the electrical behaviour of the dressing process in elid-grinding

Int. J. Nanomanufacturing, Vol. 9, No. 2, 2013 137

Copyright © 2013 Inderscience Enterprises Ltd.

The electrical behaviour of the dressing process in ELID-grinding

Bruno Kersschot*, Jun Qian and Dominiek Reynaerts Department of Mechanical Engineering, KULeuven, Celestijnenlaan, 300B b2420 3001 Leuven, Belgium E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: During ELID-grinding, the metallic wheel is kept sharp through an electrolytic dressing process in which a layer is grown on top of the wheel’s surface. The electrolysis process can be modelled electrically using the Helmholtz representation of a boundary layer. Measurements show that at the start of the (pre-)dressing the capacitance of the system is at its highest value, above 10 μF. After a while, it drops significantly to a level below 1 μF. This signifies a change in the metal-electrolyte interface and the grinding wheel is covered with a layer of (hydr-)oxides. The model is refined in two ways. Firstly, the capacitance between the two metallic electrodes is added. Secondly, the self-inductance of the power cables is included. With the proposed model it is possible to properly simulate the electrical behaviour of the growth of the layer. It also provides a fundamental insight into the electrolytic dressing process.

Keywords: electrolytic in-process dressing; ELID-grinding; passivation; electrical model; dressing; oxide layer; grinding wheel.

Reference to this paper should be made as follows: Kersschot, B., Qian, J. and Reynaerts, D. (2013) ‘The electrical behaviour of the dressing process in ELID-grinding’, Int. J. Nanomanufacturing, Vol. 9, No. 2, pp.137–147.

Biographical notes: Bruno Kersschot studied Mechanical Engineering at the KULeuven in Belgium. He obtained his Master degree in 2008 and currently works as a PhD student with the research group Micro and Precision Engineering, on the subject of ELID-grinding. He received the award for excellent contribution at the nanoMan12 conference which was held in Tokyo in July 2012.

Jun Qian is a Research Expert in Precision Engineering with the Micro and Precision Engineering group at the KULeuven in Belgium. He received his doctorate degree at the Nanjing University of Aeronautics and Astronautics, China, in 1996 and from February 1998 until April 2000 he worked as a Postdoc Researcher at the RIKEN Institute in Tokyo on micro-fabrication techniques, including ELID-grinding. He is a member of Euspen.

138 B. Kersschot et al.

Dominiek Reynaerts received his mechanical engineering degree from the Katholieke Universiteit Leuven, Belgium, in 1986. He obtained his PhD in Mechanical Engineering, also from Katholieke Universiteit Leuven, Belgium, in 1995 and became Assistant Professor at the Katholieke Universiteit Leuven in 1997. Since 2007 he is a Full Professor. He is the Chairman of the Dept. of Mechanical Engineering of the Katholieke Universiteit Leuven since 2008. His research activities are manufacturing and machine design with focus on precision engineering and micromechanical systems. He is a member of IEEE and Euspen.

This paper is a revised and expanded version of a paper entitled ‘Growth and properties of the passivation layer in ELID-grinding’ presented at The Third International Conference on nanoManufacturing: nanoMan2012, Tokyo, Japan, 25–27 July 2012.

1 Introduction

To improve the surface quality of a ground part, it is required to post-process the workpiece using very small abrasives. Under proper conditions an average grit size of a few micron leads to mirror-like surfaces with Ra roughness values ranging in the nanometre scale. Conventionally these small abrasives are embedded in polishing and lapping tools, but these finishing processes often adversely affect the final shape accuracy and require long machining cycles. When such small grits are meshed into grinding wheels, little free space remains available to remove grinding debris and this makes the grinding discs more susceptible to loading and glazing (Davis, 1974). These phenomena worsen the cutting properties and lead to chattering and a shorter wheel life. In electrolytic in-process dressing (ELID)-grinding an electrolytic dressing process is used to keep the super-abrasive wheels sharp (Ohmori and Nakagawa, 1990). A layer of oxides and hydroxides is grown on top of the metal-bonded grinding wheel which is more brittle than the wheel bonding underneath. The holding force of abrasives is lowered which facilitates the removal of worn-out grits and workpiece swarf. An electric current partly dissolves the cast iron bonding (CIB) in order to form an electrically insulating layer of iron-oxides and -hydroxides. The layer lowers the amount of current in the loop and inhibits further dissolution of the bonding. Figure 1 illustrates the reflectivity of a sample of zirconia which has been ELID-ground to a Sa roughness of 6 nm (and Sz of 50 nm). The sample has a straightness error of less than 1 μm over a length of 50 mm and a width of 20 mm. A second advantage of ELID-grinding is that it enables the use of metal-bonded grinding wheels. These are known for their superior durability compared to vitrified and resin-bonded wheels and makes them suitable to efficiently grind very hard materials.

