OPTIMISATION OF TOOL WEAR IN END MILLINGDEPARTMENT OF MECHANICAL ENGINEERING ,CMRCET Page 1 CONTENTS Chapter name page no Chapter-1 ...................... ......................... ......................................................... 1.1 Introduction.......... .................................................................................... 1.2 Types of milling ma chines....................................................................... 1.3Types of milling cutters.............................. ............................................. 1.4End milling cutter..................................................................................... 1.5Adjustable cutting factors in milling...................................................... 1.6Tool geometry of mill ing cutters.......................................... .................... Chapter-2 ...................... ......................... ......................................................... 2. Literature review....................... ................................................................ Chapter-3 ...................... ......................... .......................................................... 3.1 Classificati on of tool materials ................................................................. 3.2 Types of tool failure................................................................................... 3.3 Basic wear mechanisms........................ ...................................................... 3.4 Factors involved in tool li fe....................................................................... 3.5 Tool deformations...................... ............................................................... Chapter-4 ...................... ......................... ........................................................... 4 .1 Introduction to dampers.................... ............................................................ 4.1.1 Use cutt ers with few inserts........................ .............................................. 4.1.2 Optimiz e inserts geometry............. ............................................................
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process to add or refine features on parts that were manufactured using a different process.
Due to the high tolerances and surface finishes that milling can offer, it is ideal for adding
precision features to a part whose basic shape has already been formed.
1.1 Types of Milling Machine
1.1.1 Vertical milling machine
Fig 1.1Vertical milling machine
This study guide will cover the major working parts, functions, and machining
techniques that can be found used on most vertical milling machines. This study guide has
been designed to directly represent the questions that will be found on the open book written
assessment and as an aid for the hands-on usability assessment. Both assessments will also
include questions related to standard machine shop safety and APS internal user safety
guidelines. Answering the questions found at the end of the study guide will enable the user to successfully pass the hands-on usability and open book written assessments. Study guide
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Fig 1.4 Tool geometry of end mill cutter
The geometry of milling cutters includes four angles such as radial rake angle, angle,
radial relief angle and axial relief angle. Generally, these angles are considered for three typesof milling cutters like face mills, end mills, side and slot mills.
When angles of milling cutter are compared with the angles of single pont
tool, axial rake angle of milling cutter becomes similar to back rake angle of single point tool
whereas, radial rake angle of milling cutter becomes similar to side rake angle of single point
tool.
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Radial rake angle:
The angle measured between the slide face and the radial plane passing through the
cutter axis is reffered as radial rake angle. Radial rake angle can be positive or negative.
Angle makes the cutting edge more stronger.
Axial rake angle:
Axial rake angle is the cutting edge inclination with respect to cutter axis. It also gives
the direction of chips flow. Axial rake angle can positive or negative.
Positive axial rake angle removes the chips away from the cut when rake nose of
cutter contacts with the workpiece while negative axial rake angle traverse the chips along thedirection of work piece. It also makes the cutting edge morestronger
Mostly negative axial rake angle is applied in carbide cutters.
Approach angle:
The angle measured between the plane normal to axial cutter and the plane tangent to
the surface of revolution of thecutting edge is reffered as approach angle.
The value of approach angle is different for different types of milling cutters.
Side clearance angle:
The angle measured between the cut surface and the clearance flank on the cutter is
reffered as side clearance angle. The cutting edges becomes weak at higher clearance, but less
wear and tear occurs. Its value rely on the end mill diameter.
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Like all cast materials these alloys are relatively weak in tension and hence tend to shatter
when subjected to a shock load or if not properly supported.
3.1.5 Cemented carbides:
Carbides can be classified into two types:
1. The C grade (straight carbides) consisting of tungsten carbide with cobalt as a
binder for use in machining cast iron and nonferrous metals.
2. The S grade (steel cutting carbides) consisting of tungsten, titanium and
tanlabim carbides with a cobalt binder for using machining steels.
Cemented carbides are unusual in several respects:
1. They have high hardness over a wide range of temperatures.
2. They are very stiff (young’s modulus is nearly three times that for steel).
3. They exhibit no plastic flow (yield point) even to stresses as high as
Psi.
4. They have low thermal expansion compared with steel.
5. They have relatively high thermal conductivity.
6. They have strong tendency to form pressure welds at low cutting speeds.
3.1.6 Diamond tools:
Diamond tipped tools are sometimes used for special applications such as production
of surfaces of high finish on soft materials that are normally difficult to machine.
