Why Do CNC Machine Tools Vibrate and Crawl?
Vibration and crawl are two different faults that often show up together. This page separates them by symptom, points to the mechanical and servo causes behind each one, and gives you a check order that finds the source before you spend money on parts.
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Symptoms, causes and what to do first
Work down the table before touching parameters. Fix the mechanical item first; it usually removes the surface mark that made you look at the servo.
| Symptom | Likely cause | First action |
|---|---|---|
| Slow slide moves in jerks | Stick-slip on the guideway | Check lube film and way oil grade |
| Fine pitch marks on the floor | Tool or spindle vibration | Measure runout, then check balance |
| Crawl only on one axis | Loose thrust bearing or coupling | Clamp a dial indicator on the slide |
| Crawl at low feed, smooth at high | Servo gain too low | Raise velocity loop gain in small steps |
| Buzz that stops when idle | Servo hunting at zero speed | Check encoder signal and grounding |
| Vibration grows with spindle rpm | Imbalance or worn bearings | Run a tap test, check bearing clearance |
| Chatter only in deep pockets | Tool overhang too long | Shorten overhang, change helix angle |
| Crawl after a crash | Bent ball screw or damaged ways | Inspect screw straightness and preload |
Crawl is a feed fault, vibration is a dynamic fault
Crawl and vibration get grouped together because both spoil the surface. They are not the same problem. Crawl is a low-speed feed fault: the axis moves in stops and starts, and the surface shows a regular ripple at a pitch that matches the ball screw lead or the servo update. Vibration is a dynamic fault: the structure or the tool rings at a frequency, and the marks are finer and often change with spindle speed.
You can tell them apart in under a minute. Command a slow feed with the spindle stopped and the tool clear of the part. If the axis still jerks, the problem is in the drive train or the servo loop. If it runs smooth and only vibrates once the tool touches metal, the problem is in the cutting process or the spindle.
That single test decides where you spend the next hour. Engineers who skip it end up retuning a servo to fix a loose coupling, or replacing a spindle bearing to fix a gain setting that was too low.
The check order below follows that logic: confirm the fault class first, then work outward from the mechanical parts to the control parameters.
- 1CrawlLow-speed feed, regular ripple, present with or without cutting
- 2VibrationHigh-frequency marks, tied to rpm or tool engagement
- 3Both at onceUsually a mechanical fault exciting a servo that is already too aggressive
Why CNC machine tools vibrate and crawl from the drive train
Stick-slip is the classic cause of crawl. The sliding surfaces need a film of oil to separate them. When the film breaks down, static friction holds the slide until the motor builds enough force, then the slide jumps. The result is a sawtooth motion. Common triggers are the wrong way oil viscosity, a clogged metering unit, or a guideway that was wiped dry during maintenance.
Check the lube system before anything else. Confirm the pump cycles, confirm oil reaches the farthest metering unit, and confirm the oil grade matches the manufacturer's spec for the slide material. A machine running a heavy way oil in a cold shop will crawl until the ways warm up.
Backlash and lost motion cause a different signature. The axis reverses cleanly but the position lags, and you see a step in the surface at every direction change. A dial indicator on the slide, with the screw held still, will show how much movement is lost in the thrust bearings, the coupling and the nut.
Ballscrew preload matters at low feed. A screw with worn or lost preload has a dead band near reversal. The servo pushes, nothing moves, then the screw takes up and the axis lurches. This looks exactly like stick-slip, which is why the lube check comes first and the screw check comes second.
- 1Lube filmWrong viscosity or blocked metering unit
- 2PreloadLost preload gives a dead band at reversal
- 3CouplingA loose or worn coupling adds lost motion
- 4Thrust bearingWorn bearings let the screw float under load
Why CNC machine tools vibrate and crawl from servo settings
The velocity loop has a gain that sets how hard the drive reacts to a following error. Set it too low and the axis lags, then catches up in steps. Set it too high and the axis hunts at standstill or rings after a move. On many controls you can watch the following error trace and see the shape of the problem before you change a number.
Integral gain is the usual culprit for crawl. Too much integral action at very low feed makes the drive wind up and release. The axis creeps, then jumps. If your control lets you schedule gain by feed rate, use a lower gain set for the low-feed range instead of compromising the whole range.
Encoder and feedback faults show up as vibration that has nothing to do with load. A dirty scale, a loose encoder coupling, or a shield that is not bonded to ground can inject noise into the loop. The axis may vibrate in place with no command, or buzz only when a nearby drive starts.
Resonance is the other half. Every axis has a mechanical natural frequency. If the velocity loop bandwidth sits on top of it, the axis rings. A notch filter tuned to that frequency usually fixes it without slowing the whole loop.
Rigid tapping and contouring put the highest demand on the loop. If the machine cuts fine in a straight line but vibrates in a corner, look at acceleration limits and feedforward before you blame the mechanics.
- 1Velocity gainToo low lags, too high hunts
- 2Integral gainWind-up at low feed produces crawl
- 3Feedback noiseDirty scale or poor shield bonding
- 4ResonanceLoop bandwidth on top of a natural frequency
Why CNC machine tools vibrate and crawl during the cut
Regenerative chatter is the most common cutting vibration. The tool leaves a wavy surface, the next tooth cuts into that wave, and the force variation grows until the tool rings. It is a self-excited loop, so changing feed alone rarely kills it. You have to break the loop by changing speed, depth or the tool's dynamic stiffness.
