Rotary Axes & Probing Overview
Rotary axes and probing extend what a CNC machine can do. This page introduces A/B/C axes, touch probes, and 3+2 machining without going deep into 5-axis programming.
Concept
Rotary axes (A, B, C) rotate the tool or workpiece around the linear axes. A rotates around X, B around Y, C around Z. A touch probe measures part position and feeds back into work offsets automatically.
Why It Matters
Without probing, setting work offset on a complex fixture is manual and error-prone. A probe touches the part, calculates the edge/center, and sets G54 automatically. Rotary axes let you machine multiple sides without re-fixturing — reducing setup error and cycle time.
How It Works
3+2 vs Full 5-Axis
- 3+2 — rotary axes position once, then XYZ linear milling. Simpler programming.
- Full 5-axis — rotary and linear move simultaneously. Complex CAM required.
Probing Basics
A touch probe mounts in the spindle. It moves to the part, touches it, and records the position. The controller computes the offset. The measured coordinate is not directly G54 — it must be converted: edge touch = half stock offset from center.
Example
Probe a 60 mm square part edge: touch left edge at X-30, touch right edge at X+30. Center X = (X-30 + X+30)/2 = X0. Set G54 X to the machine position at X0. This replaces manual edge-finding.
Probe Applications
Beyond setting work offsets, probes are used for:
- Fixture verification — confirm the part is clamped before cutting.
- Work offset update — auto-set G54 after each job change.
- In-process inspection — measure a feature mid-cycle and adjust offsets.
- Tool setting — measure tool length and diameter automatically.
- First-article check — probe critical dimensions after the first cut.
Rotary Axis Naming
| Axis | Rotates Around | Typical Use |
|---|---|---|
| A | X axis | Tilt spindle or table for angled features |
| B | Y axis | Swivel head on 5-axis machines |
| C | Z axis | Rotate table for 4th-axis milling, bolt circles |
Common Mistakes
- Treating probe reading as G54 directly — the probe touches an edge, not the center. Convert: center = (edge1 + edge2)/2.
- Probing at high speed — probe feed must be slow (typically 500 mm/min or less) to avoid breaking the stylus.
- Confusing 3+2 with full 5-axis — 3+2 positions rotary once; full 5-axis interpolates simultaneously.
- Forgetting probe calibration — the stylus length and radius must be calibrated on a reference sphere before probing.
Practice
1. Probe left edge at X-25.3, right edge at X24.8. Center?
Ans
(-25.3 + 24.8)/2 = -0.25 mm. Part center is at X-0.25.
2. What is 3+2?
Ans
Rotary axes position at an angle, then XYZ linear milling — no simultaneous rotary motion.
3. Why slow probe feed?
Ans
Probes are delicate. Fast feed bends the stylus and gives false readings.