CHAPTER 4 EXPERIMENTAL PLANNING USING EDM MACHINE

Transcription

1 80 CHAPTER 4 EXPERIMENTAL PLANNING USING EDM MACHINE 4.1 INTRODUCTION In this chapter existing EDM systems like ELECTRONCIA make and SPARKONIX make EDM machines, effect of input parameters, dielectric system, selection of workpiece and tool, machining performance evaluation, machining parameters selection and design of experiments are described in detail. 4.2 EXISTING EDM SYSTEMS There are many manufacturers available worldwide for EDM systems. But, in India there are very few commercially available EDM manufacturers like Makino, Electronica, and Sparkonix. In this work, Electronica and Sparkonix EDM machines have been selected for drilling of micro-holes. At the time of purchasing the machines, there are many undisclosed parameters which are effective in the determination of machining characteristics. So this research aims to study and optimize the process parameters of the machine setup for better machining characteristics such as MRR, TWR, OC and Taper.

2 ELECTRONCIA MAKE EDM MACHINE A scheme of experiments is performed using ELECTRONICA make EDM Machine which is shown in Figure 4.1. Figure 4.1 EDM Machine (ELECTRONICA) for micro-hole machining of SS 304

3 Technical Specifications Work Table X &Y Axes Travel Z Axis Travel Maximum electrode length Electrode pipe diameter : 450X300 mm Granite : 350 & 250 mm : 350 mm : 400 mm : mm Maximum weight of the work piece Maximum coolant pressure Maximum drill depth Input Power Supply Connected Load Maximum Machining Current Machining fluid : 350 kg : 6MPa : 300 min : 415 VAC, 3 phase, 50 Hz : 3 KVA : 30 Amps : DI water 4.4 SPARKONIX MAKE EDM MACHINE The machine shown in Figure 4.2 consists of an x-y table with movements in the x-axis as well as y-axis of the horizontal plane. Both movements are achieved by means of dove-tailed x-y slides driven with lead screw and are hand wheel operated. The Z- axis comprises the servo- slide movement on which the mechanism for rotating electrode along with a chuck is mounted for holding the required tubular copper or tubular brass electrode as shown in Figure 4.3. The x and y axis movements as well as z- axis movements of electrode can be set and adjusted using digital read out (DRO) provided at the top of the control panel. The control panel appears on the right hand slide of the machine at about eye-level and the electrical controller is housed below the x-y table in the machine cabinet. The left hand side of the machine cabinet below the

4 83 x-y table is occupied by the coolant tank and the high pressure pump unit along with filter elements. An arrangement for holding and adjusting the level of electrode guides bush is provided on the left hand side of the machine pillar holding the electrode rotating mechanism and the servo slide assembly. A machine lamp is similarly located. A pressure setting valve and pressure gauge to monitor correct operating pressure are provided on the machine pillar centrally on the left hand side. The work piece or job is secured and clamped at an appropriate location on the x-y table. The location of small holes or fine deep holes to be drilled may be marked on the job. The job may be set with the help of dial stand and DRO. A suitable electrode of particular size as well as an appropriate guide bush is selected and the electrode inserted into the chuck for holding it. The electrode is then tested for coolant flow at a pressure of about 100 Kg/sq.cm. The electrode may now be positional on to job to start drilling operation Technical Specifications Power rating : 3 KVA Working voltage : 415 VAC, 3 phase, 50 Hz Maximum Machining Current : 25 Amps Electrode Movement (Z1) : 150 mm Guide movement (Z2) : 50 mm Work table size : 600x400 mm Table travel : 300x200 mm Maximum work piece size (LxWxH) : 400x300x200 mm

5 84 Maximum work piece weight : 500 kg Maximum electrode length : 400 mm Electrode pipe diameter : mm in Steps of 0.1 mm Machining fluid : DI water \Fluid tank capacity : 50 liters Type of electrode : Brass and copper tubes. Erosion transformer : Input- 440 VAC Output- 60 VAC Figure 4.2 EDM Machine (Sparkonix) (Disintegrator MICRO DRILL DSHII) for micro-hole drilling of SS 316

6 85 Figure 4.3 Tool setup Spark Generator/Controller The controller cabinet is located on the right side of the machine and can be pulled out completely if required for repairs or maintenance. This can be done by disconnecting main supply, the job cable, control panel socket and the four bolts holding the cabinet. The control cabinet houses the following:- 1) Erosion transformer : Converts 415 v input to 80v i.e. steps down transformer. 2) Rectifier Unit : Convert the stepped down AC voltage to DC. 3) Control circuit : Fitted on a chassis, it consists of control transformers, contactors for mains, pump and erosion, and rotating motor control, power MOSFET circuit and indicators circuit.

