Well, the first question is, how are the robots and CNCs going to interact? Followed by, how will the robots and CNCs communicate?
A robot reaching into a CNC without some sort of interlock is a good setup for a very expensive collision. But some old CNCs are so "standalone" that they provide nothing, or at best very little, in terms of I/O interfacing to other pieces of automation. I've literally seen people attach small pneumatic cylinders to their CNC control panel to push different buttons, and attach photo-diode sensors to different indicator lights, because the CNC controller provided no means of exchanging signals other equipment, only with human operators.
Just for starters, if your CNC has a door, you'll want a very solid way of letting the robot know that the door is fully open before reaching in. If the robot is going to grab the door and open/close it, you'll need reliable sensors for the "closed" position as well.
If the robot is loading stock, you can run into issues with stock size variance. Robot's don't usually have touch-off probing, so rely on whatever they're working with to be the same size every time. Clamping stock might be an issue if you have a manual vise -- trying to teach a robot to turn a vise handle is liable to be... interesting. And not terribly consistent in applied torque.
I would also look at the safety circuit side too and the retrofitting of robot systems, as most operate off dual channel now, with circuit analysis technology and therefore an assessment in line with your regional, national and machinery directives will be required to ensure the appropriate standards are followed relating to retrofitting to existing systems.
I know some robot systems have options for single channel, but wouldn't like to speculate whether they would be permissible as a retrofit solution or if some conversion from single to dual channel is required to upgrade the older to the newer dual channel conditioning, this you would have to research further.
Not sure if I'm off the mark here, but you mentioned 1975....
When you mentioned an interlock you meant a chuck interlock?
More like, a set of interlocks. The door would be one -- you need positive information on whether it's open, closed, or "unkown". Likewise, you need some sort of interlock to ensure that a programming error can't start the CNC while the robot is inside its working volume, or tell the robot to reach into the CNC while the CNC is still moving. Interlocking the chuck would depend on whether the robot has to be holding the stock while the chuck closes on it, or needs to grab the finished work piece before the chuck opens.
Ladder? Wouldn't a CNC machine normally control I/O through G-Code?
If you can't control interface signals through I/O, then it gets more complex. Say, for example, the CNC has an automatic door that it can open/close from inside a G-Code program. You could add sensors to the door such that the robot could detect the door opening, and trigger the robot's unload/reload program based on that. Probably you would want to wire an extra relay in series with the powered door, controlled by the robot, so that the robot could block the CNC from moving the door as long as the robot was not clear.
It all depends on what signals or hardware you have how much access to.
Ladder? Wouldn't a CNC machine normally control I/O through G-Code?
All CNC machines I know have a PLC running in the background doing things like ranking I/O, making plausibility checks for the I/O, converting M-codes to the specific I/O, handling options and so on.
The G-code can't handle I/Os direct, they all are ranked through a PLC that does some logic that's neccessary for the M functions controlling the I/O.
The G-code can't handle I/Os direct, they all are ranked through a PLC that does some logic that's neccessary for the M functions controlling the I/O.
Well, I learned something! 
My CNC experience is pretty limited, but the ones I've worked with had just a small number of relays that were controlled directly by G-Code, with no background ladder-logic processors.
First thing I would check for this type of cnc machine:
- List of M-Codes
- Circuit diagram
Normally the outputs are controlled by cnc code (M-commands) directly; maybe start and stop as well (depends of cnc machine)
Normally the outputs are controlled by cnc code (M-commands) directly; maybe start and stop as well (depends of cnc machine)
M51 and M61, and so on, on Haas CNCs, IIRC.
I've done this once before and from what I can remember the Mfin relays were one of the main communication points, its also depends one how old your cnc is, and what your using the robot for
