Surface Roughness Chart and Polishing Treatment
The surface roughness chart is a concept that expresses the amount and extent of deviation of a surface from being perfectly flat. As it is practically impossible to manufacture a completely flat surface on the microscopic level, every surface has a certain value of roughness that can be measured with the use of a profilometer. Depending on the scale of the deviations and the roughness parameters, different types of profilometers are used and different classifications are given.
Determining the roughness of a surface is quite important and useful from a practical engineering perspective. The surface roughness chart allows designers to predict a number of factors such as the amount of generated friction between two contacting surfaces, the performance of braking systems and determine the accuracy and precision of machining tools or methods. It also helps when looking to determine the quality of the result of a surface finishing process and considering the suitability of a surface to serve as a lubrication component. The roughness factor also allows engineers to predict the amount and magnitude of vibrations on a system and finally estimate the life-cycle of a part with much precision.
Physical Perspective and Measurement Standards
Before we look at surface roughness charts, this is an important area to be aware of: The surface roughness parameters that are defined by the ISO 4287:2000 standard (re-evaluation of ISO 4287:1997) and include the max valley depth (Rv), the max peak height (Rp), the skewness (Rsk), the kurtosis (Rku), and the distance between the highest peaks and lowest valleys (Rtm). All of the above are encompassed in the arithmetic total average roughness (Ra), and the total height of the maximum peak-to-valley distance for the assessed profile length (Rt) or simply max height of the profile. These are all pertain to a statistical mathematics approach which wants measured values to be compared against the center line that is theoretically determined or a mean value that is experimentally determined.
This results in Ra and Rt roughness values in μm (microns) that corresponds in classification ranges as per the aforementioned ISO standard. This classification is either expressed with N (ISO numbers) or Grit numbers (for sandpapers). For example, an Ra value of 6.3 μm corresponds to a classification of N9 and Grit 60, while an Ra value of 0.1 μm corresponds to N3 and Grit 500.
Surface Roughness Charts
ISO Classification Chart
| ISO Number | Ra (μm) | Rt (μm) |
| N12 | 50 | 200 |
| N11 | 25 | 100 |
| N10 | 12.5 | 50 |
| N9 | 6.3 | 25 |
| N8 | 3.2 | 13 |
| N7 | 1.6 | 8 |
| N6 | 0.8 | 4 |
| N5 | 0.4 | 2 |
| N4 | 0.2 | 1.2 |
| N3 | 0.1 | 0.8 |
| N2 | 0.05 | 0.5 |
| N1 | 0.025 | 0.3 |
Sandpaper Grit Designation Chart
| Grit Number | Ra (μm) | Rt (μm) |
| 60 | 6.3 | 25 |
| 80 | 1.8 | 9 |
| 120 | 1.32 | 6.6 |
| 150 | 1.06 | 5.3 |
| 180 | 0.76 | 3.8 |
| 220 | 0.48 | 2.4 |
| 240 | 0.38 | 1.9 |
| 320 | 0.30 | 1.5 |
| 400 | 0.23 | 1.3 |
| 500 | 0.1 | 0.8 |
Real Examples – manufacturing process surface roughness chart
Here is how various manufacturing, cutting, finishing, and forming processes compare, and what range of Ra values is to be expected with each one of them.
| Process | Range of Ra (μm) |
| Sand Casting, Hot Rolling, Flame Cutting | 25 – 12.5 |
| Forging | 12.5 – 3.2 |
| Sawing, Planning, Shaping, Perm Mold Casting, Investment Casting, Chemical Milling, EDM | 25 – 1.6 |
| Milling, Broaching, Reaming, Die Casting, Extruding, Cold Rolling, Drawing, Electron Beam Curring, Laser Cutting | 6.3 – 0.8 |
| Boring, Turning | 6.3 – 0.4 |
| Barrel Finishing, Electrolytic Grinding, Roller Burnishing | 0.8 – 0.2 |
| Grinding, Polishing, Honing, Electro-polishing | 1.6 – 0.1 |
| Lapping | 0.4 – 0.05 |
| Superfinishing |
0.2 – 0.025 |
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