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Blade Edge


Blade Edge - A.G. Russell

Blade Thickness

How thick a blade is will affect how strong the blade is, as well as how well it can cut. Thicker blades will be stronger. Thinner blades will have a slimmer profile - which allows for easier and better slicing. The choice of blade thickness comes down to compromising between strength and slicing ability. There is a reason we don’t all just carry around Exacto knives in our pockets. The blades are too thin. They break too often; and aren’t as reliable as a real pocket knife. The picture on the right there shows three of Jake’s Exacto blades with broken tips.

We don’t carry around a lead pipe either. You can’t call it a knife if it can’t cut. As a knife designer, I navigate the territory between the two extremes. However, the most important factor in designing a knife is the intended purpose of the blade. Sometimes the purpose is clear cut. Self-defense and combat knives need to be sturdy. They need to hold to the rigors of combat. Survival knives need to be able to take the abuses of hard use. A fillet knife needs to be able to slice with excellence and be thin enough so that the blade can flex as needed.

A G RUSSELL - 3-1/2 INCH TITANIUM LOCK BACK - 2-7/8 INCH VG-10 STEEL BLADE

Everyday carry knives have their own considerations as well. Although there is not one specific task set for them, there may be a particular niche I want to fill. The blade may be overbuilt and solid like a Zero Tolerance blade. Or a businessman may want a light, thin office blade that can slice through paper like butter - like my Titanium Lockback (pictured below). As a designer, I have to look at what kind of tasks the user will want to accomplish.

 

Once I’ve decided on a thickness, I can choose a grind that will either help reinforce the attributes of that thickness, or try to make up for any weakness. For instance, if you have a very thick and strong blade, I can choose to go with a flat or hollow grind. Even though the spine of the blade is thick, the profile will not offer any drag to the blade as it slices through a material.

Ka-Bar Companion Slicing through cardboardNotice that I have been using “slice” instead of “cut”. Any knife, properly sharpened, with almost any design or blade profile can be nearly as sharp as another knife. There are a few things that might moderately affect it, but as long as the edge has the same thickness, it will cut about as well as another knife. If you are cutting something not much taller than the width of the blade, then design won’t have an outsized impact, it’s down to the workmanship and materials at that point. However, if the blade has to pass through the material, for instance corrugated cardboard, or even an apple, then different designs in the blade will affect how much drag the blade has on the material. In this case, flat and hollow grinds will be superior to sabre grinds. I will write an article about different blade grinds in the near future. In the picture on the right, the sabre ground Ka-Bar Companion goes through some cardboard with relative ease - but not quite as well as the Titanium Lockback would. It is entirely possible that for cutting tasks the difference will not be enough for you to notice.

For a thin blade, I can choose to reinforce the blade with a sabre grind, so that the edge is a bit stronger than it would have been otherwise - due to the extra steel reaching farther down and reinforcing the edge. Or I can use a flat grind, such as the Titanium Lockback, to really reinforce the benefits of the thin blade. It has exceptional cutting ability, but it’s not designed to have the strength and weight for chopping, prying, or heavy piercing. It is designed to slice, and to slice very well. But it is not meant to be used as a screw driver.

 

Edge Thickness

The edge thickness has the same related tradeoff in strength versus cutting ability. Generally, the thinner edge will cut better, but is weaker. Thin edges risk rolling over or chipping. A thick edge is strong, but will not cut as well.

A quick note: The edge of a knife can get confusing with different terminology. There is not industry wide terminology for the grind of the edge of a knife. The edge grind and the blade grind often get confused in conversations by makers and knife enthusiasts alike. My knife engineer, Phil Gibbs, refers to it as the Canel (CAN-ELL). But for simplicity’s sake, I will refer to it as the Edge Bevel to avoid confusion.

The thickness of the Edge Bevel is determined by the designer. A big, hunky chunky knife can be extremely sharp with a thin edge. A thin stock knife can have a relatively thick edge with a wide angle. There is no right or wrong answer per se, it is simply a matter of what you want to accomplish.

The image below compares my Serpentine Stockman (bottom) to the Ka-Bar Becker Companion (top).

