Initially I used a cheap 1/4" Ryobi router from Canadian Tire. It actually did a surprisingly good job, but since then I bought a 1/2" Porter Cable router that came with two bases. I mounted the non-plunging base on the machine. If I need to use it for other jobs, I can just pull the motor out, and put it in the other plunge base. The 1/2" collet on the router allows for mounting bigger router bits, especially for larger mortises.
The lever on the front right is for moving the table in and out against the router. for side to side motion, I just push the table by the handles. I found that using 'Onsrund' spiral router bits works best. I can use them to make a series of plunge cuts using the lever, and then move the table side to side to clean out the slot.
The work piece shown is positioned for cutting an open ended mortise. Clamping of the
work piece is done very similar to how its done on a milling machine. Of course, that
table was meant for milling in a way.
To keep things precise, the router mount is very solid. Two 5/8" steel shafts are attached to the frame. The router itself is mounted in two pieces of oak that slide on the 5/8" shaft. Vertical adjustment is achieved by turning the screw at the top. The whole vertical adjustment thing has quite a bit of friction to it, so there's no risk of it moving on its own.
I'm very happy with this mortising machine. Unfortunately, I can't encourage anybody to build one the same way, because I don't know where one would get another sliding table like that. Shawn Havents did come across an identical mill/drill table though, and sent me some photos of his slot mortising machine based on that table.
I also experimented with rigging up a very minimal mortising jig on my table saw and was quite happy with the results.
I've since built a fancier slot mortising machine that's made of obtainable and relatively inexpensive parts. Its not quite as rigid as this one, but much easier to use.
I then found myself no longer using this machine for mortising, and ended up mounting a drill chuck and motor in place of the router to turn it into a horizontal boring machine