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 inter interch chan ange geab able le musk musket et part parts. s. Alth Althou ough gh crude crude,, these these mach machin ines es assis assiste ted d man man in main maintai taini ning ng accu accura racy cy and and unif unifor ormi mity ty whil whilee dupl duplic icati ating ng parts parts that that coul could d not not be manufa manufactu ctured red with with the use of a file. file. Develo Developme pment nt and improv improveme ements nts of the millin milling g machin machinee and compon component entss contin continued ued,, which which resulted resulted in the manufa manufactu cturin ring g of heavie heavier r 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. configurations of machine tool The Machine Tool – In the present climate many different configurations exist .Some machines have the table/work piece stationary whilst the X,Y and Z axes move

Department of Mechanical engineering CMRCET

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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA and others may be constructed to allow the work work piece/table to be the moving 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 considered as the longest axis,where axis,where X+ will be the table motioning motioning to the left and X- to the right. The Y-axis moves from front to back of the machine with the table motioning motioning towards the operator operator as the Y+(positive) 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 up and away from the workpiece/table 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA Other milling operations are shown in the figure.

Fig:1.7 Types of Milling Operations

1.3 WORKING PRINCIPLES OF MILLING MACHINE The workpiece workpiece is holding on the worktable worktable of the machine. machine. The table movement movement controls 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 Milling machines can be found in a variety 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 The abov abovee comp compon onen ents ts of the the mill millin ing g mach machin inee can be orien oriente ted d eithe eitherr vert vertica icall lly y 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 Carbide tools are usually usually used in the form of brazed or clamped clamped tips. High cutting cutting spee speeds ds may may be used used and and mate materi rials als diffi difficu cult lt to cut cut with with HSS HSS may may be readi readily ly

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. 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 point of the tool and is normally given in meters/min. To calculate the spindle Speed required,


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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

revolu revolutio tion n of the spindl spindlee and tool, measured measured in inches inches per revolu revolutio tion n (IPR). (IPR). In some 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 stepover 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 operat operation ionss which which may be combin combinatio ation n of several several conven conventio tional nal operat operation ions. s. Standa Standard rd milling cutters are the conventional cutters which are classified as given below.

These cutter cutterss are cylind cylindric rical al in shape shape having having teeth on their their Plain Milling Milling Cutters: Cutters: These 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.

milling cutters cutters having having very small Metal Slitting Saw: These cutters are like plain or side milling 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA These are used to machine angles other than 90 o. 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 cutters used to make contoured surfaces. surfaces.

These cutters are the single cutting point cutting tools. circumference and Convex Milling Cutters: These cutters have profile outwards at their circumference 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 Corner Rounding Rounding Milling Milling Cutters Cutters: These These cutters cutters have have teeth teeth curved curved inwards. inwards. These 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 reproduced on the gear blank. 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA designated to mill threads threads of specific form and Thread Milling Cutter: These cutters are designated 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA •

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 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.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA Fig: 1.16 Rake •

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 damp damper. er. Smit Smith[ h[3] 3],, Keyv Keyvan anma mane nesh sh[4 [4], ], and and Chen Cheng[ g[5] 5] did did an exte extens nsiv ivee resea research rch in understanding the dynamic characteristics of the tool and spindle to control chatter during machining.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA Cook et al. [6] developed developed damping damping mechanisms mechanisms to control vibrations vibrations on traffic signal structures. Traffic signal structures that are subjected to cyclic loading due to 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 used to reduc reducee the the defl deflec ecti tion on of the the laye layere red d beam beam.. One One of his his pate patent nted ed work workss [8] [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 describ described ed a contro controll strateg strategy y for chatte chatterr suppre suppressio ssion n by adjust adjusting ing the spindl spindlee speed 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 chatter reliably. reliably. M. Liang, Liang, T.Yeap, A. Hermansyah 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 autotuning 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-


