Blade Grinds By Prof. Roland Phlip
Benchmade's recent introduction of their new Model 49 with its exotic, double hollow-ground, zero-bevel blade grind has created considerable interest in the subject of blade grinds. Whole books have been written on this subject and I'm certainly not going to duplicate that here. This article is by no means a complete expose' on the subject of blade grinds. But, it will explain a bit about different blade grinds and especially those that are common on balisong knives.
By far, the most common blade grind is the simple double flat grind.
Starting at the cutting edge, the blade tapers back toward the spine at the primary angle. This angle is typically between about 15 and 35 degrees. Then, the angle changes to the secondary angle which continues back to the full thickness of the blade.
In this picture, f1 is the primary angle and f2 is the secondary angle. f1 is greater than f2. f1 determines how sharp the knife will be. The smaller f1 is, the sharper the knife will be. Surgical scalpels are about the sharpest knives made. f1 on scalpels is typically about 10 degrees. But, as sharp as they are, scalpels are notorious for not holding their edge. A surgeon typically makes only one or two short cuts with a scalpel before the blade is worn out. For long incisions, it's not unusual for a surgeon to have to change scalpels in the middle of the cut. The reason for this is simple. When the f1 is so small, the edge becomes very thin, like foil. And, like foil, just just bends right over.
For shaving razor blades, f1 is typically 15 degrees. Razor blades are also very sharp, but not quite as sharp as scalpels. But, it's not unusual for a man to get five or ten full shaves quite satisfactorily from a razor blade.
Most utility knife blades have f1 in the range of 20 to 30 degrees. Fine fillet knives are closer to 20 degrees. Heavy-duty work knives are closer to 30 degrees.
The primary angle is selected to be great enough so that the blade won't get to thin at the tip and be structurally weak. But, if the blade continued back at the primary angle the whole way, the blade would get to thick to quickly. So, once the edge thick enough to be structurally sound, most blades change angle to the secondary angle, f2, and continue at that angle until they reach the full desired thickness.
Another popular way to grind a blade is the flat chisel grind. This is sometimes called a "tanto grind" since many tanto blades are ground this way.
As you can see, it's very much like the double flat grind except that it's only ground on one side. This does four things. First, it makes the blade easier to grind; you've only got to do one side. Second,it instantly makes the edge about twice as sharp as the double-grind since it essentially cuts the edge angle in half. Third, it makes the edge weaker and easier to chip. And, fourth, it makes it harder to make a straight cut because the blade will tend to pull to one side. Chisel grinds: some people swear by 'em, some people swear at 'em.
One characteristic of a blade that makes it cut well is the fact that it's overall rather thin. Thick blades are harder to cut with. One goal is to keep the blade as think as structural integrity will allow as much as possible. One way to do that is to modify the secondary area turning it into an inward curve. This is called a "hollow grind." You can do a hollow grind on either a double or chisel ground blade. The picture below illustrates a double hollow grind. The chisel-ground version is left to the reader's imagination as an exercise.
Notice how much thinner the hollow-ground blade is. That makes it cut easier.
Making a good hollow-ground blade is very difficult. So, they're generally seen only on higher-end knives.
It is possible to make the curve of the hollow-ground blade go the other way. This is called a "convex grind."
As you can see, a convex ground blade is actually thicker and that does make it more difficult to cut through thick material with. However, that thickness makes it stronger. Convex ground blades are primarily found in industrial applications for cutting through thin pieces of material. Special jigs are required to sharpen these knives. I've never seen a convex-ground balisong.
All of the blade grinds we've looked at so far, the standard double flat grind, the common chisel flat grind, most hollow grinds, and even convex grinds, have both a primary and a secondary angles. As explained above, this allows the knife to be thin but still have a structurally sound edge. The difference between the primary and the secondary angle is called, "the bevel."
It is possible to make a blade in which the primary and secondary angles are the same, in which the bevel is zero. This is called a "zero bevel grind." Here's a "double flat zero bevel grind." The chisel version is left the reader's imagination.
Eliminating the bevel line helps the knife cut deeper easier. Obviously, the further back you carry this concept, the deeper that knife is gonna cut.
The problem here is that either the spine gets very thick, or the edge gets very thin. A thick spine makes the knife heavy and hard to slice with and a thin edge is a weak edge. Fortunately, modern steels are strong enough to stay structurally sound right up to the edge even with rather shallow angles. So, it is possible to make some very, very sharp knives this way.
Of course, you can do a hollow-ground variation of this too. IT would just get thinner, sharper, and weaker at the edge.
All of the grinds suggested above are single-edged. Double-edged knives use the same grinds, they just have two edges and no spine. The line where the one grind meets the other is called the median ridge.
Now, with that all in mind, let's look at the grind used on the Benchmade 49, the knife that started this whole discussion. It's double-edged, so it'll have two ground edges and now spine. Those edges are double-ground so they taper on both sides, not chisel ground. And, both edges are hollow ground. But, here's the really exciting thing: they're zero-bevel grinds. The result is that the blade is extremely thin and the edges are extremely sharp. It ends up looking something like this:
Just how sharp is it? Well, it's the sharpest thing I've ever seen. I was privileged to get to handle one of the prototypes which I was given permission to abuse. So, I did some cutting tests with it. I took a roll of toilet paper, one of the dense, 1000-sheet rolls, and effortlessly sliced through all of the paper, with a single stroke. This is about a four inch long blade and a single slice with it was able almost effortlessly to slice over two inches into the edge of a two-inch thick telephone directory. Of course, with the edge being as whisper thin as it is, it will not have very good durability. But, then again, I don't intend to find out.