DEVELOPMENT OF A DESKTOP SIZE ELECTROCHEMICAL

The 17th International Conference on Machine Design and Production July 12 – July 15 2016, Bursa, Turkiye

DEVELOPMENT

OF

A

DESKTOP

SIZE

ELECTROCHEMICAL

MACHINE

FOR

MICRO/MACRO MANUFACTURING

Hasan DEMIRTAS, [email protected] Kilis 7 Aralik University, 79000, Kilis Oguzhan YILMAZ, [email protected] Gazi University, 06570, Ankara Bahattin KANBER, [email protected] Bursa Technical University, 16190, Bursa

ABSTRACT Electrochemical machining (ECM) is one of the non-traditional machining processes and it has been used for specific areas where they require high-performances. In this paper, development of a desktop size EC machine is presented for micro/macro manufacturing. In order to verify the capabilities of the developed machine, the process experiments were conducted and discussed in this paper. With the developed EC machine, ECM parameters like electrolyte flow rate, temperature, concentration, voltage, current, short circuit, feed rate and initial gap can be controlled. The experimental results are compared with literature. They show that voltage is the most effective ECM parameter.

Keywords: ECM, Micro, Macro, Voltage

1. INTRODUCTION ECM is a non-traditional machining process which is theoretically based on Faraday’s and electrolysis laws. According to these laws, the electron flow from one electrode to another (connected to a direct current power supply) is obtained with the help of an electrical conductive liquid. In the ECM process, a workpiece and tool are considered as the electrodes, which are charged as positive and negative poles, respectively. Therefore, the tool is called a cathode and the workpiece is called an anode. In the machining mechanism, the material is removed from the anode without a contact between the anode and cathode. The liquid is called as electrolyte, and it is pumped through the machining gap between the anode and cathode while direct current is passed through the gap at a low voltage to dissolve metal from the anode. ECM is more advantageous than conventional machining


because ECM can be applicable regardless of material hardness, tool wear and cutting forces in some cases. In fact, ECM is very useful for producing a bright surface finish, machining difficult-to-cut materials and manufacturing complex geometry components [Rajurkar, 1999]. Because of these properties ECM has become an important manufacturing method for difficult-to-cut materials and to shape freeform surfaces. However, ECM still lags far behind other processes such as electric discharge machining (EDM) in the machining field because of the difficulty in controlling machined shape and larger machining gap during ECM. Various methods have been applied to improve the machining performance of the ECM process in the literature. [Bhattacharyya, 2003] designed an electrochemical micro machining (EMM) system that the movement was made by screw and nut, the motor type was chosen as stepper motor also electrolyte concentration and voltage effects on overcut and material removal rate (MRR) were investigated. [Li, 2008] has investigated the effect of pulse frequency, feed rate, voltage and electrolyte concentration on micro hole drilling of stainless steel. Movement of the system has been made by stepper motors. [Vanderauwera, 2013] has investigated the performance of macro electrochemical milling and experiments were performed on an electrical discharge machining (EDM) machine that has been adapted for ECM milling and have been compared continues DC machining and pulsed DC machining. [Thanigaivelan, 2013] has investigated the effect of acidified sodium nitrate of stainless steel and designed experimental setup movement made on one axis also used stepper motors were adjusted by microcontroller. [Malapati, 2011] designed an EMM system has gantry moving bridge type X-Y-Z stage and axes movements were made by steppers motors that controlled by a controller unit. [Neto, 2006] has investigated the effects of feed rate, voltage and electrolyte types on overcut and MRR for ECM. According to this study, influence of feed rate is more than the other parameters. [Mukherjee, 2005] has modeled the MRR due to over voltage of the system and compared the experimental results for the experiments the chosen parameters are equilibrium machining gap, feed rate, voltage and current density. MRR is proportional with current density and inverse proportional with over voltage. [Costa, 2009] has represented a method for texturing metallic surface with using ECM and the experiments show that the gap distance between the tool and the workpiece allow good flushing conditions with low MRR. To increase the MRR, gap distance can be decrease with increase electrolyte flow rate. [Tehrani, 2000] has investigated the pulsed voltage effect on electrochemical grinding (ECG) of two different materials die steel and stainless steel 304. Under conditions where overcut of 0.03 mm is expected in conventional ECG, with using pulsed voltage zero overcut has been obtained.


