Guide bush jig
The guide bush guide is simply a piece of 6mm, 9mm or 12mm MDF with a slot cut in it to give a snug fit to a guide bush.A piece of batten is glued to its open edge at right angles. The guide was originally made to counter-slot jigs, etc, to recess the heads of bolts and screws, but it also makes an excellent housing jig since the guide bush running in the slot prevents the router deviating from the cut. For counter slotting, different diameter cutters are used without moving the jig, each successive cut being laid exactly over the previous one.
If you have a number of guide bushes, make the guide bush guide for your largest bush.This will accommodate a wider range of cutters and will also allow you to use it with smaller guide bushes so that it acts as a template.This in turn will result in cuts wider than the cutter diameter and you can experiment with all your guide bushes and straight cutters to produce a range of groove widths.With several guide bushes and cutters you may well find that you have a set-up to give you the required width for a given housing.
A good choice for the groove width and bush is 30mm as this the size that is most readily available. If you are working with a 1/2″ shank machine then you can produce the necessary slot with a T1430M straight cutter. For smaller machines you would need to make 2 passes to achieve the required width. You could produce a selection of these simple work aids in different length/width combinations