Int. J. Manufacturing Research, Vol. 8, No. 2, 2013
121
Copyright ? 2013 Inderscience Enterprises Ltd.
Study of sudden tool failure in micro electro discharge milling
Murali M. Sundaram*
School of Dynamics Systems, University of Cincinnati, Cincinnati, OH, 45221, USA E-mail: murali.sundaram@https://www.doczj.com/doc/f914339619.html, *Corresponding author
Yu Liu
College of Mechanical Engineering, Dalian Jiaotong University, Dalian 116028, China
K.P. Rajurkar
University of Nebraska-Lincoln, Lincoln, NE, 68503, USA
Fuling Zhao
Key Lab. for Precision and
Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian, 116024, China
Abstract: Micro electro discharge milling (MED milling) is a spark erosion method capable of micromachining prismatic and complex shapes using simple non-profiled tool. Tool wear poses major hurdle in the development of MED milling. This paper discusses the types of tool wear in MED milling and analyses factors causing exfoliation and sudden failure of micro tool. Process simulation of MED milling suggests using less viscous dielectric fluid, lower tool rotational speed and higher discharge gap to reduce the chances of unpredictable tool failure in MED milling.
[Received 30 August 2011; Revised 6 February 2012; Accepted 29 March 2012] Keywords: micro EDM; micro electro discharge milling; MED milling; tool wear.
Reference to this paper should be made as follows: Sundaram, M.M., Liu, Y., Rajurkar, K.P. and Zhao, F. (2013) ‘Study of sudden tool failure in micro electro discharge milling’, Int. J. Manufacturing Research , Vol. 8, No. 2, pp.121–134.
122 M.M. Sundaram et al.
Biographical notes: Murali M. Sundaram is an Assistant Professor of
Mechanical Engineering in the School of Dynamic Systems and the Director of
Micro and Nano Manufacturing Laboratory at University of Cincinnati. He
received his BS, MS and PhD in Mechanical Engineering. He has over 15 years
of professional experience in manufacturing industry and academia. His current
research interests are in the areas of nano-manufacturing, non-traditional
machining, micromachining, hybrid machining, CAD/CAM, metrology and
process simulation.
Yu Liu is a Lecturer in the College of Mechanical Engineering in Dalian
Jiaotong University, China. He received his Doctor’s degree in Mechanical
Engineering and graduated from Dalian University of Technology in 2011. His
research field is involved in micro and non-traditional machining.
K.P. Rajurkar is a Distinguished Professor of Engineering in the Department of
Mechanical and Materials Engineering at the University of Nebraska-Lincoln.
He is the Founder and Director of the Center for Non-traditional Manufacturing
Research (established in 1989) at University of Nebraska-Lincoln. He has been
involved in non-traditional machining research for the last 30 years. His
research interests include traditional and non-traditional manufacturing
processes at macro, micro and nano scales.
Fuling Zhao is a Professor in the School of Mechanical Engineering in Dalian
University of Technology, China. He has over 20 years of professional
experience in non-traditional machining. His research areas include
non-traditional machining, micro machining and computer-aided measurement
and control.
1 Introduction
Electrical discharge machining (EDM) is a non-traditional method suitable for the machining of any conductive material, including ceramics, irrespective of its hardness (Bonny et al., 2009; Dev et al., 2009; Singh et al., 2007). Because of the ability of micro electrical discharge machining (micro EDM) to generate high precision three dimensional (3D) features (Liu et al., 2008) with low cost, it is widely used in the field of micro scale manufacturing (Masuzawa, 2000; Lambie et al., 2008). However, for 3D micro EDM die-sinking process, the fabrication of complex 3D micro tool electrode is difficult, and the parts machined by die-sinking usually cannot meet the machining requirements due to the shape change of tool electrode caused by the excessive tool wear (Zhao et al., 2004).
