Comparison table of surface roughness (finish) between China and the United States

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Comparison table of surface roughness (finish) between China and the United States. Surface roughness is expressed using parameters such as Ra, Rz, and Ry. According to GB 3505, the three commonly used surface roughness parameters and their corresponding values are: Arithmetic mean deviation of the profile (Ra) – the arithmetic mean deviation of the profile shape； Micro-irregularity peak height (Rz) – the peak height of irregularities ； Maximum profile height (Ry) – the minimum height of the profile. Ra--the arithmetic mean of the absolute values of the profile deviations within the sampling length L. Rz--the sum of the average of the 5 highest peak heights and the average of the 5 lowest valley depths within the sampling length. Ry--the distance between the top line of the contour peak and the bottom line of the contour valley within the sampling length L. If the roughness is not specified on the drawing and Ra/Rz/Ry are to be used, Ra is the default value. Comparison table of surface roughness (finish) between China and the United States: Old Chinese standard (finish), New Chinese standard (roughness) Ra; American standard (micrometers) Ra, American standard (microinches) Ra. ▽ 4: 6.3, 8.00, 320; 6.30, 250. ▽ 5: 3.2, 5.00, 200; 4.00, 160; 3.20, 125. ▽ 6: 1.6, 2.50, 100; 2.00, 80; 1.60, 63. ▽ 7: 0.8, 1.25, 50; 1.00, 40; 0.80, 32. ▽ 8: 0.4, 0.63, 25; 0.50, 20; 0.40, 16. Surface roughness refers to the small spacing and minor irregularities in the shape of a machined surface. The distance between its two peaks or two troughs (the wavelength) is very small (under 1 mm), making it difficult to distinguish with the naked eye; therefore, it belongs to micro-geometric shape errors. The lower the surface roughness, the smoother the surface. The level of surface roughness has a significant impact on the performance of mechanical parts, mainly in the following aspects: 1. Surface roughness affects the wear resistance of parts. The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, and the faster the wear occurs. ② Surface roughness affects the stability of the fit properties. In clearance fit, the rougher the surface, the more prone it is to wear, causing the clearance to gradually increase during operation ； In the case of interference fits, the micro-protrusions are flattened during assembly, which reduces the actual effective interference and thus lowers the strength of the connection. ③ Surface roughness affects the fatigue strength of parts. The surface of rough parts features large troughs, which, similar to sharp notches and cracks, are highly sensitive to stress concentration, thereby affecting the fatigue strength of the parts. ④ Surface roughness affects the corrosion resistance of parts. A rough surface allows corrosive gases or liquids to penetrate into the inner layers of the metal through the microscopic recesses on its surface, causing surface corrosion. ⑤ Surface roughness affects the sealing performance of parts. Rough surfaces cannot fit together tightly, allowing gas or liquid to leak through the gaps between them. Furthermore, surface roughness also affects the appearance and measurement accuracy of parts. Surface roughness is classified into Ra, Rz, and Ry. According to GB 3505, the three commonly used surface roughness parameters along with their corresponding values are: the arithmetic mean deviation of the profile (Ra) ； Micro-irregularity peak height (Rz) – the peak height of irregularities ； Maximum profile height (Ry) – the minimum height of the profile. Ra--the arithmetic mean of the absolute values of the profile deviations within the sampling length L. Rz--the sum of the average of the 5 highest peak heights and the average of the 5 lowest valley depths within the sampling length. Ry--the distance between the top line of the contour peak and the bottom line of the contour valley within the sampling length L. If the roughness is not specified on the drawing and Ra/Rz/Ry are to be used, Ra is the default value. Comparison table of surface roughness (finish) between China and the United States: Old Chinese standard (finish), New Chinese standard (roughness) Ra; American standard (micrometers) Ra, American standard (microinches) Ra. ▽ 4: 6.3, 8.00, 320; 6.30, 250. ▽ 5: 3.2, 5.00, 200; 4.00, 160; 3.20, 125. ▽ 6: 1.6, 2.50, 100; 2.00, 80; 1.60, 63. ▽ 7: 0.8, 1.25, 50; 1.00, 40; 0.80, 32. ▽ 8: 0.4, 0.63, 25; 0.50, 20; 0.40, 16. Surface roughness refers to the small spacing and minor irregularities in the shape of a machined surface. The distance between its two peaks or two troughs (the wavelength) is very small (under 1 mm), making it difficult to distinguish with the naked eye; therefore, it belongs to micro-geometric shape errors. The lower the surface roughness, the smoother the surface. The level of surface roughness has a significant impact on the performance of mechanical parts, mainly in the following aspects: 1. Surface roughness affects the wear resistance of parts. The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, and the faster the wear occurs. ② Surface roughness affects the stability of the fit properties. In clearance fit, the rougher the surface, the more prone it is to wear, causing the clearance to gradually increase during operation ； In the case of interference fits, the micro-protrusions are flattened during assembly, which reduces the actual effective interference and thus lowers the strength of the connection. ③ Surface roughness affects the fatigue strength of parts. The surface of rough parts features large troughs, which, similar to sharp notches and cracks, are highly sensitive to stress concentration, thereby affecting the fatigue strength of the parts. ④ Surface roughness affects the corrosion resistance of parts. A rough surface allows corrosive gases or liquids to penetrate into the inner layers of the metal through the microscopic recesses on its surface, causing surface corrosion. ⑤ Surface roughness affects the sealing performance of parts. Rough surfaces cannot fit together tightly, allowing gas or liquid to leak through the gaps between them. Furthermore, surface roughness also affects the appearance and measurement accuracy of parts.