Metrology and in-process measurement

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