DEVELOPMENT OF MILLING STRATEGY FOR OPTIMUM TOOL LIFE AND PRODUCTION COST.pdf

8/12/2019 DEVELOPMENT OF MILLING STRATEGY FOR OPTIMUM TOOL LIFE AND PRODUCTION COST.pdf

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DEVELOPMENT OF MILLING STRATEGY FOR OPTIMUM TOOL LIFE AND PRODUCTION COST

Dept. of Mechanical Engg., G.S.S Institute of Technology, 2012-13 Page 1

Chapter 1

INTRODUCTION

1.1 Background

In the Metal removal process, the cutting tool can be used until their cutting edges

produce parts within the specified surface finish and dimensional tolerances. When the

quality of cutting edge is lost because of the wear, the tool has reached its life limit and must

be replaced. This contributes to increased machining cost. To reduce the machining cost,

improve production rate and achieve world class efficiency it is essential to optimize every

possibilities.

The ultimate failure is understood to have taken place when the tool has worn out and

can machine no more and could break under the cutting forces enhanced due to the blunt

cutting edge. The Gradual wear that leads to this ultimate failure is unavoidable but

controllable. On the other hand a tool could fail due to many avoidable causes which we

would call as premature failure.

To achieve optimum tool life and reduce production cost, we need to optimize all the

cutting parameters. Depths of cut are also one of those parameters, in this study we are

focusing on influence of radial depth of cut or width of cut on tool wear and temperature.

1.2Company Introduction

1.2.1 History

In 1938, after years of research, metallurgist Philip M. McKenna created a tungsten-

titanium carbide alloy for cutting tools that provided a productivity breakthrough in the

machining of steel. "Kennametal" tools cut faster and lasted longer, and thereby facilitated

metalworking in products from automobiles to airliners to machinery. With his invention,

Philip started the McKenna Metals Company in Latrobe, Pennsylvania. Later renamed to

Kennametal, the corporation has become a world leader in the metalworking industry and

remains headquartered in Latrobe.


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McKenna Metal's first full-year sales, with a staff of 12 employees, totaled to about

$30,000. But World War II saw American heavy industry shift into high gear. Kennametal's

annual sales approached $10 million and employment was nearly 900 as the company's tools

were used extensively in the war-time economy.

When the wartime boom ended, Kennametal sought new ways to exploit the

toughness and wear resistance of tungsten carbide alloys. In the mid-1940s, the company

pioneered the use of carbide tooling for mining, which led to the development of the

continuous mining machine. Kennametal also found uses for tungsten carbide in demanding

specialty applications where resistance to wear was vital, such as in valves, dies, drill bits and

snow plough blades.

1.2.2 About Kennametal

Kennametal delivers productivity to customers seeking peak performance in

demanding environments by providing innovative custom and standard wear-resistant

solutions. This proven productivity is enabled through our advanced materials sciences and

application knowledge. Our commitment to a sustainable environment provides additional

value to our customers. Kennametals portfolio of well-respected brand names and broad

global presence enable us to help customers of all sizes in virtually every geography drive

success at every stage of their value chain. Strategically aligned across our two core

businesses - Industrial and Infrastructure - our products and services touch nearly everymanufacturing process. People around the globe can see and touch these results throughout

many aspects of their day, from the light switch they turn on to the car they drive.

Kennametal of United States of America acquired Widia India on 30th August 2002,

which is number one in Germany and India. Thus Widia enjoys the multifaceted expertise of

Kennametal. Widia (India) was incorporated in the state of Karnataka with its registered at

Bangalore on the 21st September 1964 with technical and financial collaboration from Krupp

Widia, GMBH, West Germany.

The Bangalore division went on stream in 1967 and has grown by leaps and bounds

since, then, from rupees 7.1 lakhs turnover at inception, the company has notched up an

impressive rupees 375 crores in 2010, with an active involvement of employees and officers

company as grown to greater heights and continues to be the market leader despite tough

competition, both domestic and global.


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Keeping pace with the modernization and emerging technological trends new

products are aggressively introduced. Widia (India) Ltd decided to manufacture machine

tools including CNC machines. The machine tool division Widma was thus found

specializing in the special purpose machines, to suit specific requirements of customers.

Kennametal has been named a four-time best-practice partner for excellence in our

world-class product development and portfolio management processes by the APQC, a non-

profit organization and internationally recognized leader in benchmarking, knowledge

management, measurement and quality programs.

1.2.3 Company Overview

Founded in 1938

Nearly 11,000 employees worldwide

Annual sales are approximately $2.4 billion

Headquartered in Latrobe, Pennsylvania, USA

Operations in over 60 countries

First or second in every market we serve

Global market leader in tooling for the mining and highway construction industries.

