Micro ECDM process comparison using different tool feed methods of constant gravity and spring-force

Quartz glass has been widely used in multiple frontier fields of science and technology owing to its excellent chemical and mechanical properties, such as optical communication, semiconductor, photovoltaic power, and aerospace. ECDM (electrochemical discharge machining) is a non-traditional material removal process suitable for machining non-conductive materials of high hardness and brittleness. The tool electrode feed method is a key factor affecting the ECDM process. Experimental research was carried out for comparing the conventional gravity feed method and a newly-developed spring-force feed method. Micro tool electrodes of Φ150 µm were fabricated by combining the method of TF-WEDG (tangential feed-wire electrical discharge grinding) and reverse micro EDM (electrical discharge machining). Machined microstructures of blind holes, channels, squares, and patterns were compared by the gravity feed method and the spring-force feed method respectively. The experimental results show that the spring-force feed method can improve the micro ECDM process considering the aspects of dimensional accuracy, overcuts, deteriorated edges, surface topography, and tool wear.


1.
Introduction Quartz glass has excellent chemical and mechanical properties of high-temperature resistance, outstanding electrical insulation, and high hardness.It has a wide range of applications in multiple frontier fields of optical communication, semiconductor, photovoltaic power, and aerospace, such as quartz optical fibre, quartz diffusion tube, quartz prism, and airplane porthole window [1].However, quartz glass is difficult to machine into micro structures by traditional mechanical machining processes due to its special characteristics of high hardness and brittleness.ECDM (electrochemical discharge machining) is a non-traditional material removal process suitable for machining non-conductive hardbrittle materials such as quartz glass and ceramics [2].In a ECDM process, a tool electrode and an auxiliary electrode submerged in the electrolyte are connected with the cathode and anode of the pulsed power supply respectively.The first stage of the ECDM process is electrolysis to generate bubbles into a gas film around the tool electrode, then spark discharges occur at a specific voltage to produce enough energy to melt and evaporate workpiece material [3].Actually, the ECDM process has a complex mechanism of a combination of physical and chemical phenomena.It is a challenge to achieve high machining accuracy on quartz glass.
In order to improve the ECDM process, many researchers have developed different machining techniques.The surface quality of the workpiece was improved by mixing the graphite powders into the ECDM electrolyte [4].Under the assisted magnetic field, the action of magnetohydrodynamic convection increases the electrolyte circulation, so the machining efficiency was upgraded [5].The overcut errors of micro channels were reduced by using side-insulted tool electrodes [6].As for the factors affecting the ECDM process, the tool electrode feed method is a key factor because the discharging energy onto the workpiece is directly related to the position of the tool electrode tip.Investigators used different spindle systems for feeding tool electrodes during the drilling and scanning processes.Constant gravity feed is the most common method for feeding tool electrodes by exerting a given weight onto the tool electrode.Micro holes can be drilled by the gravity feed method [7], but the exact functional relationship between the size characteristics of micro-holes and the appropriate range of gravity feed contact force is still not clear.The method of gravity feed can keep a contact force based on a simple balance mechanism, but the contact force is easily changed by the interference factors of the tool electrode weight, machining depth, uneven workpiece surface, etc.The constant velocity feed method is to feed the tool electrode with a given velocity in the direction of the tool axis for obtaining a proper machining gap.Although this method has been used to drill micro holes [8], a reasonable velocity is hardly obtained because the material removal rate in ECDM is time-varying with the increase of machining depth, resulting in the feed velocity having to select a lower speed.On the one hand, under a lower material removal rate, the faster feed velocity could break the tool electrode; on the other hand, the lower feed velocity would decrease the machining efficiency obviously.Recent research indicates that a spring contact force can improve the scanning process of ECDM [9].The tool electrode with a spring structure can reduce the tool stiffness and exert a smaller contact force onto the workpiece under the same compression, therefore the scanning process is stabilized and the bending problem is avoided.The micro cavity with lower roughness and superior accuracy of machined shape were obtained.However, previous investigations lack further research on the tool feed method based on micro spring contact force.
In this research, the machining experiments of micro ECDM are carried out to compare the effect between the gravity feed method and the spring-force feed method by machining the microstructures of blind holes, channels, squares, and patterns on quartz glass.The processing results are evaluated from the aspects of dimensional accuracy, overcuts, deteriorated edges, surface topography, and tool wear.

2.
Experimental methodology The experimental process includes tool fabrication and micro ECDM using different feed methods of constant gravity and spring-force obtaining microstructures of blind holes, channels, squares, and patterns as shown in Figure 1.

