NEAR DRY ELECTRICAL DISCHARGE MACHINING

SEMINAR REPORT

Submitted by

JEFFY JOSEPH

S7M1,06400045

To The University of Kerala

In partial fulfilment of the requirements for the award of the degree

Of Bachelor Of Technology in Mechanical Engineering

Department Of Mechanical Engineering

College of Engineering, Thiruvananthapuram – 16

NOV 2009

DEPARTMENT OF MECHANICAL ENGINEERING

COLLEGE OF ENGINEERING,THIRUVANANTHAPURAM – 16

CERTIFICATE

This is to certify that the report entitled “Near dry electrical discharge machining”,

submitted by “JEFFY JOSEPH, S7M1, 06400045” to the University of Kerala in partial fulfilment of the requirements for the award of the Degree of Bachelor of Technology in

Mechanical Engineering is a bonafide record of the seminar presented by him.

Sri. T.C.Rajan Smt. A Naseema Beevi Sri. N Sasi Asst. Professor Sel. Gr. Lecturer Lecturer

Sri. E Abdul Rasheed Sri. P Vincent Professor Professor

Senior staff adviser Head of the Department

ACKNOWLEDGEMENTS

I express my gratitude to my guides Sri. T.C.Rajan, Asst.Professor, Smt. A

Naseema Beevi, Sel. Gr. Lecturer and Sri N Sasi , Lecturer, Department of

Mechanical Engineering, College Of Engineering, Thiruvananthapuram for their expert

guidance and advice in presenting the seminar.

I express my sincere thanks to Sri. E Abdul Rasheed , Professor, Senior staff adviser,

Department of Mechanical Engineering and Sri. P Vincent, Professor, Head of the

Department, Mechanical Engineering, College Of Engineering, Thiruvananthapuram for

their kind co operation during the course of this work.

I would also wish to record gratefulness to all my friends and classmates for their help and

support in carrying out this work successfully.

Jeffy Joseph

ABSTRACT

This study investigates the near dry electrical discharge machining (EDM) process. Near dry

EDM uses liquid–gas mixture as the two phase dielectric fluid and has the benefit to tailor the

concentration of liquid and properties of dielectric medium to meet desired performance

targets. A dispenser for minimum quantity lubrication (MQL) is utilized to supply a minute

amount of liquid droplets at a controlled rate to the gap between the workpiece and electrode.

Wire EDM cutting and EDM drilling are investigated under the wet, dry, and near dry

conditions. The mixture of water and air is the dielectric fluid used for near dry EDM in this

study. Near dry EDM shows advantages over the dry EDM in higher material removal rate

(MRR), sharper cutting edge, and less debris deposition. Compared to wet EDM, near dry

EDM has higher material removal rate at low discharge energy and generates a smaller gap

distance. However, near dry EDM places a higher thermal load on the electrode, which leads

to wire breakage in wire EDM and increases electrode wear in EDM drilling. A mathematical

model, assuming that the gap distance consists of the discharge distance and material removal

depth, was developed to quantitatively correlate the water–air mixture’s dielectric strength

and viscosity to the gap distance.

