APPLICATION OF OPTICAL METHODS FOR ANALYSES OF SURFACES MADE BY ABRASIVE LIQUID JETS

Transcription

1 APPLICATION OF OPTICAL METHODS FOR ANALYSES OF SURFACES MADE BY ABRASIVE LIQUID JETS Jan Valíček 1) Milan Držík 2) Miloslav Ohlídal 3) Libor M. Hlaváč 1) 1) 2) 3) VŠB - Technical University, Ostrava, Czech Republic Institute of Construction and Architecture, Slovak Academy of Sciences, Bratislava, Slovak Republic Technical University Brno, Brno, Czech Republic Abstract The knowledge of the surface roughness is a very important parameter for machining techniques. The direct detection of surface quality is a problem of abrasive water jet technology, especially if used for turning, milling or grinding, where the surface is opened for detection. The topic of the paper is a theoretical and experimental investigation of certain optical methods and their utilisation for surface quality analyses. Methods are based on scanning by defocused laser beam. The methods are theoretically described and the most important theoretical results are discussed for wide scale of technical conditions. The experimental data are analysed from the point of utilisation of the methods for determination of roughness and waviness of surfaces prepared by abrasive water jets. 1. INTRODUCTION New and very perspective technology for material machining has been developed within the period of the last twenty years. This technology is mostly called water jetting or abrasive water jetting nowadays. The term water jetting is usually appropriate for jet formed by water without any ingredients. On the contrary, the term abrasive water jet is used for water jets containing particles of solid matter (grains of rock materials, metals, glass, slag, etc.) added into them for increasing of their efficiency. There are two possible methods for abrasive water jet generation. The first one is based on two steps generation. Primarily the pure water jet is generated making the hypotension in a special chamber. As a consequence of it, the air with dry abrasive material particles is sucked into this chamber and mixed there with water jet. Therefore, the chamber is called the mixing chamber. The abrasive water jets generated in this manner are called injection jets. The slurry resulting from the mixing process is guided by socalled mixing tube made of an extremely wear resistant material. The second method of the abrasive water jet generation is based on the direct outflow of the highly pressurized slurry made of water and abrasive material particles in a special tank. This slurry outflows from the nozzle made again of an extremely wear resistant material generating so-called suspension jet. The most important differences are the following ones: the injection water jets are active for high water pressure in the pump and the energy losses during the generation process often reach 80% and more whilst the suspension jet is efficient even for low water pressure in the pump and energy losses within generation are usually less than 20%. Nevertheless, both jets irrespective of the method of their generation make surfaces with characteristic pattern depending only on parameters of jet (pressure, abrasive material type and grain size, traverse rate, etc.), target material and type of machining

2 2. SURFACE CHARACTERISTICS The surface prepared by abrasive water jet during cutting, drilling, turning, milling, planning, slotting, shaping and grinding (with respect to correlation of the abrasive water jet technology and the classical ones in spite of the specific features) is characterized by typical pattern. There are microscopic pits (craters) on the surface prepared by abrasive water jet created by grains of abrasive material and cavitation. Abrasive particles contained by water jet remove the appropriate small parts of material according to their energy and momentum, size, stress and strain characteristics, angle of impact and other parameters or engrain itself into the surface. The cavitation effects caused by rapid water flowing over the local surface roughness are also contributing to it. Moreover, the further microscopic and even macroscopic patterns are induced by abrasive water jet impact and interaction processes striation and waviness (see Fig. 1 and 2). These last two one patterns are created as a consequence of the jet and Fig. 1. Drawing of the most typical patterns on the walls inside the slot cut by abrasive water jet. Fig. 2. The character of the walls made by abrasive water jet in the slot

3 target material vibrations, water flow instabilities, material non-homogeneities, abrasive suction and flow rate instabilities, changes in character of abrasive particle - target material interaction depending on the angle of impact, etc. Many of the specific phenomena mentioned above were described, studied and discussed by specialists on water jetting from all over the world, e.g. [1 through 6]. The measurement of surfaces generated by abrasive water jets is quite difficult. It is caused by specific surface structure. Contrary to the classical ones the surface prepared by abrasive water jet is rather diffusion. Therefore, there are certain difficulties using classical optical methods and the proper method regarding the specific optical properties of the surface are to be used [7]. Nevertheless, the effort of many research teams all over the world was concentrated on determination of the quality of walls in cuts by optical methods. Our work is aimed at development of methods usable for an interactive measurement of the surface quality during processes with opened surfaces feasible using abrasive water jet (in terms of a classical material machining it means turning, planning, slotting, shaping and/or grinding). The current state-of-the-art in realization of our intentions can be described as an appreciation of the appropriate optical methods. Our first aim is to prepare the method yielding the outputs corresponding with the main surface parameters and characteristics by course of engineering standards ČSN and ISO. 3. VISUALISATION OF THE ROUGHNESS BY OBLIQUE ANGLE ILLUMINATION Testing few optical methods used for studies of surface characteristics on several samples prepared by abrasive water jet we decided to prepare a new one. It is specially dedicated to measurements on surfaces with diffusion structure. The scheme of the laboratory investigation is presented in Fig. 3. Fig. 3. The scheme of the laboratory investigation of our method used for surface quality measurement: LS laser, O the objective, S the aperture, P - photodetector (photodiode), PC computer with fast Fourier transformation, v the traverse rate, α the angle. The method is based on the speculation graphically demonstrated in Fig. 4. The optical effect caused by light impinging the surface at an oblique angle is used for visualisation of the geometrical shapes being present at the sample surface. The shapes can be so displayed in a simulated optical plane. The intensity of laser light scattered by surface treated in this - 3 -

