Experimental study on the microelectrodes fabrication using low speed wire electrical discharge turning (LS-WEDT) combined with multiple cutting strategy

Accepted Manuscript Title: Experimental study on the microelectrodes fabrication using low speed wire electrical discharge turning (LS-WEDT) combined with multiple cutting strategy Authors: Yao Sun, Yadong Gong PII: DOI: Reference:

S0924-0136(17)30287-X http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.07.015 PROTEC 15312

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

12-11-2016 3-7-2017 10-7-2017

Please cite this article as: Sun, Yao, Gong, Yadong, Experimental study on the microelectrodes fabrication using low speed wire electrical discharge turning (LSWEDT) combined with multiple cutting strategy.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.07.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Experimental study on the microelectrodes fabrication using low speed wire electrical discharge turning (LS-WEDT) combined with multiple cutting strategy

Authors Yao Sun , Yadong Gong *

Author AFFILIATIONS: Northeastern University, Shenyang, China

Correspondence information: Corresponding author name: Y.D. Gong Affiliation: Northeastern University, Shenyang, China detailed permanent address: School of Mechanical Engineering and Automation, Northeastern University, Shenyang , P.R.China,

110819

email address: [email protected] telephone number:

86-139-4051-8488

(Check the Guide for authors to see the required information on the title page)

Abstract This paper aims at giving an insight into the fabrication of ultra-small microelectrodes and micro-cutting tools using the low speed wire electrical discharge turning (LS-WEDT). The multiple cutting strategy divides the microelectrode machining process into rough cut (RC), semi-finishing trim cut (TC) and finishing trim cut (FTC). Experimental results indicated that the breaking and bending microelectrodes are caused primarily by the improper selection of flushing pressure, peak current and open circuit voltage in FTC. The cylindrical microelectrode of 58μm in diameter is successfully fabricated with good surface quality and high machining accuracy. More importantly, the LS-WEDT method can be used to manufacture the microelectrodes and micro-cutting tools with different micro structures by combining with the numerical control technology. The D-shaped cutting tool of 65μm in diameter and the three spiral micro cutting tool are firstly and successfully manufactured by LS-WEDT method.

Keywords: LS-WEDT, Finishing trim cut, Microelectrode, Micro cutting tool

1. Introduction

Nowadays, with the development of aviation industry, national defense industry and micro-electro mechanical system, the demand for microelectrodes which are capable of realizing the micron and nanometer scale machining has been significantly increasing (Kai et

al.,2011).Electrical discharge machining (EDM) as a nontraditional machining process has a unique superiority in fabricating miniature or micro-parts due to its non-contact, no macro cutting force and not restricted by material properties, which makes it become one of the mainstream technology to manufacture micro- electrodes (Gong et al.,2016).Therefore, the fabrication of microelectrodes by EDM

has attracted more research interests. At present, there

are some methods that have been proposed and developed for fabricating microelectrodes by EDM. Masuzawa et al. (1985) invented the wire electrical discharge grinding (WEDG) which solved the problem of microelectrodes online production and successfully fabricated microelectrodes with high machining accuracy. Li et al. (2002), Kuo et al.(2004), Chern et al. (2007), Yan et al. (2010), Rees et al. (2013) and Wang et al. (2014) respectively used the WEDG method to fabricate all kinds of microelectrodes and studied the influences of different machining parameters on the surface quality. Besides, Zhang et al. (2015) proposed the tangential-feed (TF) WEDG method for improving machining precision. Sheu et al. (2008) proposed twin-wire EDM systems for enhancing machining efficiency. Ravi et al. (2002) and Lim et al. (2003) adopted block electrode discharge grinding (BEDG) method to fabricate microelectrodes but the tapered problem always existed. Jahan et al.(2010) and Hourmand et al. (2017) proposed the moving BEDG method and effectively improve the tapered problem. Yin et al. (2016) developed electrical discharge machining grinding using two block electrodes (EDG-TBE) to manufacture the microelectrode with the diameter of 86μm and consistent accuracy of less than 2μm. Minoru et al. (2004) presented the method of using self-drilled holes

