CNC Machine Servicing: How Tolerance Is Actually Held
A machine tool does not lose accuracy all at once. It drifts. This page explains what CNC machine servicing actually changes inside the machine, which checks matter for parts held to ±0.005 mm, and when servicing stops being worth it.
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Where CNC machine servicing has to intervene
Thermal growth is the biggest single error source on a running machine. A spindle that has been cutting for three hours is longer than the same spindle at 7 a.m. On aluminium, a 10 °C rise across a 500 mm part moves the geometry well past ±0.005 mm. Ballscrew preload and linear guide clearance are the second source: both loosen slowly, and the drift shows up first on interpolated circles, not on straight cuts.
The third source is the control loop itself. Servo gain, backlash compensation and pitch error compensation are stored as parameters. Those values were correct the day the machine was installed. After a hard crash or a spindle replacement they are guesses. Re-tuning them is the part of CNC machine servicing that most shops skip, and it is usually where a rejected batch is traced back to.
None of this is visible on a finished part until the tolerance is already gone. A 0.004 mm shift is inside the drawing. A 0.012 mm shift is a scrapped lot. The gap between those two numbers is what a service schedule buys you, and it is measured in hours of cut time, not calendar months.
So the useful question is not whether to service. It is which parameters you can measure, how often, and what threshold forces a correction. Everything below follows from that.
Squareness, level and the foundation under the machine
Geometry checks come first because every compensation value downstream assumes the machine sits square. A granite square and a dial indicator on a 300 mm sweep will show squareness error directly. On a machine that has not been leveled in two years, we commonly see 0.02–0.03 mm per 300 mm on the Y axis. That is four to six times the tolerance we are asked to hold.
Leveling is not a one-time event. Cast iron relaxes. Concrete floors settle, especially in a plant where forklifts run near the machine base. A 7,600 m² floor with heavy traffic will move a machine base over a single year. Re-leveling is cheap; the parts made on an unleveled machine are not.
The order matters here. Level the machine, then check squareness, then re-tune the control. If you tune compensation on a machine that is out of level, you have written a correction for a condition that will change again next month.
For large work, the effect compounds. On a 4,000 mm travel machine, a small angular error at the column becomes a visible taper at the far end of the part. That is why long-bed machines get checked more often than compact ones.
Spindle health: runout, preload and thermal behavior
Spindle runout is the number that decides your surface finish and your hole position. We measure it with a test bar at 50 mm and 300 mm from the gauge line. A spindle reading 0.002 mm at 50 mm may read 0.010 mm at 300 mm. The second number is the one that ruins a deep bore.
Preload loss is quieter. The spindle still runs, still cuts, and the operator notices nothing. What changes is stiffness under load. A finishing pass that used to hold Ra 0.8–1.6 μm starts coming out at Ra 1.6–3.2 μm, and the tool life drops with it. By the time the sound changes, the bearings have already been running loose for weeks.
Thermal behavior needs a baseline, not a single reading. Warm the spindle through a fixed cycle, then log displacement every 15 minutes for two hours. A healthy spindle reaches steady state and stays there. One that keeps growing has a cooling problem, a lubrication problem, or a bearing on the way out.
Spindle taper condition belongs in the same check. A scored taper seats the toolholder off-center, which shows up as runout you cannot fix by changing the holder. Blue the taper and look at the contact pattern before you blame the tool.
Ballscrews, guides and backlash measurement
Backlash is measured by approaching a point from both directions and reading the difference. On a healthy machine this is under 0.003 mm. Between 0.003 mm and 0.008 mm the control can compensate, and the part still holds. Above that, compensation hides the wear but does not remove it: the axis still hesitates, and interpolated bores come out lobed.
Ballscrew wear is rarely uniform. Most of the cutting happens over the middle third of the travel. So the screw is fine near the ends and loose where the work actually sits. Measure backlash at five positions along each axis, not one. A single reading at the home position will look perfect on a machine that cannot hold tolerance anywhere else.
Linear guide condition is easier to judge by feel and by load. Check for a rough spot by pushing the table by hand with the servos off, and compare motor current between axes during a rapid move. A guide that has lost preload draws more current on reversal and leaves a witness mark on a test cut.
Once you have these numbers, write them down with the date. A trend line beats any single measurement. Ten microns of drift over six months tells you when to schedule the repair. Ten microns with no history tells you nothing.
Parameters, firmware and the control loop
Back up parameters before you touch anything. That includes pitch error compensation, backlash tables, tool offsets and the PLC program. A backup you cannot restore is not a backup. We keep one copy on the machine network and one off it, and we verify a restore on a spare control before we trust the file.
Servo tuning is where accuracy is won or lost. A machine that is tuned too soft will lag on direction changes and round off corners. Tuned too hard, it will hum and leave chatter marks on a light finishing pass. The right setting shows a clean step response with a small, controlled overshoot, not zero overshoot.
Pitch error compensation should be re-measured with a laser interferometer after any mechanical repair. If you replace a bearing or re-level a machine and leave the old compensation table in place, you have stacked a new error on an old correction. The machine will be worse than before the repair.
Firmware updates are lower priority than most people think. Apply them when a fix or a feature matters to your parts. Do not update a control the week a production order is running. A control that is stable and slightly old is a better business tool than one that is current and untested.
