Parallel (engineering)

Parallels are used when machining with a mill, drill or any other machining operation that requires work to be held in a vise or with clamps - to keep work parallel or raised evenly such as in a milling vise to give adequate height for the cutting tool/spindle to pass over.

They come in a variety of thicknesses and size, allowing them to be stacked up or to support a workpiece which doesn't have a flat profile.

Parallels commonly have a series of holes drilled on the 'front' face - allowing them to be used to position a workpiece or secured using t-slot clamps, and a countersink on each side to remove any sharp edges.

Generally, workshop parallels have 4 faces that are machined and ground - the front, back and sides, although some do have the ends with a smooth surface.

There are two main grades of surface tolerance:[2] Parallels are first machined to rough dimensions, leaving a few millimeters to allow the rest to be ground.