Used CNC control evaluation
The first thing I would try to determine is how well the previous owner took care of these machines. Things to look for:
Are the insides of the CNC cabinets CLEAN. If lots of coolant, dust and goo got inside, that means that the cabinet doors were left open (Not a good sign) The CNC cabinets all use air-to-air heat exchangers, so no outside air should be circulating into these cabinets. The door gaskets should also be in good shape, and there shouldn't be any holes drilled through the cabinet for cables unless they have sealed grommets or cable fittings.
Other cabinets with transformers, motor contactors, etc. may not be as tightly sealed as the CNC cabinets, and that's not usually a problem. Sometimes outside air is allowed to circulate inside these "power" cabinets. They should still look pretty clean inside though.
Unplug a few of the connectors on the motors, or any connectors that are vulnerable to coolant and dirt. The insides of the connectors should be clean and free of oil and coolant. Motors that have fans (like spindle motors) should have a clean filter.
Does it look like somebody added any boards or electronic components inside the CNC cabinet? If so, make sure that all cables and power supply connections were done properly. Lots of crazy non-factory wiring will make troubleshooting difficult.
Are the dust filters on the cabinet's heat exchangers in good shape? These are supposed to be cleaned occasionally to keep airflow through the heat exchanger going. A clogged filter means that the control was running hotter than normal. Missing filters mean that the heat exchanger is now dirty inside, and not very efficient. Also check the fans to see if they're running.
The parts that take the most wear & tear are the operator's panel buttons. Are they functioning OK? How about the CRT display ? Does it display clearly, or is the monitor on its last legs. Monitors for these controls can be replaced, but they're not cheap.
How do the servo motors look ? If they are DC motors, remove a few of the brush caps and see if the brushes are worn down to nothing. Shops that don't do any preventive maintenance never bother to check the brushes, and the servos start giving trouble.
Rapid each axis back & forth and listen carefully to the sound of the servo. It should be smooth, and you should not hear any grumbles or raspy noises. A bad motor, ball screw, or thrust bearing will sound rough. A smooth running servo doesn't mean that the ballscrew isn't worn out, but if you reverse directions quickly in the middle of the table travel, any "clunking" noises are a bad sign. It would be easy to take along a magnetic base indicator and check for backlash in each axis. A little backlash is normal, but a lot of backlash in the middle (and less on the ends of the axis travel) is a sign of a worn ball screw. Ball screw wear and backlash is always greatest in the middle of the work area.
As for the circuit boards themselves, there's very little you can do to spot a bad board. If the control runs, and all the servos run smoothly, The CNC control is probably in pretty good shape. Excess heat and dirt/dust/goo on the boards and inside connectors are the things that will make components fail.
I'd also look in the bottom of the cabinets for a pile of discarded blown fuses. That would be a sign of crashes and/or servo problems.
Also look for the original Fanuc paperwork inside the CNC cabinet. These controls were all shipped with a factory document showing the options list. The papers are usually in a pocket of some kind inside the cabinet. Some controls also come with a spare fuse kit.
Be sure that you also get all the operators and maintenance manuals. There will be some yellow Fanuc manuals, and also some manuals printed by the machine tool builder. You will need them.
