Despite what some textbooks say, Eli Whitney didn't invent the idea of interchangeable parts (which he termed his "uniformity system"). He wasn't even the first to suggest using them in a more efficient process of musket production. Nevertheless, he deserves credit for showing that mass production was possible and for selling the idea to the U.S. government.
Whitney is also responsible for designing a highly effective milling process that made mass production work. Whitney's milling machine wasn't the first, and it probably wasn't actually built until after his death. However, when it was built, it was so successful that it remained in use for 150 years, making everything from clocks to agricultural and manufacturing tools.
Whitney's milling machine involved clamping metal to a workbench and fastening a pattern on top of it. A worker would then roll a sharp-toothed metal wheel along the pattern to cut out the part instead of chiseling it by hand. Prior to Whitney's contributions, gunsmiths made muskets by hand, with each part being custom-made for an individual weapon, which in turn took days to finish.
Eli Whitney's processes, inventions and ideas created jobs for people who lacked the extensive training and experience of artisans. They also helped the North establish a productive and world-renowned manufacturing base and transformed the South into an agricultural power. Although he died 35 years before the Civil War, Whitney unintentionally laid its groundwork by increasing the demand for slaves. He also helped fill the coffers of southern plantation owners and provide the North with the industrial output required for fighting a drawn-out conflict.