The general properties of diamond may be summarized as follows:
1. Hardest known substance (brinell hardness=7000).
2. Lowest thermal expansion of any pure substances (about 12% that for steel).
3. High heat conductivity (twice that for steel).
4. Poor electrical conductor.
5. Burns to when heated to about 1500 in air.
6. Very low coefficient of friction against metals.
Since very high hardness is always accompanied by brittleness, a diamond tool must be cautiously used to avoid rupturing the point . This usually limits the use of diamond
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3.3 Basic wear mechanisms:
Several mechanisms of tool wear have been proposed. Under certain conditions all
these mechanisms may act simultaneously as indicated in fig.2-3, after
A brief description of the various mechanisms is given below:
3.3.1 Adhesion wear mechanism:
When surfaces rub together, particularly in the absence of lubricant films, some
adhesion occurs at the rubbing contact. The friction is primarily the force required to shear
the junction so formed. The simple mechanism of friction and wear proposed by Bowden and
Is based on the concept of the formation of welded junctions and the subsequent
destruction of these. When the destruction is by sharing below the interface, a wear particle is
transferred. The plucked fragments may initially be attached to one surface but may
subsequently be back transferred onto the other. However, in machining operations this
process is probably of very minor importance since fragments plucked either from the tool or
rapidly carried away from the rubbing region.
For this reason, adhesive wear in machining operations is a relatively straightforward
concept. The tool is invariablely chosen to be harder than work. If a junction is formed at the
metal/work interface it will generally pluck out a fragment from the work. The process of
plucking-out will have the fragment in a very work-hardened condition and it may well be
hard enough to score or groove the work. The accumulation of the transferred material from
the work to tip of the tool is, of course, the origin of the built-up-edge. This nose act as an
extension of the tool, and to some extent protects the tool from water. However, the built-up-
edge may occasionally break away with a small portion of the tool itself. This is particularly
likely if the tool is heterogeneous in structure so that local regions may be appreciablyweaker in tension or shear than the overall strength. Adhesive wear of the tool is therefore
likely to be most marked if the tool is of non-uniform strength
Clearly the best way of minimizing adhesive wear is by reducing the amount of
adhesion. The commonest method is by using a lubricant. However, it is still not clear
whether the lubricant acts mainly as a coolant or as a means of reducing friction and
adhesion. If it acts as a true lubricant it is highly desirable to know how the lubricant gets into
the work/tool interface and how quickly it can interact to be effective.
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Fig 3.1 Factors influencing the tool wear
Although the shapes of metal-cutting tools used in turning, milling, drilling, etc., varywidely, The basic form is that of a wedge forced asymmetrically into the work material. It is
now accepted that in general the work piece material is deformed as indicated in fig.2-4
The secondary deformation zone is caused by the total contact length between chip
and tool. This form is dictated by the objective of the operation, which is to remove a thin
layer from a more rigid body. The layer moved in the form of fragment or a continuous bears
on the rake face of the tool and passes over it, while the more rigid body of the work material
bears against the passes over the flank or clearance face of the tool. To avoid excessive
friction between the tools and work piece a clearance angle (which may be from about 1 to
20 ) on the flank of the tool ensures that the work surface is in contact with only a narrow
band very close to the tool edge. Because of the rigidity of the work this normally remains
narrow until a new surface, more or less parallel to the work surface, is formed by wear. Such
a worn surface is called the flank wear or land is the most typical form of tool wear.
The layer removed from the work surface (the swarf or chip), being thinner and more
flexible, can conform more readily to the tool shape and normally makes contact with the
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rake face of the tool along a considerably longer path, a distance several times the thickness
of the under formed chip. Wear also takes place on the rake face of the tool although not so
universally as on the flank.
Fig 3.2 Tool wear
3.4.1 Flank wear:
This often takes the form of an even band of wear (fig.1-2), the width of which can be
measured with reasonable accuracy. Wear-land formation is not always uniform along the
side and end cutting edges of the tool. Often localized wear at one or more positions along the
edge is several times greater than the average. Two positions at which accelerated wear commonly occurs are where the work surface intersects the cutting edge of the tool and near
the nose of the tool. At the former position the surface condition of the work and the
atmosphere may influence the wear process.
