Key Principles of Surface Finish Every Mechanical Engineer Should Master

Surface roughness is crucial for most surfaces that are involved in sliding contact. This is because factors such as the rate of wear and the nature of that wear depend heavily on this property.

These surfaces are typically load-bearing and must have the appropriate roughness to ensure they are suitable for their intended use.

Many components require specific surface processing to achieve the necessary functionality. For example, the body of a car must be properly prepared before painting, and journal bearings on a crankshaft or camshaft need to meet specific roughness standards.

 

What is surface roughness?

Surface roughness refers to the texture of a surface as measured in everyday terms. It can be understood as the unevenness created by fine spacing, as well as the tiny peaks and valleys that occur during product processing.

Typically, surface roughness is defined as the small distance (wave distance) between two peaks or two valleys on a surface. Generally, this wave distance is 1 mm or less. Additionally, it can be described as the measurement of micro-contours, often referred to as micro-error values.

 

In summary, you might have a basic understanding of roughness, so the following content provides a more detailed analysis.

We typically evaluate roughness in relation to a baseline. The highest point above the baseline is referred to as the crest point, while the lowest point below the baseline is known as the trough point. The height between the crest and the trough is represented by Z, and the spacing of the microtexture of the processed product is represented by S.

 

Normally, 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 metrology verification standards specify that the three parameters, namely plate number 1, Ra, and Rmax, are typically used to assess surface roughness during verification, with the measurement unit commonly expressed in μm.

 

Relationship between evaluation parameters

Ra refers to the average arithmetic deviation of a surface curve, which is a measure of its average roughness. Rz is defined as the average height of the unevenness present on the surface. Additionally, Ry represents the maximum height of these unevenness variations. In other standards, the maximum height difference Ry of the micro-profile is also referred to as Rmax.

For the specific relationship between Ra and Rmax, please refer to the table below:

 

 

How is surface roughness formed?

The formation of surface roughness results from the workpiece CNC processing. Factors affecting surface roughness include the processing method, the workpiece material, and the process itself.

For example: discharge bumps appear on the surface of the processed part during discharge processing.

The processing technology and material of the part differ, resulting in distinct microscopic traces on the surface, such as density, depth, and shape variations.

 

The influence of surface roughness on the workpiece

The wear resistance of the workpiece
Matching stability
Fatigue strength
Corrosion resistance
Sealing
Contact stiffness
Measurement accuracy
……
The coating, thermal conductivity and contact resistance, reflection ability and radiation performance, resistance to liquid and gas flow, and the flow of current on the conductor surface will all have different degrees of influence.

 

Evaluation basis of surface roughness

① Sampling length

The unit length of each parameter is the reference line length for evaluating surface roughness. According to the ISO1997 standard, reference lengths of 0.08 mm, 0.25 mm, 0.8 mm, 2.5 mm, and 8 mm are generally used.

 

② Evaluation length

The evaluation of surface roughness consists of N reference lengths. The surface roughness of each section of a component’s surface may not accurately represent the true roughness parameters for a single reference length. To adequately assess surface roughness, it is necessary to consider N sampling lengths. According to the ISO1997 standard, the typical value for N is 5.

 

③ Reference line

The reference line serves as the contour center line for evaluating roughness parameters. There are typically two methods to establish this line: the least squares center line and the contour arithmetic mean center line.

The least squares center line is determined by performing least squares calculations on the points collected during the measurement process. In contrast, the contour arithmetic mean center line is defined such that the areas of the upper and lower contours surrounding the center line are equal over the specified sampling length.

In theory, the least squares center line is considered the ideal baseline; however, it can be challenging to obtain in practical applications. As a result, the contour arithmetic mean center line is generally used instead. During measurement, a straight line that approximates the position of the center line can also be utilized.

 

How to obtain surface roughness?

The evaluation of surface roughness is becoming more and more important in the manufacturing industry. To study surface roughness, a dedicated machine is needed, namely:

Surface roughness measuring instrument

The surface roughness measuring machine is designed to install a highly sensitive diamond stylus that scratches the surface, similar to how a phonograph pickup operates. This machine separates the small wavelength roughness from the larger ripples and contours by using electronic filtering to distinguish between shorter and longer wavelengths.

 

Schematic diagram of the structure of the probe type surface roughness measuring instrument:

 

Handheld tool measurement

Although most correct and complete surface roughness measurement methods use dedicated measuring machines, in some cases, handheld kit tools can also be used for quick and low-cost operation, as shown below:

 

Roughness comparison sheets are nickel-based samples created through an electroforming process. They are ideal for metal processing and serve as effective auxiliary tools.

 

To use these sheets, the operator simply needs to scrape the surface of each sheet within a group using their fingernail to identify the one that most closely matches the workpiece being compared. While some people use these groups of models as reference tables, it is important to remember that they do not represent a material standard.

 

Roughness measuring machines offer various functions, evaluation methods, and price points. Before selecting a model, it’s advisable to consult with a professional manufacturer to choose the most suitable option based on your specific needs.

 

 

If you want to know more or inquiry, please feel free to contact info@anebon.com

Anebon supports our purchasers with ideal premium quality products and is a substantial-level company. Becoming a specialist manufacturer in this sector, Anebon has acquired rich practical working experience in producing and managing for 2019 Good Quality Precision CNC aluminum machined parts/Precision Aluminum rapid CNC machining parts and zinc die casting parts.