
But now that the blade-edge is able to endure the strike and penetrate the target without losing much force, it needs to be able to
blast the grains apart using mechanical advantage AND not lose force to friction against the wood - here you are into the primary bevel of the axe and may find a massive flaring-out of "cheeks" that present the "concave" grind. The axe maintains it's thin profile log enough to ensure penetration, then quickly increases the grind-angle, the thickness of the head, thereby increasing the clearance area to prevent friction along the bevel and turn all the remaining force of the stroke to spreading apart the wood thus separated by the edge, acting like a ski-jump, while also allowing for a thick/heavy axe head behind the thin/penetrating edge. It's a beautiful thing
Wait, where was I? I can't remember what i was posting about.... oh yeah! blade grind and strength. in general, it is not convex/hollow/concave that determines the strength of the blade, nor is it grind-angle. All of these are merely ways of describing how the blade changes in
thickness from apex to shoulder. It is the
thickness that first and foremost determines whether or not the blade is strong enough to endure a given use. Thickness is cubically related to strength - e.g. a blade that is 2x thicker at a given distance from the apex is 8x stronger than a comparison blade.
Now, what about strength and Rockwell Hardness?
https://www.mwsco.com/kb/articles/19990630e.htm
A blade at 61Rc is ~10% stronger than one hardened to 57Rc, assuming "optimal" HT protocols for each - that is, the harder blade can endure 10% more stress before bending/deforming vs the weaker blade. This is strength, not toughness.
Toughness informs what happens
after strength is exceeded, or what happens when the strength is exceeded suddenly (impact) - how much does it bend before it fractures? A tougher steel endures greater deformation prior to fracture, fracture with "relatively little" deformation is considered "brittle", note that it is a
relative term. Comparing 1095 and CPM-3V toughness at 57-58Rc, 3V is ~2x tougher (Charpy notch testing), meaning that a test-sample of the same thickness can absorb 2X more energy through deformation prior to fracture.
It should be noted that the comparison values are generated on test-samples of a given thickness and may not maintain the proportional value at other thicknesses (i.e. 3V may be more or less than 2X tougher than 1095 at thickness greater or smaller than the test sample). In theory, CPM-3V could be made 2X thinner than the 1095 blade and retain the same toughness, but i have no evidence to bear that out. The smart move is for Guy (or someone else) to test the geometry of the knife in use, if possible to test it to destruction and so learn the limits of the tool before deciding whether to move forward with that geometry or not for a given use.
Dang, wall of text and I'm not even sure I answered the original question... Anybody?
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