7 86 The following controls and indicators are located on the front panel:- Master main ON (MCB) : For providing 415 VAC three phase supply to machine. Mains ON : For switching ON mains. Emergency OFF : For switching OFF mains Pump ON : For Switching ON pump. Pump OFF : For Switching OFF pump and Erosion. Erosion ON : For switching ON Erosion Down Indicator (Green LED) : Indicator forward (towards job) movement of electrode. Up indicator (Red LED) : Indicator reverse (away from job) Movements of electrode. Up/Down Switch : In forward mode, this control helps to bring down the electrode during setting of job without switching ON pump erosion. This should always be kept at up position except when it is required for setting of the job. Current : This is a set of six switches and will provide a total of 25Amps of current these 25 Amps are divided into six steps of 1A, 2A, 4A, 6A, 6A and 6A. So any current between 1A to 25A can be selected with the help of these switches. Rotating ON/OFF : Used for switching ON rotating (Erosion will not start unless rotating is ON). Motor ON/OFF : Used for switching ON or OFF a servo motor. Pump socket : Connects supply to coolant pump via cable and socket. Control panel socket : The control panel cables are connected here

8 87 by 48 pin plug and socket arrangement. The other end of the cable is terminated on control panel fitted on the column. Electrode and job : Electrode and job terminals constitute to the o/p and the connected respectively to the electrode and the x-y table. Both connections should be tight and proper and job should be clamped on x-y table as far as possible. ON/OFF time switches : With the help of these two rotary switches, we can change ON time OFF time of the output current in 10 steps. These can be selected with reference to the trial chart. Buzzer : This is for job setting purpose. If this switch is ON and the electrode touches the job, the buzzer will sound and be down. movement of the motor will be stopped. Capacitor : This switch is used for carbide job with copper electrodes. With increasing steps value of capacitor increases. 4.5 EFFECT OF INPUT PARAMETERS Based on the discharge phenomena discussed above, the effect of various input parameters on MRR and surface roughness is discussed below Discharge Current The discharge current is a measure of the power supplied to the discharge gap. A higher current leads to a higher pulse energy and formation of

9 88 deeper discharge craters. This increases the MRR and the surface roughness (Ra) value. Similar effect on MRR and Ra is produced when the gap voltage is increased. The current density is the most important parameter which determines the MRR and surface condition. The current density is affected by either changing the current or changing the electrode (tool) work piece gap. When the current is increased, each individual sparks removes a larger crater of metal from the work piece. But it also increases surface roughness. Increasing spark frequency results in decrease of surface roughness and reduces the removal of crater of metal from the work piece. The gap between the electrode (tool) and work piece is determined by the spark voltage and current. A small gap produces more accuracy with a better surface finish and slower MRR Voltage In EDM process with the given input voltage, a spark can be generated only at or below certain gap between the tool and work piece. The EDM power supply sense the voltage between the electrodes and then sends the relevant signal to the servo system, which maintains the desired gap value between the electrodes. It is a potential that can be measured by volt it also effects to the MRR and allowed to per cycle. The Arc gap is distance between the electrode and work piece during the process of EDM. It may be called as spark gap. The spark gap can be maintained by servo system Pulse-on Time The duration of time the current is allowed to flow per cycle is known as Pulse-on time. An illustration of on-time, off-time, pulse-time and duty cycle is shown in Figure 3.4. Material removal is directly proportional to the amount of energy applied during this on-time. This energy is really controlled by the peak

