Surface roughness

Surface roughness refers to the unevenness with small spacing and tiny peaks and valleys on the surface of the part. It is a type of surface unevenness and is mainly determined by the processing method and processing conditions. It will directly affect the fit, friction, wear, fatigue strength and appearance quality of the parts.

Ra VS Rz

Ra: The average absolute value of the deviation between the surface contour line and the center line

Rz: The average height difference between the highest 5 peaks and the lowest 5 valleys on the surface

Example comparison:

Surface A has many small undulations (high Ra, low Rz)

Surface B is very flat, but has a scratch (low Ra, high Rz)

Conclusion:

Surface A has a high overall roughness, but no obvious scratches.

Surface B has a low average value, but may fail functionally (scratches are too deep), and Rz can reveal the problem.

Engineering suggestions:

If you pursue overall processing quality (such as bearing seats, general assembly surfaces), it is recommended to use Ra;

If you focus on local functional reliability (such as seals, O-ring grooves, sliding pairs), you can also refer to Rz;

High-precision industries (such as aviation, molds) usually require both Ra and Rz parameters.

The main effects of surface roughness on parts

1. Affecting wear resistance: The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, the greater the friction resistance, and the faster the wear.

2. Affecting the stability of the fit: For clearance fit, the rougher the surface, the easier it is to wear, causing the gap to gradually increase during the working process; for interference fit, the actual effective interference is reduced and the connection strength is reduced because the micro-peaks are squeezed flat during assembly.

3. Affecting fatigue strength: There are large troughs on the surface of rough parts. Like sharp notches and cracks, they are very sensitive to stress concentration, thus affecting the fatigue strength of parts.

4. Affecting corrosion resistance: The rough surface of parts makes it easy for corrosive gases or liquids to penetrate into the inner layer of the metal through the micro-troughs on the surface, causing surface corrosion.

5. Affecting sealing: Rough surfaces cannot fit tightly together, and gases or liquids leak through the gaps between contact surfaces.

6. Affecting contact stiffness: Contact stiffness is the ability of the mating surface of parts to resist contact deformation under the action of external forces. The stiffness of the machine depends largely on the contact stiffness between the parts.

7. Influence on measurement accuracy: The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the measurement accuracy, especially in precision measurement.

In addition, the surface roughness will have different degrees of influence on the coating, thermal conductivity and contact resistance, reflection ability and radiation performance of the parts, resistance to liquid and gas flow, and current flow on the surface of the conductor.

FAQ

1. What is surface roughness?

Answer: Surface roughness refers to the microscopic geometric shape characteristics composed of small spacing and peaks and valleys on the surface of the part. It is a microscopic geometric shape error.

2. How is surface roughness generated?

Answer: The surface formed by cutting or other methods of parts always has geometric shape errors due to plastic deformation of materials, mechanical vibration, friction and other reasons during processing.

3. What effect does surface roughness have on parts?

Answer: Surface roughness has an important impact on the friction and wear, fatigue strength, corrosion resistance and matching properties of parts.

4. What are the main national standards for "surface roughness" in my country at present?

Answer: GB/T 3505 2000 Surface roughness terminology surface and its parameters; GB/T 1031-1995 Surface roughness parameters and their values; GB/T 131-1993 Mechanical drawing surface roughness symbols, codes and annotations.

5. What are the basic evaluation parameters?

Answer: The three height parameters are basic evaluation parameters, namely the arithmetic mean deviation of the profile (Ra), the ten-point height of micro-roughness (Rz) and the maximum height of the profile (Ry); the other three are additional evaluation parameters, namely the average spacing of the micro-roughness of the profile (Sm), the average spacing of the single peak of the profile (S) and the profile support length ratio (tP).

6. What is called the arithmetic mean deviation of the profile (Ra)?

Answer: Within the sampling length, the arithmetic mean of the absolute value of the distance from each point on the measured profile to the midline of the profile. The larger the Ra value, the rougher the surface. Ra can objectively reflect the geometric characteristics of the measured profile. The Ra value can be directly measured by an electric profilometer, but it is not intuitive enough.

7. What is called the ten-point height of micro-roughness (Rz)?

Answer: Within the sampling length, the sum of the average value of the 5 largest profile peak heights and the average value of the 5 largest valley depths. The larger the Rz value, the rougher the surface. Rz is used to evaluate the surface roughness height parameter with good intuitiveness and is easy to measure on optical instruments, but it has limitations in reflecting the geometric characteristics of the measured profile.

8. What is called the maximum profile height (Ry)?

Answer: The distance between the peak line and the valley line of the profile within the sampling length. The peak line and the valley line refer to the lines parallel to the midline and passing through the highest and lowest points of the profile within the sampling length. Parameter Ry is simple to measure. When the measured surface is very small and Rz and Rz are not suitable for evaluation, Ry can be used.

9. How to determine the allowable values of surface roughness height evaluation parameters (Ra, Rz, Ry)?

Answer: See GB/T 1031-1995 Surface roughness parameters and their values.

When the symbol and code of surface roughness is , what is its meaning?

Answer: The basic symbol indicates that the surface can be obtained by any method. When no roughness parameter value or relevant instructions (such as surface treatment, local heat treatment conditions, etc.) are added, it is only applicable to simplified code marking.

When the symbol or code of surface roughness is , what is its meaning?

Answer: A short dash is added to the basic symbol, indicating that the surface is obtained by removing material. For example: turning, milling, drilling, grinding, shearing, polishing, corrosion, EDM, gas cutting, etc.

When the symbol or code of surface roughness is , what is its meaning?

Answer: A small circle is added to the basic symbol, indicating that the surface is obtained by not removing material. For example: casting, forging, stamping deformation, hot rolling, cold rolling, powder metallurgy, etc. Or it is used to maintain the original supply status (including maintaining the status of the previous process).

13. When the symbol or code of surface roughness is , what is its meaning?

Answer: The upper limit of Ra of the surface roughness obtained by any method is 3.2μm.

14. When the symbol or code of surface roughness is , what is its meaning?

Answer: The upper limit of Ra of the surface roughness obtained by removing material is 3.2μm.

15. When the symbol or code of surface roughness is , what is its meaning?

Answer: The upper limit of Ra of the surface roughness obtained by the method without removing material is 3.2μm.

16. What is the meaning of the symbol and code of surface roughness when it is ?

Answer: The upper limit of Ra of the surface roughness obtained by the method of removing material is 3.2μm, and the lower limit of Ra is 1.6μm.

17. What is the meaning of the symbol and code of surface roughness when it is ? Answer: The upper limit of Ry of the surface roughness obtained by any method is 3.2μm.