to form micro-rods. Kim et al. (2006) fabricated various shaped microelectrode by the reverse electrical discharge machining (REDM).Qu et al. (2002), Haddad et al. (2008), Matoorian et al. (2008), Krishnan et al.(2012) and Janardhan et al. (2010) studied the method of cylindrical wire electrical discharge machining (CWEDM) and investigated the effect of processing parameters on machining performance instead of applying this method to fabricate ultra-small microelectrodes or micro-milling tools. Moreover, Takahata et al. (2000), Asad et al. (2007) and Kai et al. (2010) fabricated some microelectrodes using different hybrid machining processes, respectively. From literature, the EDM processes applicable for fabricating microelectrodes are the WEDG, self-drilled holes, hybrid machining, BEDG, etc. These above methods have their different characteristics as listed in Table1. The LS-WEDT method is same as the WEDG in material removal mechanism, so it possesses substantial advantages of WEDG, for example, the LS-WEDT does not exist electrode wear due to the wire electrode does one-way movement, so microelectrodes with nontaper can be obtained, which is not like the BEDG and self-drilled holes methods. Secondly, the LS-WEDT can divide the rough and finishing machining stages and it does not need to change new tool electrodes in finishing stage unlike the BEDG. Besides, the LS-WEDT can precisely evaluate microelectrode diameter by calculating the feed amounts instead of installing on-line measuring device, which greatly reduce the process complexity compared with the TF-WEDG and the hybrid process. However, there are essential differences between the LS-WEDT and WEDG methods. Firstly, the LS-WEDT doesn’t require special and expensive setup, which differs from the WEDG method. The LS-WEDT experiments can be

performed on the commercial low speed wire EDM machine by adding the rotated unit, which not only significantly saves cost, but also expands wire EDM application field. Besides, for LS-WEDT, the workpiece is fed horizontally, whereas in WEDG, the workpiece is fed in the longitudinal direction (Chern et al., 2007). The geometric accuracy of the machined workpiece is affected by the wire deflection and feed direction, so the longitudinal feed motion in WEDG makes the workpiece prone to deflect because of its gravity comparing with the LS- WEDT process (Giridharan and Samuel, 2016). Finally, the LS-WEDT is more flexible than the WEDG due to it is not restricted by idler pulley and it also can take full advantage of the numerical control program technology, so it can effectively and flexibly fabricate complicated structures. Thus, the LS-WEDT is an appropriate machining method to cover the growing need for microelectrodes and micro-cutting tool with various microstructures. In this study, investigations have been carried out to analyze the bending and breaking phenomenon, improve surface quality and dimensional accuracy, and predict the diameter of microelectrodes. More importantly, the LS-WEDT method is firstly and successfully utilized to fabricate various microelectrodes and micro cutting tools.2. Method 2.1. LS-WEDT method The configuration of LS-WEDT method is illustrated in Fig.1 (a), the brass wire as electrode tool does one-way movement at a constant speed Vw in vertical direction. The microelectrode does rotary motion at rotating speed Vn and at the same time does horizontal feed at Vs controlled by X axis of the workbench in LS-WEDM machine. So, the material is eroded by a series of discrete sparks occurring in the gap between microelectrode and brass wire, and

deionized water is supplied continuously to cool and flush debris away from the gap. Therefore, the complicated form on the rotating microelectrode can be generated based on the LS-WEDT method. 2.2. The comparison of LS-WEDT and LS-WEDM method Judging from machining principles of LS-WEDT as shown in Fig.1 (a), the brass wire conducts spark discharge like grinding on the outer cylindrical surface of the rotating microelectrode during machining, which makes it close to the point contact, so the discharge area is very small. In addition, tool electrode wear can be negligible in LS-WEDT method and it does not need to consider electrode wear compensation due to the brass wire does one-way movement of LS-WEDM machine, which in turn assures the machining precision of microelectrodes. Besides, the LS-WEDM machine adopted friction wheel to regain wire which can more easily realize the constant wire speed without jitter and improve machining accuracy. Moreover, it is not easy to produce the wire breakage due to the wire electrode moves along the workpiece’s outer cylindrical surface and differs from WEDM process where the wire electrode moves along the kerf as shown in Fig.1 (b). Finally, the rotating workpiece can speed up the flow of working fluid, which is conducive to flush debris away and improve machining speed. 2.3. The achievement of LS-WEDM process The rotation mechanism unit was designed and fabricated for adding a rotary axis to the five-axis LS-WEDM machine as depicted in Fig.2 (a).The cross-section of the rotating mechanism unit is displayed in Fig2.(b),during the machining, the micro- electrode is fixed on a

