Jointers and planers perform the same basic function on two different surfaces of a board. Jointers are designed to give the woodworker a smooth, flat edge. Planers smooth the face surface of a board and are often used to reduce the thickness of a piece of lumber to the final working dimension.
Both jointers and planers work by stabilizing the working surface of a board against a flat plane relative to the cutter head and removing a measured amount of wood. By using rapidly rotating knives and multiple cuts for each inch, they achieve a smooth, flat-finished surface.
The working surface of a jointer consists of two independently adjustable beds. The in-feed bed is adjusted to the depth of the desired cut. The out-feed bed is adjusted to the height of the top dead center of the cutting blades. This allows the board to remain flat across the working surface of the table while reducing its overall width.
The cutter head is usually a steel cylinder with from two to four slots. Steel knives are secured into these slots and set at exactly the same height to achieve a smooth cut.
The standard number of cuts per minute is 20,000. To achieve this number, the cutter head rotates at between 500 and 10,000 rpm depending upon the number of knives contained in the cutter head.
An adjustable fence is attached to the base of the jointer and runs perpendicularly to the length of the working tables to allow the woodworker to obtain the desired angle on the edge of the board.
The bed of a planer consists of one continuous flat surface. Unlike a jointer, the planer's cutting head and knives are mounted over, not in, the bed. Cutting depth is set by raising and lowering the bed attached to the body of the planer cabinet by four adjusting screws.
The planer's cutting head is usually slotted for either three or four knives and rotates at about 5,000 rpm.
Because a planer works with the face or wider surface of a board, it is usually equipped with a power feeder, a mechanism designed to feed the board past the cutting heads at a constant speed and pressure (something that would be difficult to do by hand with the greater resistance caused by the larger amount of wood being removed with each cut).
A newer style of cutter head is available for both jointers and planers. A spiral cutter head uses small edged carbide inserts in place of straight steel knives. As the name implies, these carbide cutters are arranged in a spiral around the working surface of the head. While these are considerably more expensive than traditional cutter heads, manufacturers claim that they produce smoother cuts and are more easily maintained.