Testing your 4th Axis Inhibit Circuit
4th axis - test your inhibit circuit
The inhibit circuit on your Fadal 4th axis is the one safety feature most shops never think about - until it costs them a $2,000 servo motor. Here's how to test it in under 10 minutes.
TL;DR
If you have a permanently wired 4th axis rotary table on your Fadal VMC, there is a small circuit board in your right-side cabinet called the inhibit circuit - and if it's not working, your rotary servo motor is quietly being cooked every time you clamp the brake. The inhibit circuit shuts down the servo amplifier while the brake is engaged so the motor stops hunting for position. Testing it takes about 10 minutes with a multimeter. If it fails the test, [CNCPros stocks WIR-0578 & WIR-0579 to get you back up. This is one of those checks that saves you $2,000+ for the cost of 10 minutes of your time.
The silent motor killer most shops never think about
When you wire up a 4th axis rotary table to a Fadal vertical machining center, most of the setup gets proper attention - the servo motor connections, the controller card, the amplifier, the brake plumbing. The one thing that gets skipped, more often than it should, is the inhibit circuit.
It doesn't make noise when it fails. It doesn't throw an alarm. It just silently lets your servo amplifier keep hammering current into a locked motor until something gives - and what gives is almost always the motor.
We've seen this plenty of times: a shop reports a dead rotary servo motor with no obvious cause, and when we dig in, the inhibit circuit either was never wired up correctly after a machine move, or a relay on the 1100-1 board quietly failed. The motor spent weeks slowly cooking every time the brake was applied. By the time anyone noticed, it was too late.
Here's how the circuit works, why it matters, and exactly how to test yours.
How servo motors "hunt" - and why that's usually fine
To understand why the inhibit circuit exists, you first need to understand how closed-loop servo systems hold position.
When a servo motor is told to stop and hold position, it doesn't actually go completely still. The control is constantly checking where the motor is via the encoder or resolver, comparing that to where it should be, and making tiny corrective movements. The motor moves a hair clockwise, the resolver reports back, the controller sends a correction signal to the amplifier to move counterclockwise, the motor moves back - and the cycle repeats dozens of times per second.
This is called "hunting," and it's completely normal. It's what allows a servo axis to hold position with sub-thousandth-inch precision even when you push on the table. The closed-loop feedback loop never stops working.
The problem arises when you combine that constant hunting with a mechanical brake.
Why a brake plus a hunting motor is a bad combination
The rotary brake on a 4th axis exists to lock the table rigidly during machining - holding the part against cutting forces without relying on the servo motor alone. The moment that brake clamps, the motor physically cannot move.
But the servo system doesn't know that. It still reads position via the resolver, still sees that the motor isn't exactly where it should be, and still sends correction signals through the amplifier. The amplifier ramps up current trying to force movement. The motor overcomes the brake just slightly, reverses direction, faces the same resistance again - and the cycle continues.
The amplifier keeps sending more and more power. The motor gets hotter. The amplifier gets hotter. If this goes on long enough, the motor windings cook.
A replacement rotary servo motor runs around $2,000 or more by the time you factor in parts and labor. The inhibit circuit that prevents this typically costs a fraction of that. The math is pretty obvious.
What the inhibit circuit does
The inhibit circuit is a relay board mounted in the upper left of the right-side electrical cabinet on your Fadal. Its function is exactly what the name says: when the rotary brake is applied, the inhibit circuit sends an "Inhibit" signal to the servo amplifier terminal, commanding the amp to shut down.
No drive current to the motor. No hunting. No heat buildup. The motor just waits, safely, until the brake releases and the control commands the next move.
The full operation sequence looks like this:
1. **Control commands the rotary to index** - the control releases the brake and tells the motor to turn.
2. **Brake releases** - the motor is now free to move.
3. **Motor indexes to position and stops** - the servo system closes the loop and holds.
4. **Brake clamps, inhibit activates** - the brake locks the table, and simultaneously the inhibit circuit shuts down the amplifier. The motor is now at rest - no hunting, no heat.
That last step is the one that saves motors. And if the inhibit circuit isn't doing its job, that last step just becomes: "brake clamps, motor keeps hunting, motor starts cooking."
Before you test - one SETP parameter to check
Before testing the inhibit circuit itself, verify that your Fadal control is actually configured to apply the brake automatically when the rotary stops. At the "ENTER NEXT COMMAND" prompt, type **SETP** and press **ENTER**, then navigate to the rotary-related parameters.
Look for the parameter that controls automatic brake application after the rotary indexes. If you're using a manual brake or prefer to control braking differently, skip this step - the test procedure below works either way. But if the brake isn't being applied automatically, the inhibit circuit never has a chance to do its job during normal operation.
How to test the 4th axis inhibit circuit
Testing is straightforward and takes about 10 minutes. You have three methods depending on what equipment you have on hand.
Start by commanding your rotary to index to any position and stop. Once it's stopped and the brake is applied, proceed with whichever method fits your shop:
Multimeter method
Locate the servo amplifier terminals labeled **MTR+** and **MTR-**. Set your multimeter to DC voltage and probe these two terminals.
With the inhibit circuit working correctly, you should read less than 5 VDC - ideally very close to zero. No system is perfect, so a small residual voltage reading is normal and nothing to worry about. What you don't want to see is significant voltage, which would mean the amplifier is still actively driving the motor while the brake is engaged.
Amp clamp method
If you have a clamp-style ammeter, clamp it around the MTR+ wire first, then the MTR- wire individually. With the inhibit active, you should read zero current on both. Any measurable current means the amp is still driving the motor.
AMP-0021 Visual Method
If your machine is equipped with an AMP-0021 amplifier, testing is even simpler. When the inhibit signal is active, a bright red LED on the face of the amp lights up. Index the rotary, let it stop with the brake applied, and look for that LED. If it's lit, the inhibit circuit is working. If it's dark, something in the circuit isn't doing its job.
What to do if the test fails
If your readings show significant voltage or current at the motor terminals after braking - or if the AMP-0006 LED never lights - you have a problem in the inhibit circuit. There are two likely causes:
1. The inhibit circuit board itself has failed. This is the most straightforward fix. Use WIR-0578 for 4th axis installations and WIR-0579 for 5th axis. Replacing the board is generally the fastest path back to a working circuit.
2. A relay on the 1100-1 board has failed, or there's a broken wire contact. This requires a bit more tracing - checking the relay outputs on the 1100-1 board and verifying continuity through the inhibit wiring path.
If you're not sure which path to take, call us at 208-888-9236 or email cncpros12@gmail.com. We've seen every variation of this failure and can usually point you at the right answer in a few minutes.