TABLE OF CONTENTS

NOMENCLATURE i

LIST OF FIGURES ii

LIST OF TABLES iii

1.INTRODUCTION 1

1.1 BACKGROUND 1

1.2 ELECTRICAL DISCHAREGE MACHINING 1

1.2.1 TYPES OF EDM 2

1.3DIELECTRIC MEDIUM 5

2. OBJECTIVE 6

3.EXPERIMENTAL SETUP 6

3.1 MACHINE SETUP 6

3.2 WIRE EDM CUTTING 8

3.3 EDM DRILLING 9

4.RESULTS AND DISCUSSION 10 4.1NEAR DRY WIRE EDM MRR AND GAP DISTANCE 10

4.1.1 MRR ENVOLOPE BOUNDARIES 10

4.1.2 EFFECT OF FLOW RATE ON MRR 12

4.1.3 GAP DISTANCE AND DEBRIS DEPOSITION 13

4.2 NEAR DRY EDM DRILLING 13

4.2.1 WET DRY AND NEAR DRY EDM DRILLING 13

4.2.2 EFFECT OF FLOW RATE AND PULSE CURRENT ON GAP

DISTANCE 14

4.3 MODELLING OF NEAR DRY EDM GAP DISTANCE 15

4.3.1 MATHEMATICAL MODELLING 15

4.3.2 DEVOLOPING A MATHEMATICAL MODEL 16

4.3.3PARAMETERS FOR EDM GAP DISTANCE MODEL 18

4.3.4MODEL VALIDATION 19

5. CONCLUSION 19

6. REFERENCES 21

NOMENCLATURE

EDM - electrical discharge machining

MQL - minimum quantity lubrication

MRR - material removal rate

t0 – pulse interval

te - discharge interval

ue -gap voltage

u1 - open circuit voltage

i

LIST OF FIGURES

Figure page no.

Fig 1. Schematic diagram of electrical discharge machining 2

Fig 2. Wire EDM 4

Fig 3.Sinker EDM 4

Fig 4. The delivery of liquid–gas mixture in near dry wire EDM. 7

Fig 5.BROTHER HS-5100 7

Fig 6. GROMAX MD20 8

Fig 7.Comparision of boundaries of feasible MRR envolopes for wet,dry and

near dry wire EDM 11

Fig. 8. Comparison of MRR performances of wet and near dry wire EDM under

varied t0 and te and three regions based on near dry and wet EDM

performance 11

Fig. 9. MRR envelopes of near dry wire EDM cutting at two deionized water

flow rate 13

Fig. 10. Optical micrographs on holes drilled on 1.27mm Al6061: (a) wet, (b) dry,

and (c) near dry EDM conditions 14

Fig. 11. The effect of deionized water flow rate and discharge current on the MRR

of EDM drilling 15

Fig 12. Experimental measured and model predicted gap distance vs. Volumetric

ratio of water for wire EDM cutting 18

Fig 13. Experimental measured and model predicted gap distance vs. Volumetric

ratio of water for EDM drilling 18

Fig 14. Estimated values of dielectric strength and dynamic viscosity vs.

volumetric ratio of water calculated from the wire EDM and EDM drilling 19

ii

LIST OF TABLES

Table page no.

TABLE 1.average gap distance in EDM cutting under wet,dry and near dry conditions 8

TABLE 2. average gap distance in EDM drilling under wet,dry and near dry conditions 9

iii

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1.INTRODUCTION

1.1 BACKGROUND

In spite of the advantages, conventional EDM process has certain limitations in

production application, including low material removal rate, long lead time for preshaped tool

preparation, large tool wear, environmental concern caused by toxic dielectric disposal,

etc.One of the main sources of environmental pollution during the machining processes is the

huge amount of supplied cutting fluids. The lubricants are widely considered to be a benefit

to cutting operations, but despite the recognition of their advantages, it has also been stated

the negative impact and environmental issues associated with their use. To avoid the

problems caused by the use of cutting fluids, considerable progress has been made in the last

years in the field of near-dry machining. The conversion from conventional processes to

minimal quantity lubrication methods demands new tasks classification in the tribology

system in order to guarantee the process. Near dry EDM is one such method to reduce

dielectric disposal and to obtain a better finish machining at low pulse discharge energy.

1.2 ELECTRICAL DISCHAREGE MACHINING

Electric discharge machining (EDM), sometimes colloquially also referred to as spark

machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing process

whereby a wanted shape of an object, called workpiece, is obtained using electrical

discharges (sparks). The material removal from the workpiece occurs by a series of rapidly

recurring current discharges between two electrodes, separated by a dielectric liquid and

subject to an electric voltage. One of the electrodes is called tool-electrode and is sometimes

simply referred to as „tool‟ or „electrode‟, whereas the other is called workpiece-electrode,

commonly abbreviated in „workpiece‟. When the distance between the two electrodes is

reduced, the intensity of the electric field in the volume between the electrodes is expected to

become larger than the strength of the dielectric (at least in some point(s)) and therefore the

dielectric breaks allowing some current to flow between the two electrodes. This

phenomenon is the same as the breakdown of a capacitor (condenser) (see also breakdown

voltage). A collateral effect of this passage of current is that material is removed from both

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the electrodes. Once the current flow stops (or it is stopped - depending on the type of

generator), new liquid dielectric should be conveyed into the inter-electrode volume enabling

the removed electrode material solid particles (debris) to be carried away and the insulating

proprieties of the dielectric to be restored. This addition of new liquid dielectric in the inter-

electrode volume is commonly referred to as flushing. Also, after a current flow, a difference

of potential between the two electrodes is restored as it was before the breakdown, so that a

new liquid dielectric breakdown can occur.

