Chainsaw
A chainsaw (also spelled chain saw) is a portable mechanical, motorized saw. It is most commonly used in forestry and by tree surgeons, to fell trees and to remove branches and foliage.
Construction
Chainsaws consist of a small two-stroke gasoline (petrol) internal combustion engine (although smaller versions sometimes use electric motors), the "blade" (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 which first chips a piece 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 which keeps the tooth centered between the rails of the blade, helps to pump lubricating oil around the blade, and engages with the motor's drive sprocket inside the body of the saw. The motor 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 motors, the motor 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.
The air intake filter tends to clog up with sawdust. This must be cleaned from time to time. Many saw operators clean it with petrol, but it is important to remember to use fuel that has not yet had oil added - otherwise the residual oil on the filter will make it clog up again faster.
Chains must occasionally be removed from the saw for sharpening. This may be done lightly in the field with a round round file, or more thoroughly with a specialised electric sharpening jig. The jig helps ensure that each cutting face is kept at the correct angles, which are carefully balanced to maximise the saw's efficiency. A common error is to reassemble the saw with the teeth facing back-to-front, which naturally considerably reduces its cutting power.
Safety
Since their introduction, chainsaws have become a mainstay of forestry. Despite safety improvements, chainsaws can be dangerous, and injuries can arise from their use. The most common accident arises from "kickback", when the saw blade stalls (due either to being bound by the wood or by striking a resilient object) which causes the entire saw to jerk uncontrollably. Kickback can result in serious injuries or death. Operation of chainsaws can also cause tinnitus or industrial deafness. The risks associated with chainsaw use mean that protective clothing and hearing protectors are generally worn while operating them, and many jurisdictions require that operators be certified or licensed to work with chainsaws.
Accessories
Some saws can have the blade replaced by other attachments, using the saw's "power head" as their motor. The most common such attachments are augers.
In culture
The chainsaw has gained notoriety as an instrument of murder or torture, particularly in movies such as Scarface,The Texas Chainsaw Massacre, and the Evil Dead series. They also appeared in video games including DOOM and Grand Theft Auto: Vice City. Chainsaws are ungainly weapons and are thus used only very rarely, their only recommending factor being the terror they inspire.
See also
