ANSI to ISO Tolerance ConverterB4.1 to ISO 286 Standards
Convert American ANSI B4.1 tolerance fits (RC, LC, LT, LN, FN) to international ISO 286 equivalents. Essential for communicating tolerances with international suppliers, reading metric engineering drawings, and ensuring proper fits across different standard systems.
ANSI B4.1 Specifications
ISO 286 Equivalent
B4.1 Standard
Hole Tolerance
Shaft Tolerance
Tolerance Zones (Metric)
| Feature | Nominal (mm) | Upper Limit | Lower Limit | Tolerance Zone |
|---|---|---|---|---|
| Hole (H8) | 25.4000 | 25.9588 | 25.4000 | +0.5588 / +0.0000 |
| Shaft (h7) | 25.4000 | 25.4000 | 24.8412 | +0.0000 / -0.5588 |
Manufacturing Recommendations
- Achievable with precision turning or milling
- Sharp tools and rigid setup required
- Final pass should be light (0.1-0.2mm)
- Design for lubrication channels if continuous rotation
- Consider surface finish Ra 0.8-1.6μm for bearing surfaces
- Use micrometers or CMM for inspection (±0.002mm accuracy)
- Temperature-controlled inspection room recommended
Understanding Tolerance Systems
Tolerance systems define how precise parts must be manufactured. ANSI B4.1 (American) and ISO 286 (International) are the two dominant systems, each with different nomenclature but achieving similar results.
ANSI B4.1 (Imperial)
Used primarily in the United States. Specifies fits as letter-number combinations like RC2, LC3, FN4.
- • RC = Running/Sliding Clearance
- • LC = Locational Clearance
- • LT = Locational Transition
- • LN = Locational Interference
- • FN = Force/Shrink Fit
ISO 286 (Metric)
International standard used worldwide. Specifies hole and shaft tolerances separately like H7/g6, H8/h7.
- • H = Hole tolerance (uppercase)
- • g, h, f = Shaft tolerance (lowercase)
- • 6, 7, 8, 9 = Tolerance grade (IT)
- • Lower number = Tighter tolerance
- • H-hole basis is most common
Common Tolerance Equivalents
| ANSI B4.1 | ISO 286 | Typical Application | Tolerance Range |
|---|---|---|---|
| RC-1 | H7/g6 | Precision bearings, close-running fits | IT7 (±0.015mm @ Ø25) |
| LC-2 | H8/h7 | Slip fits for easy assembly/disassembly | IT8 (±0.022mm @ Ø25) |
| LT-2 | H8/h7 | Locating pins, light press fits | IT8 (±0.022mm @ Ø25) |
| LN-2 | H7/h6 | Press fits for gears and pulleys | IT7 (±0.015mm @ Ø25) |
| FN-2 | H7/h6 | Permanent assemblies, bearing races | IT7 (±0.015mm @ Ø25) |
ISO IT Grade Precision Levels
IT6-IT7 (Precision Grade)
High precision fits requiring grinding or honing. Typical tolerance ±0.010-0.015mm for Ø25mm parts.
- ✓ANSI Equivalent: RC1, LC1, LN1-2, FN1-2
- ✓Machining: Requires grinding or precision turning
- ✓Applications: Bearings, precision gears, gauge blocks
IT8-IT9 (Standard Grade)
General manufacturing precision achievable with quality CNC operations. Typical tolerance ±0.022-0.036mm for Ø25mm parts.
- ✓ANSI Equivalent: RC2-4, LC2-4, LT2-4
- ✓Machining: Precision turning/milling sufficient
- ✓Applications: General machine parts, shafts, housings
IT10-IT12 (Coarse Grade)
Loose fits for easy assembly, non-critical dimensions. Typical tolerance ±0.058-0.150mm for Ø25mm parts.
- ✓ANSI Equivalent: RC7-9, LC7-9
- ✓Machining: Standard milling/drilling operations
- ✓Applications: Clearance holes, non-precision assemblies
Why Convert Between ANSI and ISO?
International Manufacturing
Working with overseas suppliers or reading international drawings? ISO 286 is the global standard. Convert your ANSI specs to ensure correct communication.
Engineering Drawing Review
Received a drawing with H7/g6 callout but only familiar with RC fits? Our converter helps you understand foreign tolerance specifications instantly.
Quality Inspection
Converting tolerances ensures your inspection equipment and procedures match the design intent, whether specified in ANSI or ISO standards.
Common Tolerance Conversion Mistakes
❌ Mixing Unit Systems
- • Using inches with ISO tolerances
- • Forgetting to convert nominal size to mm
- • Applying imperial tolerance to metric parts
- • Incorrect conversion factor (using 25 instead of 25.4)
❌ Misunderstanding Fit Types
- • Confusing clearance with interference fits
- • Using wrong tolerance grade for application
- • Ignoring hole vs shaft tolerance differences
- • Not considering assembly method
❌ Ignoring Size Range Effects
- • Tolerance varies with nominal size
- • Small parts need tighter absolute tolerances
- • Large parts allow looser tolerances
- • Check tables for specific size ranges
❌ Machining Capability Mismatch
- • Specifying IT6 without grinding capability
- • Expecting precision from standard operations
- • Not validating machine capability
- • Overlooking thermal expansion effects
Frequently Asked Questions
What is the difference between ANSI and ISO tolerances?
ANSI B4.1 uses fit classes (RC, LC, LT, LN, FN) to describe both hole and shaft together. ISO 286 specifies hole and shaft tolerances separately (e.g., H7 for hole, g6 for shaft). Both achieve similar fits but with different nomenclature.
What does H7/g6 mean in ISO 286?
H7 = Hole tolerance with IT7 grade (0 to +tolerance). g6 = Shaft tolerance with IT6 grade (small negative deviation). Together they create a precision clearance fit similar to ANSI RC1, commonly used for bearings.
Can I machine ISO H7 tolerance on a standard CNC mill?
H7 (IT7 grade) is challenging but possible with precision CNC equipment, sharp tools, and careful setup. For Ø25mm, H7 allows ~±0.015mm. Most shops use reaming or boring for H7 holes. H8-H9 is more realistic for standard milling operations.
Should I convert my drawings from ANSI to ISO?
If manufacturing internationally or working with metric suppliers, yes. ISO 286 is understood worldwide and prevents confusion. Our converter helps you specify the correct ISO equivalent for your ANSI callouts.
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Join thousands of engineers and machinists who use our converter for accurate ANSI to ISO tolerance conversion. Ensure proper communication with international suppliers and understand metric drawings with confidence.