1 Introduction

In recent years, precision three-dimensional micro parts and components with featured size ranged from micrometers to millimeters, and meeting the requirements of high shape accuracy and good surface quality have broad application in aerospace, defense and military, automotive electronics, biomedicine, precision instruments, etc., and Fig.1 presented some miniature products and structures including the micro aircraft, micro wall, micro trench, and micro gears [1,2,3]. These micro parts usually involved in a variety of materials such as metals, ceramics, and resins, and micro structures often relate to grooves, thin walls, micro pores, and special-shaped curved surface [4, 5]. Therefore, the realization of high precision and low cost processing of micro parts and components made of various materials and complex structures has become the key to micro-miniature processing technology.

Fig. 1
figure 1

Micro products and structures: a micro air vehicle parts, b micro wall, c microarray, d micro sensor, e micro turbine engine with diameter of about 12cm, f aerospace micro products, g micro impeller parts and rotor, h microarray

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The micro machining technology is the mainstream technique to achieve the fabrication of micro parts and components, and it mainly can be classified into micro cutting, non-cutting, and unconventional machining techniques; and the micro cutting processes contain micro cutting, micro milling, micro drilling, and micro grinding; the micro non-cutting processes include micro stamping, micro injection, etc; and the micro unconventional machining techniques include micro electrochemical, micro ultrasonic, micro-EDM, etc., and the unconventional machining and micro cutting processes have received extensive attention [6,7,8]. The micro-EDM makes the use of electro-corrosion generated by the pulse discharge immersed in the working fluid to remove conductive materials, and it has the characteristics of the non-contact, not limited by strength and hardness of material and can achieve the desired removal by controlling discharge energy, which makes it have unique advantage in fabricating micro part and structures, but it fails to fabricate micro parts and structures made of non-conductive material and consist of complex three-dimensional shapes [9,10,11,12,13,14]. Thus, the micro parts and structures made of conductive materials can be directly fabricated by EDM method. However, as for the fabrication of micro parts and structures made of non-conductive materials still relies on micro cutting technology including micro turning, micro milling, and micro grinding [15,16,17,18,19,20], and micro cutting technologies have advantages of relatively high efficiency, strong three-dimensional processing ability, and a wide range of applicable workpiece materials [21, 22].

However, the diameter of micro tools involved in micro cutting technologies is generally smaller than 1mm, and the smaller the size is, the more difficult it is to prepare, so the successful preparation of micro tool is the key to micro cutting technologies to achieve high precision and low cost manufacture of micro parts and structures [6, 23]. The non-contact nature of EDM processes makes it more competent in preparing micro tools and has been drawn wide attention of scholars around the world. This article provides a critical overview of the broad field of EDM processes involved in the fabrication of micro tools and the present achievements and limitations to provide grounds for future research works.

2 Physical process, classification, and short chronological review of EDM methods involved in fabricating micro tools

2.1 Physical process and material removal mechanism

The physical process of EDM machining is very complicated and short-lived, and the microscopic process of each spark discharge is integrative result of thermal power, electric power, electromagnetic force, and fluid power. In the process of EDM, the material erosion mainly can be divided into dielectric breakdown, channel formation, electrical discharge thermal erosion, and deionization throwing as presented in Fig. 2 [24, 25]. Firstly, an electric field is established between tool electrode and workpiece, and the field strength depends on the interelectrode voltage and distance. When the field is strong to a certain extent, working fluid molecules are polarized to form the low resistance channel. At the same time, impurities in the working fluid such as metal particles and micro bubbles will gather and form chains in the place of strongest field. When the field strength is further increased, the cathode may also emit electrons. The electrons are accelerated under action of electric field and hit neutral molecules or atoms to cause ionization. This ionization develops rapidly from the cathode to the anode in an avalanche manner. When electrons flow to the anode, the medium is broken down to form a discharge channel. The dielectric breakdown under a very thin gap is the result of the mass and dynamic interaction of field emission and static electric field, and there is a positive feedback relationship between them as shown in Eq. (1), and they can be calculated from Eq. (2) [26].

Fig. 2
figure 2

Diagrammatic sketch of material removal process for EDM process; T is tool electrode, and W is workpiece electrode; a establishment of electric field, b electron emission, c channel formation, d channel expansion and bubble formation, e channel collapse and bubble expansion, f deionization

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$$ \left\{\begin{array}{l}x=f(t)\\ {}j=\phi (t)\end{array}\right. $$
(1)
$$ \left\{\begin{array}{l}\frac{d_x^2}{d_t^2}=\frac{S(t)}{K_1\rho V(t)}\left[\frac{U^2}{x^2}-{K}_2F(t)\right]\\ {}j=1.55\times {10}^{-6}\frac{E{(t)}^2}{\phi }{e}^{-\frac{6.8\times {10}^7{\phi}^{2/3}}{E(t)}}\cdot \psi \left(\frac{3.62\times {10}^{-4}\sqrt{E(t)}}{\phi}\right)\end{array}\right. $$
(2)

where x represents the changed discharge gap (cm), j is varied field electron emission density (A/cm2), E is electric field intensity (V/cm), φ is electron escape work (eV), ρ is material density (g/cm3), V is electrode leaving volume of the made by electrostatic field (cm3), U is interelectrode voltage (V), F is mechanical stress of electrode material (N), S is cross-sectional area of electrode made by destructive force (cm2), and K1 and K2 are coefficient.

