Grind

The grind of a blade refers to the shape of the cross-section of the blade. It is distinct from the type of blade ("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 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 tradeoff 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. [ [http://forums.egullet.org/index.php?showtopic=26036 Chad Ward, Knife Maintenance and Sharpening] ]

A sharp object works by concentrating pressure, 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. [ [http://home.nycap.rr.com/sosak/convex.htm Convex Grind F.A.Q.] ]

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, scimitars, and katana, 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" (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.

Typical grinds

# Hollow ground—A common grind where a convex hollow is removed from both sides of the edge. It produces a very sharp edge but being so thin the edge is more prone to rolling or damage than other grinds. It is unsuited to heavy chopping or cutting hard materials. Straight razors are hollow ground. This grind is used extensively in mass produced knives.
# Flat ground—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. The Finnish puukko is an example of a flat ground knife. A true, flat ground knife having only a single bevel is somewhat of a rarity.
# Sabre ground—Similar to a flat ground blade except that the bevel starts at about the middle of the blade, not the spine. It produces a more lasting edge at the expense of some cutting ability and is typical of kitchen knives.
# Chisel ground—As on a chisel, only one side is ground (often at an edge angle of about 20 – 30°) whilst the other remains flat all the way to the spine. 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 double bevel 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.
# 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. In practice, double bevels are common in a variety of edge angles and back bevel angles.
# Convex ground—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. [ [http://home.nycap.rr.com/sosak/convex.htm Convex Grind F.A.Q.] ]

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.

References