What are IT Tolerance Classes?
The International Tolerance (IT) system, developed by ISO, defines the dimensional accuracy of components. The lower the IT class number, the smaller the tolerance and the higher the manufacturing precision.
IT Tolerance Classes for Nominal Size 18-30 mm
| IT Class | Tolerance [μm] | Application |
|---|---|---|
| IT01 | 0.5 | Measurement standards, precision instruments |
| IT0 | 0.8 | Workshop standards |
| IT1 | 1.2 | Measuring instruments |
| IT2 | 2 | Precision components |
| IT3 | 3 | Precision machining |
| IT4 ⭐ | 5 | Most common - precise fits |
| IT5 | 8 | Accurate fits |
| IT6 | 13 | Medium-precision fits |
| IT7 | 21 | Medium accuracy machining |
| IT8 | 33 | Standard fits |
| IT9 | 52 | Medium-accuracy machining |
| IT10 | 84 | Rough machining |
| IT11 | 130 | Non-critical components |
| IT12 | 210 | Approximate dimensions |
| IT13 | 330 | Semi-finished products |
| IT14 | 520 | Punching, stamping |
| IT15 | 840 | Castings, forgings |
| IT16 | 1300 | Raw castings |
| IT17 | 2100 | Auxiliary dimensions |
| IT18 | 3300 | Low-precision dimensions |
Why IT4 is Most Common
IT4 (±5μm for 18-30mm) provides the best balance between:
- Manufacturing precision: Suitable for most industrial applications
- Cost: Achievable on standard CNC machines
- Repeatability: Stable for mass production
- Versatility: Works in 80% of mechanical applications
How to Achieve IT4 in Practice
- Turning: Finishing with ceramic or CBN inserts, feed 0.05-0.1 mm/rev
- CNC Milling: Solid carbide cutters, positioning accuracy ±0.005mm, stable clamping
- Laser Cutting: Only suitable for approximate dimensions (IT12-IT14)
- Grinding: For high precision (IT3-IT5), using precision grinders
IT Fits for Sheet Metal
For sheet metal components, IT4-IT6 can be achieved by:
- CNC milling of holes and external contours
- Turning cylindrical components after welding or joining
- Drilling on coordinate drilling machines
- Inspection on coordinate measuring machines (CMM)
Drawing Notation Examples
- Ø20 H7 - hole IT7 with H tolerance (loose fit)
- Ø20 h6 - shaft IT6 with h tolerance (tight fit)
- Ø20 ±0.005 - direct tolerance notation (equivalent to IT4)