CNC Spindle Not Spinning? Troubleshooting Guide

When your CNC spindle will not spin the problem is usually in one of five areas: the M-code command, the VFD settings, the wiring, the spindle motor itself, or the controller configuration. This guide walks through each area systematically so you can find and fix the problem without replacing parts unnecessarily.

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Quick Diagnostic Checklist

Initial Checks

Start with these checks before diving into any single area:

☐ Is M03 in the program before the spindle should start?
☐ Is the spindle speed greater than zero? (S0 = spindle off)
☐ Is the emergency stop released?
☐ Is the machine door closed? (Door interlock switch)
☐ Does the VFD display show a fault code?
☐ Is the spindle power cable connected at both ends?
☐ Can you hear the VFD relay click when M03 is sent?

Next Steps

If any of these checks reveal an obvious problem fix it and test again. If nothing obvious appears from the quick checks work through the detailed sections below in the order they are presented.

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1. Check the G-Code

M-Code and S-Word

The most common reason a spindle does not start is missing or incorrect G-code. The spindle needs two commands to run:

M03 S3000  ; Spindle on, clockwise, at 3000 RPM

M03 tells the controller to start the spindle. S3000 tells it how fast. If either is missing the spindle will not spin.

Common G-code issues:

Issue	What happens	Fix
Missing M03	Spindle never gets the start command	Add M03 before the cutting operation
S0 (speed = 0)	Spindle gets the command but speed is zero	Change S0 to S[desired RPM]
M05 before M03	Spindle stops before it starts	Remove the extra M05
Wrong M-code	M04 instead of M03 (counterclockwise)	Check direction — use M03 for standard tools
M03 in wrong position	Spindle starts too late or too early	Move M03 to the correct program position

MDI Test

Test by sending M03 S3000 in MDI mode. If the spindle runs from MDI the problem is in the program. If not, the problem is in the machine configuration. If it still does not run the problem is in the machine configuration.

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2. Check the VFD Settings

The Variable Frequency Drive (VFD) controls the spindle speed by varying the frequency of the power supplied to the motor. If the VFD is not configured correctly the spindle will not run.

Verify the VFD is Powered

Check that the VFD display is on and shows a standby state. If the display is dark check the power input to the VFD. Most VFDs run on 220V single-phase or three-phase power.

Check for Fault Codes

A VFD in fault state will not start the spindle. Common fault codes:

Fault	Meaning	Fix
Overvoltage	Input voltage too high	Check supply voltage
Undervoltage	Input voltage too low	Check supply voltage and wiring
Overcurrent	Motor drawing too much current	Check for shorted motor windings
Overheat	VFD internal temperature too high	Clean cooling fan, reduce load
Ground fault	Current leaking to ground	Check motor wiring for shorts

Clear the fault by power cycling the VFD (wait 30 seconds after turning off before turning back on). If the fault returns immediately the problem is in the VFD or motor wiring.

VFD Parameter Check

VFDs have dozens of configurable parameters. The critical ones for spindle operation are:

Parameter	Typical Value	What It Controls
Maximum frequency	400 Hz	Max spindle speed
Minimum frequency	0 Hz	Min spindle speed
Acceleration time	5-10 seconds	How fast spindle ramps up
Deceleration time	5-10 seconds	How fast spindle slows down
Motor rated current	Match your spindle	Overload protection
Control mode	0-10V or Modbus	How controller signals speed

If the VFD was working before and stopped it is unlikely the parameters changed by themselves. If the spindle has never worked the parameters may be set incorrectly from the factory. Compare every parameter against your spindle’s specifications.

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3. Check the Wiring

Loose or damaged wiring is a common cause of spindle failure, especially on machines that have been moved or modified.

Power Wiring

Check all connections in the spindle power circuit:

1. VFD input terminals (L1, L2, L3) — should be tight
2. VFD output terminals (U, V, W) — connected to spindle motor
3. Spindle motor connector — plugged in and locked
4. Ground wire — connected at both ends

Use a multimeter to check continuity. Power off the VFD and disconnect it from power before testing continuity.

