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Transcript of Surface Roughness Optimization using Taguchi and Anova method.
Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
CHAPTER 1
MILLING
1.1 INTRODUCTION
1.1.1 Milling Machines
Milling machines were first invented and developed by Eli Whitney to mass produce
interchangeable musket parts. Although crude, these machines assisted man in
maintaining accuracy and uniformity while duplicating parts that could not be
manufactured with the use of a file. Development and improvements of the milling
machine and components continued, which resulted in the manufacturing of heavier
arbors and high speed steel and carbide cutters. These components allowed the operator to
remove metal faster, and with more accuracy, than previous machines. Variations of
milling machines were also developed to perform special milling operations. During this
era, computerized machines have been developed to alleviate errors and provide better
quality in the finished product.
Milling-Milling is the process of cutting away material by feeding a workpiece past a
rotating multiple tooth cutter. The cutting action of the many teeth around the milling
cutter provides a fast method of machining. The machined surface may be flat,angular, or
curved. The surface may also be milled to any combination of shapes. The machine for
holding the workpiece, rotating the cutter, and feeding it is known as the Milling machine.
The type of milling machine most commonly found in student shops is a vertical spindle
machine with a swiveling head. The spindle can be fed up and down with a quill feed lever
on the head. Most milling machines are equipped with power feed for one or more axes.
Power feed is smoother than manual feed and, therefore, can produce a better surface
finish. Power feed also reduces operator fatigue on long cuts.
The Machine Tool – In the present climate many different configurations of machine tool
exist .Some machines have the table/work piece stationary whilst the X,Y and Z axes move
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and others may be constructed to allow the work piece/table to be the moving part whilst
the axes are fixed.
In any condition the X, Y and Z-axes directions are always configured the same.
Fig: 1.1 MACHINE TOOL
The X-axis is always considered as the longest axis,where X+ will be the table motioning
to the left and X- to the right. The Y-axis moves from front to back of the machine with the
table motioning towards the operator as the Y+(positive) direction and away being the Y-
(negative) direction. The Z-axis where the tool normally is located,has the positive Z+
(positive) axis motioning up and away from the workpiece/table and Z-(negative)direction
down towards the workpiece/table.
1.2 CLASSIFICATION OF MILLING
Peripheral Milling: In peripheral (or slab) milling, the milled surface is generated by teeth
located on the periphery of the cutter body. The axis of cutter rotation is generally in a
plane parallel to the work piece surface to be machined.
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Fig:1.2 Peripheral Milling
Face Milling: In face milling, the cutter is mounted on a spindle having an axis of rotation
perpendicular to the work piece surface. The milled surface results from the action of
cutting edges located on the periphery and face of the cutter.
Fig:1.3 Face Milling
End Milling: The cutter in end milling generally rotates on an axis vertical to the work
piece. It can be tilted to machine tapered surfaces. Cutting teeth are located on both the end
face of the cutter and the periphery of the cutter body.
Fig: 1.4 End Milling
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1.2.1 METHODS OF MILLING
Up Milling: Up milling is also referred to as conventional milling. The direction of the
cutter rotation opposes the feed motion. For example, if the cutter rotates clockwise , the
workpiece is fed to the right in up milling.
Fig:1.5 Up milling
Down Milling:Down milling is also referred to as climb milling. The direction of cutter
rotation is same as the feed motion. For example, if the cutter rotates counterclockwise ,
the workpiece is fed to the right in down milling.
Fig: 1.6 Down Milling
The chip formation in down milling is opposite to the chip formation in up milling. The
figure for down milling shows that the cutter tooth is almost parallel to the top surface of
the workpiece. The cutter tooth begins to mill the full chip thickness. Then the chip
thickness gradually decreases.
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Other milling operations are shown in the figure.
Fig:1.7 Types of Milling Operations
1.3 WORKING PRINCIPLES OF MILLING MACHINE
The workpiece is holding on the worktable of the machine. The table movement controls
the feed of workpiece against the rotating cutter. The cutter is mounted on a spindle or
arbor and revolves at high speed. Except for rotation of the cutter has no other motion. As
the workpiece advances, the cutter teeth remove the metal from the surface of workpiece
and the desired shape is produced.
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Fig: 1.8 Working Principle of Milling Machine
1.3.1 Principle parts of a milling machine
Milling machines can be found in a variety of sizes and designs, yet they still possess the
same main components that enable the work piece to be moved in three directions relative
to the tool. These components include the following:
Base and column - The base of a milling machine is simply the platform that sits on the
ground and supports the machine. A large column is attached to the base and connects to
the other components.
Table - The work piece that will be milled is mounted onto a platform called the table,
which typically has "T" shaped slots along its surface. The work piece may be secured in a
fixture called a vice, which is secured into the T-slots, or the work piece can be clamped
directly into these slots. The table provides the horizontal motion of the work piece in the
X-direction by sliding along a platform beneath it, called the saddle.
Saddle - The saddle is the platform that supports the table and allows its longitudinal
motion. The saddle is also able to move and provides the horizontal motion of the work
piece in the Y-direction by sliding transversely along another platform called the knee.
Knee - The knee is the platform that supports the saddle and the table. In most milling
machines, sometimes called column and knee milling machines, the knee provides the
vertical motion (Z direction) of the work piece. The knee can move vertically along the
column, thus moving the work piece vertically while the cutter remains stationary above it.
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However, in a fixed bed machine, the knee is fixed while the cutter moves vertically in
order to cut the work piece.
Arbor - It holds rotating milling cutters rigidly and mounted on the spindle. Sometimes
arbor is supported at maximum distance from support of overhanging arm like a cantilever,
it is called stub arbor. Locking provisions are provided in the arbor assembly to ensure its
reliability.
Fig:1.9 Vertical milling machines
1.3.2 Manual vertical milling machine
The above components of the milling machine can be oriented either vertically or
horizontally, creating two very distinct forms of milling machine. A horizontal milling
machine uses a cutter that is mounted on a horizontal shaft, called an arbor, above the work
piece. For this reason, horizontal milling is sometimes referred to as arbor milling. The
arbor is supported on one side by an over arm, which is connected to the column, and on
the other side by the spindle. The spindle is driven by a motor and therefore rotates the
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arbor. During milling, the cutter rotates along a horizontal axis and the side of the cutter
removes material from the work piece.
A vertical milling machine, on the other hand, orients the cutter vertically. The cutter is
secured inside a piece called a collet, which is then attached to the vertically oriented
spindle. The spindle is located inside the milling head, which is attached to the column.
Milling machines can also be classified by the type of control that is used. A manual
milling machine requires the operator to control the motion of the cutter during the milling
operation. The operator adjusts the position of the cutter by using hand cranks that move
the table, saddle, and knee.
Milling machines are also able to be computer controlled, in which case they are referred
to as a computer numerical control (CNC) milling machine. CNC milling machines move
the work piece and cutter based on commands that are preprogrammed and offer very high
precision. The programs that are written are often called G-codes or NC-codes. Many CNC
milling machines also contain another axis of motion besides the standard X-Y-Z motion.
The angle of the spindle and cutter can be changed, allowing for even more complex
shapes to be milled.
The tooling that is required for milling is a sharp cutter that will be rotated by the spindle.
The cutter is a cylindrical tool with sharp teeth spaced around the exterior. The spaces
between the teeth are called flutes and allow the material chips to move away from
the work piece.
The teeth may be straight along the side of the cutter, but are more commonly arranged in
a helix. The helix angle reduces the load on the teeth by distributing the forces. Also, the
number of teeth on a cutter varies. A larger number of teeth will provide a better surface
finish. The cutters that can be used for milling operations are highly diverse, thus allowing
for the formation of a variety of features. While these cutters differ greatly in diameter,
length, and by the shape of the cut they will form, they also differ based upon their
orientation, whether they will be used horizontally or vertically.
A cutter that will be used in a horizontal milling machine will have the teeth extend along
the entire length of the tool. The interior of the tool will be hollow so that it can be
mounted onto the arbor.
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Tool materials in common use
High Carbon Steel: Contains 1 - 1.4% carbon with some addition of chromium
and tungsten to improve wear resistance. The steel begins to lose its hardness at
about 250° C, and is not favored for modern machining operations where high
speeds and heavy cuts are usually employed.
High Speed Steel (H.S.S.): Steel, which has a hot hardness value of about 600°C,
possesses good strength and shock resistant properties. It is commonly used for
single point lathe cutting tools and multi point cutting tools such as drills, reamers
and milling cutters.
Cemented Carbides: An extremely hard material made from tungsten powder.
Carbide tools are usually used in the form of brazed or clamped tips. High cutting
speeds may be used and materials difficult to cut with HSS may be readily
machined using carbide tipped tool.
1.4 Cutting parameters
As you proceed to the process of metal cutting, the relative ‘speed’ of work piece rotation
and ‘feed’ rates of the cutting tool coupled to the material to be cut must be given your
serious attention. This relationship is of paramount importance if items are to be
manufactured in a cost-effective way in the minimum time, in accordance with the laid
down specifications for quality of surface finish and accuracy. You, as a potential
supervisory /management level engineer, must take particular note of these important
parameters and ensure that you gain a fundamental understanding of factors involved.
Cutting Speed
All materials have an optimum Cutting Speed and it is defined as the speed at which a
point on the surface of the work passes the cutting edge or point of the tool and is normally
given in meters/min. To calculate the spindle Speed required,
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Where:
N = Spindle Speed (RPM)
CS = Cutting Speed (m/min)
d = Diameter of Work piece (mm)
Cutting feed: The distance that the cutting tool or work piece advances during one
revolution of the spindle and tool, measured in inches per revolution (IPR). In some
operations the tool feeds into the work piece and in others the work piece feeds into the
tool. For a multi-point tool, the cutting feed is also equal to the feed per tooth, measured in
inches per tooth (IPT), and multiplied by the number of teeth on the cutting tool.
Spindle speed: The rotational speed of the spindle and tool in revolutions per minute
(RPM). The spindle speed is equal to the cutting speed divided by the circumference of the
tool.
Feed rate: The speed of the cutting tool's movement relative to the work piece as the tool
makes a cut. The feed rate is measured in inches per minute (IPM) and is the product of the
cutting feed (IPR) and the spindle speed (RPM).
Axial depth of cut: The depth of the tool along its axis in the work piece as it makes a cut.