1.2.4 Products

The major products produce in Kennametal are metal forming tools, metal cutting

tools, which includes inserts, carbide bodies, gun drills, and end mills. Kennametal provides

the industry's best metalworking tools using advanced tungsten carbide, ceramics, and high-

speed steel materials.

Kennametal specializes in solving the unique wear problems by engineering and

manufacturing customized protective systems made of the world's toughest materials.

Kennametal is focused on delivering value to the customers for many different applications

that offer long life, maintain tolerance through multiple-use cycles, and deliver superior

overall performance. Our applications specialists can help in the design and manufacture for

your custom tooling requirements. Our customers report that our high-quality tungsten

carbide parts last a minimum of 10 times longer than steel in most applications.


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Fig.1.1: milling cutters Fig.1.2: Indexable milling cutters

Fig.1.3: Solid end mills Fig.1.4: Turning tools Fig.1.5: Inserts

(Courtesy: Kennametal Inc.)

1.3 Objectives

The main objective is to study the Effect of machining parameter and develop an

optimal machining strategy to ensure optimum tool life and production cost in face milling

operation.

Steps to achieve the objectives

To carry out literature survey on face milling, effect of machining parameters on thesurface roughness, tool wear, material removal rate and coatings.

Face milling experiment to analyze the effect by varying the radial depth of cut (Ae)and keeping constant axial depth of cut (Ap) on tool life. (When Ae=80%, 50% and

20%)

Analyze the Effect of Varying parameter on temperature and forces by Finite elementMethod using Third wave AdvantEdge software. (Ae=80% and 20%)

Identify the best strategy for enhanced tool life and production rate.


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Chapter 2

MILLING

2.1 Fundamentals of Metal Cutting2.1.1 Machining

Machining is a term used to describe a variety of material removal processes in which

a cutting tool removes unwanted material from a work piece to produce the desired shape.

The work piece is typically cut from a larger piece of stock, which is available in a variety of

standard shapes, such as flat sheets, solid bars, hollow tubes, and shaped beams. Machining

can also be performed on an existing part, such as a casting or forging.

2.1.2 Metal Removal Process

Mechanicalo Single-point cutting

TurningPlanning and shaping

o Multi-point cuttingMillingDrillingBroachingSawing

o Abrasive machiningGrindingHoningLappingUltrasonic machiningAbrasive jet machining

Chemicalo Chemical machiningo Electrochemical machining (ECM)
http://www.custompartnet.com/wu/turninghttp://www.custompartnet.com/wu/turninghttp://www.custompartnet.com/wu/millinghttp://www.custompartnet.com/wu/millinghttp://www.custompartnet.com/wu/hole-makinghttp://www.custompartnet.com/wu/hole-makinghttp://www.custompartnet.com/wu/hole-makinghttp://www.custompartnet.com/wu/millinghttp://www.custompartnet.com/wu/turning


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Thermalo Torch cuttingo Electrical discharge machining (EDM)o High energy beam machining

The popular process out of the above listed process with respect to material removal rate is

Turning Milling Drilling

In this course work we are more focusing on Milling Process.

2.2 Milling

Modern milling is a very universal machining method. During the past few years,

hand-in-hand with machine tool developments, milling has evolved into a method that

machines a very broad range of configurations. The choice of methods today in multi-axis

machinery is no longer straightforward in addition to all the conventional applications,

milling is a strong contender for producing holes, cavities, surfaces that used to be turned,

threads, etc. Tooling developments have also contributed to the new possibilities along with

the gains in productivity, reliability and quality consistency that have been made in indexable

Insert and solid carbide technology. Milling is principally metal cutting performed with a

rotating, multi-edge cutting tool which performs programmed feed movements against a

work piece in almost any direction. It is this cutting action that makes milling such an

efficient and versatile machining method. Each of the cutting edges removes a certain

amount of metal, with a limited In-cut engagement, making chip formation and evacuation a

secondary concern. Most frequently still, milling is applied to generate flat faces as in face

milling - but other forms and surfaces are increasing steadily as the number of five-axismachining centers and multi-task machines grow.

2.2.1 Basic Milling Operations

A milling cutter will basically employ one or a combination of the following basic

cutting actions: (s) Radial, (n) Peripheral and (v) Axial.


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(a) Radial (b) Peripheral (c) AxialFig. 2.1: milling operations

There are two milling process being followed

Up milling (conventional milling) Down milling (climb milling)

2.2.2 Up milling (Conventional milling)In up milling the cutter starts with zero chip thickness which increases as the cut

proceeds. At start the cutter in fact rubs against the work piece surface before actually

beginning to cut. The rubbing action generates heat at the interface. As a result, the newly

formed chip may get welded on to the rake face of the cutter tooth, thereby producing a

scratch on the work piece surface. Since the work piece motion is against the force exerted

by the cutter, any backlash present in the lead-screw of the table does not affect the process.

