experimental study on ecm with a grid cathode composed of

Journal of Mechanical Science and Technology 26 (8) (2012) 2389~2396

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-0614-1

Experimental study on ECM with a grid cathode composed of circular cells†

Yonghua Lu1,2,*, Dongbiao Zhao1 and Kai Liu1 1College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China

2Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana, 46556, USA

(Manuscript Received October 22, 2011; Revised March 9, 2012; Accepted March 9, 2012)

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Abstract In order to resolve the problem of single machining object existing in traditional electrochemical machining (ECM) with unitary cath-

ode, a grid cathode composed of circular cells is used to produce the workpieces with different shapes. Three types of circular cells, Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm, are utilized to construct the plane, slant, and blade cathode. The material of the cathode and the an-ode is CrNi18Ti9, and the ingredient of electrolyte is 15% NaCl and 15% NaNO3. The machining balance current and the balance time are acquired and analyzed, the machining process and the workpiece quality are compared between using the grid cathode and the unitary cathode. Moreover, the machining errors and the error reasons of workpiece surface are analyzed. Research shows that the grid cathode can be used to manufacture workpieces with a complex shape, and the workpiece quality is better if the circular cell is smaller. If the circular cell is small enough the workpiece quality is almost equal to it machined by the unitary cathode.

Keywords: Grid cathode; Circular cell; Balance current; Electrochemical machining (ECM); Error distribution ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Electrochemical machining (ECM) is one of the most im-portant methods in aerospace industries, weapon equipments, and other machining fields, especially for workpiece with complex shape and hard-to-cut material [1, 2]. However, com-plicated cathode design and single machining object have restricted the development of ECM in these fields. Cathode is the machining tool, whose structure, profile, and precision will mostly influence the workpiece quality, especially for the parts requiring high precision and complicated shape [3, 4].

Traditional methods, which utilize known shape of the part to obtain the cathode profile, include cosθ method [5], analyti-cal solution method [6], displacement offsetting method [7], FEM method [8], numerical model method [9], and experi-mental amending method [10]. Although these methods ad-vance the development of ECM and improve cognizance to physical machining process, some common limitations of these methods can be identified as follows:

(1) Use of unitary cathode requires considerable amount of time and personnel to amend the surface of cathode. It also demands rich experiences and high technologies on cathode design for operators.

(2) Production cycle of the cathode is long, and manufactur-

ing cost is high, especially for the parts with complex shapes. (3) After production of cathode, its profile is unvaried, i.e.,

one cathode can only be used to produce workpieces with the same shapes. The disadvantage of traditional cathode design is that machining object is single [11].

Considering the disadvantages of traditional methods of cathode design, this paper presents a new design method, called grid cathode design, in which cathode is composed of many circular cells arranged like matrix. To obtain different shapes of cathode, only amending positions of circular cells will easily copy the known shape of part. Then, cathodes with different shapes are used to machining workpieces with dif-ferent surfaces. Grid cathode method may considerably shorten the production cycle, reduce requirement of personnel, and avoid the disadvantage of single machining object in tra-ditional cathode design.

2. Experimental principle

In the paper, grid cathode composed of circular cells is used in ECM. The size of cross section of cathode is designed as 25 mm × 27 mm because anode profile refers to the shape of eight-level aero-turbine blade of some type aero-engine whose dimension of cross section is 24 mm × 26 mm. Considering the edge effect, we design cathode size in each direction 1 mm larger than anode size. With a circular cell of size Ф2.0 mm, about 168 circular cells are necessary to construct a cathode of 25 mm × 27 mm. In addition, grid cathode obtain curved sur-

*Corresponding author. Tel.: +86 025 84892506, Fax.: +86 025 84891501 E-mail address: [email protected]; [email protected]

† Recommended by Associate Editor Haedo Jeong © KSME & Springer 2012

2390 Y. Lu et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2389~2396

face though copying the known shape of part; this is called reversal copying method [12]. Machining current is direct current, and machining principle scheme is shown in Fig. 1.

In order to avoid current interference among circular cells, each cell must be electrically insulated from others. Since the use of an insulating gasket will introduce gaps to the cathode surface thereby causing a short circuit, insulated paint is sprayed on the surface of each circular cell for keeping the minimum gap between cells.

