Alarm 25 Fifth Axis Servo Fault occurs when the NGC control detects a communication loss, feedback error, or drive fault on the B-axis servo system. This alarm is triggered by the servo amplifier's internal fault detection or when encoder feedback signals fall outside acceptable parameters defined in the axis drive parameters.
HaasOvertravel/LimitBasic1.5-3 hours
25
Overtravel Negative Z
Fifth Axis Servo Fault
This alarm indicates the IOCON board has detected a fault condition from the B-axis servo amplifier during operation or attempted motion. The servo amp monitors motor current, encoder feedback, temperature, and DC bus voltage and will fault if any parameter exceeds safe limits or if encoder communication is lost. The machine will stop all motion and require a reset before the B-axis can be re-enabled.
Root Cause Summary
Alarm 25 Fifth Axis Servo Fault occurs when the NGC control detects a communication loss, feedback error, or drive fault on the B-axis servo system. This alarm is triggered by the servo amplifier's internal fault detection or when encoder feedback signals fall outside acceptable parameters defined in the axis drive parameters.
Safety
Do not attempt to manually rotate fifth axis continuously, may cause servo feedback error
Disable fifth axis motion in control panel before performing mechanical diagnostics
Some fifth axis configurations may have mechanical brakes, verify lock status before work
Causes and Fixes
1
Power Down and Inspect Encoder Connections
Turn off main power and inspect the encoder cable connections at both the motor and the drive module. Look for loose connections, bent pins, or contamination.
Always power down completely before disconnecting encoder cables to prevent damage to encoder circuits.
2
Check Encoder Cable Routing
Verify encoder cables are properly routed away from power cables and not pinched or damaged. Check for oil contamination or coolant exposure on cable jacket.
Encoder cables should maintain at least 6 inches separation from AC power cables to prevent EMI interference.
3
Test Cable Continuity
Using a multimeter, check continuity on all encoder cable conductors from motor connector to drive connector. Replace cable if any opens or shorts are found.
4
Verify Encoder Signals
Access CURRENT COMMANDS > DIAGNOSTICS and observe the B-axis encoder position feedback while manually rotating the axis. Signals should increment smoothly without dropouts.
1
Check Drive Status LEDs
Locate the B-axis servo drive module and observe the status LEDs. Red LED indicates drive fault, flashing patterns indicate specific fault codes per Haas drive documentation.
Document the LED flash pattern - different patterns indicate specific internal drive faults that help determine if drive is repairable or requires replacement.
2
Verify Drive Power Supply
Check DC bus voltage at the drive module. Should read approximately 325VDC on 230V systems or 162VDC on 115V systems. Low voltage indicates power supply issues.
High voltage present - use proper PPE and insulated test equipment when checking DC bus voltage.
3
Swap Drive Module
If available, swap the B-axis drive module with a known good drive (like A-axis) to isolate drive failure. Clear alarms and test axis motion after swap.
Always record drive parameter settings before swapping modules in case parameters need to be restored.
4
Check Drive Parameters
Access SETTING > PARAMETERS and verify B-axis drive parameters 1520-1529 match machine specifications. Incorrect parameters can cause servo instability and faults.
1
Check Motor Mechanical Condition
Manually rotate the B-axis and feel for rough spots, binding, or excessive play indicating bearing wear. Listen for grinding or unusual noises during rotation.
Ensure axis brakes are released before manual rotation to prevent brake damage.
2
Test Motor Windings
Disconnect motor power cables and measure winding resistance between motor phases. All three measurements should be within 10% of each other, typically 1-5 ohms.
Also check winding-to-ground resistance - should read >10 megohms. Lower readings indicate winding insulation breakdown.
3
Verify Encoder Mounting
Check encoder mounting at motor for looseness or damage. Loose encoder mounting causes position feedback errors that trigger servo faults.
Look for metal particles around encoder mounting area which indicates bearing wear contaminating the encoder.
4
Check Motor Brake Operation
Access CURRENT COMMANDS > DIAGNOSTICS and toggle the B-axis brake output while observing motor shaft. Brake should engage/release smoothly without binding.
1
Check Grounding System
Verify machine ground connections are tight and corrosion-free. Check continuity between machine frame and electrical panel ground with less than 1 ohm resistance.
Poor grounding is the most common cause of EMI-related servo faults in shop environments with multiple machines.
2
Identify Interference Sources
Note if alarm occurs at specific times or when other equipment operates. Check for nearby welders, large motors, or VFD-driven equipment that could generate EMI.
Use an AM radio tuned between stations near the machine - EMI sources will create static or buzzing sounds.
3
Verify Cable Shielding
Inspect encoder and servo motor cable shielding connections. Shield drains should be properly terminated at drive end only, not at both ends which creates ground loops.
4
Check Power Quality
Monitor incoming AC power with oscilloscope or power quality analyzer for voltage spikes, harmonics, or transients that could affect servo drive operation.
1
Check for Crash Damage
Inspect B-axis mechanical components for signs of crash damage including bent brackets, damaged gears, or misaligned components that increase mechanical load.
Look for metal chips or debris in rotary axis that could cause binding and motor overload.
2
Verify Lubrication
Check B-axis lubrication points per maintenance schedule. Insufficient lubrication increases friction and servo motor load, potentially causing overcurrent faults.
Over-lubrication can also cause problems by attracting chips and debris - follow specified lubrication quantities.
3
Test Axis Load
Access CURRENT COMMANDS > DIAGNOSTICS and observe B-axis motor current during slow manual movement. Current should be consistent and within normal range (typically <50% of rated).
Sudden current spikes during movement indicate mechanical binding points that need investigation.
4
Check Following Error
Monitor B-axis following error in diagnostics during movement. Excessive following error indicates mechanical resistance preventing the axis from keeping up with commands.
DIY Feasiblemedium confidence
Call a technician if:
·Drive module replacement required
·Motor replacement needed
·Encoder replacement required
·Mechanical damage found
Prevention
Regular encoder cable inspection
Maintain proper grounding
Follow lubrication schedule
Keep rotary axis clean of chips
Monitor servo drive temperatures
Common Mistakes
Swapping drives without recording parameters
Not checking cable routing after maintenance
Ignoring intermittent alarms
Over-tightening encoder connections
Not verifying ground connections after electrical work
