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A System Development Approach for ElectrolyticIn-Process Dressing (ELID) Grinding

Paper:

A System Development Approachfor Electrolytic In-Process Dressing (ELID) Grinding

Tanveer Saleh and Mustafizur Rahman

Research Chair for Advanced Manufacturing Technology, King Saud University

Riyadh 11421, Saudi Arabia

E-mail: stanveer@ksu.edu.saDepartment of Mechanical Engineering, National University of Singapore, Singapore 119260

[Received September 8, 2010; accepted December 11, 2010]

This study aims to present the development and per-

formance evaluation of an ultra-precision ELID grind-

ing machine. The machine is a 3-axis machine, includ-

ing three conventional linear axes (XXX, YYY, and ZZZ). On

top, another rotational axis has been included to as-

sist aspheric/spherical lens grinding. In order to intro-duce intelligence to the machine, several on-machine

measuring systems have been developed and incorpo-

rated. In ELID grinding, pulsed DC voltage is applied

to the metal-bonded diamond wheels to ensure the

constant protrusion of sharp cutting grit throughout

the grinding cycle. The peak dressing voltage is kept

constant irrespective of the wheel sharpness in conven-

tional ELID grinding, which may lead to over dressing

of the grinding wheel. The grinding force ratio, also

known as Kvalue, is an indicator of grit sharpness. In

this study, a new approach to wheel dressing is pro-

posed: the peak dressing voltage is varied according

to the change in the Kvalue during grinding. In con-

ventional ELID grinding, the duty ratio of the dressing

power supply is kept constant throughout the grinding

cycle. However, this method does not achieve grinding

wheel truing, which is very important to maintaining

the stability of the grinding. This research work pro-

poses a novel approach to wheel truing by controlling

the dressing voltage duty ratio for ELID grinding.

Keywords: ELID grinding, grinding wheel monitoring,

wheel truing

1. Introduction

Grinding with super abrasive wheels is an efficient

method of achieving nano surface finishes on hard and

brittle materials. However, the method has some diffi-

culties associated with it. One of the major problems is

the preparation of the bonding matrix for the super abra-

sives. The abrasives, also known as grit, need to be held

firmly by the bond material, so the bond material has to

be hard in nature. However, this reduces the grinding

wheels self-sharpening and self-truing ability. An addi-tional dressing mechanism is needed to ensure good pro-

trusion of the sharp cutting grit. To attain precision sur-

face finishes and form accuracy using grinding, having the

grinding wheel dressed in-process is an obvious require-

ment. There are several methods of in-process dressing of

the grinding wheel, though all of them can be subdivided

into the four main groups listed below.

1. Mechanical contact method

2. Electrothermal method

3. Laser technology

4. Electrochemical method

Electrolytic In-Process Dressing (ELID) grinding is

one of the latest and most appropriate techniques for

dressing the metal-bonded wheel in process by the elec-

trochemical method. The basic mechanism of ELID

grinding has been explained elsewhere [1]. Researchersall over the world have carried out extensive work to in-

vestigate the application of ELID grinding on different

engineering materials [27]. However, very little effort

has been made to develop an integrated system primar-

ily dedicated to ELID grinding. This research mainly fo-

cuses on the development of a smart technology that will

help to improve ELID grinding performance. In the next

few sections, the complete design and development of the

new intelligent ELID grinding system and its experimen-

tal evaluation shall be described thoroughly.

2. Design and Development of the Intelligent

ELID Grinding System

The main objectives of this current research is to de-

velop a sensor-assisted intelligent ELID grinding system

comprised of machine integrated with sensorial feedback

and a closed-loop control ELID power supply.

2.1. ELID Grinding Machine

The 3-axis CNC machine shown in the photograph in

Fig. 1(a)has been developed to perform the ELID grind-

ing process. All three axes are controlled by AC servomotors. The machine structure was re-engineered from

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Saleh, T. and Rahman, M.

