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Development of an original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM
Journal Pre-proof Development of an original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM Shun-Tong Chen, Li-Wen Huang, Jin-Pin Kuo, Tin-Cheng Pai
PII:
S0924-0136(19)30511-4
DOI:
https://doi.org/10.1016/j.jmatprotec.2019.116538
Reference:
PROTEC 116538
To appear in:
Journal of Materials Processing Tech.
Received Date:
24 June 2019
Revised Date:
29 October 2019
Accepted Date:
30 November 2019
Please cite this article as: Chen S-Tong, Huang L-Wen, Kuo J-Pin, Pai T-Cheng, Development of an original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM, Journal of Materials Processing Tech. (2019), doi: https://doi.org/10.1016/j.jmatprotec.2019.116538
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Development of an original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM Shun-Tong Chen1, Li-Wen Huang2, Jin-Pin Kuo2, Tin-Cheng Pai2 1 Professor, Department of Mechatronic Engineering, National Taiwan Normal University 2 Graduate student, Department of Mechatronic Engineering, National Taiwan Normal University E-mail: [email protected]
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Abstract In this study, an original ‘electromagnetic damping-controlled horizontal cutting mechanism’ is designed and proposed for precisely controlling micro-scale wire-tension for cutting a microstructure array. The electromagnetic damper uses a set of three annular electromagnets equally distributed with their end-faces orientated toward a mild-steel disc. This arrangement is located at the front of the cutting mechanism. The cutting mechanism also consists of a set of microgroove rollers, a wireelectrode guide, and an auxiliary guide designed to suppress wire-wriggling and wire-swaying in
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order to deliver a tungsten wire of Ø13 m diameter at a steady state over the long-term. Experimental verification is conducted on B-NPD (boron-doped nano-polycrystalline diamond), which possesses a high melting-point and high electrical resistivity characteristics, to establish the feasibility of cutting such difficult-to-machine materials. A 'one-cut one-skim' machining approach is used whereby the surface flatness and the dimensional accuracy of the slot-wall can be improved. Experimental results found that the wire feed-rate during the finish-cutting stage can be used at a rate greater than that of the rough-cutting stage. The resultant diamond microstructure array is of high-consistency and aspectratio at 1:22, demonstrating that the electromagnetic damping-controlled horizontal cutting mechanism can precisely and stably control the tension and running speed of the microwire.
Nomenclature:
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Keywords: Electromagnetic damper, horizontal cutting mechanism, microwire tension, one-cut oneskim, B-NPD microstructure array
Rw= distance between the electromagnet center and delivery wire-bobbin center rw= wire-bobbin radius Soffset= offset distance of the wire-electrode Te= magnetic damping moment
d= wire-electrode diameter dc= Enamel insulated wire diameter Es= single-discharge energy F= wire tension Fe= total induced magnetic force Fr = wire feed-rate fdis = work frequency
Tr= torque of the wire-bobbin tgap= single-side discharge gap ttotal= total time of processing tm= workpiece thickness Vc= capacitor voltage Vm= induced electromotive force Vr= wire running speed
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ad= Magnetic spacing between the bobbin end and mild-steel disc B= magnetic flux density in the electromagnet C= work capacitance CEC= Capacitance Erosion Capability
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Iin= total input current
Vvol= removed-material volume
Im= input current from single electromagnet Ip= peak current L= total depth of cutting
W= slot-width before one-skim Wf= slot-width after one-skim x= horizontal component of the radius of action of the magnetic force σa= allowable tensile stress of wire-electrode σu= ultimate tensile stress of wire-electrode = time-constant in R-C circuit ω= angular velocity of mild-steel disc θ= included angle of magnetic force area µ0= permeability of vacuum µm= magnetic permeability of the disc
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MA= Mechanical Advantage MRR= Material Removal Rate md= thickness of mild-steel disc n= coil number in single electromagnet Qdis= electric charge per unit of time Qtotal= total electric charge per unit of time R= resistance in the circuit
1. Introduction
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Driven by trends in product miniaturization, many associated micromanufacturing technologies have been developed whose corresponding tools, jigs and measuring tools need also be miniaturized. Wire-EDM (Wire Electrical Discharge Machining) is often used to machine gears and dies for many industries (Bouquet et al., 2014; Alhadeff et al., 2018). Miniature tools such as a micro-punch array and micro-electrode array can also be precisely cut by the microwire-EDM approach (Chen, 2008). When performing microwire electrical discharge machining (microwire-EDM), the tension of the microwire plays a key role in the outcome of cutting processes. The microwire may readily snap or fuse-break due to necking when a tight wire tension is used. On the other hand, excessively loose wire tension can reduce the geometrical accuracy of the workpiece. Due to the microwire’s low tensile
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strength, traditional methods of controlling wire tension by use of electromagnetic brake rollers have had difficulty controlling wire tension (Obara and Izumiya, 1991). Previous researchers have experimented with the best ways of controlling microwire-tension during w-EDM. Kinoshita and Hayashi (1994) used a laser beam to measure the lever through which the wire passes with wire tension adjusted based on the feedback angle of the lever. Beltrami et al. (1996) employed an optical wire position sensor, placed between the upper wire guide and the workpiece, to continuously measure wire deflection. Magnetic repulsive force has been used to control wire tension such as in Chen and Lai (2012), who employed a permanent magnetic disc to control wire tension. Although the control of microwire tension by magnetic spacing is advantageous in that there is no contact nor wear of the wire, the resolution of any adjustments has been inadequate. This phenomenon is due to the disc swaying because of backlash from the screw thread (Chen, 2007), which results in uneven damping and a non-steady-state magnetic force. Puri and Bhattacharyya (2003) thought that the solution to wire vibration could be achieved mainly by manipulating the first order mode (n=1) during machining. Under this scenario, the density of spark erosion is affected by wire-vibration mode. Habib and Okada (2016) also reported wire vibration including intense 1st and 3rd-order mode vibrations when fine cutting a thin workpiece. By means of a Proportional Integral Derivative (PID) closed-loop control system and intelligent algorithms, Chen et al. (2018) constructed a constant wire 2
tension control system to restrain wire deflection and vibration. In sum, the demands of miniaturization to conform with the functionality requirements of energysavings, cost effectiveness, and attractive appearance create inevitable challenges for the development of suitable machining tools to meet such needs. In this study, in order to precisely and stably control microwire tension to create narrow slot-widths and tight discharge gaps during machining, an original ‘electromagnetic damping-controlled horizontal cutting mechanism’ is designed and proposed. Magnetic attractive force between electromagnet and wire-bobbin can be finely manipulated by slight adjustments to the input current of the electromagnetic damper. Additionally, the following aspects are evaluated in detail: wire running mechanism, capacitance erosion capability, dimensional accuracy of resultant microstructure array, surface topography and roughness.
2. Methodology
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2.1 Design of the annular array electromagnet The setup of the annular array electromagnet is shown in Figures 1 (Fig. 1(a) (in front view)). The induced electromotive force (Vm) generated by a magnetic field can be expressed by Eq. (1) (Cadwell, 1996), where x, ω, and θ are the horizontal component of the radius of action of the magnetic force, the angular velocity of the disc, and the angle of the magnetic action area, respectively. (1)
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Bx 2 Vm cos 2
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Figure 1 (b) shows the designed prototype of an ‘annular array electromagnet’. The three sets of coils are individually convolved with enamel insulation wire (Ø0.25 mm), and the circuits are connected in parallel with each other. They are equally distributed in a circumferential angle of 120°
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with respect to their axisymmetric orientation on a mild-steel disc. The end-face of the three sets of coils and the mild-steel disc are separated from each other by a small space (ad). As shown in Fig. 1(c), the magnetic spacing produces the total induced magnetic force (Fe) generated by energizing the three sets of coils to create a smooth and uniform magnetic damping effect on the mild-steel disc. A delivery wire-bobbin is designed at the center of the mild-steel disc (dotted line in Fig. 1 (c)) to be attracted synchronously by the disc. When the wire is pulled out from the wire-bobbin, the mild-steel disc acts on the end-face of the wire-bobbin via magnetic attractive force.
