CHAPTER 4 PERFORMANCE ANALYSIS OF TOOL MATERIALS AND DIELECTRIC FLUIDS

Transcription

1 52 CHAPTER 4 PERFORMANCE ANALYSIS OF TOOL MATERIALS AND DIELECTRIC FLUIDS 4.1 INTRODUCTION The experimental setup, composition of Monel 400 TM, properties, forms and applications of monel, stages of experiment, measurement of performance attributes, equipment to assess surface characteristics were discussed in the previous chapter. Importance of performance analysis of tool materials and dielectric fluids, experimental conditions, tool materials, dielectric fluids, experimental procedure and machining tests and observations are described in the following section. 4.2 IMPORTANCE OF PERFORMANCE ANALYSIS In this stage of analysis, the effect of variation of current (8, 10 and 12 A) on performance measures of EDM process is observed. Further, the influence of properties of tool electrode materials (melting point, thermal and electrical conductivity) and dielectric fluids (viscosity, breakdown potential and electrical conductivity) on EDM performance is studied. 4.3 TOOL ELECTRODE MATERIALS brass and graphite. Tool electrode materials used for this study are copper, aluminium,

2 Copper With the development of the transistorized, pulse-type power supplies, electrolytic (or pure) copper became the metallic electrode material of choice. This is because the combination of copper and certain power supply settings enable low wear burning. Also, copper is compatible with the polishing circuits of certain advanced power supplies. Many advanced countries still prefer to use copper as the primary electrode material, due to their tool-making culture against graphite. Due to its structural integrity, copper can produce very fine surface finishes, even without special polishing circuits. This same structural integrity also makes copper electrodes highly resistant to DC arcing in poor flushing situations. The copper tools are shown in Figure 4.1. Figure 4.1 Copper tools Copper is frequently used to make female electrodes on a Wire EDM for subsequent use in reverse burning punches and cores in the sinker EDM. copper electrodes. There are a number of significant disadvantages associated with Copper electrodes will generally burn only half as fast as graphite electrodes.

3 54 Copper is a soft and gummy material to machine or grind. Copper is an extraordinarily difficult material to deburr. It can take longer to deburr a copper electrode than to manufacture it. The addition of 1-3% tellurium to copper improves its machinability to a level similar to brass, eliminating the gummy properties normally exhibited by copper when it is machined or ground. Unfortunately, the EDM performance of copper is somewhat compromised by the addition of the Tellurium. Compared to electrolytic copper, tellurium copper (also known as TELCO) exhibits 15-25% higher wear and 10% decreased metal removal rate. However, because of the ease of machining this material is preferred. Most copper comes in the as rolled condition. Being a cold rolled material, the bars exhibit a significant amount of stress movement, especially when being machined by wire EDM cutting. Copper is also commonly used for tubing for certain brands of high speed small hole machines. Copper electrodes are also the preferred material for all high speed small hole applications involving aerospace alloys as well as carbide Graphite Graphite (Figure 4.2) is the preferred electrode material for 90 % of all sinker EDM applications. Thus, it is important that we expend considerable effort to understand its properties and application to EDM. Graphite was introduced to the EDM industry approximately 50 years ago. One of the early well-known brands of graphite was manufactured by General Electric, and known by the trade name of Gentrode.

4 55 Figure 4.2 Graphite tool Graphite is made from carbon derived from petroleum. The powdered carbon is mixed with a petroleum based binder material and then compacted. How the graphite is compacted in this stage of production is vitally important to its ultimate properties. All early graphites were made by compressing the powder/binder mixture in only one direction, resulting in properties or grain similar to wood, that varied relative to the direction of pressing. As an outgrowth of the space program, methods were developed to isostatically press graphite such that its properties became isotropic, that is the same in all directions. All high quality, high performance graphites are now manufactured this way. After compacting, the green compacted material undergoes a series of thermal treatments that convert the carbon to graphite. Graphite has certain properties quite different from wrought metal- based electrode materials: Graphite has an extremely high melting point. Actually, graphite does not melt at all, but sublimes directly from a solid to a gas (just as the carbon-dioxide in dry ice) at a temperature thousands of degrees higher than the melting point of copper. This resistance to temperature makes graphite an ideal electrode material.

