How To Measure Surface Roughness
Surface roughness is an essential index for most surfaces involved in sliding contact. These surfaces are commonly load-bearing and must be marked with roughness to ensure suitability for an application. Many parts need to have a specific surface to achieve the required application—for example, the car body before painting or the journal bearing on the crankshaft or camshaft.
What is surface roughness?
Surface roughness refers to the small spacing and the unevenness of small peaks and valleys in the process of products.
It is commonly defined as the small distance (wave distance) between two wave peaks or two wave valleys. Generally, the wave distance is within 1mm or less. It can also be called the measurement of the micro profile or micro error value.
Generally, there will be a baseline for evaluating roughness. 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 trough is represented by Z, and the spacing of micro-texture of processed products is defined by S.
- S < 1mm is called surface roughness
- 1 ≤ S ≤ 10mm is called surface waviness
Chinese standard uses VDA3400, Ra and Rmax as three parameters to evaluate the surface roughness, and the unit is μ M.
Evaluate the relationship between surface roughness parameters
Ra is defined as the average arithmetic deviation (average roughness) of the curve;
Rz is defined as the average height of unevenness;
Ry is defined as the maximum height. The maximum height difference Ry of the micro profile is also expressed by Rmax in other standards.
How is surface roughness produced?
The machining process of the workpiece causes surface roughness. The processing method, the material of the workpiece and the technological revolution are all factors affecting the surface roughness.
For example, there are discharge concave-convex points on the surface of machined parts during electrical discharge machining.
If the processing technology and part material are different, the micro traces left on the surface of the processed parts are also other, such as density, depth, shape change, etc.
Influence of surface roughness on a workpiece
- Contact stiffness
- Wear resistance of the workpiece
- Corrosion resistance
- Coordination stability
- Fatigue strength
- Tightness
- Measurement accuracy
……
In addition, it will affect plating coating, thermal conductivity and contact resistance, reflection ability and radiation performance, the resistance of liquid and gas flow, current flow on conductor surface etc.
How to evaluate surface roughness
Evaluation length
It consists of N reference lengths. The surface roughness of each part of the part surface cannot truly reflect the actual parameters of roughness on a reference length, but it should take N sampling lengths to evaluate the surface roughness. Under the ISO1997 standard, the N is commonly equal to 5.
Sampling length
Under the ISO1997 standard, we can take 0.08mm, 0.25mm, 0.8mm, 2.5mm and 8mm as the reference length.
Datum line
The datum line is the contour centre line for evaluating the roughness parameters. Generally, there are least-square midline and contour arithmetic mean midline.
The least-square midline calculates the points collected in the measurement using the least square method.
The arithmetic mean midline of contour: within the sampling length, make the area of the outline’s upper and lower parts of the midline equal.
Theoretically, the least square midline is an ideal datum line, but it isn’t easy to get in practical application. Therefore, it is generally replaced by the arithmetic mean midline of the contour, and we can use a straight line with an approximate position in measurement.
How to measure surface roughness
There are qualitative and quantitative evaluation methods of surface roughness.
Quantitative evaluation measures the surface’s roughness through certain measurement methods and roughness measurement instruments.
Qualitative evaluation is to compare the surface with the standard sample with known surface roughness level and evaluate its level by visual assessment or with the help of a microscope.
Impression measurement
For large parts or internal surfaces of elements (such as deep holes, blind holes, grooves and internal threads)， we can measure it by the impression method. The peak valley value of the impression surface is always smaller than that of the measured surface, so we must modify the result. The correction coefficient is related to the materials by experiments.
Roughness sample comparison method
The surface roughness template is a set of standard surface samples with different geometric shapes made according to various processing methods, which are used to compare with the measured surface.
Take the surface roughness of the working surface of the sample block as the standard, and compare it with the surface of the workpiece by touch or vision (with the help of magnifying glass, comparison microscope, etc.) so as to evaluate whether the roughness of the surface meets the requirements.
This is a qualitative measurement method. The processing method of the selected sample block and the tested part must be the same, and the sample block’s material, shape, surface colour, etc., should be near the tested part as far as possible.
Determine whether the surface roughness profile meets the technical requirements according to the depth of the measured surface machining trace. Suppose the depth of the measured surface machining trace is equal to or less than the depth of the sample block machining trace. In that case, it means that the Ra value of the measured surface roughness contour amplitude parameter is not more than the Ra value marked by the sample block.