(a) Magnetic field generated (b) The evenly distributed coils (c) Magnetic damping force (Fe) Fig. 1 Design of the annular array electromagnet 3
2.2 Design of the electromagnetic damper coil To obtain precise cutting of microstructures, a tungsten wire of diameter Ø13 m, used in a wolfram lamp, is employed as the cutting wire-electrode in this study. The ultimate tensile stress of the microwire is 20 gf after tensile testing. The allowable stress is about 15 gf (safety factor set at 1.3), which informed the design of the coil number (n) for the electromagnet. Assuming that the annular array of the electromagnetic damper has a mechanical advantage (MA) of 3, a total induced magnetic force (Fe) of 0.05 N is required to resist a wire tension of 15 gf (F) (Eq. (2)). If the magnetic field of each electromagnet in the array is generated under a steady current (Im), according to the BiotSavart law (Jackson, 1999), the magnetic flux density (B) of all electromagnets can be calculated as 1.25×10-3 Wb/mm2, as shown in Eq. (3), where Iin is the input current of the three sets of
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electromagnets (3×0.66=1.982 A). Therefore, the coil number of a single set of electromagnets can be calculated as 500 (turns) by the Eq. (4) (Cheng, 1989). The coil number is set at 600 (turns) to improve used safety. The design-parameters of the electromagnet are listed in Table 1. F 15 , 3 , Fe 5 g 0.05 N Fe Fe
(2)
B
Fe 0.05 , B 1.25 10 3 Wb / mm2 I in x 2 20
B
n I in , 1.25 10 6 md ad
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0
n2 , n 500 (turns) 1 1 900 × 10 -9 4π × 10 -10
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MA
(3)
(4)
Table 1 Parameters of the designed electromagnet Conditions
Enamel insulated wire diameter (dc) Coil number (n) Horizontal component (x)
Ø0.25 mm 600 turns 20 mm
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Parameters
Magnetic permeability of the disc (µm) Magnetic permeability of air (µ0)
4π×10-7 H/mm
Thickness of mild-steel disc (md)
1 mm
Magnetic spacing (ad)
1 mm
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900×10-6 H/mm
2.3 Design of the annular array electromagnetic damper Figure 2(a) shows the design of the induced magnetic force on the mild-steel disc. The total induced magnetic force (Fe) generated by the three sets of electromagnets is uniformly distributed in the annular array. The magnetic damping moment (Te) generated by the induced magnetic force is expressed by Eq. (5), where Rw is the distance between the center of each electromagnet and the center of the wire-bobbin. The arrangement ensures each set of coils supports 1/3 of total induced current, balancing the stress on the disc and producing effective heat dispersion to secure a stablerunning effect. Figure 2(b) shows torque (Tr) of the wire-bobbin as obtained by Eq. (6), where rw is 4
the radius of the wire-bobbin. Wire tension (F) is obtained by Eq. (7) under static equilibrium. By substituting Eq. (3) into Eq. (7), we show that wire tension is positively correlated with total current (Iin) input into the three electromagnets (Eq. (8)). Equation (9) shows that micro-scale wire tension can be precisely regulated by finely tuning input current. Figure 2(c) shows the completed design of the annular array of the electromagnetic damper. Considering possible temperature rise of the electromagnets during operation, a heat-sink array is designed behind the electromagnetic damper to facilitate elimination of working heat. Te Fe Rw
(5) (6)
Tr F rw
Rw Fe rw
F
Rw Im B x rw
(7)
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F
(8)
F I m I in
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(9)
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(a) Magnetic damping effect (b) Generated wire tension (c) Heat dissipation design Fig. 2 Design of the annular array of electromagnetic damper
3. Relationship between electromagnetic damping force and wire tension
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In the microwire tension test, a precision counterpoise weight (W) is hung at the free end of a microwire of Ød diameter (Fig. 3(a)). The initial counterpoise weight is 1 gf. It is increased gradually by 0.5 gf for each test so that the relationship between the current input corresponding to wire tension can be found from the slope (Fig 3(b)). The power-supply used has a resolution of 0.001 A. When
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input current is at 0.01 A, the corresponding stable wire tension for a tungsten wire of Ø13 m diameter is obtained at 0.05 gf. This experiment shows that a microwire tension of 0.05 gf can be precisely and stably controlled. Assuming the weight (W) of the counterpoise can generate as much output as the electromagnetic damping force (Fe) against the mild-steel disc, and the wind-up shaft rotates at a steady constant speed to wind the wire with σa and σu being the allowable tensile stress and the ultimate tensile stress, respectively, then input current for the annular array electromagnetic damper should cause wire tension to produce four different statuses: (1) Loose wire tension when input current (Iin) to the electromagnetic damper is insufficient and magnetic damping moment (Te) applied to the mild-steel disc is smaller than wire-bobbin rotational 5
torque (Tr), Eq. (10).
Te Tr ,
4 Fe a d 2
(10)
(2) Appropriate wire tension for stable microwire delivery when input current (Iin) to the electromagnetic damper increases continuously and magnetic damping moment (Te) on the mild-steel disc is less than wire-bobbin rotational torque (Tr), Eq. (11). 4 Fe a d 2
(11)
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Te Tr ,
(3) Wire tension appropriate for cutting but wire is stationary when input current (Iin) to the electromagnetic damper increases to the point where magnetic damping moment (Te) applied to the mild-steel disc counteracts wire-bobbin rotational torque (Tr), Eq. (12).
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4 Fe u d 2
(12)
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Te Tr ,
4 Fe u d 2
(13)
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Te Tr ,
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(4) Wire-tension overcomes microwire strength and it breaks when input current (Iin) to the electromagnetic damper is too large and the magnetic damping torque (Te) applied to the mild-steel disc is greater than wire-bobbin rotational torque (Tr), Eq. (13).
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Figure 3(b) shows the relationship between total input current (Iin) and wire tension (F) in this design. The red trend curve shows an almost stable linear proportional relationship between the two. When current reaches 0.74 A, tension in the wire reaches maximal tensile stress and it is easily broken. A current of between 0.52 A and 0.60 A is appropriate for individual electromagnets to control wire-
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tension in a tungsten wire of Ø13 m diameter (Eq. (11)). In the electro-magnetic conversion process, as current runs through the coil generating damping force, the associated Joule heat resulting from eddy currents (Wang et al., 2016) causes the working temperature of the electromagnetic damper to rise. Therefore, a heat-sink array is designed behind the electromagnetic damper to maintain a working temperature of between 30-40 °C for an input power of 5 W (Fig. 3(c)).
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(a) Wire tension test (b) Input current and wire tension (c) Work temperature Fig. 3 The relationship between electromagnetic damping force and wire tension
4. Design of the horizontal cutting mechanism For micro w-EDM, in the delivery of an ultrathin wire, wire friction and wire-tension control are
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important issues. To transport a wire-electrode Ø30 m in diameter under lowered friction force, Klocke et al. (2004) designed a device with its own guides to help in bypassing some mechanical pulleys for transporting the ultra-thin wire. In this study, a horizontal cutting mechanism in which the wire tension is controlled by an annular array electromagnetic damper is designed and proposed to
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achieve the smooth delivery and stable discharge of a Ø13 m microwire (Fig. 4(a)). The annular array electromagnetic damper is positioned at the front of the mechanism. The microwire passes from the delivery wire-bobbin, through a groove roller, wire-electrode guide and auxiliary guide before arriving at the work area and being slowly wound up by the wind-up motor. The electromagnetic damping force of the mild-steel disc generates resistance opposing the direction of rotation once the wind-up motor is enabled. Hence, the microwire can be tightened due to reverse resistance coming from the damper. Figure 4(b) shows the finished cutting mechanism. To precisely regulate the feed-
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rate of the microwire electrode to avoid discharge short-circuiting, a microwire servo motion, where real-time voltage crossing the discharge gap is taken as a parameter to reflect gap conditions, is designed between the workpiece and electrode wire in the desktop machine tool. Figure 4(c) shows the horizontal cutting mechanism being mounted on the machine tool by two taper pins.
(a) Design cutting mechanism (b) Finished cutting mechanism (c) Finished machining system Fig. 4 Design of the horizontal cutting mechanism and the finished developed system
5. Experiments 5.1 Experimental planning To evaluate the feasibility of the designed ‘electromagnetic damping-controlled horizontal cutting 7
mechanism’, Boron-doped Nano-Polycrystalline Diamond (B-NPD) is employed as the material for cutting. The nature of this material’s low electro-thermal machinability makes it ideal for testing the relevant merits and performance of this study’s developed machining mechanism. The physical properties associated with B-NPD are listed in Table 2. An inverted-cutting method whereby the microwire is located at the bottom of the workpiece is proposed and implemented so that spontaneous removal of discharge-debris will occur due to gravity. The slot-width (Wf) of trial cutting and materialremoval volume (Vvol) are expressed in Eqs. (14) and (15), where L, tgap and tm are the total depth of cutting, the discharge gap on a single side, and the workpiece thickness, respectively. The sought parameters include suitable work capacitance (C), wire tension (F) and wire feed-rate (Fr). The temporary machining conditions are listed in Table 3.
