Using a router jig

You need to ensure you are using the correct accessories if you use any of the jigs available for routing applications. You should check the instructions with individual jigs for precise guides to which bushes and cutters should be used and how exactly the router should be operated when using the jig. You particularly need to pay attention to the cutter and guide bush you use as this will affect the distance between where the cut is being made and the edge of any jig or template. This distance is known as offset and needs to be taken into account for the cuts to be accurate. (Details on offset below.)

You will also get details of the thicknesses and types of material which are suited to each application, for example, worktops are usually made from laminated and resin bonded chipboard. This needs specific cutters as it is a highly abrasive material which will blunt standard cutters very quickly. Deep cuts will need to be made from a series of shallow passes and you need to make sure the router is guided along the edge of the jig at a constant, optimum, speed, without forcing the router, to ensure the waste material clears cleanly. 

What is router offset? 

The offset is the difference between the edge of any jig or template and where the cut will actually be made when using a guide bush. Most jigs need to be used with a guide bush, but some, more basic, templates may just require a guided cutter, sometimes known as a pilot cutter, which would not have any offset and will produce a cut which is the same size as the template. The offset depends on the combination of guide bush and cutter being used. There is a simple calculation you can use to work out the offset. 

How to calculate the offset

Take the diameter of the guide bush's collar, subtract the diameter of the cutter (which is the distance between the outside of the cutting edges, rather than the diameter of the cutter's shaft) and half this.

(Guide bush collar diameter - cutter diameter) ÷ 2 = Offset

This is how far away from the edge of the template the cut will be made. When routing out holes or other openings, following the inside edge of a template, this is how much smaller the hole will be compared to the template outline. When routing out shapes, following the outside edge of a template, this figure is how much larger the cut out shape will be compared to the template. 

Chart for guide bush

offset and formula

Below is a simple chart displaying the offset that will occur when using particular sizes of guide bush and router cutter.