Development of an intelligent electrolytic in process dressing (ELID) grinding system 1

Introduction

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Chapter 1

Introduction

Grinding is a machining process which uses hard abrasive particles for cutting, where the
surface speed of the abrasive tool (namely grinding wheel) is relatively faster compared
to other machining processes like turning and milling. Technically, grinding is a subset of
cutting where each grain of abrasive functions as a microscopic single-point cutting edge
with high negative rake angle. It is a very old process and has been in existence since the
Stone Age when man rubbed stones against each other to produce sharp weapons. This
process is conventionally regarded as finishing process to obtain a high degree of
dimensional accuracy and surface finish on a part. Grinding is used in a large and diverse
area of manufacturing and toolmaking. It can produce very fine finishes and very
accurate dimensions; yet stock removal rate can be quite high. It is usually better suited to
the machining of very hard materials.

1.1 Background
Rapid progress in electronic and optical devices (such as IC chips, MEMS, smart sensors,
lenses) with the increasing requirement of their performance, demand of ultra precision
surface finish of the specialized glass, silicon wafer in nano-scale is growing.
Introduction

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Conventional grinding cannot produce such surfaces with precise form accuracy;
therefore, conventional grinding is still considered to be a pre-finishing process. Typical
ways for achieving final finishing of different materials are lapping and polishing.

Though these methods have disadvantages like poor grindability, waste water problem,
mechanical damages etc. [1]. Chemical-mechanical polishing (also known as CMP) is
another final finishing process for silicon wafer preparation in the IC industries. Some of
the disadvantages associated with the CMP process [2] are:

1. Low efficiency due to low removal rates,
2. Non-uniform wafer surface due to the variation in relative cutting speed across the
wafer surface, and
3. Relatively high cost involved in this process.

In order to overcome all these drawbacks associated with the conventional finishing
processes, demand for mirror surface finish by grinding (using super abrasives) to replace
the polishing, lapping, etc. have been growing stronger and stronger in the manufacture
of optical and electrical parts. With the aim of meeting such demand, efforts are actively
being made to develop high precision grinding machines, new grinding wheels and to
review the applications of these new techniques in the field of manufacturing.

Though grinding with super abrasive wheel is an efficient method of achieving nano
surface finish on hard and brittle materials, the method has some difficulties associated
with it. One of the major problems is the preparation of the bonding matrix for the super
Introduction

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abrasives. The abrasives also known as grits need to be held firmly by the bond material.
Therefore the bond material has to be hard in nature which reduces grinding wheels’ self
sharpening and self truing ability. An additional dressing mechanism is needed to ensure
good protrusion of the sharp cutting grits. When we talk about attaining precision surface
finish and form accuracy using grinding, it is an obvious requirement to have the grinding
wheel dressed in-process. There are several methods of in-process dressing of the

grinding wheel, though all of them can be subdivided into four main groups as listed
below.

1. Mechanical contact method.
2. Electro thermal method.
3. Laser technology.
4. Electrochemical method.

In-process dressing with mechanical contact uses separate dressing tool that comes in
contact physically, with the grinding wheel. The dressing tool may be mounted on a
rotating spindle or it can also be held stationary.

Dressing of grinding wheel by electro-thermal method employs electro-discharge
machining technique. There are basically two arrangements available for this method of
wheel dressing. One is block electro-discharge method where a stationary solid block is
used as the dressing electrode and another one is wire electro-discharge method where
Introduction

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moving wire instead of a block is used for dressing the wheel. Electro thermal method of
dressing is mainly used for fine grit wheels.

Laser machining method is another technique to achieve the in-process dressing of the
wheel. It is mainly used for vitrified bond grinding wheel. As laser beam is applied bond
material becomes soft and is removed during grinding and ensures protrusion of new
sharp grits. Laser can also be used as a boosting method for mechanical contact dressing.
In this case laser softens the bond material and the following mechanical dresser removes
it with ease.