5 56 Graphite has significantly lower mechanical strength properties than metallic electrode materials. It is neither as hard, as strong, nor as stiff as metallic electrode materials. However, since the EDM process is one of relatively low macro-mechanical forces, these property differences are not often significant. Due to the significant differences between metallic electrodes and graphite, there are certain properties, unique to graphite that are commonly specified and controlled Aluminium Aluminium is a relatively soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is non-magnetic and does not easily ignite. A fresh film of aluminium serves as a good reflector (approximately 92 %) of visible light and an excellent reflector (as much as 98 %) of medium and far infrared radiation. The yield strength of pure aluminium is 7 11 MPa, while aluminium alloys have yield strength ranging from 200 to 600 MPa. Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded. Aluminium tools are shown in Figure 4.3. Figure 4.3 Aluminium tools

6 Brass Figure 4.4 Brass tools Brass was one of the first EDM electrode materials. The brass tools are shown in Figure 4.4. It is inexpensive and easy to machine. Today, however, brass is seldomly used as an electrode material in modern sinker EDMs, due to its high wear rate. In certain applications or in older machines with relaxation circuit power supplies for which wear is not a primary concern, brass still has limited use, since it exhibits a higher degree of stiffness and is easier to machine than copper. Brass however, is one of the most commonly used materials for high speed small hole machines. The properties tool materials used for the experimentation are listed in Table 4.1. Electrode Table 4.1 Physical properties of tool materials Electrical resistivity µω /cm Thermal Conductivity (W/mºK) Melting point ( C) Specific heat (cal/g C) Coefficient of thermal expansion ( 10 6 C -1 ) Copper Graphite Aluminium Brass

7 DIELECTRIC FLUIDS The dielectric fluids used for this study are kerosene, keroseneservotherm (1:1), paraffin and commercial grade EDM oil Kerosene Kerosene (Figure 4.5) was one of the first popular dielectric oils. Its primary benefit is that it has very low viscosity and flushes very well. Unfortunately, it has many drawbacks: Low flash point High volatility Odor Skin reactions Figure 4.5 Kerosene It is found that numerous EDM fires and explosions attributed to the use of kerosene. It is no longer used as a dielectric, except in third world countries. The properties of kerosene are given in Table 4.2.

8 59 Table 4.2 Properties of kerosene Appearance Clear, transparent, light Density kg/m 3 Viscosity 2 cst at 20 C Flash point 40 C Boiling point 600 C Servotherm Servotherm, as shown in Figure 4.6, is a commercial product marketed by Indian Oil Corporation and is used in present study. Servotherm oils are specially developed mineral oils for use in heat transfer industrial application. Figure 4.6 Servotherm These oils possess excellent thermal and oxidation stability, low volatility and low vapour pressure to give long and trouble-free service-life in heat transfer systems. Servotherm medium is recommended for well designed heat transfer systems up to C. Servotherm shows good

9 60 performance at high temperature due to better oxidation stability. The properties of servotherm are given in Table 4.3. Table 4.3 Properties of servotherm Appearance Clear, transparent, light-blue Kinematic viscosity, cst at 100 C Viscosity index 90 Flash point (COC) 214 C Commercial Grade EDM Oil There are currently numerous choices of mineral oils formulated specifically for EDM. They are available with a wide range of properties and pricing. These oils are currently the most commonly used sinker dielectric fluids. Electric discharge machining is a metal removal process involving the removal of material from the work-piece by a series of controlled electric discharges or sparks. The tool and the work-piece are insulated by a special dielectric fluid. These fluids are formulated with a highly refined, narrow-cut paraffinic base-stock with excellent oxidation stability which helps reduce the formation of oxidation products that will alter the dielectric strength of the fluid. The product is colourless and virtually odourless. The product exhibits a relatively high flash point that helps to reduce the possibility of fire. The low viscosity of the fluid provides good circulation through the spark gap and more rapid removal and settling of metal fines. The properties of EDM oil are shown in Table 4.4.

10 61 Table 4.4 Properties of commercial grade EDM oil Appearance Clear, light Viscosity (centistokes) 3.12 Flash-point Dielectric strength 94 C 45 KV Paraffin Oil Light liquid paraffin oil (Figure 4.7) is highly purified mixture of liquid saturated hydrocarbons obtained from petroleum and is highly paraffinic in nature. Light liquid paraffin oil is transparent, and free from fluorescence in daylight. Figure 4.7 Paraffin oil It is colourless, tasteless, and odourless when cold. Light liquid paraffin oil Ip highly refined hydro-treated oil has excellent thermal and chemical stability, having high flash point and is soluble in chloroform and solvent ether. The properties of paraffin are in Table 4.5. Table 4.5 Properties of Paraffin Appearance Clear, transparent, light yellow Dynamic viscosity at 20 C Between 25 to 80 Flash point (PMCC) C Minimum 150