Fig 1. Schematic diagram of electrical discharge machining

1.2.1TYPES OF EDM

Sinker EDM

Sinker EDM sometimes is also referred to as cavity type EDM or volume EDM.

Sinker EDM consists of an electrode and workpiece that are submerged in an insulating

liquid such as oil or dielectric fluid. The electrode and workpiece are connected to a suitable

power supply. The power supply generates an electrical potential between the two parts. As

the electrode approaches the workpiece, dielectric breakdown occurs in the fluid forming an

ionization channel, and a small spark jumps. The resulting heat and cavitation vaporize the

base material, and to some extent, the electrode. These sparks strike one at a time in huge

numbers at seemingly random locations between the electrode and the workpiece. As the base

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metal is eroded, and the spark gap subsequently increased, the electrode is lowered

automatically by the machine so that the process can continue uninterrupted. Several hundred

thousand sparks occur per second in this process, with the actual duty cycle being carefully

controlled by the setup parameters. These controlling cycles are sometimes known as "on

time" and "off time". The on time setting determines the length or duration of the spark.

Hence, a longer on time produces a deeper cavity for that spark and all subsequent sparks for

that cycle creating a rougher finish on the workpiece. The reverse is true for a shorter on

time. Off time is the period of time that one spark is replaced by another. A longer off time

for example, allows the flushing of dielectric fluid through a nozzle to clean out the eroded

debris, thereby avoiding a short circuit. These settings are maintained in micro seconds. The

typical part geometry is to cut small or odd shaped angles. Vertical, orbital, vectorial,

directional, helical, conical, rotational, spin and indexing machining cycles are also used.

Wire EDM

In wire electrical discharge machining (WEDM), or wire-cut EDM, a thin single-

strand metal wire, usually brass, is fed through the workpiece, typically occurring submerged

in a tank of dielectric fluid. This process is used to cut plates as thick as 300mm and to make

punches, tools,and dies from hard metals that are too difficult to machine with other methods.

The wire, which is constantly fed from a spool, is held between upper and lower diamond

guides. The guides move in the x–y plane, usually being CNC controlled and on almost all

modern machines the upper guide can also move independently in the z–u–v axis, giving rise

to the ability to cut tapered and transitioning shapes (circle on the bottom square at the top for

example) and can control axis movements in x–y–u–v–i–j–k–l–. This gives the wire-cut EDM

the ability to be programmed to cut very intricate and delicate shapes. The wire is controlled

by upper and lower diamond guides that are usually accurate to 0.004 mm, and can have a

cutting path or kerf as small as 0.12 mm using Ø 0.1 mm wire, though the average cutting

kerf that achieves the best economic cost and machining time is 0.335 mm using Ø 0.25 brass

wire. The reason that the cutting width is greater than the width of the wire is because

sparking also occurs from the sides of the wire to the work piece, causing erosion. This

"overcut" is necessary, predictable, and easily compensated for. Spools of wire are typically

very long. For example, an 8 kg spool of 0.25 mm wire is just over 19 kilometers long.

Today, the smallest wire diameter is 20 micrometres and the geometry precision is not far

from +/- 1 micrometre. The wire-cut process uses water as its dielectric with the water's

4

resistivity and other electrical properties carefully controlled by filters and de-ionizer units.

The water also serves the very critical purpose of flushing the cut debr is away from the

cutting zone. Flushing is an important determining factor in the maximum feed rate available

in a given material thickness, and poor flushing situations necessitate the reduction of the

feed rate.

Along with tighter tolerances multiaxis EDM wire-cutting machining center have

many added features such as: Multiheads for cutting two parts at the same time, controls for

preventing wire breakage, automatic self- threading features in case of wire breakage, and

programmable machining strategies to optimize the operation.

Wire-cutting EDM is commonly used when low residual stresses are desired. Wire

EDM may leave residual stress on the workpiece that are less significant than those that may

be left if the same workpiece were obtained by machining. In fact in wire EDM there are not

large cutting forces involved in the removal of material. Yet, the workpiece may undergo to a

significant thermal cycle, whose severity depends on the technological parameters used.

