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Surface finish conversion chart
| Ra [Micrometers] | Ra [Micro Inches] | RMS | CLA | N (ISO) |
|---|---|---|---|---|
| 0.025 | 1 | 1.1 | 1 | 1 |
| 0.05 | 2 | 2.2 | 2 | 2 |
| 0.1 | 4 | 4.4 | 4 | 3 |
| 0.2 | 8 | 8.8 | 8 | 4 |
| 0.4 | 16 | 17.6 | 16 | 5 |
| 0.8 | 32 | 32.5 | 32 | 6 |
| 1.6 | 63 | 64.3 | 63 | 7 |
| 3.2 | 125 | 137.5 | 125 | 8 |
| 6.3 | 250 | 275 | 250 | 9 |
| 12.5 | 500 | 550 | 500 | 10 |
| 25 | 1000 | 1100 | 1000 | 11 |
| 50 | 2000 | 2200 | 2000 | 12 |
What is Surface Finish
Surface finish (Also called sometimes surface roughness) is a numeric representation of a surface’s “smoothness”. There are 3 main ways to acquire the surface roughness value:
- Mechanical Scan: The most common method is to scan the surface with a needle that records the micro “Hills ” and “valleys” along a linear section. The measurement process produces a chart analyzed by a mathematical formula to yield a surface finish value.
- Visual Comparison: An old traditional way that is rarely used today. It compares visually (With a human eye) the surface against a standard pallet of different surface qualities.
- Visual Comuturized Scan: A high-definition digital camera connected to computed performs an optical scan and calculates the surface finish.
Surface Finish units dictionary
It does not matter which scanning method was used. In the end, a chart, as shown above, has to be analyzed and yield a single number that represents the surface finish. Unfortunately, there are several different units of measurement in use, and it can get confusing. We will discuss below the typical surface roughness units in use in the machining world
Ra – Roughness Average
Ra is the most commonly used unit. According to ASME B46.1, Ra is the arithmetic average of the absolute values of the profile height deviations from the centerline, recorded within the evaluation length.
Ra – Surface finish value in Micrometers/Micro-inches
R1, R2, …Rn – Peaks along the scanned surface.
n- Number of peaks along the scanned surface
Cl – Average profile height
Ra – Surface finish value in Micrometers/Micro-inches
R1, R2, …Rn – Peaks along the scanned surface.
n- Number of peaks along the scanned surface
Cl – Average profile height
Rz – Peak to Valley Average
Rz is the second most commonly used unit. Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within the scanned profile. The highest five peaks and the five deepest valleys are taken, then averaging these distances. Rz will always yield a higher value than Ra. People make a common mistake asking, “How to convert from Ra to Rz?”. It is not possible since these are two different physical properties. It is like asking how do I convert height to weight. The statistical factor between Ra and Rz is around 600%. However, it is not accurate, and the measurement should be made according to how the surface roughness was indicated on the drawing.
Rz – Surface finish value in Micrometers/Micro-inches
Rp1, Rp2, …Rpn – Peaks along the scanned surface.
Rp1, Rp2, …Rpn – Valleys along the scanned surface.
n- Number of peaks/Valleys along the scanned surface
Rz – Surface finish value in Micrometers/Micro-inches
Rp1, Rp2, …Rpn – Peaks along the scanned surface.
Rp1, Rp2, …Rpn – Valleys along the scanned surface.
n- Number of peaks/Valleys along the scanned surface
N – Numbers (N4, N5, …)
Another common unit in use is the “N” numbers. This series of arbitrary numbers from 1 to 12 (N1, N2, …N12) is defined in ISO 1302. Each number corresponds to a Ra value. Pay attention that the scale is logarithmic. Hence the difference between N8 and N7 is much more significant than between N7 and N6.
N Vs Ra
(Complete Range)
N vs Ra
(Range Relevant for Machining)
Additional Surface Finish Measuring Units
- RMS – “Root Mean Square”. Root mean square average of the profile height deviations from the mean line. Like Ra, but instead of absolute value, the square of Ri is used to normalize the sign. Yields about 10% above Ra. This unit is seldom used.
- CLA – “Center Line Average”. Equals to Ra in micro-inches and rarely used.
- Triangles – An older ISO system before the N numbers scale. (Can appear on very old drawings)