In this study, a desktop size EC machine that has capability to machine of micro/macro parts is represented. The machine has been investigated into three main units. Some experiments are conducted to find out the feasibility of understanding the ECM parameter effects on overcut and material removal rate and to verify the capabilities of the developed EC machine with using brass electrode. The work piece material is copper and the machining parameters selected for this study are feed rate, voltage and initial gap. In this work, electrolyte temperature and flow rate are kept constant. 2. ECM SETUP An ECM machine is developed independently, which is in fact a multifunctional machine tool, as shown in Fig.1. This machine can be classified into three main units. Machine base, power supply and electrolyte control units.

Figure 1 Schematic of EC machine

Figure 2 Photographic view of EC machine


2.1. ECM Machine Base Machine base of the ECM is made by aluminum profiles to avoid the corrosion effects of the electrolyte and provided from a manufacturer. Also to avoid the corrosion effects on movement elements like screw and guideways table of the machine base is covered by Plexiglas. Three-axis electrode feed mechanism is made by G codes in the multifunctional machine tool. The motion parts of X, Y and Z axes are driven by stepper motors through precision ball-race feed screw with resolution of 25 µm. The tool electrode or cathode is clamped on the Z axis by a Plexiglas plate. ECM machine base specifications are shown in Table 1. Table 1 Specifications of the ECM machine base Dimensions (mm) Working Area (mm)

X

Y

Z

350

350

100

X-Y-Z movements (Guideways)

Recirculating Rolling Guides

Max. movement speed (m/min)

4

Max. machining speed (m/min)

2

Motor type Motion transmission Table Structure

Stepper 16/5 ball screw 22.5 mm T-Channel Aluminum Aluminum

Some advantages like precise positioning and good response to starting/stopping/reversing stepper motors have been chosen for the ECM machine. As discussed below the accuracy of the movement of cathode is a key factor in ECM. Due to its advantages like more load capacity and high accuracy square type recirculating rolling guides has been chosen [López de Lacalle, 2009]. The drive by ball screw is the most widespread in the machine tool field for strokes not exceeding 4–5 m. The main characteristics which place it in such a favorable position are the high mechanical reduction provided maintaining a high efficiency and stiffness, as well as sufficient accuracy for existing machine tools. A ball screw is a mechanical device which transforms the rotary movement into linear movement [López de Lacalle, 2009] and linear drive is made by ball screw in this study.


Figure 3 Linear Motion System The communication between the machine and computer is made by Parallel Port connection. It is a very simple way of transferring data, it has been used for many things other than connecting printers and can transfer files between PCs, attach copy protection “dongles,” connect peripherals such as scanners and Zip drives, and of course control machine tools using it. The USB interface is taking over many of these functions, and this conveniently leaves the parallel port free for ECM control board. Limit switches are used to prevent any linear axis from moving too far and causing damage to the structure of the machine. You can run a machine without them, but the slightest mistake in setting up or programming can cause a lot of expensive damage. Cylindrical photoelectric sensors are used for this work as limit switches. Stepper motors have input pins or contacts that allow current from a supply source (in this application note, a microcontroller) into the coil windings of the motor. Pulsed waveforms in the correct pattern can be used to create the electromagnetic fields needed to drive the motor. Depending on the design and characteristics of the stepper motor and the motor performance desired, some waveforms work better than others. Although there are a few options to choose from when selecting a waveform to drive a two phase PM stepper motor, such as full-stepping or micro-stepping. These signals can be produced by a dedicated stepper driver. A schematic of the ECM machine axes control mechanism is shown in Figure 4 and a picture of the electrical cabinet of the ECM machine is shown in Figure 5.