A large axial depth of cut will require a low feed rate, or else it will result in a high load on
the tool and reduce the tool life. Therefore, a feature is typically machined in several
passes as the tool moves to the specified axial depth of cut for each pass.
Radial depth of cut: The depth of the tool along its radius in the work piece as it makes a
cut. If the radial depth of cut is less than the tool radius, the tool is only partially engaged
and is making a peripheral cut. If the radial depth of cut is equal to the tool diameter, the
cutting tool is fully engaged and is making a slot cut. A large radial depth of cut will
require a low feed rate, or else it will result in a high load on the tool and reduce the tool
life. Therefore, a feature is often machined in several steps as the tool moves over the step-
over distance, and makes another cut at the radial depth of cut.
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1.5 Milling cutters
Milling cutters are cutting tools typically used in milling machines or machining centres
to perform milling operations. Special milling cutters are designed to perform special
operations which may be combination of several conventional operations. Standard
milling cutters are the conventional cutters which are classified as given below.
Plain Milling Cutters: These cutters are cylindrical in shape having teeth on their
circumference. These are used to produce flat surfaces parallel to axis of rotation. Plain
milling cutter is shown in Figure 1.5. Depending upon the size and applications plain
milling cutters are categorized as light duty, heavy duty and helical plain milling cutters.
Side Milling Cutters: Side milling cutters are used to remove metals from the side of
workpiece. These cutters have teeth on the periphery and on its sides. These are further
categorized as plain side milling cutters having straight circumferential teeth. Staggered
teeth side milling cutters having alternate teeth with opposite helix angle providing more
chip space. Half side milling cutters have straight or helical teeth on its circumference and
on its one side only. Circumferential teeth do the actual cutting of metal while side teeth do
the finishing work.
Interlocking side milling cutter has teeth of two half side milling cutter which are
made to interlock to form one unit.
Metal Slitting Saw: These cutters are like plain or side milling cutters having very small
width. These are used for parting off or slotting operations. Metal slitting saw is shown in
Figure 1.6. It is of two types. If teeth of this saw resembles with plain milling cutter, it is
called plain milling slitting saw. If its teeth matches with staggered teeth side milling
cutter, it is called staggered teeth slitting saw.
Angle Milling Cutter:These cutters have conical surfaces with cutting edges over them.
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These are used to machine angles other than 90o. Two types of angle milling cutters are
available single angle milling cutter and double angle milling cutter.
End Mill: End mills are used for cutting slots, small holes and light milling operations.
These cutters have teeth on their end as well as an periphery. The cutting teeth may be
straight or helical. Depending upon the shape of their shank, these are categorized as
discussed below.
Taper Shank Mill: Taper shank mill have tapered shank.
Straight Shank Mill: Straight shank mill having straight shank.
Shell End Mills: These are normally used for face milling operation. Cutters of different
sizes can be accommodated on a single common shank.
‘T’ Slot Milling Cutters: These are the special form of milling cutters used to produce
„T‟ shaped slots in the work piece. These have cutting edges on their periphery and both
sides.
Fly cutters: Fly cutters are the simplest form of cutters used to make contoured surfaces.
These cutters are the single cutting point cutting tools.
Convex Milling Cutters: These cutters have profile outwards at their circumference and
used to generate concave semicircular surface on the work piece.
Concave Milling Cutters: These milling cutters have teeth profile curve in words on their
circumference. These are used to generate convex semicircular surfaces.
Corner Rounding Milling Cutters: These cutters have teeth curved inwards. These
milling cutters are used to form contours of quarter circle. These are main used in making
round corners and round edges of the work piece.
Gear Cutter: These cutters are used in making gears on milling machine. Gear cutting is
an operation which cannot be done otherwise. These cutters have shape of the teeth which
are to be reproduced on the gear blank. Different gear cutters are used to make teeth with
involutes profile or cycloidal profile. A gear cutter is used to cut a range of gear size with a
fixed tooth profile.
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Thread Milling Cutter: These cutters are designated to mill threads of specific form and
size on the work piece. These cutters may be with parallel shank of tapered shank and
mainly used to make worms.
Top and Reamer Cutter: Top and reamer cutters are the cutters of double angle type,
these are normally used to make grooves and flutes in taps or reamers. Taps and reamers
are used as thread cutting tools for softer material work pieces.
Fig: 1.10 Types of Milling Cutters
1.6 END MILL CUTTERS
1.6.1 Tool Geometry
An end mill is a type of milling cutter, a cutting tool used in industrial milling applications.
It is distinguished from the drill bit, in its application, geometry, and manufacture. While a
drill bit can only cut in the axial direction, a milling bit can generally cut in all directions,
though some cannot cut axially. End mills are used in milling applications such as profile
milling, tracer milling, face milling, and plunging.
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A - mill size or cutting diameter
B - shank diameter
C - length of cut or flute length
D - overall length
Fig: 1.11 Tool geometry of End mill cutters
Angular Edge - That cutting edge that is a straight line, forming an angle with the
cutter axis. The surface produced by a cutting edge of this type will not be flat as is
the case with a helical cutting edge.
Axial Run out - The difference between the highest and lowest indicator reading
taken at the face of a cutter near the outer diameter.
Chamfer - A short relieved flat installed where the periphery and face of a cutter
meet. Used to strengthen the otherwise weak corner.
Chip Breakers - Special geometry of the rake face that causes the chip to curl
tightly and break.
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Chip Splitters - Notches in the circumference of a Corn cob style End mill cutter
resulting in narrow chips. Suitable for rough machining.
Core Diameter - The diameter of a cylinder (or cone shape with tapered End mills)
tangent to the flutes at the deepest point.
Counter bore - A recess in a non-end cutting tool to facilitate grinding.
Cutter Sweep (Run out) - Material removed by the fluting cutter (or grinding
wheel) at the end of the flute.
Cutting Edge (A) - The leading edge of the cutter tooth. The intersection of two
finely finished surfaces, generally of an included angle of less than 90 degrees.
Cutting Edge Angle - The angle formed by the cutting edge and the tool axis.
Differential pitch cutters - A specifically designed variation in the radial spacing of
the cutter teeth. This provides a variation in tooth spacing and can be beneficial in
reducing chatter. This concept is based on reducing the harmonic effect of the tool
contacting the part in an exact moment of vibration.
Entrance Angle - The angle formed by a line through the center of the cutter at 90
to the direction of feed and a radial line through the initial point of contact. As this
angle approaches 90 degrees the shock loading is increased.
Entrance Angle: Ramp-in - Angle or radius value to enter the cutter into the part
surface
Fillet - The radius at the bottom of the flute, from which core diameter is found.
Flute - Space between cutting teeth providing chip space and regrinding capabilities. The
number of cutting edges. Sometimes referred to as "teeth" or "gullet". The number on an
end mill will determine the feed rate.
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Fig: 1.12 Flute
Flute Length - Length of flutes or grooves. Often used incorrectly to denote cutting
length.
Shank - Projecting portion of cutter which locates and drives the cutter from the
machine spindle or adapter
Straight Shank - Cylindrical shank, with or without driving flats or notches, often
seen on carbide end mills
Weldon Shank - Industry name for a specific type of shank with a drive and
location flat. The flat on the cutter provides positive ( non slip ) driving surface to
the End mill.
Tooth - The cutting edge of the End mill.
Tooth Face - Also known as the Rake Face. The portion of the tooth upon which
the tooth meets the part.
1.7 END MILL TECHNICAL FEATURES
Back taper - A slight taper resulting in the shank end of the cutting diameter being
smaller than the cutting end. This condition aids not only the plunging or drilling
condition but also tends to compensate for deflection.
Clearance - Space created by the removal of additional tool material from behind
the relief angle.
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Fig: 1.13 Clearance of End Mill
Clearance Angle - The angle formed by the cleared surface and line tangent to the
cutting edge.
o Clearance: Primary (1st angle, 5°-9°) - Relief adjacent to the cutting edge.
o Clearance: Secondary (2nd angle, 14°-17°) - Relief adjacent to cutting edge
o Clearance: Tertiary (3rd) - Additional relief clearance provided adjacent to
the secondary angle.
Concave - Small hollow required on the end face of an End mill. This feature is
produced by a Dish angle produced on the cutter.
Convex - An outward projection radius feature on the end face of a Ball mill.
Dish Angle - The angle formed by the end cutting edge and a plane perpendicular to the
cutter axis. Dish ensures that a flat surface is produced by the cutter.
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Fig: 1.14 Different angles shown in an End Mill
Gash (Notch) - The secondary cuts on a tool to provide chip space at corners and
ends. The space forming the end cutting edge, which is used when feeding axially.
Gash angle - The relief angle of the gash feature.
Gash width - The width of the gash feature. The space between cutting edges,
which provides chip space and resharpening capabilities. Sometimes called the
flute.
Heel - The back edge of the relieved land. It is the surface of the tooth trailing the
cutting edge.
Helical - A cutting edge or flute which progresses uniformly around a cylindrical
surface in an axial direction. The normal helical direction is a right direction spiral.
Helix Angle - The angle formed by a line tangent to the helix and a plane through
the axis of the cutter or the cutting edge angle which a helical cutting edge makes
with a plane containing the axis of a cylindrical cutter.
Hook - A term used to refer to a concave condition of a tooth face. This term
implies a curved surface rather than a straight surface. Hook must be measured at
the cutting edge, making measurement difficult.
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Land - The narrow surface of a profile sharpened cutter tooth immediately behind
the cutting edge,
o (A) Cylindrical - a narrow portion of the peripheral land, adjacent to the
cutting edge, having no radial relief.
o (B) Relieved - A portion of the land adjacent to the cutting edge, which
provides relief.
Lead - The axial advance of a helical cutting edge in one revolution.
Lead = (Cutter diameter x Pi) / Tangent Helix Angle
Length of Cut (Flute Length) - The effective axial length of the peripheral cutting
edge which has been relieved to cut.
Radial Rake angle - The angle made by the rake face and a radius measured in a plane
normal to the axis.
