Henderson Plastics Ltd
Lyng Hall Road
Tel: 01362 683 364
The guide bush  is usually supplied with the router  from new. They come in different shapes and sizes but their function is always the same.
It is fixed to the base plate  of the router so that the short tube bit is left protruding.
This short tube then locates into the slot in the template, as you machine, the guide bush slides along the edge of the template slot. This ensures that you cut exactly where you are supposed to, and you will cut the perfect joint.