According to Fig. 1, if machining workpiece has different surface, a new appropriate cathode profile is formed just by modifying the arrangement manner of the circular cells. Therefore, it resolves the shortcoming of single object for unitary cathode.

The grid cathode composed of circular cells is shown in Fig. 2, and the shapes of cathodes are a plane, a slant, and a blade, respectively.

3. Experimental scheme

Experiment scheme includes technological parameters of machining and experiment items. The technological parame-ters mainly includes electrolyte component, temperature and concentration, inlet and outlet pressure, machining current and voltage, cathode feed velocity and so on [13]. Pre-machining experiment is performed according to clamp characteristics to validate feasibility of technological parameters. These techno-logical parameters are in Table 1.

As to the experiment items, three types of circular cells, Ф1.5 mm, Ф2.0 mm, Ф2.5 mm, are utilized to construct plane, slant, and blade cathode. The inclination angle of slant cath-ode is designed as 10 degree because the altitude angle from the highest point to lowest point on blade surface is about 10 degree, and it is of benefit for comparing machining processes

of slant and blade workpieces. Different cell dimension and different cathode profile is researched to find the relation and influence on workpiece quality.

Fig. 3 shows machining tool, clamps, and experimental re-gion with grid cathode. The type of machine tool is Japax ECM-300B. Before machining, automatic feeding system makes zero calibration between cathode and anode, and ma-chine tool spindle is risen to set the initial gap at 0.5 mm.

In Fig. 3, inlet and outlet ball valves control the electrolyte pressure in machining region, inlet and outlet manometers display the pressure values [14]. Initial voltage of machining is set at 10 V by control panel of machine tool. As the machin-ing process proceeds, machining current increases gradually. At a certain time, called the balance time, the current becomes stationary, and the machining process is concluded synchro-nously.

4. Analysis of machining processes

4.1 Variety of the balance current

During a machining process, the machining current and the interelectrode gap vary until arriving at the balance status. The

Fig. 1. Machining system of grid cathode composed of circular cells.

Fig. 2. Plane/slant/blade grid cathodes composed of the circular cells.

Table 1. Machining technological parameters in experiments.

Parameters Value

Anode, cathode materials CrNi18Ti9

Electrolyte component 15% NaCl + 15% NaNO3

Conductance κ (1/Ω cm) 0.190

Inlet pressure Pin (MPa) 1.0

Machining temperature T (°C) 15 ~ 20 Current density (A/cm2） 50

Electrolyte proportion 1.18 Volume electrochemical equivalent

ω (cm3/A·min) 0.0021

Outlet pressure Pout (MPa) 0.1

Machining voltage U (V) 10

Pole potential δE (V) 1

Beginning gap Δ (mm) 0.5

Feeding velocity V (mm/min) 0.75

Fig. 3. Machine tool and clamps with grid cathode.

Y. Lu et al. / Journal of Mechanical Science and Technology 26 (8) (2012) 2389~2396 2391

balance current and the balance time are the stable values when the process arrives at the balance status. It indicates the machining process is finished. The different balance current and the different balance time are obtained when the different cathodes are used. Thus, they are the important parameters reflecting a machining process of ECM using different grid cathodes.

The distribution of the balance current is analyzed to reflect its influence on the machining processes. In this paper, three types of circular cells, Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm, are utilized to construct plane, slant, blade cathode. Thus, there are nine types of cathodes. Each type of the cathode performs machining process two times. Therefore, eighteen groups of data are recorded in experiments. The current values of two groups using the same cathode with the same circular cells are averaged, and nine groups of balance current values are ob-tained. These nine groups of balance current values are shown in Table 2. Table 2 also presents the balance time when each machining process arrives at balance status.

Fig. 4 shows the variable trend of balance current along bal-ance time under different machining conditions.

In Fig. 4, red solid line denotes machining items with plane cathode, green broken line denotes machining items with slant cathode, and blue chain double-dashed line denotes machining items with blade cathode. Three points in each line respec-tively show cathode with different circular cells, Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm from the top down. It obviously shows, in spite of cathode shape, that the balance current in-creases and balance time prolongs when the cell size is re-duced. In other words, grid cathode composed of Ф1.5 mm

circular cells has the largest balance current and the longest balance time, while that composed of Ф2.5 mm circular cells has the smallest balance current and the shortest balance time. As to the reason, there is direct relation with distribution of current density. Cell size of grid cathode is smaller, current distribution between cells is more complicated. To reach bal-ance status of machining process, larger machining current is necessary and longer machining time is required.

