I have been thinking about adding a rotary axis for a long time.  It could be used with the laser to carve pictures into water bottles or other round objects, and with the mill to make round or complex shapes that are milled on 4 faces.  

If I remove the spoiler board I have around 90 mm of Z height between the floor of the machine and the bottom of the Y carriage.  I could just barely fit a 3" chuck in that space (when fully opened).  However the cheapest 3" chuck I can find cost just a bit over $50 and that is a lot to pay for a low cost 3D printed router.

I could pick up or make a longworth style chuck.  This is made out of two flat pieces of material and could easily be 3D printed.  However the jaws on this chuck don't grip very well and it is really made for much larger material than the 90 mm we have to work with.

So a while back I ordered a small 2" chuck (48 mm) from Amazon for $12.  This is made out of some cheap cast material and is very stiff but after taking a file to it I managed to free it up enough to work.  It has a clamp diameter between 2mm and 65mm depending on how you arrange the jaws and if you are clamping on the inside or outside.  

It takes a 12 mm center bolt, or two 4 mm bolts on the back side for mounting.  It is good enough but not a great chuck. I wanted to see how it would work, and to use it as a template for a larger 3D printed chuck at some point in time in the future.

Anyway this is still just in the initial planning stage. I need to work out either how to swap the Y axis stepper with the new A axis rotary stepper, or I need a new 5 axis controller and a 5 axis version of GRBL or Marlin to go with it.  Still it has some promise, at least it will be educational.