10 89 current and the length of the on-time. Machining takes place only during the pulse-on time. When the tool electrode is at negative potential, material removal from the anode (workpiece) takes place by bombardment of high energy electrons ejected from the tool surface. At the same time positive ions move towards the cathode. When pulses with small on times are used, material removal by electron bombardment is predominant due to the higher response rate of the less massive electrons. However, when longer pulses are used, energy sharing by the positive ions is predominant and the MRR decreases. When the electrode polarities are reversed, longer pulses are found to produce higher MRR Pulse-off Time The duration of time between the sparks (i.e., on-time) is known as Pulse-off time. This time allows the molten material to solidify and to be washed out of the arc gap. This parameter is to affect the speed and the stability of the cut. Thus, if the off-time is too short, it will cause sparks to be unstable. A nonzero pulse off time is a necessary requirement for EDM operation. Discharge between the electrodes leads to ionization of the spark gap. Before another spark can take place, the medium must de-ionize and regain its dielectric strength. This takes some finite time and power must be switched off during this time. Too low values of pulse-off time may lead to short-circuits and arcing. A large value on the other hand increases the overall machining time since no machining can take place during the off-time. The surface roughness is found to depend strongly on the spark frequency. When high frequency sparks are used lower values of Ra are observed. It is so because the energy available in a given amount of time is shared by a larger number of sparks leading to shallower discharge craters.

11 Duty Cycle Duty cycle is a percentage of the on-time relative to the total cycle time. This parameter is calculated by dividing the on-time by the total cycle time (on-time plus off-time). The result is multiplied by 100 for the percentage of efficiency or the so called duty cycle. Figure 4.4 An illustration of on-time, off-time, pulse-time and duty cycle Inter Electrode Gap Inter Electrode Gap (IEG) is the distance between the electrode and the workpiece during the process of EDM. It may be called as spark gap. Since, during operation, both the work piece and electrode are eroded, the feed control must maintain a movement of the electrode towards the work piece at such a speed that the working gap, and hence, the sparking voltage remains unaltered. Since the gap width is so small, any tendency of the control mechanism to hunt is highly undesirable. Rapid response of the mechanism to hunt is highly undesirable. Rapid response of the mechanism is essential and this implies a low

12 91 inertia drive. Overshooting may completely close the gap cause a short circuit; hence, it is essential to have rapid reversing speed with no backlash. Actuation of the control drive is derived from an error indication signal obtained from an electrical sensing device responsive to either the gap voltage or the working current or both. Servo mechanisms affecting the movement of the electrode may be either electric-motor-driven, solenoid operated or hydraulically operated or a combination of these Tool Rotation Tool electrode rotation is commonly used in small-hole EDM drilling operations. Tool rotation improves flushing and leads to a more uniform electrode wear. The effects of improved flushing are increased MRR and lower Ra values. At the same time, process stability increases because tool rotation makes it easier to introduce fresh dielectric into discharge gap as the used up dielectric is thrown out due to the centrifugal force. Thus, even with low pulse off times and poor flushing conditions good machining performance is obtained. 4.6 DIELECTRIC SYSTEM It consists of dielectric fluid, reservoir, filters, pump, and delivery devices. A good dielectric fluid should possess certain properties, viz as shown below, it should: i. Have high dielectric strength (i.e. remain electrically nonconductive until the required breakdown voltage between the electrodes is attained),

13 92 ii. iii. iv. Take minimum possible time possible time to breakdown (i.e. ignition delay time) once the breakdown voltage is reached, Deionize the gap immediately after the spark has occurred, Serve as an effective cooling medium. The fluids commonly used as dielectric are transformer oil, paraffin oil, kerosene, lubricating oils, and de-ionized water which give higher MRR and functions as more effective cooling media but they also cause high electrode wear rates. Further, they suffer from the drawback of causing corrosion. To overcome this problem, corrosion inhibitors are used but they result in increased electrical conductivity to an unacceptable level. De-ionized water is commonly used as a dielectric in wire-edm and drilling of small diameter holes. The filtration of dielectric fluid before recirculation is highly essential so that a change in its insulation qualities during the process is minimal. The concentration of the debris particles in the gap increases rapidly as the machining progresses. These wear particles should be removed from the gap so that the fresh dielectric enters the IEG for spark discharges. Increase in the pollution of dielectric results in decrease of the breakdown intensity of the field. Hence, IEG at the entrance of the clean fluid is much narrower than at the exit of the flow. This significantly affects the reproduction accuracy of the process. It is possible to correct such errors only if it could be feasible to predict quantitatively the error of the shape. Effective flushing of dielectric removes by-products from the gap. Ineffective flushing results in lower MRR and poorer surface finish. The effective flushing may increase MRR as much as by a factor of 10. Poor flushing ends up with stagnation of dielectric and building-up of machining residues