Possible effects of such thermal cycles are the formation of a recast layer on the part and the

presence of tensile residual stresses on the workpiece. If the process is set up so that the

energy/power per pulse is relatively little (typically in finishing operations), little change in

the mechanical properties of a material is expected in wire-cutting EDM due to these low

residual stresses, although material that hasn't been stressed relieved can distort in the

machining process.

Fig 2.WIRE EDM Fig 3.SINKER EDM

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1.3 DIELECTRIC MEDIUM

The dielectric medium plays an essential role in the EDM process. It not only works

as the insulation medium between the polarized electrodes to induce discharge, but also

influences the plasma channel expansion and material erosion during the discharge, and the

debris flushing and discharge gap reconditioning after the discharge. Therefore,

understanding and selecting the right dielectric medium with proper electrical, mechanical

and thermal properties is very important.

According to the type of dielectric medium used, there are several categories of

EDM processes, including wet EDM, powder mixed dielectric (PMD) EDM, dry EDM and

near-dry EDM. Conventional EDM uses liquid dielectric medium, such as hydrocarbon oil or

deionized water, and it is therefore called wet EDM. Even though it is a well established

process, some problems associated with wet EDM are electrolysis corrosion when using

water as the dielectric and toxic hydrocarbon disposal when kerosene based dielectric is used

PMD EDM can enhance the machining performance of wet EDM. It utilizes powder

mixed liquid dielectrics and has the advantage of achieving good machining stability and

finishing quality, especially in the finish operation with small discharge energy ,However, the

usage of powder increases the machining cost and the consequent toxic disposal causes more

environmental concern . For production practices, the powder suspended dielectric circulation

system is also challenged by separating the machined debris from the useful powders and

maintaining a constant powder concentration.

Dry EDM, which applies high flow rate gaseous dielectric fluid, tends to alleviate the

environmental problem resulted from the liquid and powder mixed dielectrics and also

enhance the machining performance. Using inert gas to drill small holes is the first dry EDM

attempt. Oxygen has been identified as an ideal dielectric medium for high material removal

rate (MRR) in dry EDM. By applying oxygen, a discharge duration lower than 5μs can

stimulate the “quasiexplosion” mode and accelerate the material removal significantly. In

addition to the high material removal capability, extremely low tool wear ratio is also

observed. The shortcomings of dry EDM include low quality of surface finish due to debris

reattachment, odor of burning and very low MRR when using non-oxygen gases

6

As an alternative method, the near-dry EDM uses liquid-gas mixture as the dielectric

medium. The liquid content in the mist media helps to solidify and flush away the molten

debris and hence the debris reattachment is alleviated in near-dry EDM. After the first

exploitation by Tanimura ,not much study has been conducted on this process until recently

by Kao in near-dry wire EDM. It is found that near-dry EDM has the advantage in finish

operation with low discharge energy considering its higher MRR than wet EDM and better

surface finish quality than dry EDM.

2.OBJECTIVE

Wire EDM cutting and EDM drilling are investigated under the wet, dry, and near dry

conditions.A mathematical model, assuming that the gap distance consists of the discharge

distance and material removal depth, was developed to quantitatively correlate the water–air

mixture‟s dielectric strength and viscosity to the gap distance

3.EXPERIMENTAL SETUP

3.1 MACHINE SETUP

The wire EDM experiment was conducted on a Brother HS-5100 wire EDM machine

using a 0.25mm diameter brass wire electrode. For all the experiments, the axial direction

wire feed speed was set at 12 mm/s, the tension force of wire was 18 N, the gap voltage ue

was 45 V, the open circuit voltage u0 was about 72 V, and the average pulse current ie was

about 25 A. Two EDM cutting conditions, wet and near dry were tested. Results of dry wire

EDM experiments conducted on the same machine have been obtained from previous studies

.

In wet EDM, the workpiece was submerged in deionized water with water jets applied

from the top and bottom of the workpiece at 1 l/min flow rate to flush away the debris. An

MQL fluid dispenser, model T60A-2 made by AMCOL, was used to deliver the water–air

mixture. The dielectric fluid pressure was set at 0.41MPa. The delivery of the dielectric fluid

and the feed of the wire electrode are illustrated in Fig.4

7

Fig. 4. The delivery of liquid–gas mixture in near dry wire EDM.

