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Cannibalization of machine parts, in the maintenance of mechanical or electronic systems with interchangeable parts, refers to the practice of removing parts or subsystems necessary for repair from another similar device, rather than from inventory, usually when resources become limited. The source system is usually crippled as a result, perhaps only temporarily, in order to allow the recipient device to function properly again.
Cannibalization is usually due to unavailability of spare parts, due to an emergency, long resupply times, physical distance, or insufficient planning or budget. Cannibalization can also be due to surplus inventory. At the end of World War II a large quantity of high quality, but unusable war surplus equipment such as radar devices made a ready source of parts to build radio equipment.
In the electronics market, machines being cannibalized are known as parts machines.
Sometimes, removing parts from old equipment is the only way to obtain spare parts, either because they are no longer made, are obsolete, or can only be manufactured in large quantities. In logistics, this is known as Diminishing Manufacturing Sources (DMS). 
This is often the case in the military, and ships and aircraft, as well as other expensive equipment that is produced in limited quantities. Such was the case with the aircraft carrier USS Kitty Hawk, the sole survivor of a class of three ships built during the early-1960s. The ship herself is over forty years old, and having manufacturers build individual custom replacement parts would be highly impractical, and thus decommissioned ships, such as the USS Independence, have been utilized for the necessary parts to keep the Kitty Hawk in operation.
One strategy used to combat DMS is to buy additional inventory during the production run of a system or part, in quantities sufficient to cover the expected number of failures. This strategy is known as a lifetime buy.