Electrochemical discharge machining (ECDM) has emerged as one of the potential techniques in the field of glass micromachining due to higher etch rate and simple experimental set-up [1] . Glass based substrates have an important role in the fabrication of MEMS and micro-fluidics devices. Specifically, conductive metal-filled micro-holes etched in glass substrates are required in the advanced 3D packaging of inertial MEMS [2] . ECDM process removes the material by the combined action of localized thermal melting and temperature-assisted chemical etching [3] .
Even though the ECDM process has shown the capability of producing high aspect ratio and complex 2D/3D structures in the past decades [4] [5] [6] , there are still several areas that are not explored much. For example, the role of the tool electrode-substrate gap on the machining characteristics with varying machining durations is not examined well. In ECDM, the glass substrate is kept in the close vicinity to get significant material removal in the glass substrate. However, the geometric feature of removed material may get influenced by the tool-substrate gap. Hence, identification of the appropriate tool-substrate gap is necessary. In the case of high aspect-ratio, through-hole etching, measuring, and maintaining the tool-substrate gap is a very challenging task. Due to the non-conductive nature of the glass, the conventional electric current sensor-based techniques cannot be used. Optical methods have difficulty in the in-situ monitoring of the gap due to the bubbles and fume formation around the tool electrode.
Traditionally, a gravity feed approach in which the tool and substrate are always in contact (i.e., zero-gap) is used in the ECDM process [7] ; however, the zero-gap leads to severe thermal damage and drastic tool wear [8] . Fascio et al. [9] 
Electrochemical discharge machining (ECDM) has emerged as one of the potential techniques in the field of glass micromachining due to higher etch rate and simple experimental set-up [1] . Glass based substrates have an important role in the fabrication of MEMS and micro-fluidics devices. Specifically, conductive metal-filled micro-holes etched in glass substrates are required in the advanced 3D packaging of inertial MEMS [2] . ECDM process removes the material by the combined action of localized thermal melting and temperature-assisted chemical etching [3] .
Even though the ECDM process has shown the capability of producing high aspect ratio and complex 2D/3D structures in the past decades [4] [5] [6] , there are still several areas that are not explored much. For example, the role of the tool electrode-substrate gap on the machining characteristics with varying machining durations is not examined well. In ECDM, the glass substrate is kept in the close vicinity to get significant material removal in the glass substrate. However, the geometric feature of removed material may get influenced by the tool-substrate gap. Hence, identification of the appropriate tool-substrate gap is necessary. In the case of high aspect-ratio, through-hole etching, measuring, and maintaining the tool-substrate gap is a very challenging task. Due to the non-conductive nature of the glass, the conventional electric current sensor-based techniques cannot be used. Optical methods have difficulty in the in-situ monitoring of the gap due to the bubbles and fume formation around the tool electrode.
Traditionally, a gravity feed approach in which the tool and substrate are always in contact (i.e., zero-gap) is used in the ECDM process [7] ; however, the zero-gap leads to severe thermal damage and drastic tool wear [8] . Fascio et al. [9] 
Electrochemical discharge machining (ECDM) has emerged as one of the potential techniques in the field of glass micromachining due to higher etch rate and simple experimental set-up [1] . Glass based substrates have an important role in the fabrication of MEMS and micro-fluidics devices. Specifically, conductive metal-filled micro-holes etched in glass substrates are required in the advanced 3D packaging of inertial MEMS [2] . ECDM process removes the material by the combined action of localized thermal melting and temperature-assisted chemical etching [3] .
Even though the ECDM process has shown the capability of producing high aspect ratio and complex 2D/3D structures in the past decades [4] [5] [6] , there are still several areas that are not explored much. For example, the role of the tool electrode-substrate gap on the machining characteristics with varying machining durations is not examined well. In ECDM, the glass substrate is kept in the close vicinity to get significant material removal in the glass substrate. However, the geometric feature of removed material may get influenced by the tool-substrate gap. Hence, identification of the appropriate tool-substrate gap is necessary. In the case of high aspect-ratio, through-hole etching, measuring, and maintaining the tool-substrate gap is a very challenging task. Due to the non-conductive nature of the glass, the conventional electric current sensor-based techniques cannot be used. Optical methods have difficulty in the in-situ monitoring of the gap due to the bubbles and fume formation around the tool electrode.