Figure 4. Shematic of the ECM machine.

Main Board

Z Axis Driver Board X Axis Driver Board

Power Supply

Y Axis Driver Board

Figure 5. Electrical cabinet of ECM machine 2.2. Direct Current (DC) Power Supply One of the main units in ECM is the DC power supply. The DC power supply should be powerful enough to supply the necessary machining current to the working gap. According to electrolysis laws, ECM occurs at low voltage and high current values. Also control of the voltage and current with precession are important factors in ECM. EC machine is equipped with a programmable DC power supply that the power capacity is 2400W to achieve these requirements that discussed above. Maximum range of the DC power supply for voltage is 40 V – 60 A and for current is 20 V – 120 A. Also DC power supply allows reading of the voltage and current outputs to avoid the short circuits by control cards like PLC or microprocessors. Specifications of the DC power supply that is used for EC machine can be seen in Table 2.


Table 2 Specifications of the DC Power Supply Output Voltage

0 – 40 V

Output Current Output Power

0 – 120 A 1200 – 2400 W Voltage Measurement

Range Accuracy

8 – 40 V % 0.05 + % 0.05 F.S. Current Measurement

Range Accuracy

24 – 120 A % 0.1 + % 0.1 F.S.

2.3. Electrolyte Control Unit Electrolyte properties like concentration, flow rate and temperature are also important for ECM. For this reason, electrolyte pump has a special importance. Due to some advantages like smooth run and low cost, electrolyte pump chosen as a three phase type and specifications are shown in Fig. 2. As can be seen from Figure 6, the capacity of the pump or flow rate changes with the head and the power of the pump. Head of the pump cannot be changed due to design of the ECM system; therefore to change the capacity of the pump the power of the pump must be controlled. In this system to control the flow rate or the capacity of the pump a low voltage AC drive is used.

Figure 6. Specifications of the pump An AC drive is a device used to control the speed of an electrical motor in an energy-efficient way. With using the AC drive electrolyte flow rate can be adjusted from 0.1 to 14 m3/ h and AC drive is controlled by a control device that is connected to a flow meter. Due to electrolyte pump power ACS310 type AC drive is chosen and the specifications of the AC drive can be seen in Table 3. To verify the flow rate of the electrolyte a turbine type flow meter is mounted on the plumbing system and to meet the maximum flow rate capacity of the pump a flow meter that the diameter 32 mm is chosen. Table 3. Specifications of the AC drive Phase type

PN (kW)

PN (hp)

Rmax (Ohm)


Three

0.55

0.75

390

3. EXPERIMENTAL PROCEDURE The experimental observations and research studies were designed in such a way so as to carryout fruitful research analysis for deriving the effective research findings, which can be useful to the applied researchers and manufacturing industries in the area of hole drilling domain achieved through ECM. To analyze the control of the desired performance characteristics of the process parameters of the ECM system, scheme was designed so as to utilize properly the developed EC machine. The tools, made up of brass 3 mm diameter and workpiece specimens were maskless copper plates of the size of 40 mm×40 mm×5 mm. Tool and workpiece are shown in Fig. 7. A fresh aqueous solution of sodium nitrate (NaNO3) was typically chosen as an electrolyte for experimentations. Variable pulsed DC power supply was used for experimentations.

Figure 7. Brass tool and the copper workpiece The variation of MRR and the overcut were observed with the variation of predominant electrochemical process parameters. Machining voltage, feed rate and initial gap are considered to be more influential parameters in ECM. These parameters were considered for the experimentations to study their influences on machining criteria such as MRR and overcut. Table 4 shows the ECM parameters used in the operation, where of the tool (

/

), initial gap (

),

is the feed rate

is the voltage ( ).

Eq. 1 was used for calculation of the material removal rate (MRR), considering a workpiece density of 0.00896 /

. =

.