Tool overhang is the fastest lever. Stiffness falls with the cube of overhang, so pulling a tool holder 20 mm back into the spindle can raise the stable depth of cut more than any parameter change. Use the shortest holder that clears the fixture.
Spindle speed and tooth count set the tooth passing frequency. If that frequency lands near a structural mode, the machine rings. A stability lobe diagram shows the pockets of speed where the cut is stable at a given depth. Even a rough version, measured on the machine, is more useful than a generic chart.
Workholding is often ignored. A thin wall or a part held on one corner will move under cutting force and the tool will follow it. Add support, reduce radial engagement, or use a climb cut so the tooth exits into the thick section.
Some materials are simply prone to it. Titanium and stainless work-harden and push the tool harder; aluminium can gum and load the flutes. The fix differs by material, so match the tool geometry and coating to the alloy before changing the machine.
- 1OverhangStiffness drops with the cube of length
- 2Tooth frequencyKeep it away from structural modes
- 3WorkholdingUnsupported walls move with the cut
- 4MaterialTitanium and stainless need different geometry
Faults that look like machine vibration but are not
Not every ripple comes from the machine. A cutter with uneven flute spacing, a holder with a damaged taper, or a tool that has been ground off-center will produce a pattern that repeats once per revolution. Swap the tool and holder before you touch the machine.
Foundation and leveling matter more than most shops admit. A machine on a soft floor, or one that has settled since installation, will twist the bed and load the guideways unevenly. Re-level and re-check the anchor bolts after any major move.
Thermal growth is slow and easy to miss. A spindle that warms over four hours will drift, and the axis may need more torque to hold position, which shows up as a low-frequency wander rather than a sharp vibration. Measure the part at the start and end of a run to see the drift.
If the fault only appears on one part number, the problem is in the process, not the machine. Compare the tool list, the fixture and the CAM strategy against a part that runs clean. That comparison usually finds the answer faster than any diagnostic on the machine itself.
- 1Tool and holderDamage shows one mark per revolution
- 2LevelingA twisted bed loads the ways unevenly
- 3Thermal driftSlow wander over hours, not seconds
- 4ProcessOne part number bad means check the CAM
Five checks to run before you change any parameter
Run them in order. Each check tells you whether to keep going or stop and fix something.
- 11. Separate crawl from vibrationCommand a slow feed with the spindle stopped and the tool clear. If the axis jerks, it is a feed fault. If it runs smooth, the fault is in the cut. Takes two minutes and saves an hour.
- 22. Verify the lube filmConfirm the pump cycles, oil reaches the farthest metering unit, and the grade matches the slide spec. Cold shops may need a lighter way oil. A dry or over-heavy film causes stick-slip at low feed.
- 33. Measure lost motionClamp a dial indicator on the slide with the screw held still. Push the slide by hand and read the movement. Anything above a few thousandths of a millimeter points to the coupling, thrust bearings or nut.
- 44. Read the following error traceWatch the error against time at the feed where the fault appears. A sawtooth shape points to integral wind-up. A ringing shape points to gain that is too high or a resonance. Change one gain at a time, in 10 percent steps.
- 55. Test the tool and holderSwap in a known-good holder and a fresh cutter. If the mark disappears, the problem was never in the machine. Check runout at the cutting edge, and shorten overhang where the fixture allows.
- 66. Check the foundationRe-level the machine and check the anchor bolts after any move or nearby excavation. A twisted bed shows up as uneven wear on the ways and a crawl that changes with table position.
- 77. Set a stability windowFor chatter, vary spindle speed in 5 percent steps at fixed depth and note where the noise drops. Build a small lobe chart for the tools you run most. It is faster than guessing speeds on the next job.
Questions engineers ask next
Does crawl always mean the servo needs retuning?
No. In most shops the lube film or a lost ballscrew preload is the real cause. The servo is just responding to a mechanical dead band.
Fix the mechanical item first, then check whether the following error trace is still uneven. Retune only after the mechanics are clean.
Why does the machine only vibrate at certain spindle speeds?
That is the signature of a structural mode. The tooth passing frequency lands on a natural frequency and the cut becomes unstable.
Note the speeds that ring and the speeds that run clean. Use the clean ones first, and treat the ringing speeds as a limit to design around.
Can a worn tool cause vibration that looks like a machine fault?
Yes. Uneven flank wear or a chipped edge changes the cutting force every revolution. The mark then repeats at spindle frequency rather than at a structural frequency.
Swap the tool and compare. If the mark goes away, the machine was never the problem.
How do I know if the ballscrew preload is gone?
Command small moves in both directions at low feed and measure the lost motion with an indicator. A dead band near reversal that grows under load points to preload loss.
Confirm the thrust bearings and coupling are tight before you condemn the screw.
Is low gain always better for surface finish?
No. Low gain reduces hunting but increases following error, so corners and direction changes leave marks.
Set the gain as high as the mechanics allow, then use feedforward and acceleration limits to keep the corners accurate.
When should I stop troubleshooting and call the machine builder?
If the fault follows the axis regardless of tool, speed and load, and the mechanical checks are clean, the drive or the scale may be failing.
At that point the error logs and the following error traces are the useful evidence. Send both, not just a description.
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