Wire Tool Vibration Measurement Techniques In WEDM

Wire Vibration Measurement Technique in WEDM Wire Tool Vibration Measurement Techniques In WEDM Debkumar Ruidas Roll - 001411701040 4th Year Guided By Dr. B. Bhattacharyya Production Engineering JADAVPUR UNIVERSITY Debkumar Ruidas 1 Wire Vibration Measurement Technique in WEDM Acknowledgement I would like to take this opportunity to express my profound gratitude and deep regard to my Professor Dr. B. Bhattacharya, for his exemplary guidance, valuable feedback and constant encouragement throughout the duration of the report. His valuable suggestions were of immense help throughout my work. His perceptive criticism kept me working to make this report in a much better way. Working under him was an extremely knowledgeable experience for me. Debkumar Ruidas 2 Wire Vibration Measurement Technique in WEDM Content 1. Introduction 2. Parametric influence on average wire amplitude . 2.1Running Speed 2.2 Dielectric flow rate 2.3 Servo voltage 2.4 Wire Tension 3. Wire Vibration measurement 4. Mathematical Model 5. Mechanical Method: 6. Electronic Method: 6.1 Free Vibration condition 6.2 Wire vibration during cutting 6.3 Low Frequency Wire Vibrations during Cutting 6.4 Real time application 7. Using High Speed Video Camera 7.1 Evaluation of wire movement 8. Proposed measurement technique 9. Comparison between measurement methods 10.Practical Approach of Minimising the Wire Vibration 11.Conclusion 12.References Debkumar Ruidas 3 Wire Vibration Measurement Technique in WEDM 1. INTRODUCTION WEDM is a thermo-electrical process in which material is eroded from the workpiece through a series of discrete sparks occurring between the workpiece and the wire electrode (tool). The latter is separated by a thin film of dielectric fluid (deionised water) which is continuously force-fed to the area being machined in order to flush away the eroded particles. The movement of the wire is numerically controlled to achieve the desired threedimensional shape and accuracy of the workpiece. Although the average cutting speed, relative machining costs, accuracy, and surface finish have been improved several times over since the commercial introduction of the machine, further improvement is still required by different industries to meet the increasing demand of precision and accuracy. Figure 1: Schematic diagram of working of wireEDM [1] The wire electrode is supported by two guides and is moving down or up the guides at a uniform velocity. Some external forces are exerted on the wire, which include: an axial tension, an electro-static force produced by the electric field between the workpiece and the wire electrode, an electro-dynamic and explosion force caused by spark discharge, and the damping force caused by the dielectric medium. FIG 2. Reasons of wire vibration[2] In actual wire cut machining, vibrations of the wire electrode can be divided into two components: vibrations along or perpendicular to the cutting direction. As far as the latter is concerned, due to the constrains of the cutting kerf, vibration is relatively small and symmetrical. Debkumar Ruidas 4 Wire Vibration Measurement Technique in WEDM Fig. 3. Schematic diagram of workpiece positions and wire movements [3] 2. Parametric influence on average wire amplitude The influence of wire tension, wire running speed, flow rate and servo voltage on average wire amplitude can be shown in Fig. 3. Average wire amplitude decreases with the increase of wire tension and wire running speed. However, it increases with dielectric flow rate. Servo voltage has a weak influence on average wire amplitude. One of the most effecting parameters of wire vibration amplitude in wire EDM process is wire tension. Figure 4 shows wire shape difference under wire tension 0.5 and 4.0 N. Within considerable range, an increase in wire tension significantly increases the cutting speed and Fig. 4 Relationship between average wire amplitude and wire tension,wire running speed, flow rate and servo voltage [4] accuracy due to the sharp straightness of the wire. [4] 2.1 Running Speed When the wire running speed has a lower value, the amplitude slightly increases. The debris exclusion from the discharge gap is a little difficult at lower wire running speed because there is no high-speed flow of working fluid around the wire. Then, the debris stagnation occurs around the wire, which causes unstable machining and larger amplitude of wire vibration. When the wire running speed is higher, the debris is smoothly excluded. [4] Debkumar Ruidas 5 Wire Vibration Measurement Technique in WEDM 2.2 Dielectric flow rate Dielectric flow rate is the rate at which the dielectric fluid is circulated. Flow rate of the working fluid from jet nozzles is important for efficient machining. One of the forces exerted on the wire