Well, he did say:
TRA31 & 41 axis drives and FRSE spindle drive
TRA31 and TRA41 appear to be Mitsubishi servo drives, presumably original from 1986 (the date he gave for the machine). He said they are DC drives.
If I had to guess, I would say the motors have analog tachometer feedback to the drives, and resolver position feedback to the CNC. It is possible, however, that they have encoders.
It is possible that the old drives could be used -- they are almost certainly +/-10V analog velocity-mode drives. They would not necessarily perform that well, but they might. If position feedback is via resolvers, then you would have to figure out how and where to install encoders. Overall, using the old drives is likely to be more trouble than it is worth unless you are an electrical engineer with time on your hands.
It is possible that the old servo motors could be used, without the old drives. Maybe it could be done with an All-in-one DC unit, but more likely it would require Centroid's AC/DC drives. That would depend on the current draw (rated power / size) of the motors. It would again be necessary to install digital encoders on the back of the motors, but at least you would not have to retain the old tachometers there.
The path of least technical resistance would be to find out, or estimate, how much servo motor power and/or torque is required, and install new servo drives and motors. That would, of course, cost more money up front. It might end up costing less in the long run.
Using the old AC spindle drive is much more likely to be workable. That does not need to be as responsive as the servo drives. You just need to figure out what the input and output signals are (command to run forward and reverse; command to orient; optionally command to reset faults; status output for drive ok/fault; and status output for orient complete; and analog voltage input for the velocity reference). If you are lucky, those signals are shown in your original wiring diagrams for the machine. If not, you will want to see if you can locate a manual from Mitsubishi for the spindle drive unit.
This would be a pretty challenging project if it is your first CNC retrofit. Again, unless you are an electrical engineer with lots of time on your hands, be prepared and willing to hire help from people who have done it before. The research and planning required for a project like this is beyond the scope of what you can expect people to volunteer in a free support forum.