4 manner contains the information about frequency and height of geometrical shapes present at the surface. The principle can be described by equation 1) useful for evaluation of the shape height that being in a close relation with parameters of roughness R a and R q. v h= sinα 1) M f where v is the traverse rate of the sample [m.s -1 ] f is the frequency of the roughness [Hz] M is the mounting [-] Fig. 4. The principle of the surface roughness shadowing: d the length of the shadow, h the height of the unlevelness, α the complementary angle to the laser beam incidence angle. The angle α was selected from the interval <10;15> degrees. The lens speed of the objective was 1:1.4 and focused the image of the shadowed visualized roughness to the plane of the slot type shutter of the photodiode. The aperture was 0.1 x 0.3 mm. The reflected laser beam is converted to the electric signal in the photodiode. The analogue electric signal is a function of time U(t). This signal is converted to the digital form and downloaded into the computer with programme for signal analysis. The FFT (fast Fourier transformation) converts the time depending signal to its amplitude-frequency image U(f). The function U(f) is the base for calculation of the main roughness parameters R a and R q the arithmetical average of the absolute values of the height deviations of the profile and the average quadratic deviation respectively. The cut off value of the tested optical configuration [8] can be calculated from the equation Λ c = l.λ.w -1. Our parameters were as follows: the aperture width w = 0.1 mm, stand-off distance l = 0.2 m, laser beam wavelength λ = 532 nm. The cut off value calculated for our investigation is Λ c = 0.1 mm. 4. RESULTS AND DISCUSSION Up-to-date three samples were measured using both optical and contact methods. There were square samples prepared from hard steel, mild steel and aluminium alloy cut from the plates 8 mm thick. Each side of the samples was cut using another traverse rate. Each sidewall was passed by both optical signal (laser beam) and mechanical tip in zones situated round the lines perpendicular to the sample height in distances 2, 4 and 6 mm from the top (according to the direction of the abrasive water jet flow). The record of the time dependent optical signal is presented in Fig. 5. Its FFT is shown in Fig

5 Fig. 5. The record of the optical signal scattered by surface of the sample 1, sidewall 1. Fig. 6. Frequency spectrum obtained by FFT from the time dependent signal shown in Fig

6 Fig. 7. The record of the signal from the contact profilometer HOMMEL TESTER T8000. Graph in Fig. 7 shows the record of the same trace as it is shown in Fig. 5. The signal is obtained from the contact profilometer HOMMEL TESTER T8000. The initial tested length was 15 mm with cut off 2.5 mm. Signal was then processed by filter RC DIN4768 and the resulting length corresponding to the record shown in Fig. 7 is 12.5 mm. The presented record starts at about 3.75 mm from the side edge of the measured sample wall. Taking into account the cut off and the position of signal beginning we can determine the absolute position of the very typical peak situated at position approximately 7.15 mm in the graph. The 0.00 mm point (zero position of the unprocessed signal) lies in the distance one half of the cut off from the side edge of the tested wall. Simultaneously, the processed signal begins in the distance 2.5 mm from that point. This deduction and calculation tends to the conclusion that the absolute position of the peak is 8.4 mm from the side edge of the wall. Nevertheless, the typical peak, we have discussed, can be determined also in Fig. 5 at position about 5.6 ms. Considering the fact that optical signal starts at the side edge of the wall and the traverse rate is 1.5 m.s -1 we can calculate the absolute position of the peak. It is again 8.4 mm. We can see analogical behaviour of both signals in Fig. 5 and 7 provided the position of the discussed peak is known. 5. CONCLUSIONS The up-to-now results allow us to summarize the following conclusions: optical method yields signal of sufficient amplitude for further processing; signal obtained from the optical method is in a good correlation with signal obtained by commercial contact profilometer; optical method can be stamped and used for determination of the surface parameters. ACKNOWLEDGEMENTS The authors are grateful to the Grant Agency of the Czech Republic and to the Ministry of Industry and Trade supporting the work presented in the paper by projects No. 106/98/1354 and FB-C3/05 respectively. REFERENCES [1] Guo, N.S.: Cutting process and cutting quality by AWJ cutting. VDI Verlag, 1994, p.174 (in German) [2] Hashish, M.: A Modeling Study of Metal Cutting With Abrasive Waterjets. Trans. of the ASME, Journal of Eng. Mat. & Tech., 1984, Vol. 106, No.1, p

7 [3] Hashish, M.: Pressure Effects in Abrasive-Waterjet (AWJ) Machining. Trans. of the ASME, Journal of Eng. Mat. & Tech., 1989, Vol. 111, No.7, p [4] Zeng, J., Kim, T.J.: A study of brittle erosion mechanism applied to abrasive waterjet processes. Proc. the 10th Int. Symp. on Jet Cutting Technology, BHRA, England, 1990, paper B1 [5] Hlaváč, L.: Physical model of jet - Abrasive interaction. Geomechanics 93, Rakowski (ed.), Balkema, Rotterdam, 1994, p [6] Hlaváč, L.M.: Interaction of grains with water jet - the base of the physical derivation of complex equation for jet cutting of rock materials. Jetting Technology, C.Gee (ed.), Mech. Eng. Pub. Ltd., Bury StEdmunds & London, 1996, p [7] Ohlídal, M., Ohlídal, I., Tykal, M., Pražák, D., Unčovský, M.: Measuring the surface roughness by selected methods of coherent optics in engineering. JMO, 1999, No.9, p (in Czech) [8] Držík, M.: The metallic surfaces microstructure determination by measuring of scattered light. JMO, 1996, No.7-8, p