Fig 5.BROTHER HS-5100

EDM drilling tests were conducted on a Gromax MD20(shown in fig 6) micro-hole

EDM machine using a brass tubular electrode with 1mm outer diameter and 0.41mm inner

diameter. The rotational speed of the tubular electrode was 120 rpm. For stable drilling under

all EDM conditions, the gap voltage was set at 60 V, the pulse interval time was t0 = 70 ms,

and the discharge duration was te = 10 ms. The average pulse current varies for different

experiments.

8

Fig 6. GROMAX MD20

3.2 WIRE EDM CUTTING

MRR envelopes, which illustrate feasible EDM process regions, have been studied by

Miller . MRR envelopes of wet and dry EDM cutting of 1.27-mm-thick Al6061 are presented

as the baseline data for the comparison with two new envelopes of the near dry EDM. In each

envelope, t0 was varied to find the maximum achievable wire feed rate, which was then

converted to MRR. Four levels of te were selected: 4, 10, 14, and 18 ms, identical to those

used in The upper and lower boundaries of the MRR envelope correspond to the minimum

and maximum values of te (4 and 18 ms). The specific machine limits, maximum and

minimum t0 (1000 and 6 ms), as well as wire breakage and short-circuit limitations, form the

left and right envelope boundaries of the MRR envelope. The average pulse current ie is

about 25 A. To investigate the relationship between the gap distance and dielectric fluid

properties, the grooves machined at various water flow rates (0, 5, 8, 15, 21, 35, 50, 75

ml/min), as summarized in Table 1, were studied. The groove quality and groove width were

examined and measured using an optical microscope at 100x magnification. Three repeated

tests were conducted in each experimental setup.

TABLE 1.average gap distance in EDM cutting under wet,dry and near dry conditions

9

3.3 EDM DRILLING

Two sets of EDM drilling experiment were conducted. The first set was to evaluate the

drilling speed and hole quality, including the shape variation and debris deposition, of wet,

dry, and near dry EDM. The average pulse current was set at 10 A. The workpiece used was

1.27-mm-thick Al6061. For wet EDM, the flow rate of deionized water was 107 ml/min. For

dry EDM, the air jet pressure was set at 0.62MPa. For near dry EDM, the water flow rate and

the pressure of the carrying air jet were set at 21 ml/min and 0.62MPa, respectively. The hole

quality was inspected using an optical microscope at 100x magnification. The second set

investigates the effects of water flow rates on EDM drilling speeds with ie values at 10, 12,

and 15 A. Diameters of drilled holes at different water flow rates were also measured for the

investigation of the relationship between the gap distance and dielectric fluid properties. The

water flow rate was varied as 5, 8, 15, 21, and 35 ml/ min as shown in Table 2.

TABLE 2. average gap distance in EDM drilling under wet,dry and near dry conditions

10

4.RESULTS AND DISCUSSION

4.1NEAR DRY EDM MRR AND GAP DISTANCE

4.1.1 MRR ENVOLOPE BOUNDARIES

Fig. 7 shows three MRR envelopes which outline the feasible regions for the wet, dry, and

near dry wire EDM cutting of 1.27-mm-thick Al6061. The average of three repeated test

results is presented. The range of variation of three tests is within 10% of the nomina l value

and is consistent for all experimental conditions. For wet and dry wire EDM , the region of

feasible MRR is bounded by the wire breakage, short circuit, and machine limits of maximum

and minimum te (18 and 4 ms) and maximum t0 (1000 ms). The wet EDM has a significantly

higher MRR than that of the dry EDM (21.9mm3/min vs. 0.98mm3/ min). At low pulse

intervals of t0, frequent EDM pulses generate concentrated heat and lead to wire breakage.