The entire energy of pulse is released in two electrodes and discharge gap. The energy gained by anode is primarily passed by electron bombardment, and radiation heat transfer and thermal shock of gas particles are relatively small, so it can be neglected, and then anode energy per unit time can be expressed as Eq. (3); in addition, the cathode energy is supplied by positive ions, and electrons consume cathode energy; thus, the power input on the cathode Pk is represented by Eq. (4) [27].

$$ {P}_a={j}_a\left({V}_a+\frac{\phi_a}{e}+\frac{2 kT}{e}\right){S}_a $$
(3)
$$ {P}_k=\left\{{j}_p\left[d\left({V}_k+\frac{2 kT}{e}\right)+{V}_i-\frac{\phi_k}{e}\right]-{j}_e{V}_e\right\}{S}_k $$
(4)

where the ja and jp respectively are current density of anode spots and positive ions (A/cm2), je is electron current density (A/cm2), Va is anode voltage drop (V), Vi is the ionization potential of positive ion (V), φa and φk respectively are electron work of anode and cathode material (J), e is the electronic charge, k is the Boltzmann constant (k=1.38×10−23J/K), T is discharge gap temperature (K), Sa and Sk respectively are area of anode and cathode spots (cm2), and I is discharge current (A).

The extremely high temperature and pressure of discharge channel can strongly heat the cathode spot and make metal near the point become overheated material and then occur explosive melting, vaporization, and metal throwing phenomena, and the bubbles gradually expand outward. Then the cathode pressure under the action of ion flow pk and the anode pressure under the action of electron flow pa can be respectively expressed by Eq. (5) [26].

$$ \left\{\begin{array}{l}{p}_a={j}_a\sqrt{\frac{m_e\left(2{eU}_a-3{kT}_k\right)}{e^2}}\\ {}{p}_k={j}_p\sqrt{\frac{\mu \left(2{eU}_c-3{kT}_k\right)}{Ae^2}}\end{array}\right. $$
(5)

where me is electronic quality (kg), Uc is electronic voltage drop before collision (V), Ua is anode voltage drop (V), Tk is cathode boiling point (K), μ is atomic mass of metals (kg), and A is Avogadro constant.

In addition, the electrode obtained heat from the discharge channel namely surface heat source. Assuming that the radial distribution of heat flow in the surface heat source can be described by the Gaussian distribution, the solution of the temperature field can be obtained by the superposition principle and expressed as Eq. (6) [26].

$$ {T}_o\left(r,z,t\right)=\frac{8{g}_{3\varphi}\alpha k}{C\rho {\left(4\pi a\right)}^{3/2}}{\int}_0^{t_u}\frac{1}{\sqrt{t}\sqrt{4 akt+1}}{e}_{-\left(\frac{kr^2}{4 akt+1}+\frac{z^2}{4 at}\right)}{d}_t $$
(6)

To is the temperature at a point on the electrode (°C); z is the coordinate of this point; g3Φ is the effective power of the heat source (W); a is temperature coefficient; ρ is material density (g/cm3); C is the heat capacity (J/K); tu is the pulse width (μs); and k is characterized curve.

Finally, the discharge channel rapidly shrinks and collapses when the discharge pulse ends, and then deionization is performed. Meanwhile, the bubbles continue to expand to its maximum size, and the pressure in bubble drops to close to atmospheric pressure, and the overheated molten material is thrown out in EDM process. Metal particles are scattered inside and outside of the bubble to form processing chips. After the bubble expands to the maximum size, it begins to shrink due to the low pressure until it bursts. After the discharge corrosion is completed, the metal particles, carbon black, and small bubbles will be left in the liquid between the electrodes, and discharge craters are formed on the workpiece and tool electrodes, and the diameter of discharge crater can be expressed as Eq. (7) [28].

$$ d=0.151{I}_m^{0.4}{t}_u^{0.4} $$
(7)

where d is diameter of discharge crater (μm), Im is pulse current amplitude (A), and tu is pulse width (μs).

2.2 Classification of EDM involved in fabricating micro tools

The non-contact nature of EDM processes makes them competent to machine electrical conductive materials regardless of their hardness and strength with precise shape and good surface quality, which mainly can be classified into the WEDM, die-sinking EDM, EDM drilling, EDM milling, WEDG, etc.[29], but not all EDM methods are suitable for the fabrication of micro tools. For visualizing the spectrum of ongoing research in EDM methods for machining micro tools, processes are classified into four categories in this research, which is the basis of the number of tool electrodes, moving way of tool electrode, micro tool geometry, and liquid way as shown in Fig. 3.

Fig. 3
figure 3

Classification of EDM process involved in fabricating micro tool

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2.3 A short chronological review

The EDM process was invented by Soviet scholar Lazarenko in 1943, and this new processing technology has gained rapid development and wide application in the world and usually was used for the machining of various metal materials such as carbon steel, stainless steel, and high temperature alloy. Until 1985, the Japanese scholars Masuzawa proposed WEDG method which provided a new approach to the fabrication of micro tools by EDM process and meet the rapid requirements of micro tools applied in micro cutting technologies [30]. To give a holistic viewpoint of field related to EDM processes involved in fabricating micro tools, primary reports are chronologically presented in Fig. 4, and a brief description is presented as follows:

Fig. 4.
figure 4

A short chronological review of EDM process involved in fabricating micro tools

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2.3.1 Stage 1: Early works (preliminary studies and investigations) (1985–2005)

The early works mainly focused on the introduction of WEDG [30], fabricating the cylindrical micro rods and adjusting discharge parameters for minimizing its diameter [31] and preparing simple rotating parts or micro tools [32,33,34].