is the dielectric flow such that as the flow rate increases around the wire, the movement of the wire speeds up and thus the average wire amplitude increases. [4] 2.3 Servo voltage Servo voltage acts as the reference voltage to control the wire advances and retracts. Figure 4shows that there is little decrease of average wire amplitude with change of servo voltage from 50 to 70 V. After that, the average wire amplitude increases slightly. [4] 2.4 Wire Tension Wire tension is one of the most dominant factors that affect wire vibration. When tension is low wire amplitude become high and high wire tension results lower amplitude. But, if tension is too high then wire get broken. To avoid wire breakage tension must be increased to a certain limit. Fig. 5 Wire shape difference under wire tension 0.5 and 4.0 N[4] 3. Wire Vibration measurement The main functions which feature the developed wire vibration detection and analysis system are: To be able to measure the wire position and displacement during machine idling as well as during EDM cutting. To perform a vibration analysis, based on the gathered data during the wire vibration measurements. This also in both modes, machining and idling. The vibration analysis includes, graphical output of the wire displacement data in a 2-D plane, Past Fourier Transform calculation (PFT), calculation of wire damping ratios, and frequency spectrum analysis. [5] Vibration measurement model and method 1. 2. 3. 4. 5. Debkumar Ruidas Mathematical Model, Mechanical Method, Electronic Method and Using High Speed video Camera Proposed Measurement Method 6 Wire Vibration Measurement Technique in WEDM 4. Mathematical Model vibration of a stretched wire supported by the two wire guides at opposite ends satisfying the standard vibration theories of vibrating strings, has been mathematically modelled by several researchers with the following assumptions: i) the wire mass is uniformly spread along its length; (ii) the axial tension remains constant between the wire guides; (iii) the wire is static (not moving); (iv) the wire is perfectly flexible; and (v) the disturbing forces acting per unit length of the wire perpendicular to the axial force, varies as a function of time and space. Therefore, the general partial differential equation of motion (Newton’s second law of motion) for a stretched vibrating string of length L in a plane (along Z-axis) may be expressed as follows: FIG. 6. A schematic diagram of wire-tool vibration in WEDM process [6] ...............................1 Multi-discharge modelling of wire vibration and approach for solution From the geometry of the wire shape (Fig. 7), the displacement function y(z, t) may be determined by the following initial conditions (ICs Thus y(z, t) = e−βt/2ρ0 ∞  {An cos(ωn t) n=1 with  nπ  z L   nπ  8a0 L nπ H cos sin An = 2 2 2 2 L n π (L − H) β 1 Bn = An , 2ρ0 ωn Debkumar Ruidas 1 ωn = 2 FIG 7. Action of the body force before the wire is set to vibration [12] , β2 4n2 π2 F − 2 + 4α 2 ρ0 L ρ0  ..................................................................................................2 7 Wire Vibration Measurement Technique in WEDM 5. Mechanical Method: Using Probe: As shown in Fig.8, the wire is surrounded by four probes, Pl,P2,P3 and P4. Each probe is moved back and forth along L-axis or M-axis by means of the mechanism shown in Fig.11. The probe is fixed mechanically to a micro-meter screw which is driven by a stepping motor, but is isolated electrically from the screw. Each pulse supplied to the stepping motor shift the probe 0.48 micron. Assuming the probe is fixed on some adequate position in LM-plane, the vibrating wire comes into contact intermittently with it. This mechanical Fig.8 Schematic View[7] Fig.9 Area of Wire-Vibration ( XY-Plane )[7] Fiq.10 Area of W-V.( FZ-Plane[7] having peak value of 5V to the probe during Ts sec, it is checked up for every pulse whether an electrical short circuit between the wire and the probe is built up or not. If for n pulses among N “short” is detected, Debkumar Ruidas 11 Wire Vibration Measurement Technique in WEDM C = (n/N) x 100 ( % ) (1) is calculated with micro processor, TK-85 connected to the detecting circuits. As a matter of course the closer the probe is located to the wire, the higher he value of the contact ratio, C is. Monitoring the above mentioned contact ratio on every probe, a micro computer, FM-8, supplies control pulses to the stepping motor of probe unit (Fig.11) Fig.11 Probe Unit[7] Therefore, if the contact ratio, C, is greater than a constant, Co, the probe is retreated from the wire. On the contrary, if C is smaller than Co, the probe is advanced to the wire.