Re: Sharp vs "Bite" (technical and very long)
Lyn J. Mangiameli
>A great topic. Your serrated kitchen knife analogy is interesting and I believe, helpful. I've discussed something similar in the hand tools forum when I made the distinction between sharp and refined edges.
Let me offfer this as an example of what I'm talking about. Imagine two boards, twelve inches wide and one inch thick. Imagine mitering the edge of these boards so that they are joined at a 30 degree angle to each other. The outside faces of the two boards (if perfectly flat) are two planes, and where those two planes meet (intersect) is a sharp and refined edge. The edge is a straight line of infinite thinness. Note that even though the line will be of theoretically infinite sharpness, how easily those two planes (and the blade they represent) will penetrate the wood depends on the characteristics of the wood they are entering (more later) and angle at which the planes intersect (i.e, a 20 degree intersection --included angle-- is going to enter the wood a lot easier than a 65 degree intersection).
In the real world, however, the two boards will never be perfectly flat (plane), and thus will never create a truly straight edge. To go with this exagerated example, that act of sharpening creates grooves in the surfaces of the boards. The grooves are cut by the abrasive particles we use for sharpening. The smaller and more uniform the particles, the smaller and more uniform the grooves. So to go on with the board example, imagine taking three V carving gouges, say of 1/4, 3/8 and 1/2 inch size. On each board (face) make 40 randomly spaced cuts perpendicular to the edge to the full depth of the 3/8 inch gouge, 20 randomly spaced cuts with the 1/4 inch gouge and 20 randomly spaced cuts with the 1/2 inch gouge. Now imagine examining each groove on one side and following it to the edge. You will find some areas where a 1/4 inch groove meets exactly with another 1/4 inch groove, in such instances the 30 degree angle (which was specified for this example) will be maintained and the edge will be centered with respect to the overall plane of the two boards. In other areas a 1/4 inch groove on one side might meet exactly with a 1/2 inch groove from the other. Both will meet at the same angle, but the edge will be displaced from the overall centerline between the two boards (because one groove is deeper than the other and thus offsets the meeting point). This is Sideways Offset, and will occur even if all grooves were exactly 1/4 inch, as the bottoms of the grooves would not always precisely align with each other, so any offset where the bottom of the grooves meet will result in some sideways offset.
There is also Depth Offset. This is with respect to the most forward points of the edge. 1/4 inch groove meetings will be very close to the most forward points of the edge, while perfect 1/2 inch groove meetings will meet farther behind (deeper) the most forward points. Again, there are multiple variations with 1/4 inch grooves meeting halfway up the side of a 3/8 inch groove, etc.
Now everytime we have two V grooves meet from each plane, we have a sharp edge of infinite thinness. Thus even with a 60 grit abasive, if the particles make "sharp" V shaped cuts, everywhere the two sides meet can be a "sharp" edge. But it is not a refined edge. A refined edge is when the meetings of the many grooves is along a relatively (theoretically) straight line with minimal (theoretically no) variation in depth. Obviously the more closely spaced the grooves, the more minimal the groove depth (as long as the two planes will meet), and the more consistent the groove sizes, the more refined the edge. This refined edge will be functionally sharper (for most woods), require less effort (for a given overall angle) to cleave the wood, have a longer life (because there are no small points to wear unequally, and because the entire edge is is buttressed to the side and is employed to perform the task).
I qualify that the more refined edge will be functionally "sharper" for most woods because there are instances where the infinite sharp line will not have the best entry into a particular wood, much the same as the refined intersecting planes of a kitchen knife are not the best for some items like a tomato. Terry refers to this as "bite," and I have elsewhere refered to this as initial penetration. Initial penetration is enhanced by having a line of points enter the wood, rather than a continous line. A crude analogy is to imagine a pitch fork probing into the earth compared to a flat blade shovel. The pitch fork, of course will have better initial penetration, because of the lesser initial surface attempting entry. Much the same occurs with a blade edge, be it a blade sharpened with a coarse abrasive, or (and better) a deliberately serrated edge that controls the refinement of the edge despite varying depth. The dowside of this lesser surface area at entry is greatly increased wear and a rougher surface to the wood being cleaved. Both of these are lesser problems with the initial roughing out of green wood.
Green wood, indeed, is the time when ease of initial penetration is most helpful (there are times when it is helpful in dense wood too, but to keep this discussion bearable, I'll leave that for another time). This may seem counter intuitive, i.e., why should the softer green wood require an edge capable of easy penetration? It is because the softer green woods (and many softer woods in general) will deform as the edge is applied. They will deform upon contact with our (never perfectly sharp) edge, with the duller the edge, and the more obtuse the angle of the edge, the more deformation will occur in the wood. Essentially the fibers of the wood will do one, or sometimes a combination of three things: the fibers will be compressed inward away from the edge; the individual fibers will bend away from the direction of travel; and/or they will pile up in front of the edge. When this occurs, the effective approach angle of the tool to the wood changes, and in so doing, can cause the tool to behave unpredictably, usually either "bouncing off" the surface, or resulting in chatter that results from the intermittent shearing through the "piled up" fibers. This makes the tool harder to control.
Thus sometimes the most refined edge is not the most effective edge. It all depends on the characteristics of the wood one is working with, the overall geometry of the cutting edge (i.e., obtuse or acute bevels, and shape of the tip) and the quality of surface finish desired.
The key concept here is that since no abrasive we use is perfectly uniform in particle size, devoid of contamination of other particles (airborn and transfer from contact with other materials), and of infinitely small particle size, we must always recognise that the edge will never be a straight line and cannot be accurately described in either one or even two dimensions. Our understanding (and conceptual model) of the edge must be three dimensional. Sharpness is a two dimensional concept (i.e, the two dimensional planes intersect and thus there is no "rounding" of the edge) that is appropriate to a theoretical model. Refinement, as I introduce the terminology, recognizes the three dimensional nature of actual edges in the real world. Initial penetration (or "Bite") recognizes how edge geometry (both fixed and applied) must be matched to wood characteristics to achieve optimal entry into the wood.
Now mind you, I haven't even started to get into Type 1 vs Type 2 vs Type 3 (well, I actually touched a bit on the latter) chip formation after entry, nor the many issues when the abrasives are not sharp and create rounded grooves, nor have I discussed how one can have fairly evenly graded abrasive particles, but still have different groove depths due to variance in how much of the particle projects from the binding surface (it generally being less for Waterstones and more for loose diamond chips on a hard surface), which is why different binding methods have an effect on particle depth and refinement (imagine a 1/2 inch V gouge being used to only 1/8 its full depth. vs. making a full depth groove).
The issues underlying sharpening are not really all that complex when any one factor is considered in isolation, the problem is that there are so many of them (sharpness, refinement, penetration, effective cutting angle, included angle, bevel angle, approach angle, tip shape, edge retention, etc, etc.), and that any deep understanding of sharpening (and thus the ability to define optimal sharpening practice) requires the abilty to recognize the interplay of multiple factors and to prioritize them according to the needs at hand. Is it any wonder that so many are always asking for sharpening advice, so many differ on the optimal grind (to which I say, optimal for what), and so many choose to just follow the consensus of the moment.
