Fanuc Mother Board A20B-1004-0070 A20B10040070 A2OB-1OO4-OO7O

FANUC A20B-1004-0070 | CZ Spindle Motor Encoder PCB — Japan Origin

Part Number: A20B-1004-0070

Manufacturer: FANUC Corporation (Japan)

Product Type: Spindle Motor Encoder PCB (Pulse Coder Board)

Board Series: A20B-1004

Application: FANUC CNC spindle motor position and speed feedback

Overview

The A20B-1004-0070 is a spindle motor encoder PCB — a pulse coder board — manufactured by FANUC for use in CNC spindle motor feedback systems. Spindle motors require position and speed feedback for the CNC to control them precisely.

The encoder PCB is the component that generates this feedback. It reads the mechanical position of the spindle motor shaft and translates the physical rotation into electrical signals that the spindle amplifier and CNC can process.

This board is the CZ type spindle encoder. CZ-type encoders in FANUC's spindle feedback system provide both speed feedback (the C-signal, proportional to rotational speed) and position feedback (the Z-signal, one pulse per revolution for orientation).

The combination makes spindle orientation possible — the CNC can stop the spindle at a precise angular position, which is essential for tool change operations where the spindle must present the tool in a defined orientation to the automatic tool changer.

The encoder PCB mounts inside the spindle motor housing on the motor's shaft or a dedicated encoder rotor.

In FANUC spindle motors, the encoder assembly typically consists of the PCB and a separately supplied rotor. Both must be correctly matched and installed for accurate feedback.

Key Specifications

CZ Encoder Technology — Speed and Position in One

In a CNC spindle application, the control system needs two types of feedback from the motor.

Speed feedback tells the spindle amplifier how fast the motor is actually spinning, so it can compare this to the commanded speed and apply corrective current to maintain the target RPM. Without accurate speed feedback, the spindle speed would drift with load changes — the cutting force would slow the spindle below the programmed speed.

Position feedback for orientation is different.

The spindle does not need continuous absolute position feedback for normal cutting — only a reference mark that allows the CNC to determine when the spindle has reached a specific angular position. 

The Z-pulse — one pulse per revolution — provides this reference. When the CNC commands spindle orientation, it uses this pulse to stop the spindle at the correct angle relative to the Z-pulse position.

The CZ encoder on the A20B-1004-0070 PCB delivers both signals simultaneously.

The C-signal provides the speed feedback; the Z-signal provides the orientation reference. Both are generated by the encoder mechanism this board contains, driven by the encoder rotor mounted on the spindle motor shaft.

Installation and the Rotor Relationship

The A20B-1004-0070 PCB is the electronic portion of the encoder assembly. The rotor — the mechanical component that rotates with the motor shaft and passes in front of the encoder's sensing elements — is a separate part with its own FANUC part number.

When replacing the encoder PCB, the existing rotor may be retained if it is undamaged.

If the rotor has been physically damaged — from a crash, debris impact, or corrosion — it must also be replaced. 

A damaged rotor produces unreliable or absent feedback signals even with a new PCB installed.

The alignment between the PCB sensor elements and the rotor is critical. Incorrect axial or radial clearance between the PCB and the rotor produces weak or intermittent signals. Encoder installation requires confirmation of the correct air gap as specified in the spindle motor maintenance documentation.

Spindle Encoder Failure — Fault Symptoms

When a spindle encoder PCB degrades or fails, the CNC receives incorrect or absent feedback from the spindle. The fault presentation depends on which signal has been affected and whether the failure is partial or complete.

A complete loss of the C-signal makes speed control impossible. The amplifier cannot close its speed loop. The spindle either runs at uncontrolled speed or faults immediately.

Loss of the Z-signal specifically disables spindle orientation without affecting normal cutting.

The spindle runs correctly at speed but cannot orient to the tool change position. M19 (spindle orientation command) faults. Tool changes are not possible.

This symptom with otherwise normal spindle operation points specifically to the Z-signal path of the encoder.

Intermittent encoder signals produce inconsistent faults — spindle orientation that works sometimes and fails others, or speed fluctuations under cutting load.

These intermittent faults are often related to connector condition or encoder-to-rotor clearance rather than a fully failed PCB.

FAQ

Q1: The spindle runs normally but M19 (spindle orientation) always faults. Is the A20B-1004-0070 the likely cause?

Spindle orientation faults specifically, with normal cutting at speed, points to the Z-signal path. The Z-signal is the one-per-revolution pulse used for orientation. Check the Z-signal at the encoder connector with an oscilloscope — it should produce one clean pulse per revolution.

If the Z-pulse is absent or very weak, the encoder PCB or its power supply is faulty.

If the Z-pulse is present but orientation still fails, the orientation parameter settings in the CNC require checking.

Q2: The spindle motor was involved in a crash. The encoder PCB looks undamaged but the spindle reports position errors. What should be checked?

After a crash, the encoder rotor may have shifted on the shaft or been physically damaged even if not visibly cracked.

A shifted rotor changes the air gap between the rotor and the encoder PCB, producing weak or distorted signals. 

Remove the encoder cover, inspect the rotor for damage and confirm its position is correct, and verify the air gap meets specification.

Rotor damage is often the cause of post-crash encoder faults rather than the PCB itself.

Q3: Can the A20B-1004-0070 be tested outside the spindle motor?

The PCB requires its associated rotor to generate signals. Without the rotor passing across the sensor elements, no output signal is produced.

Bench testing outside the motor assembly is impractical. 

Confirm the PCB is functional by installing it in the motor, applying power, and measuring the C and Z signals at the encoder connector while rotating the shaft. 

This test requires the motor to be accessible and the shaft rotatable — either in the machine or on a test stand.

Q4: The encoder PCB was replaced but spindle speed is erratic under cutting load. The orientation works correctly. What does this suggest?

Erratic speed under load with correct orientation suggests a C-signal issue — the speed feedback is intermittent or noisy. Check the encoder connector and its cable for intermittent contact.

Also check the C-signal amplitude at the amplifier input — a signal below the amplifier's minimum threshold produces unreliable speed feedback. 

If the signal is clean at the PCB connector but degraded at the amplifier, the cable is suspect.

Q5: Can the encoder PCB be cleaned if contaminated with coolant or chips?

Light contamination on the PCB surface can be addressed with appropriate electronics-safe contact cleaner.

However, coolant or chip contamination inside the encoder housing usually indicates a sealing failure that will recur. Identify and repair the seal failure before reinstalling the encoder. 

Coolant on the encoder sensing elements produces signal degradation even without physically damaging the PCB. 

Thorough cleaning and seal repair are both required for reliable long-term operation.