A router is a power tool that spins a cutting bit, at very high speed, from a flat base with a motor above it. The bit rotates at much higher speed than the bit in most drills, as the purpose of the router is not principally for cutting holes, but rather for cutting grooves or complex shapes in surfaces, or for smoothing or shaping the edges of a sheet of material. Routers primarily are used on wood or plastic.
Routers are often used with templates that allow them to make repetitive cuts, such as the recess that takes a hinge in the edge of a door.
Standard, plunge and edge
The first power routers allowed adjustment of the depth of cut, but, if their base was parallel to the work, the bit made contact. A newer type, the plunge router, allows the base to be put parallel to the work, and then the bit gradually lowered, as with a drill press. Trammel points are jigs that attach to the base for making elliptical cuts.
Yet another variant of the tool is the edge router, which tends to be smaller and often can be manipulated with one hand; its bit primarily cuts with its side, and is used for such purposes as trimming laminate coverings glued to countertops.
Some small, precision rotary tools, which operate at even higher speed and are reminiscent of dental drills, may have an accessory router base allowing them to be used for especially delicate work.
Another variant is the biscuit joiner, which is like a plunge router in that the cutter can be advanced, but it is advanced horizontally, into the edge of a piece of wood. Matching biscuit cuts are made in two pieces of wood to be joined almost invisibly; a wooden, glue-covered "biscuit" slides into the matching slots, the pieces are clamped together, and a strong joint is made with relatively little effort.
While hand routers usually are used above the work, the router also may be temporarily or permanently mounted to a table, with the bit pointing upwards. The table may be placed on a workbench, or, for some applications, it can be very convenient to have under one of the side wings of a table saw, so the work can slide from being cut to being shaped. Appropriate guards are especially appropriate here, as it is a dangerous tool when not used with care.
The shaper is a larger woodworking tool, but essentially a floor-mounted table router. Some permanent routers also may have complex floor mounts, rather like a drill press, which allow the cutting angle to be varied, and perhaps the bit to be moved radially across the work.
So that different grooves and patterns can be cut, there is a very large variety of bit shapes. Some may be custom-made. To make certain other patterns, it may be appropriate to make multiple passes with differently shaped bits with different depths of cut. For relatively light duty, tool steel is adequate, but tungsten carbide is often preferred; the stress on a router bit is often greater than on most drill bits.