Traditionally, a gravity feed approach in which the tool and substrate are always in contact (i.e., zero-gap) is used in the ECDM process [7] ; however, the zero-gap leads to severe thermal damage and drastic tool wear [8] . Fascio et al. [9] 
Electrochemical discharge machining (ECDM) has emerged as one of the potential techniques in the field of glass micromachining due to higher etch rate and simple experimental set-up [1] . Glass based substrates have an important role in the fabrication of MEMS and micro-fluidics devices. Specifically, conductive metal-filled micro-holes etched in glass substrates are required in the advanced 3D packaging of inertial MEMS [2] . ECDM process removes the material by the combined action of localized thermal melting and temperature-assisted chemical etching [3] .
Even though the ECDM process has shown the capability of producing high aspect ratio and complex 2D/3D structures in the past decades [4] [5] [6] , there are still several areas that are not explored much. For example, the role of the tool electrode-substrate gap on the machining characteristics with varying machining durations is not examined well. In ECDM, the glass substrate is kept in the close vicinity to get significant material removal in the glass substrate. However, the geometric feature of removed material may get influenced by the tool-substrate gap. Hence, identification of the appropriate tool-substrate gap is necessary. In the case of high aspect-ratio, through-hole etching, measuring, and maintaining the tool-substrate gap is a very challenging task. Due to the non-conductive nature of the glass, the conventional electric current sensor-based techniques cannot be used. Optical methods have difficulty in the in-situ monitoring of the gap due to the bubbles and fume formation around the tool electrode.
Traditionally, a gravity feed approach in which the tool and substrate are always in contact (i.e., zero-gap) is used in the ECDM process [7] ; however, the zero-gap leads to severe thermal damage and drastic tool wear [8] . Fascio et al. [9] 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to observed that negligible material was removed when the toolglass substrate gap was above 30 μm in the NaOH electrolyte for ECD milling with a velocity feed system and 30 V DC machining voltage. Ziki and Wuthrich [10] utilized force sensor-based techniques to measure and maintain the toolsubstrate gap in a velocity-feed based ECDM. The velocity feed-based approach involves constant feed to tool in a vertical direction [11] . Nevertheless, a comprehensive study on the tool-substrate gap with varying machining time for different electrolytes along with tool wear is not thoroughly carried out.
Hence, the present work focuses on the effect of the toolglass substrate gap along with the machining times on the micro-hole overcut and depth with the KOH and NaOH electrolytes. The tool-glass substrate gap was measured by using a reference metal sheet of a thickness of 80 µm. The process output parameters investigated were micro-hole overcut and machined width for varying tool-substrate gap and machining time. Tool wear with varying tool-glass substrate gap was also analyzed. Further, a finite element (FEM) based numerical simulation was performed and validated with experimental results.
Heat flux with the gap (W/m 2 ) q zg
Heat flux with zero gap (W/m 2 ) r 0
Spark radius (µm) z
Tool-substrate gap (µm)
FEM based numerical simulations for the crater formation for the different number of electrochemical sparks were performed. The governing equations for the heat flux, mean current through circuit, assumptions, and other important parameters used in the simulation were taken from the reference [12] . These parameters include thermal conductivity, specific heat, and density of the glass substrate. Convective heat transfer coefficient, flushing efficiency of the electrolyte, and partition energy were also included from the same reference. DC power supply was used for experimentation. However, in the DC power supply, the gas film forms and breaks several times in a random fashion. Hence to include the pulse-on and off effect, discharge frequency was calculated. The f d was obtained to be 3075 Hz using equation (1) for 45 V DC power supply [13] . Accordingly, the total duration for the single-cycle was obtained to be 325 μs.
Where E is the electric field at the electrode surface. Parameters A and B are taken as 3×10 -9 /V 2 s and 5.4×10 6 V/m, respectively, for a H 2 gas film thickness of 17 µm [13] [14] . During the initial ECDM experiments with pulse DC power supply, it was observed that the duty cycle should be more than > 50 % for significant material removal from the glass substrate [15] . Therefore, for the numerical simulation, the t on was considered as 200 µs while t off was taken as 125 µs.
During the experiments with a different gap, it was observed that the crater depth reduces with an increase in the tool-substrate gap. It is due to the reduction in the fraction of heat energy transferred to the glass substrate with the increase in the gap. To include this effect, the total energy released was assumed to be constant at all machining gaps for a given machining voltage; however, the amount of heat energy transferred into the glass substrate reduces proportionally as the gap increases, as shown in equation (2).
Material removal happened due to the localized melting and vaporization in the pulse-on time, followed by the chemical etching in the pulse-off time. Fig.1 (a) showed the temperature contours for the time duration of 325 µs when 30 wt% NaOH electrolyte was used. To incorporate material removal due to chemical etching, all the regions having a temperature higher than the softening temperature of the glass, i.e., 873 K was considered to be etched. After the numerical simulation was performed, the crater depth was obtained by considering the specific regions having a temperature above 873 K ( Fig. 1(b) ). The numerical results were further validated with the experimental values in the results section. 
