What is machined parts surface roughness

2021/07/06

What is machined parts surface roughness

1,What is surface roughness?

Machined parts Surface Roughness (Surface Roughness) is what we call surface roughness in our daily measurement, which can be understood as the unevenness of fine spacing and tiny peaks and valleys in the process of processing products.

It is usually defined as the small distance between two peaks or two valley fingers (wave distance). In general, the wave distance is within 1mm or less. It can also be defined as the measurement of micro-profile, commonly known as micro-error value. .



To sum up, you may already have a general concept of roughness, so the following content is a more detailed analysis.

We generally evaluate roughness with a baseline. The highest point above the baseline is called the crest point, and the lowest point below the baseline is called the trough point. Then the height between the crest and the trough is represented by Z, which is the distance between the micro-textures of the processed product. We use S to denote.



Under normal circumstances, the size of the S value is defined in the national verification standards:

S<1mm is defined as surface roughness
1≤S≤10mm is defined as surface waviness

China's national metrological verification standard stipulates that: under normal circumstances, the three parameters of VDA3400, Ra, Rmax are used to evaluate the surface roughness of the verification, and the measurement unit is usually expressed in μm.

The relationship of evaluation parameters

Ra is defined as the average arithmetic deviation of the curve (average roughness), Rz is defined as the average height of unevenness, and Ry is defined as the maximum height. The maximum height difference Ry of the micro profile is also represented by Rmax in other standards.

Please refer to the following table for the specific relationship between Ra and Rmax:


form: Ra, Rmax parameter comparison (um)



2 How is the surface roughness formed?

The formation of surface roughness is caused by the processing of the workpiece. The processing method, the material of the workpiece, and the process are all factors of the image surface roughness.

For example, there are discharge bumps on the surface of the processed part during electrical discharge machining.

The processing technology and the material of the parts are different, and there are also various differences in the microscopic marks left on the surface of the processed parts, such as (density, depth, shape change, etc.).



3 The effect of surface roughness on the workpiece

Wear resistance of workpiece
Coordination stability
Fatigue strength
Corrosion resistance
Tightness
Contact stiffness
measurement accuracy
...

Coating, thermal conductivity and contact resistance, reflectivity and radiation performance, liquid and gas flow resistance, current flow on the conductor surface, etc. will all have varying degrees of influence.





4 Evaluation basis for surface roughness

①Sampling length

The unit length of each parameter, the sampling length is the length of a reference line for evaluating the surface roughness. Generally use 0.08mm, 0.25mm, 0.8mm, 2.5mm, 8mm as the reference length under the ISO1997 standard.

②Evaluation length

Consists of N reference lengths. The surface roughness of each part of the component surface cannot truly reflect the true roughness parameters on a reference length, but N sampling lengths are needed to evaluate the surface roughness. Under the ISO1997 standard, the evaluation length is generally N equal to 5.

③Baseline

The baseline is the center line of the profile for evaluating roughness parameters. Generally, there are the least square method centerline and the contour arithmetic average centerline.

[Middle line of least square method] is to calculate the points collected in the measurement process by the least square method.

[Contour Arithmetic Average Centerline] Within the sampling length, make the area of the upper and lower contours of the centerline equal.
Theoretically, the least squares centerline is an ideal baseline, but it is difficult to obtain in practical applications. Therefore, it is generally replaced by the arithmetic mean centerline of the contour, and a straight line with an approximate position can be used instead.



5 How is the surface roughness obtained?

The evaluation of surface roughness has been paid more and more attention in the manufacturing industry. To study the surface roughness, a dedicated machine is required, namely:
Surface roughness measuring instrument


Formtracer Avant series

The surface roughness measuring machine is to install a high-sensitivity diamond stylus across the surface, just like the pickup of a phonograph. Then the large-scale ripples and the small-wavelength roughness of the profile are separated from the longer wavelength, that is, the measuring instrument is electronically filtered.




Schematic diagram of the stylus type surface roughness measuring instrument:



Most of the correct and complete surface roughness measurement methods use a dedicated measuring machine, but in some cases, for quick and low-cost operation, you can also use a hand-held kit tool to measure, as shown in the figure below:


The roughness comparison sheet is a nickel-based sample made by electroforming. It is ideal for metal processing and is a very effective auxiliary tool.

When using, the operator only needs to scrape across each surface of a group with his nails, and look for the closest thing to the workpiece to be compared. Some people will use these model groups as a lookup table, but it is worth noting that this is not a material standard.

Roughness measuring machines can achieve different functions, different evaluation methods, and different costs. Before selecting the model, you can consult a professional manufacturer and choose the most suitable model according to your needs.