Interchangeable Parts

Interchangeable parts are parts (components) that are, for practical purposes, identical. They are made to specifications that ensure that they are so nearly identical that they will fit into any assembly of the same type. One such part can freely replace another, without any custom fitting (such as filing). This interchangeability allows easy assembly of new devices, and easier repair of existing devices, while minimizing both the time and skill required of the person doing the assembly or repair.

The concept of interchangeability was crucial to the introduction of the assembly line at the beginning of the 20th century, and has become an important element of some modern manufacturing but is missing from other important industries.

Interchangeability of parts was achieved by combining a number of innovations and improvements in machining operations and the invention of several machine tools, such as the slide rest lathe, screw-cutting lathe, turret lathe, milling machine and metal planer. Additional innovations included jigs for guiding the machine tools, fixtures for holding the workpiece in the proper position, and blocks and gauges to check the accuracy of the finished parts. Electrification allowed individual machine tools to be powered by electric motors, eliminating line shaft drives from steam engines or water power and allowing higher speeds, making modern large scale manufacturing possible. Modern machines tools often have numerical control (NC) which evolved into CNC (computerized numeric control) when microprocessors became available.

Cutting tools made of high speed steel allowed steel rather than wrought iron to be used for parts. The ability to machine hardened parts eliminated the problem of warping and dimensional changes associated with heat treatment hardening of parts after machining. Modern cutting edges also use materials such as tungsten carbide. Other innovations were drop forging and stamped steel parts, which reduced or eliminated the amount of machining in the United States.

The system for producing interchangeable parts is alternately called The American system of manufacturing because it was first most fully developed in the U.S., although contributions were made by other nations who soon implemented it.