X, Y, or Z axis limit alarm on your CNC controller? This guide covers mechanical, inductive, and proximity limit switches on UAE CNC routers and mills — with RichAuto-specific diagnostic steps.
NPN/PNP mismatch between switch type and controller input
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Loose Switch Mounting
Switch moved on mount — now triggers too early or not at all
Step-by-Step Fix Guide
Follow in order — most faults are resolved by Step 3
1
Identify Which Axis Has the Limit Alarm
The RichAuto controller displays which axis triggered the limit: X+, X-, Y+, Y-, Z+, or Z-. The sign tells you direction (positive or negative end of travel). Note the exact alarm message before doing anything.
Screenshot or photo the alarm message. This tells you which switch (and which end of travel) is the problem — saves 30 minutes of troubleshooting.
2
Manually Jog Axis Away from Limit
In the controller's override/release mode (usually hold a specific key combination), jog the triggered axis in the opposite direction (away from the limit). Move 10–20mm clear of the switch. The alarm should clear.
If the machine won't jog at all: check if there's a hardware override switch on the control panel (sometimes labeled 'Limit Override' or 'Override'). Some machines require this to jog off a limit.
3
Inspect the Limit Switch Physically
Go to the axis that triggered and find the limit switch (usually a small mechanical or inductive sensor near the axis end). Check: Is the switch arm bent or broken? Is the actuator plate that hits the switch misaligned? Is the switch loose on its mount?
Mechanical limit switches wear out faster than inductive sensors in dusty CNC environments. If the switch arm is bent from repeated impacts, the switch needs replacement.
4
Test the Limit Switch Signal
With the machine powered and the axis clear of the limit, use the controller's I/O diagnostics page to monitor the limit switch input in real time. Trigger the switch manually by pressing it — the input should toggle 0→1. If it doesn't change, the switch is faulty or wiring is broken.
Access I/O diagnostics on RichAuto: Menu → Diagnose → Input/Output monitor. Watch the input labeled with the axis limit (e.g., X+LMT, Z-LMT).
5
Check Limit Switch Wiring
Trace the wiring from the limit switch back to the controller terminal block. Look for: broken wires (especially at flex points), loose terminals, corroded connectors, or pinched cables in cable chains. UAE machines in dusty workshops often have corroded terminals.
Use a multimeter in continuity mode. Test wire by wire from switch to controller. Any open circuit = broken wire.
6
Check for False Triggering (Noise)
If the alarm triggers randomly (not at end of travel): the limit switch wiring may be picking up electrical noise from spindle VFD cables. Check that limit switch wiring is shielded and routed away from high-power cables. Also verify the switch's NPN/PNP configuration matches the controller's input type.
Limit switch wires should NEVER run parallel to VFD power cables (U/V/W). Separate routing by at least 100mm, or use shielded cable.
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Frequently Asked Questions
First, jog the triggered axis in the opposite direction to move it off the limit switch. On RichAuto, hold the limit override key combination (varies by model — check your manual) to enable jogging while the alarm is active. Once clear, the alarm should reset automatically.
Random limit alarms (not at end of travel) are almost always caused by electrical noise from the spindle VFD interfering with limit switch wiring. Check that limit switch cables are shielded and routed away from VFD power cables by at least 100mm.
CNC machines use either mechanical (micro-switch) limit switches or inductive proximity sensors. Inductive sensors are more reliable in dusty environments like UAE machine shops. Al Azhar CNC supplies both types — send us your machine model for the correct part.
Yes. If a limit switch is faulty (always triggered), the machine cannot complete its homing sequence because it thinks the axis is already at the limit. Check I/O diagnostics to see if any limit input is showing active while the axis is in the middle of travel.
Use the controller's I/O diagnostics screen (on RichAuto: Menu → Diagnose → Input/Output). Watch the input for the suspect axis. Manually press/trigger the switch — the status should change from 0 to 1 (or vice versa). If it doesn't change, the switch or wiring is faulty.
A hard limit is a physical switch at the end of travel. A soft limit is a coordinate-based software boundary set in the controller. Both prevent overtravel. If your machine triggers soft limits frequently, check that your work coordinate zero is correctly set before running programs.