(1)


where

is the mass of workpiece before machining,

machining and

is specific mass of the workpiece and

To obtain the overcut, Eq. (2) was used. Where

is the mass of workpiece after is machining time. is the hole diameter and

is the

cathode diameter. =

(2)

Initial and final weights of the workpiece were taken by a precision electronic weighing machine and to discuss the shortcut effects on MRR shortcut time is added to machining time and time was noted with help of stopwatch.

Table 4. Experimental Conditions Experiment Condition

(mm)

(mm/min)

(Voltage)

1

0.3

3

5

2

0.3

3

8

3

0.3

3

10

4

0.5

3

5

5

0.5

3

8

6

0.5

3

10

7

0.7

3

5

8

0.7

3

8

9

0.7

3

10

10

0.3

5

5

11

0.3

5

8

12

0.3

5

10

13

0.5

5

5

14

0.5

5

8

15

0.5

5

10

16

0.7

5

5

17

0.7

5

8

18

0.7

5

10


4. RESULTS AND DISCUSSIONS 4.1. Effect of Voltage Fig. 8 shows the effect of voltage on MRR for different initial gaps. Graph indicates that increase in voltage causes the MRR increase for different initial gaps. The obtained results are similar with the literature [Bhattacharyya, 2003; Vanderauwera, 2013]. This result was expected because the MRR increases with voltage, and when the voltage increases, the machining current also increases. According to Faraday laws, machining current increase with MRR and these results similar. For this condition, tool feed rate is 5 mm/min. 2,3 2,1

0,3mm 0,5mm 0,7mm

MRR (mm3/min)

1,9 1,7 1,5 1,3 1,1 0,9

Vf = 5 mm/min

0,7 0,5 4

5

6

7

8

9

10

11

Voltage (V)

Figure 8. Effect of Voltage on MRR for

=5

/ =3

Fig. 9 shows the effect of voltage on MRR for different initial gaps for

/

. As can

be seen from Fig. 9 increase in voltage causes increase in MRR but after 8 V increase in MRR is very low. The reason of this is the machining time for 8 V and 10 V are approximately equal for all initial gaps. For lower feed rates the required time for occurrence of electrical field occurs therefore formation of short circuit is minimized. 2,1

0,3mm 0,5mm 0,7mm

1,9

MRR (mm3/min)

1,7 1,5 1,3 1,1 0,9

Vf = 3 mm/min

0,7 0,5 4

5

6

7

8

9

10

11

Voltage (V)

Figure 9. Effect of Voltage on MRR for

=3

/

Fig. 10 shows the effect of Voltage on overcut for different initial gaps for

=3

/

. As

can be seen from Fig. 10, increase in voltage causes increase in overcut. Due to the increase of machining voltage, the localization effect of current flux flow decreases. Due to


less localization effect, the stray current flow actually increases in the machining zone, in turn affecting more material removal from the larger area of workpiece which causes an increase in overcut. Electrochemical reactions generate ‘H2’ gas at tool. At higher voltage, H2 gas bubbles break down resulting in the occurrence of micro-sparking. This sparking causes uncontrolled material removal from the workpiece and finally larger overcut is the consequent result. So, the overcut increases more rapidly at higher voltage zone. 1,1

1,0

0,3mm 0,5mm 0,7mm

Overcut (mm)

0,9

Vf=5 mm/min 0,8

0,7

0,6

0,5 4

5

6

7

8

9

10

11

Voltage (V)

Figure 10. Effect of Voltage on Overcut for

=5

/

4.2. Effect of Feed Rate Figure 11 shows the effect of feed rate on MRR. As can be seen from Fig. 11, MRR increases with feed rate. This result which is qualitatively similar to the literature [Klocke, 2013] can be explained as increase of feed rate cause the decrease of the gap distance between the anode and cathode and that cause the decrease of resistance between anode and cathode therefore machining time is reduced and MRR increased. 2,2 2,1

0,3 mm 0,5 mm 0,7 mm

MRR (mm3/min)

2,0 1,9 1,8 1,7 1,6 1,5

Voltage= 10V

1,4 1,3 3

4

5

Feed Rate (mm/min)