The electrical behaviour of the dressing process in ELID-grinding 139

Figure 1 ELID-ground sample of zirconia (see online version for colours)

Based on the grit size a distinction is made between ELID-roughing and ELID-finishing. In rough grinding the grits are bigger than the layer thickness, which amounts up to 25 to 40 μm, while the finishing wheels hold smaller grits. In Klocke et al. (2009), the thickness of the oxide layer was measured to have a comparable value of about 30 μm after only 7 minutes of dressing. Especially as grit sizes start to shrink, the role of the oxides in the contact zone gets more significant and the abrasive process corresponds more to polishing. The layer is crucial in obtaining a supersmooth surface finish and acts differently from any other conventional grinding contact. Therefore, it is essential to investigate the function of the layer: i.e., its formation and growth, the insulating properties and the wear behaviour.

Faraday’s law governs the electrolysis, stating that the amount of dissolved wheel bonding is determined by the total charge passed through it. With this law it is theoretically possible to predict both the wear of the wheel bonding and the growth of the passivation layer. In the literature, several theoretical considerations about the growth are based on a few chemical reactions, while in reality multiple and complex reactions occur during the precipitation phase. This makes it very difficult to predict the layer growth in practice. A first attempt to model the layer growth (and wear) using a resistor as electrical equivalent of the layer was done by Bifano et al. (1999). The authors claim that the layer growth depends on the actual oxide thickness in a logarithmic way. Klocke et al. (2007) have also investigated the thickness and resistance of the passivation layer and have analysed the growth behaviour for different types of bronze bonded wheels. Biswas (2009) also investigated the electrochemical behaviour into detail in his thesis. However, the model used is too basic and the process is modelled as being purely resistive while a stray inductance has been observed in the measurements. This model has been refined in Kersschot et al. (2012a, 2012b). This paper provides a fundamental analysis on the electrical modelling of the layer. The goal is to develop a general process model to enable the understanding and the prediction of the passivation state and to find useful ELID-grinding parameters.

2 Experimental setup

Figure 2 displays the setup which is used for the dressing experiments. The cathode is an injection electrode through which the electrolyte is supplied. Below the cathode lies a piece of CIB (25 mm × 10 mm) which can be regarded as a part of the grinding wheel. This configuration was chosen in order to simplify the experiments because it is more convenient to manipulate and measure these small blocks. The gap in between the anode and the cathode is kept at a constant value of 0.2 mm. The lower part is mounted on a linear slide which moves in a reciprocating way at a nominal velocity of 3 m/s. This motion resembles the rotation of the wheel. The alignment between both electrodes is


adjusted mechanically using two set screws and a rotational joint located above the electrode. The CIB part is the anode which is connected to the positive polarity of the power supply. Both electrodes are insulated from the environment by means of plastic holders. The experiments were done on two different types of CIB blocks bought from the Nexsys company: one of pure CIB without any diamonds and one with diamonds of mesh size JIS#4000 at a concentration of 100%. The blocks have an area of 250 mm2 from which a small amount is covered with a protective lacquer to act as a reference for the thickness measurements. The growth of the passivated samples is measured as the difference between the top of the passivated area and the original surface. The layer of oxides is porous and non-uniform in thickness and to attain statistically relevant values the growth was measured at four points. This was realised with the help of a Zeiss Discover V.20 microscope.

Figure 2 Lay-out of the ELID dressing setup (see online version for colours)

The ELID power supply, a Fuji ELIDER ED921, generates chopped DC pulses according to three adjustable parameters: the maximum current Ip, the voltage output V and the current duty ratio Rc. In this research Ip was set at 20 or 40 A, the voltage was altered between 60 and 90 V and the duty ratio was either 30, 50 or 70%. The pulses have a fixed frequency of 100 kHz.

The electrolyte is a 2% dilution of Noritake Cool CEM in demineralised water. The pH value is measured at 10.1 and the conductivity is 1.25 mS/cm.

3 Modelling of the dressing process

During the electrolytic dressing of a grinding wheel the average current drops while the average voltage rises (Ohmori and Nakagawa, 1997). This is also illustrated in Figure 3, which shows the average voltage and current in time during dressing of a block of CIB, setting the power supply parameters Ip at 20 A, V at 60 V and Rc at 50%.


Figure 3 Average voltage and current during the electrolytic dressing at 60 V, 20 A and 50%

In the first minute a rather steep change is observed in both average current and voltage. Afterwards the change is less pronounced and after 5 minutes the charge does not contribute much more to the layer growth. This is the time at which the capacitance drop is not significant anymore (see also paragraph 4 and Figure 8).