2.3.2 Stage 2: Process improvements (2005–2015)

In this stage, the significant study is reported related to improvements of EDM methods involved in fabricating micro tools, for example, in order to improve machining efficiency, the BEDG method [35], twin electro wire [36], and custom tool fabrication CAM system [37] are successively put forward. Besides, the process improvements are obtained in the fields of machining accuracy [38],surface roughness [39], super hard material [40], complexly parametric design [41] and the application of EDM fabricated micro tools in hard brittle materials [42]. Additionally, the majority of work is also reported on analyzing influencing factors and optimizing process parameters involved in micro tool fabrications of using EDM process [43,44,45,46], and a classification of parameters directly associated with process performance in the EDM fabrication of micro tools is shown in Fig. 5.

Fig. 5
figure 5

Ishikawa diagram showing the classification of process parameters in machining micro tools by EDM process

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2.3.3 Stage 3: Recent advancements and current trends (2015–2020)

In this stage, the majority of published papers are related to improvements in hybrid processes and non-standardization design of micro tools. The design of texturing micro grinding tool [47] and helical edged micro cutting tools [48] are introduced for enhancing the machining stability and surface quality. Apart from this, a significant volume of work is also reported on the fabrication of micro tools using the hybrid technology combining more than two individual processes [49, 50].

3 EDM methods applied in fabricating micro tools

3.1 Micro cutting tools

In micro machining, the micro milling is a dominant process to achieve the fabrication of micro parts and components, and micro cutting tools (MCT) involved in micro milling process are usually very slender with the diameter less than 1mm, which makes the preparation of micro cutting tools difficult, and the complex structures and ultra-hard materials will further increase the difficulty of preparation [51, 52]. The fabrication techniques of MCT are the basis of ensuring the machining accuracy and capacity of micro milling, which is also one of the main bottlenecks restricting the development of micro milling [53, 54]; at present, the fabrication technology of MCT mainly includes the precision micro-grinding, focused ion beam, laser machining and electrical discharge machining, etc [55,56,57]. Among them, the minimum diameter of MCT cannot be stably obtained with precision micro-grinding due to the limited by grinding force [43, 58]; the processing efficiency of focused ion beam is low, and the cost is expensive; the hot melt zone in laser machining is difficult to control resulting in low forming accuracy [59]; while the advantages of good controllability, non-contact, no macroscopic force, and low machining cost make EDM process more competent to fabricate micro cutting tools, it can ignore material hardness and prevent undesired problems such as chatter, mechanical stresses, and vibration [60,61,62,63]. Thus, many scholars all over the world paid attention to fabricate micro cutting tools using various EDM processes, which includes WEDG, TW(twin)-WEDG, TF/RF-WEDG, S/M-BEDG, R-EDM, Self-drilled, LS-WEDT, WEDM, etc, and specific EDM processes involved in fabricating micro tools and their machining principles, characteristic, applications, and remarks are summarized in Table 1.

Table 1 Main EDM processes involved in fabricating micro cutting or grinding tools and microelectrodes
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For reducing the preparation difficulty of micro cutting tools, some researchers adopted EDM methods to fabricate micro cutting tools with simple polygon structure and straight cutting edge. Fleischer et al. [64] found that the WEDG method has the potential to machine micro cutting tools with a diameter smaller than 100 μm, and the results meet requirements of necessary surface roughness and sharpness of the cutting edge as displayed in Fig. 3a. Egashira et al. [32, 65], Ali et al. [66], Chern et al. [67], and Fleischer et al. [68] adopted WEDG method to prepare micro cutting tools with different cross-section shape (such as D-shaped and circular) and high aspect ratio made of tungsten carbide material as presented in Fig.3. Yan et al. [43], Katahira et al. [44], Nakamoto et al. [45], Gao et al. [46], Oliaei et al. [69], Ganesh et al. [70], and Zhan et al. [71] respectively adopted the WEDM method to fabricate various kinds MCT as shown in Fig. 6 and controlled geometrical error and tool edge radius of the cutting edge by optimizing electrical discharging parameters including peak current and peak voltage.

Fig. 6
figure 6

a MCT in tungsten carbide and diameter of 100μm[64], b MCT with the clearance angle of 20° and cutting edge radius of 0.5 μm[32], c MCT with diameter of 3μm[65], d micro cutter with diameter of 50 μm[67], e semi-circular micro cutter with diameter of 100 μm, f D-shaped micro cutter [43], g custom designed single edge PCD micro end mill[45], h quadrilateral micro cutter[46]

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In recent years, the structural design of micro cutting tools has been presented a trend of non-standardization and parameterization; for example, Katahira et al. [40] fabricated a square-shaped PCD micro cutting tool with two flutes by WEDM method as presented in Fig. 7 a and b, and the micron-sized wells and groove structures on silicon carbide with surface roughness of Ra=1.7nm were successfully machined by using this PCD tool, and amorphous silicon dioxide can be detected onto the surface of the PCD tool by SEM and EDS analyses. Fonda et al. [39] investigated effects of WEDM parameters on machining speed for roughing and surface roughness for finishing by using signal to noise ratios, and a hexagonal MCT made of the PCD material was fabricated by WEDM optimized condition, and chemical composition is detected as shown in Fig. 7 c and d. In addition, Cheng et al. [37] developed an ultra-precision machining system of six-axis WEDM machine and custom tool fabrication CAM system; before fabricating micro cutters, the effectiveness of counter motion system is conducted and the detected frequency components obtained by an accelerometer located on workbench when X-axis is driven by acceleration or deceleration pattern. For cutting accurately, the wire offset is considered and calculated by Eqs. (8)~(10). Finally, they fabricated a 2-flute special cutter with diameter of 1mm and hex-polygon cutter with diameter of 0.2mm made of PCD material as shown in Fig. 8. Moreover, Cheng et al. [72,73,74] designed a new kind of PCD ball micro cutting tool and utilized WEDM tool with four axes to fabricate it and proposed to use negative rake angle to cut brittle materials for achieving the purpose of plastic cutting. Through FEM simulation analysis, it is concluded that this ball micro cutting tool has the best cutting performance when the rake angle is −60°, and three-dimensional solid model before and after optimization of ball micro cutting tool and fabricated results are displayed in Fig. 9; besides, Cheng also designed a PCD flat micro cutting tool and used it to machine brittle materials, and the quality of side and bottom of the brittle workpiece verified the effectiveness of this new designed micro cutting tools.