chuck and driven by an accuracy and anti-corrosive spindle, which together with the microelectrode are immersed in the working fluid. And the spindle is attached to the spindle housing by using two pair of deep groove ball bearing. Dynamic seals are designed in the housing for preventing water intake. Besides, a timing belt is applied between the spindle and the servo motor to deliver turning force. The servo motor is installed on the holder and located above deionized water for protecting servo motor from water intake, and the rotating speed variable range is from 0 rpm to 2000 rpm. Finally, the carbon electrical brush is fixed on the end of the spindle for increasing the conductivity of spindle and reducing parasitic capacitance of discharge circuit. Fig.3 shows size chain relationship of the microelectrode machined by LS-WEDT. Where f1 represents the total amount of feed (μm) ,and f1  nR R  nT  T  nF  F ,nR ,nT and nT respectively mean feeding times of RC,TC and FTC; δR, δT and δF respectively are the feed amount of RC,TC and FTC(μm); G1 is sensing gap (μm); Dtarget is the final diameter(μm); Dinintial is the initial diameter of microelectrode (μm).According to the empirical formula, the unilateral discharge gap (g) can be estimated as Eq.(1) 



ti

g  (u  n ) / Em  kt [  u(t )  i (t )  dt ]0.5 i t 0 i

(1)



Where ui represents the open voltage (V); ε is the constant (V.s) and related to the working fluid; t is pulse on time (μs); n is the coefficient and relevant to the kind of working fluid; E m is breakdown electric field intensity of working fluid(V/cm); u(t ) is machining voltage(V); i(t) is machining current(A); kt is the coefficient and related to the electrode materials. Therefore, the

diameter of the microelectrode can be predicted based on Eq.(2) derived from Fig.3. 



t

Dt arg et  Dinitial  2G1  2 f1  2(u  n ) / Em  2kt [  u(t )  i (t )  dt ]0.5 i t 0 i

(2)

3. Machine development and experimental setup 3.1. Machine tool design The experiments were conducted on LS-WEDM machine CA20, which was made by Beijing Agie Charmilles Industrial Electronics Co., Ltd as shown in Fig.3 (a). The machine is equipped with the precise five-axis motion table and the range of movement of the linear axes is 350mm (X-axis) ×200 mm (Y-axis) ×300 mm (Z-axis) with a resolution of 0.1 μm. From Fig.3 (b), it can be seen that the microelectrode is clamped by rotation mechanism unit which is fixed on the workbench along with Y axis. The brass wire of 0.2mm in diameter is drawn through a set of upper and lower guides. The workpiece together with the rotation mechanism unit is submerged in the deionized water during processing. The machining conditions are detailed in Table 2. 3.2. Experiment design In order to solve the contradiction of processing quality and productivity, the multiple cutting strategy is introduced to divide the machining process into RC, TC and FTC. Although every LS-WEDM machine has the recommended machining parameters aimed at different materials and thickness, the existed process parameters cannot provide available reference for LS-WEDT. Based on machining mechanism of LS-WEDT and the principal of multiple cutting strategy, main machining parameters of RC, TC and FTC are respectively determined as given in

