I have been thinking about how to make a BYCNC setup into a machine using a 4th axis for some time and I think it would be as easy as setting up the table properly the first time. This should not take any greater adjustments than extending the Z-axis drive and the table itself. The ideas I have for updating my table, are as follows:
Extend the Z-axis for the router so that you can travel beyond the surface of the table that you are doing parallel cuts upon. This will extend the overall height, but only enough to get the appropriate depth of cuts in the material you are turning. I am thinking I will invest in a longer screw for the z-axis and just cut myself some duplicate parts, just a bit longer, to facilitate the added depth.
The second part of this process is to build the table top in a fashion that not only offers the rigidity that you need while cutting flat stock on the table top, but a method by which the center can be removed, either by slide mechanisms or leafing, so that you can reveal the under portion of the table. Because the design currently does not require any mechanical parts to run under the table top, and also due to the majority of the burden being on the outside edges of the table top the center should be able to be removed without creating undue stress on the structure while giving you a space to mount a rotating piece of material.
Finally, I am imagining that there is a second NEMA 34 powering a rotational mount, so that you can mount material in the middle of the table and pin it in place, like a lathe mount. One side will be fixed in place, which is the corresponding end to the motor. I also see this system working with the same #25 roller chain and sprocket system of the other axis points. The far end for fastening the material would include a lazy susan type rotational bearing system fixed to a screw type device so you can adjust the length of the mount accepting diverse height of material.
If this method works out I will definitely share it and link it up, it will be a bit of time before I can get everything prototype d but this idea, I think would be the most cost effective and easiest if you want to convert an existing setup to accept a rotational 4th axis. All the parts that you would need to order in addition to your existing setup would be a longer drive system for the Z-axis, which may just be a longer screw, an additional motor and drive components, and a way to allow for the smooth turning of the material.
Converting the table should not be too difficult, however, I realize that when you compromise a solid flat surface for something that is modular you may run into a problem with tool resolution due to flex at the joints. I am not sure how to overcome this and allow for many uses of the same cnc for different applications, but, once you have something that is perfect, the soul of a hacker is to make it better....its just the way of life for our kind.
Hope this sparks some ideas in your brains!