2.2.3 Down milling (Climb milling)In down milling (climb milling), the chip thickness is maximum at the beginning of

the cut and gradually reduces to zero. If the work piece is a casting, the rough sandy surface

can easily abrade and make the tool blunt. The process is however good for finishing cuts.There should not be any looseness or play between the nut and lead screws of the machine

table, as otherwise the work piece would be pulled in by the cutter and this would increase

the chip thickness to such an extent that it could break the cutter tooth. Compared to up

milling, the average chip thickness is higher in this process for given values of feed and

cutting speed and there is less power consumption.


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a) b)

Fig. 2.2: a) Up milling (conventional) and b) Down milling (climb)

2.3 Milling Cutter GeometryMilling cutter geometry is comprised of three major elements

Rake Angle, Clearance Angles and Lead Angle

Fig. 2.3: Radial and axial rake angle (courtesy: Kennametal Inc.)

2.3.1 Radial Rake Angle

The radial rake angle of a milling cutter is the angle formed in a diametric plane

between the face of the tooth and a radial line passing through the cutting edge. This may be

positive, negative, or zero degree.

Impact of Radial Rake Angles

1. Cutting Forces Amount of force Direction of cutting forces


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Radial clearance

2. Strength of the Cutting Edge3. Controls the radial direction of Chip Flow4. Has a major impact on radial clearance

Impact on cutting Edges Strength

Cutting forces enter the cutting edge at right angles to the rake surface. Radial rake angles absorb the impact of interruption on each revolution of the cutter.

Positive Radial Rake:The Positive radial rake exposes cutting edge to transfer rupture.

Negative Radial Rake:The Negative radial rake places the cutting edge into compression.

Chip flow Characteristics in Positive Radial Rake

Chip flow is inboard, up the incline plane formed by the positive radial rake. Cutter tends to recut chips Chips tend to weld to the chip slot Finish is marred by chip flow

Chip flow Characteristics in Negative Radial Rake

Chips clear the periphery.

Chips are thicker. Chip flow is outboard along the negative incline plane.

2.3.2 Radial Clearance

Fig. 2.4: Radial clearance in milling cutter (courtesy: Kennametal Inc.)


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The cutter is designed to provide a set amount of clearance (based on work piecesmaterial) under the heel of the insert.

The smaller the cutter diameter the greater the negative rake required to generate theprescribed clearance.

As diameter of the clearance increased the radial rake becomes more negative.

2.3.3 Lead Angle (Bevel Angle)Lead angle is dependent on work piece configuration, machine rigidity and fixture

rigidity. Lead angle controls the direction of cutting forces, chip thickness and nose radius

impacts on the lead angles.

2.3.4 Axial Rake Angle or Helical Rake

When a milling cutter has helical teeth, that is, when its cutting edge is formed along

a helix about the cutter axis, the resulting rake is called helical rake. If the cutting edge is

straight, its rake is axial rake. The axial rake or helical rake angle is the angle formed

between the line of the peripheral cutting edge and the axis of the cutter, when looking

radially at the point of intersection. This applies in the case of helical mills, half-side mills,

staggered tooth mills, face mills, and metal slitting saws having face cutting edges.

Axial Rake angle controls the cutting forces generated by the cutter.

Cutting forces decrease as the axial rake angle becomes more positive. Controls the axial direction of chip flow

Fig. 2.5: Nomenclature of Face milling cutter


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2.4 Milling Parameter

There are three major cutting parameters to be controlled in any milling operation.

These three parameters are cutting speed, feed rate and depth of cut. These parameters are

described below.

2.4.1 Cutting Speed

Cutting speed of a milling cutter is its peripheral linear speed resulting from

operation. It is expressed in meters per minute. The cutting speed can be derived from the

above formula.

Vc = Dn/1000 m/min (2.1)

where D= Diameter of milling cutter (mm)Vc= Cutting speed (linear) (meter per minute, m/min)n= Cutter speed in revolution per minute.

2.4.2 Feed Rate

It is the rate with which the work piece under process advances under the revolving

milling cutter. It is known that revolving cutter remains stationary and feed is given to the

work piece through worktable. Generally feed is expressed in three ways.

Feed per Tooth

It is the distance traveled by the work piece (its advance) between engagement by the

two successive teeth. It is expressed as mm/tooth and denoted by fz.

Feed per Revolution

Travel of work piece during one revolution of milling cutter. It is expressed as

mm/rev. and denoted by frev

Feed per Unit of Time


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Feed can also be expressed as feed/minute or feed/sec. It is the distance advances by

the work piece in unit time fm.

Above described three feed rates are mutually convertible.

fm= n x frev (2.2)

where n= rpm of cutter.

It can be extended further as

fm= n x frev = Z x n x fz (2.3)

where Z= Number of teeth in milling cutter.

