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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A860-2120-V003 |
Part Number: A860-2120-V003
Series: A860-2120
Manufacturer: Fanuc Corporation
Description: AiBZ SENSOR W/MOUNTING RING
Application: Fanuc built-in AC spindle motor — position and speed feedback
Includes: Sensor unit + integrated mounting ring
Condition: New / Refurbished
The Fanuc A860-2120-V003 is an AiBZ encoder sensor for Fanuc's built-in AC spindle motor series — the sensing device that gives the spindle drive its real-time rotor position and velocity data.
This variant ships complete with the mounting ring, which is the precision mechanical interface that fixes the sensor body at the correct radial position relative to the motor's tone wheel or magnetic target track, establishing the air gap and alignment that accurate encoder readout depends on.
In a Fanuc built-in spindle motor, the motor rotor is mounted directly on the machine spindle shaft — there is no intermediate coupling, no belt, no separate motor axis.
The spindle runs true to the motor's rotor, and the AiBZ sensor provides the position feedback that makes this arrangement function as a closed-loop servo axis rather than a simple variable-speed drive. Orientation control, C-axis contouring, tapping cycles, and thread cutting all require this feedback loop to be working correctly; a failed or degraded AiBZ sensor takes all of these spindle functions offline simultaneously.
What sets the A860-2120-V003 apart from the V004 variant (which omits the mounting ring) is the hardware completeness. In machine tool maintenance, having the mounting ring eliminates a sourcing and fitment variable: the ring supplied with the sensor is matched to the sensor body's mechanical tolerances, ensuring the assembled unit meets the air gap specification without shimming or improvised spacer hardware.
| Parameter | Value |
|---|---|
| Part Number | A860-2120-V003 |
| Series | A860-2120 |
| Includes | Sensor unit + mounting ring |
| Application | Fanuc built-in AC spindle motor |
| Function | Rotor position / speed feedback (AiBZ encoder) |
| Manufacturer | Fanuc Corporation, Japan |
The AiBZ designation is Fanuc's naming convention for the absolute-capable incremental encoder sensor used on the αi-series built-in spindle motors. The "Ai" portion identifies the motor series; the "BZ" identifies the encoder interface type — B referring to the incremental pulse output channels (A, B phase quadrature) and Z the once-per-revolution zero pulse that establishes the absolute reference position.
On each power-up, the spindle control initialises the encoder by seeking the Z pulse — rotating the spindle through an orientation move to find the reference. Once the Z pulse is found, the drive knows the absolute shaft position within one revolution, and the A/B incremental counting maintains tracking from that point forward. This initialisation sequence is the "spindle orientation" function that must complete successfully before the machine is ready for operation.
The sensor detects a physical pattern — a magnetic or optical target track — on the motor shaft or a tone wheel mounted to it.
The air gap between sensor face and target track determines the signal amplitude and duty cycle. If the sensor is installed with insufficient gap, mechanical contact with the rotating target is possible; if the gap is too large, the signal amplitude drops below the receiver threshold and the drive loses position. The mounting ring in the A860-2120-V003 establishes this gap accurately as part of the installation.
The spindle drive uses the AiBZ encoder feedback in several distinct ways depending on the operating mode commanded by the CNC program. In speed control mode — the default for straight cutting passes — the encoder provides velocity feedback to the drive's speed regulator, which maintains programmed RPM against variable cutting loads.
In spindle orientation mode, the drive switches to position control mode and uses the Z pulse and incremental position to bring the spindle to its programmed angular stop position for tool change.
In C-axis or tapping mode, the CNC uses the spindle position as a servo axis in its interpolation calculations, requiring the full position feedback loop to be active.
Any encoder-related alarm on the CNC — AL-300 series on Fanuc 0i/30i controls — that points to the spindle encoder circuit typically indicates either the sensor, the cable, or the amplifier's encoder receive circuit.
The A860-2120-V003 is the correct replacement part for sensor-specific failures in the A860-2120 sensor series with the original mounting ring still serviceable in the machine.
The A860-2120-V003 is used in Fanuc's built-in spindle motor product line — spindle motors designed for direct installation into machine tool spindle cartridges rather than external belt or coupling connections.
The motor's model designation and the machine builder's documentation determine whether the A860-2120 sensor series is the correct match for the specific spindle motor variant.
Installation requires access to the motor's sensor cavity, typically from the rear of the spindle cartridge, and involves removing the failed sensor, fitting the A860-2120-V003 with its mounting ring into the sensor housing, verifying the air gap, and reconnecting the encoder cable.
After reassembly, the spindle must complete an orientation move to verify the Z pulse is detected correctly before returning the machine to production.
Q1: What is the difference between the A860-2120-V003 and the A860-2120-V004?
The V003 includes the mounting ring; the V004 does not. Both are the same sensor element from the A860-2120 series and serve the same AiBZ position feedback function.
When the original mounting ring on the installed sensor is intact and undamaged, the V004 provides a cost-effective sensor replacement — the existing ring remains in the machine and the new sensor element is installed into it.
When the mounting ring is damaged, corroded, or unavailable, the V003 provides the complete assembly with a new ring matched to the sensor.
Q2: What CNC alarms indicate an AiBZ sensor failure?
On Fanuc Series 0i, 30i, 31i, and 32i controls, spindle encoder alarms typically appear as SP-xxxx alarm codes displayed on the operator panel. Common codes related to the sensor feedback include SP-9001 (spindle encoder disconnection), SP-9002 (spindle encoder signal failure), and similar codes in the SP-9xxx range.
The CNC's alarm history and the spindle amplifier's diagnostic LED can both point to the encoder feedback circuit. Before replacing the sensor, verify that the encoder cable connector at both the motor and amplifier ends is clean and fully engaged.
Q3: How is the air gap set when installing the A860-2120-V003?
The mounting ring in the A860-2120-V003 sets the air gap mechanically — the ring positions the sensor face at a fixed offset from the motor housing face when correctly seated. The actual gap between sensor face and the magnetic target on the rotating shaft is determined by the combination of the ring's geometry and the shaft-to-housing dimension.
After installation, verify correct operation by monitoring the spindle's encoder signal on the CNC's diagnostic screen during a slow orientation move and confirming the Z pulse is received cleanly.
Q4: Is the A860-2120-V003 compatible with all Fanuc αi built-in spindle motors?
The A860-2120 sensor series is designed for specific Fanuc built-in spindle motor models. Compatibility depends on the motor's frame size and the sensor cavity's mechanical dimensions. Fanuc's spindle motor documentation for each motor model specifies the compatible sensor part number.
Using the incorrect sensor series — different physical dimensions — risks incorrect air gap, mechanical interference, or incompatible signal levels. Always confirm the sensor part number against the motor's nameplate or Fanuc's spindle motor spare parts list before ordering.
Q5: What is the service life of the AiBZ sensor, and when should it be proactively replaced?
Fanuc does not publish a fixed time-based replacement interval for AiBZ sensors; they are replaced on failure or when diagnostic evidence suggests degradation. In practice, sensors in machine tool environments accumulate wear from vibration, thermal cycling, and coolant contamination over years of service.
A preventive replacement policy — replacing the sensor during a scheduled overhaul of the spindle motor assembly — is reasonable for machines operating double or triple shifts, where unplanned spindle downtime has high production impact.
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