**Desktop Micro-EDM System for High-Aspect Ratio Micro-Hole Drilling in Tungsten Cemented Carbide by Cut-Side Micro-Tool**

#### **Yung-Yi Wu <sup>1</sup> , Tzu-Wei Huang <sup>2</sup> and Dong-Yea Sheu 2,\***


Received: 15 June 2020; Accepted: 7 July 2020; Published: 11 July 2020

**Abstract:** Tungsten cemented carbide (WC-Co) is a widely applied material in micro-hole drilling, such as in suction nozzles, injection nozzles, and wire drawing dies, owing to its high wear resistance and hardness. Since the development of wire-electro-discharge grinding (WEDG) technology, the micro-electrical discharge machining (micro-EDM) has been excellent in the process of fabricating micro-holes in WC-Co material. Even though high-quality micro-holes can be drilled by micro-EDM, it is still limited in large-scale production, due to the electrode tool wear caused during the process. In addition, the high cost of precision micro-EDM is also a limitation for WC-Co micro-hole drilling. This study aimed to develop a low-cost desktop micro-EDM system for fabricating micro-holes in tungsten cemented carbide materials. Taking advantage of commercial micro tools in a desktop micro-EDM system, it is possible to reach half the amount of large-scale production of micro-holes. Meanwhile, it is difficult to drill the deep and high aspect ratio micro-holes using conventional micro-EDM, therefore, a cut-side micro-tool shaped for micro-EDM system drilling was exploited in this study. The results show that micro-holes with a diameter of 0.07 mm and thickness of 1.0 mm could be drilled completely by cut-side micro-tools. The roundness of the holes were approximately 0.001 mm and the aspect ratio was close to 15.

**Keywords:** Tungsten cemented carbide (WC-Co); desktop micro-electrical discharge machining (micro-EDM) system; cut-side micro-tool; micro-holes

#### **1. Introduction**

Tungsten carbide (WC) is a materiel which is widely applied in military industrial composite, metallurgy, aerospace, and other important fields because of its excellent physical and chemical properties [1]. Pure WC is very brittle. If doped with small amounts of titanium, cobalt, or other metals, the incorporated brittleness can be reduced. The interaction between Co-based binders and WC grains on early stages of liquid-phase sintering can be strongly affected by the carbon content in the binders [2]. Tungsten cemented carbide (WC-Co) has a series of excellent properties, such as hardness, strength, toughness, wear resistance, and corrosion resistance [3,4].

Micro-holes made fromWC-Co are widely used as spraying nozzles, injection nozzles, and spinning nozzles, owing to their low wear and hardness [5]. Machining processes, such as micro-mechanical drilling, laser machining (LBM), and electron beam machining (EBM), are typically used for the massor semi-mass production of micro-holes in WC-Co materials. The micro-electrical discharge machining (micro-EDM) process is highly suitable for micro-hole fabrication because it is burr-free and efficient

irrespective of the workpiece hardness, especially since the development of the WEDG technology [6–8]. However, micro-EDM still has some limitations for the mass production of micro-holes due to the low productivity of the micro-tools used for fabrication [9,10].

In order to achieve semi-mass production of micro-hole drilling using EDM in WC-Co material, a low-cost desktop micro-EDM system was developed in this study. Off-the-shelf spindle electrode tools with diameters of 0.15 mm were used directly as microelectrode tools. These tools have proven to be commercially successful in low-cost mechanical drilling. Using these commercially available micro-spindle tools, it was possible to achieve the semi-mass production of a WC-Co material with micro-holes drilled via a desktop micro-EDM system. However, small diameter and long electrode tools with diameters less than 0.1 mm are not commercially available. Therefore, in this study, a WEDG unit was attached to the desktop micro-EDM system to fabricate micro-spindle tools with diameters less than 0.01 mm by using commercially available 0.15 mm tools. In order to produce micro-holes with a high aspect ratio, a single-side notch electrode method was applied to flush debris. The machining parameters, such as machining time, aspect ratio, spindle tool wear, and micro-hole quality, were investigated in this paper. It is expected that the desktop micro-EDM system will be potentially useful for drilling micro-holes in tungsten carbide materials.

#### **2. Structure of the Desktop Micro-EDM**

#### *2.1. Desktop Micro-EDM Structure*

The micro-EDM with three axis computer numerical control (CNC) controllers has been commercialized in the industry market [11]. The high accuracy controlling system makes micro-EDM more expensive. However, most micro-EDM systems are still designed for micro-holes drilling in industry applications. Low productivity of micro-holes drilling is still the main challenge for micro-EDM due to the significant tool wear and micro-tools fabrication [12,13]. To achieve mass-production of micro-hole fabrication, these two factors need to be addressed in this paper.

The desktop micro-EDM system was developed and designed in this research. The block diagram of the operation relationship of each unit is shown in Figure 1. The system has only three axes, X, Y, Z for the micro-hole drilling, as shown in Figure 2. The X-Y stages were controlled manually with digital indicators, and the Z-axis was controlled by the microcontroller unit (MCU). The most important components of the desktop EDM system were the V-shaped block and the spindle electrode tools. The structure of the spindle tool is shown in Figure 3. The spindle tool was mounted on the V-shaped block and rotated by a direct current (DC) motor. The rotation speed was variable. The linear straightness and roundness of the spindles were important to ensure highly accurate micro-hole drilling. This machine was mainly suitable for micro-holes with diameters of 0.15 mm or less with micro resistor capacity electro discharge circuit (RC circuit). In this paper, the desktop EDM was designed for micro-holes drilling with diameters less than 0.15 mm. The desktop micro-EDM system was available for electro-conductive materials and used conductive materials such as tungsten (W), tungsten cemented carbide (WC), die steel (SKD), and stainless steel (SUS). The discharge energy of the desktop micro-EDM system simply adopts a RC discharge circuit with DC power supply of 80 to 100 V, and the workpiece cannot be touched during discharge machining. The discharge power must be turned off when replacing the microelectrode tool. The main specifications of the desktop micro-EDM system is shown in Table 1 [14].

