A thickness planer (or surface planer) is a machine used to reduce a rough or pre-flattened board to a specific, uniform thickness across its entire length and width.
It works by passing the workpiece under a rotating cutterhead equipped with multiple knives, which shaves off a precise amount of material.
The critical feature is that the machine ensures the finished board has two perfectly parallel faces.
It is typically used after one face of a board has been flattened on a jointer. The planer then references that flat face to create a parallel opposite face.
This process is essential for preparing stock for fine furniture making, cabinetry, and any project requiring lumber of consistent dimensions.