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Table 2 B-NPD physical properties (FACT, 2019) Parameters
Conditions
Grain size Hardness
100~500 nm 110~130 GPa
8 MPa m < 100 nm ~1600 °C 50~370 Ω·m 680~880 W/m·K
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Toughness Edge accuracy Heat resistance Electrical resistivity Thermal conductivity
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Table 3 Machining conditions for B-NPD cutting test Parameters
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Workpiece material Wire material Wire tension (F) Wire feed-rate (Fr) Running speed (Vr) Capacitor voltage (Vc) Work capacitance (C) Work frequency (fdis) Total depth of cutting (L) Workpiece thickness (tm) Dielectric
Conditions B-NPD Tungsten wire (Ø13μm) (-) 8, 10, 12 gf 0.01, 0.015, 0.02, 0.025, 0.03 mm/min 36 mm/min 100 V 200, 330, 470 pF 1 MHz 200 μm 500 μm Lamp-oil
W f 0.013 2 t gap
Vvol
Wf W f L 2
(14)
W f 4
2
tm
(15) 8
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5.2 The appropriate wire tension When the wire draws back, short-circuiting reduces and the insulation gap between the wire and workpiece is re-established. The wire drawing back, however, could result in unwanted secondary discharge events leading to a wider than desirable slot being cut through the workpiece. The level at which these unwanted secondary discharge events occur is governed by wire tension. In this experiment, a study for optimal wire tension is carried out on a previously constructed desktop microtooling machine (Chen and Lin, 2011). The test conditions are listed in Table 3. When tension is 6 gf, the cut slot’s width is 27 μm, and the discharge gap is 7 μm (Fig. 5(a)). This result indicates wire tension is too loose resulting in poor form accuracy and slot size. At a wire tension of 8 gf, slot-width narrows to 20.6 μm and the gap 3.8 μm (Fig. 5(b)), indicating wire-sway has been significantly reduced. At a wire tension of 10 gf, slot-width is further reduced to 15.4 μm and the corresponding gap reaches just 1.2 μm (Fig. 5(c)), implying effective suppression of wire vibration giving much
(a) Wire tension of 6 gf
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improved slot narrowness and straighter slot walls. Further increasing wire tension to 12 gf increases the probability of wire breakage due to wire necking and high temperature spark erosion.
(b) Wire tension of 8 gf
(c) Wire tension of 10 gf
Fig. 5 Experiment for appropriate wire tension
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5.3 Capacitance erosion capability The capacitor releases its electric charge (Qdis) per unit of time as an exponential decay equation (Eq. (16)). In the case of a single discharge, maximum energy release of the capacitor occurs in the
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initial stage of the discharge function curve (about 1, i.e. one time-constant), at which point the voltage (Vc) of the capacitor drops instantaneously by about 63.2 % and energy released (Es) is about 86.46 % of maximum potential release (Bishop, 2011) (Eq. (17)). This means the capacitor can create a very high-peak current (Ip) in a very short time and the discharge forms a very high-speed plasma beam with a temperature up to 8,000~12,000 °C (Sommer, 2000). Different machined materials have different melting-points and electrical resistivity, also the energies released (according to capacitance) may have different material erosion rates (Chen and Chen, 2017). This difference is defined as Capacitance Erosion Capability (CEC)’ in this study. It is the amount of material that can be removed by unit of electricity released by the capacitor in mm3/Coulomb. In other words, it is the volume of removed-material (Vvol) per unit of electric charge (Qdis) (Eq. (18)). The lower the CEC, the more coulombs of electricity required to melt the same volume of material. Hence, the total electric charge (Qtotal) released by the capacitor per unit of time can be obtained by the amount of electric charge per 9
unit of time (Qdis) input from the capacitor and work frequency (fdis) (Eq. (19)). By evaluating CEC, a suitable work capacitance for precisely cutting a B-NPD microstructure can be attained.
Qdis C VC e
t RC
(16)
1 CV 2 2
(17)
Vvol Q
(18)
Qtotal Qdis f dis
(19)
Es
CEC
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5.4 The appropriate work capacitance Experimental testing to determine appropriate work capacitance is carried out with capacitors of 200 pF, 330 pF and 470 pF, respectively. Table 3 lists the remaining machining conditions. Figures 6(a) and 6(b) show the corresponding total amount of electric charge (Qtotal) released. The results of two experiments show that a work capacitance of 330 pF has better capacitance erosion capability for B-NPD although its Material Removal Rate (MRR) is slightly lower than 470 pF (Fig. 6(c)). This implies that with a work capacitance of 330 pF, the wire can more effectively cut B-NPD microstructures. In addition, the amount of coulomb released per unit of time is low at a work capacitance of 200 pF, but it has a higher capacitance erosion capability and lower MRR, inferring that a work capacitance of 200 pF is more suitable for finish-cutting than other tested capacitances.
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(a) Amount of electric charge (b)Capacitance erosion capability Fig. 6 Appropriate work capacitance
(c) Material removal rate
5.5 The proper wire feed-rate in rough-cutting In this set of experiments to determine the optimal feed-rate for rough cutting five wire feed-rates: 0.01, 0.015, 0.02, 0.025 and 0.03 mm/min are examined (Fig. 7(a)). Figure 7(b) lists examined slotwidths and their corresponding straightness. Better straightness of slot can be obtained at feed-rates of 0.01, 0.015 and 0.02 mm/min, indicating that a wire feed-rate in this range is appropriate for machining B-NPD material. Slot-width consistency diminishes significantly when the wire feed-rate is increased to 0.025 and 0.03 mm/min used. An excessively fast wire feed-rate tends to cause the discharge gap to narrow, resulting in worse debris expulsion, increased probability of arc discharging, 10
and wire-drawback events. This leads to the slot-wall surface being more wave patterned. Experimental results demonstrate that the optimally efficient rough-cutting feed-rate for slot straightness is obtained at a feed-rate of 0.02 mm/min (Fig. 7(c)).
(a) Scheme of the rough-cutting (b) Straightness of the cut slot (c) Slot-width (F=0.02 mm/min)
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Fig. 7 Optimal wire feed-rate in rough-cutting
The above experimental results show B-NPD microstructures can be precisely cut by a tungsten wire
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of Ø13m under the developed horizontal cutting mechanism with electromagnetic damping control. The most suitable wire tension is 10 gf. The proper work capacitance and wire feed-rate are 330 pF and 0.02 mm/min in rough-cutting and 200 pF and 0.05 mm/min in finish-cutting, respectively.
6. Experimental verification
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Odake et al. (2009) reported that B-NPD is highly suitable for use as a material for cutting tools due to its higher hardness, toughness, abrasive resistivity and heat resistance than general polycrystalline diamond. To verify and confirm the feasibility of the designed ‘electromagnetic damping-controlled horizontal cutting mechanism in the machining of a B-NPD workpiece, a micro
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diamond tool with a trapezoidal-shaped microstructure plate array of 20 m at its tip and an aspectratio of 1:20 is designed and cut (Fig. 8(a)). Such a structure is suitable for transcribing microgroove arrays (Yan et al., 2009) and micropyramid arrays (Fig. 8(b)). A 'one-cut one-skim' machining approach is used whereby the first path is used to cut open the workpiece while the second path performs finish-cutting on the slot-wall to improve surface flatness and eliminate dimensional errors. Table 4 lists the remaining machining conditions. The resultant microstructure array is highly consistent with very little waviness of cut (Fig. 8(c)).