Control Wiring

The signal that tells the VFD to run comes from the CNC controller through control wires:

1. Check the wire from the controller output to the VFD input terminal
2. Verify the 0-10V speed control signal (if used) reaches the VFD
3. Check the common ground between controller and VFD

On GRBL-based machines the spindle control is typically through the PWM output on the Arduino or a separate spindle control board. Verify that the PWM signal is present when M03 is active.

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4. Check the Spindle Motor

If the VFD is working correctly and the wiring is intact the problem may be in the spindle motor itself.

Brush Wear (DC Spindle Motors)

DC spindle motors (common on budget CNC routers) have carbon brushes that wear down over time. Worn brushes lose contact with the commutator and the motor stops running.

Signs of worn brushes:

• Spindle runs intermittently
• Spindle runs slow even at full speed
• Visible sparking at the brush area
• Motor runs hot

Fix: Replace the brushes. They cost $5-$15 per pair and take about 10 minutes to replace on most motors. Keep spare brushes on hand for production machines.

Bearing Seizure

Spindle bearings that have lost lubrication or been contaminated with debris can seize and prevent the spindle from turning.

Signs of bearing failure:

• Grinding noise when spindle is turned by hand
• Rough feel when rotating the spindle
• Excessive heat after running
• Visible rust or contamination

Fix: Replace the bearings. This requires disassembling the spindle and pressing out the old bearings. On low-cost spindles replacing the entire spindle assembly may be more cost-effective than rebuilding it.

Winding Failure

Motor windings can short circuit or open circuit due to overheating, voltage spikes, or manufacturing defects. A motor with failed windings will not run regardless of what the VFD does.

Test: Measure the resistance between each pair of motor wires (U-V, V-W, W-U). All three measurements should be similar. If one measurement is very different or shows an open circuit the windings are damaged and the motor needs replacement.

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5. Check the Controller Configuration

If the hardware is all working correctly the problem may be in the software configuration.

GRBL Settings for Spindle

GRBL-based machines use specific settings to control the spindle:

Setting	Function	Typical Value
$30	Max spindle speed	1000-12000
$31	Min spindle speed	0
$32	Laser mode (disable for spindle)	0

Send $$ to see all current GRBL settings. Verify that $30 is set to your maximum spindle RPM.

Mach3 / Mach4 Configuration

In Mach3 and Mach4 the spindle is controlled through the Pulse Width Modulation (PWM) output. Verify that the spindle setup tab has the correct pin assignments and that the PWM frequency matches your VFD requirements.

LinuxCNC Configuration

In LinuxCNC spindle control is configured in the HAL file. Verify that the spindle-on signal is connected to the correct output pin and that the PWM generator is configured for the correct frequency.

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When to Call a Professional

Signs You Need Help

Some spindle problems require specialized knowledge or tools to diagnose and repair:

• VFD internal component failure (requires electronics troubleshooting skills)
• Motor winding damage requiring rewinding
• Bearing replacement requiring spindle disassembly and balancing
• Controller board failure requiring component-level repair

Next Steps

If you have worked through all the checks above and the spindle still does not run, consider taking the VFD or spindle to a repair shop. The cost of professional diagnosis is often less than the cost of replacing parts that are not actually broken.

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Prevention: Keeping Your Spindle Running

Maintenance Schedule

Maintenance Task	Frequency	Time Required
Check brush wear (DC motors)	Every 50 hours	5 minutes
Clean VFD cooling fan	Monthly	10 minutes
Check spindle cable connections	Monthly	5 minutes
Lubricate bearings	Per manufacturer spec	15 minutes
Replace brushes	When worn to half length	10 minutes

Benefits

Regular maintenance prevents most spindle problems. A spindle that receives regular care runs for thousands of hours without unexpected failures. A spindle that receives regular maintenance according to the schedule above will run for thousands of hours without unexpected failures.

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What’s Next?

Related Guides

Once your spindle is running again learn how to use it correctly:

Prevention Summary

Regular maintenance prevents most spindle problems. Follow the maintenance schedule and address small issues before they become big problems.

• Feeds and Speeds for Beginners
• CNC Troubleshooting Guide
• G-Code for Beginners

Spindle Runs but Stops During Operation

Thermal Overload

A spindle that starts correctly but stops during a job has different causes. Thermal overload is most common — if the VFD or motor overheats, the thermal protection circuit shuts down the spindle. Let the machine cool for 30 minutes and try again.