Touch comparison is suitable for the outer surface with a Ra value of 0.63 ~ 10mm; The visual comparison method is ideal for the surface with a Ra value of 2.5 ~ 80mm;
The 5 ~ 10x magnifying glass is suitable for the surface with a Ra value of 0.32 ~ 2.5mm;
The comparison microscope is suitable for the surface with a Ra value of 0.08 ~ 10mm.
Pros：
Simple and easy
Cons：
Not very high accuracy
It is commonly used in factories, especially in workshop inspection.
Light cutting measurement
It is to irradiate a narrow flat beam on the measured surface at a certain tilt angle, and the beam is reflected on the measured surface, and observe the micro surface roughness by a microscope—light cutting microscope instruments, including JSG and 9J etc.
It can be used to measure the outer surface of metal processed by turning, milling, planing and other similar methods. It is suitable for measuring the surface roughness Rz with 0.8 ~ 100 μ M (equivalent to Ra value of 0.16 ~ 20) μ m) of a plane and outer cylindrical surface.
Pros：
High accuracy
Cons：
Complex calculation
Contact measurement
It uses the measuring probe of the instrument to contact the surface and makes the measuring probescratch gently along its surface to measure the surface roughness.
Place a sharp probe vertically on the measured surface for lateral movement. Because the working surface is rough and uneven, the probe will move vertically with the contour of the measured surface. The micro displacement is converted into an electrical signal through a circuit, amplified and calculated, and get the parameter value of workpiece surface roughness ;
It is suitable for measuring the Ra value of 0.025 ~ 12.5mm and RZ value of 0.02 ~ 160 μM.
Pros：
Simple and easy
Cons:
It can not be used in soft materials and high-speed online occasions.
It is not suitable for measuring the super smooth surface.
Optical probe method
In principle, it is similar to the mechanical probe measurement method. The probe uses a focused beam to replace the diamond tip. It can measure the change in surface contour height by detecting the focus error. There are laser triangulation probes, optical critical angle probes, astigmatism probes, confocal scanning probes, optical probe scanning methods based on optical fiber and so on.
Pros:
The measurement range is much more extensive than that of other methods.
It can measure the roughness of the local surface, the surface shape within 1mm and minor defects on the surface.
Scattering measurement
It can evaluate the surface roughness by light intensity comparison. The light wave emitted by the light source is incident on the surface of the measured workpiece in parallel or divergent through the optical system. The optic of the measured surface workpiece reflected by the reflected light wave on the surface of the measured workpiece is processed by various forms of photoelectric sensors and post-processing circuits related to the optical information.
For the surface with minor surface roughness, the scattered light energy is weak, and the reflected light energy is strong; On the contrary, for the surface with large surface roughness, the scattered light energy is vital, and the reflected light energy is weak.
It is suitable for measuring a Ra value of 0.012 ~ 2.0 μ M plane, cylindrical and spherical surface, sample block, etc. It can also be used to measure the depth of scribing and coating on the surface of parts.
Pros：
Simple structure, small volume, easy integration of instrument
Good dynamic response
It is suitable for online measurement and so on.
Cons：
Not high accuracy
It is not suitable for super-smooth surface roughness.
Interferometric measurement
It measures the surface roughness by interference microscope and the combination of interference and micro amplification principle.
We can measure the micro unevenness in the vertical height direction of the measuring surface by the light wave interferometry and the horizontal parameters of the surface roughness by the micro amplification system.
There are Michelson, Mirau and Link of spectroscopic interference microscopy according to different spectroscopic methods.
It is mainly used to measure Ry, and Rz surfaces roughness parameters, such as RZ value of 0.063 ~ 1.0 μm (equivalent to Ra value of 0.01 ~ 0.16 μm) of the plane, cylindrical surface and spherical surface.
Pros:
Intuitive surface information
High measurement accuracy
Can measure an area at one time.
In addition to the above roughness measurement methods, there are laser methods (laser holography, laser spot method, laser speckle method), pneumatic method, capacitance method, thermal comparison method, microwave method, infrared radiation method, electron microscope method, optical fiber sensing method, atomic force method, X-ray interference technology etc.
