boring mill, machine tool used to increase the size of a hole previously made in a workpiece, usually with the purpose of obtaining a required degree of finish and accuracy in the final hole. In a horizontal boring mill the workpiece is held stationary on a vertical table whose position can be adjusted. A spindle attached to a vertically adjustable head holds the cutting tool, which is fed horizontally into the work. In a vertical boring mill the workpiece is made to revolve on a horizontal circular table as the tool is fed in.
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