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA based shape design sensitivity formulation for a frictional contact problem with arigid body using using mesh mesh less less method method.. Tony Tony L. Schmit Schmitz, z, John John C. Zieger Ziegert, t, [18] [18] Charles Charles Stanis Stanislau lauss 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 varyin varying g cuttin cutting g force force coeffic coefficien ients. ts. Budak Budak and Altint Altintas as [20] [20] derive derived d the finite finite order 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 finish can be charact characteriz erized ed by variou variouss parame parameters ters such such as averag averagee roughn roughness ess (Ra), (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. improved. S. Rajesham et al.[25] al.[25] stresses that Process knowledge knowledge is the prerequisite prerequisite to applying Taguchi Method/D Method/D O E. E. W.H. Yang, Y.S. Tarng [26]

highlighted on the Taguchi Taguchi method, 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.. Engel Engelhar hardt dt et al [29] [29] have have been been demons demonstrat trated ed the techniq technique ue of spindl spindlee speed 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 We ck et al [30] [30] atte attemp mpte ted d to asse assess ss the the meri merits ts of usin using g the the spin spindl dlee spee speed d modulation and for that matter any other technique for chatter suppression, one needs to detect detect the onset onset chatte chatterr reliab reliably. ly. This This blurrin blurring g is amplifi amplified ed drastic drasticall ally y when when applyi applying ng


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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. T. Schm Schmit itzz et al [31] [31] pred predic icte ted d that that the the stabl stablee cutt cuttin ing g regio regions ns are are a criti critica call 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 meth method od was was appl applie ied d and and foun found d that that holl hollow ow end end mill millin ing g cutt cutter er with with 2 damp damper erss was 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. Elim Elimin inat atin ing g vibr vibrat atio ion n also also redu reduces ces wear wear on cutt cuttin ing g tool toolss and and mach machin inin ing g cent center erss to minimize minimize machine machine downtime. downtime. Poor fixturing, fixturing, work holding and machine machine maintenance maintenance all contribute to vibration and its associated problems. The best way to quiet chatter is often a combin combinatio ation n of remedi remedies. es. Howev However, er, machin machinee operat operators ors and manufa manufactu cturin ring g engine engineers ers


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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. 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 reducing chatter in milling operations 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. be. Conditions permitting, larger cutters cutters provide provide more choices choices about how to approach approach the work piece. Varying the relative 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 Optimize insert insert geometry 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 r ake cutters reduces cutting pressure by more than 20 percent versus zero- or negative-rake negative-ra ke 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.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA c) Choose inserts coatings carefully: Coatings on inserts perform many functions, but their primary jobs are protecting against heat, maintaining lubricity and preventing buildup 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 r educes 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA generate vibration as too heavy a feed pressure. Use the loading recommended by the tool supplier to minimize chatter and maximize tool life. 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 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 oneinch end mill, the offset may be 0.0050". Again, experimentation can determine the lowvibration 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. Especia Especially lly on millin milling g jobs jobs with with long long overh overhang ang,, machin machinee operato operators rs should should avoid avoid tool 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 Department of Mechanical engineering CMRCET

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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA sources sources that can lead to harmon harmonic ic respons responses. es. Run the job on the "tight "tightest" est" machin machinee available. The more that the machine's ways and spindle are tight and robust, the less 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. Vibr Vibrati ation on is most most like likely ly in work work piec pieces es with with a long long overh overhan ang. g. As a rule rule of thum thumb, b, whenever whenever a cutter's cutter's shank aspect ratio - its length-to-dia length-to-diameter meter ratio - exceeds exceeds three to one, the the risk risk of vibr vibrat atio ion n rises rises rapid rapidly ly.. With With ratio ratioss over over five five to one, one, vibr vibrati ation on-d -dam ampi ping ng adapters/exte adapters/extenders nders and modular modular tool holders can help. Unlike solid adapters adapters that transmit vibrations readily, today's vibration-damping adapters have an internal chamber containing a heavy heavy body body suspen suspended ded on rubber rubber bushin bushings. gs. Machin Machinee operat operators ors should should positi position on 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 highspeed speed machin machining ing due due to its detrim detriment ental al effect effect on the tool and the machin machined ed surface 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 dynami dynamicc behavi behavior or of the spindl spindle/h e/hold older/t er/tool ool at the tool point point was studie studied d by obtaining 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 measur measureme ements nts was plotted plotted.. The effect of the damper damper on the cuttin cutting g tool tool dynami dynamics, cs, 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 during the cut. The results were then plotted to show show the effect of spindle speed on stability limits providing a measure of performance of the three tools.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 lobes developed developed from dynamic measurements measurements are more realistic realistic than statically generated, 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 consid considerab erable le improv improveme ement nt in tool tool perfor performan mance. ce. An accepta acceptable ble agreem agreement ent betwee between n 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 Materia Material, l, that that beams beams underg undergo o intern internal al shear shear deform deformati ation on along along their their axes axes during 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:Department of Mechanical engineering CMRCET