The minimum value of t0 that can be reached at high level of te without wire breakage, is

greatly dependent on the dielectric fluid used. For wet EDM, due to the higher thermal

conductivity of the bulk water than that of the water–air mixture, t0 can be as low as 100 ms

at te = 18 ms. For the near dry EDM using water–air mixture at a water flow rate of 5.3

ml/min, the envelope boundary falls between the wet and dry EDM. The maximum MRR is

improved, from 0.98mm3/min in dry EDM, to 2.53mm3/ min. The near dry EDM has a

consistently higher MRR than that of dry EDM for all t0 and te. However, the wire breakage,

due to the lower capability of water–air mixture to relieve the concentrated heat from the wire

electrode, still limits the MRR in near dry EDM at low t0. Nevertheless, near dry EDM shows

two advantages. First, there is no short circuit limit at the lower boundary. Second, in the

region of very low-energy input (te = 4 ms and t0>150 ms), the MRR in near dry EDM is

higher than that of the wet EDM.

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Fig 7.comparision of boundaries of feasible MRR envolopes for wet,dry and near dry wire

EDM

Fig. 8. Comparison of MRR performances of wet and near dry wire EDM under varied t0 and

te and three regions based on near dry and wet EDM performance (ie = 25 A, ue = 45 V).

12

The close up view of MRR (below 4mm3/min) vs. t0 for the wet and near dry EDM is

shown in Fig. 8. Three regions, designated as I–III, are identified. In Region I (t0>650 ms),

the near dry EDM has higher MRR than that of wet EDM because the lower thermal

conductivity and heat capacity of the water–air mixture contribute to less heat dissipation

during discharge and a larger portion of discharge energy for material removal. At the very

low discharge energy setup, te = 4 ms, wet EDM fails to cut due to the short circuit, but near

dry EDM still works with fairly low MRR. The higher dielectric strength of the water

medium generates a narrow gap distance and causes a frequent short circuit in wet EDM. In

Region II (250 <t0<650 ms), the MRR of near dry and wet EDM is roughly the same. At the

highest te ( = 18 ms), the MRR of wet EDM starts to exceed that of near dry EDM. Under

higher energy input, the higher viscosity of the water dielectric fluid in wet EDM generates

larger explosion force, which contributes to the high MRR .

In Region III (t0<250 ms), a significant MRR difference exists between wet and near

dry EDM. The MRR drops in near dry EDM and, wire breakage occurs as t0 is further

reduced. The dielectric fluid viscosity is critical to the MRR in Region III.

4.1.2 EFFECT OF FLOW RATE ON MRR

The MRR in near dry EDM under 5.3 and 75 ml/min water flow rates is shown in Fig.

9. In near dry EDM, high water flow rates increases the MRR because of improved cooling,

more efficient debris flushing, and higher dielectric fluid viscosity due to the higher

concentration of water. It improves the MRR at low t0 (below 500 ms) for all values of te, and

is particularly beneficial when te is high ( = 18 ms). The peak MRR rises to 3.9mm3/min at

75 ml/ min flow rate. A much higher flow rate is required to increase the MRR because the

nozzle is set near the discharge gap and thus not all water droplets are successfully delivered

into the gap.

13

Fig. 9. MRR envelopes of near dry wire EDM cutting at two deionized water flow rates (5.3

and 75 ml/min, ie = 25 A, ue = 45 V).

4.1.3 GAP DISTANCE AND DEBRIS DEPOSITION

The groove width in wire EDM is used to estimate the gap distance. The average gap

distance under the wet, dry, and near dry wire EDM and the associated dielectric strength and

viscosity of the dielectric fluid are listed in Table 2. The gap distance of wet EDM is wider

than that of near dry EDM. This is likely caused by the lower viscosity of the water–air

mixture. Similarly, in near dry EDM,

higher water flow rate generates larger gap distance. This gap distance data will be analyzed

in section 6 for the modeling of gap distance based on dielectric fluid properties in near dry

EDM.

No debris deposition is observed for near dry EDM. This occurs because water–air

mixture has a better flushing capability than the air jet in dry EDM

4.2 NEAR DRY EDM DRILLING

4.2.1 WET DRY AND NEAR DRY EDM DRILLING

Optical micrographs of top and cross-sectional side views of EDM drilled holes and

the drilling time under the wet, dry, and near dry conditions are shown in Fig. 10. The dry

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EDM takes 428 s to drill a hole through the 1.27-mmthick Al6061. This is very long

compared to the 11 and 13 s

drilling time for the wet and near dry EDM, respectively. The dry EDM also has a severe

debris deposition problem, which subsequently creates a tapered hole. The taper in wet EDM

also exists but is not as significant as in dry EDM. The smallest taper exists in holes drilled

by near dry EDM, which generates a straight hole with sharp edges.