An in-house developed ECDM experimental set-up with a velocity feed system was utilized for experimental study (see Fig. 2 ). Electrolytic tank unit, XYZ linear stages with motion controlling unit, and DC power supply are the main constituents. The electrolytic tank unit contains a substrate holder, circular counter electrode ring, and electrolyte level maintaining valve. The flow of electrolyte was maintained using a simple submersible pump with flow control valves. XYZ motion controller with a resolution of 1 µm provides precise control over the movement of the tool electrode. DC power supply was used in all the experiments. A four-channel, digital storage oscilloscope was used to observe the voltage and current signals. The geometric characterization of the fabricated micro-features was performed using a focus variation optical microscope (Zeta instruments). 
As mentioned earlier, measuring and maintaining a constant tool-substrate gap is a challenging task in ECDM due to the non-conductive glass substrate. Initially, a thin sheet of 80 µm thicknesses was kept in between the tool and substrate, and the tool electrode was allowed to move downwards until it touched the glass substrate. Once the tool electrode was in physical contact with the substrate, the vertical downward feed was paused. The thin metal sheet was then removed, and the tool electrode was moved downwards as per the required gap. In this way, the different tool-substrate gap was maintained, and experiments were carried out with the experimental input parameters, as shown in table 1.
The tool-substrate gap is varied from 0 µm to 60 µm for NaOH and KOH electrolyte with three different sets of machining time, i.e., the 20s, 40s, and 60s. Each experiment was repeated three times to check the repeatability of the results. A new tool was used for every experiment to avoid the effect of tool wear on the subsequent experiments. The etched glass samples were cleaned in DI water and then in acetone using ultrasonic agitation and then characterized using a 3D opto-digital microscope (Olympus DSX100). 
The micro-hole overcut is the difference between the machined hole size at the entrance and the tool tip size. 3D profiles and optical images of micro-holes etched by 30 wt% NaOH and 30 wt% KOH electrolytes for varying gaps are shown in Fig. 3 and Fig. 4 , respectively. All the optical images shown were etched for the machining time of 60s. Images showing the micro-holes etched at varying gaps, i.e., 0 μm, 30 μm, and 40μm T-S gaps for NaOH electrolyte, are shown in Figure 3 (a-c). The 3D profiles and optical images are shown for 0 μm, 10 μm, and 20μm T-S gaps for the KOH electrolyte. It is clear from Fig. 4(a-c) that in the case of the KOH electrolyte, the hole overcut was less in comparison with the NaOH electrolyte. Variation of the hole overcut with respect to the tool-substrate gap (T-S gap) is presented in Fig. 5a and 5b for NaOH and KOH electrolyte, respectively. Though the material removal was observed up to the 60 μm and 30 μm T-S gap in case NaOH and KOH electrolytes, respectively, the rate reduces when the gap was increased. Hence, the T-S gap range was selected as 0-60 μm and 0-30 μm for NaOH and KOH electrolytes, respectively. Overcut was observed to be increased up to 20 μm T-S gap and then reduced after that in case of NaOH. As the T-S gap increases, the spread of the thermal energy due to electrochemical discharges over the surface of substrate increases as the plasma channel formed due to electrochemical discharges gets widened. Hence, higher material removal and overcut observed until a particular gap was maintained. It happens up to a 20 µm gap in NaOH electrolyte and a 10 µm gap in KOH electrolyte, beyond which substrate surface experiences a relatively lower amount of thermal energy, which led to a decrement in the hole overcut. Out of three machining times viz., the 20s, 40s, and 60s, the higher overcut were observed at 60s for both NaOH and KOH electrolytes. The application of thermal energy for a higher duration of time led to a higher amount of material removal and overcut. The maximum value of hole overcut was measured to be 1122 μm at 20 μm T-S gap and 60s for NaOH electrolyte. In the case of KOH, the maximum overcut was obtained at a 10 μm gap, i.e., 216 μm at 60s machining time. As compared to NaOH electrolyte, lower overcut, and lower surface damage (cracks, heat affected zones) was observed in the case of KOH electrolyte. The molar conductivity of KOH is higher, which leads to faster and stable gas film formation. KOH electrolyte's viscosity is low; hence, debris flushing efficiency the machining zone is more, which results in lower surface damage [4] . No machining was observed in the glass substrate above 30 µm and 60 µm for KOH and NaOH, respectively. It may be due to the fact that higher T-S gaps may lead to very less amount of machining. As the T-S gap is large thermal energy produced due to electrochemical discharges was not sufficient to remove the material. In the case of larger gaps, thermal energy was utilized for heating the surrounding electrolyte, which was not desirable. Though lower overcuts were observed for higher T-S gaps, the depths obtained were very less.