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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 are chos chosen en,, so that that diffe differe rent nt inte interac ractio tions ns betw between een inde indepe pend nden entt vari variab able less coul could d be effectively investigated. The diameter of the damper insert was such that the solid portion provided a light press fit into the tool body. 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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 most exciting and diversified machine tool 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 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 The heat heat of of each each ele elect ctric rical al spar spark, k, est estim imat ated ed at aro aroun und d 15,0 15,000 00



to 21, 21,00 000 0



Fahr Fahren enhe heit it,,

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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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA nozzles. The water also prevents heat heat build-up in the workpiece. Without this cooling, thermal expansion of the part would affect size and positional 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 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 3.6

REDU REDUCI CING NG

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.

•

Repr Reprog ogram ram the the cutt cutter er path path to shif shiftt cutt cuttin ing g forc forces es into into stiffe stifferr port portio ions ns of the the workpiece.

•

Look for ways to improve spindle speeds then adjust feed accordingly.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA Chatte Chatterr is common common when machin machining ing corners. corners. As the end mill mill enters enters the corner, 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.

CHAPTER 4 SURFACE ROUGHNESS 4.1 INTRODUCTION Characterizati Characterization on of surface topography topography is important important in application applicationss involving 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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). 1999). Therma Thermall or electri electrical cal conduc conductio tion n between between two surface surfacess in contact contact occurs occurs only only 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA Fig:4.1 .Contact resistance due to construction of flow lines of the isothermal lines,

Thermal Thermal contact contact resistance resistance is an important important issue in space applications, applications, such as satellites, Wher Wheree the the heat heat gene genera rated ted by the the elect electro roni nicc devi device cess can can only only are are driv driven en away away by conduction. Surface roughness is also a topic of interest in fluid dynamics (Thomas, 1999). The The roug roughn hnes esss of the the inte interi rior or surfa surface ce of pipe pipess affec affects ts flow flow para parame mete ters, rs, such such as the 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 geomorphometry (Thomas, 1999).

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.


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Nominal Surface:

The intended surface boundary (exclusive of any intended

surf surfac acee roug roughn hnes ess) s),, the the shap shapee and and exte extent nt of whic which h is usua usuall lly y show shown n and 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 defined as closely closely spaced, irregular deviations 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)

Fig 4.2 schematic diagram diagram of surface characteristics 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,


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Forces or temperature sufficient to cause warping,

•

Uneven lubrication,

•

Vibration, or

•

Any periodic mechanical or thermal variations in the system during manufacturing operations.

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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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.

Parameters

R a is the arithmetic average of the absolute values and R t 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.


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Parameter

Description

R a

arithmetic average of absolute values

R q, R RMS RMS

root mean squared

R v

maximum valley depth

R p

maximum peak height

R t

Maximum Height of the Profile

R sk sk

Skewness


Formula

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Table 4.1 surface roughness parameters

The Parameter that we dwell upon the most is Average Roughness –Ra: • Roughnes This para parame mete terr is also also know known n as the the arit arithm hmeti eticc mean mean Roughnesss average average (Ra): (Ra): This 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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

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 called profilometers.. Non-contact methods include: interferometer , confocal microscopy, microscopy, focus variation, variation, structured light, light, electrical capacitance, capacitance, electron microscopy, microscopy, and photogrammetry 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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.

