A machine assembly plant is a factory devoted to the manufacture of complex machines, which consume energy in order to perform various forms of work. Products include engines, robots, turbines, generators, elevators, and various other forms of machines. Vehicle assembly plants are not included, but factories for mechanical parts of vehicles are. This usage does not include facilities devoted to the manufacture of electronics, although electronic parts may be incorporated into the products of machine assembly.

The products are complex mechanical objects that consume energy to perform work. Manufacture of mechanical vehicle parts is included, while assembly of entire vehicles is separate.

Electronic components can be incorporated into these machines. A facility devoted to producing electronics belongs to electronics factory instead.