Purchase now the A860-2120-T411 — Atlantechdrives provides this FANUC AiBZ Sensor Assembly complete with 256-tooth gear and mounting ring, the spindle speed and position feedback sensor for FANUC built-in AC spindle motors. Essential for restoring accurate spindle feedback in CNC machining centres and turning centres. Available with express worldwide delivery.
What Is the A860-2120-T411?
The A860-2120-T411 is a FANUC AiBZ Sensor Assembly — a complete kit comprising the AiBZ magnetic sensor head (A860-2120-V004), a precision 256-tooth gear ring, and the associated mounting ring hardware. This assembly provides the spindle speed, position, and orientation (Z-pulse) feedback required by FANUC Alpha i series spindle amplifiers and CNC controllers for accurate spindle control. The AiBZ sensor system uses magnetic sensing technology to detect the passing teeth of the gear ring as the spindle rotates, generating speed pulses (the «Bi» signal) and a single reference pulse per revolution (the «Z» signal) that the spindle amplifier uses for speed regulation and spindle orientation (CSS — Constant Surface Speed, and rigid tapping). The 256-tooth gear produces 256 pulses per revolution for the speed feedback channel. The A860-2120-T411 complete assembly is the replacement kit for machines where the gear ring, mounting ring, or sensor head has been damaged — typically by mechanical impact, coolant contamination, or chip ingress in turning and machining centre environments. FANUC built-in spindle motors (integrated into the machine spindle housing rather than belt-driven) are particularly common in high-speed vertical machining centres and turning centres where spindle feedback accuracy directly determines surface finish quality, thread pitch accuracy in rigid tapping, and tool-change orientation precision.
Key Technical Specifications
- Part Number: A860-2120-T411 (complete assembly)
- Assembly Contents: AiBZ sensor head + 256-tooth gear ring + mounting ring
- Sensor Head Reference: A860-2120-V004 (AiBZ sensor unit)
- Gear Tooth Count: 256 teeth
- Sensor Type: Magnetic proximity (non-contact)
- Output Signals: Bi (speed pulse, 256 PPR) + BZ (Z-pulse, 1 PPR reference)
- Application: FANUC built-in AC spindle motors (Alpha i series)
- Compatible Amplifiers: FANUC Alpha i series spindle amplifiers (A06B-6141, A06B-6044 series)
- Compatible Controllers: FANUC Series 16i, 18i, 21i, 30i, 31i, 32i, 0i-D/F
- Manufacturer: FANUC Ltd., Oshino-mura, Yamanashi, Japan
- Weight (complete assembly): approximately 5.4 kg
Spindle Feedback Architecture and Signal Function
FANUC built-in spindle motors using the AiBZ sensor system operate with a separate-type speed feedback encoder rather than a shaft-mounted pulse coder. The gear ring is typically pressed onto or secured to a rotating element of the spindle assembly — either the spindle shaft itself or the motor rotor housing — while the AiBZ sensor head is mounted in a fixed bracket at a precise air gap from the gear ring teeth, typically 0.5 to 1.0 mm. As the spindle rotates, the sensor head detects the magnetic flux variation as each of the 256 gear teeth passes, generating the Bi speed pulse train. A single reference tooth on the gear ring (typically a missing or modified tooth) generates the BZ Z-pulse once per revolution, used for spindle orientation during automatic tool changes and the phase reference for rigid tapping synchronisation. Misalignment of the air gap — too small causing sensor contact with the gear ring, or too large causing signal loss — is the most frequent cause of spindle encoder alarms (SP9001, SP9002 on Series 30i/31i) and must be set precisely during installation. The 256-tooth count is one of several standard configurations in the A860-2120 series — other variants in the same family include 128-tooth (T211), 64-tooth (other T-suffix variants), and different mounting ring configurations — so always confirm the tooth count required for your machine before ordering.
Installation Advice
When installing the A860-2120-T411 assembly, the most critical parameter is the air gap between the AiBZ sensor head and the gear ring tooth tips — set this to 0.5 mm ±0.1 mm using a feeler gauge with the spindle stationary. Excessive air gap causes weak signal amplitude and spindle encoder alarms at high speed; insufficient gap risks physical contact between the sensor and the rotating gear ring, which will immediately destroy the sensor head. After setting the air gap, rotate the spindle slowly by hand through at least two full revolutions before powering up, checking that the gear ring does not contact the sensor at any rotational position — axial and radial runout of the spindle assembly can cause contact at specific angular positions even if the static air gap is correct. Secure the sensor cable with strain relief to prevent vibration-induced connector damage — loose sensor cables are a common cause of intermittent spindle encoder alarms that are difficult to diagnose without oscilloscope verification of the Bi signal. After installation, confirm correct signal output by monitoring the spindle encoder signal on the FANUC diagnostic screen (Series 16i/18i: parameter screen, spindle monitor) at low speed (100–200 RPM) before running the spindle at full speed.
Frequently Asked Questions
Q: What spindle alarms indicate a failed or misaligned A860-2120-T411 AiBZ sensor?
A: On FANUC Series 16i/18i/21i controllers, typical alarms include SP0749 (spindle sensor disconnected), SP9001 (spindle motor overspeed), and SP9030 (feedback signal error). On Series 30i/31i/32i, the equivalent alarms are in the SP9xxx range. Intermittent alarms that appear only at high spindle speeds or after thermal expansion of the machine indicate an air gap that is borderline rather than a completely failed sensor.
Q: Can I replace just the AiBZ sensor head (A860-2120-V004) without replacing the entire A860-2120-T411 assembly?
A: Yes, if the gear ring and mounting ring are undamaged. The sensor head A860-2120-V004 is available as a separate replacement component. However, if the gear ring teeth are chipped, worn, or the ring is cracked from mechanical impact, the complete A860-2120-T411 assembly including the new gear ring must be replaced to restore accurate feedback signal quality.
Q: Is the A860-2120-T411 compatible with all FANUC built-in spindle motors?
A: The A860-2120-T411 (256-tooth configuration) is designed for specific FANUC built-in spindle motor models in the Alpha i series. Not all built-in spindle motors use the same tooth count — some use 128 or 64-tooth gear rings. Always verify the tooth count of the original gear ring on your machine before ordering to ensure signal compatibility with the spindle amplifier parameter settings.
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