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A blade is the flat part of a tool, weapon, or machine (such as a fan) that normally has a cutting edge and/or pointed end typically made of a flaking stone, such as flint, or metal, most recently steel. A blade is intentionally used to cut, stab, slice, throw, thrust, position and/or place (an example of this is razor wire), shoot (an example of this is the ballistic knife), scrape (an example of this is an ink eraser) or strike an animate or inanimate object.
Physics
The basic idea of a blade is very similar to a sharp point.
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Encyclopedia
A blade is the flat part of a tool, weapon, or machine (such as a fan) that normally has a cutting edge and/or pointed end typically made of a flaking stone, such as flint, or metal, most recently steel. A blade is intentionally used to cut, stab, slice, throw, thrust, position and/or place (an example of this is razor wire), shoot (an example of this is the ballistic knife), scrape (an example of this is an ink eraser) or strike an animate or inanimate object.
Physics
The basic idea of a blade is very similar to a sharp point. The shape concentrates all the force onto a very small area, resulting in a high amount of pressure which allows it to penetrate matter.
A serrated blade (a blade which has many small "teeth") takes this further as each individual tooth concentrates the force on a smaller area which helps cut through denser materials. A serrated knife can cut through objects solely with a sliding motion with little pushing force, this is useful for tools which require these attributes such as bread knives.
Some bladed weapons (and tools) have curved blades. A curve can serve two purposes, the first is that it allows for slicing by continuing to "push" on the surface as it is drawn across it. The other effect is to allow the force to be concentrated in an even smaller area.
As a rule the blade must be made of a substance which is harder than (or as hard as) the material it is intended to cut. If this isn't the case the blade will either be unable to cut (as it absorbs all the energy as it is damaged) or will wear away very quickly (if it is hard enough to transfer enough of the energy to damage the material). In practical terms the material must also be tough enough to last (e.g. glass is very hard but it shatters easily and thus isn't very effective as a material for a blade).
The problem is further compounded by the fact that heat treatments, which increase hardness for better edge-holding, inevitably reduce the material's toughness. Essentially speaking, a balance must be found between how well the edge must hold, and how well it can last. Methods that can circumvent this somewhat do exist however; for instance differential hardening allows for an edge that can hold well, and a body that can withstand mechanical stress.
Geometry
An ideal blade would come to a perfect edge—not at all rounded—but that says nothing of the angle of that edge. The ideal angle is a function of the material being cut. For example, a tool bit for cutting metal may have nearly a 90° edge; it would probably not even be considered a blade. With very rigid materials such as metal, cutting deep into a piece with a blade would be impossible so deep cutting is done with a saw or grinder which provides kerf through which the cutting device can pass. With less-rigid materials such as a butternut squash, an acute blade prevents the blade from being pinched by the material. When cutting biomaterials such as tomatoes (which tend to have a low elastic modulus but high yield strain), the angle of the blade is less important since the material will bend, but the sharpness of the edge is important because if too much force is required, the material will be squashed rather than cut (or alternatively a serated blade could be used as mentioned above).
Material
Material for weapon blades has to be carefully selected to achieve a balance between hardness and toughness and their ratio to each is dependent upon the intended use of a blade. In antiquity, the main metal used was copper, then of bronze and later iron. Perhaps the most well known is pattern welding, a technique used for katanas (samurai swords) and blades made to resemble damascus steel blades. This was a very labor-intensive technique - and thus such swords were very expensive.
Various techniques may also be employed to make the blade stronger or harder. Copper and bronze can be "work-hardened" by simply hitting the blade with a hammer while it is cold. Blades made of steel with a high enough carbon content (greater than 0.2%) can be heat-treated by heating the steel up to a critical point (most simple carbon alloys become non-magnetic slightly below that point), then quenching it with forced air, oil, or water depending on the steel. Quenching puts an enormous amount of stress on the metal, and often a sword would break into pieces during that step. If the sword survived heat-treating, it would be tempered by heating it to a relatively low temperature for an extended period of time. The tempering process would make it slightly softer, but also tougher and "springier", and thus less likely to break or chip during everyday usage.
Case hardening is a process of increasing the carbon content at the surface of very low carbon steel. It is done by placing the object to be hardened in a sealed container along with carbon-containing material; in antiquity, this material was usually horn or hide. The container would then be heated until it was glowing red, and held at that temperature for a while, based on the size of the part being hardened, allowing carbon to penetrate the steel by a few thousandths of a centimeter. At that point, the object would be dumped out of the container into a water bath to quench it, resulting in a very hard surface, but completely unhardened core. There is very little evidence of this having ever been done to swords except, perhaps, the very earliest of iron blades. Due to the inherent weakness of a sword's cutting edge, coupled with the high-impact stresses of combat, such a thin hardened surface over a soft core would provide very little advantage in terms of edge-holding, other than mild wear resistance.
Another important aspect of many blades are so-called "fullers". Despite popular belief, fullers were not "blood grooves" that facilitated quicker bleeding of the victim and easier removal after insertion. Rather fullers helped to make a blade stronger and more durable at the core by giving it an I beam cross section, thus reducing the amount of steel needed to keep the spine stiff. This was very important in ancient times when high quality steel was more labor intensive to make, smiths would scrape the fuller with a U shaped tool before hardening and reuse the scraps. Modern day fullers are made by positioning a heated blade over a bottom fuller, setting a like sized top fuller on the top side of the sword, and hitting the top fuller with a hammer.