14 93 which are apart from low MRR also lead to short circuits and arcs. A good flushing system is the one that shoots the dielectric to the place where the sparking occurs. It is felt that adequate flushing in case of blind cavities is difficult. Various methods have been proposed by different researchers to ensure proper and adequate flushing of the gap and thereby obtaining a better process output. i. Suction through electrodes ii. Pressure through electrodes iii. Jet flushing iv. Alternating forced flushing v. Ultrasonic vibration of electrodes vi. Rotating electrode flushing These are the some of the dielectric flushing techniques. In some cases, hypodermic needles have been used as nozzles for flushing in tight quarters. In case of machining of blind cavities, flushing through a hole in the tool is most effective but it rises to protruding bump (or spike). This is the situation similar to the one faced during ECM. To avoid this problem, a rotating tool with a eccentric hole (off-centred holes) for dielectric supply can be used. Jet flushing is less effective hence it should be used only when none of the other methods can be used due to tool or workpiece configuration. In case of inflammable dielectric fluids, the workpiece should always be immersed in the dielectric fluid to minimize any chance of accidental fire. As machining continues, the dielectric gets contaminated with more and more amount of debris. Presence of such debris in the IEG, results in bridging the gap. It yields more number of arcs. Arcs are undesirable in EDM because they damage both tool and workpiece. Occurrence of such arcs can be eliminated by proper filtration of the dielectric as well as appropriate flushing of the IEG.

15 Functions of Dielectric It acts as an insulation medium. It cools the spark region and helps in keeping the tool and work piece cool. It carries away the eroded metal particles along with it It maintains a constant resistance across the gab. It remains electrically non-conducting until the required breakdown voltage is reached. It cools the machining zone by carrying away excess heat from the tool electrode and the workpiece Properties of Dielectric The most important properties of dielectric are its dielectric strength, viscosity, thermal conductivity and thermal capacity. Dielectric strength characterizes the fluid s ability to maintain high resistivity before spark discharge and the ability to recover rapidly after the discharge. High dielectric strength leads to a lower discharge gap which in turn leads to a low gap resistance. Hence, high discharge currents may flow leading to a higher MRR. Also, fluids with high dielectric strength need lower time for the recovery of dielectric strength. Thus, low pulse-off times are sufficient. This not only improves the MRR but also provides better cutting efficiency because of a reduced probability of arcing. Liquids with low viscosity generally provide better accuracies because of a better flow ability of the oil leading to improved flushing. Also, the sideward expansion of the discharge plasma channel is restricted by high viscosity fluids. This focuses the discharge energy over a small region and leads to a deeper crater which reduces the surface finish. Dielectric fluids with high thermal conductivity

16 95 and thermal heat capacity can easily carry away excess heat from the discharge spot and lead to a lower thermal damage. 4.7 SELECTION OF WORKPIECE AND TOOL The brass electrode of diameter 500 m is used as a tool electrode. The melting point of the tool material should be high for machining difficult-tocut materials. Normally copper, brass, and tungsten tools are used for this purpose. However, brass tool is more economical than copper and tungsten tool and mostly used in industrial applications. Also, brass tools can withstand high tensile stress compared to pure copper tools. The workpiece used is SS 304 plate of size mm and SS 316. In the case of SS, its mechanical strength and chemical resistance can easily be controlled by changing the composition of the alloy. Therefore, SS is one of the most widely used materials for micro mechanical structures. The composition of SS is given in Table 4.1 and the physical properties of workpiece and tool materials are presented in Table 4.2 Table 4.1 Chemical composition of Stainless steel component Fe C Mn Si P S Cr Ni Weight % (SS 304) Weight % (SS 316)

17 96 Table 4.2 Physical properties of workpiece and tool materials Material Stainless Steel Grade 304 Stainless Steel Grade 316 Brass Thermal Conductivity(W/m-K) Density (kg/m 3 ) Poisson's Ratio Specific Heat (J/kg-K) Yield Strength (MPa) Melting point ( C) The properties of De-Ionized water at room temperature are given in Table 4.3 Table 4.3 Electrical, thermal, and mechanical properties of De-ionized Water at room temperature De-ionized Water Dielectric strength (MV/m) 13 Dielectric constant 80.4 Dynamic viscosity (g/m s) 0.92 Thermal conductivity (W/m K) Heat capacity (J/g K) 4.19