Fig. 4 also shows that balance current is largest in machin-ing process with plane cathode, and balance time is longest in the process with blade cathode. To find its cause, plane cath-ode is used to machining plane workpiece, current density between anode and cathode is even and machining gap is co-herent relatively. Thus, there has good electric conductivity between cathode and anode, and the balance current with plane cathode is the largest one. However, blade cathode has complex shape and longer machining time is required to arrive at balance status. Therefore, the balance time with blade cath-ode is the longest one.

4.2 Analysis on the surface quality of workpieces

The grid cathodes with different profiles composed of circu-lar cells are shown in Fig. 2. Fig. 5 shows the plane, slant, and blade workpieces produced by grid cathode composed of the circular cells. It is obvious that plane workpiece has smooth surface, quality of slant workpiece is worse than that of plane workpiece, and blade workpiece has the worst profile com-pared with the others.

For analyzing surface quality of workpieces effectively, we measure 3-D space coordinates of each workpiece in Coordi-nate-Measuring Machine (CMM) with precision of 0.001 mm. Moreover, measured values are compared respectively with theoretical values to obtain machining error. Taking slant workpiece for example, measured value minus corresponding theoretical value is its machining error. If we measure one hundred points represented by a matrix with 10 × 10 elements on workpiece surface, one hundred theoretical values should

Table 2. Machining balance current and balance time.

Plane Slant Blade Cell dimension

(mm) Current

(A) Time

(s) Current

(A) Time

(s) Current

(A) Time

(s) Φ1.5 410 148 350 153 330 253

Φ2.0 400 141 330 140 310 232

Φ2.5 380 130 300 135 300 224

Fig. 4. Machining balance current along balance time with different cathodes and different cells.

Fig. 5. Plane/slant/blade work pieces machined by grid cathode com-posed of circular cells.


be calculated corresponding to each measured point including their height and position information. Because the machining error is pure numerical value without height, position, and other information, machining error is of benefit for analyzing from statistical point of view. In addition, the error distribution may be described in a 3-D graph.

Fig. 6 shows the error distribution of plane workpieces ma-chined by grid cathodes composed of Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm circular cells. 210 numerical values shown by red “*” denote surface errors of workpiece produced by Ф1.5 mm grid cathode, 156 numerical values shown by blue “o” denote surface error of workpiece produced by Ф2.0 mm grid cathode, and 100 numerical values shown by green “” denote surface error of workpiece produced by Ф2.5 mm grid cath-ode. In Fig. 6, besides seven stray points with large error val-ues whose largest value is about 0.5 mm, the error points are located in the range of ±0.08 mm for the cell size of Ф1.5 mm and Ф2.0 mm, and the errors of the cell size Ф2.5 mm vary from -0.2 mm to +0.2 mm. It indicates that plane machining using grid cathodes composed of circular cells has high preci-sion.

Fig. 7 presents the error distribution of slant workpieces machined by grid cathodes composed of Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm circular cells. 272 numerical values shown by red “*” denote surface errors of workpiece produced by Ф1.5 mm grid cathode, 156 numerical values shown by blue “o” denote surface errors of workpiece produced by Ф2.0 mm grid cathode, 100 numerical values shown by green “” denote surface error of workpiece produced by Ф2.5 mm grid cath-ode. Error points on workpiece of size Ф2.5 mm are located in the range of -0.3 ~ 0.2 mm besides two points with large error, denoting the largest error range with the worst surface quality in Fig. 7. Error points on workpiece of size Ф2.0 mm are lo-cated in the range of ±0.1 mm, it is better than error span of Ф2.5 mm workpiece. The errors for workpiece of size Ф1.5 mm are located in the range of ±0.09 mm besides three points with large errors, expressing the smallest error range with the best surface quality.

Fig. 8 shows the error distribution of blade workpiece ma-chined by grid cathode composed of Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm circular cells. 210 numerical values shown by red “*” denote surface errors of workpiece produced by Ф1.5 mm grid cathode, 156 numerical values shown by blue “o” denote surface errors of workpiece produced by Ф2.0 mm grid cath-ode, 100 numerical values shown by green “” denote surface error of workpiece produced by Ф2.5 mm grid cathode. To sum up, more than 95% error points are located in the range of ±1.0 mm, it is larger than error span of plane and slant ma-chining. The structural complexity of blade cathode composed of circular cells results in structural error shown in Fig. 5 and machining error shown in Fig. 8.