The most common materials used nowadays are various carbon and stainless steels, though strictly speaking anything that's fairly hard can be used. This has led to exotic blade materials being used since history, such as obsidian, flint and bone. Joining them in the modern day are more types such as synthetic sapphire, zirconium dioxide and even very hard plastics.
Dulling
Blades dull with use and abuse. This is particularly true of acute blades and those made of soft materials. Dulling usually occurs due to contact between the blade and a harder substance such as a ceramic, stone or a tougher metal. To a first approximation, a harder material cannot be deformed by a softer material at their interface because the stress on both materials is the same at the interface and so the softer material will yield first. One exception to this is when the highest stress isn't at the contact point; this is why one can easily bend a steel paper clip even though an end of the same paper clip could scratch one's skin.
Patterns of knife blades
There are a variety of knife blade shapes; some of the most common are listed below.
(1) A normal blade has a curving edge, and flat back. A dull back lets the wielder use fingers to concentrate force; it also makes the knife heavy and strong for its size. The curve concentrates force on a small point, making cutting easier. This knife can chop as well as pick and slice.
(2) A curved, trailing-point knife has a back edge that curves upward. This lets a lightweight knife have a larger curve on its edge. Such a knife is optimized for slicing or slashing. Trailing point blades provide a larger cutting area, or belly, and are common on skinning knives.
(3) A clip-point blade is like a normal blade with the back "clipped" or concavely formed to make the tip thinner and sharper. The back edge of the clip may have a false edge that could be sharpened to make a second edge. The sharp tip is useful as a pick, or for cutting in tight places. If the false edge is sharpened it increases the knife's effectiveness in piercing. The Bowie knife has a clipped blade and clip-points are quite common on pocket knives and other folding knives.
(4) A drop point blade has a convex curve of the back towards the point. It handles much like the clip-point, though with a stronger point less suitable for piercing. Swiss army pocket knives often have drop-points on their larger blades.
(5) A spear-point blade is a symmetrical blade with a spine that runs along the middle of the blade. The point is in line with the spine. Spear-points may be single-edged (with a false edge) or double-edged or may have only a portion of the second edge sharpened. Pen-knives are often single-edged, non-spined spear-points, usually quite small, named for their past use in sharpening quills for writing. Pen-knife may also nowadays refer to somewhat larger pocket knives which are often drop-points. Some throwing knives may have spear-points but without the spine, being only flat pieces of metal.
(6) A needle-point blade is a symmetrical, highly tapered, twin-edged blade often seen in fighting blades, such as the Fairbairn-Sykes commando knife. Its long, narrow point offers good penetration but is liable to breakage if abused. Although often referred to as a knife, this design may also be referred to as a stiletto or (slender variety of) dagger due to its use as a stabbing weapon albeit one very capable of slashing as well.
(7) A spay-point (once used for spaying animals) has a single, mostly straight edge that curves strongly upwards at the end to meet a short, dull, straight clip from the dull back. With the curved end of the blade being closer to perpendicular to the blade's axis than other knives and lacking a point, making penetration unlikely, spay points can be suitable for skinning.
(8) A Westernized tanto style knife has a somewhat chisel-like point that is thick towards the point (being close to the spine) and is thus quite strong. It is superficially similar to the points on most Japanese long and short swords (katana and wakizashi). The traditional Japanese tanto knife uses the blade geometry of (1). The Westernized tanto is often straight but may also be gently curved. The point is actually a second edge on the end of the blade, with a total edge angle of 60 – 80 degrees. Some varieties may have the back edge angled to the point slightly and sharpened for a short distance from the point.
(9) A sheepsfoot knife has a straight edge and a straight dull back that curves towards the edge at the end. It gives the most control, because the dull back edge is made to be held by fingers. Sheepsfoot look like a sheep's hoof. They were used mostly by sailors in old times, as the shape of the tip prevented accidental penetration of the work or a person when the ship rolled suddenly.
(10) A Wharncliffe blade is similar in profile to a sheep's foot but the curve of the back edge starts closer to the handle and is more gradual. Its blade is much thicker than a knife of comparable size.
(11 and 12) An ulu (Inuit woman's knife) knife is a sharpened segment of a circle. This blade type has no point, and has a handle in the middle. It is good for scraping, and sometimes chopping. It is the strongest knife shape. The semi-circular version appears elsewhere in the world and is called a head knife. It is used in leatherworking both to scrape down leather (reducing thickness), and to make precise, rolling cuts for shapes other than straight lines.
Not pictured is the undulating style found on items like the kris or flame-bladed sword. These blades have a distinct wavy design and are sharpened on both sides, typically tapering to (or close to) a symmetrical point.
Patterns of sword blades
Swords may have either a straight blade or a curved one. A straight sword was thought to primarily intended for hacking and stabbing, yet recent studies have shown this to be untrue, as many slicing techniques were used. The difference between a hacking cut and a slashing one is essentially the same as the difference between using a butcher's knife and a chef's knife; one forces an edge straight into a material while the other is pulled along the material to get more of a slicing action. Hacking cuts were usually followed by a slicing action, where the sword is drawn backwards to maximize the cut. For more information see Western Martial Arts or kenjutsu.
Some variations included
- the flame blade (undulated blade, for both psychological effect and some tactical advantage of using a non-standard blade: vibrations and easier parry)
- the colichemarde, essentially found in smallsword
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