**Figure 1.** The block diagram sketch of the desktop micro-electrical discharge machining (micro-EDM) system.

**Figure 2.** Complete structure of the desktop micro-EDM system.

**Figure 3.** Structure of the spindle tools on the V-shaped block.


**Table 1.** Main specification of the desktop micro-EDM system.

#### *2.2. Commercially Available Spindle Tools*

The desktop micro-EDM system uses off-the-shelf spindle electrode tools. The micro-tool length and diameter are 5 mm and 0.15 mm, respectively, as shown in Figure 4. It was successful and commercially available for the mass production of electrode tools and had a shank diameter of 3.0 mm and no screw slots due to the mechanical grinding process. The micro-tool accuracy of both the diameter and the length was approximately 0.003 mm for tungsten cemented carbide. Figure 5 shows the spindle micro-tool with pulley [15]. The pulley was fixed onto the spindle tool in the system, and it rotated directly on the V-shaped block. The rotation roundness of spindle tool was approximately 0.5 µm without any vibration. Due to a low tool wear ratio of the tungsten carbide by micro-EDM, the tool electrode is not available for commercial polishing process if it is made by tungsten. The WC-Co material demonstrates the possibility of mass production of the micro-tool by mechanical grinding process [16]. Even though it was possible to fabricate spindle micro-tools with a diameter of 0.05 mm by conventional grinding process, the aspect ratio was only 3 or 4 due to the mechanical grinding force. This low aspect ratio of spindle tools makes them unsuitable for mass-drilling micro-holes by micro-EDM. The other critical challenge involved in grinding is the fabrication of micro-tools with diameters less than 0.03 mm. In this paper, WEDG technology was used for micro-tools fabrication with diameters less than 0.03 mm by using commercial tools directly for high aspect ratio micro-hole drilling. μ μ

**Figure 4.** Commercially available spindle micro-tool without the screw slot. Diameter of shank: 3 mm. Diameter of tool: 0.15 mm.

**Figure 5.** Spindle micro-tool with pulley.

#### *2.3. WEDG Technology Unit*

As described in the previous section, mechanical grinding process has been successful for the mass production of micro-spindle tools. The diameter was approximately 0.003 mm. However, it is still difficult to fabricate spindle tools with diameters less than 0.1 mm by using the grinding process, due to the mechanical grinding force [17]. It is possible to produce small spindle tools with a high precision grinding machine, however, the aspect ratio is only 3 or 4 with diameters less than 0.1 mm. Hence, in this study, in order to fabricate ultra-micro-holes with diameters of less than 0.05 mm, the wire-electro-discharge grinding (WEDG) technology unit was attached to the desktop micro-EDM system, as shown in Figure 6 [18,19]. The off-the-shelf spindle tools with diameters of 0.15 mm and lengths of 5 mm from the commercial market were used directly. The material of the spindle tool was tungsten carbide, which provides sufficient toughness and rigidness. Compared to the conventional WEDG process, the micro-electrode tool fabrication technique employed in this study will be more efficient, due to the finishing process post-machining. The x-axis was manually controlled by a digital micrometer head with the position control display resolutions as low as 1 µm, as shown in Figure 7. By aligning the position of the x-axis, the micro-electrode tools could easily be fabricated using WEDG technology. It is thus possible to shape the tools through one machining process only. μ μ

**Figure 6.** Wire-electro-discharge grinding (WEDG) technology unit attached to the desktop micro-EDM system.

**Figure 7.** X direction manual control by digital micrometer head.

#### *2.4. Control Pad Micro-Controller Chips (dsPIC)*

The conventional micro-EDM system is usually operated by numerical control (NC) or computer numerical control (CNC) controllers [20]. In addition, the position alignment and the scanning process with tool compensation are also possible. However, the large cost of conventional micro-EDM systems makes EDM unpopular for micro-hole drilling. To reduce the cost, this study used the MCU digital signal peripheral interface controller (dsPIC), to control the movement of the spindle tool and to detect the discharge gap [21]. The I/O connection provides drilling depth selection. The z-axis of the spindle micro-tool feeding rate is controlled by detecting voltage of the gap discharge. By only pushing the start button, the desktop micro-EDM system can automatically produce micro-holes. The diameter of the microelectrode tool could be manually adjusted using the x-axis position alignment with high resolution indicator. The micro-tools and micro-hole fabrication can be carried out on the same desktop micro-EDM system. Complete internal structure of the desktop micro-EDM system is as shown in Figure 8.

**Figure 8.** Internal structure of the desktop micro-EDM system.

#### **3. Micro-Holes Drilling by Desktop Micro-EDM System**

#### *3.1. Micro-Electrode Tool Fabrication by WEDG*

In this study, two types of micro-spindle tools with diameters 0.15 mm and 0.035 mm were used to fabricate the WC-Co micro-holes by micro-EDM drilling. To achieve the semi-mass production of micro-hole drilling using micro-EDM, commercial micro-spindle tools with a diameter of 0.15 mm can be used directly, but there is no supply for diameters less than 0.050 mm. However, fine micro-spindle tools may be produced by the WEDG grinding process, as shown in Figure 5. In order to fabricate micro-tools with diameters less than 0.035 mm, the WEDG unit mounted on the table was still used to reform the commercial spindle tools using only the finishing process. Figure 8 shows the potential of using the desktop EDM system to fabricate micro-tools with diameters of only 0.007 mm with tungsten carbide, as shown in Figure 9. It is thus possible to fabricate micro-holes with diameters less than 0.01 mm using this low-cost desktop micro-EDM system. By using commercial tools directly, the desktop EDM is able to produce micro-spindle tools more efficiently without rough machining.