Grinding wheel dressing by electrochemical method actually employs the electrochemical
machining principle to dress the wheel where the wheel is set as an anode and another
metal electrode is set as a cathode. Electrolytic In-Process Dressing (ELID) grinding is
one of the latest and most appropriate techniques to dress the wheel in process by
electrochemical method. Electrolytic dressing of the wheel applies the basic mechanism
of electrolysis. This process converts electrical energy to chemical energy in an
electrolytic solution in the presence of two electrodes (anode and cathode) as shown in
figure 1.1. An electrical power supply is used to energize anode positively and cathode
negatively. As the electric energy is applied ionic dissolutions occurs in the electrolytic
solution and the negative and positive ions in the solution move to the positive and
negative electrodes respectively. This will cause anodic metal dissolution and formation
of anodic oxide.

Introduction

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Fig 1.1: Basic principle of electrolysis

The mathematical form of metallic dissolution rate is governed by Faraday’s law of
electrolysis, which concludes that dissolution from anode material is directly proportional
to the integration of current supplied to the electrolytic cell. The mathematical form of
anodic dissolution can be written as follows,
0
T
idt
M
m
F z

= ⋅
∫


where,
m is the mass of the substance altered from anode
i is the electric current passed through the cell
F is Faraday constant
M is the molar mass of the substance
z is the valence number of ions of the substance (electrons transferred per ion)

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Basic elements of ELID grinding are shown in figure 1.2. ELID cell comprises of a metal
bonded grinding wheel, a cathode electrode, a DC power supply and electrolyte. The
wheel is connected to the positive terminal of the DC power supply thru

Fig 1.2: Basic elements of ELID grinding

a carbon brush whereas the electrode is connected to the negative pole of the power
supply. Usually alkaline liquids are used as both electrolytes and coolant for grinding. A
nozzle is used to inject the electrolyte into the gap between wheel and electrode. The gap
is usually maintained to be approximately 0.1mm to 0.3 mm. An anodic oxide layer is
formed on the circumference of the grinding wheel as a result of electrochemical
reaction. The formation of this oxide layer is very crusial for the success of ELID
grinding. It is soft and brittle in nature as compared to the original metal bond and gets
easily worn off because of the excessive grinding force. The basic mechanism of ELID
grinding has been explained in the figure 1.3[30].

After truing [Figure 1.3(a)], the grains and bonding material (metal) of the wheel surface
are flattened. It is necessary for the trued wheel to be electrically pre-dressed to protrude
the grains on the wheel surface. When pre-dressing starts [Figure 1.3(b)], the bonding
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Fig 1.3: Principle of ELID grinding [30]
material flows out from the grinding wheel and an insulating layer composed of the
oxidized bonding material is formed on the wheel surface [Figure 1.3(c)]. This insulating
layer reduces the electrical conductivity of the wheel surface and prevents excessive
flow-out of the bonding material from the wheel. As grinding begins [Figure 1.3(d)],
diamond grains wear out and the layer also becomes worn out [Figure 1.3(e)]. As a result,
the electrical conductivity of the wheel surface increases and the electrolytic dressing
restarts with the flow-out of bonding material from grinding wheel. The protrusion of
diamond grains from the grinding wheel therefore remains constant. This cycle is
repeated during the grinding process to achieve stable grinding.

The above mentioned technologies for in-process dressing of grinding wheel have their
own advantages and disadvantages. The comparative analysis of all these processes can
be understood from the table 1.1 given below,


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Table 1.1: Comparative analysis of different in-process dressing method

Mechanical
contact dressing
Electro
thermal
dressing
Laser
machining
dressing
Electrochemical
dressing
System
complexity
Bulky system Die sinking
method is
simpler than
WEDM
method.
Bulky and
complex
system
Fairly simple design
System noise
level
Noisy Quiet Quiet Quiet
Any damage
to the wheel
May cause
mechanical
damage from
over feeding etc

May cause
thermal
damage
May cause
thermal
damage
No thermal or
mechanical damage
to the wheel
Dressing tool
wear
Tool wear exists Tool wear
exists
No tool
wear.
No tool wear.