Flank wear occurs under almost all conditions of cutting, but metallographic evidence
shows that more than one wear process is involved so that simple laws relating the rate of
wear to variables such as speed, feed, tool geometry, etc., can be expected only under
conditions where the wear process remains substantially unaltered. Cutting tools are generally
used most efficiently when the only form of wear is an even land on the tool flank, but factors
other than flank wear influence the life of carbide tools in practice.
The surface finish produced in a machining operation usually deteriorates as the
flank-land wear increases although there are circumstances in which a wear land may burnish
the work piece and produce a good finish. Cutting forces are normally increased by flank
wear of the tool. Flank wear also influence the plan geometry of o tool. This may affect the
workpiece
tool
crater wear
flank wear
chip
workpiece
tool
crater wear
flank wear
chip
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Fig 3.4 crater wear
On the rake face a cavity or crater frequently forms a short distance from the cuttingedge, as shown in fig.1-2. Once the crater is established, its depth KT grows more rapidly
than its top width KB. The edge of the crater approaches the cutting edge, both by wear of the
crater and by clearance-face wear. This weakens the tool close to the cutting edge and a
major failure may occur by fracture from the crater through to the clearance face. This is
more likely under discontinuous cutting conditions. Cutting forces are normally increased
by wear of the tool. Crater wear may, however, under certain circumstances, reduce forces by
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Chapter-4
Introduction to dampers
4.1 Introduction to dampers:
A mechanical damper has been introduced to reduce tool vibration during the high-
speed milling process. The mechanical damper is composed of multi-fingered cylindrical
inserts placed in a matching cylindrical hole in the center of a standard end-milling cutter.
Centrifugal forces during high-speed rotation press the flexible fingers against the inner
surface of the tool. Bending of the tool/damper assembly due to cutting forces or chatter
vibration causes relative axial sliding between the tool inner surface and the damper fingers,
and dissipates energy in the form of friction work.
Damper consists of a multi-fingered cylindrical insert placed inside a matching
axial hole along the center line of the milling cutter. During high speed rotation, centrifugal
forces press the outer surface of the insert fingers against the inner surface of the tool. During
lateral vibrations of the tool, relative sliding occurs at the interface between the damper and
tool inner surface, and the resulting frictional work in the contact interface dissipates energyand reduces vibration amplitude. They developed a simplified analytical model for the multi
fingered cylindrical damper and performed experiments. In this paper, non-linear finite
element analysis with frictional contact is used to study the mechanical damper, and calculate
the amount of friction work during lateral bending of the tool. Although chatter vibration is a
dynamic
Phenomenon, the amount of damping in the proposed system is directly
dependent on the energy dissipated during lateral vibrations. If we assume that the contact
pressure between the damping elements inside the tool is primarily due to centrifugal forces,
i.e. The bending stiffness of the damping elements is small, then static finite element analysis
is sufficient to predict frictional work during static bending.
4.1.1 Use cutters with fewer inserts:
Although it may seem counterintuitive, the first step to reducing chatter in milling
operations is to switch to a cutter with fewer teeth. In general, the coarser the cutter pitch, the
lesser the chance of harmonic vibration. Sometimes, replacing a 16-tooth cutter with a 12-
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tooth tool ends chatter altogether. A differential-pitch cutter may be required in more difficult
cases to eliminate troublesome harmonics.
The larger the cutter, the better the performance will be. Conditions permitting,
larger cutters provide more choices about how to approach the work piece. Varying the
relative position often helps damp vibration. Manufacturing engineers should try to keep the
cutter diameter 20 to 50 percent larger than the width of the cut. The cutter should be sized so
that no more than two-thirds of the inserts are engaged in the cut at any time. These
guidelines help produce an ideal entry angle, thereby reducing cutting forces and vibration.
4.1.2 Optimize insert geometry:
The shape of the cutting inserts often determines their vibration tendency. Round
inserts are most vibration prone, while those with 45-degree lead angles are the least prone to
chatter. The smaller the entry angles of the cutting edge to the work, the lower the tendency
to vibrate.
Cutting tool specifies can reduce overall cutting force and resulting vibration by using
positive rake insert geometry. The shearing action of positive rake cutters reduces cutting
pressure by more than 20 percent versus zero- or negative-rake milling tools. The sharper
edge and angle of entry of this type of insert also helps to reduce the power needed to
penetrate the surface of the work piece.
