Thanks @krzysztof especially for the DaVinci link. I’m really not thinking about applying ink to to anything at this point though. I have considered using a translucent glass powder in CMYK. This could be printed in a bed of refractory support material and then fired in a kiln. Porcelain and bone china could also be done like this. Do you think that the 90 degree turned Y shape that you describe would have enough reach inward for smaller or more complex shapes?

I suggested it because it could easily reach into center, you don't need more. It's just a matter of rail height.  Try drawing it on paper in different positions. Unfortunately with this setup max print height will be approximately half of printer height at biggest distances from center (and almost full height at center). Also I used small letter 'y' on purpose.

@krzysztof that does sound like a more manageable approach. I was letting my thoughts run wild and was thinking that I might see if I could print the inside of an object white and then use four extruders for a cmyk dot printing as if I were using a color cartridge. Anybody here know how to do that?

For color you would need mixing extruder (http://www.reprap.org/wiki/Diamond_Hotend). It's very hard to do this with different extruders. You could of course have printer cartridge instead of one extruder but this has been already done and effects are not that good. Using single extruders for this will have even worse effect, because you will have too small resolution, 0.2 at best but this will be very hard to control accurately (severe stringing with such short extrusions).

hmmm. I understand. But I don’t like thinking that something can’t be done. The powder based systems are getting pretty good color. Perhaps looking into a material that needs to be sintered afterward would be the way to go. But that particular idea can be tabled until the arms are operational. 

Another issue with using 3 color as a building material is the inability to add black and white. On paper you have a white background so there is no issue with its lack. There is a separate black cartridge which the software substitutes when the cmy reaches a certain threshold. Otherwise the print looks muddy. So I don’t see where the diamond hotend could be utilized for full color on its own. 

It's not like it can't be done, but was already done and effects of printing ink over plastic were ratther poor. Maybe you can do something with this, better method is yet to be invented, I'm just giving you ways things were done poorly. As for printing ink over plastic: https://www.xyzprinting.com/en-US/product/da-vinci-color I've seen early versions, latest seem rather good.

If you want to have rotating build plate, I suggest only Z/R movement for each extruder (Z - vertically, R - radially). This will use only 2 steppers for each arm. This could be done with two sliders on two parallel rails (so that they don't rotate), upper slider would have longer arm and lower slider would support upper arm with shorter arm connected in middle of upper one (think letter 'y', but rotated 90deg CCW). Build plate could be rotated from outside.

I would like to develop this concept into a functioning 3d printer. Would anyone out there be interested in collaborating? The picture shows the armature reaching all the way across just to demonstrate that it could. I am more interested in developing a build plate that rotates to the active tool so that they will always be operating in their most stable range. 

Interesting design, though the working area is in the worst part of the delta's reach in this case, where you have the largest errors — I hope this can be compensated with the large vertical travel of the arms.

I wonder if a similar design could be useful for a pick-and-place machine.

Yes. In the model I was trying to demonstrate that the arm could reach across if necessary. However I’m also considering having a rotating build plate that can turn with the tool change to keep the active extruder in its most stable range. I’m pretty new so all thoughts and contributions are welcome. I need to read more about pick-and-place technology.