ISO 230 Accuracy Standards Summary
ISO 230-1 and 230-2 geometric and positioning accuracy requirements
What is ISO 230?
ISO 230 is the international standard for testing the accuracy and performance of CNC machine tools. It ensures machines meet specified tolerances for geometric accuracy, positioning precision, and thermal stability.
📘 Standard Parts:
- ISO 230-1: Geometric accuracy of machines operating under no-load or quasi-static conditions
- ISO 230-2: Determination of positioning accuracy and repeatability
- ISO 230-3 through 230-11: Thermal effects, circular tests, noise, etc.
ISO 230-1: Geometric Accuracy
ISO 230-1 tests verify the machine's geometric accuracy without cutting load. These tests check straightness, squareness, parallelism, and angular deviations of linear and rotary axes.
Key Geometric Tests
| Test | Description | Typical Tolerance | Tool Used |
|---|---|---|---|
| Straightness (X, Y, Z) | Linear deviation of axis travel in horizontal/vertical planes | ±0.01mm per 300mm | Laser interferometer or precision level |
| Squareness | Angular deviation between two axes (e.g., X vs Y) | ±0.02mm per 300mm | Granite square + dial indicator |
| Parallelism | Axis travel parallel to reference surface | ±0.015mm per 1000mm | Precision level |
| Flatness (Table) | Surface deviation of work table | ±0.02mm over full travel | Precision level + grid measurement |
| Spindle Runout | Radial and axial runout at spindle nose | Radial: ±0.005mm Axial: ±0.003mm | Dial indicator + test bar |
| Angular Deviation | Pitch, yaw, roll of linear axes | ±20 arcseconds per 1000mm | Electronic level or autocollimator |
🔧 When to Perform ISO 230-1 Tests:
- • Installation: Verify machine meets spec before acceptance
- • Annual: Check for wear and alignment drift
- • After major repair: Confirm accuracy restored
- • After collision: Assess geometric damage
ISO 230-2: Positioning Accuracy & Repeatability
ISO 230-2 measures how accurately and consistently a CNC machine can position its axes at commanded locations. This directly affects part dimensional accuracy.
Positioning Accuracy vs. Repeatability Visual
❌ Poor Accuracy & Repeatability
High scatter + Off-target
Accuracy: Poor (mean ≠ target)
Repeatability: Poor (wide spread)
✅ Good Accuracy & Repeatability
Tight cluster on-target
Accuracy: Excellent (mean = target)
Repeatability: Excellent (tight)
Key Insight:
Repeatability measures the spread of measurements (precision).Accuracy measures how close the mean is to the target (trueness). You need both for quality parts!
Positioning Accuracy (A)
Maximum deviation between commanded position and actual mean position
A = |commanded - actual mean|
Typical spec: ±0.005-0.015mm
Repeatability (R)
Range of position variation when returning to same point multiple times
R = 6σ (6 standard deviations)
Typical spec: ±0.003-0.008mm
ISO 230-2 Test Procedure
Select 5 target positions
Evenly distributed across axis travel (e.g., 0%, 25%, 50%, 75%, 100%)
Approach from both directions
Test each position approaching from + and - direction to capture backlash
5 measurements per position
Return to each target position 5 times to calculate repeatability (statistical sample)
Record with laser interferometer
Use laser system (e.g., Renishaw XL-80) with 0.001mm resolution or better
Calculate A and R values
Use statistical formulas per ISO 230-2 Annex A
⚠️ Test Conditions (Critical!):
- • Temperature: Stabilize machine for 4+ hours at 20°C ±2°C
- • Warm-up: Run axes through full travel 3 times before testing
- • Load: Perform under no-load (empty spindle) conditions
- • Speed: Use positioning feedrate (not rapid traverse)
Typical Accuracy Specs by Machine Class
| Machine Class | Positioning Accuracy | Repeatability | Typical Applications |
|---|---|---|---|
| High Precision | ±0.002-0.005mm | ±0.001-0.003mm | Aerospace, medical devices, precision molds |
| Standard CNC | ±0.005-0.015mm | ±0.003-0.008mm | General machining, automotive parts, tooling |
| Industrial Grade | ±0.015-0.030mm | ±0.008-0.015mm | Heavy fabrication, structural components |
| 5-Axis Machines | ±0.010-0.025mm | ±0.005-0.012mm | Complex surfaces, turbine blades, dies |
Note: 5-axis machines have looser tolerances due to additional kinematic error sources (rotary axes).
Machine Acceptance Testing Guide
Before Accepting a New CNC Machine
Always perform ISO 230 acceptance tests before signing off on a new machine purchase. This verifies the machine meets manufacturer's stated specifications.
Acceptance Test Checklist:
🚫 Do NOT Accept Machine If:
- • Any ISO 230 value exceeds manufacturer's published spec by >10%
- • Geometric errors show systematic bias (e.g., all positive or all negative)
- • Repeatability exceeds 50% of positioning accuracy (indicates instability)
- • Spindle runout >0.010mm (will cause poor surface finish)
Error Compensation Strategies
Modern CNC controls can compensate for systematic errors discovered during ISO 230 testing.
Pitch Error Compensation
Corrects positioning errors at specific points along axis travel
- • Measure with laser interferometer
- • Input correction table into CNC parameters
- • Can reduce positioning error by 80-90%
Backlash Compensation
Compensates for play in ball screw/nut interface
- • Measure difference between +/- approach
- • Set backlash parameter (typical 0.01-0.05mm)
- • Re-test to verify improvement
Thermal Compensation
Adjusts for axis expansion due to temperature change
- • Use thermal sensors on key components
- • Create compensation model (ISO 230-3)
- • Critical for long production runs
Volumetric Compensation
3D correction map for entire work envelope
- • Requires laser ballbar or tracer system
- • Advanced feature (Fanuc Ai, Siemens CCS)
- • Can achieve ±0.002mm over full volume
Cost vs. Accuracy Trade-Off
Higher precision machines cost significantly more. Choose accuracy level based on actual part requirements.
Ultra-Precision
$200K-$500K+
Aerospace, medical
Standard CNC
$80K-$200K
80% of machining needs
Industrial
$40K-$100K
Heavy/structural work
Rule of thumb: Don't pay for ±0.005mm if your parts tolerance is ±0.05mm
Related Resources
Use our calculators to evaluate equipment with different accuracy specifications: