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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 3, August 2014 125
Microwave Drilling with Litz Wire using a
Domestic ApplicatorNitin K. Lautre, Apurbba K. Sharma, Pradeep Kumar and Shantanu Das
Abstract ---The work presented in the paper shows a way
to drill a blind hole on perspex material at 2.45 GHz
frequency of microwaves using a domestic applicator of 700
W maximum powers. The approach was based on the
approach was based on the application of a Litz wire coax as
a monopole, to act like a drill bit with a heat source. The
methodology involves no rotary but, single linear movement of
tool inside a microwave applicator for drilling. A computer
simulation on COMSOL for about 50 minute each run, was
also used to show the heat distribution inside microwave
applicator. The result of computer simulation helped to locate
the optimized position of Litz wire inside the applicator for
optimum heat generation. The experimental results show the
feasibility of microwave drilling and limitations on perspex
specimen in the used applicator. The work presented involves
a new approach in the area of microwave drilling of materials
on a customized domestic microwave setup.
Keywords--- Applicator, COMSOL, Drilling, Litz,
Microwave, Perspex
I. INTRODUCTION
ICROWAVE energy and drilling process are two
different popular areas in the field of engineering.
Microwave concepts are relatively newer to that of drilling.Many of the researchers have tried to overcome the limitation
of traditional drilling process with the newer emerging
concepts. Microwave drilling is one such approach presented
in the year 2000 [1]. The importances of some of applications
in the area of microwave material processing are rising interms of their popularity and consequently researched [2]. The
active research areas include microwave assisted sintering,material interaction, modeling, equipment and its design,
measurements, etc. Most of the research works in microwave
assisted material processing however are limited to processing
of ceramics followed by composites.
Nitin K. Lautre, Research Scholar, Department of Mechanical and
Industrial Engineering, Indian Institute of Technology, Roorkee, India. E-
mail:[email protected]
Dr. Apurbba K. Sharma, Associate Professor, Department of Mechanical
and Industrial Engineering, Indian Institute of Technology, Roorkee, India. E-
mail:[email protected]
Dr. Pradeep Kumar, Professor, Department of Mechanical and Industrial
Engineering, Indian Institute of Technology, Roorkee, India. E-
mail:[email protected]
Shantanu Das, Scientist - H, Reactor and Control Division, Bhabha
Atomic Research Center, Mumbai, India. E-
mail:[email protected]
DOI: 10.9756/BIJIEMS.6053
Majority of the work reported was confined to
metallurgical processing of material. However, microwaveassisted mechanical processes received very less attention.
Some of the developments in the areas of application of
microwaves for various processes like sintering, cladding,
melting, coating, glazing, joining, cladding and co-axial cableassisted drilling, etc. are reported [3] [8]. The area of
microwave drilling on various materials like concrete, glass,
ceramics, basalts, silicon, and bones are well envisaged and
explored [9] [12]. The application of bone drilling in
biomedical engineering, electronics industry and aeronauticalengineering involves the majority of recent drilling
applications for small size holes [13] [18].
Researchers have tried to drill hole in perspex of 2.5 mmthick through a 1064 nm diode laser. The power of 1 W
without decrease in pressure and 2.45 W at a low pressure of
60 mm Hg of air was used to drill a hole [19]. A carefulcontrol of various parameters like the preparation of precise
nozzle diameter, its focal point, lens effect, speed of cut,
cutting power, standoff distance, hole circularity, spatter
thickness, material removal rate and assists gas pressure
adjustment add complexity. These controls hinder the drilling
process for attaining a high aspect ratio holes with low taperratio [20] [22]. An attempt is made to use different
wavelength the degrade perspex to cause burning, cracking,
boiling in and around the imposed laser part and nearby. Thewavelength varied from near infrared (1064 nm) to visible
(532 nm) to ultraviolet ( 266 nm) wavelength. At 266 nm the
photo chemical dissociation by covalent bond breaking
(photo-ablation) was more dominating than thermal ablation in
perspex [23] [24]. A high aspect ratio of 300- 600 for holeranging from 10 100 micron is reported [25] [26]. For
blind holes a surface modification is prepared by exposing
perspex to laser beam. The thermochemical damages due tocarbonization of perspex into small spots are reported [27]
[28]. Conventionally the drilling of perspex establishes the
significant parameter to be controlled for effective drilled hole
as the speed, feed rate, thrust force, tool wear, delamination
[29] [30] and hole shrinkage. A high speed steel drill bit is
related significantly with power and acoustic emission. Theacoustic emission is unavoidable and is due to assembly of
mechanically vibrating and rotating system [31]]. Microwave
drilling makes an attempt to minimize the problem observed inconventional and non conventional (laser) drilling process.