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Table 4 Machining conditions for B-NPD microstructure cutting Parameters Workpiece material Wire material Wire tension (F) Wire feed-rate (Fr) Running speed (Vr) Capacitor voltage (Vc)
Conditions B-NPD Tungsten wire (Ø13μm) (-) 10 gf 0.02 mm/min (Rough-cutting) 0.05 mm/min (Finish-cutting) 36 mm/min 100 V 11
Work capacitance (C)
200 pF (Finish-cutting) 1 MHz 500 μm Lamp-oil
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Work frequency (fdis) Workpiece thickness (tm) Dielectric
330 pF (Rough-cutting)
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(a) B-NPD microstructure array (b) Microgroove array (c) The finished microstructure design transcribing simulation array with high-consistency Fig. 8 Verification of B-NPD microstructure array cutting
7. Discussion
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7.1 Degrees of freedom in microwire running To combat wire-swaying, a 10 μm slot is cut into a tungsten carbide rode to act as a wire guide. This guide is in addition to the machine’s existing wire guide and auxiliary guide designs. The slot is cut using low energy on the surface of a tungsten carbide rod (no electricity) in the working area (Fig. 9(a)). The wire-guide is used in combination with the electromagnetic damper to limit the wire to one
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degree of freedom along the direction of the running wire. Trial results show that when the wire passes over the rod without a microgroove to constrain wire-sway, slight swaying of the wire causes discharge short-circuiting and side-erosion to occur in the cut slot of the workpiece (Fig. 9(b)). By contrast, when the slot in the tungsten carbide guide rod is used, the wire is firmly restrained by the microgroove, reducing the potential for wire-sway and vibration (Fig. 9(c)). By utilizing the designed microgroove, even if the wire breaks during machining, after rethreading, the wire can continue to operate accurately on the original path, creating improved microwire cutting performance with highstraightness and reproducibility.
(a) Microgroove design
(b) Without microgroove assist 12
(c) With a microgroove assist
Fig. 9 Degrees of freedom in microwire running 7.2 Effect of skim on the slot-wall Inverted-cutting facilitates discharge-debris removal from the narrow discharge gap (Hwang et al., 2010; Chen and Chang, 2013) and the use of low-energy cutting reduces excessive Joule heating in the microstructure workpiece (Luo, 1995); however, the use of low-energy machining means the discharge gap can be overly narrow, which leads to debris expulsion difficulties and an increased possibility of short-circuiting and secondary discharging. This results in an uneven slot-wall surface because of increased wire-drawback. A 'one-cut one-skim' strategy for improving the geometrical accuracy of the slot-wall surface is proposed and schemed (Fig. 10(a)). At the ‘one-cut’ stage, the microstructure is first cut open according to a planned path using higher discharge energy and a lower wire feed-rate. Figure 10(b) shows the machined microstructure with an uneven cut surface after the
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initial one-cut process. At the ‘one-skim’ stage, a new cutting path is established based on the original path whereby the wire-electrode is offset at a micro distance to trim as little as 1-2 m of material from the rough-wall. One-skim cutting is implemented under a lower discharge energy and higher wire feed-rate. The resultant slot-width (Wf) is expressed in the Eq. (20), where Soffset is the offset
(20)
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W f W 2Soffset , Soffset 2 m
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distance of the wire-electrode equal to less than 2 m. Low-energy cutting is helpful in removing the protrusions and micro erosion-scars from the rough-wall, improving the flatness of the slot-wall (Fig. 10(c)). In order to maintain slot-width dimensional accuracy, offset distance of the wire must be controlled at an extremely low micro level. This demonstrates that the developed ‘electromagnetic damping controlled horizontal cutting mechanism’ can tightly and stably control the micro offset distance of the microwire.
(a) one-cut one-skim strategy (b) After one-cut processing (c) After one-skim processing Fig. 10 Effect of skim on the slot-wall 7.3 Effect of wire feed-rate on surface roughness At the one-skim stage, four different wire feed-rates higher than that of the one-cut stage are tested to confirm cutting performance at the skim stage. When the wire feed-rate is Fr=0.025 mm/min, the corresponding surface roughness is Ra0.082 m, meaning discharge energy can effectively act on 13
pockmark protrusions, making the pockmarks shallower (Fig. 11(a)). With an increase in feed-rate to Fr=0.05 mm/min, roughness of the machined surface is improved to a level of Ra0.088 m (Fig. 11(b)), demonstrating that MRR is still close to that corresponding to a wire feed-rate of 0.025 mm/min, even though the feed-rate has doubled. When the feed-rate is increased to Fr=0.075 mm/min, the
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corresponding surface roughness is Ra0.171 m, suggesting spark erosion energies at such a wire feedrate do not effectively act on the protrusions around discharge-pockmarks; i.e., a worsened surface roughness result (Fig. 11(c)). An unduly high wire feed-rate of 0.1 mm/min leads to the discharge gap shrinking, increasing the probability of arc discharging, abnormal discharging, and short-circuiting (Fig. 11(d)).
(b) Fr = 0.05 mm/min
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(a) Fr = 0.025 mm/min
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(c) Fr = 0.075 mm/min (d) Fr = 0.1 mm/min Fig. 11 Effect of skim on cutting performance
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8. Conclusion
An original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM is successfully developed and proposed in this study. The cutting mechanism has a high-degree of stability in the control of an extremely fine wire of Ø13m in diameter during cutting of difficult-tomachine materials. Both quantitative and qualitative approaches to the microwire-cutting process have been studied to provide complete information on cutting B-NPD microstructures. The conclusions from this study are as follows: 1.
An electromagnetic damper with a coil design of 600-turns is able to stably and precisely control the wire tension of Ø13 m micro tungsten wire. 14
2.
Compared to tension control by a permanent magnet, the electromagnetic damper possesses wire
3.
tension with high controllability. There is no need to adjust magnetic spacing. The designed horizontal cutting mechanism can suppress potential wire-swaying and vibration.
4. 5. 6.
The appropriate wire tension is between 10-12 gf corresponding to a sway error within 1 m. Through the evaluation of capacitance erosion capability, suitable work capacitances for cutting B-NPD microstructures are established at 330 pF for rough-cutting and 200 pF for finish-cutting. A ‘one-cut one-skim’ strategy makes discharge-debris expulsion easy. In addition, it is shown that the wire feed-rate in the one-skim stage can be higher than that of the one-cut stage. Experimental results confirm that material with low electro-thermal machinability can be precisely cut by the proposed technology since microwire tension can be stably controlled.
Acknowledgement
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The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan for financially supporting this research under Contract No. MOST 106-2221-E-003-017-MY2.
Reference
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Alhadeff, L.L., Curtis, D.T., Marshall, M.B., Slatter, T., 2018. The Application of Wire Electrical Discharge Machining (WEDM) in the Prototyping of Miniature Brass Gears. Procedia CIRP 77, 642-645. Beltrami, I., Bertholds, A., Dauw, D., 1996. A simplified post process for wire cut EDM. Journal of Materials Processing Technology, 58, 385-389. Bishop, O., 2011. Electronics - circuits and systems, 4th Edition, Elsevier Ltd., 31-38. Bouquet, J, Hensgen, L, Klink, A, Jacobs, T, Klocke, F, Lauwers, B, 2014. Fast production of gear
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prototypes - a comparison of technologie. Procedia CIRP, 14, 77-82. Cadwell, L.H., 1996. Magnetic damping: Analysis of an eddy current brake using an airtrack. American Journal of Physics, 64, 917-923. Chen, S.T. 2007. A high-efficiency approach for fabricating mass micro holes by batch micro EDM. Journal of Micromechanics and Microengineering, 17, 1961-1970. Chen, S.T., 2008. Fabrication of high-density micro holes by upward batch micro EDM. Journal of Micromechanics and Microengineering, 18, 085002, (9pp). Chen, S.T., Lin, S.J., 2011. Development of an extremely thin grinding-tool for grinding microgrooves in optical glass. Journal of Materials Processing Technology, 211, 1581-1589. Chen, S.T., Lai, Y.C., 2012. Development of micro co-axial diamond wheel-tool array using a hybrid process of electrochemical co-deposition and RWEDM technique. Journal of Materials Processing Technology, 212, 2305-2314. Chen, S.T., Chang, C.H., 2013. Development of an ultrathin BD-PCD wheel-tool for in-situ microgroove generation on NAK80 mold steel. Journal of Materials Processing Technology, 213, 740-751. Chen, S.T., Chen, C.H., 2017. Development of a novel micro w-EDM power source with a multiple Resistor-Capacitor (mRC) relaxation circuit for machining high-melting point, -hardness and 15
resistance materials. Journal of Materials Processing Technology, 240, 370-381. Chen, Z., Zhang, G., Yan, H., 2018. A high-precision constant wire tension control system for improving workpiece surface quality and geometric accuracy in WEDM. Precision Engineering, 54, 51-59. Cheng, D.K., 1989. Field and Wave Electromagnetics, 2nd Edition, Addison-Wesley Longman, 264266.