Fig. 7
figure 7

a Square-shaped PCD end mill, b micro cutting tool after machining, c PCD tool fabrication process, d a hexagonal micro cutting tool

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Fig. 8
figure 8

a Micro tool fabrication CAM system, b fabricated results of micro cutting tool, c wire offset calculation, d frequency response components

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Fig. 9
figure 9

a The FEM static analysis, b new hexagonal end mill made of PCD, c static FEM analysis, d fabricated new ball end mill.

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$$ {Offset}_x=\left|{E}_1{E}_2\right|={R}_E\sin (B)\ \sin (D) $$
(8)
$$ {Offset}_y=\left|{E}_2{E}_4\right|={R}_E\cos (B) $$
(9)
$$ {Offset}_z=\left|{E}_3{E}_4\right|={R}_E\sin (B)\ \cos (D) $$
(10)

where B is cutting point central angle, D has the same value as wire rotation angle A, C1C2 the tangential line between workpiece and wire discharging envelope, and RE is the wire radius plus the discharge gap under current machining conditions.

Ball micro cutting tools have good geometric adaptability and simple tool path algorithm, which is the main micro cutting tool for machining complex curved micro surfaces involved in micro parts and components used in aerospace, automobile, etc. Nowadays, some studies are focused on preparing ball micro cutting tools using EDM processes. Oliaei et al. [75] proposed a new method combining BEDM and WEDG techniques to fabricate ball micro cutting tools with different geometries as shown in Fig. 10 and studied the effects of EDM process parameters on MRR and Ra of MCT, and they found that this MCT can be used to fabricate microfluidic channels on fused silica with acceptable quality and accuracy. Wu et al. [50] proposed multistep fabricated process of MCT made of PCD for satisfying the efficiency and tool quality; firstly, the PCD plate is welded on the tool shank. Then, the PCD micro cutting tool is machined with WEDM method to generate its main shape and structure. Finally, the MCT is further sharpened with a diamond wheel. Besides, Zhang et al. [57] used continuously feeding wire electrode through a wire guider to fabricate PCD tools as presented in Fig. 11, and the resistor-capacitor discharge circuit is adopted, and for realizing extremely low discharging energy, the stray capacitance of the circuit C0 (~1pF) is used, and PCD tool with the radius of the spherical surface of about 400 μm and the edge radius of about 1.8μm is obtained. Su et al. [76] proposed two different schemes to machine ball MCT with four straight cutting edges using LS-WEDT method; after the electrode wire path and parameter optimization, the ball MCT with the radius less than 200μm, Ra of 1.13μm, and cutting edge radius of 5.16μm can be obtained.

Fig. 10
figure 10

a Stepped WC micro tool, b D-shaped flat end cylindrical PCD micro tool, c micro ball end mills, d lollipop end mill

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Fig. 11
figure 11

a Schematic of tool shape generation; b detailed diagram showing graphitization, dissolution, and gradual removal of a diamond grain; c EDM-fabricated hemispherical tool; d PCD micro cutting tool; e tool edge and flank

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Compared to the conventional micro cutting tools, which are produced through grinding process to have helical flute geometry, these tools usually have straight edges, which improves the stiffness but limits the chip evacuation. The use of straight edges can be justified by considering low depth of cut values in micro milling. Samad et al. [77] adopted Sodick AP250L high precision WEDM machine with a brass wire of 0.1 mm diameter to fabricate a single cutting edge micro cutting tool as displayed in Fig. 12, and the best surface roughness of 0.15μm and edge radius of 1.5~2.5μm can be obtained after a design of experiments approach on WEDM process parameters. Sun et al. [48] realized visual predictability of helical micro end mills fabricated by LS-WEDT method, and they found that feeding amount of electrode wire, feeding, and rotating speed have key influence on the shape of spirals micro end tool, and their experimental results showed that the three spiral MCT with 123.8μm in diameter and 800μm in length, MCT with six spirals, and varied spiral angle MCT are efficiently prepared using this method as presented in Fig. 13.

Fig. 12
figure 12

a Geometry of designed micro cutting tool, b schematic of micro cutting tool fabricated

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Fig. 13
figure 13

a Simulated results and b experimental results of micro cutting tool with three spirals, c three edge spirals micro end tool with varied spiral angle, d micro cutting tool with six spirals

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process, c fabricated results of WC single edge micro cutting tools

In addition, Oliaei et al. [78] adopted WEDM method to fabricate a PCD end mill with a hexagonal geometry, and its micro cutting geometry has been modified to have a parallelogram shape featuring a large negative rake angle on the bottom of the tool, and they found that the PCD micro cutting tool geometry significantly affects the process outputs as displayed in Fig. 14. Based above mentioned contents, the type of EDM processes involved in fabrication of micro cutting tools and their performance, application, and remarks are summarized in Table 2.

