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ISO 10816 is the international standard for evaluating machine vibration. It defines four severity zones based on vibration velocity, measured in millimeters per second (mm/s). For CNC machine shops, it is the difference between catching a failing bearing on Tuesday and replacing a spindle on Friday.
This article covers what the standard actually says, which machine class applies to your CNC equipment, how to set alert thresholds, and where to put the sensors. No fluff, no theory that does not translate to your shop floor.
ISO 10816 measures broadband vibration velocity — specifically, the RMS (root mean square) velocity in mm/s across a frequency range of 10 Hz to 1,000 Hz. This captures the dominant vibration energy produced by rotating machinery: imbalance, misalignment, bearing defects, gear mesh, and structural resonance.
The standard defines four evaluation zones — A through D — with threshold values that vary by machine class. The zones represent a progression from newly commissioned equipment (Zone A) to imminent damage (Zone D). The idea is simple: measure vibration, compare to the thresholds, take action accordingly.
Note: ISO 10816 has been partially superseded by ISO 20816 for some machine categories. For general industrial machinery including CNC equipment, the ISO 10816-3 thresholds for Class II machines remain the most widely used reference. That is what we cover here.
Each zone maps to a specific operational response. This is not a suggestion — it is what decades of rotating machinery failure analysis have established as safe operating practice.
Vibration velocity below 2.8 mm/s. This is the condition of newly commissioned or freshly overhauled machinery. The machine is running within its designed vibration envelope. No action required. This is your baseline — the number you want to see.
Vibration between 2.8 mm/s and 4.5 mm/s. The machine can operate indefinitely in this zone without restriction. It has moved beyond new-machine condition but is still well within safe limits. Monitor the trend. If vibration is stable in Zone B, there is nothing to fix. If it is rising toward 4.5, start planning maintenance.
Vibration between 4.5 mm/s and 7.1 mm/s. The machine should not operate for extended periods in this zone. Investigate the cause within the current shift. Common causes: bearing wear, spindle imbalance, loose mounting, or workholding issues. You can finish the current job, but schedule maintenance before the next one.
Vibration above 7.1 mm/s. Continued operation risks damage to the machine. Stop the machine. This is not a “when you get a chance” situation — it is a “right now” situation. Running a CNC spindle at this vibration level can cause bearing seizure, shaft damage, or structural cracking in the spindle housing.
Class II covers medium-sized machines from 15 kW to 75 kW (20-100 HP) on rigid foundations. This includes the vast majority of CNC vertical mills, horizontal mills, and lathes in job shop environments.
| Zone | Velocity (mm/s RMS) | Condition | Action Required | Typical Cause |
|---|---|---|---|---|
| A | < 2.8 | Good | None — normal operation | New or freshly maintained |
| B | 2.8 – 4.5 | Acceptable | Monitor trend — plan if rising | Normal wear, minor imbalance |
| C | 4.5 – 7.1 | Unsatisfactory | Investigate this shift | Bearing wear, misalignment, looseness |
| D | > 7.1 | Unacceptable | Stop machine immediately | Severe bearing damage, structural fault |
Values are RMS vibration velocity at the bearing housing. Measurements taken in the radial direction (horizontal and vertical) at the drive end and non-drive end bearings.
ISO 10816-3 defines four machine classes. Here is how to match your equipment:
| Class | Power Range | Foundation | Typical CNC Equipment |
|---|---|---|---|
| I | < 15 kW | Any | Small bench mills, micro lathes, grinders |
| II | 15-75 kW | Rigid | Most CNC VMCs, lathes, HMCs — this is the one |
| III | > 75 kW | Rigid | Large boring mills, 5-axis gantries, heavy lathes |
| IV | > 75 kW | Flexible | Large machines on isolator mounts |
A Haas VF-2 has a 22.4 kW spindle motor — Class II. A Haas ST-10 has a 11.2 kW spindle — Class I, though most shops apply Class II thresholds conservatively. A DMG Mori DMU 80 with a 35 kW spindle is solidly Class II.