Fig : 1.15 Tool terminology
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Rake - The angular relationship between the tooth face or a tangent to the tooth
face at a given point and a reference plane or line. An angular feature ground onto
the surface of an end mill.
o Axial rake - The angle formed by a plane passing through the axis and a line
coinciding with or tangent to the tooth face.
o Effective rake - The rake angle influencing chip formation most is that
measured normal to the cutting edge. The effective rake angle is greatly
affected by the radial and axial rakes only when corner angles are involved.
o Helical rake - For most purposes the terms helical and axial rake can be
used interchangeably. It is the inclination of the tooth face with reference to
a plane through the cutter axis.
o Negative Rake - Exists when the initial contact between tool and workpiece
occurs at a point or line on the tooth other than the cutting edge. The rake
surface leads the cutting edge.
o Positive Rake - Exists when the initial contact between the cutter and the
work piece occurs at the cutting edge. The cutting edge leads the rake surface.
Fig: 1.16 Rake
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Relief-Space - Provided by removing material immediately behind the cutting edge.
Done to eliminate the possibility of heeling or rubbing.
o Axial angle relief - The angle made by a line tangent to the relieved surface
at the end cutting edge and a plane normal to the axis.
o Axial relief - The relief measured in the axial direction between a plane
perpendicular to the axis at the cutting edge and the relieved surface. Helps
to prevent rubbing as the corner wears.
o Concave relief - The relieved surface behind the cutting edge having a
concave form. Produced by a grinding wheel set at 90 degrees to the cutter
axis.
o Eccentric relief - The relieved surface behind the cutting edge having a
convex form. Produced by a type I wheel presented at an angle to the cutter
axis.
o End relief - Relief on the end of an end mill. Needed only for plunging
cutters and to relieve rubbing as the result of corner wear.
o Flat relief - The relieved surface behind the cutting edge having a flat
surface produced by the face of a cup wheel.
o Radial relief - Relief in a radial direction measured in the plane of rotation.
It can be measured by the amount of indicator drop at a given radius in a
given amount of angular rotation.
Tangential rake angle - The angle made by a line tangent to a hooked tooth at the
cutting edge and a radius passing through the same point in plane normal to the
axis.
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CHAPTER 2
LITERATURE STUDY
Researchers in the area of high-speed milling have implemented various chatter
recognition techniques. Professor Jiri Tlusty[1] developed a method that detects chatter
during machining, and in turn, suggests a new speed for the same depth. Cobb [2] found
after testing, that impact dampers served better in controlling the vibrations. The types
ofimpact dampers used were a spring/mass liquid impact damper and a tapered impact
damper. Smith[3], Keyvanmanesh[4], and Cheng[5] did an extensive research in
understanding the dynamic characteristics of the tool and spindle to control chatter during
machining.
Cook et al. [6] developed damping mechanisms to control vibrations on
traffic signal structures. Traffic signal structures that are subjected to cyclic loading due to
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the wind and fast moving vehicles, sometimes, result in premature fatigue failures. They
investigated this problem and proposed devices to provide damping to the structures. The
research damper model was based on the work done by Slocum [7] on damping bending in
beams. In his book, Slocum introduced the concept of friction damping between layered
elements. The book explains that, when two cantilevered beams stacked on top of each
other undergo bending there occur a relative shear motion between the inner surfaces of the
layered elements causing friction energy to be produced at the interface, which in turn,
used to reduce the deflection of the layered beam. One of his patented works [8]
implements this idea. He developed a method to damp bending vibrations in beams and
similar structures T. Schmitz, J.C. Ziegert, C. Stanislaus [9] Charles Stanislaus predicted
that the stable cutting regions are a critical requirement for high-speed milling operations.
M.Alauddin, M.A.EL Baradie, M.S.J.Hashmi [10] has revealed that when the cutting speed
is increased, productivity can be maximized, and surface quality can be improved. F.
Ismail and E.G. Kubica [11] proposed the maximum quantity of material that can be
removed by the milling operation which is often limited by the stability of the cutting
process, and not by the power available on the machine. Smith and Dilio[12] have
described a control strategy for chatter suppression by adjusting the spindle speed to
operate in high stability lobe. Experimentally, they achieved a remarkable increase in metal
removal rates. Weck et al [13] attempted to assess the merits of using the spindle speed
modulation and for that matter any other technique for chatter suppression, one needs to
detect the onset chatter reliably. M. Liang, T.Yeap, A. Hermansyah [14] reported a fuzzy
logic approach for chatter suppression in end milling processes. Vibration energy and the
peak value of vibration frequency spectrum are jointly used as chatter indicators and inputs
to the proposed fuzzy controller.
Kosuke Nagaya, Jyoji Kobayasi, Katuhito Ima i [15] gave a method of
micro-vibration control of milling machine heads by use of vibration absorber. An auto-
tuning vibration absorber is presented in which the absorber creates anti-resonance state.
Ziegert John C. Stanislaus Charles, Schmitz Tony L. Streling [16] Robert found that the
limiting chatter free depth of cut in milling is dependent on dynamic stiffness of the tool or
spindle system. N.H.Kim, K.K.Choi, J.S.Chen and Y.H.Park [17] proposed a continuum-
based shape design sensitivity formulation for a frictional contact problem with arigid body
using mesh less method. Tony L. Schmitz, John C. Ziegert, [18] Charles Stanislaus
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predicted that the stable cutting regions are a critical requirement for high-speed milling
operations. Sridhar et al [19] presented the first detailed mathematical model with time
varying cutting force coefficients. Budak and Altintas [20] derived the finite order
characteristic equation for the stability analysis in milling. Recent investigation performed
by Alauddin [21] has revealed that when the cutting speed is increased, productivity can be
maximized, and surface quality can be improved. According to Hasegawa [22] surface
finish can be characterized by various parameters such as average roughness (Ra),
smoothening depth (Rp), root mean square (Rq), and maximum peak-to-valley height (Rt).
EI-Baradie [23] and Bandyopad [24] have shown that by increasing cutting speed, the
productivity can be maximized, an and the surface quality can be improved. S. Rajesham et
al.[25] stresses that Process knowledge is the prerequisite to applying Taguchi Method/D
O E. W.H. Yang, Y.S. Tarng [26] highlighted on the Taguchi method, a powerful tool to
design optimization for quality, is used to find the optimal cutting parameters for turning
operations. J.Z. Zhang et al [27] says that Taguchi design is an efficient and effective
experimental method in which a response variable can be optimized, given various control
and noise factors, using fewer resources than a factorial design.
Several efforts were made to reduce the chatter on the products produced by the
milling process. Sridhar et al [28] presented the first detailed mathematical model with
time varying cutting force coefficients..
Engelhardt et al [29] have been demonstrated the technique of spindle speed
modulation to be very effective in suppressing chatter in milling at regular cutting speeds.
The speed modulation parameters are application specific and may not be suitable for
entire job. The static force variation that results from modulated feed per tooth could
produce undesirable effects where constant speed cutting may suffice. Hence this
technique on its own lacks broad applicability..
Weck et al [30] attempted to assess the merits of using the spindle speed
modulation and for that matter any other technique for chatter suppression, one needs to
detect the onset chatter reliably. This blurring is amplified drastically when applying
certain chatter suppression techniques like spindle speed modulation method. The limit of
stability is defined as the axial depth of cut at which chatter commences.
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T. Schmitz et al [31] predicted that the stable cutting regions are a critical
requirement for high-speed milling operations. J.C.Ziegert et al (2004) found that the
limiting chatter-free depth of cut in milling is dependent on dynamic stiffness of the tool or
spindle system. A method for increasing the dynamic stiffness by providing additional
damping is demonstrated. The proposed damper is multi-fingered cylindrical insert placed
in an interior bore located inside conventional milling cutters. Spindle rotation forces these
flexible fingers against the inner surface of the tool, bending of the tool during cutting
dissipate energy through friction, leading to improved damping and dynamic stiffness. This
presents an analytical model of the damper, experimental measurements of tool response
and comparison between stable cutting depths using both conventional tool and with the
damping insert.
P.Ravi kumar and G.Krishna mohana Rao [32] conducted experiments on end
milling in aluminium and mild steel using solid end milling cutters .It was observed that
surface roughness decreases as the cutting speed increases.
P.Ravi Kumar and G.Krishna mohana Rao [33] conducted experiments on damper
inserted end milling cutters . Influence of cutting speed and type of damping insert on the
roughness of surface produced by damper inserted end mill cutter was studied. Taguchi
method was applied and found that hollow end milling cutter with 2 dampers was
optimum.
In the present study, experiments are conducted on work material Cast iron to
investigate the effect of damper inserted end milling tools on surface roughness.
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CHAPTER 3
REDUCTION OF VIBRATION IN MILLING CUTTERS
3.1 MILLING CUTTER CHATTER
Eliminating chatter or noisy vibration in mold making and other cavity milling operations
pays off in greater productivity. It increases metal removal rates, enhances surface finishes
with fewer finishing steps and reduces scrap.
Eliminating vibration also reduces wear on cutting tools and machining centers to
minimize machine downtime. Poor fixturing, work holding and machine maintenance all
contribute to vibration and its associated problems. The best way to quiet chatter is often a
combination of remedies. However, machine operators and manufacturing engineers
generally look first at their cutting tools. A knowledgeable supplier of both segmented and
solid carbide cutting tools can integrate total solutions to stop the chatter.
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Vibration in cavity milling creates uneven wear on cutting tools and shortens tool life.
While indexable insert milling cutters and solid carbide end mills differ in construction,
they are both vulnerable to chatter and share some common vibration remedies. Indexable
insert milling cutters are generally available in diameters down to one-half inch. They use
replaceable inserts with a choice of geometries and coatings. Smaller openings call for
solid carbide end mills with two, three or four cutting edges. There are steps that users can
take to end vibration with both milling cutters and end mills.
a) 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-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.
b) 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.
c) Choose inserts coatings carefully: Coatings on inserts perform many functions, but
their primary jobs are protecting against heat, maintaining lubricity and preventing build-
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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.
3.1.1 STIFFER TOOLS, LESS VIBRATION
The same anti-vibration principles true for indexable milling cutters also apply to solid end
mills. To reduce vibration, users should select end mills with fewer teeth and a high helix.
A steeper helix corresponds to a more positive rake. A shallow helix is equivalent to a
negative rake. To minimize vibration, end mill users should examine using helix angles
from 30 to 60 degrees relative to the centerline of the tool.
a) Minimize length; maximize diameter: In addition to positive rake and high helix
angles, both milling cutters and end mills should be as stiff as possible. Machine operators
and manufacturing engineers should do everything possible to minimize the bending or
deflection of cutting tools. A rule of thumb states that reducing the length of the tool by 20
percent reduces the amount of bending in the tool by 50 percent. Likewise, increasing the
diameter of a cutting tool by 20 percent cuts deflection in half. In practical terms, this
usually means that you should try to use the largest diameter tool you can to do the job.