Table 3, where electrical parameters are gradually reduced. The lowest wire speed of 30mm/s is selected for reducing the brass wire consumption and vibration. The lowest feed speed of 1mm/min is set in RC and TC, which is helpful to increase the utilization ratio of discharge energy. The higher feed speed of 4 mm/min is selected in FTC, which is conductive to discharge point transfer and reduce secondary discharge and then improve the surface quality. In order to protect the workpiece from the electrolytic corrosion, the alternating pulse is set, which can make average voltage to be zero by generating a negative voltage during pulse off time to counteract the positive voltage of pulse on time. The rotational speed as the new processing parameter is set 20rmp to ensure the workpiece rotates stably. The too big flushing pressure will make the microelectrode bend and even break, but too small flushing pressure could burn the machined surface, so the flushing pressure was determined after multiple tests. 4. Fabrication results and discussions 4.1. The defects of microelectrodes fabricated by the LS-WEDT method 4.1.1. The phenomenon of break and bend The microelectrode will appear break or bend phenomenon when the machining parameters are improperly adopted. In FTC, the discharge force and flushing pressure are main factors in determining the machining accuracy of microelectrodes. The broken microelectrode with length of 386μm and diameter of 75μm is obtained when the flushing pressure of 1bar is set in FTC as shown in Fig.5. The break phenomenon can be explained that the microelectrode produces bending deformation caused by the large flushing pressure and then the deforming portion will be eroded by the electrical discharge, which results in the broken microelectrode.

The microelectrode appeared bending phenomenon as presented in Fig.6(a) when the peak current of 120A, open circuit voltage of 100V and the flushing pressure of 0.3 bar are adopted in FTC. This is due to the microelectrode of less than 100μm in diameter with an aspect ratio of about 10 is similar to the slender axle, so the poor stiffness and low bent-resistant ability of this thin microelectrode makes it susceptible to discharge force, thermal stress and flushing pressure. Besides, the brass wire moves along the microelectrode on one side which is not symmetrically arranged as shown in Fig.6 (b), and this will make the microelectrode bend opposite to the machining side when the force acting on the thin microelectrode is beyond the bear range. Fig.6 (a) also disclosed that the microelectrode has the poor surface quality and low diameter consistent accuracy, which can be explained by the existence of big craters caused by high discharge energy. To solve the problem of bending and breaking, the peak current, open circuit voltage, pulse on time and flushing pressure are reduced and adjusted as descripted in Table.3.This is because of the fact that the instant discharge force decreases with the decrease of the peak current, pulse on time and open circuit voltage according to the Eq.(3) (Yu.,2011). t

Fd   (  u(t )i (t )dt  P)t 0.6 0

(3)

Where Fd represents the discharge force (N); β is the constant of 2.05×102 (μs); α is the constant of 5×104 (N.μs-1.J-1); P is the constant of 2.5×103(N. μs-1); t is the pulse on time (μs); △ is discharge gap (mm); u (t) is voltage (V); i (t) is peak current (A). On the other hand, the LS-WEDT is essentially a thermal material removal process and the thermal stress will be present after machining. The thermal stress mainly depends on the

discharge energy, which can cause the local failure and even deformation of the microelectrode. In addition, the root overcut phenomenon will occur due to discharge sparks are intensive at the root of the microelectrode, which will severely weaken the root structure and induce the bending deformation. Lowering discharge energy can mitigate the root overcut problem. The large flushing pressure will contribute to flush away debris from the machining area and reduce secondary discharge, but it also can aggravate the vibration of the brass wire and thin microelectrode, which adversely affects the dimensional accuracy of the microelectrode and even results in bending deformation. Therefore, considering the above reasons, the lower peak current, pulse on time, open circuit voltage and the flushing pressure are adopted for reducing the discharge energy, discharge force, thermal stress, root overcut and vibration, which can further protect the microelectrode from bending and breaking. Experimental results identified that the straight microelectrode with high dimensional accuracy and good surface quality can be successfully fabricated after reducing the relevant parameters as shown in Fig.8. 4.1.2. The phenomenon of bamboo-like waviness The microelectrodes machined after RC are observed and measured by VHX- 1000E microscope as displayed in Fig.7 (a), it can be found that the microelectrode machined after RC appeared obvious bamboo-like shape and the surface quality is poor which is related to the high discharge energy adopted in RC. The length of this bamboo-like waviness is measured as displayed in Fig.7(c) and the average length is 95.31μm. Fig.7 (b) disclosed the microelectrode’s surface distributed large amount of craters, and the surface profile fluctuation height is 15.23μm,

