The Seal That Leaked (Rough Surface)
We machined a shaft (for a seal, article 175). The surface was rough (Ra = 3.2 µm, from a milling cut). The seal (an O-ring, or a rod seal, article 175) didn’t seal (it leaked). The rough surface (the peaks) let the air (or oil) through (the seal couldn’t cover the peaks). We machined the surface (turned it, to Ra = 0.8 µm, fine). The seal sealed. The mistake was not specifying the surface finish (the seal needs a fine surface, Ra = 0.8, not 3.2).
Surface finish (Ra) — match the finish to the function. This article covers the selection.
The Ra (Roughness Average)
Ra is the roughness (the average of the peaks/valleys). A high Ra (3.2 µm) is rough (visible tool marks). A low Ra (0.8 µm) is fine (smooth). The machining process determines the Ra:
- Rough milling: Ra = 6.3–12.5 µm (rough).
- Finish milling: Ra = 1.6–3.2 µm.
- Turning (fine): Ra = 0.8–1.6 µm.
- Grinding: Ra = 0.2–0.4 µm (very fine).
Step 1: The Function (Seal?)
Is the surface a seal (article 175)? Use a fine finish (Ra = 0.8 µm, turning, or grinding). The seal needs a smooth surface (no peaks to leak). Is it a bolt hole (clearance)? Use a rough finish (Ra = 3.2 µm). Is it a decorative surface? Use a medium finish (Ra = 1.6 µm).
The surface finish rule: Match Ra to the function. The seal that leaked had Ra = 3.2 (rough). Use Ra = 0.8 (fine) for a seal. For a clearance hole, 3.2 is fine. Don’t over-specify (grinding costs).
Step 2: The Seal (Article 175)
A rod seal (article 175) needs Ra = 0.4–0.8 µm (fine). A rough surface (3.2 µm) leaks. A ground surface (0.2 µm) is best (but expensive). For a standard cylinder (article 175), the rod is ground (Ra = 0.4). Don’t use a milled rod (rough, leaks).
Step 3: The Bearing Surface
A bearing (article 46) needs a fine surface (Ra = 0.8 µm, on the shaft). A rough surface wears the bearing fast. The shaft (in the bearing) is ground (fine).
Step 4: The Cost
A rough finish (3.2 µm) is cheap (milling). A fine finish (0.8 µm) costs more (turning). A very fine (0.2 µm) is expensive (grinding). Use the coarsest finish that works (don’t over-specify).
| Ra | Process | Use |
|---|---|---|
| 6.3–12.5 | Rough milling | Non-critical |
| 1.6–3.2 | Finish milling | Clearance, general |
| 0.8 | Fine turning | Seal (article 175), bearing |
| 0.2 | Grinding | Precision (expensive) |
A Surface Finish Checklist
- Is the surface a seal? (Ra = 0.8?)
- Is it a bearing? (Fine?)
- Is it a clearance? (Ra = 3.2?)
- Is it decorative? (Ra = 1.6?)
- Is the process right? (Mill? Turn? Grind?)
- Does the seal leak? (Test? Article 175?)
- Is the cost right? (No over-spec?)
- Is the drawing marked? (Ra?)
- Is it measured? (Roughness gauge?)
- Is it consistent? (Repeats?)
The Bottom Line
Surface finish (Ra) matches the finish to the function. The seal that leaked had Ra = 3.2 (rough). Use Ra = 0.8 (fine) for a seal. For a clearance, 3.2 is fine. The seal that didn’t leak wasn’t the smoothest surface — it was the right Ra.