18 97 The positive terminal is connected to the workpiece and made as anode and the negative terminal is connected to the tool and made as cathode. When the negative terminal is given to the tool, the tool wear is less due to low sparking energy distribution at the tool, and this helps in improving the micromachining accuracy. De-ionized water is used as the dielectric medium in EDM and is an alternative to traditional hydrocarbon oil to promote a better working environment from the perspective of health and safety, as it does not decompose and release harmful vapours (CO and CH 4 ). given in Table 4.4 The Experimental condition for micro-hole machining on SS 304 are Table 4.4 Experimental conditions for micro-hole machining on SS 304 Workpiece Tool electrode Dielectric fluid Polarity Pulse-on time Peak current Gap voltage mm SS 304 plate Brass tool of diameter 500 m De-Ionized water Positive (workpiece +ve and tool -ve ) 100 to 200 s 2 to 4 amps 20 to 40 volts 4.8 MACHINING PERFORMANCE EVALUATION To observe the surface quality and dimensions of micro holes a Scanning Electron Microscope (SEM; JEOL-6390) having 3, 00,000X magnification and an accelerating voltage of 30KV is used. In this research study, EDM characteristics such as MRR, TWR and OC are considered as the

19 98 output responses for through micro-hole machining. The MRR is calculated as the workpiece weight loss over the machining time, which is expressed as milligrams per minute. As during the EDM process both the workpiece and tool electrode is eroded simultaneously, TWR is expressed as the ratio of tool weight loss to the machining time. A digital vernier caliper with 0.01 mm accuracy is used to measure the length of the tools before and after machining. The loss in length of the tools is calculated by taking the difference between initial and final length measurements. Measurement of MRR: MRR = Work piece weight loss (g) Machining Time (min) (3.1) Measurement of TWR: TWR = Tool weight loss (g) Machining Time (min) (3.2) OC of the machined micro-hole is calculated by subtracting the diameter of the tool electrode from the diameter of the machined micro hole. Due to the debris movement and secondary discharge occurring on the side of hole in addition to front end tool wear, the machined hole is not ideally cylindrical but tapered one, which is undesirable in precision machining processes. The typical shape of micro-drilled hole resulting from electrodischarge machining is shown in Figure 4.5. The diameter of the drilled hole is larger at the top, i.e., at the entry and decreases along the depth and is minimum at the bottom (exit). The diameters of machined micro-hole at the entry, D t and at the exit, D b are measured with the help of optical microscope at the magnification of 10X and the taper of the machined micro-holes are calculated as follows. Dt Db Taper = 2 L (3.3)

20 99 Figure 4.5 Schematic cross-sectional view of EDMed micro-drilled hole 4.9 MACHINING PARAMETERS SELECTION In this study, the experimental plan has three controllable variables namely current, pulse-on time, and gap voltage. The range of the current, pulseon time, and gap voltage are selected as 2 to 4 A, 100 to 200 s and 20 to 40 V respectively based on the available settings in the machine set-up as shown in Table 4.5. Table Machining parameters and their levels for micro-hole machining of SS 304 Symbol Parameters Level 1 Level 2 Level 3 A Pulse-on time, s B Current, A C Voltage, V

21 The selection of machining parameters for SS 316 is presented in Table Table Machining parameters and their levels for micro-hole machining of SS 316 Symbol Parameters Level 1 Level 2 Level 3 A 1 Current, A B 1 Voltage, V C 1 Pulse -on time, s D 1 Pulse -off time, s DESIGN OF EXPERIMENTS Design of Experiments (DoE) refers to planning, designing and analyzing an experiment so that valid and objective conclusions can be drawn effectively and efficiently. To perform a designed experiment, changes are made to the input variables and the corresponding changes in the output variables are observed. The input variables are called factors and the output variables are called response. Factors may be either qualitative or quantitative. Qualitative factors are discrete in nature (such as type of material, color of sample). Each factor can take several values during the experiment. Each such value of the factor is called a level. A trial or run is a certain combination of factor levels whose effect on the output is of interest. It is essential to incorporate statistical data analysis methods in the experimental design in order to draw statistically sound conclusions from the experiment. Some of the advantages of DoE over One-Variable-At-a-Time approach (OVAT) are that a DoE approach enables to separate the important factors from the unimportant ones by comparing the factor

22 101 effects. Also, interaction effects among different factors can be studied through designed experiments CONCLUDING REMARKS In this chapter constructional features and specifications of the existing EDM systems like ELECTRONCIA make and SPARKONIX make EDM machines are explained. Furthermore the effect of input parameters, dielectric system, selection of workpiece and tool, machining performance evaluation, machining parameters selection have been explained. Design of experiments is described in detail.