Micro electro discharge milling (MED milling) is a spark erosion method of micromachining of prismatic and complex shapes using simple shaped non-profiled tool. MED milling uses simple electrodes and CNC machining paths (Rajurkar and Yu, 2000), to machine complex 3D micro features (Sundaram and Rajurkar, 2008), thus greatly saves the machining time and cost of complex electrode machining (Bleys et al., 2002). It is well known that the tool wear poses serious challenge in electro discharge micro machining(Bigot et al., 2004)and its hybridisations (Pham et al., 2007), and attempts such as Uniform wear method have been reported elsewhere to reduce the adverse effect of tool wear on generated part accuracy, when the depth of cut is of the order of few micro meters (Pham et al., 2004). However, for economical micro machining, it is essential to perform micro machining with substantially higher depth of cut (in the order
Study of sudden tool failure in micro electro discharge milling
123
of hundreds of micrometers). Tool wear under such high depth of cut in MED milling has not yet been reported adequately. This paper discusses the tool wear in MED milling with high depth of cut (200 μm) and analyses factors causing tool wear and unpredictable tool failure based on process simulation and experimental results.
2 Motivation
Micro EDM using uniform wear method and uniform tool shape methods have demonstrated their potential for machining complex features by layer-by-layer machining in X-Y plane using face of the micro tool (Yu et al., 1998). However, the tedious and time-consuming tool path planning process reduces the machining efficiency and a large number of layers (with typically 1 to 2 μm layer thickness) increase the complexity of part programming (Wang et al., 2009). In order to reduce the number of layers in X-Y plan, especially in the machining of less complex features such as slots and pockets, a simpler method of micro machining using the side wall rather than face of the tool has been attempted as shown schematically in Figure 1. Depth of cut as high as 200 μm significantly reduces the part programming effort and increases the machining efficiency.
Figure 1
MED milling using side wall of tool (see online version for colours)
During the MED milling using tool side wall, certain unique tool failure was observed. Following section reviews types of micro EDM tool wear and subsequently unique tool failure observed in MED milling are discussed.
3 Review of tool wear in micro EDM
In general the micro EDM tool electrode is subjected to three types of wear namely the side wear, corner wear and end wear as shown in Figure 2.
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Figure 2
Types of tool wear in micro EDM (see online version for colours)
Source: Sundaram and Rajurkar (2008)
The side wear is caused by the discharges between the tool and the escaping debris or side of the workpiece (Kumagai et al., 2006). Providing an insulating sheath is one attempt to overcome this problem in micro drilling (Zhang et al., 2009). The corner wear is mainly caused by the excessive electric field intensity in the corner of the tool electrode. When voltage is applied between two electrodes the electric field will be more intense in the edges due to the finite dimensions of the electrodes. The distribution of electric field intensity caused by non-uniform fields near the edges (called fringing fields) is illustrated in Figure 3.
Figure 3 Illustration of excessive electric field intensity at the edge of the tool electrode
(see online version for colours)
Source: Sundaram and Rajurkar (2007)
The higher field density causes excessive tool wear in the corner of the electrode and the electrode is eventually expected to obtain an equipotential surface, the profile of which depends on the applied potential and actual machining conditions. The problem of corner wear to some extent can be overcome by careful planning of tool path in such a way that the shape of the electrode is retained or by using thin walled hollow electrodes (Kumagai et al., 2004). The end wear is the most prominent tool wear and occurs due to the spark erosion between tool face and the workpiece.
In MED milling in addition to the above mentioned tool wear types, tool wear due to exfoliation and unpredictable mechanical failure were also noticed. Compared with the
Study of sudden tool failure in micro electro discharge milling 125
conventional tool wear mentioned before, the exfoliation and sudden tool failure is a
special type of tool wear. Their characteristics are:
1 the primary material removal effect is not the electrothermal effect but the effect of
certain mechanical force
2 the instantaneous amount of material removal is very large
3 the material removal process is without phase transition.
These are discussed in the next section.
4 Tool wears in MED milling
4.1 Experiment work
Panasonic micro EDM (MG-ED72W) machine was used to conduct the experiments.
