Router tables are fantastic for doing tenons and also some kinds of mortises. They truly shine when doing tenons, and they work fairly well for doing centered a rmortises parallel to the length of the material.
To route mortises like on the picture, square to the long edge and on the flat side, some special setup is required. Yet, it does not take very much when you have got the hang of it.
As can be seen on the picture, you require a fence (①) to keep the distance between mortise and bottom. In the picture is the standard Festool CMS router fence. To do the tenons, the fence would need to be closed as on the picture, but for the mortises here, also an open fence would work.
The stop bit (②) on the picture can be of any kind, here it is just a bit of scrap wood, fastened to the CMS table with a Wolfcraft one-hand clamp. The purpose of the stop bit is to assure you can not do the mortise longer than intended. Since the stop bit can be placed and tested in advance, this is a safe part of the routing.
A mortise like the one on the picture (③) is created using the drop method. This is done by taking the piece that should have the mortise on it, and carefully lower it onto the already spinning router bit (④). This is done to assure that the start point occurs inside the edge of the piece which should have the mortise. The drop has to be done very carefully, so the piece is pressed against both the fence, and something that keeps i square to the fence, at the same time as it is prevented from rotating. This could have been another piece of wood, but in the picture it is the mitre guide (⑤) which is fastened onto the sliding table, allowing it to guide the piece forward while keeping it at a fixed angle. To assure the drop starts at the correct position, a mark (⑥) has been made with a pen on the fence. When the piece has been lowered onto the router bit, it is moved forward until the stop bit prevents it from going further.
Although the setup works, it has a number of small drawbacks:
- The router table is used for both the mortise and the tenon, making it harder to have two work stations, and forcing repeated setups
- The drop methos is a bit unprecise. It can be improved with an additional stop bit, but it still requires the piece that should have the mortise to slide downwards on the fence, changing angles.
- If needing to adjust the height between runs on the same bit, a number of drops need to be made per piece