Figure 11 Effect of Feed Rate on MRR for 10V. The relationship of overcut at different feed rates is given in Fig. 12. Figure 12 shows that an increase in tool feed rate reduces the overcut. With increase in tool feed rate the void fraction increases and the electrolyte conductivity reduces resulting in decrease in overcut. The accumulation of gas bubbles on the side surface of the cathode and the precipitation of the


metal ions removed from the workpiece on the side-wall of the hole (or anode) together reduce the passage of current in the radial direction, which reduces side dissolution of the work material. 1,1

5V 8V 10V

1,0

Initial Gap=0,5 mm Overcut (mm)

0,9

0,8

0,7

0,6

0,5 3

4

5

Feed Rate (mm/min)

Figure 12. Effect of Feed Rate on Overcut for 0.5 mm Initial gap. 4.3. Effect of Initial Gap The relationship of MRR with different Initial Gap distances is given in Fig. 13. Figure 13 shows that the effect of initial gap for MRR is very low. But experiments show that after 5 V initial gap has a significant role on overcut. Fig. 14 shows that overcut does not change with initial gap for different initial gaps. The reason of this is the distance between anode and cathode is too long for the occurrence of the electrical field. If the initial gap is reduced under 0.3 mm initial gap can be more effective for 5 V. 2,2

5V 8V 10V Vf= 3 mm/min

2,0

MRR (mm3/min)

1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,2

0,3

0,4

0,5

0,6

0,7

0,8

Initial Gap (mm)

= 3

Figure 13. Effect of Initial Gap on MRR for 1,1

3 mm/min 5 mm/min

1,0

Voltage= 5V OverCut (mm)

0,9

0,8

0,7

0,6

0,5 0,2

0,3

0,4

0,5

Initial Gap (mm)

0,6

0,7

0,8

/

.


Figure 14. Effect of Initial Gap on MRR for 5V. But after 5 V the influence of Initial gap on Overcut increases. As can be seen from Fig. 15 for 5 mm/min feed rate change with Overcut for 8 V and 10 V becomes similar and for 0.7 mm Initial gap distance cause rise in Overcut. The reason of this is the time that it takes for travels initial gap distance of 0.7 mm is enough for to dissolve the metal atoms. 1,1

8V 10V

1,0

Vf=5 mm/min Overcut (mm)

0,9

0,8

0,7

0,6

0,5 0,2

0,3

0,4

0,5

0,6

0,7

0,8

Initial Gap (mm)

Figure 15. Effect of Initial Gap on Ovurcut for

= 5 mm/min

The time for travel the initial gap distances are also long enough to dissolve the metal atoms for lower feed rates. Fig. 16 shows that initial gap has a significant role on overcut and cause increase in overcut. 1,1

1,0

Overcut (mm)

0,9

0,8

0,7

8V 10V

0,6

Vf=3 mm/min

0,5 0,2

0,3

0,4

0,5

0,6

0,7

0,8

Initial Gap (mm)

Figure 16. Effect of Initial Gap on Ovurcut for

= 3 mm/min

5. CONCLUSSIONS In the present research, a developed desktop size EC machine that has micro/macro machine capabilities is represented. With the developed the following ECM parameters can be controlled. 

Electrolyte properties  Flow rate  Temperature


 Concentration 

Feed rate



Voltage



Current



Short circuit

The experiments on hole drilling have been conducted for verification of the EC machine capabilities and the experiment results are compared with the literature. According to experiments following results for ECM parameters were obtained; 

Voltage has a significant role on MRR and Overcut and takes place in direct proportional to each other.



With lower feed rate and higher voltages short circuits can be minimized.



Increase in feed rate cause lower overcut and higher MRR.



For lower voltages Initial gap is not an important factor but for higher voltages due to electrical field occurrence initial gap becomes more effective.

6. ACKNOWLEDGEMENT This work was supported by the Gaziantep University Scientific Research Project (BAP) Department under grant number MF.14.19.

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