Using various machining parameters, a series of experiments was carried out to machine
micro slots on an aluminium plate by MED milling. The experimental conditions used in
the micro slot machining are listed in Table 1. The machined slots and the micro tools
were observed with Nikon optical measuring microscope (MM-40) and the results are
shown in Figures 4 to 6.
conditions
Table 1 Experimental
Workpiece material Aluminium
Tool material Tungsten
Dielectric Commonwealth
185 Voltage 70(V), 110 (V)
Capacitance 1,000 (pF), 3,300 (pF)
Polarity Cathode tool, Anode workpiece
Tool rotation 0 RPM, 3,000 RPM
Tool diameter 300 (μm)
Depth of cut 50 (μm), 200 (μm)
Length of cut 30 (mm)
During the experimental work to study the influences of different MED milling
parameters on the shapes of micro tool tip after machining, an unexpected tool failure
phenomenon as shown in Figure 4 was observed when one of the experiments was
carried out with the following experimental parameters: the discharge voltage as 110 V,
the capacitance as 3,300 pF, the tool rotation speed as 3000RPM, and the depth of cut as
200 μm. Substantial material loss on tool tip can be seen in Figure 4. This kind of tool
failure is very rare in micro EDM, so to verify the repeatability of such a tool failure,
another six micro slots were machined on aluminium with the same experimental
parameters. However, the tool failure shown in Figure 4 did not repeat again. This
indicates that the occurrence of sudden tool failure is a random event.
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Figure 4
Tool failure after MED milling (see online version for colours)
Large pieces of material left over machined surface as seen in Figure 5 is uncommon in typical micro EDM. The large lump of material is estimated at about 148.5 × 104 μm 3 which is much higher than the conventional micro EDM tool wear in the range of
1 × 104 – 10 × 104 μm 3
. Moreover, the sharp edges of the tool tip suggest tool failure due to fracture rather than the typical EDM mechanism of melting and vaporisation, thus suggesting the involvement of certain kind of force in the tool failure. Figure 5 Slot surface after med milling
(see online version for colours)
Figure 6
Surface exfoliation of tungsten tool after MED milling
Study of sudden tool failure in micro electro discharge milling 127 Additionally, a higher magnification SEM image shown in Figure 6 reveals discontinuous surface with peeling off in top surface (surface exfoliation) of tungsten tool in MED milling. From Figure 6, the thermal cracks under the recast layer separate the top surface and sub-surface. The possible reason of surface exfoliation is the interlacing and expansion of pre-existing cracks on the micro tool caused during the formation of micro rods by cold rolling, and the disturbance caused by varying fluid pressure. To understand these phenomena in MED milling, experimental analysis and simulation studies were conducted as described in the following section.
4.2 Impact of material defects of tungsten tool
Figure 7 shows the SEM picture of tungsten tool surface morphology after MED milling. In Figure 7, the entire electrode, both the side and the end is covered with cracks, and the cracks sprawl along the length of the electrode. It looks like that the tool is loosely composed of many thin strips of material. In fact, the production processes of tungsten tools cause the structural defects of tools before they are used in MED milling. Figure 8 shows the SEM picture of strip type surface on purchased cold drawn tungsten electrodes. As the tools used in micro EDM are both thin and long, they are very difficult to produce with conventional methods. At present, the widely used method is cold drawing to make the thin and long tungsten tools. Under the cold drawing force, the micro grains of tungsten material are stretched. While the grains extend significantly along the longitudinal direction, the forces between them and others grains around become weak gradually, and then the gaps are formed in the grain boundaries, in macro scale, the longitudinal crack are shaped. Meanwhile, the cold drawing force can either make the grains broken or cause the formation of gaps between the vertical grains, in macro scale, the transverse cracks are generated. The nature of these cracks is that they are not only present on the tool surface, but also deep into the internal electrode impacting on a large range of electrode material. Because of this structural defect, even a small disturbance of force has the potential to cause large amount of material removal during the machining process. Therefore, the material defects of tungsten tool formed in the cold drawing process is the root cause that results in the sudden failure of tungsten tool in MED milling.
