Why Does a Knife Cut The Way it Does?
Answer? Edge Geometry
Like most things, the answers come when you test your knife designs. When you are happy with the way your knives cut, you will have come to understand what works for the application. Hopefully we can start on the right track with some concepts and examples of cutting edges, what they are good for and why they work for some purposes and don't work for others.
This essay is not intended to give you perfect edge geometry, rather to give you the general knowledge so that you can determine where to start when designing knives for a single or range of purposes.
A filleting knife is not a cleaver, just as a ax is not a hammer!I feel that two main factors influence how you will shape the blade:
- Steel / Hardness
A blade's steel and hardness influence the appropriate angles that make up good cutting edge geometry, but most importantly, the application of the knife should primarily drive the choice of edge geometry. Is it being used to precision slice raw fish or hack bones?
In garden variety knifemaking, we are faced with only a small number of different grinds, but it is interesting to know what characteristics are encouraged by various shapes. For most knives we see primary only or primary and secondary grinds (bevels).
PRIMARY AND SECONDARY
PRIMARY ONLYPrimary Only Grinds have two faces that meet at the cutting edge. That's it. Some Japanese and Scandinavian knives are famous for having a primary bevel only. I have broken these into single grind and double grind.
The Single Grinds shown below.
A chisel concave will be a crazy fine slicing edge, but will be fragile and chip easily. A chisel convex or chisel flat can be seen in sushi and sashimi slicing knives. These slice through softer materials like razor blades. Chisel flats are ubiquitous in mass produced safety razor blades and some utility knife blades. These are incredibly sharp but delicate cutting edges and cutting into a hard material will damage the edge.
These so called chisel grinds are not limited to chisels as you can find high angle versions of them on scissors, lawn mower blades, planer blades and paper shears. In knives, single grinds are not very common.
Although technically there will always be some secondary bevel, even if only visible with a microscope; for the purposes of discussion, let's call bevels INTENTIONAL.
This brings us next to the primary only double grinds. These feature only one bevel that comes to a point at the cutting edge, but are ground on both faces.
Note that some of the latter three grinds are asymmetrical, meaning the grind differs from one side of the blade to the other (again intentionally), like the chisel. Although they look like a chisel, I like to differentiate these as one face is not 90° to the spine like they are in the single grind chisel types.
A common variety primary only grind is the Scandinavian or "Scandi" grind often found in Swedish Mora knives and Finnish Puukko knives. In order to maintain the durability of the edge the angles are higher than dual grind (primary and secondary beveled) knives.
Scandi blades are robust and relatively easy to sharpen with a flat stone. The blade only need be moved across and into the stone at an angle consistent with the primary bevel. The primary bevel helps 'hold' the edge true to the stone so it is less likely to wobble and create a poorly formed cutting edge.
Right or Left Handed?
Asymmetrical grinds are often intended for right handed or left handed use. A rule for determining whether a knife is right or left handed can be this: When holding the knife by the handle and pointing the tip away from you; if the bevel is ground predominantly on the right side of the blade, it is a right-handed knife. The converse is true for left handed knives.
Author's KN16 Petty asymmetrical convex ground for right-handed use.
There are potentially dozens of different shapes of asymmetrical grinds, often made for specialized applications. In some cases, these asymmetrical grinds are not intentional, rather they are a result of inaccurate grinding.(wink)
PRIMARY AND SECONDARYWhen two distinct bevels make up the cutting edge geometry, the cutting part is called the secondary bevel. Many knives have this basic primary/secondary geometry from hunting knives to kitchen knives. The purpose of the higher angle secondary bevel is to increase the durability of the edge.
The Convex grind offers durability with mass behind the cutting edge. It sacrifices some cutting ability for this durability.
TERTIARY?Yes, there can be more than two bevels. A third bevel would be called a tertiary and the forth a quaternary bevel, and so on, but these are rare in knifemaking.
ANGLE OF CUTTING EDGEWhen setting the cutting edge and sharpening we say the angle of the cutting edge is 1/2 of the total angle. That is, when we say 15 degrees, we mean 15° on each side. For symmetrical blades, the total angle is 15° + 15° or 30°. We don't normally set our sharpening equipment up for 30° though as we hone one side of the blade at a time.
"I pop my knife into my Lansky and use the 20° slot and start."
This 20° is really only ONE SIDE. The total angle for a common knife will be two times 20° or 40°.
There are cases with primary only single (such as chisels) where the total angle is the same as the angle of the cutting edge.
The angle of the cutting edge is largely set according to the intended use and the hardness of the steel. A very low angle cutting edge will slice very well, but is more prone to being damaged when impacting hard objects. This is where the blade hardness works with the angle to reduce the possibility of rolling or chipping.
An edge intended to take abuse will need a higher angle to have a chance at surviving. Here are some example ranges for cutting edge angles and their uses.