[13] Repeating preliminary experiments the value of Ts and Co were selected at 0.1 sec and 0 %, respectively. And it was confirmed that the boundary of the area of wire-vibration could be detected both on L-axis and M-axis through the controlled location of the probe without disturbing the motion of wire. Fig.12 shows the location of each probe at two different times, tl and t2, on LMplane. Two ellipses represent the corresponding areas of wire-vibration. Counting the number of pulses sent to the stepping motor during a period, (t2tl), each shift of the four probes, AL1 or AL2 or AM3 or AM4 is calculated. Here, the following four values are defined: Fig.12 Probe vs. Area of Wire-Vibration (LM-Plane)[7] ΔEL = 0.5 X (ΔL1 - ΔL2) ……………………………...….. (3) ΔEM = 0.5 x (ΔM3 - ΔM4) …………………………………... (4) ΔAL = 0.5 x (ΔL1 + ΔL2) …………………………………... (5) ΔAM = 0.5 x (ΔM3 + ΔM4) …………………………………...(6) having peak value of 5V to the probe during Ts sec, it is checked up for every pulse whether an electrical short circuit between the wire and the probe is built up or not. If for n pulses among N “short” is detected, relations are true, too. On the contrary, if the coordinates of the ellipse are not parallel to L, M-coordinates, the above relations Debkumar Ruidas 12 Wire Vibration Measurement Technique in WEDM are not guaranteed. Fortunately in wire- EDM the eccentricity of the ellipse is close to 1.0, therefore it is possible to consider that ΔEL=m, and that Δ E M - m In this case, according to the same reason, ΔAL and ΔAM in equations (5) and (6) are nearly equal to L-component and M-component of the increment of the amplitude of wire-vibration, respectively, on LM-plane.[1] Using ΔEL and ΔEM the area of wire-vibration is estimated approximately on work-surface defined as XY-plane in Fig.8 through the following equations: Shift of the center(X-component); ΔEX = (l/L) x ΔEL…………………..(7) Shift of the center (Y-component); ΔEY= (l/L) x ΔEM Increment of the amplitude (X-component); ΔAX= (sin(Ln/S)/sin(Pn/S)) x ΔAL…. ......(8) Increment of the amplitude (Y-component); ΔAY= (sin(Ln/S)/sin(pn/S)) x ΔAM…. ....(9) Where l, L and S were already defined in Fig.10.[7] 6. Electronic Method: Using light emitting diode and light detecting diode: Fig.13 shows the general system set-up. The wire displacement device is mounted between the upper wired guide head and the workpiece. The system is built to operate in a water based dielectric, such that real cutting conditions can be analyzed Fig.13. Main system setup[8] Debkumar Ruidas 13 Wire Vibration Measurement Technique in WEDM The measured analog data is then digitalized and transmitted to a parallel dual port interface which strobes this data into a Personal Computer (PC) for further analysis. After data analysis, control actions required are fed back to the EDM machine trough a serial interface. The measurement principles are explained in Fig.14. The wire position is instantaneously and optically measured in the x-y plane. Indeed, two light emitting diodes (x and y) transmit a small light beam which is partially interfered by the wire towards the opposite light detecting diodes. One must be aware that EDM wires may vary from some 25μm to 300μm in diameter and hence, the light beam accuracy and diameter are of very importance. One of the basic reasons why optical diode components were used in our design is that this measuring device must be reliable and more accurate when functioning in a dielectric fluid. The major reason why we did not applied "Charged Coupled Device" sensors (CCD's) is due to the fact that they hardly perform in humid and dirty environments. Fig.14. Wire displacement measurement principle.[8] The system described so far is able to keep track peak to peak wire displacements and vibrations. Initialization and calibration is made when no wire vibration is detected. In this position, half of the light beam diameter passes along the wire, the other part of the beam is cut off by the wire. See fig.14.[8] 6.1 Free Vibration condition: wire is put into excitation by several sources: the unwinding system, internal friction of driving belts, friction at the wire guiding heads etc. This picture shows wire displacements in X and Y direction versus time. Both graphs on the picture illustrate that wire displacements of some 200 micro-meter peak to peak can be reached. 6.2 Wire vibration during cutting: Fig. 15. Wire position along X and Y direction versus time [8] Including th forces in free vibration, force due to spark generation and dielectric flow are seen in this case Debkumar Ruidas 14 Wire Vibration Measurement Technique in WEDM Fig. 16. X and Y wire position versus time. Cutting direction along the X axis.[8] Fig. 17. FFT of the data of Fig. 16.[8] 6.3 Low Frequency Wire Vibrations during Cutting Low frequency vibrations of the order of 1 Hz have also been observed during cutting, as given in Fig. 18. 