[Dan Fritz]

One thing I forgot to mention before was that many Z axis motors have electric brakes inside them. The brake is activated by REMOVING the power from the brake coil, which is usually 100 volts AC. When you turn the control on and the servo comes up ready, the brake is supposed to get power and release so the motor can turn easily. If the brake is stuck, or if it's not getting power, you'll have a motor that's fighting it's own brake. That'll trip the overloads pretty fast.

Also, if you have an ocilloscope, you can see the current feedback on CH11 on the Z axis servo board. These older Fanuc servos were 3-phase full-wave SCR drives, and they work pretty well when they're tuned up.

If you look at CH11 on the servo board with a scope, you will see a series of "bumps" that match up with the 3-phase current. You'll see 3 phases of 60-cycle bumps, which means 180 bumps per second on the scope. These bumps get taller and shorter depending on how much current is going to the motor. A flat line at 0 volts means no current, and bumps that range up to +10 or -10 volts means full current forward (or reverse). At jog speeds, these bumps shouldn't be much more than one volt either way. You will be able to compare the current feedback on the Z axis with the X and Y axes to see if things are normal. A scope display that shows two bumps and a "gap" indicates a blown fuse, a bad SCR, or a missing phase. I don't think this is your problem, because you would probably be blowing the other fuses rather than tripping the overload sensor.

The overload sensor is monitoring the DC current to the motor, not the AC current into the SCRs. A shorted motor could also do this, so also look for things like coolent goo in the motor, carbon brush dust from worn-out brushes, etc.

Here's a simple test you can perform (if you're careful!). Start with the power off, and:

1) Put a set of wooden blocks under the Z axis to keep it from dropping very far (leave them loose, but don't let things move down more than an 1/8 inch or so).

2) On the back of the Z servo amplifier, you will find some fuses. Pull them all out. Do not remove the X or Y fuses, just the Z fuses. If you have small motor (0 or 5 size) you will only have 3 black plastic 15A indicator fuses. If you have a larger motor (10, 20, or 30 size) you will have 3 cartridge fuses and 3 1.3A indicator fuses in parallel. In that case, remove all 6 fuses.

3) Turn the CNC on and observe the Z axis. If it slowly drifts downward, the ballscrew is turning easily (that's good). If it drifts a bit too far, you'll get an "Excess error" alarm in the Z axis. This would be normal, but you'll have to turn the CNC off/on again to continue with the test. If the Z ballscrew still can't be rotated by hand, you've either got a stuck brake, an unlubricated ballscrew, or a set of dry ways. Your mission is to find out which one is causing the friction.

The fact that the Overload sensor keeps tripping out indicates that the Z axis is just too hard to move. I'll bet there's a mechanical or lube problem that's making that Z motor work too hard.