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FACT abrasives Taiwan Co. LTD., 2019. http://www.factdiamond.com/ Habib, S., Okada, A. 2016. Study on the movement of wire electrode during fine wire electrical discharge machining process. Journal of Materials Processing Technology, 227, 147-152. Hwang, Y.L. Kuo, C.L., Hwang, S.F., 2010. Fabrication of a micro-pin array with high density and high hardness by combining mechanical peck-drilling and reverse-EDM, Journal of Materials Processing Technology, 210, 1103-1130.
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Jackson, J.D., 1999. Classical Electrodynamics. 3rd Edition, John Wiley & Sons, New York. Chapter 5. Kinoshita, H., Hayashi, Y., 1994. Study in micro-wire EDM, In: EDM Technology 2. EDM Technol Transfer 24-29. Klocke, F., Lung, D., Thomaidis, D., Antonoglou, G., 2004. Using ultra thin electrodes to produce micro-parts with wire-EDM. Journal of Materials Processing Technology, 149, 579-584. Luo, Y.F., 1995. An energy-distribution strategy in fast-cutting wire EDM. Journal of Materials Processing Technology, 55, 380-390. Obara, H., Izumiya, S., 1991. Apparatus for wire tension control and disconnection detection. US Patent: 5039834.
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Odake, S., Ohfuji, H., Okuchi, T., Kagi, H., Sumiya, H., Irifune, T., 2009. Pulsed laser processing of nano-polycrystalline diamond: A comparative study with single crystal diamond. Diamond & Related Materials, 18, 877-880. Puri, A.B., B. Bhattacharyya, 2003. Modelling and analysis of the wire-tool vibration in wire-cut EDM. Journal of Materials Processing Technology, 141, 295-301. Sommer, C., 2000. Non-traditional machining handbook, Advance Publishing, Inc., 117-124. Wang, Y., Tian, G.Y., Gao, B., 2016. Eddy Current and Thermal Propagation for Quantitative NDT&E. 19th World Conference on Non-Destructive Testing, 9pp. Yan, J., Oowada, T., Zhou, T., Kuriyagawa, T., 2009. Precision machining of microstructures on electroless-plated NiP surface for molding glass components. Journal of Materials Processing Technology, 209, 4802-4808.
16
PII:
S0924-0136(19)30511-4
DOI:
https://doi.org/10.1016/j.jmatprotec.2019.116538
Reference:
PROTEC 116538
To appear in:
Journal of Materials Processing Tech.
Received Date:
24 June 2019
Revised Date:
29 October 2019
Accepted Date:
30 November 2019
Please cite this article as: Chen S-Tong, Huang L-Wen, Kuo J-Pin, Pai T-Cheng, Development of an original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM, Journal of Materials Processing Tech. (2019), doi: https://doi.org/10.1016/j.jmatprotec.2019.116538
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Development of an original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM Shun-Tong Chen1, Li-Wen Huang2, Jin-Pin Kuo2, Tin-Cheng Pai2 1 Professor, Department of Mechatronic Engineering, National Taiwan Normal University 2 Graduate student, Department of Mechatronic Engineering, National Taiwan Normal University E-mail: [email protected]
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Abstract In this study, an original ‘electromagnetic damping-controlled horizontal cutting mechanism’ is designed and proposed for precisely controlling micro-scale wire-tension for cutting a microstructure array. The electromagnetic damper uses a set of three annular electromagnets equally distributed with their end-faces orientated toward a mild-steel disc. This arrangement is located at the front of the cutting mechanism. The cutting mechanism also consists of a set of microgroove rollers, a wireelectrode guide, and an auxiliary guide designed to suppress wire-wriggling and wire-swaying in
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order to deliver a tungsten wire of Ø13 m diameter at a steady state over the long-term. Experimental verification is conducted on B-NPD (boron-doped nano-polycrystalline diamond), which possesses a high melting-point and high electrical resistivity characteristics, to establish the feasibility of cutting such difficult-to-machine materials. A 'one-cut one-skim' machining approach is used whereby the surface flatness and the dimensional accuracy of the slot-wall can be improved. Experimental results found that the wire feed-rate during the finish-cutting stage can be used at a rate greater than that of the rough-cutting stage. The resultant diamond microstructure array is of high-consistency and aspectratio at 1:22, demonstrating that the electromagnetic damping-controlled horizontal cutting mechanism can precisely and stably control the tension and running speed of the microwire.
Nomenclature:
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Keywords: Electromagnetic damper, horizontal cutting mechanism, microwire tension, one-cut oneskim, B-NPD microstructure array
Rw= distance between the electromagnet center and delivery wire-bobbin center rw= wire-bobbin radius Soffset= offset distance of the wire-electrode Te= magnetic damping moment
d= wire-electrode diameter dc= Enamel insulated wire diameter Es= single-discharge energy F= wire tension Fe= total induced magnetic force Fr = wire feed-rate fdis = work frequency
Tr= torque of the wire-bobbin tgap= single-side discharge gap ttotal= total time of processing tm= workpiece thickness Vc= capacitor voltage Vm= induced electromotive force Vr= wire running speed
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ad= Magnetic spacing between the bobbin end and mild-steel disc B= magnetic flux density in the electromagnet C= work capacitance CEC= Capacitance Erosion Capability
1
Iin= total input current
Vvol= removed-material volume
Im= input current from single electromagnet Ip= peak current L= total depth of cutting
W= slot-width before one-skim Wf= slot-width after one-skim x= horizontal component of the radius of action of the magnetic force σa= allowable tensile stress of wire-electrode σu= ultimate tensile stress of wire-electrode = time-constant in R-C circuit ω= angular velocity of mild-steel disc θ= included angle of magnetic force area µ0= permeability of vacuum µm= magnetic permeability of the disc
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MA= Mechanical Advantage MRR= Material Removal Rate md= thickness of mild-steel disc n= coil number in single electromagnet Qdis= electric charge per unit of time Qtotal= total electric charge per unit of time R= resistance in the circuit
1. Introduction
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Driven by trends in product miniaturization, many associated micromanufacturing technologies have been developed whose corresponding tools, jigs and measuring tools need also be miniaturized. Wire-EDM (Wire Electrical Discharge Machining) is often used to machine gears and dies for many industries (Bouquet et al., 2014; Alhadeff et al., 2018). Miniature tools such as a micro-punch array and micro-electrode array can also be precisely cut by the microwire-EDM approach (Chen, 2008). When performing microwire electrical discharge machining (microwire-EDM), the tension of the microwire plays a key role in the outcome of cutting processes. The microwire may readily snap or fuse-break due to necking when a tight wire tension is used. On the other hand, excessively loose wire tension can reduce the geometrical accuracy of the workpiece. Due to the microwire’s low tensile
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strength, traditional methods of controlling wire tension by use of electromagnetic brake rollers have had difficulty controlling wire tension (Obara and Izumiya, 1991). Previous researchers have experimented with the best ways of controlling microwire-tension during w-EDM. Kinoshita and Hayashi (1994) used a laser beam to measure the lever through which the wire passes with wire tension adjusted based on the feedback angle of the lever. Beltrami et al. (1996) employed an optical wire position sensor, placed between the upper wire guide and the workpiece, to continuously measure wire deflection. Magnetic repulsive force has been used to control wire tension such as in Chen and Lai (2012), who employed a permanent magnetic disc to control wire tension. Although the control of microwire tension by magnetic spacing is advantageous in that there is no contact nor wear of the wire, the resolution of any adjustments has been inadequate. This phenomenon is due to the disc swaying because of backlash from the screw thread (Chen, 2007), which results in uneven damping and a non-steady-state magnetic force. Puri and Bhattacharyya (2003) thought that the solution to wire vibration could be achieved mainly by manipulating the first order mode (n=1) during machining. Under this scenario, the density of spark erosion is affected by wire-vibration mode. Habib and Okada (2016) also reported wire vibration including intense 1st and 3rd-order mode vibrations when fine cutting a thin workpiece. By means of a Proportional Integral Derivative (PID) closed-loop control system and intelligent algorithms, Chen et al. (2018) constructed a constant wire 2
tension control system to restrain wire deflection and vibration. In sum, the demands of miniaturization to conform with the functionality requirements of energysavings, cost effectiveness, and attractive appearance create inevitable challenges for the development of suitable machining tools to meet such needs. In this study, in order to precisely and stably control microwire tension to create narrow slot-widths and tight discharge gaps during machining, an original ‘electromagnetic damping-controlled horizontal cutting mechanism’ is designed and proposed. Magnetic attractive force between electromagnet and wire-bobbin can be finely manipulated by slight adjustments to the input current of the electromagnetic damper. Additionally, the following aspects are evaluated in detail: wire running mechanism, capacitance erosion capability, dimensional accuracy of resultant microstructure array, surface topography and roughness.