6.4 Real time application: Fig. 18. Low power frequency spectrum for wire vibration [8] The X, Y, wire position is measured regularly when machining goes on. This data is digitalized and fed into a PC-AT where a FFT is executed. This allows to receive wire vibration data during cutting. Based on the frequency analysis and the position information, control actions can be fed back to the EDM machine's interface for a better and more accurate machining execution. 7. Using High Speed Video Camera: High-speed observation system of fine wire EDM is used in this work as shown in Fig. 19. The wire movements during the process are observed by a high-speed video camera. The digital high-speed video camera system (KEYENCE VW-6000) is used for the recording with the aid of a Halogen light source. The recording conditions are listed in Table 1. Table.1 Digital video camera recording condition Recording condition Value Recording speed Shutter speed Recording time view size 8000fps 1/40,000 s 20 s 0.4 x 0.2 mm The wire vibration during the process is analyzed with the motion analysis program (DITECT DIPP Motion Pro). This software is used for the analysis of recorded images obtained using a high-speed video camera (DIPP Motion Pro User s Manual). Debkumar Ruidas 15 Wire Vibration Measurement Technique in WEDM Fig. 19 Experimental setup of the developed high-speed observation system of WEDM[2] Average wire amplitude, which is simply the arithmetic average of the signal level over a certain length of time. 7.1 Evaluation of wire movement: The observed area is above the upper edge of workpiece. The wire is observed from the rear side for machining direction. The observation area is 1.0 mm above the top edge of workpiece, and the view size is 0.4×0.2 mm. The wire vibration during the process is analysed with the motion analysis program (DITECT DIPP Motion Pro) (DIPP-Motion Pro user’s manual). This software is used for the analysis of recorded images obtained using a high-speed video camera. In order to clearly catch the wire vibration in the image analysis process, the recording process is started after the kerf of 0.1 mm in length is machined. The recording speed is set to both 8000 and 24,000 fps.[2] This can be carried out during wire movements during process and during only wire running without machining. Result look like Fig. 20. Wire movements during process and during only wire running without machining. [9] Debkumar Ruidas 16 Wire Vibration Measurement Technique in WEDM 8. Proposed measurement technique Fig.21 shows wire at distance of m from the workpiece , can be measure using Vernier calliper. Workpiece wire m Fig. 21 wire moving direction w/p d
 Sg n Fig.22 shows wire is moving towards the Workpiece. Now wire movement is Controlled manually. After a distance of n travel spark will strart. So the Spark gape Sg = m – n – d…………………............10 Here, d = diameter of the wire Fig. 22 Fig. 23 shows wire inside the the work piece. Here a is amplitude of the wire vibrating with a fixed frequency. Now thikness of the cut can be measure with microscope if thickness is x, then x = 2Sg + 2a + d...............................11 Now we know Sg, d and x so amplitude a can be measure from the equation 11. Debkumar Ruidas Sg a d a Sg Fig. 23 26 Electronic Method Using High Speed Video Camera Proposed method Tool Neel Requirement Probe, micrometer, stepper motor, Light emitting diode, light detecting diode, Analog to digital converter High speed video camera, photo analysing software, computer Vernier caliper, microscope, Set-up No external setup require Setup is very complex Not so difficult Easier than other Easiest than other Cost Neel In the range of Rs. 20,000 – 30,000 In the range Rs. 5,000 – 6,000 More than lacs. Less than using high speed video camera Time Time taken for calculation only Too much time for setup and experiment Lesser time than mechanical method Quite same to the electronic method Minimum time taken than any other method Output estimated accurate More accurate Most accurate Not measured Use as a real time feedback system to the machine Can’t be use Can’t be use Can be use Can be use Can’t be use Wire Vibration Measurement Technique in WEDM Mechanical Method 9.Comparison between measurement methods Debkumar Ruidas Mathematical Model 18 Wire Vibration Measurement Technique in WEDM 10. PRACTICAL APPROACHES THE WIRE VIBRATION OF MINIMISING a) Optimizing the machining parameters It can be found that the machining parameters have a great impact on the wire vibration, so seeking a good parameter combination is an effective measure to suppress the wire vibration phenomenon. (1) The smaller discharge energy per pulse (decreasing pulse-on time, increasing pulse-off time, decreasing peak discharge voltage) produces the smaller discharge impulsive force. While, the discharge energy per pulse cannot be too small to satisfy the material removal