SEM picture of tool surface showing surface cracks (see online version for colours)
Figure 7
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Figure 8
Strip type surface on cold drawn tungsten electrode
4.3 MED milling process simulation
Unlike micro EDM drilling, the gap between a given point in the tool and workpiece in MED milling continuously changes from minimum to maximum to minimum as shown in Figure 9 due to tool rotation. This side gap change will cause pressure change on any particular point on the side wall of the tool. Based on the theory of incompressible Navier-Stokes flow condition, a simulation of the pressure change due to dielectric fluid flow in the interelectrode gap caused by tool rotation in MED milling is studied using COMSOL multiphysics software.
Figure 9
Physical model of med milling process
4.3.1 Physical description
The physical model of MED milling is shown in Figure 9. Machined slot is filled with dielectric fluid medium. The tool electrode is rotating in the dielectric with varying interelectrode gap. The following assumptions are made for simplification of the MED milling process simulation:
Study of sudden tool failure in micro electro discharge milling
129
1 the dielectric medium is considered as continuous medium and incompressible
2 debris in the interelectrode gap is neglected and the flow field is considered as single-phase liquid flow field
3 there exist micro roughness on the surfaces of tool and workpiece 4
during the rotation of tool, there is no radial run out.
4.3.2 Mathematical description
The governing equations used in incompressible single-phase flow field analysis are Navier-Stokes equations whose generalised version in terms of transport properties and velocity gradients are given by William (2006):
()()()T ρ
ηρp t
???????+?+??+?=???u
u u u u F (1) 0??=u (2)
where ρ is the density (kg/m 3), η is the dynamic viscosity (Pa · s), u is the velocity field, p
is the pressure (Pa), F is a volume force field.
The equation (1) is the momentum transport equation, and the equation (2) is the continuity equation for incompressible fluids. The influences that tool rotation exerts on gap flow can be described with the following boundary conditions:
0 (,)
W
y x U n n ?=??
?=??=??n u u t t (3) where n is the radial direction vector of tool, while t is the circumferential direction vector.
The particular point around the circumference of tool electrode has only tangential velocity and no radial velocity. The simulation parameters are shown in Table 2.
Table 2
Parameters used in the simulation of gap flow
Density of dielectric (ρ) 725 (kg/m 3) Dynamic viscosity of dielectric (η)
2.4 × 10–3 (Pa · s)
Gap width (d ) 1–8 (μm) Tangential velocity of tool electrode edge (v ) 0.03 (m/s)
4.3.3 Simulation results and discussion
The pressure distribution of dielectric flow for tool rotation speed of 3,000 RPM and a gap of 8 μm is shown in Figure 10. This pressure change from highest to the lowest level occurs in each circle of rotation. Thus, in the case of the tool rotating at the speed of 3,000 RPM, the surface of electrode experiences stress changes 3,000 times per minute. Figure 11 shows the continuous pressure variation on a given point in the tool as it passes along points A-B-C-D-E-A. This continuous stress change can cause fatigue failure of the electrode surface, especially when the electrode is covered with cracks.
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The effect of the dynamic viscosity on the highest (H) and lowest (L) fluid pressure for discharge gap of 3 μm and tool rotation speed of 3,000 RPM is shown in Figure 12 which reveals that the absolute value of highest and lowest fluid pressure increases as the dynamic viscosity of dielectric increases. Therefore, to reduce the chances of mechanical failure, MED milling using dielectric with low dynamic viscosity such as deionised water (dynamic viscosity of 1.005 × 10–3 Pa · s) is suggested by this simulation.
Figure 10
Pressure distribution graph (see online version for colours)
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Figure 11 Pressure curve along the arc-length of tool for a minimum gap of 1 μm
(see online version for colours)
Figure 12 Effect on the fluid pressure function of the flow dynamic viscosity (see online version
for colours)
The relationship between rotational speed, gap distance and fluid pressure is shown in Figure 13 which reveals that the pressure of fluid is directly proportional to the rotation speed and inversely proportional to the discharge gap. Following two significant observations can be made from these results.
1 Unlike mechanical micromachining processes such as micro milling, MED milling
does not require high speed spindles to perform micromachining. In fact, the chance of mechanical failure of tool is lesser at lower RPM.
