A router plate is the very flat, very smooth surface on a hand-held router or a table-mounted router that comes in contact with the workpiece while the router bit itself is engaged. Router plates can be purchased and are usually made from aluminum or synthetic compounds like acrylic or phenolic. However, many do-it-yourselfers (DIYers) will make their own router plates from plywood, acrylic or phenolic.
A router is one of the essential woodworking tools in any woodshop and it's critical that it be properly equipped. The router plate that's installed as original equipment is usually an aluminum disc, about 4 - 6 inches (about 10 - 15 cm) in diameter and about 3/8 in (9.5 mm) thick. One of the main reasons woodworkers have for replacing a router plate is that it's easier to monitor the progress of work using a clear acrylic router plate than the opaque aluminum plate. Some will also want a larger plate to increase the amount of surface area that comes in contact with the workpiece, contributing to the stability of the process and accuracy of the work, as well as the overall safety of the project. Although clear acrylic router plates are commercially available, many DIYers have the equipment and competence to work with acrylic and will prefer to fabricate a new clear router plate themselves.
Many jobs, such as routing wood molding, will require that the router be mounted upside down on a router table. Depending on the nature of the work being done, the tabletop may be very large, but in most home shop settings, the router table top will be approximately 24 x 32 inches (about 61 x 81 cm). Many home or professional shop router tabletops, especially those built in the shop, will have an opening for a rectangular router plate of approximately 10 x 12 inches (about 25 x 30 cm), and this router plate will have a hole in the center, about 4 - 6 inches (about 10 - 15 cm) into which a circular insert is fixed that's sized for the particular router bit to be used. For airflow, stock support, and sawdust management, the clearance between the router bit itself and the insert should be between 0.125 and 0.25 inches (3.2 – 6.4 mm), which means that a well-equipped shop will have an assortment of different-sized inserts.
The essential qualities of a tabletop router plate, and of the circular inserts used with it, are that they be perfectly flat, perfectly smooth, and perfectly flush with each other when installed. Anything at all that impedes the smooth and even movement of the workpiece &emdash; dents and scratches in the router plate surface, an insert that's not perfectly flush with the surrounding plate, or even a slight warp in the router plate's surface &emdash; will result in flaws and errors in the work performed. It's the possibility of these imperfections occurring in an active woodshop that's the underlying reason for having replaceable router plates.