which reflects that the surface of the microelectrode is not smooth. The diameter of the microelectrode is measured in 32 different positions as shown in Fig.7 (d) and the average diameter of 345μm can be obtained, the diameter variation range is large due to the existence of bamboo-like waviness. 4.2. The surface microstructure and surface roughness of the microelectrode The microelectrode with good surface quality and high machining accuracy is fabricated by the LS-WEDT after RC, TC and FTC as described in Fig.8 (a), which is observed by JSM-7001F scanning electron microscope (SEM).The average diameter of microelectrode after FTC is 90μm as displayed in Fig.8 (b), it can be observed that the microelectrode is straight without bending and breaking. Moreover, the surface is very smooth without bamboo-like waviness and obvious craters, which identifies the rationality of processing stages division and parameters selection shown in Table.3. In order to observe the surface microstructure, the microelectrode surface is magnified 2000 and 5000 times as displayed in Figs.9 (a) and (b), respectively. It can be found that the small craters and microvoids distributed in the microelectrode surface after FTC. In addition, the confocal laser scanning microscope is used to measure surface outline and surface roughness (Ra) of microelectrodes obtained by the LS-WEDT. Fig.10 (a) showed the measuring process of the Ra. The Ra values of microelectrodes machined after RC, TC and FTC are respectively measured at different angles and the results are plotted in Fig.10(b). Fig.10(c) showed the surface outlines of the micro- electrodes machined after FTC in 30°, 90°, 150°and 210° direction, it can be observed that the surface outlines are flat and approximate to be a line, which reflects the small surface undulation of the microelectrode.

The average Ra of microelectrode surface machined after RC is larger than 8μm, more importantly the Ra distribution in different angles is similar to irregular polygon, suggesting the Ra values fluctuation is large and the surface consistency is not good. Whereas, the surface consistency gradually becomes better due to the Ra distribution tends to be a circle. After FTC, the Ra values are confined to be lower than 1μm and the surface consistency is the best judging from the Ra distribution approached a circle as presented in Fig.10(b). Therefore, the method of LS-WEDT combined with multiple cutting strategy can successfully fabricate microelectrodes with good surface quality and machining accuracy. This is attributed to the multiple cutting strategy can make the microelectrodes surface undergo gradually decreased discharge energy and rectify the inaccuracy and large craters produced in the last process, More importantly, the brass wire does one-way movement and its wear defect would not be mirrored on the surface of the microelectrode. Besides, the LS-WEDM machine adopted friction wheels to regain wire which can guarantee the unchanged wire speed without jitter and is conducive to improve machining accuracy. 4.3. The diameter prediction of the microelectrode The online measurement is difficult to the microelectrodes’ fabrication due to the machining process is conducted under deionized water. In order to timely converse to the RC, TC and FTC and reasonablely plan the machining allowance, the diameter accuracy prediction is essential. Based on the Eq.(2) and the feed amount of various machining stages as descripted in Table.3, the diameter prediction of microelectrodes machined after RC,TC and FTC are respectively displayed in Figs.11(a), (b) and (c), it can be found that the prediction deviation of RC is the

largest and then is gradually decreased in TC1,TC2 and FTC. This is due to the existence of bamboo-like waviness in microelectrode machined after RC damages the diameter consistency accuracy and then results in the poor prediction accuracy. The average absolute deviation of FTC can be confined to 3μm as displayed in Fig.11(d), this is due to the electrode wear in LS-WEDT can be ignored, which is unlike the block electrode with unpredictable wear as studied by Yin et al.(2016). Therefore, it can realize the effective control and exact prediction of the microelectrode diameter machined by the LS-WEDT method, which will reduce the blindness in setting feed amount. 4.4. Application machining using the LS-WEDT method 4.4.1. The fabrication of the slender cylindrical microelectrode The above experimental results indicated that the LS-WEDT method is capable of fabricating cylindrical microelectrodes with a diameter of less than 100μm, but further improvement is necessary for fabricating the thinner microelectrode without taper. One possible method is to minimize the discharge energy by lowering the open circuit voltage to be 50V in FTC. Experimental results indicated that the micro- electrode with diameter of 58μm can be successfully fabricated by LS-WEDT without inclination and waviness as presented in Figs.12 (a)-(d), which further reduces the diameter size of the microelectrode fabricated using the LS-WEDT method. 4.4.2. Microelectrodes and micro-cutting tools with various micro structures Although, the WEDG method can machine non-cylinder structures in principle, the driven way of the idler pulley greatly restricts the space motion flexibility, which makes it difficult to machine complex micro rotary structures. But as for LS-WEDT, the motion of electrode wire not