In addition to large diameter tools, try to use the shortest tool possible for each application.
Many operators tend to choose a tool that meets the most demanding case on a work piece
requiring multiple operations. For a work piece with several hole depths, the same long
tool selected to make the deepest hole is also used to make shallower penetrations. Using a
longer-than-necessary tool in shallow holes contributes instability to the entire operation
and invites chatter. Programming the machine to use the right tool for each step minimizes
vibration and maximizes productivity for the entire job.
3.1.2 FEEDS, SPEEDS AND ANGLES
a) Maintain feed pressure per tooth: To minimize vibration, don't try to go easy on tools
by reducing feed pressure. Too light a feed allows the tool to slip and is just as prone to
generate vibration as too heavy a feed pressure. Use the loading recommended by the tool
supplier to minimize chatter and maximize tool life.
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b) Increase feed rate: Machine operators commonly respond to a vibration problem by
reducing the cutting speed and leaving the table feed alone. Speeding up the machine or the
feed may seem like a recipe for disaster. However, an increase in feed at the same rpm may
turn out to be the ideal solution. Anyone who has experienced harmonic vibrations in a car
on the highway knows either speeding up or slowing down can end the noise. Similar
experimentation can counter the complex harmonics of milling chatter.
c) Vary entry points: Moving the centreline of the cutter slightly too either side of the
entry point on the work piece can often reduce the tendency to chatter or vibrate. The
offset creates a finer entry angle and prevents forces from oscillating from one side of the
cutter to the other. For a two- to three-inch face mill, the offset may be 3/16". For a one-
inch end mill, the offset may be 0.0050". Again, experimentation can determine the low-
vibration setting.
3.2 TOOLHOLDING OPTIONS
a) Balance and true cutting tools: Cavity milling operators seeking to minimize vibration
should make sure that their tool is properly balanced and that it is mounted true to the
spindle centre. Strategies for connecting the tool to the machining centre vary widely.
Especially on milling jobs with long overhang, machine operators should avoid tool
holders that rely only on setscrews or keyways to transmission of torque. Modern tool
holding solutions, like a modular tool holding system, can help ensure balance and true
mounting
One system reduces tool run out to less than eighty millionths of an inch. The holder
design maintains 100 percent contact in the clamping area where torque is transmitted to
the tool. For shanked tools, a hydro mechanical chuck improves tool balance and stability,
and thereby reduces uncontrolled vibration.
3.3 INTEGRATED SOLUTION
Chatter is the product of every element in the cavity milling process, including the tools,
the machine and the work piece. The total system remedy is to eliminate all vibration
sources that can lead to harmonic responses. Run the job on the "tightest" machine
available. The more that the machine's ways and spindle are tight and robust, the less
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vibration will occur. Keep the structure rigid from spindle to cutting edge. Clamp the part
to minimize movement, vibration and deflection. Add support close to the areas to be
machined.
Vibration is most likely in work pieces with a long overhang. As a rule of thumb,
whenever a cutter's shank aspect ratio - its length-to-diameter ratio - exceeds three to one,
the risk of vibration rises rapidly. With ratios over five to one, vibration-damping
adapters/extenders and modular tool holders can help. Unlike solid adapters that transmit
vibrations readily, today's vibration-damping adapters have an internal chamber containing
a heavy body suspended on rubber bushings. Machine operators should position the
milling cutter as close to the tuned adapter as possible. Tooling is just one element in the
campaign against vibration.
3.4 INTRODUCTION TO FRICTION DAMPERS
Self-excited vibration in cutting tools has been a significant problem in the area of high-
speed machining due to its detrimental effect on the tool and the machined surface.
Theoretical models were developed and the magnitude of frictional work produced by the
damper was obtained by optimizing the physical dimensions of the design. Three different
tools, solid, hollow, and damped, were selected for investigation and were fabricated with
identical profiles. Initial tests to understand the tool characteristics were performed by
measuring the frequency response function (FRF) of the tools. The effect of spindle speeds
on the dynamic behavior of the spindle/holder/tool at the tool point was studied by
obtaining the rotating FRF at different speeds. Stability lobes were obtained based on the
measurements and the difference in stability limits between the static and rotating FRF
measurements was plotted. The effect of the damper on the cutting tool dynamics,
compared with the solid and the hollow tools, was also determined based on measured
FRFs at the tool point .To verify the preliminary results, a series of cutting tests were
performed on the three tools, and a method to identify the stability limits was developed by
recording the audio signal during the cut. The results were then plotted to show the effect
of spindle speed on stability limits providing a measure of performance of the three tools.
The concept of frictional damping was verified when the damped tool achieved a sixty-six
percent improvement in cutting depth over the solid tool. The results also showed that
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lobes developed from dynamic measurements are more realistic than statically generated,
non-rotating FRFs.
.
In an effort to control vibration in cutting tools, a method is developed to stabilize
the high frequency chatter vibration in end mills by employing a friction damper. It is
observed that end mills during machining, when unstable, produced chatter frequency of
more than 1 KHz. This caused a reduced tool life and a bad surface quality on the
machined surface. In order to improve the tool life and to reduce chatter, implemented a
frictional dampers are introduced.
Frictional damper is proposed for suppression of chatter in slender end mill tool.
This damper is made of a core and multi fingered hollow cylinder .The core is press fitted
into the hollow cylinder and they both are press fitted into an axial hole inside the tool.
This combination produces the resisting frictional stress against the stress reaction. An
analytical model including accurate modelling of friction in sliding and pre-sliding region
is developed for this damper. Finally, the optimal damped tool with damper inside is
fabricated and experimentally tested in comparison with traditional tool. The results show
a considerable improvement in tool performance. An acceptable agreement between
analytical and experimental results is obtained which show the effectiveness of damped
tool in improvement of tool performance.
The damping caused by the structure in the model is due to the principle of axial
shear in beams. It is well known from the elementary engineering subject called Mechanics
of Material, that beams undergo internal shear deformation along their axes during
bending. Members of a composite beam that are not securely fixed together will slide over
each other in proportion to their distance from the neutral axis of the composite beam. It is
this same sliding which would occur in the model beam while bending as long as the
neutral axis of the internal members, or fingers, does not coincide with the neutral axis of
the composite beam.
3.4.1Friction damper inserts:-
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Fig: 3.1 Solid End mill tool Fig: 3.2 Hollow
End mill Tool
Testing was performed on the solid end mill, and later the dampers were inserted
into the hollow tool, and the tests were repeated. The damper insert had fingers and was
constructed from a 9.5 mm diameter mild steel blank. The damper insert was slit down
76mm of its 105mm length by wire electro discharge machining in order to form the
separate fingers. As shown in the figures tools with one, two, three, four and five dampers
are chosen, so that different interactions between independent variables could be
effectively investigated. The diameter of the damper insert was such that the solid portion
provided a light press fit into the tool body.
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Fig:3.3 Hollow tool with one damper
Fig:3.4 Two dampers to be inserted in a Hollow Tool
Fig:3.5 Three Dampers to be inserted in a Hollow Tool
For the purpose of the project and to obtain better results dampers with four and five slots
in number were fabricated using Wire Cut EDM machining process. Dampers with
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increased number of slots have increased area of friction surface which increases the
amount of energy dissipation and hence reduced chatter and vibration.
Fig:3.6 Four dampers fabricated using wire cut EDM machining process
Fig:3.7 Five dampers fabricated using wire cut EDM machining process
3.5 WIRE CUT EDM MACHINING:
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Wire EDM (Vertical EDM's kid brother), is not the new kid on the block. It was
introduced in the late 1960s', and has revolutionized the tool and die, mold, and
metalworking industries. It is probably the most exciting and diversified machine tool
developed for this industry in the last fifty years, and has numerous advantages to offer.
The accuracy, surface finish and time required to complete a job is extremely
predictable, making it much easier to quote.
PRINCIPLE OF WIRE ELECTRICAL DISCHARGE MACHINING
The Spark Theory on a wire EDM is basically the same as that of the vertical EDM
process. In wire EDM, the conductive materials are machined with a series of electrical
discharges (sparks) that are produced between an accurately positioned moving wire (the
electrode) and the workpiece. High frequency pulses of alternating or direct current is
discharged from the wire to the workpiece with a very small spark gap through an
insulated dielectric fluid (water). Many sparks can be observed at one time. This is
because actual discharges can occur more than one hundred thousand times per second,
with discharge sparks lasting in the range of 1/1,000,000 of a second or less. The volume
of metal removed during this short period of spark discharge depends on the desired
cutting speed and the surface finish required.
The heat of each electrical spark, estimated at around 15,000 to 21,000 Fahrenheit,
erodes away a tiny bit of material that is vaporized and melted from the workpiece.
(Some of the wire material is also eroded away) These particles (chips) are flushed away
from the cut with a stream of de-ionized water through the top and bottom flushing
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nozzles. The water also prevents heat build-up in the workpiece. Without this cooling,
thermal expansion of the part would affect size and positional accuracy. Keep in mind
that it is the ON and OFF time of the spark that is repeated over and over that removes
material, not just the flow of electric current.
STARTING A CUT FROM THE EDGE OF A WORKPIECE
When starting a cut from the edge of a workpiece, cutting a form tool, slicing a tube
or bar stock, or starting a cut from a large diameter start hole, is a slower process without
submerged machining capabilities. There is a greater risk of breaking a wire if the flush
is not set properly or if too much power is used. This condition is greatly reduced when
cutting the part submerged.
Fig 3.8 working of wire EDM 3.6 REDUCING VIBRATIONS AND CHATTER IN END MILLING:
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When chatter occurs, it can be self-sustaining until the problem is corrected. Chatter
causes poor finish on the part, and will damage and significantly reduce the life of end
mills. Carbide end mills are particularly susceptible to damage.
3.6.1 Typical methods to reduce chatter include reducing cutting forces by:
Reducing the number of flutes. Decreasing the chipload per tooth by reducing the feed or increasing the speed or
RPM. Reducing the axial or radial depth of cut.
Though these steps will reduce chatter, slowing down the cutting process is not always the best course of action, and reducing the chipload can be detrimental to the cutter.
