Blade Geometry FAQ
Author: Joe Talmadge
Source: rec.knives Newsgroup
June 1998
Welcome to the blade geometry FAQ. Our objective is to provide a working knowledge of blade shapes and grinds. After reading the FAQ, I hope you'll be better able to answer the question, "I need to do job X, what kinds of knives should I consider?". We will start with some general characteristics of blade shapes, then talk about blade grinds, then put it all together to discuss specific blade patterns. The last section, a brief analysis of some common designs, will build upon the previous sections. It's important to keep in mind that the characteristics can be as important as blade shape. When you're done with the FAQ, you should be able to make decisions not only on blade shape but on other attributes. If you need to slice, you'll know to look for a nice curving belly -- and not get hung up on what the ostensible blade shape is supposed to be.
I. Blade Grinds
II. Blade Characteristics
III. Blade Shapes, and What They're Good For
IV. Steel and Blade Geometry
V. Putting It All Together
I. Blade Grinds
The Hollow Grind
The hollow grind is done by taking two concave scoops out of the side of the blade. Many production companies use this grind, because it's easier to design machines to do it. But many custom makers grind this way as well. Its great advantage is that the edge is extraordinarily thin, and thin edges slice better. The disadvantage is that the thinner the edge, the weaker it is. Hollow ground edges can chip or roll over in harder use. And the hollow ground edge can't penetrate too far for food-type chopping, because the edge gets non-linearly thicker as it nears the spine.
For designs where slicing is important, but the slice doesn't need to go too deep, this grind is an excellent choice. Many hunting knives are hollow ground, because field dressing is often best done with a knife that slices exceptionally well through soft tissues. Unfortunately, if you hit a bone, you can chip the edge, so the flat grind (see below) is also used often.
Another advantage of the hollow ground knife, at least at the beginning, is ease of sharpening. Most hollow grinds thicken slightly towards the edge. That means that as you sharpen (at least at first), the blade gets thinner and easier to sharpen. After this, however, the blade begins thickening non-linearly and sharpening will become more difficult.
The ultimate push cutter, the straight razor, is usually hollow ground.
The Chisel Grind
The chisel grind is a knife which is not ground at all on one side. So it is completely flat on one side, and has a bevel on the other. It is simple to produce (the maker need only grind one side), and simple to sharpen (it is sharpened on one side only, then the burr is stropped off the other side). It is also typically very sharp, due to the single bevel design. Whereas a blade ground on both sides might be sharpened at 20 degrees per side, for a total of 40-degrees edge angle, a chisel ground blade is often ground at around 30 degrees, making for a thin (and thus sharp) edge.
Accurate slices are very difficult with the chisel grind, due to the fact that the non-symmetrical design forces the knife to curve in the medium being cut.
The Sabre Grind
The sabre grind is a strong edge format. The bevel starts around the middle of the blade, and proceeds flatly towards the edge. This leaves a strong edge for chopping and other hard use. But it also means the edge will be fairly thick, so this design will not necessarily slice all that well.
The sabre grind is found on many military classic designs such as the Randall #1 and the kabar.
The Flat Grind
The flat grind endeavors to provide an edge that is both thin and strong, and leaves a strong thick spine. The grind is completely flat, going from the spine to the edge. This grind is harder to make, because a lot of steel needs to be ground away. However, the edge ends up being fairly thin and so cutting very well. Because the bevels are flat, there is plenty of metal backing the edge, so it's much stronger than a hollow grind. It is not as strong as a sabre grind, but will outcut that grind.
The edge on this design also penetrates better for slicing and chopping. The hollow grind expands non-linearly as you go up the blade, the sabre grind expands linearly but very quickly. The flat grind expands linearly and slowly. Kitchen knives are usually flat ground, because when chopping/slicing food you need to push the blade all the way through the food. This grind is an outstanding compromise between strength and cutting ability, sacrificing little for either.