It has been found that very few researchers are active in
the domain of microwave drilling at present. The behavior of
microwave drilling in a closed applicator is infact, highlycomplex as it is governed by various phenomena. Concepts
like dielectric behavior of material in high temperature, hot
spot formation, thermal runaway as well as safety
M
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ISSN 2277
Science, Vol. 4, o. 3, August 201 126
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 3, August 2014 127
Figure 4: EDS Spectrum of PMMA Specimen
The drill bit chosen for the experimentation was a coax
cable with twisted strands of copper wire commonly known asLitz wire as shown in figure 5(a). These wires are commonly
used for carrying AC (Alternating Current) at low frequency
upto 1 MHz. It has a group of seven copper wire strands eachof diameter 0.6 mm. After winding of these individual
wires, the diameter of the drill bit tool was maintained at 2.1
mm.
The diameter of individual wire was so selected to reducethe skin effect as its effectiveness increases if wire diameter is
less than the twice the skin depth. For copper at 2.45 GHz, the
skin depth is 1.35 m. A copper wire of 2.7 m will be very
thin and less stiff to act as a drill bit. The Litz wire used was
of Suhner make, RG 213/U, 50 ohm as shown in figure 5(b).
The bunch of wounded wire of 2.1 mm was surrounded by adielectric material shield of 7.3 mm. This diameter ( 7.3
mm) was less than the top hole drilled 12 mm, which allows
the monopole of group of wire to enter along with dielectriccover inside the applicator. The dielectric cover was further
surrounded by criss cross woven thin copper straps as shown
in figure 5(a), which are finally covered by a protective plastic
cover.
Figure 5: (a) Coax litz wire (b) Specification on Coax Cable
The criss cross or basket weaved copper straps donot keep
two straps or wire parallel to each other causing cancellation
of wire magnetic field between successive twists. Also, at
higher frequencies more and more signals are transferred in
the outermost layer of the coax becomes effective. The finalouter diameter after the plastic cover for the coax was 10.26
mm. Since the arrangement was of monopole entry the copper
straps are spread to increase the overall diameter beyond 12mm to restrict the entry of copper strap and outer protective
cover inside the applicator.
The cross section of the Litz wire has typically arranged
wire relative to each other as shown in figure 6. The wire iswound in such a way that they form a hexagonal honeycomb
shape grid with central wire inside the grid. This arrangement
has the capability to induce eddy current on the nearby wire,
and hence improves the current flow over the Litz wire. Since
the wire cross sections is circular and twisted over each otherand are equally spaced wire, it leads to six void patches as
shown in figure 6. These voids are acting as insulator and are
unavoidable.
Figure 6: Cross section of Litz wire
III. COMSOLSIMULATION
The experimentation for microwave irradiation requires an
idea of the temperature distribution inside the applicator
without the presence of stirrer and rotary plate. A simulationtool (COMSOL of 4.3 version 0.151) was used to simulate the
temperature profile for effectively positioning the workpiece
and drill bit in the applicator. The geometrical model with
exact size of the microwave cavity is shown in figure 7. Thedimensions of the applicator used are 285 X 277 X 182 (L X B
X H) mm. The grids of predrilled hole on either side of
vertical face was ignored in simulation, as theses holes are to
release the gases trapped in the cavity through forcedconvection.
2.1 mm
0.6 mm
(a)(b)
Wire bunch,
2.1 mm
Copper straps
Dielectric
cover,7.3 mm
Outer plastic,
10.26 mm
cover
void
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 3, August 2014 128
Figure 7: Geometrical Model of the Experimental Setup
A frequency transient study was performed at power ratingof 700 W with power application cycle time of less than 3
minute. In order to estimate the variation of heat in theapplicator, a series of simulation runs were performed for
microwave heating. Each run inside the microwave oven
module was iterating and converging for a cycle time of about50 minute. The other parameters selected during simulation
were model definition of air, no heat transfer, port boundary,
meet boundaries to respective domains.
The variation of electric field and resistive losses as
obtained through simulation are shown in figure 8(a). It is
clear from the figure that throughout the irradiation trial, the
tool and the workpiece will be within the safe limit and theheat variations are cyclic. It was also seen that the higher
heating envelope was away from the center of the applicator.
As pointed out, the temperature distributions are more uniformaway from the center axis of the cavity [32]. The overheatingof the coax was thus not possible for low exposure time (30 s).
The resistive heating in W/m3was more around the brick asshown in figure 8(b). The smaller size of specimen also helps
the working atmosphere to be maintained at lower initial
temperature variation without interfering the resistive heating
area.
Figure 8: Variation in (a) Electric Field and (b) Resistive
Losses in Microwave Applicator
In order to find out the optimum coordinates for effectivemicrowave heating inside the applicator, Table 1 has been
constructed from the simulation results. The temperature
variation of three times the melting point of PMMA was
assumed from the center to around the brick. It is concluded
from the data that for the application of less than 480 OC (~
160 * 3 times) temperature near the drilling zone, the ideal
location for the drill bit will be about the center, as it will notinterfere the high electric field generated around the brick
area.