4.1.3 Choose inserts coatings carefully:
Coatings on inserts perform many functions, but their primary jobs are protecting
against heat, maintaining lubricity and preventing build-up on the insert. To reduce edge
rounding and chatter, you should look to replace inserts protected by thick CVD coatings
with those wearing thinner PVD coatings. Though CVD treatments are formulated for wear
resistance, PVD coatings provide a sharper insert edge and a more positive rake angle to help
minimize vibration.
4.2 End mill cutter damper geometry
The cutting tool (end mill) used in conventional end mills have a solid
cylindrical cross section, the proposed mechanical damper requires an axial hole along the
tool center line . When the multi-fingered cylindrical damper is inserted into the hollow tool,centrifugal forces from the high-speed spindle rotation cause high contact pressures between
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the damper fingers and the inner surface of the tool. When lateral bending of the system
occurs, it causes a Relative sliding motion between the damper and the tool due to their
differences in neutral axis locations. This relative motion in conjunction with the contact
pressure causes a friction stress at the interface, which dissipates the vibration energy. In this
paper, this damping mechanism will be referred to as a mechanical damper. While the
geometry of the cutting edges of the tool is very important for cutting performance, it does
not affect damper performance. Therefore, the tool can be simplified as End mill . Geometry
of end mill and four-fingered mechanical damper.
The simplified ‘damper’ is also modelled as a hollow cylinder, slit along its
length to form individual ‘fingers’. The inner diameter of the tool shank is set to 9.525 mm
and it cannot be made larger because enough material must be left on the shank to allow
cutting teeth to be formed. Thus, although a larger inner diameter of the tool might provide
better damper performance; this is not considered as a design variable since these dimensions
could not be used to produce the actual cutting tool. The damper has an outer diameter of
9.525 mm. The inner diameter of the damper can be changed to maximize the frictional
energy dissipation. The number of fingers can also be altered to improve damping
performance. The parameter study detailed in Section 4 will examine the effect of varying the
number of fingers as well as the damper inner diameter. Because a damper with one finger will not work in the manner described above, this case will not be considered.
.
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tilted for angular cuts in any vertical position. It is not so rigid and hence used only for light
and precision work.
Fig 5.9 Plain vise
5.6 Measuring instrument used:
5.6.1 Tool makers microscope
A tool maker microscope is a type of a multi functional device that is primarily used
for measuring tools and apparatus. These microscopes are widely used and commonly seen
inside machine and tools manufacturing industries and factories. These microscopes are also
inside electronics production houses and in aeronautic parts factories. A tool maker
microscope is an indispensable tool in the different measurement tasks performed throughout
the engineering industry.
The main use of a tool maker microscope is to measure the shape, size, angle, and the
position of the small components that falls under the microscope’s measuring range. Moreoften than not, a tool maker microscope is outfitted with a CCD camera that has the ability to
capture, collect, and store images into specialized computer software. Certain computer aided
design software is commonly used for such applications. The image produced by the camera
and processed by the software is normally a two dimensional image.
But what makes a tool maker microscope fully functional are its glass grading and
optics system. Since what are being viewed under these microscopes are metals and precision
instruments, it is important that the objectives and the eye piece lenses are made of fine
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quality glasses only. These essential parts are what makes the device very durable and gives it
the ability to withstand the wear and tear associated with the everyday stress of factory usage.
And much because of this, it is also important that the body, structure, and
mechanisms of a tool maker microscope are created with highly durable materials, most
preferably good quality metals. Because the conditions inside an industrial laboratory are not
as good as a home or office laboratory setup, the microscope’s body should be capable of low
heat production. It should also be able to resist corrosion, oscillation, and pollution – because
all of these elements are present inside an industrial laboratories and production plants.
There are tool maker microscopes that are equipped with a cross hair reticle on the
eye piece, coupled with a protractor on the tube. These are good instruments used toaccurately measure the distance or the diameter of the tool under observation. The
microscope’s stage is also built with a millimeter measuring system that also allows for the
measurement of the specimen. The stage when moved, produce the distance traveled with
which the microscope effectively measures.
Right now, quality tool maker microscopes are using semiconductor laser devices as
directors. Instead of the cross hairs, a red point is virtually marked on the microscope’s
working surface in order to locate the parts that have to be measured by the microscope. The
CCD imaging system can also be used as a measurement system as well. This is another
advanced feature of the newer versions of a tool maker microscope models. A CCD camera
that has the ability to measure diameters and distances is a lot more convenient to use,
especially to beginners.