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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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. 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 reentrant 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:


Page 48


Fig 4.8 Principle of talysurf 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 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.


Page 49


CHAPTER 5 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). Second Secondary ary chatte chatterr may be caused caused by the regene regenerati ration on of wavine waviness ss on the workpi workpiece ece 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 refers to regene regenerat rative ive chatter chatter.. Howev However, er, it has to be mentio mentioned ned that it is possib possible le to distin distingui guish sh betwee between n frictio frictional nal chatte chatter, r, thermo thermo-mec -mechan hanical ical chatte chatterr and mode mode coupli coupling ng 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.


Page 50


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 source of vibrat vibration ion amplifi amplificati cation on .The .The cutter cutter vibrat vibration ionss leave leave a wavy wavy surface surface.. During During mill millin ing g the the exte extern rnal al toot tooth h in cut cut atta attack ckss this this wavy wavy surf surface ace and gene generat rates es an

wavy 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 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 dynami dynamicc chip chip thickn thickness ess variat variation ion is maximu maximum. m. Conseq Consequen uently tly,, the force force on the cutter 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 s peeds and provides the stability due to the short undulations left on the part’s surface by highfrequency vibrations. These surface waves interfere with the cutting tool flank face and


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 metho method d ,in which which modify modifying ing certain certain machin machinee tool tool elemen elements ts to passiv passively ely change change the behaviour of the system composed of the machine machine tool, the cutting tool and tool holder. 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 reduc reducee the the exce excessi ssive ve vibr vibrat atio ions ns of an endend-mi mill ll cutt cutter er,, a mech mechan anic ical al damp damper er 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, r esearch, 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. 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.


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Costs Costs of recycli recycling ng ,repro ,reprocess cessing ing or dumpin dumping g non-va non-valid lid final final parts parts to recycli recycling ng 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 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 nonfluted 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.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 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 muchsimplified 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.


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CHAPTER 6 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. Fact Factor oria iall Desi Design gn 2. Respon Response se Surface Surface method methodolo ology gy 3. Mix Mixture ture Desi Design gn 4. Tagu aguchi chi Des Desig ign n 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.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 6.1.1 Taguchi design phases

To achieve economical product quality design, Taguchi proposed three phases: 1. System design, 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: Toleran Tolerance ce design design finall finally y identi identifie fiess the compon component entss 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.

Establish tolerances, statistical tolerance & design Establishment of design target – dimensions, properties, statistical design and sensitivity analysis

TOLERANCE DESIGN

PARAMETER DESIGN Establishment of basic design and engineering concepts

SYSTEMS DESIGN ENGINEERING EXPERTIZE


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA Fig 6.1 Taguchi design phases

6.2 Taguchi approach 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


Page 58

Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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.

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. Smal Smalle ler-T r-The he-Be -Bett tter, er, 2.

Larg Larger er-T -The he-B -Bet ette ter, r,

3. Nomi Nomina nall-Th Thee-Be Best st.. •

The Smaller-The-Better: 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)

 ∑ y 2   S / N = −10 log 10   n    . •

The Larger-The-Better: 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.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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)

 1 1   ∑ 2  n y   .

S / N = −10 log10  

•

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 give given n part part of a mach machin ine, e, fail failur uree to have have the the leng length th of the the pist piston on to matc match h 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)

 y 2  S / N = 10 log10  2    s  . 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.


Page 60

Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 design stage. Orthogonal Arrays (OA) are a special set of Latin squares, constructed by Taguchi to lay out the product design experiments. 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 ‘L 9’ 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 L 9 (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.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 required to conduct the Taguchi method can be calculated based on the degrees of freedom approach. NV

N Taguchi

= 1 + ∑( Li − 1) i =1

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 usually applied applied in the design design of engine engineeri ering ng produc products, ts, test and quality 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 techniques are not applicable, applicable, such as a process process involving involving influencing 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.