The electrode wear in near dry EDM is 3.7 mg per hole, which is larger than the 2.7

mg per hole in wet EDM. The higher thermal load on the e lectrode in near dry EDM likely

causes the higher electrode wear. The same phenomenon also exists in near dry wire EDM.

As shown in Fig. 9, at low t0 the wire breakage due to electrode wear limits the MRR in near

dry wire EDM.

Fig. 10. Optical micrographs on holes drilled on 1.27mm Al6061: (a) wet, (b) dry, and (c)

near dry EDM conditions (ie = 10 A, te = 10 ms, t0 = 70 ms, ue = 60 V).

4.2.2 EFFECT OF FLOW RATE AND PULSE CURRENT ON GAP DISTANCE

Effects of water flow rate and pulse current ie on the MRR in near dry EDM drilling are

shown in Fig. 11. The efficiency of near dry EDM drilling improves with a higher water flow

15

rate under all three levels of ie. The MRR is low at ie = 10A due to the low-energy input. The

highest energy input (ie = 15 A), however, does not generate the highest MRR as expected.

This is caused by the debris flushing problem at high-energy input. The medium level of ie (

= 12 A) has the highest MRR by balancing the debris flushing and power input. The

measured average gap distance is calculated using the difference between the average hole

diameter and electrode diameter. Table 2 lists the average gap distance in wet, dry, and near

dry EDM at five water flow rates. Following the same trend observed in Table 1 for the wire

EDM, higher water flow rate corresponds to larger gap distance. A model is developed in the

following section to investigate the effect of dielectric strength and dynamic viscosity on the

gap distance.

Fig. 11. The effect of deionized water flow rate and discharge current on the MRR of EDM

drilling (te = 10 ms, t0 = 70 ms, ue = 60 V).

4.3 MODELLING OF NEAR DRY EDM GAP DISTANCE

4.3.1 MATHEMATICAL MODELLING

A mathematical model uses mathematical language to describe a system.The process

of developing a mathematical model is termed 'mathematical modelling' (also

modeling).Eykhoff (1974) defined a mathematical model as 'a representation of the essential

16

aspects of an existing system (or a system to be constructed) which presents knowledge of

that system in usable form

Often when engineers analyze a system to be controlled or optimized, they use a

mathematical model. In analysis, engineers can build a descriptive model of the system as a

hypothesis of how the system could work, or try to estimate how an unforeseeable event

could affect the system. Similarly, in control of a system, engineers can try out different

control approaches in simulations.

A mathematical model usually describes a system by a set of variables and a set of

equations that establish relationships between the variables. The values of the variables can

be practically anything; real or integer numbers, boolean values or strings, for example. The

variables represent some properties of the system, for example, measured system outputs

often in the form of signals, timing data, counters, and event occurrence (yes/no). The actual

model is the set of functions that describe the relations between the different variable

4.3.2 DEVOLOPING A MATHEMATICAL MODEL

A mathematical model is developed to predict and understand the effect of an air–

water mixture on the gap distance in near dry EDM. The model is based on four

assumptions:

1. The gap distance, d, is assumed to be affected primarily by the dynamic viscosity and

dielectric strength of the dielectric fluid.The effect of dynamic viscosity and dielectric

strength on d can be decoupled into d1 and d2, which are the gap distance contributed

by discharge distance and material removal depth, respectively.

2. The critical distance at which the applied gap voltage will cause the breakdown in the

dielectric fluid is d1. The discharge distance d1 is assumed to be inversely proportional

to the dielectric strength, Sm, in the unit of MV/m, i.e. d1=α/Sm, where α is a constant.

3. The dynamic viscosity affects the magnitude of explosive force and material removal

depth in the dielectric fluid. The distance d2 is proportional to the dynamic viscosity

ƞm of the water–air mixture, i.e., d2=βƞm, where b is a constant.

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4. Values of ƞm and Sm are not readily available but they are expected to be between

those of air and deionized water and dependent upon the concentration of water in the

mixture dielectric fluid. The viscosity and dielectric strength of air, ƞa and Sa, and

water, ƞw and Sw, are available.. Values of ƞm and Sm can be predicted using the

following formula:

where s is the volumetric ratio of water in the air–water dielectric fluid and m and n are the

exponent coefficients to be solved by curve fitting the experimental data. In Eqs. (1) and (2),

the water–air mixture properties are bounded between those of the air and water at s = 0

(100% air) and 1 (100% water), respectively. The validity of these assumptions is evaluated

in EDM experiments.