(a) (b)

Fig. 1. (a) Sensor-integrated ELID grinding machine. (b) Specially designed injection electrode.

an existing wire-cut EDM machine and configured as a

gantry structure comprised of two pillars, a crossbeam,

and a base. ELID grinding is meant to produce nano-level

surface quality with high dimensional and form accuracy,so machine structural rigidity is a big concern. That is

why a gantry structure has been chosen: to develop a ma-

chine that provides better rigidity. A special type of elec-

trode is used, one which has a cavity inside the body, as

shown inFig. 1(b), for better flushing of the electrolytes.

A pump is used to inject electrolyte right at the gap be-

tween the wheel and electrode [8].

A rotary turntable has also been designed and fabri-

cated to give a rotary motion to the workpiece to machine

curvilinear and axi-symmetric surfaces. Sealed-type ball

bearings are used in the turntable to protect them from

corrosion.

2.2. Sensory Arrangements and Feedback

Algorithm Associated with the Machine

2.2.1. Closed-Loop Controlled ELID Power Supply

In order to carry out the electrochemical dressing of the

grinding wheel, most of the power comes from a pulsed

DC power source. In ELID grinding, an insulating oxide

layer is formed during the dressing of the wheel because

of the electrochemical reaction. However, the formation

of the layer actually softens the wheel bond material, lead-

ing to higher tool wear. The conventional ELID powersupply continuously dresses the grinding wheels without

monitoring its condition, which may lead to over dress-

ing of the expensive grinding wheels. Moreover, the con-

ventional ELID power supply is incapable of carrying out

truing of the grinding wheels. In this research work, we

propose a novel ELID power supply. The two main ob-

jectives of this power supply are as follows.

a. To carry out dressing on demand by controlling the

dressing voltage amplitude according to the grinding

wheel condition

b. To carry out the wheel truing by controlling thedressing voltage duty ratio

In this section, the theory and the design concept of this

power supply are described extensively.

Controlling the dressing voltage amplitudeIt is a well accepted phenomenon that force ratio K

(Normal Force (Fn)/ Tangential Force (Ft)) in grinding

promotes the dulling and wearing out of the grit of the

grinding wheel [9]. Thus, a grinding force ratio moni-

toring (K) approach was implemented in this newly de-

veloped ELID power supply to control the peak dressing

voltage. A three-component Kistler dynamometer was in-

tegrated in the machine for this purpose.

The acceptable value ofKfor grinding depends on the

material of the cutting grit. In the current study, the au-

thors use a diamond grinding wheel for which the stan-

dard range ofKfor a sharp wheel is 11 to 15 [9]. There-fore, the maximum permissible limit for Kis set to 15.

The overall algorithm is described in the block diagram

shown in the Fig. 2(a).Fig. 2(b) shows the variation in the

peak dressing voltage and grinding force ratio in one ma-

chining cycle. The two peaks shown in the figure describe

the high value of grinding force ratio during the initial en-

gagement between wheel and workpiece because of the

sudden impact at the start of the machining cycle.

Controlling the dressing voltage duty ratio

In order to develop a controlled wheel-truing system by

ELID grinding (discussed later), a wheel metal bond pro-

file monitoring system was developed using an inductivesensor. The inductive sensor used in this study measures

the distance between the sensor head and wheel metal

bond. It is mounted on the setup in such a way that the

distance measured is equal to the distance between the

cathode electrode and wheel metal bond. These data were

fed back to the truing controller after proper signal condi-

tioning. In the practical implementation, 500 data points

were taken for one wheel revolution. After the signal pro-

cessing, 40 data points for one wheel revolution were used

as the feedback signal for the proposed truing controller.

The objective of the proposed control system is to vary

the dressing voltage duty ratio in such a way that it en-sures consistent wheel-workpiece contact. This can be

22 Int. J. of Automation Technology Vol.5 No.1, 2011