**Figure 9.** Microelectrode tool with diameter of 0.007 mm.

#### *3.2. Micro-Hole Drilling by Micro-EDM System*

μ μ μ μ μ The desktop micro-EDM system uses an RC discharge circuit and DC power with an open voltage of 80 V [22]. The machining conditions of the desktop micro-EDM system is as shown in Table 2. The V-shaped block was mounted onto the desktop micro-EDM system with high accuracy. Commercial micro-tools were used directly without the WEDG technology and the electrode tools are available from commercial market. However, for micro-tools with diameters less than 50 µm, the WEDG technology was still applied for reforming in this study. The main characteristic of the low-cost desktop micro-EDM system is that micro-tool and micro-hole convenience fabrication can be achieved conveniently on the same machine. After shaping the microelectrode tools, it is possible to drill micro-holes directly on the same micro-EDM. Experimental results of micro-hole drilling using tools with diameters of 150 µm and 35 µm are shown in Figure 10. It is facile to drill micro-holes on the WC-Co using this desktop micro-EDM system without any burr. It takes less than 2 min to drill a micro-hole of 150 µm diameter by the 1000 pf capacitor; and it takes approximately 5 min to drill a micro-hole of 40 µm when using discharge capacitor of 100 pf. Thus, when conducting less discharge capacitance, the machining time takes longer, and the electrode wear increases. Therefore, a small capacitance value will result in optimal micro-holes machining.

**Table 2.** Machining conditions of the desktop micro-EDM system.


grind μ


**Figure 10.** Micro-hole drilling on tungsten cemented carbide (WC-Co) through a desktop micro-EDM system.

μ μ μ μ μ μ The surface observation of the micro-holes by Scanning Electron Microscope (SEM) is shown in Figure 11 [23]. Compared to another machining process, the desktop micro-EDM process can meet the requirement of micro-holes drilling on WC-Co material without any burrs. The experimental results show micro-tools with diameters less than 0.01 mm and micro-holes with diameters less than 0.05 mm. In micro-hole machining, the horizontal diameter (Dx) for a 0.3 mm thick workpiece is 39 µm, and the vertical diameter (Dy) is 40 µm; whereas the horizontal diameter (Dx) for 0.5 mm, the thick workpiece, is 66 µm, and the vertical diameter Dy is 66 µm. Therefore, the machining roundness is approximately 1 µm. For machining workpieces with apertures of 40 and 66 µm and thicknesses of 0.3 and 0.5 mm, the aspect ratio is about 8. μ μ μ μ μ μ

( **Figure 11.** Surface of WC-Co micro-holes observed by Scanning Electron Microscope (SEM). (

#### *3.3. Electrode Tool Wear and Roundness*

Figure 12 shows the length of tool wear of EDM micro-holes drilling with different electrical capacites. The micro-tools maintain their shape with only little tool wear by using small electric discharge capacities [24]. However, the tool wear increases significantly when the diameter is smaller than 0.05 mm. Even though the larger electric discharge capacities could increase the efficiency of micro-hole drilling, it leads to greater tool wear and deterioration of the micro-hole roundness. Therefore, it is necessary to consider all parameters to identify a suitable discharge capacity. In this study, for a work piece thickness of 0.3 mm and tool electrode diameter of 150 µm, approximately 1000 pF is the ideal drilling capacitance, whereas, for a tool electrode diameter of approximately 35 µm, a capacitance of 100 pF is the optimal value for micro-hole drilling. μ μ

**Figure 12.** Tool wear in micro-hole drilling of WC-Co.

#### *3.4. Limitations of the Machining Depth*

μ μ μ Normally, transistor discharge circuits are used in commercial EDM because the discharge energy is controllable by adjusting the pulse generator and duty cycle [25]. However, micro-EDM requires extremely small electric discharge energy, especially for micro-tools with diameters less than 50 µm. The resistor capacity (RC) pulse generator is popular and widely used for the micro-EDM electric discharge energy [26]. The discharge energy depends on the capacity of the RC pulse generator. Therefore, the main parameter of the RC discharge circuit is the magnitude of capacity. Theoretically, it is possible to increase the machining speed with large electric capacity due to the large discharge energy. The large capacitors could cause bigger discharge sparks, however, the large discharge energy will lead to significant tool wear. The high aspect ratio of micro-holes will not be drilled to penetrate completely as the micro-tool wear will exceed the feed depth. As shown in Figure 13, the spindle feeding speed is efficient without any stagnation while the feeding depth is below 800 µm. However, the machining speed decreases significantly when the spindle feeding depth is larger than 800 µm due to insufficient debris flushing. Even the spindle feeding depth increases. However, this phenomenon means a large amount of micro-tool wear. It is clear that the micro-EDM drilling process is a method to fabricate micro-holes with aspect ratios less than 5 exclusively. The high aspect ratio micro-spindle tools are not possible for deep micro-hole drilling using micro-EDM due to the significant tool wear [27].

**Figure 13.** Micro-tools wear on WC-Co by the desktop micro-EDM.

#### *3.5. High-Aspect Micro-Holes Fabrication*

μ μ μ μ μ Micro-EDM encounters another critical problem for high-aspect ratio on deep micro-holes machining, especially when the diameter is less than 100 µm and the aspect ratio is larger than 10 [28]. The cut-side shaped micro-electrode tool was capable of fabrication using the desktop micro-EDM system. The purpose is to enhance the machining efficiency for micro-holes drilling with high aspect ratio. The 50 µm diameter tool-electrode can be ground out to 10 µm in depth by the WEDG technology machining unit without any tool spindle rotation, shown as in Figure 14. The side view and front view of cut-side shaped micro-tool after the WEDG process is shown as in Figures 15 and 16. The cut-side electrode tools of special shapes are able to improve the debris removal problem, but there is still no elevation for higher aspect ratio due to high tool wear [29,30]. Figure 17 shows the comparison between the feeding depth of the cylinder tool and cut-side tool. The experiment shows that initially the smaller cut-side electrode brought more electrode wear than the bigger cylindrical electrode. Later, the cut-side electrode contrarily brought less electrode wear than the bigger cylindrical electrode, for its larger space removed debris faster. Under such circumstances, to reduce high tools wear and increase machining efficiency of micro-holes EDM drilling, the cylinder and cut-side shaped micro-electrode tool should be shifted alternately for deep micro-holes drilling. In micro-holes machining with high-aspect ratio, the feeding depth was drilled alternately by cylindrical and cut-side micro-electrode tools. At first, the cylindrical micro-tool with less electrode wear started to drill micro-holes to reach the feeding depth limit area, and then shifted to the cut-side electrode with larger space to process it at the same speed without moving the workpiece. Continuing the process until the micro-holes completely drilled might improve the machining efficiency. The best machining method was achieved by using the cylinder- and cut-side shaped micro-tools alternately; a micro-hole with high aspect ratio will be drilled completely at 15 times larger in about 10 min of the machining time. The results show that it can be completely drilled by cut-side micro-tools, and high aspect ratio drilling can be performed on WC-Co material with a thickness of 1.0 mm, with an inlet diameter of 73 µm and an outlet diameter of 58 µm, as shown in Figure 18. The micro-holes surface with high aspect ratio on WC-Co was observed through SEM, as shown in Figure 19.

