Parallels are precision-ground bars used in a machine vise to elevate and support a workpiece at a known height above the vise bed. They are typically used in pairs, positioned on either side of the vise, so that the workpiece rests on their top surfaces while being clamped by the jaws.
The primary function of parallels is to establish a consistent and repeatable reference plane. By lifting the workpiece off the vise floor, they provide clearance for cutting tools to pass through or beyond the bottom surface without contacting the vise. This is especially important in milling operations where through-slots, pockets, or drilled holes extend below the workpiece.
Parallels are manufactured to tight dimensional tolerances, with their opposing faces ground parallel to one another. They are commonly supplied in matched sets of varying heights, allowing the user to select a size appropriate for the workpiece thickness and required tool clearance. The accuracy of the setup depends on clean contact surfaces; chips or debris between the parallels and the workpiece will introduce error.
In use, the parallels should sit flat on the vise ways, and the workpiece should be tapped down firmly to ensure full contact before final tightening. Many sets include small relief grooves or wire springs along the sides to aid in handling and to reduce the chance of trapping chips