For the grid cathode with the same cell size of Ф1.5 mm in Figs. 6-8, the plane workpiece has the smallest error range of ±0.08 mm, the slant workpiece has error range of ±0.1 mm, and the blade workpiece has the largest error range of ±1.0 mm.

Therefore, for the same cell dimension, the plane workpiece has the smallest error with the best surface quality, the slant workpiece has larger error with worse surface quality, and the

Fig. 6. Error distribution of plane work pieces using grid cathode com-posed of different circular cells.

Fig. 7. Error distribution of slant work piece using grid cathode com-posed of different circular cells.

Fig. 8. Error distribution of blade workpieces using grid cathode com-posed of different circular cells.


blade workpiece has the largest error with the worst surface quality. In other words, the machining quality degrades with increasing of complexity degree of the workpiece shape.

4.3 Influence of cell dimension to surface quality

We now study the influence of cell dimension on machining precision and surface quality. Fig. 9 presents the plane, the slant and the blade workpieces produced by three different cathodes with different cells of size Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm from left to right. Each workpiece surface almost copies the cathode profile. Because there is an altitude differ-ence between adjoining cells existing on the end of grid cath-ode, after copying the cathode surface, the workpiece surface is also not smooth being bestrewed with small hunches. The height of hunch decreases with reducing of cell dimension in spite of plane, slant, and blade machining. Fig. 9 shows obvi-ously that Ф1.5 mm workpiece has the smallest hunch, Ф2.0 mm workpiece has bigger one, and Ф2.5 mm workpiece has the biggest one.

Error distribution of workpiece in Figs. 6-8 also reflects the surface quality of workpieces produced by different grid cath-odes shown in Fig. 9. Taking slant workpiece for instance, machining error of Ф1.5 mm, Ф2.0 mm, and Ф2.5 mm work-piece respectively varies in the range of ±0.09 mm, ±0.1 mm, and -0.3 ~ 0.2 mm.

Therefore, for the same surface to be produced, if the cell dimension of grid cathode is smaller, the surface quality of workpiece is better and the machining precision is higher. If the cell dimension is small enough, the workpiece can be ma-chined with high precision and smooth surface.

5. Reasons resulting in machining error

5.1 Machining principle introducing error

The advantage of grid cathode is multi-machining objects. We can obtain cathode with different machining profiles just by modifying the position and height of the circular cells. Grid cathode also introduces a disadvantage: altitude difference, existing among circular cells in order to construct different surfaces. Fig. 10 shows the altitude difference in slant and blade cathode. Altitude difference can not be avoided, thus, machining error is unavoidable. It is dependent on design and machining principle of grid cathode. However, altitude differ-ence becomes smaller as the cell dimension is decreased, and may be ignored if the circular cells are small enough. It forms the basis of a perfect workpiece with smooth surface and high precision.

5.2 Cell gaps introducing error

There are gaps in the connection of circular cells, which re-sult in the formation of hunches on the workpiece surface. Fig. 11 shows the cross section of grid cathode composed of circu-lar cells along with the cell gaps.

Fig. 10 also shows the gaps among cells, which make cur-rent density in partial gap positions be smaller than that in other entitative positions. Thus, the hunches come into being in gap positions in machining processes. Moreover, with in-creasing of the gap size, the hunch is more pronounced. In an extreme case, large enough gap will hinder machining in par-tial place between cathode and anode, cause short circuit, and the workpiece surface will be ablated. To reduce such errors influenced by the cell gaps, keeping good stiffness and straightness of each cell, ensuring coherence of all cells, and avoiding physical cross and interference between cells is im-portant.

Fig. 9. Plane/slant/blade workpieces machined with different cell cath-odes.

Fig. 10. Altitude difference among circular cells in slant and blade cathode.

Fig. 11. Gaps existed among cells in vertical view.


5.3 Uneven current density introducing error

ECM realizes a machining process through an ion effect and a chemical reaction in which current density is one of the most important factors. Whether even or not, the current den-sity in circular cells is the key factor that influences machining quality and error for grid cathode.

To avoid machining error introduced by uneven current density, keeping on-state circuit and steady voltage of all cells is the first step, replacing short-circuit cells when reforming grid cathode is the next step. Upon two steps may be the main factors according to doing experiments up to now.