When in doubt, use Class II. The thresholds are well-established and appropriate for the majority of production CNC machines in a job shop.
Sensor placement matters more than sensor quality. A $50 sensor in the right location outperforms a $500 sensor in the wrong one. ISO 10816 specifies measurement at the bearing housing in the radial direction. For CNC machines, this translates to:
For wireless LoRaWAN sensors (what we deploy), magnetic mount is the standard for most CNC applications. Stud mounting provides better high-frequency response but requires drilling and tapping the machine. For broadband velocity monitoring per ISO 10816, magnetic mount is sufficient and non-invasive.
See real vibration data from CNC machines
Our live lab has vibration sensors on 3 CNC machines reporting in real time. Watch the data, check the thresholds, and see what an alert looks like when a machine crosses from Zone A into Zone B.
Watch live vibration data from our lab →ISO 10816 broadband velocity is the first line of defense — it tells you something is wrong. Frequency analysis tells you what is wrong. Different fault types produce vibration at characteristic frequencies relative to the shaft speed (1x RPM):
| Frequency | Fault Type | What It Looks Like |
|---|---|---|
| 1x RPM | Imbalance | Dominant radial vibration at shaft speed — tool holder imbalance, uneven insert wear |
| 2x RPM | Misalignment | Strong axial component — spindle/motor coupling, belt tension, shaft bend |
| BPFO/BPFI | Bearing defect | Non-synchronous frequencies — outer/inner race damage. Calculated from bearing geometry |
| High (2-5 kHz) | Early bearing wear | High-frequency energy increase before broadband velocity rises — earliest warning |
| Gear mesh | Gear damage | Teeth count x RPM — sidebands indicate specific tooth damage |
For most CNC job shops starting with vibration monitoring, broadband velocity per ISO 10816 is enough. It catches 80% of developing faults with a simple, well-understood threshold. Frequency analysis is the next level — useful for root cause diagnosis once an alarm triggers.
Start with ISO 10816 Class II thresholds. Then refine based on your machine's actual baseline. Here is the approach:
The key principle: a sudden change in vibration level is more concerning than a consistently elevated level. A machine that has run at 3.0 mm/s for 2 years is fine. A machine that jumped from 1.2 to 3.0 mm/s in a week has a developing problem.
Need custom thresholds for your machines?
Our downtime calculator reviews your machine count, shift schedule, and hourly costs to show exactly what unplanned downtime costs your shop — not a generic estimate.
See what downtime is costing you →ISO 10816 thresholds are defined for machines running at steady state without load. In a CNC environment, vibration during heavy cutting will exceed these thresholds — that is normal. The meaningful measurement is taken during spindle warmup, light cuts, or rapid traverse. Most monitoring systems (including ours) sample continuously and use the lowest readings in each window for threshold comparison, filtering out cutting transients.
CNC machines operate across a wide RPM range. Vibration amplitude changes with speed — a machine that reads 1.5 mm/s at 4,000 RPM might read 2.5 mm/s at 10,000 RPM. For trending purposes, compare measurements at the same spindle speed over time. Some monitoring systems normalize for RPM automatically; others require the user to compare like-for-like.
In a CNC shop, machines transmit vibration through the floor. A large horizontal boring mill can elevate vibration readings on nearby VMCs. If your baseline seems high, check whether adjacent machines are the source. Time-correlation (does the reading spike when the machine next door is in a heavy cut?) identifies environmental contamination quickly.
ISO 10816 gives you a proven, internationally accepted framework for deciding when a machine needs attention. For CNC shops, the Class II thresholds (2.8 / 4.5 / 7.1 mm/s) are the starting point. Baseline your machines, set alerts at the zone boundaries, and watch the trends.
You do not need a vibration analyst on staff. You do not need a PhD in signal processing. You need a sensor on the spindle, a threshold that triggers an alert, and the discipline to investigate when the alert fires. That is the entire program.
A $50 sensor and a 2.8 mm/s threshold will catch 80% of developing mechanical faults before they become catastrophic. That is the highest ROI investment on your shop floor.