18° to 25° - General purpose, European kitchen knives, hunting knives.
26° to 30° - Pocket knives, hunting knives, bushcraft knives, where more likely to be abused.
31° and higher - Machete, camp knives, hatchets. Durable, hacking edges.
Before you set the secondary edge on sabers and flat grinds, check the thickness. It should be uniform over the length of the soon-to-be cutting edge. I could use coins and hairs to describe the thicknesses, but an inexpensive digital caliper is far more precise and really has become essential to the knifemaker.
BEFORE YOU SET THE EDGE
Inexpensive (under $20) and accurate enough for knifemaking needs.
ANGLE OF PRIMARY BEVEL
The angle of the primary bevel is chosen based on the intended use of the knife, the blade thickness and grind height. This angle is in relation to the thickness of the stock and the grind height or how far the grind "goes up" the blade.
A heavy use bush chopper may have a thick spine and rigid blade. A paring or filleting knife will have a thin spine and low primary angle bevel. You can compare the two shown here to get an idea of what the two extremes offer.
There is a simple formula for finding the angle that is required to achieve a set grind height based on the thickness of the stock you are using.
degrees = atan((thickness/2) / height) * (360/pi)
This grind angle calculator can be helpful if you are using a jig to set the primary bevel angle.
CROSS SECTION EXAMPLES
I have made several chef's knives so I thought I would start there with the most basic of grinds, the full flat grind. Typically, a larger German style chef's knife is made from 1/8" (0.125") stock and the primary bevel is flat ground. Some prefer to grind this bevel to about 0.04" (around the thickness of a dime) before heat treating.
After heat treat, one will further grind the primary bevel to around 0.02" or about 1/2 the thickness of a dime.
Then finally grind the secondary bevel or cutting edge at between 20° and 22° for about a 45° total angle.
Spine: Not exactly related to cutting edge, unless you cut your fingers, a slightly rounded spine will be favorable on the hands when pinch gripping the blade on a chef's knife. However, at least one maker has told me his customer likes a sharp spine to better pinch grip the knife. To each his/her own.
Some bushcrafters love a very square sharp spine for scraping bark from trees and roots. Avoid rounding the spine if this sharpness is desired.
Some guides are available that clamp on the blade to guarantee the angle. Others like to use a matchbook as a guide. Unfortunately, matchbooks are a common as hen's teeth nowadays. The goal here is a consistent angle. When the angle changes, the secondary is rolled over and not as precise as it should be.
In the case of a classic bushcraft knife, the blade is thick, often about 5/16" (0.303"). Generally these knives feature only one bevel, that is the primary grind goes all the way to the cutting edge. Sometimes this grind is called a Scandinavian or "Scandi" grind.
As the bushcraft knife is used for general woodworking duties including, scraping, splitting, carving, chopping and slicing it's blade is quite thick. The angle allows for a reasonable variety of tasks the thick spine for batoning or splitting. This cutting edge is however less durable against hard materials.
Here are some basic edge profiles that I like to use for kitchen knives. These dimensions are taken at the heel as viewed from the rear of the blade.
These profiles are based on cutting tests and perform how admirably for their applications. Do experiment with your blade designs and make something that works. You will know when it works by how effortlessly it cuts.
Here are three basic edge profiles for hunting knives based on 3/32", 1/8" and 3/16" stock. The angles are based on the thickness of the blade and potential uses. Hunting knives are not specialized in the sense that they are going to be used for many field tasks.
A concave in convex (sometimes called an S grind) is well suited for slicing wet foods. The overall convex offers great slicing characteristics while the concave provides the vacuum release to prevent wet foods from sticking to the blade. The graphic shows a right handed concave in convex as seen from the heel of the blade.
Scalloping is the intentional removal of part of the primary bevel, which in turn creates a surface that acts like a vacuum release when cutting certain foods. Wet foods want to stick to the blade and these scalloped areas break the vacuum and allow the food to fall free from the blade. Santoku style knives commonly feature scallops or cullens, however the scalloping can run along the blade as well.
Serrations are teeth-like ridges that transform a knife essentially into a saw. We quite often see serrated edges on bread knives, steak knives and combination blades on hunting/survival knives.
A knife must perform its intended function. There are many factors to consider that are related to a blade's shape and edge geometry. A knife that's wielded all day long at a hog processing plant isn't going to weigh 2 lbs. The operator will simply pit it aside and find something that cuts that involves far less effort.
The length, thickness and bevels of the blade directly influence the weight of a knife. A backpacker may not want to carry a two pound "Rambo" knife with a 12" blade as every ounce counts when you're walking all day.
As a rule, thin knives cut better. So if you can, make your knives as thin as practical for the application. Obviously a millimeter thin cleaver is not going to go over so well, but use your judgment and test your designs. Collect feedback from your users and strive to make the best possible knives you can.