Is there a rule of thumb for fixture thickness vs span? Or do people just eyeball it?
Adding a middle support between parts would probably help reduce fixture deflection, but nobody chimed in to clarify if its the fixture that needs to be "stiffer" or the clamping method...
I took a CAD model of a 9x35" stock trunnion and modified it to fit some centric grippers and a pair of 16" rotors. Using Fusion360 I ran a quick analysis. The stock trunnion in cast iron had .0001" max deflection, using the same parameters on my modified design it showed .002" deflection (so .004" total with a flip) which doesn't seem too horrible, especially considering I switched it to 6061. Spending more than 15 minutes designing it, or just switching to steel could cut that in half which would be acceptable.
I'm no stress analyst, so I'm not going to bet my life on this but its a little confidence inspiring.
I know Schunk makes both robot grippers and pneumatic vises, are those grippers capable of holding parts for machining? They have longer strokes than a lot of the pneumatic 3 jaw chucks I see which would help cut down on jaws, using a step in each set of jaws to do 2 sizes I could make 5 sets of jaws to cover the entire range.
Just Googling for comparisons I found this
Precision Stationary (Milling) Air Chuck 125mm chuck has 970 KGF and 3mm stroke
A Schunk PZN 125 has 3330 N and 12mm stroke. According to Google that would only be 340 KGF, but it would only require 1/4 as many jaws.
PZN 125 specifies a workpiece weight up to 15.5kg, that's quite a bit more than the largest rotor.