Figure 4 depicts the time evolution of the instantaneous pulses after 0.1, 1, 5 and 20 minute(s). Although the current pulses always drop down to 0 A, the voltage never reaches the level of 0 V. The voltage signal is floating and this effect is strengthened over dressing time.

Figure 4 Measured voltage and current pulses during dressing of a CIB block at 60 V, 20 A and 50%

The voltage drop consists out of two different regimes: first a steep voltage drop is noticeable and the second part is characterised by an exponential decay. This effect gets stronger after several minutes of dressing. The steep drop is caused by stopping the current from running through the loop: when the voltage starts to drop the power supply


suddenly does not provide any current anymore. The voltage dissipation across the passivation layer suddenly ends, which explains the sharp drop in voltage. Not the whole amount of the voltage difference is dissipated, which means that still some of it remains stored in the layer. In fact, a capacitor is charged during the ON-state of the pulses and is slowly discharged during the OFF-state. The charge is then dissipated in the passivation layer.

The Helmholtz representation of a boundary layer, see Figure 5, is a good approximation of this process. In this model the boundary layer is represented by a parallel circuit of a capacitor C and a resistor Rp, in series with a second resistance Rs. There are always two boundary layers in an electrochemical cell, both at the anode and at the cathode, but in ELID-grinding the one at the cathode is assumed to remain constant. The cathodic boundary layer therefore has little influence on the transient behaviour of the signals.

Figure 5 Electrical equivalent scheme of the electrolytic dressing process

Figure 5 also indicates the positions of the current and voltage sensors and of the power supply.

Based on the acquired data, the transient total resistance Rt of the oxide layer is derived. This resistance is the sum of resistors Rs and Rp (see Figure 6) and is calculated using Ohm’s law at the time that the pulses are in the ON-state. The activation and concentration overpotentials and the voltage drops due to hydrogen and oxygen gas generation are assumed to be very low (i.e., lower than 2 V). The potential drop is almost entirely attributed to ohmic losses in the electrolyte and in the insulation layer. More considerations on these electrochemical subjects can be found in Biswas (2009).

With the previous insights the total resistance is mathematically divided into the series and parallel values. The abrupt voltage drop is equal to ΔI.Rs, with ΔI the current drop. Knowing the total and the series resistances, it is straightforward to calculate the parallel resistance. Figure 6 illustrates the evolution of the different resistance values for a dressing experiment in which the power supply is set at 60 V, 20 A and 50%. The change in parallel resistance amounts up to about 70 Ω for the given power settings and is much more significant than the rise in serial resistance, increasing from 9 to 18 Ω after 20 min. This rise in Rp has the major contribution to the rise in the total resistance. Furthermore, it is possible to derive the capacitance value from the exponential voltage decay. The slow voltage decay is caused by discharging the capacitor C over the parallel


resistance. By calculating the time constant, which is equal to Rp.C, the capacitor is known.

Figure 6 Change of different resistor values in time at 60 V, 20 A and 50%

4 Dressing experiments

4.1 Electrical behaviour of the passivation layer

Figure 7 shows the increase in total resistance during 20 minutes of dressing for different settings of the power supply. The word ‘CIBD’ means that the piece of CIB is mixed with diamonds (‘D’). The initial resistances for all of these measurements are determined by the conductivity of the electrolyte, the area of the electrodes and the gap width. The values lie in between 13.6 and 20.7 Ω, giving a theoretical averaged gap width ranging from 0.25 to 0.5 mm. This is an overestimation of the gap widths because the overpotentials and the ohmic losses in the interfaces are neglected. The initial value was subtracted from the data to obtain a clearer comparison in Figure 7. In Bifano et al. (1999), the resistance of the electrolyte on its own (which is in fact equal to the initial resistance) is measured to be in between 9.5 and 18 Ω for a series of parameter settings. The authors hereby changed the following parameters: the grinding wheel speed is set at 6 to 10 m/s, the electrolyte supply rate at 1 to 1.8 liter per minute and the gap width at 0.175 to 0.450 mm.

The transient evolution of the capacitance follows a decreasing trend. This can be seen in Figure 8 which has a logarithmic scale for the ordinate. The magnitude of the capacitor starts at values in the order of 10 μF (until about 100 μF) and drops down to the order of magnitude of 0.1 μF after 20 minutes. At the beginning, when the piece of CIB has no oxides on top of it, the capacitance is very high. A metallic electrode in an electrolytic environment typically has a double layer capacitance of 10 to 40 μF/cm2 (Wang, 2006). Given the area of the CIB blocks, which is 2.1 cm2, the total capacitance


amounts up to an estimated value of 20 to 80 μF, which is very close to the calculated values. In the initial dressing phase the capacitive voltage decay is very small compared to the ohmic voltage drop of the parallel resistor. This makes it more difficult to accurately calculate the time constant of the decay and therefore it is difficult to obtain reliable initial capacitance values. However, changing the initial capacitance value from 20 to 80 μF has a negligible influence on the simulated pulses. After a few minutes the distinction between the two voltage drop regimes gets clearer, resulting in more accurate values.

