The concept of surface roughness
Surface roughness refers to the unevenness of a machined surface characterized by small pitches and tiny peaks and valleys. The distance (pitch) between two peaks or valleys is typically small, less than 1mm, and is considered a micro-geometric error.
The degree of surface roughness is specifically determined by the Z (height) and S (spacing) conditions of the tiny peaks and valleys, with S being the most commonly used method of categorization.
- S＜1mm is the surface roughness;
- 1≤S≤10mm is the waviness;
- S>10mm is f shape.
Comparison table of VDI3400, Ra and Rmax
In national standards, three indicators are commonly used to assess surface roughness (unit is μm).
- Average arithmetic deviation of the contours: Ra
- The average height of unevenness: Rz
- Maximum height: Ry
The Ra index is widely used in actual production. The maximum microscopic height deviation of a contour, Ry, is commonly referred to as Rmax in Japan and other countries, while the VDI index is commonly used in Europe and America. The following table compares VDI3400, Ra, and Rmax.
Table: Comparison of Ra, Rmax parameters (μm)
|VDI3400||Ra ( μm )||Rmax (μm )|
Surface roughness formation factors
Surface roughness is typically formed by various factors, including the processing method used.
For instance, factors contributing to surface roughness include friction between the tool and the surface of the part during machining, plastic deformation of the surface layer metal during chip separation, high-frequency vibrations in the processing system, and discharge pits in electrical machining.
The depth, density, shape, and texture of the marks left on the processed surface can vary due to differences in processing methods and workpiece materials.
Main effects of surface roughness on parts
The Impact on Wear Resistance:
The rougher the surface, the smaller the effective contact area between mating surfaces, the higher the pressure, and the higher the frictional resistance, resulting in faster wear.
The Impact on Gap Fit Stability:
For gap fit, a rougher surface leads to increased wear and a gradual increase in the gap during operation. In the case of interference fit, the actual effective interference is reduced due to the flattening of microscopic convex peaks during assembly, leading to a decrease in connection strength.
The Impact on Fatigue Strength:
The rough surface of a part has large troughs that are susceptible to stress concentration, similar to sharp-edged notches and cracks, affecting the part’s fatigue strength.
The Impact on Corrosion Resistance:
Rough part surfaces can easily allow corrosive gases or liquids to penetrate the microscopic valleys on the surface and reach the metal inner layer, causing surface corrosion.
The Impact on Sealability:
Rough surfaces do not fit tightly against each other, allowing gases or liquids to leak through the gaps between contact surfaces.
The Impact on Contact Stiffness:
Contact stiffness refers to a part’s bonding surfaces’ ability to resist deformation under external forces. The stiffness of a machine is largely dependent on the contact stiffness between its parts.
The Impact on Measurement Accuracy:
Parts are measured by their surface and the roughness of the measuring tool’s surface will directly affect measurement accuracy, especially in precision measurement.
Additionally, surface roughness has a variable effect on a part’s coating, thermal and contact resistance, reflectance and radiation properties, resistance to liquid and gas flow, and current flow through the surface of a conductor.
Evaluation basis of surface roughness
- Sampling length
The sampling length is the length of a designated reference line used to evaluate surface roughness.
To accurately reflect the surface roughness characteristics of a part, the sampling length should be selected based on the formation and texture of the actual surface. The sampling length should be measured in accordance with the general profile of the actual surface.
The purpose of specifying and selecting the sample length is to minimize the impact of surface ripple and shape errors on surface roughness measurement results.
The evaluation length is a required length used to assess the contour and may include one or multiple sampling lengths.
Since the surface roughness of a part’s surface is not always uniform, it may not be possible to accurately reflect a specific surface roughness feature with just one sampling length. Hence, multiple sampling lengths on the surface are necessary to evaluate the surface roughness.
Typically, the evaluation length consists of five sampling lengths.
The baseline is the centerline of the profile used to evaluate surface roughness parameters. There are two types of baselines:
- Least-Squares Centerline of the Contour: This baseline is the line within the sampling length for which the sum of the squares of the contour offset of each point on the contour line is the smallest, and has a geometric contour shape.
- Arithmetic Mean Centerline of the Contour: This baseline is the line within the sampling length for which the area of the upper and lower contours on the centerline is equal.
While the least-squares centerline is an ideal baseline in theory, it is challenging to obtain in practical applications. As a result, the arithmetic mean centerline of the contour is commonly used instead and can be measured using a straight line with an approximate position.
Evaluation parameters of surface roughness
1. Altitude characteristics
Contour Arithmetic Mean Deviation (Ra):
Ra is the arithmetic mean of the absolute value of the contour deviation within the specified sampling length (lr).
In actual measurements, a higher number of measurement points results in a more accurate Ra value.
Contour Maximum Height (Rz):
Rz is the distance between the top and bottom lines of the contour.
In common range of magnitude parameters, Ra is preferred.
Prior to 2006, the national standard included an evaluation parameter known as “the height of ten points of micro-unevenness,” which was expressed as Rz and the maximum height of the profile was expressed as Ry.
However, after 2006, the national standard abolished the “ten-point height of microcosmic unflatness” and instead expressed the maximum height of the profile as Rz.
2. Pitch characteristics
Rsm is the average width of the contour unit, representing the average of the microscopic unevenness spacing over the sampled length.
The microscopic unevenness distance refers to the length between a profile peak and the adjacent profile valley on the midline.
Even with the same Ra value, the Rsm value may not be the same, resulting in a different reflected texture.
Surfaces that prioritize texture usually consider both Ra and Rmr metrics.
The Rmr shape feature parameter is expressed as the contour support length ratio, which is the ratio of the contour support length to the sampling length.
The profile support length is calculated as the sum of the length of each section of the profile obtained by drawing a straight line parallel to the centerline and at a distance of “c” from the top line of the profile within the sampling length.
Measurement methods of surface roughness
1. Comparative method
It is used for on-site measurement in workshops and is often employed for measuring medium to rough surfaces.
The method involves comparing the measured surface to a roughness model marked with a specific value to determine the roughness value of the measured surface.
Roughness comparators, which are nickel-based electroformed specimens, are ideal for metalworking and serve as an effective aid. The operator simply scrapes their fingernail across each surface in a group to find the closest match to the part being compared.
While some people use these model groups as reference tables, it’s important to note that they are not official material standards.
There are various roughness measuring machines available, each with different functions, evaluation methods, and costs. Before choosing a model, it’s recommended to consult with a professional manufacturer to select the most suitable option for your needs.
2. Stylus method
Surface roughness measurement involves using a diamond stylus with a tip curvature radius of approximately 2μm to move along the measured surface.
The upward and downward displacement of the diamond stylus is converted into an electrical signal by an electrical length sensor. After amplification, filtering, and calculation, the surface roughness value is displayed on an instrument and the measured profile curve can also be recorded by a recorder.
Instruments that only display surface roughness values are called surface roughness gauges, while those that record surface profile curves are referred to as surface roughness profilers.
Both types of tools have electronic calculation circuits or computers that automatically calculate the contour arithmetic mean deviation (Ra), ten-point height of microscopic unevenness (Rz), maximum contour height (Ry), and other evaluation parameters. These tools have high measurement efficiency and are suitable for measuring surface roughness with Ra values ranging from 0.025 to 6.3 μm.