6. Machining comparison between grid cathode and

unitary cathode

Blade workpieces produced by grid cathode composed of Ф2.0 mm circular cells and unitary cathode are taken for in-stance. Two workpieces machined by two different cathodes are shown in Fig. 12. It is obvious that workpiece surface pro-duced by unitary cathode is smooth and fully copies cathode profile. Accordingly, the workpiece surface produced by grid cathode basically copies blade profile, but with coarse surface. Referring to foregoing error statistics, workpiece quality pro-duced by grid cathode satisfies machining requirement under the error range of -1 ~ 0.8 mm. Of course, machining preci-sion can be improved by decreasing cell dimension of grid cathode.

In order to compare two workpieces produced by grid cath-ode and unitary cathode more clearly, 3D surface error distri-bution describing measured data of workpieces in CMM is depicted in Fig. 13. In Fig. 13(a), colored meshed lines, formed by 210 points with coordinates, denote the profile of workpiece machined by unitary cathode, black points with poles according with upon 210 points show the profile of workpiece machined by grid cathode composed of Ф1.5 mm circular cells.

Black points distribute nearby meshed surface, some over-lapping with cross points on meshed surface, some above surface, and some below surface. The distribution of points shows surface errors between grid workpiece and unitary workpiece. Large distance between black points and cross points on meshed surface means large machining error when

Fig. 12. Surface comparison machined by grid cathode and unitarycathode.

(a) Ф1.5 mm

(b) Ф1.5 mm side view

(c) Ф2.0 mm

(d) Ф2.0 mm side view

Fig.13. 3D Error distribution of work piece surface machined by uni-tary cathode and grid cathode composed of circular cells.


using grid cathode. If all the black points overlap with cross points on meshed surface, perfect workpiece with high preci-sion and smooth surface will be produced.

Fig. 13(b) shows the position of points in side view, and it is obvious that black points surround meshed surface.

Fig. 13(c) and (d) has the similar distribution of points just like Fig. 13(a) and (b). Fig. 13(c) and (d) shows the profile of workpiece machined by unitary cathode and grid cathode composed of Ф2.0 mm circular cells. There are 156 measured points less than these in Fig. 13(a) and (b) because its cell size is larger than Ф1.5 mm. Fig. 13(d) also shows the positions of points surrounding meshed surface in side view.

7. Conclusions

(1) The grid cathode composed of the circular cells can be used in ECM, the machining quality degrades with increasing of complexity degree of the workpiece surface.

(2) For the same surface to be produced, if the size of circu-lar cell is smaller, the workpiece quality is better and the ma-chining precision is higher. If the size of circular cell is small enough, the workpiece can be machined with high precision and smooth surface.

(3) When using the grid cathode composed of the circular cells, the plane workpiece has the smallest error range of ±0.08 mm, the slant workpiece has error range of ±0.1 mm, and the blade workpiece has the largest error range of ±1.0 mm.

(4) The balance current varies from 300 to 410A, and the balance time varies in the range of 130~253s. The balance current increases and the balance time prolongs when the size of the circular cell is reduced. The balance current is the larg-est one in machining process with the plane cathode, and the balance time is the longest one in the process with the blade cathode.

Acknowledgments

This work was supported by National Natural Science Foundation of China (No. 51005122) and Aeronautics Science Foundation of China (No. 2008ZE52049).

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Yonghua Lu received his M.S. and Ph.D from Nanjing University of Aero-nautics and Astronautics (NUAA), China. He is currently an Associate Professor at the College of Mechanical and Electrical Engineering at NUAA in Nanjing, China. Dr. Lu's research inter-ests include measurement system, ECM,

and sensor.


Dongbiao Zhao received his M.S. and Ph.D from Nanjing University of Aero-nautics and Astronautics (NUAA), China. He is currently a Professor at the College of Mechanical and Electrical Engineering at NUAA in Nanjing, China. Dr. Zhao's research interests include CNC system, robotics, and con-

trol system.

Kai Liu received his M.S. and Ph.D from Nanjing University of Aeronautics and Astronautics (NUAA), China. He is currently an Associate Professor at the College of Mechanical and Electrical Engineering at NUAA in Nanjing, China. Dr. Liu's research interests in-clude CNC system and robotics.

experimental study on ecm with a grid cathode composed of

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