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2
Typically in micro EDM, roughing and finishing are performed at higher and lower discharge energies respectively. This results in higher discharge gap and higher tool wear during roughing and lower discharge gap and lower tool wear during finishing. MED milling simulation results indicate that the chance of mechanical failure and tool material loss is actually higher at lower gap (finish machining condition) rather than at higher gap (rough machining condition).
Figure 13 Influence of tool rotational speed and discharge gap distance on flow pressure
(see online version for colours)
It should be noted that the relationship between fluid pressure and discharge gap distance is not linear as shown in Figure 13. With the gap distance decreases, the pressure of fluid increases rapidly. When the tool electrode is rotating at a high level of speed, it makes the dielectric around moving with it because of internal friction of dielectric molecules. With the sudden change of channel gap, intermolecular repulsion is generated in incompressible fluid. The smaller the cross section area is, the stronger the generated intermolecular repulsion and thus higher the flow pressure exerted on the tool surface. Therefore, too small discharge gap in MED milling is not preferable.
Further, the pressure difference around the circumference of the micro EDM tool generates a non-zero lateral force causing a certain deflection at the end of tool. This deflection depends on the tool’s stiffness which is significantly reduced in the condition of large aspect ratio of micro tool, especially when the electrode is covered with cracks. The deflection will break the dynamic balance of rotating tool. In the case of high rotating speed, the deflection will continue to increase until it reaches the new balance. Zhang et al. (2010) discussed the relationships between the deflection of tool electrode and length of tool, rotation speed of tool respectively. The existence of deflection could cause that the high speed rotating tool contacts with the side wall of workpiece and cause tool failure.
Sudden mechanical failure causes uncertainty of tool wear prediction and complicates tool electrode compensation techniques. It is difficult if not impossible to foresee when and where the failure will happen, and once the exfoliation appears, the original strategy of electrode wear compensation would become invalid, leading to the poor precision in micro EDM milling.
Study of sudden tool failure in micro electro discharge milling 133 5 Conclusions
This paper reports on the tool wear in MED milling. In addition to the typical side, end, and corner tool wears, surface exfoliation and sudden tool failure are noticed in MED milling. The reasons causing these tool failures are analysed and the following conclusions are drawn:
1 The pre-existing crack defects acquired in the cold drawing process of the micro tool
formation is the root cause resulting in the sudden tool failure of tungsten tool.
2 Continuously, varying fluid pressure in the interelectrode gap during MED milling
exerts cyclic loading on any given point in the tool electrode and thus increases the chances of sudden failure of tool.
3 Using dielectric with low dynamic viscosity and higher discharge gap are suggested
to reduce the chances of sudden tool failure in MED milling.
4 Unlike mechanical micromachining processes such as micro milling, MED milling
does not require high speed spindles to perform micromachining. In fact, the chance of tool failure is lesser at lower RPM.
Acknowledgements
The financial support from the National Science Foundation (NSF) under grant number CMMI-0928873 and National Natural Science Foundation of China (NSFC) under grant number 50635040 are acknowledged.
References
Bigot, S., Ivanov, A. and Popov, K. (2004) ‘A study of the micro EDM electrode wear’, Proceedings of First International Conference on Multi-Material Micro Manufacture (4M), pp.355–358.
Bleys, P., Kruth, J.P. and Lauwers, B. et al. (2002) ‘Real-time tool wear compensation in milling EDM’, Annals of the CIRP, Vol. 51, No. 1, pp.157–160.
Bonny, K., De Baets, P., Vleugels, J., Van der Biest, O., Lauwers, B. and Liu, W. (2009) ‘EDM machinability and dry sliding friction of WC-Co cemented carbides’, International Journal of Manufacturing Research, Vol. 4, No. 4, pp.375–394.
Dev, A., Patel, K.M., Pandey, P.M. and Aravindan, S. (2009) ‘Machining characteristics and optimisation of process parameters in micro-EDM of SiCp-Al composites’, International Journal of Manufacturing Research, Vol. 4, No. 4, pp.458–480.
