cnc_mill — vibration_health (spindle-accelerometer)
Physics
Section titled “Physics”A CNC mill spindle’s health shows up in vibration as tool imbalance (1x spindle speed), misalignment or coupling wear (2x), and developing spindle-bearing defects at higher-order BPFO/BPFI/BSF/FTF frequencies that scale with actual spindle RPM. A high-frequency accelerometer on the spindle housing picks up this broadband energy; integrating to velocity and taking RMS over the 10-1000 Hz band gives an overall severity trend, while order-tracked peaks at 1x/2x spindle speed and bearing defect frequencies point to the specific fault. Real spindle speed varies job to job (a common range spans roughly 1000-12000 RPM depending on tooling and material), so shaft_hz here defaults to a representative low-end value (3000 RPM = 50 Hz) that the operator/integrator tunes per job or per the dominant production speed; the detector’s order-tracking re-derives the actual running frequency from the spindle drive signal where available. Because spindle bearing defect frequencies can run several times spindle speed, pushing into the low kHz range at high RPM, the accelerometer is sampled at 8000 Hz (comfortably above 2x the highest fault frequency of interest even at high spindle speeds), with the edge node reducing this to 1 Hz velocity-RMS and order features for the historian rather than streaming raw waveform continuously.
Sensor
Section titled “Sensor”- Spec: high-frequency piezoelectric accelerometer, IEPE output, 1 Hz-20 kHz bandwidth, stud or adhesive mount
- Signal: scalar (mm/s), range n/a
Example parts
Section titled “Example parts”- PCB Piezotronics 352C33 miniature high-frequency IEPE accelerometer (10 mV/g, 1-20 kHz)
- ifm electronic VSE100 vibration sensor with integrated velocity RMS output
- Banner Engineering QM30VT1 vibration/temperature sensor
Mounting
Section titled “Mounting”Stud-mount the accelerometer on the spindle housing as close as practical to the front (tool-side) spindle bearing, on a flat machined surface, oriented radially. Avoid mounting on the spindle motor housing itself if it is isolated by a flexible coupling from the bearing assembly being monitored, since that attenuates the relevant high-frequency signal; confirm the mount location with the machine builder’s vibration-monitoring provision if one exists.
Wiring
Section titled “Wiring”2-wire shielded IEPE cable routed along the spindle head’s existing cable carrier/conduit to a dedicated vibration input module on the edge box; keep the cable away from the spindle drive’s VFD power leads to avoid high-frequency electrical noise coupling into the accelerometer signal.
Validation
Section titled “Validation”run the spindle at a known representative RPM and compare the detector’s 1x-order amplitude and overall velocity-RMS against a handheld calibrated vibration meter at the same point on the spindle housing
Expected: detector velocity-RMS within +/-10% of the handheld meter reading, and the 1x-order peak frequency matching the commanded spindle RPM within the FFT bin resolution
Safety notes
Section titled “Safety notes”- lock out the mill’s spindle drive (LOTO) before mounting a stud-type sensor on the spindle housing, and keep hands clear of the tool holder/collet during install
- verify no tool is loaded, or that the spindle is fully stopped and the control is in a safe maintenance mode, before working near the spindle nose
- route the sensor cable clear of the axis travel envelope and any tool-change mechanism so it cannot be sheared or caught during normal machine operation
180–750 USD, difficulty 4/5, ~40 min install downtime.