Loose Connections

Loose connection. A wire that barely makes contact may work cold but lose connection as vibration works it loose. Check all spindle connections for tightness.

Intermittent Signal

Intermittent speed signal. If the 0-10V speed control signal from the controller to the VFD is unstable, the spindle may stop when the signal drops. Measure the control voltage at the VFD input while running — it should be steady.

Spindle Runs at Wrong Speed

Possible Causes

If the spindle runs but at the wrong speed check these factors:

S-word in the program. Verify that the S value is correct. S3000 means 3000 RPM. S300 means 300 RPM. A missing decimal point changes the speed by a factor of ten.

VFD maximum frequency setting. The VFD parameter for maximum frequency determines the spindle speed at full RPM. If this is set incorrectly the spindle speed will be wrong at all S-values. Verify that the maximum frequency matches your spindle rating.

GRBL and PWM Settings

PWM configuration. On GRBL-based machines the spindle speed is controlled by a PWM signal. If the PWM frequency or resolution in the GRBL configuration is set incorrectly for the VFD input specifications the spindle speed may not match the commanded S-value. Verify GRBL settings $30 and $31 are correct for your spindle.

Spindle Runs Rough or Noisy

Mechanical Causes

A spindle that runs but with unusual noise or vibration has a mechanical problem rather than an electrical one. Stop the machine immediately if you hear grinding or knocking sounds.

Check the collet and tool. A tool that is not seated correctly in the collet causes vibration that sounds like a bearing problem. Remove the tool, clean the collet, reinstall the tool, and tighten securely. Test again.

Check for bent tool. A bent cutting tool causes vibration at all speeds. Replace the tool and test again. Running with a bent tool damages the spindle bearings over time.

Diagnostic Steps

Check collet and tool — remove, clean, reinstall. Check for bent tool — replace and test. Check bearing condition — worn bearings cause rumbling or growling noises. Apply light pressure to the tool with the spindle off and try to feel movement in the bearings. Any perceptible play means the bearings need replacement.

Spindle Wiring Diagrams

DC and VFD Wiring

Understanding your spindle wiring helps diagnose problems faster. The basic wiring configurations are:

DC spindle (common on 3018 routers): The DC motor connects directly to a controller board or relay. The PWM signal from the controller sets the speed by varying the DC voltage. Wiring is simple: positive wire to the controller output, negative wire to ground.

VFD and AC Wiring

VFD spindle (common on larger routers and mills): The VFD receives power from the wall and sends variable-frequency power to the spindle motor. The controller sends a 0-10V analog signal or Modbus digital signal to the VFD to set the speed. A separate relay signal tells the VFD to start and stop.

AC spindle (older machines): The spindle runs directly from AC power through a contactor. Speed control is mechanical through belt and pulley changes.

Spindle Maintenance Schedule

Overview

A well-maintained spindle runs longer and fails less often than one that is ignored between breakdowns.

After Every Job

• Clean chips and debris from the spindle housing
• Check that the collet nut is tight
• Wipe the spindle shaft clean of coolant residue

Weekly

• Listen for bearing noise during operation
• Check the VFD display for fault codes
• Verify that the spindle cable is not chafing against the machine frame

Monthly

• Clean the VFD cooling fan and heatsink
• Check brush length on DC spindles
• Measure spindle runout with a dial indicator
• Inspect the spindle mounting bolts for tightness

Annually

• Replace spindle bearings (high-use machines)
• Replace DC motor brushes
• Check VFD parameter settings against spindle specifications
• Test ground continuity for electrical safety

Maintaining a log of spindle maintenance helps you predict when parts will need replacement. A spindle that has run 500 hours since the last brush change is approaching the service interval. Replacing brushes before they fail prevents unplanned downtime.

Spindle problems can be frustrating but most have straightforward causes that are inexpensive to fix. Work through the diagnostic steps in order, check the simplest possibilities first, and do not replace parts until you have confirmed they are actually faulty. A systematic diagnostic approach saves money and reduces downtime compared to the guess-and-replace method that beginners often fall back on.

If you have worked through every section in this guide and the spindle still does not run, consider posting the specific symptoms and the results of each diagnostic step to a CNC forum. Include your machine model, controller type, VFD model, and what you have already checked. The community can often identify the issue from the pattern of symptoms alone.