Kumagai, S., Misawa, N. and Takeda, K. et al. (2004) ‘Plasma-applied machining of a narrow and deep hole in a metal using a dielectric-encased wire electrode’, Thin Solid Films, Vol. 457, No. 1, pp.180–185.
Kumagai, S., Sato, N. and Takeda, K. (2006) ‘Combination of capacitance and conductive working fluid to speed up the fabrication of a narrow, deep hole in electrical discharge machining using
a dielectric-encased wire electrode’, International Journal of Machine Tools and Manufacture,
Vol. 46, Nos. 12–13, pp.1536–1546.
Lambie, J.E.J., Xi, F., Tan, B. and Jubran, B.A. (2008) ‘An overview on micro-meso manufacturing techniques for micro-heat exchangers for turbine blade cooling’, International Journal of Manufacturing Research, Vol. 3, No. 1, pp.3–26.
134 M.M. Sundaram et al.
Liu, K., Ferraris, E. and Peirs, J. et al. (2008) ‘Micro-EDM process investigation of Si3N4-TiN ceramic composites for the development of micro fuel-based power units’, International Journal of Manufacturing Research, Vol. 3, No. 1, pp.27–47.
Masuzawa, T. (2000) ‘State of the art of micromachining’, Annals of the CIRP, Vol. 49, No. 2, pp.473–488.
Pham, D., Ivanov, A. and Bigot, S. et al. (2007) ‘An investigation of tube and rod electrode wear in micro EDM drilling’, The International Journal of Advanced Manufacturing Technology, Vol. 33, Nos. 1–2, pp.103–109.
Pham, D.T., Dimov, S.S. and Bigot, S. et al. (2004) ‘Micro-EDM – recent developments and research issues’, Journal of Materials Processing Technology, Vol. 149, Nos. 1–3, pp.50–57. Rajurkar, K.P. and Yu, Z.Y. (2000) ‘3D micro-EDM using CAD/CAM’, Annals of the CIRP, Vol. 49, No. 1, pp.127–130.
Singh, S., Maheshwari, S. and Pandey, P.C. (2007) ‘Optimisation of multiperformance characteristics in electric discharge machining of aluminium matrix composites (AMCs) using Taguchi DOE methodology’, International Journal of Manufacturing Research, Vol. 2, No. 2, pp.138–161.
Sundaram, M.M. and Rajurkar, K.P. (2007) ‘Micro electro discharge machining by uniform tool shape method – study of relative wear’, Transactions of the 35th North American Manufacturing Research Conference/SME, pp.137–144.
Sundaram, M.M. and Rajurkar, K.P. (2008) ‘Towards freeform machining by micro electro discharge machining process’, Transactions of the 36th North American Manufacturing Research Institution of SME, pp.381–388.
Wang, Y.G., Zhao, F.L. and Wang, J. (2009) ‘Wear-resist electrodes for micro-EDM’, Chinese Journal of Aeronautics, Vol. 22, No. 3, pp.339–342.
William, B. (2006) Multiphysics Modelling with Finite Element Methods, World Scientific Publishing Co. Pte. Ltd., Singapore.
Yu, Z.Y., Masuzawa, T. and Fujino, M. (1998) ‘Micro-EDM for three dimensional cavities – development of uniform wear method’, Annals of the CIRP, Vol. 47, No. 1, pp.169–172. Zhang, M., Guo, D.M. and Jin, Z.J. (2009) ‘EDM performance of electroformed Cu-ZrB2 shell electrodes’, Rapid Prototyping Journal, Vol. 15, No. 2, pp.150–156.
Zhang, Y., Li, J. and Luan, J. et al. (2010) ‘Influence of the electrode length on the diameter of a micro hole in micro EDM’, Proceedings of the 16th International Symposium on Electromachining (ISEM), pp.629–632.
Zhao, W., Yang, Y. and Wang, Z. et al. (2004) ‘A CAD-CAM system for micro-ED-milling of small 3D freeform cavity’, Journal of Materials Processing Technology, Vol. 149, Nos. 1–3, pp.573–578.