restricted by idler pulley, which makes it more flexible to fabricate complex micro structures compared to the WEDG. Therefore, microelectrodes with different micro structures can be fabricated based on processing parameters selection, stages division and the numerical control technology. Experimental results indicated that the free-form microelectrode and the external conical microelectrode can be successfully and flexibly fabricated by LS-WEDT method as displayed Fig.13. In addition, the D-shaped micro-cutting tool with the diameter of 65μm also can be manufactured by LS-WEDT based on the machining parameters and feed amounts as listed in Table3. Experimental results are observed and measured by VHX-1000E microscope and SEM, which indicate that the D-shaped micro-cutting tool has good geometry accuracy and sharp cutting edges as demonstrated in Figs.14 (a) and (b). Besides, the three spiral micro-cutting tool can be firstly and successfully fabricated by the LS-WEDT method, which indicates that the LS-WEDT is more competent in fabricating complicated micro structures compared to the WEDG method. 5. Conclusion The method of using LS-WEDM machine combined with the multiple cutting strategy to fabricate microelectrodes is firstly proposed. Experiment results disclosed that the lower flushing pressure, peak current and open circuit voltage are conductive to prevent the microelectrode from bending or breaking. The desired diameter of the microelectrode can be obtained by the LS-WEDT without the online measurement. The cylindrical microelectrode with 58μm in diameter and the external conical microelectrode can be successfully obtained. Besides, the

D-shaped micro cutting tool with 65μm in diameter and the three spiral micro cutting tool are firstly fabricated by LS-WEDT method. Acknowledgment

The authors would like to thank the support of National Natural Science Foundation of China (No. 51375082)

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Figures:

Micro-rod

Wire feed direction

Brass wire

Brass wire Rotation direction

Vn

Workpiece Wire feed direction

Workpiece feed direction Vs

Vw Wire feed direction

(a) LS-WEDT

(b) LS-WEDM

Fig.1 The schematic diagram of LS-WEDT method and LS-WEDM

servo motor rotation mechanism unit LS-WEDM machine small pulley

big pulley

timing belt deep groove ball bearing precise spindle seal ring

bearing housing chamber

bearing end cover

holder

chuck micro-rod

(a)The overall layout

carbon brush

bottom plate

(b) Rotation mechanism unit

Fig.2 3D-Models of LS-WEDT

Fig.3 Size chain relationship of microelectrode machined by LS-WEDT

upper guide rotating mechanism unit

lower guide

chuck

microelectrode

deionized water

e

workbench

brass wire

(a) Machine tool

(b) Setup of LS-WEDT Fig.4 Experiment setup of LS-WEDT

Diameter:75μm

break

Length:386μm

(a) The broken microelectrode

(b) The enlarged view

Fig.5 The broken microelectrode

(a) The bent microelectrode

(b) Schematic diagrams of discharge force

Fig.6 The bent microelectrode and discharge force schematic diagrams

craters craters

(a) The microelectrode obtained after RC

(b) The microelectrode surface profile

The average length is 95.31μm

(c) The bamboo-like waviness length measurement

(d) The diameter measurement

Fig.7 The microelectrode machined by LS-WEDT after RC

Φ90μm

(a) The microelectrode machined after FTC

(b) The enlarged view

Fig.8 The microelectrode machined by LS-WEDT after FTC

craters

(c) 2000 times

(d) 3000 times

Fig.9 The microelectrode surface microstructure machined by LS-WEDT after FTC

(a) The measuring process

(b) The Ra values at different angle

(c) The surface outlines of microelectrode machined after FTC Fig.10 The surface outline and Ra of microelectrode at different angle