The Convex Grind
Also called the Moran grind, after Bill Moran. This grind is as you would expect, the grind arcs down in a convex curve down to the edge. This means the point can be very sharp, because there's no secondary bevels to create the edge itself, just two intersecting arcs. There is also a fair amount of steel behind the edge, because the convex arcs cause the edge to widen non-linearly. This is a strong-edge format, which won't penetrate like a flat grind but will be stronger. Knifemakers form this grind on a flat-belt grinder. A disadvantage of this grind is if you don't have a flat-belt grinder yourself, it is difficult to touch up the edge.
The Dual-Ground Reinforced Tanto
The Americanized tanto as executed by Cold Steel shows multiple grind types. Along the long flat, the knife is hollow ground, for a thin edge and incredible sharpness. However, along the front up to the point, the grind switches to a flat grind. This provides incredible tip strength. The result is a knife with a very keen bottom edge, but a strong profile towards the front where it pierces. Of course, the reinforced front edge is strong but doesn't pierce easily.
II. Blade Characteristics
The Belly
The belly of a blade is the curving section under the point. Some knives do not have a curving section (e.g., Americanized tanto), others are mostly curve (e.g., skinner). The belly increases the knife's ability to both slice and slash. It presents an ever-changing angle to the material being cut, and this means slicing efficiency is preserved across the cut.
If slicing and slashing are important to you, you want to look for a nice curving belly. However, there are always tradeoffs. Typically, the more belly a knife has, the less acute its point. So you get better slicing, but piercing ability goes down. A knife with tons of belly is the Emerson Commander (which actually has a recurved belly), and you can see the point on this knife is not very acute. Trailing point skinners are basically all belly, because you do nothing but slice with them. A knife with little belly is the classic F-S dagger, and this knife has an incredible point for piercing but is not a great slicer/slasher.
So you trade off belly (slicing) for point (piercing). There are some games that can be played here. For example, if the knife design has lots of belly for slicing, the designer can clip the point and add a falsed edge to make it a bit sharper.
The Re-curved Belly
When the belly of the knife is S-shaped, it is called a re-curved belly. The Emerson Commander has a very sharp re-curve, so does the Cold Steel Vaquero Grande. The Darrel Ralph Krait has a more subtle recurve.
The recurved belly presents more edge to the material being cut, and in slicing forces the material into the edge. It is an even better slicer/slasher, and in a big knife (see some of Walter Brend's knives), can make for excellent chopping geometry as well.
The downside of this design is that it is *very* difficult to sharpen. Freehand on a big stone, it's nearly impossible. Some of the sharpening rigs available make the job easier, but in any case it's still difficult.
Bellies and Angles
Another way to improve edge angles without introducing the hard-to-sharpen recurved blade is by playing with the angles between the edge and handle. Ond way to do this is with a positive included angle. This means that if you're holding the knife in your hand, spine parallel to the ground, the knife's edge is *not* parallel to the ground, but rather angles down toward the ground (from handle as it goes towards the tip) before it goes up towards the tip. Another way to say this is, the edge at its lowest point is below the handle, and must angle back upwards to meet the handle. This provides more edge for the blade size. The Mad Dog ATAK is a knife with a positive included angle. The BM spike shows the opposite -- a negative angle -- where the edge just goes straight up towards the tip right from the beginning. A positive included angle generally provides more edge and belly, a negative one provides a sharper point.
Another method to change the angles is to join the blade and handle at an angle. This can change the ergonomics for the better on some designs.
Both methods exaggerate the angle change for slicing, slashing, or chopping, and this in turn increases performance. The chopping ability of the kukri, the chopping/slashing/slicing ability of the ATAK, and the slicing ability of the AFCK are due in part to belly angles.
The Point
The point is, obviously, what the knife pierces with. Like everything else, designing the point is a game of compromises. To pierce really well, there needs to be as little metal as possible up front, so a piercing point is thin and incredibly sharp. The downside is that the sharper the point, the weaker it is.