11 CHOOSING A DIELECTRIC OIL Choosing the best dielectric oil for your particular application is an exercise in compromise, since optimizing one property will be accomplished at the expense of another property as outlined below: Oil with very low viscosity usually will have a low flash point, high volatility, and invade the pours of the skin. Oil with a high flash point may also have high viscosity. Oil with the lowest cost may not have any redeeming qualities. The experimental conditions for performance of different tool materials and dielectric fluids at different current settings are listed in Table 4.6. Table 4.6 Experimental conditions Working conditions Description Work-piece Monel 400 TM (Density g/cm 3 ) Melting point Work-piece hardness Work-material polarity Discharge current in steps Dielectric fluids Tool electrode materials power supply in voltage Pulse on-time Pulse off-time 1329 C 32 HRC Positive 8, 10 and 12 A EDM oil, paraffin, kerosene and kerosene-servotherm (1:1) Copper, graphite, aluminium and brass 30 V 7 sec 5 sec

12 MACHINING TESTS AND OBSERVATIONS Four electrodes made of copper, aluminium, graphite and brass were ground to a cylindrical shape with 10 mm in diameter and 60 mm in length. Composition test for the Monel 400 TM was carried out. Eight circular plates of 100 mm in diameter and 5 mm in thickness made of Monel metal were taken. One surface of the work-piece was ground on a surface grinder to remove surface irregularities and minor sealing. Initial weights of the electrodes and work-piece were measured by digital balance (Serial no. BL- 220 H, Accuracy = g; Made: Shimadzu corporation). The copper tool and test piece were mounted on the machine and immersed in EDM oil. The machining operation is shown in Figure 4.8. Figure 4.8 Machining of work-piece Further, the depth of machining was set to 3 mm. The work-piece was machined with at 8 A (Shankar singh et al., 2004) discharge current and other standard machine settings as given in Table 4.6. The time of machining

13 64 was recorded in minutes and final weights of the electrode and work-piece were measured. The electrode was machined again to remove distortion due to erosion and obtain an accurate face and diameter. The machining cycle was repeated for the next value of discharge current i.e. 10A and 12A (Shankar singh et al., 2004). Similarly, the observations were made for the other electrodes i.e. aluminum, graphite and brass. After this, entire process was repeated by using paraffin oil, kerosene and kerosene-servotherm (1:1 - in equal proportion) subsequently. Thus, 48 holes were machined in the work-pieces using different tool materials and dielectric fluids as mentioned above. It is noted from the Table 4.7 that the copper tool electrode and kerosene-servotherm (1:1) dielectric is a better combination, since it offers highest materials removal rate, less tool wear rate and more surface roughness than kerosene. This may be due to higher thermal and electrical conductivity of the copper; in addition, mixture of kerosene-servotherm increases the sparking frequency. The level of surface roughness is ensured from SEM image as shown in Figure 4.9. Figure 4.9 SEM image of machined work surface for copper and kerosene servotherm (1:1) Table 4.7 lists the values of performance measures at different current settings, tool materials and dielectric fluids.

14 SR in µ TWR in mg/min MRR in g/min Dielectric fluid Performance measure 65 Table 4.7 Values of performance measures different tool materials and dielectric fluids Copper Aluminium Brass Graphite 8A 10A 12A 8A 10A 12A 8A 10A 12A 8A 10A 12A EDM oil Paraffin Kerosene Kerosene- Servotherm EDM oil Paraffin Kerosene Kerosene- Servotherm EDM oil Paraffin Kerosene Kerosene- Servotherm a. Cu tool b. Brass tool c. Gr tool d. Al tool Figure 4.10 Machined test pieces for (a) Cu tool, (b) Brass tool (c) Gr tool (d) Al tool

15 66 Machined test pieces using different tool materials and dielectric fluids at various current settings are shown in Figures Tools used for experimentation are shown in Figure a. Cu tool b. Brass tool c. Gr tool d. Al tool Figure 4.11 Tools used for experimentation 4.7 SUMMARY The performance analysis of different electrode materials, dielectric fluids at different current settings was discussed in this chapter. It is observed that the copper tool electrode and kerosene servotherm (1:1) dielectric offers better performance in EDM process. The performance analysis of kerosene-servotherm of sixteen combinations in EDM process is discussed in the next chapter.