Machining depth is characterized as the linear distance from the opening of the micro-hole cavity to the bottom surface of the micro-hole cavity. Variation of the machining depth to the tool-substrate gap (T-S gap) is shown in Fig. 6a and 6b. As can be seen, the depth of the microchannel decreased with an increase in the T-S gap for both NaOH and KOH electrolyte. The highest machined depth was observed at 0 μm T-S gap for the machining time of 60s. For each T-S gap, machining depth was observed to be highest at 60s and lowest at 20s in both NaOH and KOH electrolyte. In the case of 0 μm gap, the electrochemical discharges are in close vicinity of the glass substrate. So highly directed discharges try to penetrate the substrate in a vertical direction. As the T-S gap is increased, the penetration in the substrate due to the thermal energy of electrochemical discharges reduces. Fig. 6 . Effect of the tool-substrate gap on the machined depth Ultimately, at large T-S gap, no penetration was observed due to discharges and led to negligible depths. The maximum value of machined depth was 162.7 μm at 0 μm gap and 60s for NaOH electrolyte. For KOH electrolyte, maximum machined depth was obtained at 0 μm gap, i.e., 81.96μm. The cross-sectional profiles of the micro-holes, indicating the depths at different T-S gaps and machining time are shown in Fig. 7 . 
Variation of the tool wear in terms of length reduction to the tool-substrate gap (T-S gap) for varying machining time is shown in Fig. 8a and 8b . As can be seen, the tool wear is observed to be decreased with an increase in the T-S gap for both NaOH and KOH electrolyte. The highest tool wear was observed at 0 μm T-S gap and 60s machining time in case both electrolytes. As the machining time increases, the electrochemical discharge period increases resulting in increased tool wear. The temperature of the tool tip during the electrochemical discharges can reach up to 2800°C [16] , which mainly contributes to the tool erosion. In the case of lower toolsubstrate gaps, the tool was more exposed to the electrolyte, and a higher amount of electrochemical discharges were produced, which increases the temperature of the tool tip and the tool wear. As compared to NaOH electrolyte, tool wear was found to be relatively lower in the KOH electrolyte. It might be due to the fact that H 2 gas film and electrochemical discharges stability are more for KOH electrolyte [14] . The maximum value of the tool wear was measured to be 36 µm when the NaOH electrolyte was used for 60s. However, the tool wear was only 19.67 µm in the case of KOH for the same machining duration. The higher amount of machining time led to higher tool wear in both electrolytes. Least value of tool wear, i.e., 8 µm was observed in the case of KOH electrolyte with 20s machining time. Fig. 9 shows the tool conditions before and after the machining for both electrolytes with 60s machining time. Tool wear along with debris deposition on the sidewalls of tool tip can be seen in Fig. 9b . The debris deposited on the tool tip mainly consists of the glass substrate and tool electrode constituents such as Boron, Silicon, Sodium, Iron, Nickel, etc. [17] .
A comparison of the experimental and numerically simulated machined depth for 30 wt% KOH and 30 wt% NaOH electrolyte is shown in Fig. 10 . The 0 µm, 20 µm, and 40 µm T-S gaps for NaOH were selected for the simulation. Similarly, for KOH, the T-S gaps were selected to be 0 µm, 10 µm, and 20 µm. 6 Author name / Procedia Manufacturing 00 (2019) 000-000 Numerical results are in good agreement with experimental results. As mentioned earlier, machined depth was observed to be decreasing with the increment in the tool-substrate gap due to the lowering impact of thermal energy due to discharges on the glass substrate. It has been observed that the experimental machining depth values were higher than simulated values for the medium tool-substrate gap (i.e., 20 μm for NaOH and 10 μm for KOH). It might be because simulations were performed considering material removal by thermal energy alone. However, actual material removal involves thermal melting along with chemical etching at elevated temperatures.
The experimental demonstration of the tool-substrate gap and its significance over the machining behavior of glass substrate by the ECDM process is reported. Tool wear dependence on the tool-substrate gap was also demonstrated. The following conclusions can be drawn: a) Compared to the physical contact or zero gap, an optimal gap must be maintained between the tool and substrate to have significant material removal, good surface quality, optimum hole depth, and low tool wear. b) The optimal gap which must be maintained was µm in case of NaOH electrolyte, and 10-20 µm when KOH electrolyte was used. c) The amount of overcut is smaller in microholes etched by the KOH electrolyte; therefore, usage of KOH is preferred over the NaOH electrolyte. d) Tool wear was followed by a decreasing trend with the increase in the tool-substrate gap.