SP9027 Means: Communication between the Spindle Position coder and the spindle amplifier Module is disconnected or abnormal.
External encoder (Position Coder) is to provide direct real-time feedback from the actual mechanical components( spindle nose) rather than inferring it from the motor, eliminating errors caused by belts & gears. It is typically mounted directly on the spindle shaft or connected via a 1:1 timing belt.
Use of External encoder(Position Coder)
Synchronization for Threading: An external encoder is used for synchronization of threading. For the machine to cut a thread, Z axis motion must be perfectly synchronized with the spindle’s rotation. Built in motor encoder can not account for belt slippage or transmission error between the spindle motor and spindle. External encoder tells to the CNC the exact angle of the spindle, allowing the tool to track the thread pitch precisely.
Many times we have seen that due to vibrations in external encoder threading problem generated in component so also need to check vibration of external encoder if threading problem is occurring in component.
C-Axis Control (Turn Mill):If your lathe has live tooling (Milling capabilities), Spindle must act as a rotary servo axis(C axis) to index to specific angles(e.g. drilling holes at 0degree,90 degree,180 degree). An external encoder provides high-resolution feedback required to hold the spindle stationary at precise angles against cutting force.
Rigid Tapping :Rigid Tapping requires the spindle to reverse rotation at a precise depth while the z-axis reverses simultaneously, external encoder ensure the tap follows the exact same path out of the holes as it went in, preventing broken tapes or stripped threads. In many Milling machines MTB do not use external encoder in that case if spindle belt slippage occurs then thread may be stripped or tap may be broken.
True RPM display: On belt-driven spindles, Motor RPM and Spindle RPM can differ due to pulley ratios or belt. The external encoder displays the actual RPM on the screen.
Likely causes of alarm SP 9027
- The coder cable at the spindle amplifier coder port (JYA3/JYA4) may be unplugged or loose.
- The external encoder timing belt may be loose or slipping.
- The shaft of the external encoder may be loose, damaged, or broken.
- The pulley teeth of the external encoder may be worn out.
- The pulley or timing belt of the main spindle may be worn out.
- The external encoder itself may be faulty.
- The external encoder cable may be faulty.
- Electronic noise or poor shielding/grounding on the feedback cable may be reducing signal amplitude.
How to resolve the alarm SP9027
1.Confirm the symptom:
Note down when the alarm appears: at power-up, at spindle stop, or when the cable moves with the axis travel. Intermittent appearance during motion often points to cable/connector movement or EMI issues.
2.Visual inspection ( Panel and Motor Side)
- Inspect the external encoder connector. Oil mist and coolant aerosols can condense inside the connector housing, creating conductive residue that degrades signal integrity between the external encoder and the spindle amplifier. Clean and dry the connector thoroughly to restore reliable encoder communication.
- Inspect the spindle sensor/encoder feedback cable from the motor to the spindle amplifier: look for cuts, crushed sections, oil/coolant wicking, and bent or oxidized pins at both ends. reseat firmly, and ensure keyed orientation.
- Verify the feedback cable is not routed in the same bundle as the spindle power cable; route separately and ensure proper cable shield termination at the designated shield clamp/ground.
3) Check connector designations and seating at the spindle amplifier
- Identify the coder connector (commonly JYA3) on your spindle amplifier; reseat these connectors and inspect for pin push-back or damage.
4) Encoder signal quality
- External encoders serve as critical feedback devices in CNC systems, providing position and velocity data to machine controllers. Over extended operational periods, encoder signal quality progressively deteriorates through environmental contamination, mechanical wear, electrical degradation, and thermal stress.
- Signal degradation manifests as reduced peak-to-peak voltage amplitude, increased signal-to-noise ratio (SNR) degradation, and phase jitter. As signal amplitude approaches controller input threshold sensitivity levels, pulse detection becomes unreliable, resulting in missed edge recognition events.
- This deterioration directly compromises system performance through multiple failure mechanisms. Reduced signal integrity causes cumulative positioning errors, servo loop instability, and spindle synchronization failures. Degraded encoder feedback triggers protective alarms such as “Disconnect Position Coder” suspending programmed operation and halting spindle rotation.
- Physical degradation occurs through metal chip and coolant residue accumulation on optical surfaces, bearing wear inducing runout and alignment drift, connector corrosion increasing contact resistance, and thermal cycling causing material fatigue in optical and electronic components.
- Preventive maintenance includes regular inspection of encoder optics and connectors, implementation of protective covers to minimize contamination and scheduled encoder replacement based on operational hours and environmental severity. Systematic signal quality monitoring ensures sustained dimensional accuracy and operational reliability in CNC machine tools
5) Verify sensor type vs. parameters
- Confirm the spindle sensor/encoder type installed matches the spindle parameter bits that select the sensor/coder type. A mismatch (e.g., wrong encoder type selection) will present as an abnormal/disconnected coder signal. Correct the parameter and cycle power if required.
- Parameter to set external encoder is as below with detail.
- To run the machine in emergency you can bypass the external coder by parameter 4002 but be clear that threading operation will not work in turning machine .After bypassing the external encoder you have to set feed and rpm of the program because now you are using rpm of spindle motor.
For Bypassing the external encoder we need to make zero of bit no.1 of parameter 4002 as per below image.
We will read in detail how many different types of encoder ,sensor we can set by Parameter 4002 .
6) Shielding and EMI mitigation
- Ensure 360-degree shield termination at the amplifier end, proper ground bonding, and physical separation from high-current motor/power cables to prevent electromagnetic interference that can mimic disconnection.
7) Cable replacement test
- If the alarm persists after reseating/adjusting and EMI checks, substitute a known-good feedback cable between motor sensor and spindle amplifier to eliminate cable faults. Cable failures are a frequent cause of “position coder disconnection.”
8) Sensor and amplifier checks
- If a good cable does not resolve it, swap in a known-good spindle motor encoder (or Position coder) if practical. Persistent faults after cable and sensor replacement may indicate an issue on the spindle amplifier control PCB and warrant amplifier repair/replacement.
9) Power-up and verify
- Restore power, clear alarms, and test low-speed spindle jog, then run through typical RPM ranges while monitoring that SP9027 does not recur. Pay attention to positions where cable flexes or where coolant spray is present.
10)Alarm is coming intermittently
If Disconnect Position coder alarm is coming intermittently then you have to ensure encoder pulley,encoder timing belt is ok or not.
Most probably cause of intermittent alarm is due to spindle pulley teeth is worn out or encoder timing belt may be damaged.
Practical tips
- If the alarm appears only when the spindle is stopped, focus on parameter/sensor selection and connector integrity; if it appears when axes move, focus on cable flex points and shielding/EMI.
- Keep feedback connectors protected from coolant; reroute or sleeve cables to prevent future ingress and intermittent signal loss.
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