I referred this to the wood working section due to the use of a router and a special inlay guide bush set. Otherwise it is a very typical DIY exercise.
We have an old house, and with that comes the need to repair window frames of wood that has some bad or rotten wood and need maintenance. The wood is old and good, so given the right care, the repaired frame will last longer than new windows. For the window in the picture, there was an old and badly done repair of the window frame applied to the wood. I removed the unwanted part of the window frame wood, and replaced it with a piece of fresh wood as seen in the picture. There is a number of approaches to replacing the wood that can be followed.
For this job, I have used a router inlay guide bush set and template. The work to repair the frame is straight forward. The template I needed was a rectangle. For that, I used the the Festool MFS which is an excellent routing aid (but not as speedy as templates you have already created and do not need to tune.
I decided for the size I wanted. A size which removed enough material from the window frame. I dialed in the lengths of the sides on the Festool MFT (at ② in the picure).
To be sure I did not make mistakes with the size, I select to create a wooden template (at ③ in the picture) from the MFT. That wooden template was later the template used to route the actual hole in the window frame to remove the bad wood. Doing this extra (not needed) template does not make the job easier. But it gave me a clear visible hole telling me if I had thought right when tuning in the MFS. First time I had done a mistake, and I needed to change the size before I was sure it would give me the right size hole when routing in the window frame. This way felt safer than risking that the window frame hole became bigger than intended due to a miscalculation.
With the right size template created, route away all bad material from the window frame within the template. The depth was increased in steps. I stopped increasing the depth when I thought the frame beneath the hole only contained good enough solid wood.
It was not possible to immediately set the final height to remove. In this case, the bad wood (seen at the small picture) was a terrible done previous repair. It could also have been some rotten wood. On the top, there was cavities between the differen round wooden repair bits sen in the picture. I needed to go so deep that these cavities where eliminated, or so small they could be ignored.
No matter what the bad wood is, you have to tune down the router in steps until what you see after the routing is good enough wood. If instead of an old reapir, what you would see is wood damaged from moisture, you would still not know how deep to go until you tried.
When I had routed away enough wood (all cavities from the previous fix removed), I measured the height of the hole created. This height is the minimum thickness needed for the plug to be inserted into the window frame.
Next step was to create the new wood plug to use for an inlay. I used a piece of wood that was about 1 mm higher than the hole. The very same template as used for routing the hole in the frame was now used to route a suitable piece of wood to fit in the hole (an inlay).
With the piece in hand comes the most scary part of the exercise. The piece need to fit in the whole created in the frame. I did not use rounded corners in the template for the inlay. I selected instead to chisel the corners square in the window frame. This is in my mind easier to do than rounding the corners of the piece.
With the corners squared, the piece if tested for fit. And it fit perfectly. In fact, it fitted too perfectly. It needed to be adjusted a bit. And that is scary. Adjusting too little, and the piece will get stuck when pressed halfway down in the hole it is to heel. Adjusting too much, and it will not take the glue well. An old rule is that it should be so tight you cannot press it down with your thumb, but so loose you do not need a hammer. The rule does not help too much, because if the piece is put too far down the hole while testing, it will not be possible to take up again. So I straightened the edges of the hole, and the edges of the inlay. Until the point when it was time to try.
On with glue (I use Titebond III when the fit is tight and I need some additional opening time. I would have used white glue otherwise, unless a more filling glue would be required). The glue was spread evenly using a brush. Do not spend to much time. The glue will cause a slight swelling of the wood, so it should not get too much time for doing too much swelling. Also, it is important the glue is still open when the piece is fitted in place (open time is defined by the glue manufactorer and differs from glue to glue) .
In my case, I needed some blows from my hammer, and the piece was down in correct depth. Best way I have found to remove the excess wood is by using a plane. A special purpose japan saw could be a good start, but a plane is perfect for the final adjustement.
All done, a perfect fit and no risk it will be visible in the future.