A planer jig is a device used to guide either the piece of wood to be planed or the planing device itself throughout the planing process. The planer jig can be made from a variety of sturdy and straight materials, and it may be either purchased prefabricated, or it can be made from scratch. Many woodworkers will use a jig to prevent a piece of wood from moving in an errant manner, thereby allowing the wood to be cut at the wrong depth or in the wrong shape. The jig may also be used to prevent the entire length or width of a board from being planed.
Planing is the process of flattening one face of a board or piece of wood. It is usually done using a specially designed planer, which can be a handheld device or a benchtop device. Much larger, industrial models may be larger and heavier duty, accommodating the planing of very long or wide boards. A planer jig can be used differently according to the type of planer being used. If the woodworker is using a handheld planer, for example, the planer jig may be used to steady the piece being planed, or it may be used to prevent the woodworker from planing certain areas of the board.
A planer jig used with a benchtop planer or a larger machine may allow a woodworker to plane the side of the board rather than the face. The jig will steady the board on its end as it progresses through the machine, preventing the board from kicking, swaying, or otherwise becoming unstable. If the jig is not used, the board may move, causing an imperfect cut as well as a dangerous situation in which a bystander may be struck by the board. Damage to the machine itself is also possible if the board is not steadied.
Sometimes a specially designed planer jig may be used to make planing possible with other tools. A circular saw, for example, may be used to plane a board, though the process is difficult to do by hand without any guides. A jig can therefore be made from scraps of wood to steady the blade of the circular saw at a specific depth in relation to the board being cut. This is usually an imprecise process even when using a jig, so it is not recommended for boards that will be used for aesthetic purposes.