Fig. 14
figure 14

a Schematic of micro tool fabricated steps, b material removal mechanisms of PCD during WEDM process, c 3D profile of the fabricated PCD tools, d hexagonal PCD micro end mill

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Table 2 The specifications of various micro tools fabricated using EDM processes
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3.2 Micro grinding tool

In grinding process, the surface quality, grinding hardening, residual stress, micro hardness, and machining accuracy of workpiece directly affect its machining performance and service life [79,80,81,82,83]. In recent years, with the increasing requirements of machining size and surface quality of micro parts, the micro grinding technology has been widely concerned. The design and preparation of grinding tools are the most important factor in grinding process. At present, the main preparation methods of micro-grinding tools mainly include the super hard abrasive coating method and the diamond or CBN usually selected as abrasives and coating methods always involved in the electroplating [84], electroless plating [85, 86], cold spraying [87], chemical vapor deposition [88], etc. And the other fabricated methods refer to EDM-fabricated polycrystalline diamond [34] as displayed in Fig. 15a–h. Besides, Pratap et al. [47] analyzed effects of discharge energy and wire tension on surface roughness and diametrical error involved in the fabrication of micro-grinding tool using WEDM as shown in Fig.16, and they found that defects will be increasing at low wire tension and high discharge energy, and diametrical error of micro tools is found to be large at low wire tension.

  1. (a)

    Coating[82] (b) coating[83] (c) coating[84] (d) electroless plating [85]

Fig. 15.
figure 15

Cylindrical micro-grinding tools fabricated by various methods

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Fig. 16
figure 16

a Wire electrical discharge grinding setup for polycrystalline diamond tool, b machining time and wire breakage at wide range of wire tension and capacitance, c machining processes of PCD micro tool, d 3D view and surface leveling of micro tools

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(e) electroless plating [86] (f) cold spraying [87] (g) chemical vapor deposition [88] (h) EDM[34]

Based on above analysis, it can be seen that the design of micro-grinding tools is still dominated by cylindrical shape without chip groove. But, in recent years, the design of micro-grinding tool has been presented a trend of non-standardization and parameterization; for example, Perveen et al. [42], Yan et al. [89], Chen et al. [41, 90], Morgan et al. [34, 91], Masaki et al. [92], and Pratap et al. [93], respectively, adopted various EDM processes to prepare the D-shaped, four prism, conical, cross, ball, fluted, ladder shaped micro grinding tools as displayed in Fig. 17; in another research of Chen et al. [94] employed micro WEDM to fabricate thin the micro grinding wheel with tip edge width of 15μm as shown in Fig.17k, and then it is used to grind microgrooves on optical glass. Additionally, Sun et al. [95] proposed hybrid process combining the LS-WEDT and electroplating process to fabricated diamond MGT with helical chip removal slots and diameter of about 800μm, and firstly the WEDM with high precision rotating unit to prepare MGT substrate and slots and then diamond abrasive is uniformly coated by electroplating process as displayed in Fig.17l. In addition, Oliaei et al. [49] used micro WEDM and EDM methods to fabricate two kinds of PCD micro tools with micro-pyramid lattice structures as presented in Fig.18, and these micro tools can be employed as post-processing tools in a wider range of feed per revolution and cut depth. Based on above mentioned contents, the type of EDM processes involved in the fabrication of micro grinding tools and their performance, application, and remarks are summarized in Table 3.

Fig. 17
figure 17

Special shape micro-grinding tools: a “D” shape[42], b square shape[42], c triangle shape[42], d cross-shaped[89], e conical shape[90], f a conical tipped PCD micro-grinding tool[92], g ball end MGT [93], h fluted MGT[90], i ladder shaped MGT[90], j five types of MGT[93], k formed extremely thin diamond wheel[94], l MGT with helical slots[95]

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Fig. 18
figure 18

a Schematic of micro tool fabricated steps and results of micro tools with micro-pyramid lattice structure b micro tool fabricated steps and results of hole drilling tools

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Table 3 The specifications of various micro grinding tools fabricated using EDM processes
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4 EDM methods involved in preparing microelectrodes

4.1 Single microelectrode

The size and surface quality of tool microelectrodes directly affect the machining of micro holes and complex micro three-dimensional structure cavity. Therefore, the preparation of tool microelectrodes with high efficiency, high quality, and high precision is the key to achieve the fabrication of micro hole and complex micro structure. The efficiency of preparing the tool microelectrode by traditional machining method is very low or even cannot be processed at all. Therefore, at present, the preparation of tool microelectrode mainly relies on the non-traditional techniques especially the EDM method. The microelectrode can be divided into cylindrical, edge cutting, D shaped, spiral electrode, and so on. Scholars at home and abroad have done a lot of research on the preparation of tool microelectrode, and Table 4 demonstrated the type of EDM processes involved in the fabrication of microelectrodes, machining size and shapes, and characteristics studied by previous researchers.

Table 4 The specifications of various microelectrode fabricated using EDM processes
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The WEDG method was introduced by Professor Masuzawa of the Tokyo University in 1985 [31], which provides an effective approach for fabricating micro pins, micro spindles, and micro tools, and they adopted this method successfully to fabricate several microelectrode with a diameter of about 2.5μm, 15μm, and 25μm, respectively, and their measured results indicated that the accuracy and repeatability of WEDG method are high with the error of less than 1μm [32]. Li et al. [96], Kuo et al. [97], Egashira et al. [98], Chern et al. [67], Yan et al.[99], Rees et al. [100], and Li et al. [130], respectively, used the WEDG method to fabricate all kinds of microelectrodes of different cross-sectional shapes and the sacrificial electrode involved in wire, block, disk etc. and studied the influences of different machining parameters on the surface quality and machining accuracy of microelectrodes by statistical methods and parameter optimization. For improving machining repeatability, Wang et al. [101] studied primary error sources of diameter accuracy of microshafts obtained by WEDG, and the empirical models of evaluating the relationship of infeed and radial direction (△R) are set up as shown in Eq. (11), and based on empirical models and machining strategy, the micro shafts with diameters of 45±2μm are successfully fabricated as displayed in Fig.19, which verifies the model to be effective and can be applied in improving machining repeatability.

$$ 2\Delta {R}_f=2\Big(\Delta {Y}_P+\Delta {Y}_T+{S}_r+\left(\sqrt{2}+1\right)\Delta {R}_W+{V}_W+\Delta G $$
(11)
Fig. 19
figure 19

a Effects of vibration of wire electrode, b factors affecting △YD, c effects of relaxed wire electrode when electrical contact occurs on △R, d fabricated results [2]

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where the G and △G are spark gap and its variations, respectively, Vw is wire electrode vibration amplitude, Sr is the runout of spindle, △YP is position variations of WEDG unit, and 2Rf and 2ΔRf are diameter and its tolerance of micro shaft, respectively.

