Methods overview
| Method | Category | IT grade | Ra [µm] | Typical application |
|---|
IT grade reference (ISO 286-1) — example for ∅ 50 mm
Surface roughness Ra scale
Ra = arithmetic mean deviation of the surface profile (ISO 4287). Lower Ra = smoother surface. Note: Ra alone is insufficient — always specify the measurement cutoff length λc.
Good to know
IT grade system (ISO 286)
- IT01, IT0, IT1 — gauge making, precision instruments
- IT2–IT5 — precision fits, bearings, spindles
- IT6–IT8 — standard engineering fits (most common)
- IT9–IT11 — general machining, loose fits
- IT12–IT18 — sheet metal, castings, free machining
Ra vs Rz vs Rmax
- Ra — arithmetic average deviation (most common)
- Rz — average peak-to-valley (≈ 4–7 × Ra)
- Rmax — maximum single peak-to-valley
- For sealing surfaces use Rz; for fatigue use Rmax
- Older DIN/Czech drawings use Rz as primary
Rule of thumb: IT → Ra
Approximate relationship between IT grade and Ra for general machining:
Ra ≈ 0.05 × IT_tolerance [µm]
- IT6 at ∅50 mm → tolerance ≈ 16 µm → Ra ≈ 0.8 µm
- IT8 at ∅50 mm → tolerance ≈ 39 µm → Ra ≈ 1.6 µm
Cost vs accuracy
- Each step tighter ≈ 30–50 % cost increase
- IT6 and finer require grinding or fine boring
- IT4 and finer require honing, lapping, or superfinishing
- Specify the loosest tolerance that satisfies the function
- Cylindricity and roundness add cost independently of IT