Surface Finish Degree (Smoothness) Comparison Table between Rz, Ra and RMS for Iron and Steel Castings
What standard can be used to express the surface finish or smoothness of steel castings?
The comparison table we made as below is for your reference. You can find the comparison of the new Chinese Ra and Rz standards with the American micron and micron inch Ra in this table. In addition, we have also borrowed from China’s old grading system for your reference. The old grade is still widely used in China. We usually call the downward-pointing triangle “hwar”. However, we usually do not recommend buyers to indicate the old grades of China on the drawings or inquiry sheets. The comparison is for reference only, not as an inspection standard.
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| China Old Grade | China New Ra |
China New Rz |
USA micron Ra |
USA microinch Ra |
| ▽ 1 | 50 | 200 | ||
| ▽ 2 | 25 | 100 | ||
| ▽ 3 | 12.5 | 50 | ||
| ▽ 4 | 6.3 | 25 | 8.00 | 320 |
| 6.30 | 250 | |||
| ▽ 5 | 3.2 | 12.5 | 5.00 | 200 |
| 4.00 | 160 | |||
| 3.20 | 125 | |||
| ▽ 6 | 1.6 | 6.3 | 2.50 | 100 |
| 2.00 | 80 | |||
| 1.60 | 63 | |||
| ▽ 7 | 0.8 | 6.3 | 1.25 | 50 |
| 1.00 | 40 | |||
| 0.80 | 32 | |||
| ▽ 8 | 0.4 | 3.2 | 0.63 | 25 |
| 0.50 | 20 | |||
| 0.40 | 16 | |||
| ▽ 9 | 0.2 | 1.6 | 0.20 | 12.5 |
| 10 | ||||
| 8 | ||||
| ▽ 10 | 0.1 | 0.8 | 0.10 |
| RMS (microinch) | RMS (µm) |
RA (microinch) | RA (µm) |
Grit Size |
| 80 58 47 34 17 14 |
2.03 1.47 1.2 0.6 0.43 0.36 |
71 52 42 30 15 12 |
1.90 1.32 1.06 0.76 0.38 0.30 |
80 120 150 180 240 320 |
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