GUIDE BUSH CUTTER OFFSET
The crucial thing about using guide bushes is that the cut takes place at a distance from the outside edge of the guide bush. This distance depends on the diameter of the bush and the diameter of the cutter and is known as the ‘offset’. With some standard applications e.g. dovetailing, mitring kitchen work tops, you do not have to worry about this offset; you simply use the jig with the specified guide bush and cutter. When making your own templates however, the offset must be calculated and taken into account in the construction.
The simple formula for calculating the offset is: (Outside diameter of guide bush – diameter of cutter)/2.
For example, with a 30mm guide bush and a 10 mm cutter, the offset is (30-10)/2 = 10mm.
The calculation can become awkward when we have metric guide bushes and Imperial diameter cutters. Remember when making your templates that if the cut is around the outside of the template it will be bigger than the template; if it is round the inside the cut will be smaller.
If you are still puzzled, the easiest way to grasp it is to put a cutter and guide bush into your router, make a cut against a straight edge and measure the distance from the edge of the cut to the straight edge.
The illustration shows the above calculation diagrammatically.