But aside from all of these, a tool maker microscope should also have a good
illumination system. It is the lights that allows for the superior viewing of tools andspecimens. The higher the luminance value of the light provided by the microscope, the better
its performance is. If necessary, an incandescent lamp should not be used for these
applications. The light that is ideal is the one that produces a nice level of brightness with less
heat. Lamps have life spans too. And because most of a tool maker microscope uses a built-in
lighting system, the light to be used should last for an extended period of time, if and when
possible.
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occurring profiles. E.g. Threads or rounding – standard line pattern for comparison with the
shadow image of the text object is projected to a ground glass screen. The text object is
shifted or turned on the measuring in addition to the comparison of shapes. The addition to
this method (shadow image method), measuring operations are also possible by use of the
axial reaction method, which can be recommended especially for thread measuring. This
involves approached measuring knife edges and measurement in axial section of thread
according to definition. This method permits higher precision than shadow image method for
special measuring operations.
5.6.2 Stop watch:
Fig 5.11 stop watch
A stopwatch is a handheld timer used in sporting events such as track meets, swim
meets, triathlons and other time-lapse games. It is designed to be manually started, for example at the beginning of an event, then stopped with the press of a button at the exact
moment a runner crosses a finish line, a swimmer reaches the end lap or a skier sails past the
final gate. Aside from officials, coaches use stopwatches for training and practices.
A stopwatch can be mechanical, resembling a pocket watch with an analog face, or
digital. Digital stopwatches are more accurate, though some people prefer the traditional look
of a silver-cased analog stopwatch. Winning team members or students often choose this type
of stopwatch to engrave as a gift to a coach or trainer.
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Chapter-6
Introduction to ANOVA analysis and Taguchi design
6.1 Definition of 'Analysis Of Variance - ANOVA'
A statistical analysis tool that separates the total variability found within a data set
into two components: random and systematic factors. The random factors do not have any
statistical influence on the given data set, while the systematic factors do. The ANOVA test is
used to determine the impact independent variables have on the dependent variable in a
sregression analysis.
6.2 Analysis of variance (ANOVA):Purpose:
The reason for doing an ANOVA is to see if there is any difference between groups
on some variable. For example, you might have data on student performance in non-assessed
tutorial exercises as well as their final grading. You are interested in seeing if tutorial
performance is related to final grade. Anova allows you to break up the group according to
the grade and then see if performance is different across these grades. Anova is available for
both parametric (score data) and non-parametric (ranking/ordering) data.
6.3 Types of anova:
One-way between groups:
The example given above is called a one-way between groups model. You are looking
at the differences between the groups. There is only one grouping (final grade) which you areusing to define the groups. This is the simplest version of anova. This type of anova can also
be used to compare variables between different groups - tutorial performance from different
intakes.
One-way repeated measures:
A one way repeated measures anova is used when you have a single group on which
you have measured something a few times. For example, you may have a test of
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Performing a series of experiments each of which gives some understanding. This
requires making measurements after every experiment so that analysis of observed data will
allow him to decide what to do next - "which parameters should be varied and by how much".
Many a times such series does not progress much as negative results may discourage or will
not allow a selection of parameters which ought to be changed in the next experiment.
Therefore, such experimentation usually ends well before the number of experiment reach a
double digit! The data is insufficient to draw any significant conclusions and the main
problem (of understanding the science) still remains unsolved.
2. Design of experiments:
A well planned set of experiments, in which all parameters of interest are varied over
a specified range, is a much better approach to obtain systematic data. Mathematically
speaking, such a complete set of experiments ought to give desired results. Usually the
number of experiments and resources (materials and time) required are prohibitively large.