Interval, runtime and condition-based servicing
Calendar-based servicing is a starting point, not a plan. A machine running two shifts wears four times faster than one running one shift, and both get the same quarterly checklist. Runtime hours are a better clock. For most of our machines, 2,000 spindle hours is a reasonable interval for a full geometry and backlash check on a machine holding ±0.005 mm.
Condition-based servicing means you measure and let the number decide. Instead of replacing a spindle bearing at a fixed interval, you log runout and vibration every month and act when the trend crosses a line. This costs more in measurement time and less in unexpected downtime. For a machine on a hot job, that trade is usually worth it.
There is a practical middle ground for job shops. Do the quick checks, the ones that take an hour, on a routine schedule: level, runout, backlash at the work zone. Do the expensive checks, laser interferometry and full servo tuning, only when a quick check goes out of range or after any mechanical repair.
Whatever schedule you choose, tie it to the parts, not the calendar. A machine that only cuts soft aluminium to ±0.1 mm does not need the same attention as one cutting 17-4PH to ±0.005 mm. Service intensity should track the tolerance you are selling.
When servicing is no longer the answer
There is a point where the numbers stop being recoverable. If backlash is spread unevenly across all five measurement positions and the ballscrew shows scoring, compensation will not bring the machine back to ±0.005 mm. It will hide the problem for a few months and then fail on a tight order.
The same applies to a spindle with a worn taper and rising runout. Grinding the taper can help once. After that, the bearing bores and the housing are the limit. Replacing a spindle on a 20-year-old machine is often more expensive than the machine's remaining value, and the geometry of the rest of the machine is usually not far behind.
The honest test is this: can the machine hold the tolerance on a test part that represents your hardest work? Not a simple block. A part with an interpolated bore, a deep pocket, and a tight position callout. Run it cold and run it after warm-up. If both pass, service the machine and keep going. If the cold part passes and the warm part does not, you have a thermal problem worth fixing.
If neither passes, and the measured errors are widespread rather than local, the money is better spent on a machine that can hold the tolerance. We would rather tell a customer that than sell hours of servicing that will not solve their problem.
What each check tells you, and when it forces action
Thresholds are the ones we use on our own 127 machines.
| Check | Normal range | Action threshold | What it protects |
|---|---|---|---|
| Level and squareness | Under 0.01 mm per 300 mm | Above 0.02 mm per 300 mm | Geometry on long parts |
| Spindle runout at 300 mm | Under 0.005 mm | Above 0.010 mm | Bore position, finish |
| Axis backlash | Under 0.003 mm | Above 0.008 mm | Interpolated circles |
| Ballscrew wear spread | Under 0.004 mm end to middle | Above 0.010 mm | Mid-travel accuracy |
| Servo step response | Small controlled overshoot | Sustained oscillation | Corner accuracy |
| Thermal drift after warm-up | Steady within 0.005 mm | Still growing at 2 hours | Long-cycle parts |
Service the machine or replace it
If errors are local and measurable, service it: level, re-tune, re-measure compensation, and log the trend. If errors are spread across every axis and the spindle taper is gone, replace the machine. Servicing buys accuracy back; it does not create it.
Questions engineers ask about CNC machine servicing
How often should a machine holding ±0.005 mm be checked?
Around every 2,000 spindle hours for a full geometry and backlash check. That is roughly quarterly on two shifts and biannually on one. Log the numbers each time, because the trend matters more than the absolute value.
Between full checks, run the quick ones: level, spindle runout at 300 mm, and backlash measured at the work zone rather than at home position.
Can compensation fix a worn ballscrew?
Up to a point. Backlash under 0.008 mm can be compensated and the machine will still hold tolerance. Past that, the screw is loose in a way that changes with load and direction, and a fixed table cannot follow it.
You will see the failure as lobed interpolated bores and inconsistent position on reversal, even though the compensation table looks correct on paper.
Does a spindle replacement require re-tuning the control?
Yes. Any mechanical change to the spindle or the axes invalidates the existing pitch error and backlash values. Re-measure with a laser interferometer before you run production parts.
If you skip this, the machine carries a correction for a condition that no longer exists, and accuracy can be worse than before the repair.
Why does accuracy drop after a few hours of cutting?
Thermal growth. The spindle and the ballscrews expand as they warm, and the tool tip moves relative to the table. This is normal. The question is whether the machine reaches steady state or keeps drifting.
Warm the machine through a fixed cycle before the first tight tolerance cut, and check whether displacement stabilizes within two hours. If it does not, look at cooling and lubrication before you look at the control.
Is CNC machine servicing worth it on an older machine?
Only if the errors are local. Re-leveling, a backlash adjustment or a compensation re-measure on a sound machine is cheap and effective.
If backlash is uneven across the whole travel and the spindle taper is worn through, the remaining errors are structural. Service hours will not recover accuracy that the iron no longer has.
What records should we keep?
Date, spindle hours, level readings, runout at 50 mm and 300 mm, backlash at five positions per axis, and the compensation values in use. Keep the parameter backup with the same date stamp.
With that history, a service decision becomes a comparison instead of a guess, and you can show the trend to whoever signs off on the repair.
Send us the drawing and the tolerance
Tell us the part and the tolerance band. We will quote it, flag the features that are hard to hold, and tell you which machine and which checks the job needs.
12-hour quote±0.005 mm100% inspectionNo minimum order quantity
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