Figure 7 Transient overall resistance of the passivation layer

Figure 8 Transient capacitance of the passivation layer

At the start of dressing there is only one interface at the grinding wheel, i.e., between the purely metallic bonding and the electrolyte. When the layer begins to grow two interfaces are present: between the CIB and the oxides and between the oxides and the electrolyte. This sequence is a series chain of capacitors, for which the smallest capacitor determines the overall equivalent value. Because the oxides slow down the flow of ions, its interfaces can build up less charge than in the case of a metal-electrolyte interface. This explains the decrease in capacitance value over time.


4.2 Growth of the passivation layer

After the dressing experiments the lacquer was removed from the samples to measure the growth of the passivation layer. Figure 9 lists the means and standard deviations of these measurements. Again, ‘CIBD’ stands for a CIB part with diamonds. All mean values lie in a range of 20 to 35 μm. According to the data there is no significant difference observed if the power supply parameters are adjusted. This means that even for the lowest power settings the layer is fully grown in thickness. Some passivation layers however contain more voids and imperfections than others, explaining the differences in total resistance. The dissolved depth of the CIB part is assumed at 5 μm, as measured in Klocke et al. (2009, 2007). Therefore, the total layer thickness is 25 to 40 μm. Furthermore, the presence or absence of diamonds in the CIB seems to have little effect on the growth.

Figure 9 Measured growth of the dressed cast iron blocks

5 Modelling of the dressing mechanism

Table 1 lists the derived electrical parameters of the curves for 60 V, 20 A and 50%, after 0.1, 1, 5 and 20 minutes. Given these values of capacitor and resistors, the electrical equivalent scheme is used to check the validity of the pulses. Table 1 Calculated electrical parameters at different times derived for 60 V, 20 A and 50%

0.1 min. 1 min. 5 min. 20 min.

Rs [Ω] 8.6 12.7 16.2 18.4

Rp [Ω] 3 2.2 22.5 175.7

C [μF] 20a 2.7 0.27 0.16

Notes: aBecause the initial capacitance is difficult to calculate, a value of 20 μF has been estimated. However, changing it to 80 μF has little effect on the final shape of the pulses.


Figure 10 Simulated pulses using the scheme of Figure 5

Figure 11 Simulated pulses using the improved scheme

A simulation with these parameters leads to the pulses of Figure 10 and shows a moderate correlation with the measurements of Figure 4. The actual electrochemical configuration differs from the proposed theoretical model in Figure 5. Firstly, the long electrical cables are characterised with a not to be neglected inductive impedance. The self-inductance of the cables is large in this application because both the amount of current and the frequency of the pulses are relatively high. The self-inductance of one of the two straight connecting cables of 0.7 m length is 1.1 μH while the self-inductance of the two 5 m power cables amounts to 4.4 μH. The total self-inductance of the circuit is therefore estimated at 6 μH. Secondly, the capacitance between the two metallic electrodes has to be considered too. This capacitor is added in parallel to the circuit of the double layer. The capacitance is estimated at 5 nF. With these adaptations the new simulations lead to the instantaneous pulses in Figure 11. Comparing these results to the measured pulses of Figure 4 shows a high resemblance between experimental and simulated results, confirming the validity of the model.


6 Conclusions

In this paper a series of electrolytic dressing experiments has been analysed. Altering the power supply settings leads to differences in the transformation of the total resistance and the capacitance of the passivation layer. In the beginning the capacitance is high and the total resistance is low. After 5 minutes the capacitance has dropped significantly due to the formation of a thin covering layer of oxides. In the meanwhile the total resistance keeps on rising until about 180 Ω (in the case of the highest power supply settings). The power supply settings are observed to have little influence on the growth of the passivation layer after a dressing time of 20 minutes. The growth of the CIB part is measured to be in between 20 to 35 μm.

Furthermore, a model of the electrolytic dressing process in ELID-grinding has been formulated. The model is based on the Helmholtz approximation of a boundary layer. The transient behaviour of the instantaneous pulses can be explained using this model. The scheme is refined by including both the self-inductance of the power cables and the parallel plate capacitance between the two electrodes. The simulations of the voltage and current pulses show a very good correlation with the measured signals. This model provides a better understanding of the electrolytic dressing process and can be used for monitoring or controlling the state of the oxide layer.

Acknowledgements

This research is funded by the Research Foundation Flanders – FWO Vlaanderen: Project G.A076.11: ‘Predictive ELID process for high-efficiency damage-free grinding’.

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the electrical behaviour of the dressing process in elid-grinding

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