To improve the prepared efficiency of microelectrodes, Sheu et al. [36] described a new method combining twin electrode wire with two discharge circuits to fabricate microelectrode tools as presented in Fig. 20, and their experimental results found that this twin-wire EDM system can significantly improve machining efficiency of microelectrodes compared to conventional WEDG technology.

Fig. 20
figure 20

a View of microelectrode tool machining by the twin-wire EDM system, b micro tool machining time of the twin-wire EDM system

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For reducing the effect of positioning error on machining accuracy of microelectrodes, the tangential-feed WEDG (TF-WEDG) method is proposed by Zhang et al. [38], and its principle is illustrated as shown in Fig. 21, the microelectrode is fed from position A to B and to C, and the distance of A and C position is defined as ST and can be calculated as Eq. (12). Several microelectrodes with diameter less than 70μm were machined repeatedly, and the consistent accuracy of microelectrode along axial direction can be confined to be 1μm, and the negative polarity machining involved in TF-WEDG process can further enhance the resolution and surface roughness of microelectrodes...

$$ {d}_A=2\left( OA-R\right)=2\ \left(\sqrt{{\left(R+r\right)}^2+{S}_T^2}-R\right) $$
(12)
Fig. 21
figure 21

a The TF-WEDG process, b three steps of TF-WEDG method, c error analysis of TF-WEDG, d microelectrodes with diameter of 27μm, e microelectrodes with diameter of 64μm, f diameter deviation

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Based on the reports mentioned above, it can be seen that machining efficiency of WEDG is low, and in order to improve the prepared efficiency of micro tools, Ravi et al. [102], Lim et al. [103], Zhao et al [35], and Siddhartha et al. [104] proposed block electrode discharge grinding (BEDG) to fabricate microelectrodes or micro rods at a higher machining speed.

Although machining efficiency is significant by the use of block electrodes and the process is simple, but, the fabricated micro tools fail to possess dimensional accuracy due to the existence of block electrode wear. Therefore, in order to solve this problem, Jahan et al. [105], Rahman et al. [106], Hourmand et al. [107], and Jahan et al. [108] proposed moving BEDG method, and the illustration of horizontal MBEDG (moving BEDG) process developed by Hourmand as shown in Fig. 22, and the microelectrodes with diameter of 97.14μm and 78.19μm are fabricated, and their experimental results disclosed that this MBEDG process is more beneficial for machining straight and thin microelectrodes with smooth surface, short time, and low cost. In order to decrease the discharge crater diameter of micro rods machined by MBEDG process, Koyano et al. [109] proposed HER (high electrical resistivity)-MBEDG method namely adopting the silicon tool electrodes for increasing the electrical resistivity, and they found the discharge craters diameter formed on micro-rods can be decreased to approximately 0.4 μm, and finally the cemented tungsten carbide micro-rod with diameter of 0.8 μm was manufactured.

Fig. 22
figure 22

a Illustration of horizontal moving BEDG process, b micro-electrode with diameter of 97.14μm [107]

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In order to enhance both the efficiency and accuracy of microelectrode using the conventional machine tool with low cost, Yin et al. [110] developed the electrical discharge machining grinding by two block electrodes (EDG-TBE) method without measuring device on-line as shown in Fig. 23 and desired diameter of microelectrodes mainly determined by the width of narrow slit and the discharge gap. They found that machining efficiency of microelectrode machined by EDG-TBE is higher than that of BEDG, and the error between machined diameter and target diameter is less than 2μm, and the cylindrical error of microelectrode also can be confined to be 2μm. In addition, Gil et al. [111, 112] presented the inverse slab electrical discharge milling (ISEDM) method and studied the influence of process variables including rotational speed and discharge energy on the efficiency, surface finish, and accuracy of micro pins machined by the ISEDM process. Their results showed that the micro pins with diameter of 200μm and aspect ratio of 90 can be economically manufactured. Minoru et al. [113] proposed self-drilled holes to form micro-rods which does not need initial positioning of the tool electrode, and the operation is easy and short as shown in Fig. 24. Their experimental results indicated that microelectrodes obtained by this method still existed taper errors after reverse copying many times. Naotake et al. [114] and Tani et al. [115] demonstrated a new method of microelectrode formation by scanning EDM, and a micro pin WC-Co electrode is formed with a diameter of about 40μm, and the machining time of tool electrode with diameter of 300μm that was changed to be 50μm only takes 3 min. Kim et al. [116] and Singh et al. [117] adopted reverse electrical discharge machining (REDM) to prepare microelectrodes or micro rods with various shapes and investigated the effect of capacitance and voltages on machining characteristics of REDM, and then optimal conditions for obtaining stable machining were provided.