AG Russell Serpentine Stockman slipjoint with white bone compared to Ka-Bar Companion fixed blade

The pocket knife shows a flat ground, Sheepsfoot blade. I chose my preferred 30 degree (15 edge side) blade edge for this knife. This accentuates its slicing ability, and allows for an extremely sharp blade. The Ka-Bar Companion is a big, hunky chunky blade. The spine is ¼” thick steel, and it features a sabre grind rather than a flat grind. The sabre grind reinforces the benefits of a thick knife stock. The blade is tough, and extremely unlikely to break. The Edge Bevel is also ground thicker than the sheepsfoot. So the Companion is not as sharp as the sheepsfoot. But the Companion isn’t designed to be a slicer. It can pry, chop, and pierce things that the sheepsfoot cannot.

 

The Companion could be sharpened to the same angle and sharpness of the sheepsfoot. Then it would be as sharp as the sheepsfoot. But it would have a lot more drag, as the knife is a lot thicker and has a sabre grind. It would cut well, but it wouldn’t slice into material as well. I like that the designer went with the strengths of the knife, rather than try to make up for the “weaknesses.” However, if a user wanted to, changing the edge bevel thickness would be as simple as sharpening it to a more acute angle.

 

The thickness of the blade’s edge is one of the few things that can be easily changed by the knife owner (rather than simply being predetermined by the maker). If you want your knife to cut smoother and easier, you can sharpen the edge and thin it out. I personally prefer my edges to be at a total of 30 degrees. Some makers disagree with me, and prefer a thicker edge of 40 degrees. There are a few who go even thinner than I do. If you go thinner than 30 degrees, and your blade is chipping or rolling out too much, that will indicate that you need to thicken your edge. A little more sharpening at a wider angle and it will strengthen as the blade thickens. Don’t eat too much of your blade away though, or you won’t have much left! If you are interested in learning more about sharpening, take a look at my Sharpening Instructional Videos. Or if you prefer reading, my sharpening article will take you slowly step by step through the process.

 

The Primary Bevel and Secondary Bevels

Primary Bevel and Edge Bevel (Secondary Bevel) shown

Many knives have two bevels. If you look at, say, a Randall, you will first see a bevel that starts from the middle of the knife and goes most of the way towards the edge. 

This is the primary bevel. Even if the blade is flat ground, the steel still slopes downward towards the edge in the Primary Bevel. At the very edge, there is a secondary. Edge Bevel. Most knives have this kind of geometry where a shallower Primary Bevel meets a little bit wider Edge Bevel. This leaves the edge just a bit thicker for strength and durability.

 

Some knives do not bother with the Edge Bevel at all. Scandanavian knives like the Finnish Puuko only have a single bevel that goes all the way down to the edge. Technincally the puukko is a sabre grind, but it only has the one primary bevel that goes all the way. Because of this the edge ends up being a thin, high performance edge rather than being a strong edge. The steel stock (the widest thickness on the blade) on a puukko is normally thin, add in that the shallow Primary Bevel is the only bevel, and you have a knife that can cut really well. The Swedish Mora knife is also very similar in design to the Finnish Puukko.Small lightweight hatchet with handforged Swedish Gransfors Bruks head and American Hickory handle.

Convex Grind Example

Certain chisel ground knives also use the Primary Bevel for the edge. Again, this usually means the edge has a thin angle and allows for a very sharp edge. Of course there are many chisel ground knives that also incorporate a secondary Edge Bevel.

Another example of a single Primary bevel acting as the edge bevel is the convex grind. The convex grind rounds into the edge. Axes commonly use the convex grind. The image above shows the Gransfors Mini Hatchet which uses the convex grind. The Esee Knife company also uses the convex grind extensively, and has built up a loyal fan base. The convex grind, done properly, does strengthen the edge of the knife. They can be difficult to sharpen if you do not have experience and or access to several grades of slack sandpaper belt however.

Finally, certain cutting competition knives have also taken to using the convex grind. For example the Jerry Lairson Utility Camp Knife pictured below is designed for cutting competitions.Mr. Lairson featured his design in a rope cutting contest to show off the quality of his design and work. The convex grind can certainly perform very well. 

AG RUSSELL - LAIRSON CUTTING COMPETITION KNIFE - SK5 CARBON STEEL WITH TUNING FORK UNDER THE HANDLE

In conclusion, the edge presents similar trade offs for the designer as the other design decisions. He must decide if he wants strength or cutting ability - and must find a balance between the two. While the blade's performance can be affected by blade shape and profile, often it comes down to the angles of the Edge Bevel. In the future I will write a full article on blade grinds - in this article I am focusing on just the edge.

 

Thank you for reading. I hope you now understand more about how blade and edge thickness can affect performance. Stay tuned for my next article in my Knife Characteristics series.