Values of s can be calculated based on the flow rate of water and air. Details of the

calculation of s are summarized in the Appendix.

The gap distance d can be expressed as

Substituting Zm and Sm from Eqs. (1) and (2), the d can

be expressed as

In Eq. (4), m, n, a, and b are four unknown variables, which can be determined by curve

fitting the experimental data. Values of viscosity and dielectric strength of air (ƞa and Sa) and

deionzied water (ƞw and Sw) are known. Experimental measured values of d under different

water flow rates in wire and drilling EDM are summarized in Tables 1 and 2.

4.3.3 PARAMETERS FOR EDM GAP DISTANCE MODEL

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The curve fitting toolbox in Matlab is utilized to calculate a, b, m, and n using the

experimental data of d and s for both the wire and drilling EDM. Reasonable approximation

in curve fitting is achieved, as illustrated in Fig. 12 for wire EDM and in Fig. 13 for EDM

drilling. For the wire EDM, a = 4.5*10^-6 V, b = 4.6*10^-5 m^2s/g, m = 0.90, and n = 5.0.

For the EDM drilling, a = 0.10*10^-6 V, b = 7.7*10^-5 m^2s/g, m = 0.82, and n = 4.7.

Fig 12. Experimental measured and model predicted gap distance d vs. volumetric ratio of water for wire EDM cutting

Fig 13. Experimental measured and model predicted gap distance d vs. volumetric ratio of water for EDM drilling.

4.3.4 MODEL VALIDATION

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Since the viscosity and dielectric strength are the intrinsic properties of the dielectric fluid,

they should be independent of the EDM setups. The values m and n are used in Eqs. (1) and

(2) to estimate the viscosity and dielectric strength of the air–water mixture for the wire and

drilling EDM. Hence, it is expected theoretically that m and n should be the same for wire

and drilling EDM. The closely matched modeling results of m = 0.90 and 0.82 and n = 5.0

and 4.7 for the wire and drilling EDM, respectively, indicates the validity of the model. Fig. 9

plots the estimated viscosity and dielectric strength of the air–water mixture at different level

of water volumetric ratio. Good match has been observed for the estimated properties yielded

from the two independently developed models, wire EDM model and EDM drilling model. It

indicates that the model estimated liquid–gas mixture property is independent of the EDM

setup, either wire EDM or EDM drilling, but only dependant on the liquid volumetric ratio.

Thus, the theoretical model is indirectly validated.

Fig 14. Estimated values of dielectric strength Sm and dynamic viscosity vs. volumetric ratio of water calculated from the wire EDM and EDM drilling

5. CONCLUSION

In this study, MRR envelopes of near dry wire EDM at two flow rates were compared

with those of wet and dry EDM. Near dry EDM improved the MRR and eliminated the

problem of debris deposition. It was observed that floating metallic debris was effectively

collected and burning fumes were greatly suppressed with the application of water–air

20

mixture. The same benefits were also observed in near dry EDM drilling, which achieved

better hole consistency with almost no taper.

A mathematical model predicting the gap distance based on the dielectric strength and

dynamic viscosity of the water–air mixture was proposed and validated for both wire and

drilling EDM. The semi-empirical model provided a quantitative prediction of the gap

distance and gained better insight into wet, dry, and near dry EDM

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6. REFERENCES

1.Near dry electrical discharge machining.C.C. Kao, Jia Tao, Albert J. Shih, International

Journal of Machine Tools & Manufacture 47 (2007) 2273–2281

2.C.-C. Kao, J. Tao, S. Lee, A.J. Shih, Dry wire electrical discharge machining of thin

workpiece, Transactions of NAMRC 34 (2006) 253–260

3.M. Kunieda, C. Furudate, High precision finish cutting by dry WEDM, Annals of the CIRP

50/1 (2001) 121–124.

4.M. Kunieda, M. Yoshida, Electrical discharge machining in gas,

Annals of the CIRP 46/1 (1997) 143–146.