Surface roughness measurement method
Surface roughness is an important indicator reflecting the microscopic morphology of the surface of the workpiece, which refers to the processing of the surface has a small pitch and tiny peaks and valleys of unevenness; the smaller the surface roughness, the smoother the surface of the workpiece.
The smaller the surface roughness, the smoother the surface of the workpiece. Surface roughness indicators applied to the processing industry to improve the quality of the workpiece and productivity are significant.
At the same time, the wear resistance of the workpiece also has a direct impact. The rougher the surface, the effective the area of contact with the surface will be reduced, and the pressure will become more prominent so that the frictional resistance becomes larger, leading to accelerated wear but also affecting the stability of the workpiece with the fatigue strength of the indicators and so on.
People for the surface roughness of the comprehensive and systematic research so far only one or two hundred years of history. In recent years, research has mainly focused on the surface roughness of the workpiece measurement methods and molding mechanism.
Although high roughness values are usually undesirable, controlling high roughness values is difficult and expensive.
Currently, there are many methods to measure the surface roughness of the workpiece. This paper will introduce the surface roughness from the surface roughness measurement theory and its instrumentation development, the surface roughness detection method research status, and other aspects of surface roughness.
Surface roughness measurement theory and its instrument development
Surface roughness, formerly known as surface finish, is an important indicator to assess mechanical surface quality.
In the study of surface roughness, the Russian professor Chebyshev is the first to carry out a systematic and comprehensive study of scholars. 1874, based on his previous research, he pioneered the calculation of surface roughness of the maximum formula applied to cylindrical milling.
At the beginning of the 20th century, in industrially developed countries, some scholars also began publishing articles on surface finish’s impact on mechanical parts performance, life, etc.
In 1929, a light-cut microscope for measuring the surface roughness of the workpiece was invented by a member of the USSR Academy of Sciences called Linnik.
In the same year, a scientist from Germany Schmaltz first began to evaluate the height of the surface microscopic unevenness quantitatively, and in this way published a monograph on the surface roughness, the book put forward the evaluation parameters Humax and the measurement of the baseline two concepts.
This opened a precedent for scientists to measure surface roughness and began the quantitative description of surface roughness.
In 1936, Abbott (E.J. Abbott) developed the first successful instrumentation for measuring the surface roughness of the workshop workpiece, through which the instrument can be used to measure the depth of the peak of the contour and the support area and to obtain the relationship between the two curves, i.e., the Abbott curve to characterize the surface roughness.
A few years later, British scholars successfully developed the Taylor Shelf (Talysurf) stylus surface roughness measurement instrument. 1951, the Federal Republic of Germany, Opto factory produced for measuring surface roughness of the interference microscope.
In 1975, the Taylor-Hobson company even developed the Talysurf-5 surface profiler. This kind of computerized data processing and the computer can be directly displayed on the 15 evaluation parameters at the same time profiler, far ahead of all the world surface roughness research level [6].
Since then, various other surface roughness measurement instruments continue to emerge, for further research on surface roughness provides a powerful means of testing.
The development of science and technology, especially in the machinery manufacturing industry, has made great breakthroughs. The manufacturing industry in developed countries is critical to the relationship between the surface quality and the function of the part, launched a lot of research on the surface quality of the micro-geometric parameters, and to this end a series of relevant standards have been formulated.
In the 21st century, with the emergence of the scanning electron microscope (SEM), the measurement of surface roughness opened up a new way.
With the increasingly high requirements on the surface quality of parts, especially to promote the development of optical and ultra-precision machining technology and make the surface roughness toward the nanometer level technology.
Surface roughness detection method research status
With the continuous development of computer technology, the main means of measuring the surface roughness of parts has gradually shifted from visual observation to a numerical form of quantitative measurement of surface roughness.
At present, the surface roughness measurement method mainly has a type of electric contour meter and other contact measurement methods, laser scattering method, and another non-contact measurement method, two categories.
1. Contact measurement method
The most used in quantitative measurement is the stylus-type motorized profilometer. These instruments are more stable, display accurate and reliable values, and are quite convenient.
However, along with the development of mechanical and optical, especially in the laser technology breakthroughs, for the processing of the surface of the workpiece, quality requirements are higher and higher. The traditional stylus-type contour meter for the measurement of the surface roughness of the workpiece in some aspects of the measurement of the surface of the workpiece and the existence of many insurmountable problems, the specific problems are mainly manifested in:
(1) the stylus of the profilometer in the microscopic surface topography detection, in the lateral resolution, is easily subject to the stylus head width radius limitations. Nowadays, the minimum width of the tip of the stylus is 0.1μm, so it isn’t easy to apply this instrument to measure ultra-smooth surface workpieces.
(2) The stylus of the profilometer also has some limitations in the application. It can not be applied to the detection of soft materials. In the measurement process, the needle tip will exert a certain pressure on the measured object’s surface, and the stylus will easily scratch the measured object’s surface. Based on the optical effect, some scholars have created a method of optical measurement of surface roughness.