**Figure 14.** Cut-side shaped micro-tool process by WEDG.

**Figure 15.** Side view of cut-side shaped micro-tool.

**Figure 16.** Front view of cut-side shaped micro-tool.

**Figure 17.** Comparison between the feeding depth of the cylinder tool and cut-side tool.

**Figure 18.** Micro-hole drilling of high aspect ratio on WC-Co through a desktop micro-EDM system.

**Figure 19.** Surface of micro-hole drilling of high aspect ratio on WC-Co observed by SEM.

#### **4. Conclusions**

A review of the research trends in micro-EDM about various electrode tools and their effects in the characteristics of micro-EDM are presented. A low-cost desktop micro-EDM system was explored and developed for rapid drilling of micro-holes through tungsten cemented carbide in this study. Using commercial electrode tools of 50 up to 150 µm, for about 2 to 4 min, it is possible to achieve semi-large-scale production of micro-holes. The desktop micro-EDM system is also able to drill micro-holes by mechanical drilling using a micro-spindle tool with a screw slot. Besides, the WEDG technology can even be employed for more meticulous micro-electrode and shaping tool fabrication. In addition to superb microelectrode tool fabrication, micro-hole drilling could be also done by the same machine. Compared to the nearly one million dollars and high prices of commercial micro-EDM systems, there are more potential applications for the drilling of tungsten cemented carbide in the low-cost desktop micro-EDM system developed in this study, which is able to produce roundness of a micro-hole of approximately 1 µm and a 9 times standard aspect ratio, enhanced to 15 times in high-aspect ratio. We hope that research and analyses on machining characteristics of double cut-side tool electrodes will be continued in the future.

**Author Contributions:** Conceptualization, D.-Y.S. and Y.-Y.W.; methodology, Y.-Y.W., T.-W.H. and D.-Y.S.; software, Y.-Y.W. and T.-W.H.; validation, D.-Y.S.; software, Y.-Y.W. and T.-W.H.; formal analysis, D.-Y.S. and Y.-Y.W.; software, Y.-Y.W. and T.-W.H.; investigation, Y.-Y.W., T.-W.H. and D.-Y.S.; resources, D.-Y.S.; data curation, Y.-Y.W.; writing—original draft preparation, Y.-Y.W.; writing—review and editing, D.-Y.S.; visualization D.-Y.S. and Y.-Y.W.; supervision, D.-Y.S.; project administration, D.-Y.S. and Y.-Y.W.; funding acquisition, T.-W.H. and D.-Y.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the MOST of Taiwan grant number [MOST 107-2221-E-027-050-MY2].

**Acknowledgments:** The authors explain the gratefully acknowledge to the Ministry of Science and Technology (MOST) Taiwan.

**Conflicts of Interest:** The authors declare no conflict of interest

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Shaping Soft Robotic Microactuators by Wire Electrical Discharge Grinding**

**Edoardo Milana <sup>1</sup> , Mattia Bellotti <sup>1</sup> , Benjamin Gorissen 1,2, Jun Qian <sup>1</sup> , Michaël De Volder 1,3 and Dominiek Reynaerts 1,\***


Received: 13 June 2020; Accepted: 3 July 2020; Published: 4 July 2020

**Abstract:** Inflatable soft microactuators typically consist of an elastic material with an internal void that can be inflated to generate a deformation. A crucial feature of these actuators is the shape of ther inflatable void as it determines the bending motion. Due to fabrication limitations, low complex void geometries are the de facto standard, severely restricting attainable motions. This paper introduces wire electrical discharge grinding (WEDG) for shaping the inflatable void, increasing their complexity. This approach enables the creation of new deformation patterns and functionalities. The WEDG process is used to create various moulds to cast rubber microactuators. These microactuators are fabricated through a bonding-free micromoulding process, which is highly sensitive to the accuracy of the mould. The mould cavity (outside of the actuator) is defined by micromilling, whereas the mould insert (inner cavity of the actuator) is defined by WEDG. The deformation patterns are evaluated with a multi-segment linear bending model. The produced microactuators are also characterised and compared with respect to the morphology of the inner cavity. All microactuators have a cylindrical shape with a length of 8 mm and a diameter of 0.8 mm. Actuation tests at a maximum pressure of 50 kPa indicate that complex deformation patterns such as curling, differential bending or multi-points bending can be achieved.

**Keywords:** wire electrical discharge grinding (WEDG); micromoulding; soft microrobotics; electrical discharge machining (EDM)

#### **1. Introduction**

Soft robotic systems are capturing the interests of scientists and engineers with characteristics that are breaking with conventional robot traditions. Softness, compliancy and cost-effective manufacturing make soft robots preferable in applications where gentle manipulation and human interaction occur [1]. Thanks to their low mechanical stiffness, soft robots can safely operate in unstructured environments by adapting to unforeseen collisions and reduce the risk of harmful events [2]. For those reasons, soft robotic technology is extensively utilised to make universal grippers [3], some of which have been commercialised [4]. Other promising soft robotic applications include robots for search and rescue operations [5,6] as well as innovative techniques to make untethered and entirely soft machines [7]. There is an increasing interest in downscaling soft robotic technology to micrometre scales to drive advances in applications where the operational environment is unpredictable or extremely delicate, such as minimally invasive surgery and drug delivery [8–10]. Further, soft microrobotics has already been applied in microfluidics for making flexible active valves [11] and artificial cilia for biomimetic micromixing and micropumping [12].

Soft robots necessitate actuators that display large deformations as a response to a generalised force input. Typically, soft actuators are designed such that their deformation corresponds to a desired kinematic trajectory, behaving as a compliant mechanism with one degree of freedom. Many types of soft actuators can be distinguished according to the nature of the applied forces [8], which can vary from electrical (dielectric or ionic polymers) to magnetic fields (magnetic polymers), from solvent concentrations (hydrogels) to pressurised fluids (inflatable structures), etc.