Even though the ELID is a suitable process to obtain mirror surface finish and it has
proven its advantages over other in-process dressing methods, the technology of
electrolytic in-process dressing is yet to be fully optimized. ELID grinding is always a
complex process in which the tool shape is changing while machining. Therefore it is
necessary to monitor the change in the tool shape caused by the tool wear with some
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intelligent sensory system and generate the tool path by compensating the tool wear, in
order to maintain high dimensional accuracy. In ELID grinding, electrolytic dressing
condition is another significant factor to get successful results. In most of the earlier
studies, the dressing conditions were mostly set manually and the process greatly relied

on the experience of the operators. Open loop control of the ELID power supply, which is
currently used, could not give the best results always. So, an intelligent electrolysis
control technique using in-process sensor feedback is needed.

1.2 Main objectives
This research mainly focuses to develop a smart technology that will help to improve
ELID grinding by addressing the above mentioned difficulties currently faced in this area
of ultra precision grinding. In order to accomplish this main objective several sub
objectives have been set to be achieved.

1. The first goal is to build an intelligent machine tool for ELID grinding. In
order to attain this; an existing wire cut EDM machine structure shall be
reinforced with new control and sensory system to enhance the performance
of ELID grinding.
2. Open loop control of ELID power may not be the best solution as described
in the previous section hence implementation of dressing on demand concept
for the ELID power control and comparison of this new idea with the
conventional ELID grinding is another objective for this study.
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3. Grinding wheel truing is the prerequisite to maintain uniform cutting. One of
the prime objectives in this research is to develop an in-process truing method
by introducing segmented dressing of the grinding wheel. The performance of
this proposed wheel truing concept shall also be studied.
4. In ELID grinding wheel wear is very significant which introduce form
inaccuracy for curved surface machining. Therefore a wheel wear
compensation method shall be implemented for spherical surface grinding.

1.3 Organization of this thesis
This thesis comprises of six chapters. Chapter 1 gives a brief overview of the new
concept of ELID grinding and its history. The advantages of ELID grinding over other in-
process dressing methods of grinding wheel are described in this chapter. Finally the
objectives of this study are summarized. This chapter also outlines the organization of
this dissertation.

A comprehensive literature review is given in the Chapter 2 which includes three parts.
First section describes different other in-process dressing and truing techniques for
grinding. In the second part a detailed discussion on the current research trend in ELID
grinding is given. The review of different approaches to monitor grinding wheel wear is
next presented. Finally some concluding remarks on the literature review are made.

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Chapter 3 describes the development of a prototype machine-tool for ELID grinding.
Detailed discussions on the experiments and results for evaluating the performance of the
machine are also included in this chapter. This chapter also explains in details the
concept of tool wear compensation in ELID grinding for spherical surface machining.
The implementation of this important idea on the developed ELID grinding machine is
discussed in this chapter as well. Experiments to evaluate the performance of the tool
wear compensation method and their results are explained here.

Chapter 4 describes implementation and performance evaluation of dressing on demand
concept for ELID grinding. This technique aims to optimize the dressing power by
monitoring the grinding condition and thus to save the grinding wheel from excessive
dressing. The theoretical background as well as the experimental study on the comparison
between this novel concept and conventional ELID grinding is discussed elaborately in

this chapter.

Chapter 5 presents the theoretical framework of closed loop controlled in-process truing
for ELID grinding. In-process truing of the grinding wheel can be achieved by controlling
pulse duty ratio of the ELID power supply. The algorithm for practical implementation
of this concept is also included in the chapter. This chapter also describes the
experimental approach to assess the performance of the proposed system. Finally Chapter
6 concludes this dissertation with the summary of contributions. The suggestions for the
future research are also recommended in this chapter.