Page 62

Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 optimizing any process.

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 response variable to be observed. Examples are weight,


Page 63

Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 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 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 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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.

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 D r. 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.


Page 65

Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 piece by varying 4 parameters.

Fig 7.1 slot cutting on Cast Iron


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 SPEE

FEE

S.No.

TYPE OF TOOL

D

D

DOC

1 2 3

solid end mill solid end mill solid end mill hollow with one damper hollow with one damper hollow with one damper hollow with two damper hollow with two damper hollow with two damper hollow with three damper hollow with three

3 85 6 85 9 60

18 29 41

0.25 0.35 0 .5

38 5

18

0.35

68 5

29

0 .5

96 0

41

0.25

38 5

29

0.25

68 5

41

0.35

96 0

18

0 .5

38 5 68 5

41 18

0 .5 0.25

4 5 6 7 8 9 10 11


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12 13 14 15 16 17 18

damper hollow with three damper hollow with four damper hollow with four damper hollow with four damper hollow with five damper hollow with five damper hollow with five damper

96 0

29

0.35

38 5

29

0 .5

68 5

41

0.25

96 0

18

0.35

38 5

41

0.35

68 5

18

0 .5

96 0

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


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Fig: 7.3 Surface roughness analysis 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).


Page 70


CHAPTER 8 RESULTS AND DISCUSSIONS The Cast Iron work piece of 50mm X 50mm is machined 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 Average Surface Surface Roughness Roughness (Ra). The Roughness Roughness values for each corresponding corresponding tool, tool, speed, speed, feed and depth of cut is tabula tabulated ted.. Taguch Taguchii design design identi identifies fies 18 uniqu uniquee combinations of type of tool, feed, doc and speed. SPEE S.No.

TYPE OF TOOL

D

FEED

DOC

Ra

1 2 3 4

solid end mill solid end mill solid end mill hollow with one

3 85 6 85 9 60 38 5

18 29 41 18

0.25 0.35 0.5 0.35

5 .6 5 3.83 3.94 2.52


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5 6 7 8 9 10 11 12 13 14 15 16 17 18

damper hollow with one damper hollow with one damper hollow with two damper hollow with two damper hollow with two damper hollow with three damper hollow with three damper hollow with three damper hollow with four damper hollow with four damper hollow with four damper hollow with five damper hollow with five damper hollow with five damper

68 5

29

0.5

3.36

96 0

41

0.25

3.71

38 5

29

0.25

3.66

68 5

41

0.35

3.81

96 0

18

0.5

3.08

38 5

41

0.5

4.15

68 5

18

0.25

3.34

96 0

29

0.35

3.69

38 5

29

0.5

4.81

68 5

41

0.25

3.91

96 0

18

0.35

3.33

38 5

41

0.35

2.67

68 5

18

0.5

2.36

96 0

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 increased area of friction surface which increases the amount of energy dissipation and hence reduced chatter chatter and vibration. 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 Taguchi design, the procedure procedure of the Taguchi design is described described in the Fig. The complete procedure in Taguchi design method can be divided into three stages: system design, parameter parameter design, design, and tolerance tolerance design Of the three design stages, the second stage – the parameter design – is the most important stage.

Fig:8.1 Taguchi Design

8.2 Orthogonal array and experimental experimental factors: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


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L18 (6**1 3**3) Factors: 4 Runs: 18

Table 8.2-Orthogonal array

Trial No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Type of tool 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6

Speed 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

Feed 1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2

Depth of cut 1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 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 • Surface roughness measurement device: Taly surf (measures Ra 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


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 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 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6

Table- 8.3

FEED 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

DOC 1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2

Ra 1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1

5 .6 5 3 .8 3 3 .9 4 2 .5 2 3 .3 6 3 .7 1 3 .6 6 3 .8 1 3 .0 8 4 .1 5 3 .3 4 3 .6 9 4 .8 1 3 .9 1 3 .3 3 2 .6 7 2 .3 6 2 .2 5

SNRA1 -15.041 -11.664 -11.9099 -8.02801 -10.5268 -11.3875 -11.2696 -11.6185 -9.77101 -12.361 -10.4749 -11.3405 -13.6429 -11.8435 -10.4489 -8.53023 -7.45824 -7.04365

Surface roughness parameter, Roughness average Ra values, S/N values values for machining the Cast Iron work piece at eighteen runs

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.