rate; (2) Increasing tension means increased stiffness of wire electrode; (3) Medium wire speed (v¼0.12 m/s) may make the wire vibration frequency away from wire natural frequency. Finally, in machining 1 mm-thickness steel process, the optimal parameter combination may be as follows: tON =4μs, toff =12μs, Tf=20N, v=0.12m/s, U=30 V. [10] b) Improving wire transportation system According to the multi-physics coupling model, wire tension and velocity are important factors on wire vibrational character. While, the wire tension and velocity always fluctuate within some range due to the inertia of wire reel, disturbance load, non-uniformity of unreeling wire. Hence, improving the wire transportation system mainly aims to keep the wire speed and tension constant. In this paper, two improvements have been applied in wire transportation system. (1) A closed-loop wire tension control system is developed by importing a tension sensor. Through comparison of responsive tension and control signal, tension motor is real-timely adjusting the wire tension to agree with control signal. (2) Two dynamic absorbers are designed to absorb fluctuation of wire tension and velocity from the source. [10] c) Applying external vibrational excitation on work piece The wire vibration characteristic (period, amplitude and phase position) can be calculated. An external vibrational excitation can be applied on work piece, and the lateral vibration can be significantly restrained under the following two aspects: (1) The frequency of external vibrational excitation is the same as the lateral vibrations; (2) The differences of two phase positions are maintained as half period. [10] Debkumar Ruidas 17 Wire Vibration Measurement Technique in WEDM 11. Conclusion 1. From mathematical model, amplitude and frequency of the wire tool vibration are estimated using some assumptions and avoiding some negligible forces. 2. Mechanical method is a primitive technique to measure wire vibration amplitude with good accuracy. But it gives no idea of frequency and relatively costlier and complex. 3. Electronic method is simpler and better in accuracy for measuring amplitude as well as frequency. Optical diodes are designed to work in actual working condition during machining. 4. High speed video camera is the latest technique to measure vibration with best accuracy but it requires huge cost involvement. 5. Above Proposed method is yet to be tested practically, but it can be practically helpful to study wire tool vibration with certain equipment which can be easily arranged. This method can find easy way to measure vibration with some research and advancements Debkumar Ruidas 19 Wire Vibration Measurement Technique in WEDM 12.Reference 1. Kalyani S. Kanekar, Diwesh B. Meshram, 2015, IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308. 2. Sameh Habib, Akira Okada. Experimental investigation on wire vibration during fine wire electrical discharge machining process, The International Journal of Advanced Manufacturing Technology, Volume 84, Issue 9–12, pp 2265– 2276(2016) 3. Sameh Habib, Akira Okada. Study on the movement of wire electrode during fine wire electricaldischarge machining process, Journal of Materials Processing Technology 227 (2016) 147–152 4. Sameh Habib. Optimization of machining parameters and wire vibration in wire electrical discharge machining process, Mechanics of Advanced Materials and Modern Processes (2017) 3:3 5. Dauw, D. F., Sthioul, H., Delpretti, R., Tricarico, C. Wire analysis and control for precision EDM cutting, Ann. CIRP 38 (1),pp. 191–194 (1989). 6. A.B. Puri, B. Bhattacharyya. Modelling and analysis of the wire-tool vibration in wire-cut EDM, Journal of Materials Processing Technology 141 (2003) 295–301 7. Kinoshita, N.,Fukui, M., Kimura, Y. Study on wire-EDM: inprocess measurement of mechanical behaviour of electrode-wire, Ann. CIRP 33 (1), pp 89 – 92 (1984). 8. Dauw, D. F., Beltrami, I.High-precision wire- EDM by online wire positioning control, Ann. CIRP 43 (1) pp. 193–197, (1989). 9. Sameh Habib, Akira Okada. Study on the movement of wire electrode during fine wire electricaldischarge machining process, Journal of Materials Processing Technology 227 (2016) 147–152 10. Zhi Chen, Yu Huang, Hao Huang, Zhen Zhang, Guojun Zhang. Threedimensional characteristics analysis of the wire-tool vibration considering spatial temperature field and electromagnetic field in WEDM, International Journal of Machine Tools & Manufacture 92 (2015) 85–96 Debkumar Ruidas 20 Wire Vibration Measurement Technique in WEDM Debkumar Ruidas 21 Wire Vibration Measurement Technique in WEDM Debkumar Ruidas 22 Wire Vibration Measurement Technique in WEDM Debkumar Ruidas 23 Wire Vibration Measurement Technique in WEDM Debkumar Ruidas 24 Wire Vibration Measurement Technique in WEDM Debkumar Ruidas 25