Surface finish drawings show Ra values but engineers and customers may want Rz or Rt. Knowing the differences prevents over-machining.
Common Roughness Parameters
- Ra (Roughness average) — Most common. Average of absolute deviations from mean.
- Rz (Mean peak-to-valley) — Average of 5 highest peaks + 5 deepest valleys.
- Rt (Total height) — Single largest peak-to-valley distance.
- Rmax (Maximum) — Same as Rt, older notation.
- Rq / Rrms — Root-mean-square (similar to Ra but more sensitive to outliers)
Approximate Conversions
- Rz ≈ 4-6 × Ra
- Rt ≈ 6-10 × Ra
- So Ra 0.4 µm ≈ Rz 1.6-2.5 µm ≈ Rt 2.5-4 µm
Target Surface Finish by Operation
| Operation | Ra (µm) | Equivalent SAE |
|---|---|---|
| Rough turning/milling | 3.2-12.5 | 125-500 µin |
| Standard machining | 1.6-3.2 | 63-125 µin |
| Precision finishing | 0.8-1.6 | 32-63 µin |
| Reaming | 0.4-0.8 | 16-32 µin |
| Cylindrical grinding | 0.2-0.4 | 8-16 µin |
| Surface grinding | 0.1-0.4 | 4-16 µin |
| Honing | 0.05-0.2 | 2-8 µin |
| Lapping | 0.02-0.1 | 1-4 µin |
| Polishing | 0.01-0.05 | 0.5-2 µin |
Drawing Symbol
The check-mark symbol shows roughness requirement:
- Open V — Surface as machined OK
- V with bar — Material removed required (no scale)
- V with circle — No material removal allowed
- Number above the V — Ra value (µm or µin)
Measurement Methods
- Surface roughness tester (Mitutoyo SJ-210, $1k-3k) — Portable stylus profilometer
- Bench profilometer ($5k-30k) — Higher accuracy, lab use
- Optical profilometer ($30k+) — Non-contact, sub-nanometer
- Visual comparison plates ($50-200) — Quick check, less accurate
Tooling Effect on Surface Finish
| Factor | Effect on Ra |
|---|---|
| Larger nose radius | Smoother (lower Ra) |
| Wiper insert | Major improvement (often 50%) |
| Lower feed rate | Smoother (lower Ra) |
| Higher cutting speed | Slightly smoother |
| Worn tool | Rougher |
| Vibration/chatter | Much rougher with periodic pattern |
Wiper Insert Magic
A wiper-geometry insert can achieve Ra 0.4 µm at fn=0.20 mm/rev — twice the typical feed for that finish. Standard insert at same feed gives Ra 1.6 µm. Major productivity boost.
For wiper inserts, browse our insert catalog.
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