Sigma recently released a (second) video of its large AIZU manufacturing facility in Japan. This large factory produces most of the lenses the company produces in a long but very organized process.
Back in late 2012, Sigma released its first AIZU facility video. The idea was to show the general landscape of the factory as well as the people who make the lenses and the facility itself (a kind of a contrast between the natural landscape and the high tech facility).
You can find many more video showing the manufacturing process of photography equipment from Leica, Zeiss, Broncolor and more on or dedicated “how do they make it” section here on LensVid.