Fig. 23
figure 23

a Schematic diagrams of EDG-TBE process, b comparison of efficiency of BEDG and EDG-TBE, c schematics of removal resolution according to machining types of BEDG and EDG-TBE, d experimental results of microelectrode of 86μm, e diameter deviation

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Fig. 24
figure 24

a Off-centering concept chart, b fabricated results of cylindrical microelectrodes

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As we all know, the WEDM process including LS (low speed)-WEDM and HS (high speed)-WEDM can realize the machining of parts with abnormalities and complex ruled surfaces, but it fails to manufacture parts with incomplete penetration surface or partial micro rotating structure. So in order to achieve the function of machining rotary surfaces on the WEDM workbench, Qu et al. [33], Matoorian et al. [118], Haddad et al. [119], Krishnan et al. [120], and Janardhan et al. [121] developed the cylindrical wire electrical discharge machining (CWEDM) to fabricate simple rotary parts and investigated effects of parameters on surface quality, roundness, and material removal rate; on this basis, Sun et al. [122, 123] make full use of the advantages of multiple cutting and the electrode wire unidirectional motion for LS-WEDM machine and proposed the LS-WEDT (low speed wire electrical discharge turning) method and adopted this method to fabricate slender cylindrical and spiral microelectrodes as displayed in Fig. 25, and the measured results disclosed that the errors of pitch length and thread angle respectively can be confined to less than 3μm and 3° due to in-existence of electrode loss, which indicated that the LS-WEDT method has superiority in fabricating complicated structure not just limited to the simple cylindrical parts, but in this method, the wire does one-way movement and cannot be reused, resulting in the relatively high cost.

Fig. 25
figure 25

a Slender microelectrode with diameter of 58μm, b spiral microelectrode with diameter of 150μm, c three-dimensional topography and section contour

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The wire electrode used in WEDG and CWEDM methods has the disadvantages of low material removal rate and easy wire breaking, so for the sake of increasing the material removal rate and avoiding wire electrode broken, Song et al. [124, 125] proposed the strip-EDM turning method, and a wide conductive strip is selected as an EDM electrode, and the strip was fed continuously as shown in Fig. 26; compared to CWEDM and LS-WEDT processes, the strip-EDM turning does not cause electrode vibration, electrode breakage or cusps on machined surfaces due to the strip are wide and flat. Their experimental results disclosed that the MRR of the strip-EDM turning was 74.3% higher than of CWEDM, and this method is competent to fabricate complex shapes, like a taper pin, a cylinder, and a polygon.

Fig. 26
figure 26

a Schematics of strip-EDM turning, b material removal rates according to radial depth of cut, c the SEM images of the machined surfaces of micro rods

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Moreover, for circumventing the limitation of single processing method and balancing the contradiction between processing efficiency and surface quality, some researchers developed the hybrid technique to prepare microelectrodes. Takahata et al. [126] developed a new process combining LIGA and micro-EDM process to fabricate a high-aspect-ratio tungsten carbide super hard alloy micro structure. Asad et al.[127] combined the micro-EDM and micro turning technology to produce the high aspect ratio electrodes. Kai et al. [128] fabricated some tungsten electrodes by combination of WEDG and electrochemical machining. Zhang et al. [38] developed the hybrid process combining the TF-WEDG and self-drilled hole method to fabricate microelectrodes for improving efficiency. Li et al. [129] prepared microelectrodes with paraboloid-shaped tip by four steps including the rough BEDG, semi-finish and finish WEDG, bottom trimming, and formation of paraboloid-shaped tip. In previous three steps, the microelectrode worked as the anode, and in the fourth step, the positive polarity machining was adopted, and microelectrode with paraboloid-shaped tip is formed after machining of 86 holes as displayed in Fig. 27.

Fig. 27
figure 27

a Schematic of microelectrode preparation processes, b rough grinding by BEDG, c semi-finish and finish grinding by WEDG, d self-drilling holes process and image of microelectrode with a paraboloid-shaped tip.

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4.2 Microelectrode array

At present, the microelectrodes array mainly can be prepared by the LIGA, R-EDM and micro WEDM techniques, and the LIGA process contains several different processes which makes the equipment cost high and machining efficiency low; micro WEDM method is primarily utilized in fabricating microelectrodes array with polygon structures but failure of rotary structures; R-EDM method namely forming microelectrodes array by replacing polarity of workpiece and tool electrodes, which is mainly used in the preparation of cylindrical microelectrodes array. Chen et al [131, 132] proposed a set of micro WEDM mechanisms and mounted it on the developed precise tabletop CNC machine tool, and in order to reduce wire electrode jitters, and the vibration suppression system is developed for effectively suppressing micro vibrations of the wire during discharging and then preparing the 10×10 squared microelectrodes with width of 21μm, height of 700μm, and spacing of 24μm between two electrodes as shown in Fig. 28. Zeng et al. [133] fabricated 10×10 squared microelectrode array with the width of about 30μm and the length of about 600μm and the distance between neighborhood electrodes of about 70μm using WEDM method as shown in Fig.29. Rakwal et al [134] adopted WEDM method to prepare high length-width ratio silicon microarray electrodes as displayed in Fig.19, where the tip size of taper electrode is about 95×94 μm and the cross section size of wave electrodes is about 220×220 μm. Arab et al [135] used reciprocating wire-EDM machine to fabricate multi-tip array tool and selected sufficient tension for avoiding wire wobbling and reducing overcut, and then the high aspect ratio 3×3 multi-tip array tool electrodes were obtained as shown in Fig.30. Fofonoff et al [136] fabricated an 8×8 microelectrode array WEDM process, and these 64 microelectrode array were used to record the voltage changes in the cerebral cortex of mice for studying the inhibitory and excitatory activities of neurons in the cerebral cortex of mice as shown in Fig. 31; besides, in another study [137], they also prepared array electrode with length of 1mm, width of 80μm, and spacing of 500μm by WEDM process, the number of electrodes was 8×8 after chemically etching, and the material was titanium, and microelectrode array of 1141 are obtained as displayed in Fig. 32.