2. Methodology
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2.1 Design of the annular array electromagnet The setup of the annular array electromagnet is shown in Figures 1 (Fig. 1(a) (in front view)). The induced electromotive force (Vm) generated by a magnetic field can be expressed by Eq. (1) (Cadwell, 1996), where x, ω, and θ are the horizontal component of the radius of action of the magnetic force, the angular velocity of the disc, and the angle of the magnetic action area, respectively. (1)
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Bx 2 Vm cos 2
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Figure 1 (b) shows the designed prototype of an ‘annular array electromagnet’. The three sets of coils are individually convolved with enamel insulation wire (Ø0.25 mm), and the circuits are connected in parallel with each other. They are equally distributed in a circumferential angle of 120°
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with respect to their axisymmetric orientation on a mild-steel disc. The end-face of the three sets of coils and the mild-steel disc are separated from each other by a small space (ad). As shown in Fig. 1(c), the magnetic spacing produces the total induced magnetic force (Fe) generated by energizing the three sets of coils to create a smooth and uniform magnetic damping effect on the mild-steel disc. A delivery wire-bobbin is designed at the center of the mild-steel disc (dotted line in Fig. 1 (c)) to be attracted synchronously by the disc. When the wire is pulled out from the wire-bobbin, the mild-steel disc acts on the end-face of the wire-bobbin via magnetic attractive force.
(a) Magnetic field generated (b) The evenly distributed coils (c) Magnetic damping force (Fe) Fig. 1 Design of the annular array electromagnet 3
2.2 Design of the electromagnetic damper coil To obtain precise cutting of microstructures, a tungsten wire of diameter Ø13 m, used in a wolfram lamp, is employed as the cutting wire-electrode in this study. The ultimate tensile stress of the microwire is 20 gf after tensile testing. The allowable stress is about 15 gf (safety factor set at 1.3), which informed the design of the coil number (n) for the electromagnet. Assuming that the annular array of the electromagnetic damper has a mechanical advantage (MA) of 3, a total induced magnetic force (Fe) of 0.05 N is required to resist a wire tension of 15 gf (F) (Eq. (2)). If the magnetic field of each electromagnet in the array is generated under a steady current (Im), according to the BiotSavart law (Jackson, 1999), the magnetic flux density (B) of all electromagnets can be calculated as 1.25×10-3 Wb/mm2, as shown in Eq. (3), where Iin is the input current of the three sets of
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electromagnets (3×0.66=1.982 A). Therefore, the coil number of a single set of electromagnets can be calculated as 500 (turns) by the Eq. (4) (Cheng, 1989). The coil number is set at 600 (turns) to improve used safety. The design-parameters of the electromagnet are listed in Table 1. F 15 , 3 , Fe 5 g 0.05 N Fe Fe
(2)
B
Fe 0.05 , B 1.25 10 3 Wb / mm2 I in x 2 20
B
n I in , 1.25 10 6 md ad
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n2 , n 500 (turns) 1 1 900 × 10 -9 4π × 10 -10
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MA
(3)
(4)
Table 1 Parameters of the designed electromagnet Conditions
Enamel insulated wire diameter (dc) Coil number (n) Horizontal component (x)
Ø0.25 mm 600 turns 20 mm
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Parameters
Magnetic permeability of the disc (µm) Magnetic permeability of air (µ0)
4π×10-7 H/mm
Thickness of mild-steel disc (md)
1 mm
Magnetic spacing (ad)
1 mm
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900×10-6 H/mm
2.3 Design of the annular array electromagnetic damper Figure 2(a) shows the design of the induced magnetic force on the mild-steel disc. The total induced magnetic force (Fe) generated by the three sets of electromagnets is uniformly distributed in the annular array. The magnetic damping moment (Te) generated by the induced magnetic force is expressed by Eq. (5), where Rw is the distance between the center of each electromagnet and the center of the wire-bobbin. The arrangement ensures each set of coils supports 1/3 of total induced current, balancing the stress on the disc and producing effective heat dispersion to secure a stablerunning effect. Figure 2(b) shows torque (Tr) of the wire-bobbin as obtained by Eq. (6), where rw is 4
the radius of the wire-bobbin. Wire tension (F) is obtained by Eq. (7) under static equilibrium. By substituting Eq. (3) into Eq. (7), we show that wire tension is positively correlated with total current (Iin) input into the three electromagnets (Eq. (8)). Equation (9) shows that micro-scale wire tension can be precisely regulated by finely tuning input current. Figure 2(c) shows the completed design of the annular array of the electromagnetic damper. Considering possible temperature rise of the electromagnets during operation, a heat-sink array is designed behind the electromagnetic damper to facilitate elimination of working heat. Te Fe Rw
(5) (6)
Tr F rw
Rw Fe rw
F
Rw Im B x rw
(7)
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F
(8)
F I m I in
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(9)
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(a) Magnetic damping effect (b) Generated wire tension (c) Heat dissipation design Fig. 2 Design of the annular array of electromagnetic damper
3. Relationship between electromagnetic damping force and wire tension
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In the microwire tension test, a precision counterpoise weight (W) is hung at the free end of a microwire of Ød diameter (Fig. 3(a)). The initial counterpoise weight is 1 gf. It is increased gradually by 0.5 gf for each test so that the relationship between the current input corresponding to wire tension can be found from the slope (Fig 3(b)). The power-supply used has a resolution of 0.001 A. When
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input current is at 0.01 A, the corresponding stable wire tension for a tungsten wire of Ø13 m diameter is obtained at 0.05 gf. This experiment shows that a microwire tension of 0.05 gf can be precisely and stably controlled. Assuming the weight (W) of the counterpoise can generate as much output as the electromagnetic damping force (Fe) against the mild-steel disc, and the wind-up shaft rotates at a steady constant speed to wind the wire with σa and σu being the allowable tensile stress and the ultimate tensile stress, respectively, then input current for the annular array electromagnetic damper should cause wire tension to produce four different statuses: (1) Loose wire tension when input current (Iin) to the electromagnetic damper is insufficient and magnetic damping moment (Te) applied to the mild-steel disc is smaller than wire-bobbin rotational 5
torque (Tr), Eq. (10).
Te Tr ,
4 Fe a d 2
(10)
(2) Appropriate wire tension for stable microwire delivery when input current (Iin) to the electromagnetic damper increases continuously and magnetic damping moment (Te) on the mild-steel disc is less than wire-bobbin rotational torque (Tr), Eq. (11). 4 Fe a d 2
(11)
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Te Tr ,
(3) Wire tension appropriate for cutting but wire is stationary when input current (Iin) to the electromagnetic damper increases to the point where magnetic damping moment (Te) applied to the mild-steel disc counteracts wire-bobbin rotational torque (Tr), Eq. (12).
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4 Fe u d 2
(12)
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Te Tr ,
4 Fe u d 2
(13)
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Te Tr ,
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(4) Wire-tension overcomes microwire strength and it breaks when input current (Iin) to the electromagnetic damper is too large and the magnetic damping torque (Te) applied to the mild-steel disc is greater than wire-bobbin rotational torque (Tr), Eq. (13).
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Figure 3(b) shows the relationship between total input current (Iin) and wire tension (F) in this design. The red trend curve shows an almost stable linear proportional relationship between the two. When current reaches 0.74 A, tension in the wire reaches maximal tensile stress and it is easily broken. A current of between 0.52 A and 0.60 A is appropriate for individual electromagnets to control wire-
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tension in a tungsten wire of Ø13 m diameter (Eq. (11)). In the electro-magnetic conversion process, as current runs through the coil generating damping force, the associated Joule heat resulting from eddy currents (Wang et al., 2016) causes the working temperature of the electromagnetic damper to rise. Therefore, a heat-sink array is designed behind the electromagnetic damper to maintain a working temperature of between 30-40 °C for an input power of 5 W (Fig. 3(c)).