3.6.2 First steps are to improve rigidity and stability:
Use a larger end mill with a larger core diameter. Use end mills with reduced clearance or a small circular margin. Use the shortest overhang from spindle nose to tip of tool. Use stub length end mills where possible Use balanced tool holders. Rework fixture to hold the workpiece more securely. Reprogram the cutter path to shift cutting forces into stiffer portions of the
workpiece. Look for ways to improve spindle speeds then adjust feed accordingly.
Chatter is common when machining corners. As the end mill enters the corner, the
percentage of engagement increases the number of teeth in the cut. This drastically
increases the cutting forces, causing chatter. To reduce chatter when machining corners,
consider using circular interpolation to produce a bigger corner radius than indicated by the
part print. Then remove the remaining stock with a smaller end mill using circular
interpolation.
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CHAPTER 4
SURFACE ROUGHNESS
4.1 INTRODUCTION
Characterization of surface topography is important in applications involving friction,
lubrication, and wear (Thomas, 1999). In general, it has been found that friction increases
with average roughness. Roughness parameters are, therefore, important in applications
such as automobile brake linings, floor surfaces, and tires. The effect of roughness on
lubrication has also been studied to determine its impact on issues regarding lubrication of
sliding surfaces, compliant surfaces, and roller bearing fatigue. Finally, some researchers
have found a correlation between initial roughness of sliding surfaces and their wear rate.
Such correlations have been used to predict failure time of contact surfaces.
Another area where surface roughness plays a critical role is contact resistance (Thomas,
1999). Thermal or electrical conduction between two surfaces in contact occurs only
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through certain regions. In the case of thermal conduction, for example, the heat flow lines
are squeezed together at the areas of contact, which results in a distortion
Fig:4.1 .Contact resistance due to construction of flow lines of the isothermal lines,
Thermal contact resistance is an important issue in space applications, such as
satellites,
Where the heat generated by the electronic devices can only are driven away by
conduction. Surface roughness is also a topic of interest in fluid dynamics (Thomas, 1999).
The roughness of the interior surface of pipes affects flow parameters, such as the
Reynolds number, which is used to evaluate the flow regime (i.e., laminar or turbulent).
The performance of ships is also affected by roughness in the form of skin friction, which
can account for 80-90% of the total flow resistance. In addition, the power consumption
can increase as much as 40% during the service life of a ship as a result of increased
Surface roughness caused by paint cracking, hull corrosion and fouling. The examples
mentioned above are just a few of the applications in which surface roughness has to be
carefully considered. However, the influence of roughness extends to various engineering
concerns such as noise and vibration control, dimensional tolerance, abrasive processes,
bioengineering, and geomorphometry (Thomas, 1999).
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4.1.1 Surface Finish:
Surface finish is the allowable deviation from a perfectly flat surface that is made
by some manufacturing process
4.1.2 Terminology of surface roughness
Surface: The boundary that separates an object from another object, substance, or
space.
Real Surface: The actual boundary of an object. Its deviations from the nominal
surface stem from the processes that produce the surface.
Measured Surface: A representation of the real surface obtained by the use of a
measuring instrument.
Nominal Surface: The intended surface boundary (exclusive of any intended
surface roughness), the shape and extent of which is usually shown and
dimensioned on a drawing or descriptive specification (Figure 1.8).
Flaws: Flaws, or defects, are random irregularities, such as scratches, cracks,
holes, depressions, seams, tears, or inclusions as shown in Figure 1.8.
Lay: Lay, or directionality, is the direction of the predominant surface pattern and
is usually visible to the naked eye. Lay direction has been shown in Figure 1.8.
Roughness: It is defined as closely spaced, irregular deviations on a scale smaller
than that of waviness. Roughness may be superimposed on waviness. Roughness is
expressed in terms of its height, its width, and its distance on the surface along
which it is measured.(figure 1.8.1)
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Fig 4.2 schematic diagram of surface characteristics
Waviness: It is a recurrent deviation from a flat surface, much like waves on the
surface of water. It is measured and described in terms of the space between
adjacent crests of the waves (waviness width) and height between the crests and
valleys of the waves (waviness height). Waviness can be caused by,
Deflections of tools, dies, or the work piece,
Forces or temperature sufficient to cause warping,
Uneven lubrication,
Vibration, or
Any periodic mechanical or thermal variations in the system during manufacturing
operations.
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Fig 4.3 schematic diagram of surface characteristics
4.2 DEFINITION AND PARAMETERS
The concept of roughness is often described with terms such as ‘uneven’, ‘irregular’, ‘coarse in texture’, ‘broken by prominences’, and other similar ones (Thomas, 1999). Surface roughness is a measure of the texture of a surface. It is quantified by the vertical deviations of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small the surface is smooth. Roughness is typically considered to be the high frequency, short wavelength component of a measured surface.
Roughness plays an important role in determining how a real object will interact with its environment. Rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces . Roughness is often a good predictor of the performance of a mechanical component, since irregularities in the surface may form nucleation sites for cracks or corrosion. On the other hand, roughness may promote adhesion.
Although roughness is often undesirable, it is difficult and expensive to control in manufacturing. Decreasing the roughness of a surface will usually increase exponentially its manufacturing costs. This often results in a trade-off between the manufacturing cost of a component and its performance in application.
There are many different roughness parameters in use, but Ra is by far the most common. Other common parameters include Rz, Rq, and Rsk.
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Parameters
Ra is the arithmetic average of the absolute values and Rt is the range of the collected roughness data points.
The average roughness, Ra, is expressed in units of height. In the Imperial (English) system, 1 Ra is typically expressed in "millionths" of an inch. This is also referred to as "microinches" or sometimes just as "micro" The parameters are by far the most common surface roughness parameters found in the India on mechanical engineering drawings and in technical literature.
Table 4.1 surface roughness parameters
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Parameter Description Formula
Ra arithmetic average of absolute values
Rq, RRMS root mean squared
Rv maximum valley depth
Rp maximum peak height
Rt Maximum Height of the Profile
Rsk Skewness
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The Parameter that we dwell upon the most is Average Roughness –Ra: -
• Roughness average (Ra): This parameter is also known as the arithmetic mean
roughness value, AA (arithmetic average) or CLA (centre line average). Ra as shown in
thefig is universally recognized and the most used international parameter of roughness.
Fig 4.4 Roughness average of surface texture
Where Ra = the arithmetic average deviation from the mean line
L = the sampling length
y = the ordinate of the profile curve
It is the arithmetic mean of the departure of the roughness profile from the mean line.
An example of the surface profile is shown in Figure 6.6.
Fig: 4.5 Surface profile
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4.3 MEASURMENT TECHNIQUES
Surface finish may be measured in two ways: contact and non-contact methods. Contact
methods involve dragging a measurement stylus across the surface; these instruments are
called profilometers. Non-contact methods include: interferometer, confocal
microscopy, focus variation, structured light, electrical capacitance, electron microscopy,
and photogrammetry.
The most common method is to use a diamond stylus profilometer. The stylus is run
perpendicular to the lay of the surface.
4.3.1 Taylsurf instrument:
The Taylor-Hobson Talysurf.
The Talysurf is an electronic instrument working on carrier modulating principle. This
instrument also gives the same information as the previous instrument, but much more
rapidly and accurately. This instrument as also the previous one records the static
displacement of the stylus and is dynamic instrument like profile meter.
The measuring head of this instrument consists of a diamond stylus of about 0.002 mm tip
radius and skid or shoe which is drawn across the surface by means of a motorized driving
unit (gearbox), which provides three motorized speeds giving respectively x 20 and x 100
horizontal magnification and a speed suitable for average reading. A neutral position in
which the pick-up can be traversed manually is also provided. In this case the arm carrying
the stylus
forms an armature which pivots about the centre piece of E-shaped stamping as shown in
Fig. 11.9. On two legs of (outer pole pieces) the J5-shaped stamping there are coils
carrying an a.c. current. These two coils with other two resistances form an oscillator. As
the armature is pivoted about the central leg, any movement of the stylus causes the air gap
to vary and thus the amplitude of the original a. c. current flowing in the coils is
modulated. The output of the bridge thus consists of modulation only as shown in Fig.
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Fig. 4.6 Schematic Layout of Talysurf.
This is further demodulated so that the current now is directly proportional to the vertical
displacement of the stylus only. The demodulated output is caused to operate a pen
recorder to produce a permanent record and a meter to give a numerical assessment
directly. In recorder of this statement the marking medium is an electric discharge through
a specially treated paper which blackens at the point of the stylus, so this has no distortion
due to drag and the record strictly rectilinear one.
Now-a-days microprocessors have made available complete statistical multi-trace systems
measuring several places over a given area and can provide standard deviations and
average over area-type readings and define complete surface characterization. These
systems lend themselves to research applications where specialized programming can
achieve autocorrelation, power spectrum analysis and peak curvature.
Stylus
Phonograph needles, though used in some cases are found to be too large and too heavily
loaded. It also causes damage. Diamond styli are used universally. Some of them are cones
of 90° include dangle and tip radius 4-12 urn. A popular stylus with truncated pyramid is
shown in Fig. 11.10. The angle between the faces is 90°. The short edge is parallel to the
direction of motion. Thus this stylus cannot resolve a wavelength shorter than 6 \xm, and
integrates over a narrow strip of surface 8 \im wide.
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It may be noted that this pick up has finite dimensions, and it is constrained to move in a
nearly vertical plane, relative to the moving pickup. Thus the stylus cannot record re-
entrant features, an unimportant drawback for engineering investigations as re-entrant
structures are absent on most machined surfaces. This stylus will fail to follow peaks and
valley faithfully and produces a distorted record of the surface.
Since the dimensions of the stylus are finite, so also is the load on it. The load is of the
order of 70 mg force. But as the area of contact is too small, the local pressure may be
sufficiently high to cause significant local elastic downward deformation of the surface
under examination.
Fig: 4.7 Talysurf
4.3.2 Principle:
Fig 4.8 Principle of talysurf
A profile measurement device is usually based on a tactile measurement principle.
The surface is measured by moving a stylus across the surface. As the stylus moves up and
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down along the surface, a transducer converts these movements into a signal which is then
transformed into a roughness number and usually a visually displayed profile. Multiple
profiles can often be combined to form a surface representation.
