Double column long milling machines, also called duplex milling machines, are a special type of milling machine that has two opposing worktops. This allows the workpiece to be machined from both sides simultaneously, increasing machining time and accuracy. These types of machines are commonly used in the automotive and aerospace industries, as well as in the tool and die industry. They are designed to machine large, heavy workpieces and can withstand large loads. They are also capable of complex machining operations, such as groove milling and surface grinding. However, duplex milling machines are very expensive and require special training and experience of the operator, as they have complex technical requirements.