Measurement knowledge - What is roughness? How to measure it efficiently?

1.What is surface roughness?

During the machining process of parts, due to the plastic deformation of the metal surface during cutting, the vibration of the machine tool, and the tool marks left on the surface, the surfaces of the parts, no matter how smooth they are processed, can be seen as uneven peaks and valleys under a microscope, as shown in the figure.

The microscopic geometric shape features composed of peaks and valleys with small spacing on the machined surface are called surface roughness. Different machining methods will produce different roughness:

Problems caused by surface roughness:

# Wear resistance: The rougher the surface, the smaller the effective contact area and the faster the wear.

# Stability of fit properties: The rougher the surface, the easier it is to wear; for interference fit, the actual effective interference is reduced, and the connection strength is reduced.

# Fatigue strength: The larger troughs on the surface are very sensitive to stress concentration, which affects fatigue strength.

# Corrosion resistance: Rough surfaces make it easy for corrosive gases or liquids to penetrate into the inner layer of the metal, causing corrosion.

# Sealing: Rough surfaces cannot fit tightly together, and gases or liquids leak through the gaps between the contact surfaces.

# Contact stiffness: The stiffness of the machine depends to a large extent on the contact stiffness between the parts, and surface roughness will lead to a decrease in stiffness.

# 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.

# Other issues…: It will have varying 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 conductor surface.

2.How to efficiently measure surface roughness?

Roughness is a very microscopic parameter, so it was difficult to measure in the early days.

In situations where the roughness requirement is not high, the roughness level can usually be qualitatively evaluated by visual inspection or touch, or by comparing with a standard roughness sample. However, since it is subjective and difficult to quantitatively analyze, it is not widely used in the precision manufacturing industry.

In the early days of quantitative analysis, light-section microscopes were mainly used for measurement. However, with the upgrading of measurement technology, this method has gradually been replaced. Now, the most common measurement methods are pin-tracing method-contact or optical-non-contact.

Taking the widely used pin-tracing method as an example, this method is also called stylus method.

As the name suggests, a fine diamond needle (the shape and angle of the needle tip are precisely fixed) is slowly drawn across the measured surface, and a high-precision sensor records the ups and downs of the needle tip.

This allows the microscopic profile of the surface to be measured to be obtained, thereby evaluating the roughness.

As shown in the figure above, the FIMS-R flexible roughness measurement system can use flexible control functions to flexibly mobilize the world-renowned measurement brand roughness measuring instrument connected to the manipulator or servo mechanism.

While quickly completing the workpiece roughness measurement requirements, it can also store all the measurement data and graphics, saving a lot of positioning time before roughness measurement.

It is reported that the efficiency can be improved by about 90% while ensuring the measurement accuracy.

3.How to evaluate roughness?

Roughness parameters are different from some common length measurements. They are not measured directly, but are obtained by analyzing and evaluating the profile curve of the material surface to finally obtain the roughness value we need.

Of course, the early roughness evaluation standards were not unified. Each country and company had its own set of standards. In order to facilitate mutual communication, the International Organization for Standardization ISO formulated internationally recognized roughness standards and unified the calculation method of roughness parameters.

Commonly used roughness parameters and calculation methods are as follows:

Average roughness Ra:

Root mean square roughness Rq:

Average roughness depth Rz:

Maximum roughness depth Rmax:

Average spacing of contour elements RSm:

Where:

Core roughness depth Rk - the depth of the roughness core profile

Minimum peak height Rpk - the average peak height generated by the roughness core profile

Minimum valley depth Rvk - the average valley depth generated by the roughness core profile

In addition, there are some evaluation criteria based on ISO1997 standards, for reference only:

--Benchmark length: 0.08mm, 0.25mm, 0.8mm, 2.5mm, 8mm;

--Evaluation length: N=5 (usually);

--Least squares midline: perform least squares calculation on the points collected during the measurement process;

--Arithmetic mean midline of the contour: within the sampling length, make the areas of the upper and lower parts of the contour of the midline equal;

Note: In theory, the least squares midline is an ideal baseline, but it is difficult to obtain in practical applications. Therefore, the arithmetic mean midline of the contour is generally used instead, and a straight line with an approximate position can be used instead during measurement.