Diameter:345 μm

Diameter:243 μm

(a) RC

(b) TC1

Diameter:128μm Diameter:93μm

(c) RC

(d) TC1

Fig.11 The diameter prediction deviation of different machining stages

Φ58μm

(a) The slender microelectrode

(c) The slender microelectrode

(b) Enlarged view

(d) Enlarged view

Fig.12 The SEM picture of the microelectrode machined by LS-WEDT

Column-like accretion Accretion

(a) Microelectrode with free-form surface

(b) Zoomed image

21°

(c) External conical microelectrode

120μm

(d) Zoomed image

Fig.13 SEM picture of different microelectrodes machined by LS-WEDT

Clearance angle of 20°

780μm 65μm 80μm 215μm

Bottom cutting edge

Main cutting edge

(a) The micro-cutting tool

(b) The SEM picture

Fig.14 The D-shaped micro-cutting tool observed by VHX-1000 microscope and SEM

Fig.15 SEM picture of three spiral micro-cutting tool machined by LS-WEDT

Table 1 The specifications of various microelectrode fabricated processes Performance

Microelectrode fabricated processes WEDG TF-WEDG

Advantages Dimensions About 2.8μm in diameter About 26μm in diameter

Various Cylindrical

Twin-wire EDM

About 5μm in diameter

Cylindrical

Self-drilled holes

About 4μm in diameter

Various

BEDG

Moving BEDG

EDG-TBE

About 150μm in diameter About 40μm in diameter About 45μm in diameter

Various

High machining accuracy and nontaper

diameter and 1.5mm in About 125μm in

rotating disk

diameter

Micro-turning and

About 19 μm in

micro- EDM

diameter

good, low efficiency, high investment Complex setup, high investment, low

roughness

efficiency

High machining accuracy and nontaper, high

Complex setup, surface roughness is not

machining efficiency

good, high investment

Simple setup, good surface roughness, low

Low machining accuracy and taper, low

investment

machining efficiency

Simple setup, good surface roughness, low

Low machining accuracy and taper, low

investment

machining efficiency

Simple setup, good surface roughness, high Various

machining accuracy and nontaper, low

Low machining efficiency

investment Low investment, simple setup, high machining Cylindrical

efficiency, high machining accuracy and nontaper

Various

Good surface roughness

length Micro-EDM with

Complex setup, surface roughness is not

High machining accuracy ,good surface

About 35μm in REDM

Disadvantages

Shapes

Cylindrical

Good surface roughness, low investment, high machining accuracy and nontaper

Surface roughness is not good, high demands for positional accuracy Complex setup, low machining accuracy and taper, low machining efficiency Complex setup, low machining efficiency

Good surface roughness, high machining Cylindrical

accuracy and nontaper, high machining efficiency

Complex setup, high investment

WEDG and ECM LIGA micro-EDM

and

About 0.3μm in diameter About 200μm in diameter

Cylindrical Various

Good surface roughness Good surface roughness, high machining accuracy

Complex setup, high investment Complex setup, high investment

Table 2 Experimental conditions Factors

Description

Workpiece

Carbon steel rod, machining length:1mm or 0.5mm

Electrode

Brass wire,diameter:0.2mm

Working fluid

Deionized water, temperature: 25℃,Flux:4 Conductivity:5μS/cm

Table 3 Machining parameters and levels of multiple cuts Parameters

Unit

Rough cut

Semi-finishing trim cut

Finishing trim cut

RC

TC1

TC2

FTC

A

320

180

120

40

Open voltage( u )

V

170

136

100

85

Pulse on time(ti)

μs

15

13

10

4

μs

25

25

25

25

Negative pulse

1

1

1

1

1

Positive pulse

1

1

1

1

1

Rotating speed(Vn)

rmp

20

20

20

20

Wire tension (Fw)

N

12

15

18

18

Wire speed(Vw )

mm/s

30

30

30

30

Feed speed (Vs)

mm/min

1

1

1

4

Flushing pressure (P)

bar

10

1

1

0.3

Feed amount (f)

μm

15

10

5

1



Peak current ( Ie ) 

i

Pulse off time(tO)