Figure 1. Schematic diagram of experimental process.
The tool fabrication is a hybrid process of reverse micro EDM (electrical discharge machining) and TF-WEDG (tangential feed-wire electrical discharge grinding) for complementary advantages of high efficiency and accuracy [10].The initial diameter of the tool is 500 µm, and the finished diameter of a tool with almost negligible taper error along the axis is 150 µm, as shown in Figure 2. 8 tools were fabricated for performing machining experiments.
Two spindle feed methods of constant gravity and spring-force are configured as shown in Figure 1.For the gravity feed method, the weight of the spindle/tool holder is balanced by additional weight on the opposite side.After balancing, a specific amount of weight is removed to get the proper contact force of 0.7 g between the tool tip and the workpiece.For the spring-force feed method, the force sensor with high resolution and frequency response is used to detect the real-time contact force between tool and workpiece.This force signal is compared with the set force value, constituting a feedback control loop.Thereby the contact force is continuously kept at the set value.The set contact force is 0.00686 N, which is close to the contact force value of the gravity feed method.
Table 1 shows the parameters of micro ECDM.The technique of measuring 4-point Z-axis positions by indirect electric contact is adopted for levelling workpiece [11].The auxiliary electrode fixed at the bottom of the electrolyte tank is connected to the anode of the pulsed power supply, while the tool electrode is connected to the cathode.Microstructures of blind holes, channels, squares, and patterns are respectively machined by the feed methods of constant gravity and spring force.

Results and discussions
The moving distances of the tool electrode in the horizontal plane are set as follows: 0 for blind hole, 1500 μm for channel, 1500 μm×1500 μm for square, and 1600 μm×1600 μm for pattern.The machining time decides the depth of microstructures, which is set at 5 min., 3 min.30 sec., 12 min., and 2 min.for blind hole, channel, square, and pattern respectively.12-time scanning is performed for the channel and square.The pattern is scanned only one time to investigate the effect of contact force in a single scanning.Constant gravity feed method Figure 3 shows the machining results of blind hole, channel, square, pattern, and the corresponding tool end shape under the constant gravity feed method.Due to the kinematic instability of the tool tip, overcuts are obviously produced when machining the blind hole and channel as shown in Figure 3 (a) and (b).This is because the constant contact force of the tool cannot properly act on the workpiece surface, thus the fluctuating contact force may exceed the appropriate range.The inlet diameter of blind hole is much larger than the tool diameter, resulting in a fairly significant taper error.The edges of channel are out of shape and deteriorated.In Figure 3 (c), the square widths at point a and b are different because of the tool misalignment from the original path of scanning during the machining process.Tool may strike on the side edges, causing craters and rough machined surface.When scanning the longer path in shorter time, as shown in Figure 3 (d), the depth becomes smaller with well-kept consistency and the surface is finer because of the shorter in situ processing time.Tool wear in the gravity feed mechanism is large.

3.2.
Spring-force feed method Figure 4 shows the machining results of the blind hole, channel, square, pattern, and the corresponding tool end shape under the spring-force feed method.The diameter and depth of the blind hole become smaller with a perfect circular shape compared with that of the constant gravity feed, as shown in Figure 4 (a).The contact force imposed on the tool tip is servo stabilized, thus the deflection of tool from the programmed path is reduced, obtaining a narrower channel with a fine edge and smooth side wall, as shown in Figure 4

Conclusion
In the current research, the specific microstructures of micro blind hole, channel, square, and pattern on quartz glass are machined by the conventional gravity feed method and the newly-developed springforce feed method in micro ECDM using the tool fabricated by combining the methods of reverse micro EDM and TF-WEDG.The main conclusions are drawn as follows: (1) The Φ150 µm finished tools with almost negligible taper error along the axis were obtained by using the hybrid process of reverse micro EDM and TF-WEDG.
(2) Although the gravity feed is easier to employ compared with the spring-force feed, the acting contact force on the workpiece tends to fluctuate and exceed the appropriate range, causing the kinematic instability of the tool tip and the misalignment from the original programmed path.Large taper error of blind hole, overcuts and deteriorated edges of channel, and rough surface of square are monitored.
(3) The spring-force feed can keep the contact force at a fixed value by using the force sensor and feedback control loop, thus servo stabilizing tool tip and reducing tool deflection.The circular blind hole of smaller inlet diameter, the narrower channel of smooth edges, and well dimensional consistency of square and pattern are obtained.

Figure 2 .
Figure 2. SEM photo of finished tool electrode.
(b).Similarly, the machined square in Figure 4 (c) has approximately the same measured widths at points a and b.The surface quality is greatly improved.The feature of the scanning pattern with a longer path in Figure 4 (d) resembles the machined results under the constant gravity feed method.The tool wear is less by using the spring-force feed method.

Figure 3 .
Figure 3. Machining results of microstructures and corresponding tool end shape under constant gravity feed method: (a) blind hole; (b) channel; (c)square; (d) pattern.

Figure 4 .
Figure 4. Machining results of microstructures and corresponding tool end shape under spring-force feed method: (a) blind hole; (b) channel; (c)square; (d) pattern.