Elastic inflatable actuators (EIAs) are one of the most widespread soft actuators, that rely for their motions on the morphology of the actuators [13]. Such actuators at small-scale were first introduced by Suzumori [14,15] and Konishi [9,16] for biomedical applications. For a comprehensive review of the different types of EIAs we referred to previous studies (see [13,17]). Current miniaturised EIAs are made with a single inflatable elastomeric cavity which leads to a simple motion that can be bending [18], twisting [19], contracting [20] or extending [21]. Large-scale actuators, on the other hand, have been presented with richer deformations, originating from a more complex design. However, these complex designs are challenging to copy at smaller scales due to manufacturing limitations. At a larger scale, inflatable actuators are typically made out of different elastomeric parts, which are subsequently bonded or glued together. This approach makes it possible to create intricate actuator geometries that drastically modify the actuator performance as shown by Mosadegh et al. [22] for pneumatic networks (PneuNets) bending actuators. However, at smaller scales, creating similar actuators by combining parts and bonding them is an uphill task, due to more stringent requirements on the tolerances of the parts to be assembled, possible misalignments and handling problems. A commonly used bonding process for sub-centimetre soft actuators in polydimethylsiloxane (PDMS) consists of oxygen plasma treatment to activate the PDMS surfaces before bonding [23]. However, these bonds are often the weakest part of the structure which eventually causes the actuator to rupture.

In order to circumvent these issues, we propose a bonding-free technique to fabricate millimetre-scaled soft bending microactuators, using out-of-plane moulding [18]. These microactuators consist of PDMS cylindrical structures with a simple cylindrical inflatable cavity, which is placed eccentric to the axis of the outer cylinder of the actuator. This eccentricity introduces an asymmetry in the cross-section of the actuator that causes the actuator to bend. The mould is composed of two micro-milled parts and a cylindrical microrod placed in between the two parts, where the shape of the microrod is replicated as the inflatable cavity. We used these devices for different applications such as artificial cilia [24] and flexible endoscopes [25]. However, given the simple morphology of the inflatable cavity, these microactuators have a limited operating range with a maximum bending angle of up to 45◦ [25].

In this paper we introduce an additional manufacturing step to this bonding-free technique in order to fabricate more complex microactuators. During this additional step, the cylindrical microrods are machined using a wire electrical discharge grinding (WEDG) process. WEDG is a manufacturing process in which material is removed from a rotating tool using a running wire through high-frequency sequences of electric discharges. WEDG, which was early developed by Masuzawa et al. [26], has been established over the past years as a proven technology for machining axisymmetric microrods down to less than 10 µm in diameter [27]. Nowadays, WEDG is used for machining not only cylindrical, but also tapered microrods [28] as well as microrods with a spherical tip [29]. WEDG'ed microrods find a wide variety of applications in industry such as touch probes for contact measurement systems [30] or microtools for drilling tapered microholes [31] and microhole arrays [32].

WEDG was already used for the fabrication of classic pneumatic microactuators [33], but not for soft actuators. Here, WEDG is used for machining axisymmetric microrods with more complex shapes to be placed in the internal cavity of inflatable soft robotic actuators in a micromoulding process. Dimensional and surface metrology are used to check that the accuracy and surface quality of the

fabricated microrods are compatible with the moulding requirements. Further, an analytical model is used to predict the deformation of the moulded actuator. The model is validated by experimental tests on prototypes. These tests clearly show the benefits in terms of deformation patterns of using structured microrods over their unstructured counterparts. The paper is structured as follows: in the first section we report on three different morphologies of inflatable cavities and the details about the WEDG process to realise the respective internal shapes. Subsequently the manufacturing steps involved in the moulding process are described. Finally, the microactuators are characterised and compared to the state-of-the-art.

#### **2. Materials and Methods**

#### *2.1. Inflatable Cavity Morphologies*

All microactuators described in this work have the same general architecture and only differ in the shape of the inflatable cavities. Nevertheless, the deformation changes significantly as further reported. The microactuators are cylindrical pillars, 8 mm in length and 0.8 mm in outer diameter, with inflatable cavities that are placed at an eccentricity, *e*, of 110 µm. We designed the inflatable cavities to be compatible with the WEDG process as described in Section 2.2. Thus, all cavities are axisymmetric with local variations of the radius across the length. We identified three different actuators with distinct rod shapes that lead to different functional deformations. In the paper we refer to them as: (i) *Saw* (Figure 1a); (ii) *Totem* (Figure 1b); (iii) *Halter* (Figure 1c).

**Figure 1.** Inflatable cavity morphologies: (**a**) *Saw*, (**b**) *Totem*, (**c**) *Halter*. The cavities are axisymmetric, and the colour code corresponds to the local diameter of each segment. The segment lengths are reported in Table 1.


The inflatable cavities of the three actuators can be divided into segments of different length and diameter. The segment lengths are reported in Table 1, while the diameters can be found in Figure 1, where the colour codes distinguish the different segments. In total, the combined length of the inflatable cavities equals 7.5 mm.

Given the eccentricity of the cavity with respect to the axis of symmetry of the structure, all actuators undergo a bending motion upon pressurisation, as explained in our previous paper [18]. Due to the asymmetric placement of the central void (Figure 2a), the centroid of pressure is shifted with respect to the neutral (bending) axis of the structure, resulting in a large bending deformation. In Figure 2b,c, the 3D representation and longitudinal cross-section of a microactuator of *Totem* type are shown as an example. The eccentricity, *e*, corresponds to the distance between the axis of the inflatable cavity and that of the rubber structure.

**Figure 2.** (**a**) Cross section of the microactuators. The distance between the neutral axis and the centre of the inflatable cavity (in white) is the moment arm *d*. (**b**) 3D representation of the microactuator with a *Totem* morphology of the inflatable chamber. Section planes are displayed. (**c**) Longitudinal section of the microactuator. The distance between the axis of the inflatable cavity and that of the rubber structure corresponds to the eccentricity, *e*.

