What is the RA and RMS surface finish in machining of injection mold toolings?
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What is the RA and RMS surface finish in machining progress of injection mold toolings?
RMS and Ra are based on different methods of calculating the roughness. Both are done with a profilometer, but the profilometer calculates the roughness differently for Ra and RMS.
Ra is the arithmetic average of surface heights measured across a surface. Simply average the height across the microscopic peaks and valleys.
Ra and RMS are both representations of surface roughness, but each is calculated differently.
Ra is calculated as the Roughness Average of a surfaces measured microscopic peaks and valleys.
RMS is calculated as the Root Mean Square of a surfaces measured microscopic peaks and valleys. Each value uses the same individual height measurements of the surfaces peaks and valleys, but uses the measurements in a different formula.
A single large peak or flaw within the microscopic surface texture will effect (raise) the RMS value more than the Ra value, which is why Ra is more commonly used today as a measurement.
Machining surface finish chart: Ra vs RMS
The following is the conversion chart for Ra (um), Ra (micro inch), Rz (um), RMS.
| USA Ra(um) | USA Ra(micro inch) | USA RMS(micro inch) | Machining Finish Method |
| 50 | 2000 | 2200 | The most coarse machining or good rough casting surfaces. |
| 25 | 1000 | 1100 | The machining marks are very obvious.Rough turning,planning,milling and drilling. |
| 12.5 | 500 | 550 | The machining marks are obvious.Normal turning,planning,milling and drilling. |
| 8 | 320 | 352 | The machining marks are visible.Normal turning,boring,planning,milling and drilling. |
| 6.3 | 250 | 275 | |
| 5 | 200 | 220 | The machining marks are not obvious,but still visible.Normal turning,boring,planning,milling and drilling. |
| 4 | 160 | 176 | |
| 3.2 | 125 | 137.5 | |
| 2.5 | 100 | 110 | The machining marks are blur,but direct obvious.Control of the turning,boring,planning,drilling and grinding. |
| 2 | 80 | 88 | |
| 1.6 | 63 | 69.3 | |
| 1.25 | 50 | 55 | The machining marks are direct blur,but still visible,controlled turning,boring,planning,drilling and grinding. |
| 1 | 40 | 44 | |
| 0.8 | 32 | 35.2 | |
| 0.63 | 25 | 27.5 | The machining marks are direct blur.Reaming,grinding,boring and rolling. |
| 0.5 | 20 | 22 | |
| 0.4 | 16 | 17.6 | |
| 0.2 | 12.5 | 13.75 | Machining marks are direct invisible,grinding and super machining. |
| 10 | 11 | ||
| 8 | 8.8 | ||
| 0.1 | 4 | 4.4 | Surface is dark gloss,super machining. |
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