CHAPTER 5
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METHODOLOGY
5.1 CHATTER DETECTION AND SUPPRESSION
Chatter is generally classified in two categories: primary and secondary. Primary chatter
can be caused by the cutting process itself (i.e. by friction between the tool and the
workpiece, by thermo-mechanical effects on the chip formation or by mode coupling).
Secondary chatter may be caused by the regeneration of waviness on the workpiece
surface.
This regenerative effect is the most important cause of chatter. For this reason it has
become a convention and been followed by a lot of the publications that ’’chatter’’ only
refers to regenerative chatter. However, it has to be mentioned that it is possible to
distinguish between frictional chatter, thermo-mechanical chatter and mode coupling
chatter and regenerative chatter depending on the self-excitation mechanism that causes the
vibration.
Frictional chatter occurs when rubbing on the clearance face excites vibration in the
direction of the cutting force Fc and limits in the thrust force Ft direction.
Thermo-mechanical chatter occurs due to the temperature and strain rate in the
plastic deformation zone.
Mode coupling chatter exists if vibration in the thrust force direction generates
vibration in the cutting force direction and vice versa. This result in simultaneous vibration
in the cutting and thrust force directions .Physically, it is caused by a number of sources
such as friction on the rake and clearance surfaces, chip thickness variation, shear angle
oscillations and regeneration effect.
Regenerative chatter is the most common form of selfxcited vibration. It can occur
often because most metal cutting operations involve overlapping cuts which can be a
source of vibration amplification .The cutter vibrations leave a wavy surface. During
milling the external tooth in cut attacks this wavy surface and generates an wavy
surface .The chip thickness and, hence ,the force on the cutting tool vary due to the phase
difference between the wave left by the previous teeth (in turning it is the surface left after
the previous revolution) and the wave left by the current ones . This phenomenon can
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greatly amplify vibrations, become dominant and build up chatter. If the relative phase
difference is zero, the dynamic chip thickness is also zero. If the relative phase is p, the
dynamic chip thickness variation is maximum. Consequently, the force on the cutter
depends, among other factors, on the displacement of the previous tooth.
At high speeds, the stabilizing effect of process damping diminishes, making the
process more prone to chatter. Process damping usually occurs at low spindle speeds and
provides the stability due to the short undulations left on the part’s surface by high-
frequency vibrations. These surface waves interfere with the cutting tool flank face and
dampen the cutting tool vibration.
Fig: 5.1 Regeneration of waviness in a milling model with two degrees of freedom.
5.1.1 Strategy for ensuring stable machining processes:
In detecting, identifying ,avoiding, preventing, reducing ,controlling, or suppressing chatter
a review of the great deal of literature regarding the chatter problem leads to a existing
method ,in which modifying certain machine tool elements to passively change the
behaviour of the system composed of the machine tool, the cutting tool and tool holder.
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In this method, the design of the machine tool is changed to improve its performance
against vibration or on the use of extra devices that can absorb extra energy or disrupt the
regenerative effect. Examples of these are passive damping devices installed in machine
tool elements with lower rigidity: friction dampers, mass dampers or tuned dampers. This
is focused on ensuring chatter-free operations by using passive strategies to damp, reduce
and control the phenomenon.
To reduce the excessive vibrations of an end-mill cutter, a mechanical damper is
introduced into a cylindrical hole in the centre of a standard end-milling cutter to dissipate
chatter energy in the form of friction. Chatter is a highly complex phenomenon due to the
diversity of elements that can compose the dynamic system and its behaviour the cutting
tool, the tool holder, the work piece material, the machine tool structure and the cutting
parameters. Predicting its occurrence is still the subject of much research, even though the
regenerative effect, the main cause of chatter, was identified and studied very early on.
Moreover, chatter can occur in different metal removal processes: milling, turning, drilling,
boring, broaching and grinding.
Chatter occurrence has several negative effects:
Poor surface quality.
Unacceptable inaccuracy.
Excessive noise.
Disproportionate tool wear.
Machine tool damage.
Reduced material removal rate (MRR).
Increased costs in terms of production time.
Waste of materials.
Waste of energy.
Environmental impact in terms of materials and energy.
Costs of recycling ,reprocessing or dumping non-valid final parts to recycling
points
For these reasons, chatter avoidance is a topic of enormous interest. In workshops, machine
tool operators often select conservative cutting parameters to avoid chatter and, in some
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cases, additional manual operations are required to clean chatter marks left on the part
surface. This common practice usually results in a decrease in productivity.
Chatter causes poor surface quality, therefore an investigation is made to test
surface roughness of work piece with solid end milling cuter, hollow milling cutter and
damper inserted milling cutters at various.
5.2 FRICTION DAMPER STRUCTURE
Friction damper is to increase the damping in end mills used for high-speed milling by the
addition of internal features into the tool. The increased damping is achieved by hollowing
the tool body and inserting a multi fingered damper into the center opening. The fingers on
the damper insert are created by cutting axial slits along most of the length of a cylinder
whose outer diameter matches the inner diameter of the tool body, thus forming multiple
fingers a damper may also be inserted into a solid end mill that has a blind hole in the non-
fluted end.
When the tool bends, the neutral surface of the outer cylindrical tool body is
located on the tool centreline. However, the fingers will bend with their neutral surfaces
passing through their own centroids. The net result is that the axial strain experienced on
the outer surface of the fingers will be different than the axial strain experienced on the
inner surface of the tool body, causing a relative sliding between them. When the tool
rotates at high speed, large centrifugal forces press the fingers against the inner surface of
the tool body. Press fitting causes an enhanced pressure between the parts that makes the
effect of damper more sensible. This is because of increasing the amount of frictional force
between the surfaces. The effect of press fit pressure seems to be much more than
centrifugal effect. Wire electro discharge machining is used to form fingers of the damper.
The damper was primarily designed to fit into the tool through a blind hole made
on the shank. When the tool rotates, the centrifugal forces generated at high speeds tend to
push the fingers of the damper outwards against the inner surface of the tool shank. During
this event, when the tool experiences bending vibrations, the fingers slide over the inner
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surface of the tool body. The relative sliding is proportional to the distance from the neutral
axis of the tool to the neutral axis of the individual fingers. The frictional forces,
Which arise during this sliding of the fingers over the inner surface of the tool body?
Dissipate energy and produce damping.
Two designs of the damper were developed in this research. The second is a
modified design, which was developed to attempt to improve the damper performance.
Since both the designs were based on same fundamental concept, the basic equations for
calculating the friction work remain the same except that the second model has a much-
simplified geometry and assumes that the contact between the tool’s inner surface and the
damper is only at the end. The equations for calculating the frictional work will be derived
for the original model followed by the modified equations that were used to calculate the
frictional work of the new design.
Of course, when the center section of a tool is removed, the stiffness will decrease.
This stiffness loss must, at minimum, be compensated by increases in the damping ratio
provided by the centrifugal damper. However, the stiffness loss is minimal for holes of
reasonable size. For example, in the tools developed for this work, the diameter of the
central hole is one-half of the outer diameters of the tool body, which reduces the bending
stiffness of the tool by only 7%. The frictional forces, which arise during this sliding of the
fingers over the inner surface of the tool body, dissipate energy and produce damping.
CHAPTER 6
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DESIGN OF EXPERIMENTS
A Design of Experiment (DOE) is a structured, organized method for determining the
relationship between factors affecting a process and the output of that process.
Other Definitions:
1. Conducting and analysing controlled tests to evaluate the factors that control the value
of a parameter or group of parameters.
2. "Design of Experiments" (DOE) refers to experimental methods used to quantify
indeterminate measurements of factors and interactions between factors statistically
through observance of forced changes made methodically as directed by mathematically
systematic tables.
Design of Experiment Techniques
1. Factorial Design
2. Response Surface methodology
3. Mixture Design
4. Taguchi Design
Among those we had selected Taguchi Design for optimizing surface finish and cutting
forces in end milling Operation.
6.1 Introduction to Taguchi Method
Competitive crisis in manufacturing during the 1970’s and 1980’s that gave rise to
the modern quality movement, leading to the introduction of Taguchi methods to the U.S.
in the 1980’s. Taguchi’s method is a system of design engineering to increase quality.
Taguchi Methods refers to a collection of principles which make up the framework of a
continually evolving approach to quality. Taguchi Methods of Quality Engineering design
is built around three integral elements, the loss function, signal-to-noise ratio, and
orthogonal arrays, which are each closely related to the definition of quality.
6.1.1 Taguchi design phases
To achieve economical product quality design, Taguchi proposed three phases:
1. System design,
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2. Parameter design,
3. Tolerance design.
1. Systems Design: Systems design identifies the basic elements of the design, which
will produce the desired output, such as the best combination of processes and
materials, selection of machine, the type of tool are considered.
2. Parameter Design: Parameter design determines the most appropriate, optimizing
set of parameters covering these design elements by identifying the "settings" of
each parameter which will minimize variation from the target performance of the
product.
3. Tolerance Design: Tolerance design finally identifies the components of the
design which are sensitive in terms of affecting the quality of the product and
establishes tolerance limits which will give the required level of variation in the
design.
Fig 6.1 Taguchi design phases
6.2 Taguchi approach
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SYSTEMS DESIGN
PARAMETER DESIGN
TOLERANCE DESIGN
ENGINEERING EXPERTIZE
Establishment of basic design and engineering concepts
Establishment of design target –dimensions, properties, statistical design and sensitivity analysis
Establish tolerances, statistical tolerance & design
Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
The objective of the robust design is to find the controllable process parameter
settings for which noise or variation has a minimal effect on the product's or process's
functional characteristics. It is to be noted that the aim is not to find the parameter settings
for the uncontrollable noise variables, but the controllable design variables. To attain this
objective, the control parameters, also known as inner array variables, are systematically
varied as stipulated by the inner orthogonal array. For each experiment of the inner array, a
series of new experiments are conducted by varying the level settings of the uncontrollable
noise variables. The level combinations of noise variables are done using the outer
orthogonal array.
The influence of noise on the performance characteristics can be found using the
ratio. Where S is the standard deviation of the performance parameters for each inner array
experiment and N is the total number of experiment in the outer orthogonal array. This
ratio indicates the functional variation due to noise. Using this result, it is possible to
predict which control parameter settings will make the process insensitive to noise.
Taguchi method focuses on Robust Design through use of
Signal-To-Noise ratio
Orthogonal arrays.
