head spins. The cutter head's axis of rotation is parallel to the table surfaces and perpendicular to the feed direction. The knives cut into the board in the direction opposite to the feed. Some, more expensive, jointer models contain a spiral, or helical, cutting head. This configuration has many individually mounted, self-indexing knives that can be rotated to a new edge when necessary. Other, older, models have cutter heads that are not cylindrical but instead square. This leaves a significantly larger open region below the level of the blade edges and creates a larger hazard as hands, etc., can be pulled in further and cut more deeply. The infeed and outfeed tables can be raised or lowered independently of each other and in relation to the cutter head although the outfeed table is normally set so that it is level with the knives when at the top dead centre of the rotation of the cutter head. The infeed table is adjusted so that it is lower than the outfeed table and this gives the depth of cut. Jointers for home workshops usually have a 4–6 inch (100–150mm) width of cut. Larger machines, often 8–16 inches (200–400mm), are used in industrial settings. Operation In operation, the board to be jointed is held with its face against the fence and the edge to be jointed resting on the infeed table. The board is fed across the cutter head and onto the outfeed table. The knives in the revolving cutter head remove an amount of material and the relationship of the two tables and the fence keeps the board oriented in such a way that the result is an edge which is flat along its length and perpendicular to the board's face. A jointer may also be used to flatten the face of a board, in which case the sole focus is to produce a flat surface on the face of the board and the fence is not used. This procedure is often performed prior to edge jointing so that the board has a flat reference face for subsequent operations. To straighten a piece of bowed timber, the guard is temporarily swung out of the way. The machine is switched on and the timber is slowly lowered to the machine table, with the concave side down. A few cuts are made out of the red section "A". The timber is turned end for