CNC Mill Concept: Vibration, Sensing & Metrology

Introduction

This post is part of the CNC Mill Concept hub. It covers the vibration sensing strategy, sensor types and placement, the distributed sensor PCB architecture, and the laser-based tool measurement subsystem.

Vibration and Sensing Strategy

Purpose

• Observe machine dynamic and quasi-static behavior
• Detect chatter and dynamic instability
• Provide inputs for compliance estimation and process supervision
• Favor redundant, inexpensive sensing over sparse instrumentation

Piezo Surface Microphones (Sensing Only)

• High-impedance piezo elements used exclusively for sensing
• No electrical shunt damping attached
• Used for:
  • high-bandwidth vibration measurement
  • chatter detection
• Non-directional measurement

Explicit placement:

• multiple microphones on spindle tower plates
• one microphone mounted directly on the spindle housing
• additional microphones permitted elsewhere as needed

Notes:

• microphones are distinct from piezo damping elements
• microphones are not bonded at modal antinodes by requirement

Accelerometers

• Triaxial MEMS accelerometers
• Directional vibration measurement (X, Y, Z)
• Used for:
  • modal analysis and validation
  • directional vibration insight
  • quasi-static tilt estimation

Explicit placement:

• spindle tower
• spindle mounting plate
• optional additional locations where useful

Dual-use strategy:

• high-rate data:
  • vibration analysis
  • frequency-domain features
• strongly low-pass-filtered data:
  • tilt estimation relative to gravity
  • quasi-static deflection observation

Other Sensors (Contextual)

• Temperature sensors:
  • monitor structural and spindle-related temperatures
  • provide context for drift and boundary-condition changes
• Sensors are not used for real-time compensation in this phase

Integration

• Sensors connected to distributed sensor PCBs
• Local preprocessing performed on sensor PCBs:
  • filtering
  • FFT or envelope extraction
• Event-driven reporting to ECU via CAN-FD
• Raw data access available for debugging and validation

Sensor Placement and Measurement Coverage

Sensor PCB Placement

Two identical sensor PCBs are used in the machine.

• PCB A — mounted on the left X-beam near the spindle tower.
• PCB B — mounted on the right X-beam near the spindle tower.

Both boards use identical hardware and firmware. The distributed placement reduces analog wiring length for strain gauges and improves signal integrity. An optional synchronization line between the boards allows deterministic simultaneous sampling.

X-Beam Instrumentation

Strain Gauges (DMS)

• One full bridge on the left beam (top and bottom surface)
• One full bridge on the right beam (top and bottom surface)
• Dynamic capable measurement

Purpose:

• Measure beam bending
• Support dynamic load estimation
• Enable structural excitation analysis and compliance observation

Spindle Plate Instrumentation

Accelerometer #1

• Mounted directly on the spindle plate
• Used for high-frequency vibration measurement
• Supports low-frequency tilt estimation

Spindle Housing Instrumentation

Microphone #1

• Mounted on the spindle housing
• Used for bearing and chatter acoustic monitoring

Temperature Sensor #1

• Mounted on the spindle housing
• Used to monitor spindle thermal growth

Tower Instrumentation

Microphone #2

• Mounted on the tower structure
• Used for structural acoustic monitoring

Accelerometer #2

• Mounted on the tower body
• Measures tower structural vibration

Temperature Sensor #2

• Installed in a drilled, thermally coupled pocket inside the tower plate
• Used for structural temperature monitoring

Measurement Coverage

The sensing system provides coverage of:

• Beam static and dynamic bending
• Spindle plate vibration and tilt
• Bearing and chatter acoustics
• Tower structural vibration
• Spindle thermal growth
• Tower structural temperature
• Digital monitoring of piezo shunt circuits

Metrology

Purpose

• Establish accurate knowledge of tool geometry
• Reduce uncertainty from tool runout and length variation
• Enable high repeatability without relying on conservative margins
• Decouple accuracy from spindle and tool holder quality

Laser-Based Tool Measurement

• Dedicated laser emitter and receiver
• Emitter and receiver housed separately
• Optical aperture implemented as a slit:
  • width: 5–10 µm
• Measurement performed with spindle rotating

Measurement Procedure

• Spindle moves tool into laser beam
• Initial detection:
  • threshold-based (not binary light/no-light)
• Secondary fine measurement:
  • sensor housing mounted on precision flexure
  • flexure actuated independently of machine motion
• Flexure motion resolution:
  • sub-100 nm repeatability
• Tool remains rotating during measurement

Measured Quantities

• Tool length
• Effective tool diameter
• Radial runout envelope
• Repeatable reference position for tool geometry

Thermal Stabilization

• Laser emitter and sensor actively heated
• Target temperature: fixed elevated setpoint (e.g. 50 °C)
• Measurement only enabled once thermal equilibrium is reached
• Thermal history not used for compensation
• Stability prioritized over absolute temperature accuracy

Calibration Strategy

• All absolute values obtained through calibration
• Measurement system optimized for:
  • repeatability
  • consistency
• Absolute accuracy derived from:
  • known calibration artifacts
  • reference tools

Integration

• Measurement system treated as a metrology subsystem
• Not part of:
  • force loop
  • vibration control
• Results communicated to:
  • Raspberry Pi (tool library management)
  • Duet (tool offsets)
• Compatible with:
  • manual tool changes
  • future automated tool handling

Scope Limitations

• No real-time tool deflection measurement
• No thermal growth compensation of tool during cutting
• No wear prediction or life estimation

License

This project (with exceptions) is published under the CC Attribution-ShareAlike 4.0 International License.