The main measuring device is a reflective device, where the laser light is emitted to the surface of the object to be measured in a parallel or divergent way through the system. The laser light reflected from the object’s surface to be measured has optical information that can reflect the shape of the object’s surface to be measured, which is accepted, transformed, and recorded by the optical sensors and processing circuits.
Among them, the contact measurement method is dominated by the tiny probe scanning method, which aims to amplify the small changes detected by the optical circuit.
It first controls the vibration component through a microcontroller to produce a fixed resonance frequency on the shrapnel to drive the probe’s vibration.
At the same time, the X-axis and Y-axis direction of the two motors so that the sample produces a plane on the flat movement in the process of moving due to changes in the surface of the material undulation, the vibration frequency of the probe to produce a change in the probe’s amplitude, after the amplification of the laser light path, so that the probe’s slight amplitude changes amplified by using the line array CCD detected by the micro-controller will be detected by the results transmitted to the computer, and finally use MATLAB software graphical simulation, showing the small changes in the MATLAB software graphical simulation, showing the small changes in the probe. The software’s graphical simulation shows the microscopic level’s three-bit image. It calculates the roughness value or the upper computer application software is used to receive and process the data and directly calculate its roughness.
2. Non-contact measurement method
1) Laser scattering method.
The formation of scattered spots is mainly attributed to the interference effect. When a beam of coherent light irradiation to the surface is to be measured, the surface of different parts of the reflected light interference forms the intensity distribution for the granular scattering.
Meanwhile, the scattering phenomenon is when light is reflected from a rough surface in the scattering pattern on the observation screen, composed of reflected scattering and scattered light bands.
The resulting scattering’s luminance distribution, contrast, etc., is closely related to the surface roughness. Still, it is quite different because the phase difference of the reflected light is modulated by the surface microform.
2) Light cutting method.
According to the principle of light cutting, parallel light can be measured on the surface at the indicated projection angle.
We can derive the micro-geometry of the surface to be measured from the intersection curve of the light band and the surface contour. This avoids contacting the surface to be measured and solves the measurement problem of the workpiece due to the existence of micro peaks and valleys depth on the surface.
Due to the light-cutting principle, the depth of field and the resolution of the objective lens limit the maximum and minimum height of the contour peaks and valleys on the measurable surface.
3) Interferometry.
It mainly refers to the use of light interference phenomenon to measure the roughness and the contour of high-quality optical surfaces. At present, eliminating the influence of external factors on measurement results has become the focus of interferometric research.
To overcome air interference, orbital error, and vibration, which may impact the online measurement, some experts have developed a laser outlier interferometer using a common laser optical path common mode suppression, allowing online surface measurement technology to achieve a zero breakthrough. The machine realizes a large dynamic measurement range and high measurement resolution and processes the outlier signal by combining large numbers with decimal numbers.
We will not see spatial fringes if a non-coherent broadband light source is used and the optical range difference between the two arms is larger than the coherence length. However, the spectra in the overlapping spectral region will change with the surface roughness of the workpiece. The accuracy of surface roughness measurement is positively related to the optical range difference and the relative visibility of the spectra.
4) Optical probe method.
This method is similar in principle to the mechanical probe-type measurement method, replacing the probe with a focused beam, thus realizing non-contact measurement.
The measurement range of the optical stylus-type measurement system is larger than that of the other methods, and it can not only measure the roughness of the localized surface but also accurately measure the surface shape changes within a range of 1 mm, as well as detect tiny defects on the surface.
Conclusion
With today’s processing industry continuing to mature, as well as strict control of product utilization, even in the sustainable development of today’s impact on the environment, there is a need to put a new processing technology on the stage, especially for the mastery of surface roughness characteristics, it will make the processing industry technology chain tend to mature.
Industry experts have done a lot of research on the surface characteristics of processing tools, and the tiny probe scanning method of measuring the surface state of the object has many advantages.
On the one hand, compared with the traditional stylus detection method, probe scanning in the workpiece surface detection for the measured object surface flatness requirements is smaller. At the same time, a small number of burrs on the measured object can also be detected smoothly.
Compared with non-contact optical measurement, probe scanning of the detection environment is widely applicable, do not have to consider the dust in the air and other factors will have an impact on the results of the inspection, the inspection system is small, low-cost, suitable for a wide range of input and use.
On the other hand, the traditional stylus scanning results are only limited to a line of parameters, not in the entire plane for continuous scanning, the data collected is not very representative, and probe scanning can effectively solve the above problems, with the probe to do a “Z” movement, you can be the entire object to be measured macro-plane are swept surface Complete, the entire scanning process does not require manual participation, a high degree of automation.
Comprehensive consideration of various factors, with the increase in demand for high-precision instruments, will be increasingly demanding for high-quality parts. The study of surface roughness will be more and more critical.
The use of the tiny probe type detection surface roughness method is cost-effective and has a high degree of popularity; the surface roughness of the workpiece and the development of the entire processing industry will have a great role in promoting it.