Grind
The grind of a blade refers to the shape of the cross-section of the blade. It is distinct from the blade profile (e.g., clip point or drop point knife, sabre or cutlass, axe or chisel, etc.), though different tools and blades may have lent their name to a particular grind.
Grinding involves removing significant portions of metal from the blade and is thus distinct from honing and polishing. It is notably done when first sharpening the blade or when a blade has been significantly damaged or abused (such as breaking a tip, chipping, or extensive corrosion) A well maintained blade will need less frequent grinding than one which is not treated well.
The terms edge angle and included angle can be important when talking about grinding. The edge angle is measured between the surface of an edge and a line running from the point of the cutting edge to the centre of the back edge. The included angle is the sum of the edge angles. All other things being equal, the smaller the included angle the sharper the blade and the easier it is to damage the edge.
An appropriate grind will depend upon what the blade is to be used for and the material from which the blade is made. Knife manufacturers may offer the same model of knife with different grinds on the blade and owners of a blade may choose to reshape it as a different grind to obtain different blade properties. A trade off exists between a blade's ability to take an edge and its ability to keep an edge. Various grinds are easier to maintain than others or can provide a better shape over the life of the blade as the blade is worn away by repeated sharpening. In material science terms, harder steels take sharper edges, but are more brittle and hence chip more easily, while softer steels are tougher, and are used for knives such as cleavers which must be tough but do not require a sharp edge. In the range of hardnesses used for knives, the relationship between hardness and toughness is fairly complex and high hardness and high toughness are often possible at the same time.
As a rough guide, Western kitchen knives generally have a double-bevel (approximately 15° on the first bevel and 20–22° on the second), while East Asian kitchen knives are made of harder steel and are ground at 15–18°, being either wedge-shaped (double-ground) or chisel-shaped (single-ground).
Process
A sharp object works by concentrating forces which creates a high pressure due to the very small area of the edge, but high pressures can nick a thin blade or even cause it to roll over into a rounded tube when it is used against hard materials. An irregular material or angled cut is also likely to apply much more torque to hollow-ground blades due to the "lip" formed on either side of the edge. More blade material can be included directly behind the cutting edge to reinforce it, but during sharpening some proportion of this material must be removed to reshape the edge, making the process more time-consuming. Also, any object being cut must be moved aside to make way for this wider blade section, and any force distributed to the grind surface reduces the pressure applied at the edge.[1]
One way around this dilemma is to use the blade at an angle, which can make a blade's grind seem less steep, much as a switchback makes a trail easier to climb. Using the edge in this way is made easier by introducing a curve in the blade, as seen in sabers, tulwars, shamshirs, and katanas, among many others. Some old European swords (most memorably Hrunting) and the Indonesian style of kris have a wavelike shape, with much the same effect in drawing or thrusting cuts.
When speaking of Japanese edged weapons, the term niku (肉 meat) refers to the grind of the blade: an edge with more niku is more convex and/or steep and therefore tougher, though it seems less sharp. Katana tend to have much more niku than wakizashi.
If it is required to measure the angles of cutting edges, it is achieved by using a blade edge protractor (or goniometer) see CATRA Hobbigoni
Typical grinds
- Hollow grind — a knife blade which has been ground to create a characteristic concave, beveled cutting edge. This is characteristic of straight razors, used for shaving, and yields a very sharp but weak edge which requires stropping for maintenance.
- Flat grind — The blade tapers all the way from the spine to the edge from both sides. A lot of metal is removed from the blade and is thus more difficult to grind, one factor that limits its commercial use. It sacrifices edge durability in favor of more sharpness. A true, flat ground knife having only a single bevel is somewhat of a rarity.
- Sabre grind — Similar to a flat grind blade except that the bevel starts at about the middle of the blade, not the spine. Also sometimes referred to as a "V Grind", made with strength in mind and found on tactical and military knives. A Sabre grind without a secondary bevel is called a "Scandinavian Grind," which is easier to sharpen due to the large surface. The Finnish puukko is an example of a Scandinavian ground knife.
- Chisel grind — As on a chisel, only one side is ground (often at an edge angle of about 20 – 30°); the other remains flat. As many Japanese culinary knives tend to be chisel ground they are often sharper than a typical double bevelled Western culinary knife. (A chisel grind has only a single edge angle. If a sabre grind blade has the same edge angle as a chisel grind, it still has two edges and thus has twice the included angle.) Knives which are chisel ground come in left and right-handed varieties, depending upon which side is ground. Japanese knives feature subtle variations on the chisel grind: firstly, the back side of the blade is often concave, to reduce drag and adhesion so the food separates more cleanly; this feature is known as urasuki.[2] Secondly, the kanisaki deba, used for cutting crab and other shellfish, has the grind on the opposite side (left side angled for right-handed use), so that the meat is not cut when chopping the shell.[3]
- Double bevel or compound bevel — A back bevel, similar to a sabre or flat grind, is put on the blade behind the edge bevel (the bevel which is the foremost cutting surface). This back bevel keeps the section of blade behind the edge thinner which improves cutting ability. Being less acute at the edge than a single bevel, sharpness is sacrificed for resilience: such a grind is much less prone to chipping or rolling than a single bevel blade. This profile is commonly found in Japanese swords, such as the familiar katana. The shape of the bevel is much more efficient in reducing drag than the sabre grind typically found on Western sword blades, which is part of how the katana is able to slice through very tough targets. In practice, double bevels are common in a variety of edge angles and back bevel angles, and Western kitchen knives generally have a double bevel, with an edge angle of 14-16° (included angle of 28-32°) and a maximum of 40° as specified by International standard ISO 8442.1 ( knives for the preparation of food).
- Convex grind — Rather than tapering with straight lines to the edge, the taper is curved, though in the opposite manner to a hollow grind. Such a shape keeps a lot of metal behind the edge making for a stronger edge while still allowing a good degree of sharpness. This grind can be used on axes and is sometimes called an axe grind. As the angle of the taper is constantly changing this type of grind requires some degree of skill to reproduce on a flat stone. Convex blades usually need to be made from thicker stock than other blades.[1] This is also known as hamaguriba in Japanese kitchen knives, both single and double beveled. Hamaguriba means "clam-shaped edge".
It is possible to combine grinds or produce other variations. For example, some blades may be flat ground for much of the blade but be convex ground towards the edge.