Chainsaw
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A chainsaw (also spelled chain saw) is a portable mechanical, motorized saw. It is most commonly used in logging activities such as felling, delimbing, and bucking; by tree surgeons to fell trees and remove branches and foliage; to fell snags and assist in cutting firebreaks in wildland fire suppression, and to harvest firewood. Chainsaws with specially designed blades have been developed as tools for use in chainsaw art.
Construction
Chainsaws consist of a small two-stroke gasoline (petrol) internal combustion engine (although smaller versions sometimes use electric motors), the "guide bar" (essentially a long metal frame of a very hard wearing alloy) and the cutting chain itself. Usually each segment in this chain (which is constructed from riveted metal sections similar to a bicycle chain, but without rollers) features a small sharp blade, called a "tooth." "Skip tooth" chain has a tooth only on every second link, and is used for reduced risk of the chain clogging when cutting very soft wood. In modern saws the teeth are not straight blades; they have a forward section that first chips a piece of wood from the bottom of the cut, then another section, at a right angle to the first, which chips a piece from the wall of the cut. There are left and right handed teeth, depending on which wall of the cut they will chip. Left and right teeth are alternated in the chain.
The underside of each link features a small metal finger that keeps the tooth centered between the rails of the bar, helps to carry lubricating oil around the bar, and engages with the engine's drive sprocket inside the body of the saw. The engine drives the chain around the track at a high speed, providing an effective (if rather rough) cutting action.
Maintenance
Chainsaws usually require two sources of lubrication. Like most two-stroke engines, the engine is lubricated by its fuel, which contains about 2 ~ 5% (depending on model) oil dissolved in the fuel. Since this mixture is highly flammable, a completely separate oil reservoir is used for the external lubrication of the blade and chain. This latter oil is depleted quickly because it tends to be thrown off the chain by centrifugal force. Failing to keep this reservoir topped up, or using an oil of incorrect viscosity, is a common source of damage to saws and tends to lead to the chain coming off of the bar. With some types of wood, the operator will need to occasionally stop and unplug the holes where the oil is dispensed to the chain and guide bar
The air intake filter tends to clog up with sawdust. This must be cleaned from time to time, but is not a problem during normal operation. Many saw operators clean it with petrol, although manufacturers recommend using compressed air to blow the dust off the filter mesh from the inside, or warm, soapy water, and then left to air dry. If these resources are not available, pure, clean gasoline should be used, otherwise the residual oil on the filter will make it clog up again faster.
Chains must be kept very sharp to perform well. This may be done with a round file or a jig (without removing the chain from the saw), or with a specialised electric sharpening jig with the chain removed from the saw. The jigs help ensure that each cutting face is kept at the correct angles, which are carefully balanced to maximise the saw's efficiency. Proper hand-sharpening may produce a better result, but electric sharpeners are faster, particularly when sharpening very dull chains (especially those which have been subjected to dirt, metal or rocks). Carbide chainsaw blades cannot be sharpened by conventional sharpeners and must be sharpened by a diamond sharpener.
Safety
Despite safety improvements, chainsaws can be dangerous, and injuries can arise from their use. The most common accident arises from "kickback," when a chain tooth at the upper quadrant of the the guide bar tip cuts into wood without cutting through it. The chain cannot continue moving, and the bar is driven in an upward arc toward the operator. Kickback can result in serious injuries or death. Another dangerous situation occurs when heavy timber begins to fall or shift when a cut is nearly complete - the chainsaw operator can be trapped or crushed. Operation of chainsaws can also cause vibration white finger, tinnitus or industrial deafness. The risks associated with chainsaw use mean that protective clothing and hearing protectors should be worn while operating them, and many jurisdictions require that operators be certified or licensed to work with chainsaws. Injury can also result if the chain breaks during operation due to poor maintenance or attempting to cut inappropriate materials.
A more complete description of the design features built into chainsaws to enhance safety is given in the chainsaw safety features article. Personal protective equipment to be worn by chainsaw users is described at chainsaw safety clothing.
Accessories
Some saws can have the blade replaced by other attachments, using the saw's "power head" as their motor. Some common attachments are augers, circular saws, winches, hedge trimmers and capstans.
History
The origin is debated, but the first chainsaw was probably made around 1830 by the German orthopaedist Bernard Heine. This instrument, the osteotome, had links of a chain carrying small cutting teeth with the edges set at an angle; the chain was moved around a guiding blade by turning the handle of a sprocket wheel. As the name implies, this was used to cut bone.
Two important contributors to the modern chainsaw are Joseph Buford Cox and Andreas Stihl; the latter patented and developed a chainsaw in 1926 and a gasoline-powered chainsaw in 1929, and founded a company to mass-produce them. In 1927, Emil Lerp, the founder of Dolmar, developed the world's first gasoline-powered chainsaw and mass-produced them. McCulloch and Industrial Equipment Corp in North America started to produce chainsaws. The early models were heavy, two-person devices with long bars. Often chainsaws were so heavy that they had wheels like dragsaws. Other outfits used driven lines from a wheeled power unit to drive the cutting bar.
After WW2, improvements in aluminum and engine design lightened chainsaws to the point where one person could carry them. In some areas the skidder / chainsaw crews have been largely replaced by the feller buncher and harvester.
Chainsaws have almost entirely replaced simple man-powered saws in forestry. They come in many sizes, from small electric saws intended for home and garden use, to large "lumberjack" saws. Members of military engineer units are trained to use chainsaws.
Popular Culture
Although designed as a tool, the chainsaw is often portrayed as a gruesome weapon in movies and videogames of the horror genre. In Army of Darkness the main character, Ash, retrofits a chainsaw as a replacement hand, after he is forced to cut his off. Dead Rising features the chainsaw and mini-chainsaw as the most powerful zombie-killing melee weapons in the game. Chainsaws also feature prominently in the horror series, Texas Chainsaw Massacre.