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A planer is a type of metalworking machine tool that is analogous to a shaper, but larger, and with the entire workpiece moving beneath the cutter, instead of the cutter moving above a stationary workpiece. The work table is moved back and forth on the bed beneath the cutting head either by mechanical means, such as a rack and pinion gear, or by a hydraulic cylinder.

Planers and shapers were used generally for two types of work: generating accurate flat surfaces and cutting slots (such as keyways). Planers and shapers are now obsolescent, because milling machines have eclipsed them as the machine tools of choice for doing such work. However, they have not yet entirely disappeared from the metalworking world.

Modern planers are used by smaller tool and die shops within larger production facilities to maintain and repair large stamping dies and plastic injection molds. Additional uses include any other task where an abnormally large (usually in the range of 4'×8' or more) block of metal must be squared when a (quite massive) horizontal planing or floor mill is not available, too expensive, or unpractical for the situation. While not as precise as grinding, a planer can remove a tremendous amount of material in one pass and still maintain a high degree of accuracy. Metal planers come in two kinds: double-housing and open-side. The double-housing variety has vertical supports on both sides of its long bed; the open-side variety has a vertical support on only one side, allowing the workpiece to extend beyond the bed. Metal planers can vary in size from a table size of 30"×72" to 20'×62', and in weight from around 20,000 lbs to over 1,000,000 lbs.

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