6.2.1 Signal-To-Noise Ratio
The signal-to-noise concept is closely related to the robustness of a product design.
A Robust Design or product delivers strong ‘signal’. It performs its expected function and
can cope with variations (“noise”), both internal and external. In signal-to-Noise Ratio,
signal represents the desirable value and noise represents the undesirable value.
Uses
o S/N ratios can be used to get closer to a given target value, or to reduce variation in
the product's quality characteristic(s).
o Signal-To-Noise ratio is used to measure controllable factors that can have such a
negative effect on the performance of design.
o They lead to optimum through monotonic function
o They help improve additives of the effects.
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o To quantify the quality.
There are 3 Signal-to-Noise ratios of common interest for optimization of Static
Problems. The formulae for signal to noise ratio are designed so that an experimenter can
always select the largest factor level setting to optimize the quality characteristic of an
experiment. Therefore a method of calculating the Signal-To-Noise ratio we had gone for
quality characteristic.
They are
1. Smaller-The-Better,
2. Larger-The-Better,
3. Nominal-The-Best.
The Smaller-The-Better: Impurity in drinking water is critical to quality. The less
impurities customers find in their in their drinking water, the better it is. Vibrations
are critical to quality for a car, the less vibration the customers feel while driving
their cars the better, the more attractive the cars are.
The Signal-To-Noise ratio for the Smaller-The-Better is:
S/N = -10 *log (mean square of the response)
.
The Larger-The-Better: If the number of minutes per dollar customers get from
their cellular phone service provider is critical to quality, the customers will want to
get the maximum number of minutes they can for every dollar they spend on their
phone bills.
If the lifetime of a battery is critical to quality, the customers will want their batteries to
last forever. The longer the battery lasts, the better it is.
The Signal-To-Noise ratio for the bigger-the-better is:
S/N = -10*log (mean square of the inverse of the response)
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.
Nominal-The-Best:
When a manufacturer is building mating parts, he would want every part to match
the predetermined target. For instance when he is creating pistons that need to be anchored
on a given part of a machine, failure to have the length of the piston to match a
predetermined size will result in it being either too small or too long resulting in lowering
the quality of the machine. In that case, the manufacturer wants all the parts to match their
target.
When a customer buys ceramic tiles to decorate his bathroom, the size of the tiles is
critical to quality, having tiles that do not match the predetermined target will result in
them not being correctly lined up against the bathroom walls.
The S/N equation for the Nominal-The-Best is:
S/N = 10 * log (the square of the mean divided by the variance)
.
6.2.2 Orthogonal Arrays
Introduction:
In order to reduce the total number of experiments “sir Ronald Fisher” developed
the solution:” orthogonal arrays”. The orthogonal array can be thought of as a distillation
mechanism through which the engineers experiment passes (Ealey, 1998). The array
allows the engineer to vary multiple variables at one time and obtain the effects which that
set of variables has an average and the dispersion.
Taguchi employs design experiments using specially constructed table, known as
"Orthogonal Arrays (OA)" to treat the design process, such that the quality is build into the
product during the product design stage. Orthogonal Arrays (OA) are a special set of Latin
squares, constructed by Taguchi to lay out the product design experiments.
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An orthogonal array is a type of experiment where the columns for the independent
variables are “orthogonal” to one another. Orthogonal arrays are employed to study the
effect of several control factors. Orthogonal arrays are used to investigate quality.
Orthogonal arrays are not unique to Taguchi. They were discovered considerably
earlier (Bendell, 1998). However Taguchi has simplified their use by providing tabulated
sets of standard orthogonal arrays and corresponding linear graphs to fit specific projects
(ASI, 1989; Taguchi and Kenishi, 1987).
A typical orthogonal Array:Sno A B C
1 1 1 1
2 1 2 2
3 1 3 3
4 2 1 3
5 2 2 1
6 2 3 2
7 3 1 2
8 3 2 3
9 3 3 1
Table 6.1 L9 Orthogonal array
In this array the columns are mutually orthogonal. That is for any pair of columns
all combination of factors occurs; and they occur an equal number of times. Here there are
4 parameters, A, B, and C each at three levels. This is called an ‘L9’ design; with the 9
indication the nine rows, configurations, or prototypes to be tested. Specific test
characteristics for each experimental evaluation are identified in the associated row of the
table. Thus L9 (34) means that nine experiments are to be carried out to study four variables
with three levels. There are greater savings in testing for larger arrays.
6.2.3 Minimum number of experiments to be conducted
The design of experiments using the orthogonal array is, in most cases, efficient when
compared to many other statistical designs. The minimum number of experiments that are
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required to conduct the Taguchi method can be calculated based on the degrees of freedom
approach.
For example, in case of 8 independent variables study having 1 independent variable with
2 levels and remaining 7 independent variables with 3 levels (L18 orthogonal array), the
minimum number of experiments required based on the above equation are 16. Because of
the balancing property of the orthogonal arrays, the total number of experiments shall be
multiple of 2 and 3. Hence the number of experiments for the above case is 18.
Application of Orthogonal Array
Taguchi's OA analysis is used to produce the best parameters for the optimum
design process, with the least number of experiments.
OA is usually applied in the design of engineering products, test and quality
development, and process development.
Advantages and disadvantages of orthogonal array:
Conclusions valid over the entire region spanned by the control factors and their
settings
Large saving in the experiment effort
Analysis is easy
OA techniques are not applicable, such as a process involving influencing factors
that vary in time and cannot be quantified exactly.
6.3 Steps in Taguchi Methodology
Taguchi method is a scientifically disciplined mechanism for evaluating and
implementing improvements in products, processes, materials, equipment, and facilities.
These improvements are aimed at improving the desired characteristics and simultaneously
reducing the number of defects by studying the key variables controlling the process and
optimizing the procedures or design to yield the best results. Taguchi proposed a standard
procedure for applying his method for optimizing any process.
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Fig 6.2 Steps in Taguchi methodology
6.3.1 Determine the Quality Characteristic to be optimized
The first step in the Taguchi method is to determine the quality characteristic to be
optimized. The quality characteristic is a parameter whose variation has a critical effect on
product quality. It is output or the response variable to be observed. Examples are weight,
cost, corrosion, target thickness, surface roughness, strength of a structure, and
electromagnetic radiation etc.
6.3.2 Identify the Noise Factors and Test Conditions
The next step is to identify the noise factors that can have a negative impact on
system performance and quality. Noise factors are those parameters which are either
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uncontrollable or are too expensive to control. Noise factors include variations in
environmental operating conditions, deterioration of components with usage, and variation
in response between products of same design with the same input.
6.3.3 Identify the Control Parameters and Their Alternative Levels
The third step is to identify the control parameters thought to have significant
effects on the quality characteristic. Control parameters are those design factors that can be
set and maintained. The levels for each test parameter must be chosen at this point. The
number of levels, with associated test values, for each test parameter defines the
experimental region.
6.3.4 Design the Matrix Experiment and Define the Data Analysis Procedure
The next step is to design the matrix experiment and define the data analysis
procedure. First, the appropriate orthogonal arrays for the noise and control parameters to
fit a specific study are selected. Taguchi provides many standard orthogonal arrays and
corresponding linear graphs for this purpose.
After selecting the appropriate orthogonal arrays, a procedure to simulate the
variation in the quality characteristic due to the noise factors needs to be defined. A
common approach is the use of Monte Carlo simulation. However, for an accurate
estimation of the mean and variance, Monte Carlo simulation requires a large number of
testing conditions which can be expensive and time consuming. As an alternative, Taguchi
proposes orthogonal array based simulation to evaluate the mean and the variance of a
product response resulting from variations in noise factors as shown in fig. the results of
the experiment for each combination of control and noise array experiment are denoted by
Yii.
6.3.5 Conduct the Matrix Experiment
The next step is to conduct the matrix experiment and record the results. The Taguchi
method can be used in any situation where there is a controllable process. The controllable
process can be an actual hardware experiment, systems of mathematical equations, or
computer models that can adequately model the response of many products and processes.
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6.3.6 Analyze the Data and Determine the Optimum Levels
After the experiments have been conducted, the optimal test parameter configuration
within the experiment design must be determined. To analyze the results, the Taguchi
method sues a statistical measure of performance called signal-to-noise (S/N) ratio
borrowed from electrical control theory. The S/N ratio developed by Dr. Taguchi is a
performance measure to choose control levels that best cope with noise. The S/N ratio
takes both the mean and the variability into account. In its simplest form S/N ratio is the
ratio of the mean (signal) to the standard deviation (noise). The S/N equation depends on
the criterion for the quality characteristic to be optimized. While there are many different
possible S/N ratios, three of them are considered standard and are generally applicable in
the situations below.
6.3.7 Predict the Performance at these Levels
Using the Taguchi method for parameter design, the predicted optimum setting need
not correspond to one of the rows o f the matrix experiment. This is often the case when
highly fractioned designs are used therefore, as the final step; an experimental
confirmation is run using the predicted optimum levels for the control parameters being
studied.
6.4 Analysis Of Variance (Anova)
Analysis of variance (ANOVA) is a statistical method for determining the existence
of differences among several population means. While the aim of ANOVA is the detect
differences among several populations means, the technique requires the analysis of
different forms of variance associated with the random samples under study- hence the
name analysis of variance.
The original ideas analysis of variance was developed by the English Statistician Sir
Ronald A. Fisher during the first part of this century. Much of the early work in this area
dealt with agricultural experiments where crops were given different treatments, such as
being grown using different kinds of fertilizers. The researchers wanted to determine
whether all treatments under study were equally effective or whether some treatments were
better than others.
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CHAPTER 7
EXPERIMENTAL SETUP
To experimentally test the performance of the damper insert, two end mills are
designed. The tool is made of high speed steel (HSS) with 19.05mm outer diameter,
125mm length, and has 4 cutting flutes. Their external geometry was identical. One of the
tool had an internal blind hole of 9.5mm diameter with a length of 105 mm.
7.1 EXPERIMENTAL SETUP AND CONDITIONS
The experiment was carried out into two stages.
I. Cast Iron pieces of 50*50*35mm were used as the workpieces for the machining
process. Longitudinal slots were machined on the work piece by varying 4 parameters.
Fig 7.1 slot cutting on Cast Iron
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Tests were performed at different spindle Speeds, Feeds and Depth of cuts for each tool. In
total there were about 6 different tools with various damper inserts for the present study.