#### *2.2. WEDG Process*

WEDG is used for shaping the microrods to the desired shapes. In this research, we developed a WEDG processing strategy using the WEDG unit of a SARIX® SX-100-HPM micro-EDM machine (Figure 3a). This unit is equipped with a brass wire of 200 µm in diameter (*Dwire*), which is used for machining the microrods. The brass wire runs continuously during the WEDG process. Cylindrical microrods in tungsten carbide provided by SARIX® are used. The microrods, which have a nominal diameter (*Drod*) of 500 µm, are clamped in the spindle of the micro-EDM machine tool. In Figure 3b, a schematic illustration of the performed WEDG process is shown. During the WEDG process, hydrocarbon oil of viscosity equal to 2.4 mm<sup>2</sup> /s at room temperature (HEDMA® 111) is applied as a dielectric fluid.

**Figure 3.** (**a**) Experimental setup and (**b**) schematic view of the wire electrical discharge grinding (WEDG) process. In the magnified view in (b), the wire diameter (*D*wire) and the depth of cut (*a*p) are indicated. The figure is adapted from [34].

In order to reduce machining time, WEDG processing is carried out using two machining regimes: roughing and finishing. Table 2 lists the processing parameters applied in each machining regime. In both cases, a positive polarity is applied to the microrod. In roughing, a relatively high amount of energy per discharge (average discharge energy: 46.5 µJ) is applied in order to increase the machining efficiency. The amount of energy per discharge is reduced during finishing (average discharge energy: 3.8 µJ). The average energy per discharge is computed from samples of the voltage and current signals including roughly 2000 pulses following the same methodologies we applied in a previous study [35].


**Table 2.** Wire electrical discharge grinding (WEDG) processing parameters.

<sup>1</sup> Estimated value.

The WEDG process for shaping the microrods consists of multiple roughing steps and a single finishing step. A radial depth of cut *(ap*) equal to 20 µm is applied during the roughing step. This value is chosen based on the results of some preliminary experiments, which were carried out to maximise the material removal rate (MRR) during roughing [34]. In the finishing step, a 10 µm radial depth of cut is applied. The finishing step is meant to ensure a higher machining accuracy and improve the surface quality of the microrods. In order to assess the accuracy and quality of the WEDG'ed sections on the microrods, post-process metrology is carried out by optical microscopy (ZEISS® SteREO Discovery V20), confocal microscopy (Sensofar® S lynx) and scanning electron microscopy (Phenom® Pro).

#### *2.3. Moulding*

The microactuators are fabricated through a bonding-free out-of-plane moulding process, as sketched in Figure 4. The mould consists of two aluminium micromilled parts. The bottom part contains the designated holes for the microrods (Figure 4a) as well as features for releasing the microactuators, while the top part presents through-holes with a diameter equal to the external diameter of the microactuators (0.8 mm). The holes in the two parts are drilled in such positions so that when aligned and assembled they create an eccentricity of 110 µm of the inflatable cavity. Before pouring the uncured rubber, the mould surfaces are coated with a layer of release agent (Devcon).

**Figure 4.** Fabrication process steps. (**a**) WEDG'ed (wire electrical discharge grinding) microrod is placed in the designated hole of the aluminium bottom half of the mould. (**b**) Uncured rubber is poured on the bottom half. (**c**) The top half is aligned and tightened to the bottom half. (**d**) After curing, the top part is removed. (**e**) Microactuator demoulding. (**f**) Microrod removal (figure adapted from [36]).

The two liquid prepolymers of the silicone rubber (Dragon Skin™ 30 by Smooth-On) are thoroughly mixed in a 1:1 ratio for 2 min. The uncured rubber is subsequently placed in a vacuum chamber for 5 min to make sure no air is trapped inside. Indeed, this is a fundamental step as the presence of microscopic air bubbles in the cured rubber dramatically affects the mechanical properties, and due to the small size of the actuators also cause imperfections and unwanted voids.

After filling the bottom part of the mould (Figure 4b), we degas it again for 5 min in the vacuum chamber. The top part of the mould is aligned with respect to the bottom part through alignment pins and firmly tightened (Figure 4c). This tightening ensures that the uncured rubber flows in all the features of the mould. The mould is subsequently placed in the oven at 60 ◦C for 1 h to let the elastomer cure. After curing, the mould is opened, using ethanol as lubricant (Figure 4d). Given their intricate shape, the microrods are stuck in the microactuators after demoulding (Figure 4e). Removing the microrods is the last and most delicate step, which requires the use of ethanol to slightly swell the silicone and allow a safe removal of the rod (Figure 4f). Since the rubber cures around the microrod, the absence of air generates a negative relative pressure that locks the microrod inside the microactuator. The function of ethanol is to both lubricate and swell the rubber so that air can penetrate the cavity and eliminate the negative relative pressure. This effect combined with the compliancy of the soft material enables the removal of the microrod. After removing the microrod, the microactuators are dried at room temperature to fully evaporate the ethanol. Then, they can be connected to pressure supply tubing.

Microactuators of the *Halter* type require a different process to remove the microrod. Indeed, due to the large diameter variation (from 500 µm to 100 µm), the swelling-induced removal of the microrod is not effective. Alternatively, the microrod is broken at the thin section and the two parts are extracted from the base and the tip of the microactuator. The tip is subsequently sealed with a drop of uncured rubber.

#### *2.4. Analytical Model*

In order to evaluate the deformation patterns associated with the different inflatable cavity morphologies of the actuators, we apply a multi-segment linear analytical model consisting of Euler–Bernoulli beam segments that are each loaded with a constant bending moment. For one segment, this model has already been applied to capture the overall bending deformation of a soft bender [18,37,38], while here we introduce a segmented approach. The different actuator types that we analyse in this work can be divided into *n* segments (Table 1), where each radial variation *r<sup>i</sup>* is considered as the *i*-th segment (*i* = 1, . . . , *n*) of length *l<sup>i</sup>* . In our model, each segment is subjected to a constant moment *M<sup>i</sup>*

$$M\_{\rm i} = p\_{\rm E} \pi r\_{\rm i}^2 d\_{\rm i} \tag{1}$$

where *d<sup>i</sup>* is the distance between the centre of the cavity section and the neutral bending axis, passing through the centre of mass of the section (Figure 2 and Equation (2)), while *p<sup>E</sup>* is a non-dimensional parameter corresponding to the normalised pressure with respect to the Young's modulus of the material (*p<sup>E</sup>* = *p*/*E*).

$$d\_{\bar{i}} = e + \frac{er\_{\bar{i}}^2}{R^2 - r\_{\bar{i}}^2} \tag{2}$$

Therefore, the curvature of the *i*-th segment is equal to

$$k\_i = \frac{M\_i}{I\_i} \text{.} \tag{3}$$

where *I<sup>i</sup>* is the second moment of area of the section:

$$I\_i = \frac{\pi \left(R^4 - r\_i^4\right)}{4} + \pi R^2 (d\_i - e)^2 - \pi r\_i^2 d\_i^2 \tag{4}$$

The curved profiles of the *n* segments are numerically computed and assembled in MATLAB to show the overall deformed configuration of the actuator.