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Main Effects Plot for SN ratios

Data Means TYPE TY PE OF OF TOOL TOOL

SPEED

-8 -10 s o i t -12 a r N S f o n a -8 e M

1

2

3

4

5

6

1

2 DOC

3

1

2

3

FEED

-10 -12 1

2

3

Signal-to-noise: Smaller is better Figure:8.2 Graph of S/N to various factors According to Taguchi design:Factor levels for predictions TYPE OF TOOL 6

SPEED 3

FEED 1

DOC 2

Predicted S/N Ratio = -6.07629 According to Taguchi method, the optimum value of surface roughness can be obtained with the 6th tool, the 3 rd speed, the 1 st feed and the 2 nd depth of cut. From the orthogonal array, Factor levels for predictions TYPE OF TOOL Hollow end mill with 5 damper

SPEED 960rpm


FEED 18mm/min

DOC 0.35mm

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8.5 SUMMARY OF ANOVA RESULTS

TABLE 8.4.Analysis of Variance

Source TYPE OF TOOL SPEED

DF 5

Seq SS 7.4665

Adj SS 7.4665

Adj MS 1.4933

F 3.46

P 0.081

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

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.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA 

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 optimal surface surface roughness roughness of the 18 confirmatio confirmation n samples samples 2.35µm. 2.35µm. which was very very close to the smallest value value optimal value value of surface roughness roughness 2.012 µm by Taguchi method .

References

[1] S. Smith, J. Tlusty, Efficient simulation programs for chatter in milling, CIRP Annals —Manufacturing Technology 42 (1) (1993) (1993) 463–466.


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA [2] Cobb, W. T., 1989, “Design of Dampers for Boring Bars and Spindle Extensions,” Master’s Thesis, Mechanical Engineering, University of Florida, Gainesville, FL. [3] Smith, Kevin Scott, 1985, “Chatter, Forced Vibrations, And Accuracy In High- Speed Milling,” Master’s Thesis, Mechanical Engineering, University of Florida, Gainesville. [4] Keyvanmanesh, Amir, 1990, “Evaluation of Chatter Detection and Control System,” Master’s Thesis, Mechanical Engineering, University of Florida, Gainesville, FL. [5] Cheng, Cheng, E., 1992, 1992, “A Chatte Chatter-Fr r-Free ee Pocket Pocketing ing Routin Routinee For Any Two-An Two-And-A d-A HalfHalfDimensional Dimensional Pocket With Islands,” Master’s Thesis, Mechanical Mechanical Engineerin Engineering, g, University University of Florida, Gainesville, FL. [6] Cook, R. A., Bloomquist, D., Richard, D. S., and Kalajian, M.A., 2001, “Damping Of Cantilevered Traffic Signal Structures,”Journal of Structural Engineering, Vol. 25, pp 1476-1483. [7] Slocum, H. Alexander, 1992, Precision Machine Design, Society Of Manufacturing Engineers, Dearborn, MI. [8] Slocum, H. Alexander, Concord, N. H., Marsh, R. Eric, Smith, H. Douglas, 1998, “Rep “Repli licat cated ed In-P In-Pla lace ce Inte Intern rnal al Visco Viscous us Shear Shear Damp Damper er For For Mach Machin inee Stru Struct ctur ures es And And Components,” U. S. Patent No. 5, 799, 924 (September 1, 1998). [19] T. Schmitz, J.C. Ziegert, C. Stanislaus, “A Method for Predicting Chatter Stability for Systems with Speed-Dependent Spindle Dynamics” SME Technical Paper TP04PUB182, Transa Transactio ctions ns of the 2004 2004 North North American Manufacturing Research Institute of SME , vol.32 2004 pp.17–24. [22] M.Alauddin, M.A.EL Baradie, M.S.J.Hashmi, "Prediction of tool life in end milling by response surface surf ace methodology", Vol 71,1997, 456-465. [11] [11] E.G. E.G. Kubica Kubica,, F. Ismail Ismail,, Active Active suppre suppressio ssion n of chatte chatterr in periph peripheral eral millin milling. g. 2. application of fuzzy control, International Journal of Advanced Manufacturing Technology 12 (4) (1996)236–245. [12] Delio T., Tlusty J., Smith S. (1992) Use of audio signals for chatter detection and control. Journal of Engineering for Industry 114: 146-157


