What is surface roughness?
Surface roughness refers to the processing of the surface has a small pitch and small peaks and valleys of unevenness. The distance between the two peaks or two valleys (wave pitch) is very small (below 1mm), which belongs to the micro-geometric shape error. The smaller the surface roughness, the smoother the surface.
Specifically refers to the tiny peaks and valleys Z high and low degree and spacing S condition. Generally divided by S:
S <1mm for surface roughness
1 ≤ S ≤ 10mm for the degree of corrugation
S>10mm for f-shape
How is surface roughness formed?
Surface roughness is formed by the machining method used and other factors, such as friction between the tool and the part surface during machining, plastic deformation of the metal in the surface layer when the chips are separated, as well as high-frequency vibration in the process system, and discharge craters from electrical machining.
Due to the different machining methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface vary.
Surface roughness evaluation basis
Sampling length
The unit length of each parameter, the sampling length is the evaluation of surface roughness stipulates the length of a section of the baseline. In the ISO1997 standard generally use 0.08mm, 0.25mm, 0.8mm, 2.5mm, 8mm for the benchmark length.
Ra, Rz, Ry sampling length L and evaluation length Ln selected values:
Evaluation length
Composed of N reference lengths. Parts of the surface roughness of each part of the surface, in a benchmark length can not truly reflect the roughness of the real parameters, but need to take N sampling length to assess the surface roughness. In the ISO1997 standard under the evaluation of the length of the general N is equal to 5.
Baseline
Baseline is used to assess the surface roughness parameters of the contour of the center line .
Surface roughness evaluation parameters
Maximum height (Rz)
It is the sum of the maximum contour peak height Zp and the maximum contour valley depth Zv within a sampling length.
- In JIS” 94, it is expressed as Ry.
- Contour peak: The part of the contour above the center line (X-axis) of the contour (from the material).
- Contour valley: The part of the contour below the center line (X-axis) of the contour (from the surrounding medium).
Pz Maximum height of the original contour
Wz Maximum height of corrugation
Points to note
The maximum height, although often used, is strongly influenced by scratches, contamination and measurement noise because it is related to the peak value.
Average width (RSm)
The average value of the width Xs of a contour unit over a sampling length.
- Expressed as Sm in JIS’94.
- Minimum height and minimum length of peaks (valleys) to be distinguished
- Minimum height discrimination: 10% of Rz value
- Minimum length discrimination: 1% of the reference length
Psm Average width of original contour unit
Wc Average width of corrugated contour cell
Points
This parameter replaces the height parameter and is used to evaluate the horizontal dimensions of parallel grooves and particles.
Surface Roughness Measurement Methods
Comparative method
Comparative methods are easy to measure and are used in the shop floor, often for medium or rough surfaces.
It is a method of determining the surface roughness by comparing the measured surface with a roughness sample that has a certain value marked on it.
Methods that can be used for comparison: visual inspection for Ra > 1.6μm, magnifying glass for Ra1.6~Ra0.4μm, and comparative microscope for Ra < 0.4μm.
Follow the processing method of the sample, processing texture, processing direction, and the same material as the surface of the tested part.
Surface roughness measurement instruments can be categorized into contact-based and noncontact-based instruments.
There are pros and cons to both methods, and it is important to select the most suitable instrument based on your application.
Method | Measurement instrument | Advantages | Limitations |
---|---|---|---|
Contact-based measurement | Stylus roughness instrument | Enables reliable measurement as the sample surface is physically traced with a stylusHas been used for a long time | Limited to measuring a single section with a reduced quantity of measurement informationIncapable of measuring adhesive surfaces and soft samplesDifficult to precisely position the probeIncapable of measuring details smaller than the stylus probe tip diameter |
Non contact-based measurement | Coherence scanning Interferometers | Quick measurementsEnables sub-nanometer measurement of smooth surfaces at low magnification | Has trouble measuring rough surfacesHas trouble measuring samples with significant differences in brightnessLow contrast makes it difficult to locate the areas subject to measurementLow XY resolution |
Laser microscope | High angle detection sensitivity, enabling analysis of steeply inclined slopesHigh XY resolution, providing for clear, high-contrast images | Incapable of conducting sub-nanometer measurementsInferior height discrimination capabilities at lower magnification rates | |
Digital microscope | Enables many kinds of observations and a simple level of measurement | Not suitable for measuring component roughness (suitable for measuring waviness)Incapable of measuring sub-nanometer irregularitiesLow XY resolution | |
Scanning probe microscope (SPM) | Enables measurement of sub-nanometer surfacesEnables measurement of samples with a relatively high aspect ratio | It’s difficult to precisely position the probeSlowNot suitable for measuring μm irregularities |
Surface roughness on the parts of the main performance
Wear resistance
The rougher the surface, the smaller the effective contact area between the surface of the fit, the greater the pressure, the greater the frictional resistance, the faster the wear.
Stability of the fit
For clearance fit, the rougher the surface, the easier it is to wear, so that the gap gradually increased in the working process; for the interference fit, due to the assembly of the microscopic peaks will be flattened, reducing the actual effective interference, reducing the strength of the connection.
Fatigue strength
Rough parts of the surface of the presence of large valleys, they are like sharp notches and cracks, sensitive to stress concentration, thus affecting the fatigue strength of the parts.
Corrosion resistance
Rough surfaces of parts are prone to allow corrosive gases or liquids to penetrate into the inner layers of the metal through microscopic valleys on the surface, causing surface corrosion.
Sealing
Rough surfaces can not be tightly fit between the gas or liquid leakage through the gap between the contact surface.
Contact Stiffness
Contact stiffness is the ability of the parts bonding surface to resist contact deformation under the action of external forces. The stiffness of the machine depends largely on the contact stiffness between the parts.
Measurement accuracy
The surface roughness of both the measured surface of the part and the measuring surface of the measuring tool will directly affect the accuracy of the measurement, especially in precision measurement.
In addition, the surface roughness of the parts of the plating and coating, thermal conductivity and contact resistance, reflective ability and radiation properties, the resistance of liquid and gas flow, the current flow on the surface of the conductor will have a different degree of influence.