Fig. 28
figure 28

a Machining diagram of microelectrode array, b electrode arrays from an isometric view, c electrode arrays from a front view, d machining live by an upward mode, e 10×10 microelectrode array, f enlarged view

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Fig. 29
figure 29

The squared microelectrode array fabricated by the WEDM: a top view, b 10×10 squared microelectrode arrays, c enlarged view

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Fig. 30
figure 30

The microelectrode array with different shape fabricated by the WEDM: a experimental setup for micro-electrode array fabrication using WEDM, b designed electrode geometries, c hourglass, d variable length, e wavy

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Fig. 31
figure 31

a Assembly equipment, b 8×8 microelectrode array made by WEDM, c voltage curve

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Fig. 32
figure 32

a Chemically etched array electrodes, b 1141 microarray electrodes, c 10×10 microarray electrode fabricated by the WEDM

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Additionally, cylindrical array micro holes are commonly used in components and parts of micro electro mechanical system, such as instrument components in aerospace inertial gyroscope, chemical fiber spinnerets, and high speed printer nozzles; for preparing these array micro holes, some scholars have prepared cylindrical array electrodes by REDM method. Kim [116], Mastud [138], Talla [139], Zhang [140], and Zeng [141], respectively, adopted reverse EDM process to fabricate cylindrical array electrodes, and the three microelectrodes of 35μm in diameter, the 3×3 tungsten electrode arrays, and 5×5 tungsten electrode arrays are shown in Fig. 33 a and b.

Fig. 33
figure 33

a Microelectrodes of 35 μm in diameter, b WC micro-rods, c 5 × 5 microelectrodes

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The microelectrode array fabricated by R-EDM process existed the problem of electrical corrosion products removal difficulty, secondary discharge, and instability discharge due to the large number of microelectrode array and cannot be rotated. Therefore, in order to improve chip removal performance and reduce the occurrence of electric arc and short-circuit involved in process of fabricating microelectrode array machined by R-EDM, the ultrasonic vibration is introduced to R-EDM by Zeng et al. [141], and they prepared 5×5 arrays of microelectrode with the diameter of less than about 30μm and height-to-width aspect ratios of larger than 8, and measured results disclosed that these microelectrode arrays have good coaxiality and surface quality as shown in Fig. 33c. For the preparing efficiency of microelectrode array, Hwang et al. [142] proposed the method of combining mechanical peck-drilling and R-EDM to fabricate micro-pin array, and their results indicated that the 1600 micro-pin array with an average diameter of 30μm, a length of 625μm, and a pitch of 100μm can be efficiently produced introducing working fluid spraying, vibration assisted electrode, and shake-down type workpiece as shown in Fig. 34.

Fig. 34
figure 34

The 1600 micro-pin array with an average diameter of 30μm and a length of 625μm

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5 Conclusion and outlook

This paper extensively reviews the application of EDM-based processes in fabricating micro tools as well as their characteristics, performances, measurements, remarks, existing problems, and future development direction based on previous researchers, and the following conclusions can be identified:

The EDM-based process is the most widely used and proved to be a promising technique for fabricating micro tools due to its versatile and non-contact nature. Dimensional accuracy, machining efficiency, and surface quality of micro tools are significantly affected by various tool and workpiece electrode factors, power factors, flushing fluid parameters, tool/workpiece electrode feed rate or way, etc. Besides, the analytical and predicted models for diameter, edge radius, runout error, and surface roughness of micro tools machined by EDM process are established, but the predicted results differ greatly from the test results. The preparation of micro tools has to enter into the scope of micro machining scale, the influences of micro defects or in-homogeneity of material, the flow of working liquid in discharge area, and the formation and annihilation of spark discharge channel on the machining precision and quality of micro tools fabricated by EDM processes are not clear; until now, the effective theoretical model for the EDM fabrication process of micro tools has not yet been established, which needs further investigation.

Some hybrid techniques have been proposed to fabricate micro tools, like WEDG along with micro-EDM, combination of WEDM, and diamond grinding. So it needs to vigorously develop the micro tool preparation techniques of compound processes in the future, such as integrating the laser, EDM, electrochemical, and ultrasonic vibration, and meanwhile combining the real-time monitoring, on-line detection, and CAD/CAM advanced technologies, which can satisfy the processing requirements of high precision and continuous development of new materials for micro tools as well as improving the machining efficiency and machining quality.

The WEDG method is firstly proposed to fabricate micro tools; for enhancing its machining accuracy, the TF-WEDG and RF-WEDG methods are developed; for improving fabricated efficiency, the twin-WEDG, strip-EDM, S/M-BEDG, etc. have been successively put forward; for getting rid of the limit of regulating wheel and realizing the process of complicatedly rotary surface, the CWEDT, EDT, LS-WEDT, etc. have been invented. Based on continuous enhancements of EDM-based process, the micro tools with straight or helical cutting blades and various cross sectional geometry made of super hard material (including PCD, cemented carbide, etc) can be successfully prepared. However, the experimental studies of micro tools prepared by EDM based processes are carried out the EDM test platform at home and abroad, but there is little research on overall development of special EDM machine tool for micro tool preparation. In order to meet the requirements and the actual production needs of micro tools made of increasingly diverse materials, gradually specialized structures, and continuous high-precision size, the micro tool EDM-fabricated techniques will develop towards the direction of high precision in nanometer scale and high efficiency in macro scale, which needs to develop high-performance micro tool special EDM machine.