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Surface Roughness Optimization in End Milling using Taguchi method and ANOVA [13] Weck, M., Altintas.Y. Beer.C,1994,CAD assisted chatter free NC tool path generation in milling ,International journal of machine tools and manufacture ,34,879-891.

[14] M. Liang, T. Yeap, A. Hermansyah, A fuzzy system for chatter suppression in end millin milling, g, Procee Proceedin dings gs of the Instit Instituti ution on of Mechani Mechanical cal Engin Engineers eers Part Part B: Journa Journall of Engineering Manufacture (2004) 403–417. [15] K. Nagaya, J. Kobayasi, K. Imai, Vibration control of milling machine by using autotuning magnetic damper and auto-tuning vibration absorber, International Journal of Applied Electro magnetics and a nd Mechanics 16 (1–2) (2002) 111–123. [16]J.C [16]J.C.. Zieger Ziegert, t, C. Stanis Stanislau laus, s, T. Schmit Schmitz, z, R. Sterlin Sterling, g, Enhanc Enhanced ed Dampin Damping g in Long Long Slender End Transactions of the 2004 North American ,Manufacturing Research Institute of SME, vol. 32 2004 pp. 1–8. [17] [17] N.H. N.H. Kim, Kim, K.K. K.K. Choi, Choi, J.S. J.S. Chen, Chen, Y.H. Y.H. Park, Park, Meshle Meshless ss shape shape design design sensitivi sensitivity ty analysis and optimization for contact problem with friction, Computational Mechanics 25 (2/3) (2000) 157–168.

[18] T. Schmitz, J.C. Ziegert, C. Stanislaus, A Method for Predicting Chatter Stability for Systems with Speed-Dependent Spindle Dynamics SME Technical Paper TP04PUB182, Transa Transacti ctions ons of the 2004 2004 North North Americ American an Manufa Manufactu cturin ring g Researc Research h Instit Institute ute of SME, SME, vol.32 2004 pp. 17–24. [19] Sridhar, R., Hohn, R. E., and Long, G. W., 1968, ‘‘A General Formulation of the Milling Process Equation—Contribution to Machine Tool Chatter Research—5,’’ ASME J. Eng. Ind., pp . 317–324. [20] E. Budak, Y. Altintas, Analytical prediction of chatter stability in milling-part I: general formulation; part II: application to common milling systems, Transactions of the ASME, Journal of Dynamic Systems, Measurement, Me asurement, and Control 120 (1998) 22–36 [21] M.Alauddin, M.A.EL Baradie, M.S.J.Hashmi,"Prediction of tool life in end milling by response surface methodology", Vol 71,1997, 456-465. [22] M. Hasegawa, A. Seireg, R.A. Lindberg, "Surface roughness model for turning", Tribology International, December Vol,1976, 285–289. [23] M.A. EI-Baradie, EI-Baradie, "Surface "Surface roughness model for turning turning grey cast iron 1154 BHN)", Proc.IMechE, 207 11993) 43-54.


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Surface Roughness Optimization using Taguchi and Anova method.

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