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(a) Wire tension test (b) Input current and wire tension (c) Work temperature Fig. 3 The relationship between electromagnetic damping force and wire tension
4. Design of the horizontal cutting mechanism For micro w-EDM, in the delivery of an ultrathin wire, wire friction and wire-tension control are
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important issues. To transport a wire-electrode Ø30 m in diameter under lowered friction force, Klocke et al. (2004) designed a device with its own guides to help in bypassing some mechanical pulleys for transporting the ultra-thin wire. In this study, a horizontal cutting mechanism in which the wire tension is controlled by an annular array electromagnetic damper is designed and proposed to
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achieve the smooth delivery and stable discharge of a Ø13 m microwire (Fig. 4(a)). The annular array electromagnetic damper is positioned at the front of the mechanism. The microwire passes from the delivery wire-bobbin, through a groove roller, wire-electrode guide and auxiliary guide before arriving at the work area and being slowly wound up by the wind-up motor. The electromagnetic damping force of the mild-steel disc generates resistance opposing the direction of rotation once the wind-up motor is enabled. Hence, the microwire can be tightened due to reverse resistance coming from the damper. Figure 4(b) shows the finished cutting mechanism. To precisely regulate the feed-
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rate of the microwire electrode to avoid discharge short-circuiting, a microwire servo motion, where real-time voltage crossing the discharge gap is taken as a parameter to reflect gap conditions, is designed between the workpiece and electrode wire in the desktop machine tool. Figure 4(c) shows the horizontal cutting mechanism being mounted on the machine tool by two taper pins.
(a) Design cutting mechanism (b) Finished cutting mechanism (c) Finished machining system Fig. 4 Design of the horizontal cutting mechanism and the finished developed system
5. Experiments 5.1 Experimental planning To evaluate the feasibility of the designed ‘electromagnetic damping-controlled horizontal cutting 7
mechanism’, Boron-doped Nano-Polycrystalline Diamond (B-NPD) is employed as the material for cutting. The nature of this material’s low electro-thermal machinability makes it ideal for testing the relevant merits and performance of this study’s developed machining mechanism. The physical properties associated with B-NPD are listed in Table 2. An inverted-cutting method whereby the microwire is located at the bottom of the workpiece is proposed and implemented so that spontaneous removal of discharge-debris will occur due to gravity. The slot-width (Wf) of trial cutting and materialremoval volume (Vvol) are expressed in Eqs. (14) and (15), where L, tgap and tm are the total depth of cutting, the discharge gap on a single side, and the workpiece thickness, respectively. The sought parameters include suitable work capacitance (C), wire tension (F) and wire feed-rate (Fr). The temporary machining conditions are listed in Table 3.
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Table 2 B-NPD physical properties (FACT, 2019) Parameters
Conditions
Grain size Hardness
100~500 nm 110~130 GPa
8 MPa m < 100 nm ~1600 °C 50~370 Ω·m 680~880 W/m·K
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Toughness Edge accuracy Heat resistance Electrical resistivity Thermal conductivity
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Table 3 Machining conditions for B-NPD cutting test Parameters
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Workpiece material Wire material Wire tension (F) Wire feed-rate (Fr) Running speed (Vr) Capacitor voltage (Vc) Work capacitance (C) Work frequency (fdis) Total depth of cutting (L) Workpiece thickness (tm) Dielectric
Conditions B-NPD Tungsten wire (Ø13μm) (-) 8, 10, 12 gf 0.01, 0.015, 0.02, 0.025, 0.03 mm/min 36 mm/min 100 V 200, 330, 470 pF 1 MHz 200 μm 500 μm Lamp-oil
W f 0.013 2 t gap
Vvol
Wf W f L 2
(14)
W f 4
2
tm
(15) 8
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5.2 The appropriate wire tension When the wire draws back, short-circuiting reduces and the insulation gap between the wire and workpiece is re-established. The wire drawing back, however, could result in unwanted secondary discharge events leading to a wider than desirable slot being cut through the workpiece. The level at which these unwanted secondary discharge events occur is governed by wire tension. In this experiment, a study for optimal wire tension is carried out on a previously constructed desktop microtooling machine (Chen and Lin, 2011). The test conditions are listed in Table 3. When tension is 6 gf, the cut slot’s width is 27 μm, and the discharge gap is 7 μm (Fig. 5(a)). This result indicates wire tension is too loose resulting in poor form accuracy and slot size. At a wire tension of 8 gf, slot-width narrows to 20.6 μm and the gap 3.8 μm (Fig. 5(b)), indicating wire-sway has been significantly reduced. At a wire tension of 10 gf, slot-width is further reduced to 15.4 μm and the corresponding gap reaches just 1.2 μm (Fig. 5(c)), implying effective suppression of wire vibration giving much
(a) Wire tension of 6 gf
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improved slot narrowness and straighter slot walls. Further increasing wire tension to 12 gf increases the probability of wire breakage due to wire necking and high temperature spark erosion.
(b) Wire tension of 8 gf
(c) Wire tension of 10 gf
Fig. 5 Experiment for appropriate wire tension
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5.3 Capacitance erosion capability The capacitor releases its electric charge (Qdis) per unit of time as an exponential decay equation (Eq. (16)). In the case of a single discharge, maximum energy release of the capacitor occurs in the
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initial stage of the discharge function curve (about 1, i.e. one time-constant), at which point the voltage (Vc) of the capacitor drops instantaneously by about 63.2 % and energy released (Es) is about 86.46 % of maximum potential release (Bishop, 2011) (Eq. (17)). This means the capacitor can create a very high-peak current (Ip) in a very short time and the discharge forms a very high-speed plasma beam with a temperature up to 8,000~12,000 °C (Sommer, 2000). Different machined materials have different melting-points and electrical resistivity, also the energies released (according to capacitance) may have different material erosion rates (Chen and Chen, 2017). This difference is defined as Capacitance Erosion Capability (CEC)’ in this study. It is the amount of material that can be removed by unit of electricity released by the capacitor in mm3/Coulomb. In other words, it is the volume of removed-material (Vvol) per unit of electric charge (Qdis) (Eq. (18)). The lower the CEC, the more coulombs of electricity required to melt the same volume of material. Hence, the total electric charge (Qtotal) released by the capacitor per unit of time can be obtained by the amount of electric charge per 9
unit of time (Qdis) input from the capacitor and work frequency (fdis) (Eq. (19)). By evaluating CEC, a suitable work capacitance for precisely cutting a B-NPD microstructure can be attained.
Qdis C VC e
t RC
(16)
1 CV 2 2
(17)
Vvol Q
(18)
Qtotal Qdis f dis
(19)
Es
CEC
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5.4 The appropriate work capacitance Experimental testing to determine appropriate work capacitance is carried out with capacitors of 200 pF, 330 pF and 470 pF, respectively. Table 3 lists the remaining machining conditions. Figures 6(a) and 6(b) show the corresponding total amount of electric charge (Qtotal) released. The results of two experiments show that a work capacitance of 330 pF has better capacitance erosion capability for B-NPD although its Material Removal Rate (MRR) is slightly lower than 470 pF (Fig. 6(c)). This implies that with a work capacitance of 330 pF, the wire can more effectively cut B-NPD microstructures. In addition, the amount of coulomb released per unit of time is low at a work capacitance of 200 pF, but it has a higher capacitance erosion capability and lower MRR, inferring that a work capacitance of 200 pF is more suitable for finish-cutting than other tested capacitances.
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(a) Amount of electric charge (b)Capacitance erosion capability Fig. 6 Appropriate work capacitance
(c) Material removal rate
5.5 The proper wire feed-rate in rough-cutting In this set of experiments to determine the optimal feed-rate for rough cutting five wire feed-rates: 0.01, 0.015, 0.02, 0.025 and 0.03 mm/min are examined (Fig. 7(a)). Figure 7(b) lists examined slotwidths and their corresponding straightness. Better straightness of slot can be obtained at feed-rates of 0.01, 0.015 and 0.02 mm/min, indicating that a wire feed-rate in this range is appropriate for machining B-NPD material. Slot-width consistency diminishes significantly when the wire feed-rate is increased to 0.025 and 0.03 mm/min used. An excessively fast wire feed-rate tends to cause the discharge gap to narrow, resulting in worse debris expulsion, increased probability of arc discharging, 10
and wire-drawback events. This leads to the slot-wall surface being more wave patterned. Experimental results demonstrate that the optimally efficient rough-cutting feed-rate for slot straightness is obtained at a feed-rate of 0.02 mm/min (Fig. 7(c)).