Since the top face drilled hole may not permit any x-ychange; the data of z-axis along the drill bit height was
effective. The z axis data help to allow a particular length ofcoax inside the applicator for optimized heat concentration for
drilling. The optimum entry distance for the drill bit was H -142 mm = 40 mm as shown in figure 9. The condition of Litz
wire become sensitive, if the entry distance of about 40 mm
was not maintained. A burnout was observed due to sudden
rise in temperature at coax in 2 minute, as shown in figure 10.The burnout had not only burnt the start portion of drill bit but,
also melted the dielectric cover over it. This clears the
temperature range generated at drill bit end in the microwave
applicator. A need of research for good and tolerable dielectric
cover to withstand such elevated temperature is considerableto prevent the dielectric cover burn outs.
Table 1: Variation of Temperature with Coordinated ofApplicator
x y z Temp (OC)
0.1079 0.1472 0.14146 438.1342
0.09441 0.09193 0.14214 643.2254
0.25801 0.1199 0.14247 741.0294
0.255 0.11098 0.14276 827.4488
0.06919 0.053 0.14277 832.0689
0.08321 0.13952 0.14283 850.6143
0.08078 0.06883 0.14285 856.7601
Figure 9: Optimized Entry of the Drill Bit from Top
Figure 10: Burnt Litz Wire
B
coax
Magnetron
PMMA
L
H
Brick
Resistive
heatinElectric
field
variation
(a) (b)
40 mm
H
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BoBo fring Internation l Journal of Industrial Engineering
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ISSN 2277-
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waveguider to starting
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700 W maxim
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Science, Vol. 4, o. 3, August 201 129
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 4, No. 3, August 2014 131
ISSN 2277-5056 | 2014 Bonfring
[30] A. Faraz and D. Biermann, "Subsequent Drilling on Pilot Holes inWoven Carbon Fibre Reinforced Plastic Epoxy Laminates: The Effect of
Drill Chisel Edge on Delamination", Proceedings of the Institution of
Mechanical Engineers, Part B: Journal of Engineering Manufacture, pp.
1493 1504, 2011.
[31] S. Arul, L. Vijayaraghavan and S. K. Malhotra, "Online Monitoring ofAcoustic Emission for Quality Control in Drilling of Polymeric
Composites", Journal of Materials Processing Technology, vol. 185, pp.
184 190, 2007.
[32] K. Pitchai, S. L. Birla, J. Subbiah and D. D. Jones, HeatingPerformance Assesment of Domestic Microwave Ovens, International
Microwave Power Institutes 44th Annual Symposium, pp. 1-7, 2010.Nitin K. Lautre is working as a Research Scholar in
the department of Mechanical and Industrial
Engineering, Indian Institute of Technology, Roorkee,India. He obtained his graduate and post-graduate
degree in Mechanical and Industrial engineering
respectively from Nagpur University. He has an
industrial experience of about 01 year seven month inone of the Bharat Ratna defence company. He has a
teaching experience of about 12 years and handled
various student projects. He is member of various societies and presented
paper in Japan and USA. He has in his credit about 30 conference and six
journal papers. His research interest is Non-conventional machining andhybrid machining processes.
Dr. Apurbba K. Sharma, currently working as
Associate Professor in the Department of Mechanicaland Industrial Engineering at the Indian Institute of
Technology Roorkee, India. He obtained his Bachelor'sDegree in Mechanical Engineering from Dibrugarh
University and Master's and Doctoral Degrees from
Indian Institute of Technology Madras, India. He has
guided five Ph.D students and currently supervising sixdoctoral theses. He has published more than 55 articles
in journals and 67 articles in international conferences. He is a reviewer to
various international journals. He holds one Indian Patent to his credit and has
filed four more recently. Currently, he is Principal Investigator for three
externally funded projects.
Dr. Pradeep Kumar is working as a Professor in the
Department of Mechanical and Industrial Engineering
at Indian Institute of Technology Roorkee, (formerly
University of Roorkee), India. He earned his BS inIndustrial Engineering in 1982, his MS in Production
Engineering in 1989 and his PhD in 1994, all from the
University of Roorkee, Roorkee, India. He has also
served DRF Industries and the Steel Authority of India
Ltd as a Production Engineer for four years. Hisresearch interests include quality engineering, reliability analysis, product and
process design, optimization, non-conventional machining processes, and
metal casting. He has published 55 papers in national and international
journals and conference proceedings. He has supervised 25 Ph.D theses, 82M.Tech dissertations and several Graduate theses. He was honored as
outstanding teacher in the year 2005 and 2009. He completed 9 sponsored
projects and currently working on 8 research projects. He also holds various
administrative post and membership of various esteemed societies.
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