The tools were
1) Solid End mill tool
2) Hollow End mill tool with one damper insert
3) Hollow End mill tool with two damper inserts
4) Hollow End mill tool with three damper inserts
5) Hollow End mill tool with four damper inserts
6) Hollow End mill tool with five damper inserts
Three prominent cutting speeds were selected among the 9 standard milling speeds, they
were 385Rpm, 685Rpm and 960Rpm. Similarly three Feed values were selected namely
18mm/min, 29mm/min and 41mm/min and three depth of cuts which are 0.25mm, 0.35mm
and 0.5mm. Taguchi's orthogonal array suggests a suitable combination of Speeds, Feeds
and Depth of cuts (doc) with the tool and damper arrangements. For the purpose of
variation of speeds, feeds and doc's with the various tools the Taguchi design gives outputs
as various combinations of these four parameters. Thus tests begin with each combination
of Type of tool, Feed, Speed and Depth of cut.
Consider the following table:-The details of these experimental conditions are shown
S.No. TYPE OF TOOL SPEED FEED DOC1 solid end mill 385 18 0.252 solid end mill 685 29 0.353 solid end mill 960 41 0.54 hollow with one damper 385 18 0.355 hollow with one damper 685 29 0.56 hollow with one damper 960 41 0.257 hollow with two damper 385 29 0.258 hollow with two damper 685 41 0.359 hollow with two damper 960 18 0.5
10hollow with three
damper 385 41 0.5
11hollow with three
damper 685 18 0.25
12hollow with three
damper 960 29 0.3513 hollow with four damper 385 29 0.514 hollow with four damper 685 41 0.25
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15 hollow with four damper 960 18 0.3516 hollow with five damper 385 41 0.3517 hollow with five damper 685 18 0.518 hollow with five damper 960 29 0.25
Table : 7.1 Experimental Details for Surface Roughness Analysis
II. After machining with the different cutting conditions, the surface roughness were
measured using surface measuring instrument TALYSURF shown in Figure.
Fig: 7.2 Taly Surf
Fig: 7.3 Surface roughness analysis
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The experiments were carried out on vertical milling machine. The physical and
mechanical properties of work piece are 50mm in length, 50mm in width and 35mm in
thickness. The work piece material is Cast Iron. The end milling cutter is of High Speed
Steel (HSS).
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CHAPTER 8
RESULTS AND DISCUSSIONS
The Cast Iron work piece of 50mm X 50mm is machined on vertical milling
machine with an end milling cutter of 19.05mm diameter and 125mm length at varying
feeds speeds and depth of cuts. Each individual slot or cut imparts a certain texture on the
newly exposed surface of the work piece. The slot is tested with a Talysurf instrument to
find the Average Surface Roughness (Ra). The Roughness values for each corresponding
tool, speed, feed and depth of cut is tabulated. Taguchi design identifies 18 unique
combinations of type of tool, feed, doc and speed.
S.No. TYPE OF TOOL SPEED FEED DOC Ra1 solid end mill 385 18 0.25 5.65 2 solid end mill 685 29 0.35 3.833 solid end mill 960 41 0.5 3.944 hollow with one damper 385 18 0.35 2.525 hollow with one damper 685 29 0.5 3.366 hollow with one damper 960 41 0.25 3.717 hollow with two damper 385 29 0.25 3.668 hollow with two damper 685 41 0.35 3.819 hollow with two damper 960 18 0.5 3.08
10 hollow with three damper 385 41 0.5 4.1511 hollow with three damper 685 18 0.25 3.3412 hollow with three damper 960 29 0.35 3.6913 hollow with four damper 385 29 0.5 4.8114 hollow with four damper 685 41 0.25 3.9115 hollow with four damper 960 18 0.35 3.3316 hollow with five damper 385 41 0.35 2.6717 hollow with five damper 685 18 0.5 2.3618 hollow with five damper 960 29 0.25 2.25
Table 8.1 Tabulated values of surface roughness at various cutting speeds, feeds, Doc’s with different tool inserts
Table 8.1 Depicts the variation of surface roughness Ra with cutting speed, feed and depth
of cut. It shows the influence of number of damper inserts also. Among all the tools used,
cutter with five fingered input resulted in better surface finish. This may be due to the
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increased area of friction surface which increases the amount of energy dissipation and
hence reduced chatter and vibration. When the tool bends, the neutral surface of the outer
cylindrical tool body is located on the tool center line. However, the fingers will bend with
their neutral surfaces passing through their own centroids. The net result is that the axial
strain experienced on the outer surface of the fingers will be different than the axial strain
experienced on the inner surface of the tool body, causing a relative sliding between them.
When the tool rotates at high speed, large centrifugal forces press the fingers against the
inner surface of the tool body. Press fitting causes an enhanced pressure between the parts
that makes the effect of damper more sensible. This is because of increasing the amount of
frictional force between the surfaces. The effect of press fit pressure seems to be much
more than centrifugal effect.
8.1 .TAGUCHI DESIGN METHOD:-
To better understand Taguchi design, the procedure of the Taguchi design is described in
the Fig. The complete procedure in Taguchi design method can be divided into three
stages: system design, parameter design, and tolerance design Of the three design stages,
the second stage – the parameter design – is the most important stage.
Fig:8.1 Taguchi Design
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8.2 Orthogonal array and experimental factors:-
Following the procedure described in the Fig, the first step in the Taguchi method is to
select a proper orthogonal array. A L18 orthogonal array was used in this study and is
shown in Table. This basic design makes use of up to four control factors, with three levels
each. A total of eighteen experimental runs must be conducted.
Table specifications -Taguchi Orthogonal Array Design
L18 (6**1 3**3)
Factors: 4Runs: 18
Table 8.2-Orthogonal array
Trial No. Type of tool Speed Feed Depth of cut1 1 1 1 12 1 2 2 23 1 3 3 34 2 1 1 25 2 2 2 36 2 3 3 17 3 1 2 18 3 2 3 29 3 3 1 310 4 1 3 311 4 2 1 112 4 3 2 213 5 1 2 314 5 2 3 115 5 3 1 216 6 1 3 217 6 2 1 318 6 3 2 1
8.3 Experimental set-up and procedure
After the orthogonal array has been selected, the second step in Taguchi parameter design
is running the experiment. This experiment was conducted using the hardware listed as
follows:
• End Milling Machine
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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
• Surface roughness measurement device: Taly surf (measures Ra in μm; stylus travel 0.4
mm).
• Cutting tools
8.4 Results of Taguchi analysis:-
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. Smaller is better S/N ratio was used in this study because less
surface roughness 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 surface roughness
values are measured and tabulated.
TYPE OF TOOL SPEED FEED DOC Ra SNRA1
1 1 1 1 5.65 -15.0411 2 2 2 3.83 -11.6641 3 3 3 3.94 -11.90992 1 1 2 2.52 -8.028012 2 2 3 3.36 -10.52682 3 3 1 3.71 -11.38753 1 2 1 3.66 -11.26963 2 3 2 3.81 -11.61853 3 1 3 3.08 -9.771014 1 3 3 4.15 -12.3614 2 1 1 3.34 -10.47494 3 2 2 3.69 -11.34055 1 2 3 4.81 -13.64295 2 3 1 3.91 -11.84355 3 1 2 3.33 -10.44896 1 3 2 2.67 -8.530236 2 1 3 2.36 -7.458246 3 2 1 2.25 -7.04365
Table- 8.3 Surface roughness parameter, Roughness average Ra values, S/N values for
machining the Cast Iron work piece at eighteen runs
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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
On analysing the data, inference can be made that the S/N ratio keeps decreasing with more number of dampers inserted into the hollow end mill tool on account of the vibration energy absorbed as friction.
After calculating S/N Ratios, the effect of control parameters on S/N ratio is shown below.
Figure:8.2 Graph of S/N to various factors
According to Taguchi design:-
Factor levels for predictions
TYPE OF TOOL SPEED FEED DOC 6 3 1 2
Predicted S/N Ratio = -6.07629
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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
According to Taguchi method, the optimum value of surface roughness can be obtained with the 6th tool, the 3rd speed, the 1st feed and the 2nd depth of cut. From the orthogonal array,Factor levels for predictions
TYPE OF TOOL SPEED FEED DOCHollow end mill with 5 damper 960rpm 18mm/min 0.35mm
8.5 SUMMARY OF ANOVA RESULTS
TABLE 8.4.Analysis of Variance
Source DF Seq SS Adj SS Adj MS F PTYPE OF
TOOL5 7.4665 7.4665 1.4933 3.46 0.081
SPEED 2 1.1370 1.1370 0.5685 1.32 0.335
FEED 2 0.3188 0.3188 0.1594 0.37 0.706
DOC 2 0.6235 0.6235 0.3118 0.72 0.523
Error 6 2.5860 2.5860 0.4310
Total 17 12.1319
S = 0.656510 R-Sq = 78.68% R-Sq(adj) = 39.60%
Predicted Optimal S/N value from Taguchi method= -6.07629
Predicted Surface roughness value Corresponding to S/N = -6.07629 is 2.012 µm
Experimental surface roughness value = 2.35 µm
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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
CONCLUSIONS
In the present study, experiments are conducted on work material Cast iron to
investigate the effect of damper inserted end milling tools on surface roughness.
The results indicated that the surface roughness decreases with increasing cutting
speed. The selection of appropriate cutting conditions and the use of sharp cutting
tools with adequate edge preparation are critical to achieve good surface finish.
Friction damper will lead to increase in the material removing rate in the milling
process via increasing stable chatter free depth of cut. It can also cause better
surface finish that is investigated here.
From the results obtained, it was found that the damped tool outperformed the
solid tool. Hence the overall performance of the damped tool with five damper
inserts was exceptional compared to the rest of the tools and consequently the solid
end mill tool.
Surface finish achievement of the confirmation runs under the optimal cutting
parameters indicated that of the parameter settings used in this study, those
identified as optimal through Taguchi parameter design were able to produce the
best surface roughness in this milling operation.
The optimal levels for the controllable factors were spindle speed 960 rpm, feed
rate 28 mm/rev, depth of cut 0.35 mm. Compared with the experiment results in
Table 8.1, the optimal surface roughness of the 18 confirmation samples 2.35µm.
which was very close to the smallest value optimal value of surface roughness
2.012 µm by Taguchi method .
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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA
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