(a) Scheme of the rough-cutting (b) Straightness of the cut slot (c) Slot-width (F=0.02 mm/min)
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Fig. 7 Optimal wire feed-rate in rough-cutting
The above experimental results show B-NPD microstructures can be precisely cut by a tungsten wire
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of Ø13m under the developed horizontal cutting mechanism with electromagnetic damping control. The most suitable wire tension is 10 gf. The proper work capacitance and wire feed-rate are 330 pF and 0.02 mm/min in rough-cutting and 200 pF and 0.05 mm/min in finish-cutting, respectively.
6. Experimental verification
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Odake et al. (2009) reported that B-NPD is highly suitable for use as a material for cutting tools due to its higher hardness, toughness, abrasive resistivity and heat resistance than general polycrystalline diamond. To verify and confirm the feasibility of the designed ‘electromagnetic damping-controlled horizontal cutting mechanism in the machining of a B-NPD workpiece, a micro
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diamond tool with a trapezoidal-shaped microstructure plate array of 20 m at its tip and an aspectratio of 1:20 is designed and cut (Fig. 8(a)). Such a structure is suitable for transcribing microgroove arrays (Yan et al., 2009) and micropyramid arrays (Fig. 8(b)). A 'one-cut one-skim' machining approach is used whereby the first path is used to cut open the workpiece while the second path performs finish-cutting on the slot-wall to improve surface flatness and eliminate dimensional errors. Table 4 lists the remaining machining conditions. The resultant microstructure array is highly consistent with very little waviness of cut (Fig. 8(c)).
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Table 4 Machining conditions for B-NPD microstructure cutting Parameters Workpiece material Wire material Wire tension (F) Wire feed-rate (Fr) Running speed (Vr) Capacitor voltage (Vc)
Conditions B-NPD Tungsten wire (Ø13μm) (-) 10 gf 0.02 mm/min (Rough-cutting) 0.05 mm/min (Finish-cutting) 36 mm/min 100 V 11
Work capacitance (C)
200 pF (Finish-cutting) 1 MHz 500 μm Lamp-oil
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Work frequency (fdis) Workpiece thickness (tm) Dielectric
330 pF (Rough-cutting)
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(a) B-NPD microstructure array (b) Microgroove array (c) The finished microstructure design transcribing simulation array with high-consistency Fig. 8 Verification of B-NPD microstructure array cutting
7. Discussion
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7.1 Degrees of freedom in microwire running To combat wire-swaying, a 10 μm slot is cut into a tungsten carbide rode to act as a wire guide. This guide is in addition to the machine’s existing wire guide and auxiliary guide designs. The slot is cut using low energy on the surface of a tungsten carbide rod (no electricity) in the working area (Fig. 9(a)). The wire-guide is used in combination with the electromagnetic damper to limit the wire to one
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degree of freedom along the direction of the running wire. Trial results show that when the wire passes over the rod without a microgroove to constrain wire-sway, slight swaying of the wire causes discharge short-circuiting and side-erosion to occur in the cut slot of the workpiece (Fig. 9(b)). By contrast, when the slot in the tungsten carbide guide rod is used, the wire is firmly restrained by the microgroove, reducing the potential for wire-sway and vibration (Fig. 9(c)). By utilizing the designed microgroove, even if the wire breaks during machining, after rethreading, the wire can continue to operate accurately on the original path, creating improved microwire cutting performance with highstraightness and reproducibility.
(a) Microgroove design
(b) Without microgroove assist 12
(c) With a microgroove assist
Fig. 9 Degrees of freedom in microwire running 7.2 Effect of skim on the slot-wall Inverted-cutting facilitates discharge-debris removal from the narrow discharge gap (Hwang et al., 2010; Chen and Chang, 2013) and the use of low-energy cutting reduces excessive Joule heating in the microstructure workpiece (Luo, 1995); however, the use of low-energy machining means the discharge gap can be overly narrow, which leads to debris expulsion difficulties and an increased possibility of short-circuiting and secondary discharging. This results in an uneven slot-wall surface because of increased wire-drawback. A 'one-cut one-skim' strategy for improving the geometrical accuracy of the slot-wall surface is proposed and schemed (Fig. 10(a)). At the ‘one-cut’ stage, the microstructure is first cut open according to a planned path using higher discharge energy and a lower wire feed-rate. Figure 10(b) shows the machined microstructure with an uneven cut surface after the
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initial one-cut process. At the ‘one-skim’ stage, a new cutting path is established based on the original path whereby the wire-electrode is offset at a micro distance to trim as little as 1-2 m of material from the rough-wall. One-skim cutting is implemented under a lower discharge energy and higher wire feed-rate. The resultant slot-width (Wf) is expressed in the Eq. (20), where Soffset is the offset
(20)
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W f W 2Soffset , Soffset 2 m
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distance of the wire-electrode equal to less than 2 m. Low-energy cutting is helpful in removing the protrusions and micro erosion-scars from the rough-wall, improving the flatness of the slot-wall (Fig. 10(c)). In order to maintain slot-width dimensional accuracy, offset distance of the wire must be controlled at an extremely low micro level. This demonstrates that the developed ‘electromagnetic damping controlled horizontal cutting mechanism’ can tightly and stably control the micro offset distance of the microwire.
(a) one-cut one-skim strategy (b) After one-cut processing (c) After one-skim processing Fig. 10 Effect of skim on the slot-wall 7.3 Effect of wire feed-rate on surface roughness At the one-skim stage, four different wire feed-rates higher than that of the one-cut stage are tested to confirm cutting performance at the skim stage. When the wire feed-rate is Fr=0.025 mm/min, the corresponding surface roughness is Ra0.082 m, meaning discharge energy can effectively act on 13
pockmark protrusions, making the pockmarks shallower (Fig. 11(a)). With an increase in feed-rate to Fr=0.05 mm/min, roughness of the machined surface is improved to a level of Ra0.088 m (Fig. 11(b)), demonstrating that MRR is still close to that corresponding to a wire feed-rate of 0.025 mm/min, even though the feed-rate has doubled. When the feed-rate is increased to Fr=0.075 mm/min, the
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corresponding surface roughness is Ra0.171 m, suggesting spark erosion energies at such a wire feedrate do not effectively act on the protrusions around discharge-pockmarks; i.e., a worsened surface roughness result (Fig. 11(c)). An unduly high wire feed-rate of 0.1 mm/min leads to the discharge gap shrinking, increasing the probability of arc discharging, abnormal discharging, and short-circuiting (Fig. 11(d)).
(b) Fr = 0.05 mm/min
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(a) Fr = 0.025 mm/min
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(c) Fr = 0.075 mm/min (d) Fr = 0.1 mm/min Fig. 11 Effect of skim on cutting performance
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8. Conclusion
An original electromagnetic damping-controlled horizontal cutting mechanism for microwire-EDM is successfully developed and proposed in this study. The cutting mechanism has a high-degree of stability in the control of an extremely fine wire of Ø13m in diameter during cutting of difficult-tomachine materials. Both quantitative and qualitative approaches to the microwire-cutting process have been studied to provide complete information on cutting B-NPD microstructures. The conclusions from this study are as follows: 1.
An electromagnetic damper with a coil design of 600-turns is able to stably and precisely control the wire tension of Ø13 m micro tungsten wire. 14
2.
Compared to tension control by a permanent magnet, the electromagnetic damper possesses wire
3.
tension with high controllability. There is no need to adjust magnetic spacing. The designed horizontal cutting mechanism can suppress potential wire-swaying and vibration.
4. 5. 6.
The appropriate wire tension is between 10-12 gf corresponding to a sway error within 1 m. Through the evaluation of capacitance erosion capability, suitable work capacitances for cutting B-NPD microstructures are established at 330 pF for rough-cutting and 200 pF for finish-cutting. A ‘one-cut one-skim’ strategy makes discharge-debris expulsion easy. In addition, it is shown that the wire feed-rate in the one-skim stage can be higher than that of the one-cut stage. Experimental results confirm that material with low electro-thermal machinability can be precisely cut by the proposed technology since microwire tension can be stably controlled.
Acknowledgement
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The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan for financially supporting this research under Contract No. MOST 106-2221-E-003-017-MY2.
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