Surface roughness is crucial for most surfaces involved in sliding contact because factors such as the initial rate of wear and its ongoing nature are highly dependent on this property. These surfaces are typically load-bearing and require roughness markings to ensure suitability for their intended use.
Many components require specific surface finishes to achieve their required functions. Examples include the journal bearings on a car body before painting or on a crankshaft or camshaft.
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What is Surface Roughness?
Surface roughness, as we commonly refer to it in everyday measurements, can be understood as the unevenness of fine gaps and tiny peaks and valleys during product manufacturing.
It is usually defined as the minute distance (wavelength) between two peaks or two valleys. Typically, the wavelength is within 1 mm or less. It can also be defined as a measurement of micro-profiles, commonly known as micro-error values.
In summary, you may now have a general understanding of roughness. The following section provides a more detailed analysis.
In general, we use a baseline to evaluate surface roughness. The highest point above the baseline is called the crest, and the lowest point below the baseline is called the trough. The height between the crest and trough is represented by Z, and the spacing of the micro-texture of the processed product is represented by S.
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The value of S is typically defined in national verification standards as follows:
S < 1 mm: Defined as surface roughness
1 ≤ S ≤ 10 mm: Defined as surface waviness
Chinese national metrological verification standards stipulate that surface roughness is usually evaluated using three parameters: VDA3400, Ra, and Rmax, with the unit of measurement usually expressed in μm.
Relationship between evaluation parameters
Ra is defined as the average arithmetic deviation of the curve (average roughness), Rz is defined as the average height of the 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 table below for the specific relationship between Ra and Rmax:
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Table: Comparison of Ra and Rmax parameters (µm)
How is surface roughness formed?
Surface roughness is caused by the machining process of a workpiece. The machining method, the workpiece material, and the process all influence surface roughness.
For example, during electrical discharge machining (EDM), discharge bumps appear on the surface of the machined part.
Different machining processes and workpiece materials result in various microscopic marks left on the surface of the machined part, such as density, depth, and shape variations.
The Influence of Surface Roughness on Workpieces
Workpiece wear resistance, fit stability, fatigue strength, corrosion resistance, sealing performance, contact stiffness, measurement accuracy, coatings, thermal conductivity and contact resistance, reflectivity and radiation properties, resistance to liquid and gas flow, and current flow on conductor surfaces all have varying degrees of influence.
Evaluation Criteria for Surface Roughness
① Sampling Length: The unit length of each parameter. The sampling length is the length of a reference line specified for evaluating surface roughness. Under the ISO 1997 standard, 0.08mm, 0.25mm, 0.8mm, 2.5mm, and 8mm are generally used as reference lengths.
② Evaluation Length: Consists of N reference lengths. The surface roughness of different parts of a component cannot be accurately represented by a single reference length; therefore, N sampling lengths are needed to evaluate the surface roughness. Under the ISO 1997 standard, the evaluation length is generally N equal to 5.
③ Reference Line: The reference line is the profile centerline used to evaluate roughness parameters. Generally, there are least squares centerlines and profile arithmetic mean centerlines.
[Least Squares Centerline] is calculated using the least squares method from points collected during measurement.
[Profile Arithmetic Mean Centerline] ensures that the areas of the two profile parts above and below the centerline are equal within the sampling length.
Theoretically, the least squares centerline is the ideal reference line, but it is difficult to obtain in practical applications. Therefore, the profile arithmetic mean centerline is generally used instead, and a straight line with an approximate position can be used during measurement.
How is surface roughness obtained?
Surface roughness evaluation is increasingly valued in manufacturing. To study surface roughness, a specialized machine is required: a surface roughness measuring instrument. (Image: Formtracer Avant series) A surface roughness measuring instrument uses a highly sensitive diamond stylus to traverse the surface, much like a phonograph pickup. It then separates the small-wavelength roughness of large-scale ripples and contours from the longer wavelengths-a process known as electronic filtering.
*The characteristics of stylus-type roughness measuring instruments are defined in ISO 3274:1996.
(Image: Schematic diagram of a stylus-type surface roughness measuring instrument)
While most accurate and complete surface roughness measurements use specialized measuring machines, in some cases, handheld tool kits can be used for faster and lower-cost operation, as shown below:
Roughness comparison plates are nickel-based samples made by electroforming. They are ideal for metalworking and are highly effective auxiliary tools.
The operator simply runs their fingernail across each surface in a set to find the closest match to the workpiece being compared. Some people use these model sets as lookup tables, but it's important to note that these are not material standards. Roughness measuring machines offer different functions, use different evaluation methods, and vary in cost. Before selecting a machine, it's advisable to consult with professional manufacturers to choose the most suitable model based on your needs.