Often the experimenter decides to perform a subset of the complete set of experiments to save
on time and money! However, it does not easily lend itself to understanding of science behind
the phenomenon. The analysis is not very easy (though it may be easy for themathematician/statistician) and thus effects of various parameters on the observed data are
not readily apparent. In many cases, particularly those in which some optimization is
required, the method does not point to the best settings of parameters. A classic example
illustrating the drawback of design of experiments is found in the planning of a world cup
event, say football. While all matches are well arranged with respect to the different teams
and different venues on differ rent dates and yet the planning does not care about the result of
any match (win or lose)!!!! Obviously, such a strategy is not desirable for conducting
scientific experiments
6.6 Taguchi method
Dr. Taguchi of Nippon telephones and telegraph company, Japan has developed a
method based on “orthogonal array" experiments which gives much reduced " variance " for
the experiment with " optimum settings " of control parameters. Thus the marriage of design
of experiments with optimization of control parameters to obtain best results is achieved in
the Taguchi method. "orthogonal arrays" (oa) provide a set of well balanced (minimum)
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Step-5: select the orthogonal array matrix experiment
Step-6: conduct the matrix experiment
Step-7: analyze the data predict the optimum levels and performance
Step-8: perform the verification experiment an plan the future action
A technique for designing and performing experiments to investigate processes where
the output depends on many factors (variables; inputs) without having to tediously and
uneconomically run the process using all possible combinations of values of those variables.
By systematically choosing certain combinations of variables it is possible to separate their
individual effects.
A special variant of design of experiments (doe) that distinguishes itself from classic
doe in the focus on optimizing design parameters to minimize variation before optimizing
design to hit mean target values for output parameters.
6.11 Signal to Noise (S/N) Ratios:
The product/process/system design phase involves deciding the best values/levels for the control factors. The signal to noise (S/N) ratio is an ideal metric for that purpose.
The equation for average quality loss, Q, says that the customer’s average quality loss
depends on the deviation of the mean from the target and also on the variance. An important
class of design optimization problem requires minimization of the variance while keeping the
mean on target.
Between the mean and standard deviation, it is typically easy to adjust the mean on
target, but reducing the variance is difficult. Therefore, the designer should minimize the
variance first and then adjust the mean on target.Among the available control factors most of
them should be used to reduce variance. Only one or two control factors are adequate for
adjusting the mean on target.
The design optimization problem can be solved in two steps:
1. Maximize the S/N ratio, h, defined as
H = 10 log10 ( h2~ / sigma2 )
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The experiments were carried out on vertical milling machine. Each experiment was repeated
using a new cutting edge every time to obtain accurate reading of tool wear. The physical and
mechanical properties of work piece are 50mm in length, 50mm in width, 10mm in depth.
The work piece material is Aluminum. The end milling cutter is of high speed steel (HSS).
7.3 Experimental design
7.3.1. Orthogonal array and experimental factors
Following the procedure described in fig. 1, the first step in the Taguchi method is to select a
proper orthogonal array. The standardized Taguchi-based experimental design, a l18 (3^4)
orthogonal array was used in this study and is shown in table 1. This basic design makes use
of up to four control factors, with three levels each. A total of nine experimental runs must be
conducted, using the combination of levels for each control factor as indicated in table 2. The
control factors are the basic controlled parameters used in a milling operation. The spindle
speeds and type of tools were selected from within the range of parameters for milling of
Aluminum. The feed and depth of cut used for milling Aluminum work pieces are constant.
7.3.2. Experimental set-up and procedure
After the orthogonal array has been selected, the second step in Taguchi parameter design(see fig.1) is running the experiment. This experiment was conducted using the hardware
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With various cutting tools, speeds, feeds and depth of cuts shown in above table .then tool wear is
measured using is measured on tool makers microscope and values are given in table7.1.
In the Taguchi method, the term ‘signal’ represents the desirable value (mean) for the
output characteristic and the term ‘noise’ represents the undesirable value for the output characteristic.Taguchi uses the s/n ratio to measure the quality characteristic deviating from the desired value. There
are several s/n ratios available depending on type of characteristic: lower is better (lb), nominal is best
(nb), or higher is better (hb) .smaller is better s/n ratio was used in this study because less tool wear
was desirable.
Quality characteristic of the smaller is better is calculated in the following equation
Experiments are conducted in the order given by Taguchi method and tool wear values are measured
and tabulated Table 7.5
Trail
no type of tool
Speed in
rpm
Feed in
mm/min
Depth of
cut in mm
Tool wear in
mm
1 Solid end mill 385 18 0.25
2 Solid end mill 685 29 0.35
3 Solid end mill 960 41 0.5
4 Hollow with one damper 385 18 0.35
5 Hollow with one damper 685 29 0.5
6 Hollow with one damper 960 41 0.25
7 Hollow with two damper 385 29 0.25
8 Hollow with two damper 685 41 0.35
9 Hollow with two damper 960 18 0.5
10 Hollow with three damper 385 41 0.5
11 Hollow with three damper 685 18 0.25
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