#### *2.5. Microactuators Experimental Setup*

The experimental setup to characterise the microactuator response consists of a pressure regulator valve (Festo LR-D-7-I-Mini) fed with compressed air coupled to a manometer (Festo FMAP-63-1-1/4-EN) and connected to the tested microactuator. A 500 µm outer diameter (OD) tube is inserted in the inflatable cavity of the microactuator and fixed with uncured silicone rubber. In the experiment, the pressure input is manually increased by 10 kPa increments, while a camera (Nikon 1 V3) captures

the actuator deformation in the bending plane once a static equilibrium is reached at each pressure increment. As the actuators have different curvatures in accordance to their segmentation, the deformed configurations are characterised using the tip trajectory as parameter.

#### **3. Results and Discussions**

#### *3.1. WEDG Accuracy and Quality*

In order to evaluate the accuracy of the WEDG process for the fabrication of soft robotic microactuators, the WEDG'ed sections of the microrods are measured by means of a ZEISS® SteREO Discovery V20 microscope. The measurement results for ten different sections, which were machined on microrods with a *Saw* morphology, are shown in Figure 5. The target diameter for the measured sections is 400 µm.

**Figure 5.** Measurement of the diameter of the WEDG'ed sections of the microrods. Ten different sections are measured, and eight measurements are carried out per section. The data points refer to the mean diameter of each measured section, while the error bars indicate the standard deviation of the eight measurements.

From Figure 5, it can be observed that the WEDG'ed sections deviate less than ±3 µm from the target diameter. In particular, the average diameter of the measured sections is 400.4 µm, while the standard deviation is 2.59 µm. These results highlight the high precision and accuracy of the proposed WEDG processing method. In order to study the effect of the processing accuracy on the performance of soft robotic microactuators, the analytical model presented in Section 2.4 is used to analyse the influence of variations to the diameter of the inflatable cavity of a segment on the curvature relative error (CRE). Equation (5) is used to compute the CRE from the curvature *k*<sup>0</sup> of a segment of nominal diameter and the curvature *k*<sup>e</sup> of a segment of diameter affected by a machining error.

$$\text{CRE} = \frac{|k\_e - k\_0|}{k\_0} \tag{5}$$

 In Figure 6, the effect of the machining error on the CRE is shown. Segments of diameter equal to 100 µm and 400 µm are considered, which are the diameters of the segments of the three types of microactuators presented in this work. It can be seen that the CRE linearly increases with the machining error. WEDG processing, which allows the machining of segments having a deviation of less than ±3 µm from the nominal diameter, results in maximum CREs of about 2.2% and 5.3% for segments of diameter equal to respectively 400 µm and 100 µm. These maximum errors are relatively small and confirm that WEDG processing can be considered as a viable technique for machining axisymmetric microrods to be used in the bonding-free fabrication process of soft robotic microactuators. The trends shown in Figure 6 also suggest that the finishing step is crucial and unavoidable for ensuring high performance of the soft robotic microactuators since deviations from the nominal diameter in the order of 4–10 µm are observed after roughing. These deviations would result in an increase of the CRE up to 20% for a section diameter of 100 µm.

**Figure 6.** Effect of the machining error on the curvature relative error (CRE). In the graph, the calculated CRE of segments with inner cavity diameter *D* equal to 100 µm and 400 µm is shown.

The main drawback of the proposed WEDG method is the relatively long processing time. For instance, it takes approximately 5 min for machining a section of 400 µm diameter and 0.4 mm length on a cylindrical microrod of 500 µm diameter. In this case, the two roughing steps take approximately 3 min in total, while roughly 40% of the total machining time is spent in finishing. However, long processing time are acceptable, taking into consideration that a microrod can be used for moulding multiple soft robotic microactuators. A possible solution for reducing the WEDG processing time could be to increase the discharge energy during the roughing steps. This can be accomplished, for example, by increasing the capacitance or open voltage parameters [39]. Nevertheless, an increase of the discharge energy is likely to result in a more aggressive and less repeatable removal of material by electric discharges, thus decreasing the overall accuracy and precision of the WEDG process.

The surface roughness of the WEDG'ed sections of the microrods is analysed qualitatively by means of scanning electron microscopy (SEM) and confocal microscopy. In order to study the effects of finishing on the surface quality, the surface analysis is carried out on the same sections considered in Figure 5 and on other sections, which were machined by interrupting the WEDG process before performing the final finishing step. The benefits of the finishing step are clear when observing the SEM micrographs of the microrods before and after the finishing step (Figure 7). In particular, it can be seen that a less uneven surface morphology can be achieved once the single-step finishing is performed after roughing. The observed difference corresponds to a decrease of the surface roughness from *S<sup>a</sup>* = 0.84 µm to *S<sup>a</sup>* = 0.37 µm. These values refer to the average values of the *S<sup>a</sup>* surface parameter, which are computed from 20 samples measured on 10 different grooves by a Sensofar® S lynx microscope in confocal mode (magnification: <sup>×</sup>50, field of view: 350 <sup>×</sup> <sup>260</sup> <sup>µ</sup>m). The *S<sup>a</sup>* parameter represents the arithmetical mean height of a surface. It is the extension of the *R<sup>a</sup>* parameter (arithmetical mean height of a line) to the surface. In light of the bonding-free fabrication process of the microactuators, a reduction in surface roughness is advantageous, since the demoulding forces in microreplication processes depend on friction [40]. Despite the relatively long machining time, it can be concluded that the finishing step is crucial not only to achieve the required processing accuracy, but also to facilitate the removal of the microrod after moulding.

