This is not about doing beautiful inlays, alhough it is using the very same technique. It is about replacing wood that is already in place, by removing it and taking other wood from somewhere else to fill the gap, using a router for both exercises. This is in truth doing an inlay, but the focus here is on how to do the replacement of wood using a router with a guide bush set and a template. I am using it to repair window frames, not to create nice inlays for the sake of looking good.
The basic principle is simple as for any inlay. The router is used with a guide bush set on which there is a removable collar (seen at ① in the picture). I have bought such guide bush set for my Festool. It is purchased from Trend which has a better supply of such accessories for routers than Festool has. So what is seen seen at ① in the picture is a Festool router with the Trend inlay routing guide bush set mounted. It is just a small bottom plate with a guide bush, onto which a much thicker, but removable, guide bush collar is fastened.
With the collar in place, the router traces a template to remove wood (a possible such template is seen at ② in the picure). The edges of the wood is removed by tracing the guide collar along the template. The wood inside the edges is removed by moving the router inside the template until all parts of the wood has been removed (seen at ③ in the picture). How much wood is removed is decided by how deep the router bit goes.
When the hole is created, it is time to create the routed inlay piece to insert to replace the removed wood. To do that, the collar is removed (seen in the picture).
The width of the collar is the same as the diameter of the router. That is the key to the success. Previously, with the collar ring in place, the hole was created with its outer edge being defined by the outside of the router bit. With the collar removed, the piece of wood to insert in the hole is created with its outer edge being defined by the inside of the router bit. Since the router bit is the same size as the collar, the piece of wood routed out with the collar removed fits exactly in the hole previously created with the collar on the guide bush. Thus the inlay fits almost perfectly.
The whole inlay routing exercise intends to cut a piece of wood free from the board where it is taken from. Something need to be done to prevent the inlay piece from moving around when the router cuts it loose. The simple solution is double sided tape which keeps it in place On the plate it is placed on when routing.
The double sided tape is placed on the piece of wood before routing starts. The picture shows the board after routing was completed. As can be seen, the tape is still keeping it in place. Even if you routed deeper and the tape would be fully routed away, the piece it self would stay in place, since it is taped in place also on the plate where it lies during routing.
The template shown at ② in the picure is a rectangle. For that, the Festool MFS is an excellent routing aid. All needed to do is to tune in the size on the built in scale, and lock it at that size. The size dialed need to be big enough to remove enough material. Sizing this is not that hard, but it ends up as a mathematical exercise where you can go wrong. You need to tune the size so big that the inner router guide bush ring thickness, plus the thickness of the removable guide bush collar reduces the routed hole to perfectly fit what you want to remove.
Any template of suitable form or size works. The only trick to think about besides how to find the right size, is how sharp turns it does. If it contains turns (like corners or roundings) and those have an angle smaller than the outer guide bush collar, then there will be manual adjustments needed. When the outer collar is removed and the inlay bit is created, the inner ring has a smaller diameter. Thus it can reach further into the turns than the collar. This only beocmes a problem if the turns are so small that the outer collar cannot track them. For square corners it is often ok, because you can adjust them with a chisel.