For some designs, like a dagger, the objective of the design is to pierce. So a dagger has a thin point, sharp on both sides to decrease the profile and to enable the knife to cut its way in from all sides. Other designs, like the skinner, put the point up and out of the way since the objective of the design is to slice. The Americanized tanto has a very strong point, due to the spine being full thickness very close to the point. This means it won't penetrate anything like a dagger into a soft target, but the massively strong point can survive a thrust into a very hard target that would break a dagger point.
Some tricks can be employed to make the point stronger (and worse at piercing) or sharper (and weaker). A false edge can be ground into a point to make it pierce better, for example.
The other important decision about the point is where to put it. It can be placed to provide a number of characteristics. Some knives place the point down almost at the edge. For example, the Japanese-style chef knife, the santuko, has this format. The knife is used to chop food and do long slices, so a low point means maximum straight edge length. The trailing point hunter, which is used for slicing in a way which requires a belly, puts the point way up high and out of the way. Knives whose points require maximum control -- a hunter used for dressing out game, or a defensive knife -- want the point to be in line with the users hand. This usually means the point must be below the spine of the blade. There are a number of methods to achieve this, such as dropping the point in a convex curve (drop point format), a concave curve (clip point format), or straight line (still called a clip point, usually).
Blade Thickness
Blade thickness or thinness is important to both strength and cutting ability of the knife. A thick blade will generally be stronger. But a thin blade will generally have a thinner edge -- and thin edges cut easier and better. So the choice of blade thickness is driven by the compromise of strength vs. cutting ability, just like the choice of point type.
Once the blade thickness is chosen, the particular grind type (see below) can reinforce the attributes of that thickness, or try to make up for any weakness. For example, on a thick blade, a flat or hollow grind can be utilized, so that even though the spine is thick & strong, the edge is thin and cuts a bit better than expected. Or on a thin blade, a sabre grind can be used to make the edge a bit stronger than it would otherwise be. Of course, the grind can reinforce rather than counteract the blade thickness. For example, on the AG Russell Deerhunter, the flat grind is done on a thin blade, to make for exceptional cutting ability, but the design lacks weight for chopping and strength for prying, being exceptionally thin.
Edge Thickness
The thickness of the edge is another tradeoff in strength vs. cutting ability. The thinner the edge, generally the better it will cut, but a thin edge is weak and can chip out or roll over faster than a thick edge. A thick edge is strong, but doesn't cut as well.
The blade shape, plus the thickness of the blade spine, combined with the grind type, determines the edge thickness.
The edge thickness is one of the only factors that can be modified easily by the knife owner (rather than the maker). Remember that if you're not happy with the way a knife is performing, there's no reason you're stuck with the factory edge. Feel free to re-sharpen, grinding a lower-angle thinner edge into the knife. At some point, if you go too thin the edge will start chipping out, that's an indication that you need to thicken the edge back up. Also see the Sharpening FAQ.
The Primary-Bevel Edge
Normally, a knife has two bevels. If you look at, say, a kabar, you will first see a bevel at starts from the middle of the knife and goes most of the way towards the edge. I'll call this the primary bevel. Then at the very edge itself, there is *another* bevel. at a higher angle, that forms the actual edge. I'll call this the secondary bevel. Most knives have this kind of geometry, where a shallow primary bevel meets a bigger secondary edge bevel. This leave the edge a bit thicker for robustness.
However, some knives do not have a secondary bevel to form the edge. Scandanavian knives, like the Finnish puukko, only have a single bevel. So ostensibly, the puukko is a sabre grind (see below). However, because the primary bevel goes all the way down to the edge, the edge ends up being a thin high-performance edge rather than a thicker stronger edge. Combined with the normally thin blades on the puukko, the edge ends up being thin enough to cut really well.
Similarly, some chisel-ground knives also feature a primary-bevel edge. Again, it guarantees thinness and sharpness at the edge. The most popular chisel-ground folder, Benchmade's CQC7, has an edge that is formed by a secondary bevel. However, many other chisel-ground knives do not have the secondary bevel.