**Figure 7.** Surface morphology of a microrod (**a**) before and (**b**) after the finishing step. The images are taken by means of Phenom® Pro scanning electron microscope.

Figure 8 shows a microrod after WEDG processing. The enlarged views taken by SEM reveal that a flat tip and straight edges can be achieved by WEDG. It can also be seen that the WEDG'ed sections have round chamfers, of which the radius depends on the radius of the wire which is used in the WEDG process. Round chamfers are crucial for demoulding the microactuators. Very sharp edges should indeed be avoided as they can damage the microactuators.

**Figure 8.** WEDG'ed microrod for moulding of soft pneumatic microactuators. The image of the microrod is taken using a ZEISS® SteREO Discovery V20 optical microscope, while the enlarged views are taken by means of Phenom® Pro scanning electron microscope.

#### *3.2. Microactuators Analytical Model*

The analytical model described in Section 2.4 is solved for the three different inflatable cavities. The normalised input pressure (*pE*) is equal for the three actuators, varying linearly from 0 to a maximum of 0.2. Figure 9 displays the deformation of the three actuators for six equidistant pressure values along the input ramp.

**Figure 9.** Analytical model results. Six configurations are depicted for each actuator at the same normalised pressure inputs (*pE*) varying from 0 (undeformed) to 0.2 (maximum deformation). Segments are distinguished with the same colour code as used in Figure 1.

The different segments of each inflatable cavity are distinguished using the same colour code as for Figure 1. The sections of the inflatable cavity with a reduced diameter (light blue) undergo a lower curvature for two reasons. First of all, the bending stiffness is higher due to the increase of the second moment of area as the cross-sectional void (Figure 2) is smaller. Secondly, the normal force driving the bending moment scales linear with this cross-sectional area (Equation (1)).

The diverse responses of the segments along the microactuators determine the complex deformation patterns that we aim to achieve. As such, each low-stiffness segment acts as a compliant joint. For example, we expect *Saw* to achieve a full-curled configuration due to the higher distribution of joints, whereas *Totem* has a discrete deformation, with only two low-stiffness segments working as the main bending points. On the other hand, *Halter* bends only at the extremities of the microactuators while the central part stays undeformed.

#### *3.3. Microactuators' Characterisation*

The three microactuators are experimentally characterised using the setup described in Section 2.5. Figure 10 shows the deformation of the three microactuators at a pressure of 20, 40 and 50 kPa. The deformed shapes are in agreement with the results of the analytical model, showing the predicted segmented curvatures according to the shape of the inflatable cavity, as discussed in the previous paragraph.

However, the experimental displacement starts to deviate from the model at large displacements. This is more significant for actuators *Saw* and *Halter* as they have a higher distribution of thinner membranes that undergo large strains. Indeed, the assumptions of the linear model (such as linear elasticity and undeformed cross-sections) hold at small deformations, but for large displacements silicone rubbers follow hyperelastic models, where stress and strain are nonlinearly related, and cross sections deform. Moreover, circumferential strains become important at large deformations, leading to nonlinear phenomena that occur in rubber structures, such as ballooning and elastic instabilities [41]. This is one of the main reasons why soft bending actuators are manufactured with fibre-reinforcements or bellows shapes to limit circumferential strains [22,38]. Given the asymmetric geometry of the cross-section of our microactuators, the analytical formulation of the nonlinear problem is not trivial, and finite element method (FEM) is commonly used to deal with these nonlinearities [42].

**Figure 10.** Microactuator inflation tests. Deformed configuration at four different pressures is reported for each microactuator. The white dash in the subfigures of the first column corresponds to a length of 0.8 mm. The experimental and modelled tip trajectory with respect to the initial position at 0 pressure is reported in the graphs.

Thanks to the segmented shape of the inner chamber introduced with the WEDG process, the ballooning effect is limited to the segments with lower stiffness and does not affect the whole actuator. For example, the 400 µm segments in *Saw* work as circumferential strain limiters and prevent the propagation of the ballooning from the 500 µm segments. This decreases the risk of bursting and allows the microactuator to safely achieve a fully curled configuration.

Another interesting feature that we obtained with WEDG can be observed in the *Totem* deformation (Figure 10, second row), where the low-stiffness segments balloon and bend while the rest of the cavity is less deformed, resulting in a finger-like motion. Therefore, larger bending deformations can be locally concentrated in the actuator.

#### **4. Conclusions**

In this paper we investigated a new production technology to improve the design of soft inflatable bending microactuators and achieving more complex deformations. WEDG processing was shown to accurately machine micromoulds whose shapes are replicated in the soft actuators through a bonding-free micromoulding process. As demonstrators, we proposed three different actuators that share the same global geometry and material except for the shape of the inflatable cavity. The actuators showed very different kinematics. To predict the response, we applied a simple analytical model based on a multi-segment approximation and linear beam theory. Experimental results agreed with the model-based predictions, within the limits posed by the linear approximation. We manufactured microactuators that exhibit application-relevant behaviours such as full curl, flexible joint-like fingers and undeformed segments. Indeed, this type of actuator has been used to develop flexible microgrippers [36], as well as biomedical devices [25]. In the future we envision a reverse

kinematics approach in the form of an optimisation algorithm that, starting from a given trajectory of the end effector, is able to deduce the right morphology of the inflatable cavity which is compatible with WEDG manufacturing.

**Author Contributions:** E.M. and M.B. conceived the research and wrote the manuscript; M.B. characterised the WEDG process parameters and manufactured the microrods; E.M. created the analytical model, manufactured and characterised the microactuators; B.G. and J.Q. critically analysed the experimental results; M.D.V. and D.R. supervised and supported the research. All authors revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Fund for Scientific Research-Flanders (FWO). The authors also gratefully acknowledge the financial support from the European Union's Horizon 2020 projects ProSurf (grant agreement number 767589) and Microman (Project ID: 674801) and from Flanders Make vzw.

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **References**


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