2.4.3 Depth of Cut

Depth of cut in milling operation is the measure of penetration of cutter into the work

piece. It is thickness of the material removed in one pairs of cutter under process. One pairs

of cutter means when cutter completes the milling operation from one end of the work piece

to another end.

a) Axial depth of cut is axial advance of milling cutter into work piece. Axial depth is

represented by Ap and measured in mm.

(b) Radial depth is radial advance of milling cutter into work piece. Its also called as width

of cut, represented by Ae and measured in mm.

Fig. 2.6: milling process showing radial(Ae) and axial depth of cut (Ap)


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Coarse Pitch Cutter has less number of teeth compared to Fine Pitch Cutter and used for

larger depths of cut.

Fine Pitch Cutter has more teeth engagement with less chip clearance and used for lighter

Depths of cut.

Effect of Pitch on Feed: A simple calculation shows the effect of pitch on feed

Metal Removal rate of Coarse Pitch= 7 teeth x 0.127 mm/tooth x 500 RPM = 444.5 mm3/min

Metal Removal rate of Fine Pitch = 12 teeth x 0.127 mm/tooth x 500 RPM = 762 mm3/min

2.5.4 Cutter Hand

There are two types: LH cutter and RH Cutter

LH cutter is application specific and RH Cutter is most widely used for General purpose

2.5.5 Geometry

Fig. 2.10: Insert geometry (courtesy: Kennametal Inc.)

Edge Preparation

There are mainly 4 types of Edges preparations

Fig. 2.11: Edge configuration of insert (courtesy: Kennametal Inc.)

Out of above four types of edges we choose Honed Edge because of uniform

distribution of cutting forces.


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2.5.6 Carbon and Cobalt contents for machining Steel work piece

Fig. 2.12: Effect of carbon and cobalt (courtesy: Kennametal Inc.)

More the percentage of tungsten (wc) more is the wear and thermal shock resistance. The

strength of the insert increases with percentage of cobalt (co).

2.6 Factors affecting the machining parameters

Attention should be paid on the factors that are influencing the cutting parameters

cutting speed, feed rate and depth of cut.

2.6.1 Factors affecting speed

Work piece Hardness Work piece Condition (scale, sand) Condition of the Machine Horsepower Available Ability of the Grade to withstand Heat (Hot Hardness)

2.6.2 Factors affecting feed rates

Machine Horsepower Machine Rigidity and Fixture Rigidity Positive vs. Negative Geometry Cutter Pitch Surface Finish Required


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2.6.3 Factors affecting Depth of cut

Machine Horsepower Machine Rigidity and Fixture Rigidity Material to be Removed

2.7 Cutting Tool Materials

The cutting tool materials that are commonly used are:

Plain carbon and low alloy steels High-speed steels Cemented carbide, cermet and coated carbide

Ceramics Synthetic diamond (Poly Crystalline Diamond-PCD) and cubic boron nitride

(CBN)

Fig. 2.13: Cutting tool materials, speed vs. feed, doc (courtesy: Kennametal Inc.)

2.7.1 Evolution of Cutting Tool Materials

1910-1920: High speed steel 1920s: Cemented carbide 1950s: Cermet (TiC-based) 1960s: Alumina-based ceramic

Speed

(Thermal

Deformatio

n Resist)

Feed, DoC, Interruptions (Fracture resistance)


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1970: CVD coated carbide 1980: First engineered carbide

substrate (cobalt-enrichment)

1982: First SiAlON ceramic 1985: First PVD coated carbide Mid 80s: Modern cermets (TiCN-based) Late 80s: SiC whisker reinforced Al2O3 ceramic Early 90s: Advanced Sialons Mid 90s: Thin film diamond coated carbide Late 90s: PVD coated PCBN 2000: Advanced Pre-coat & post-coat treatments

2.7.2 Commonly used cutting tool materials

Common cutting tool materials are described below:

Carbon steels:

Carbon steels have been used since the 1880s for cutting tools. However carbon steels

start to soften at a temperature of about 180oC. This limitation means that such tools are

rarely used for metal cutting operations.

Plain carbon steel tools, containing about 0.9% carbon and about 1% manganese,

hardened to about 62 Rc, are widely used for woodworking and they can be used in a router

to machine aluminum sheet up to about 3mm thick.

High speed steels (HSS):

HSS tools are so named because they were developed to cut at higher speeds. These

steel have excellent hardenability and retain harness upto 650 oC. F.W. Taylor and M.White

in 1900 developed this steel for the first time. It typically contains 12-18% tungsten, 4-5.5%

chromium as principal alloying elements and retained hardness upto red heat temperature.

Other common alloying elements are vanadium, molybdenum and cobalt.

There are two basic types of high speed steels, tungsten (T-series) and molybdenum

(M-series).


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Most grades contain about 0.5% molybdenum and 4- 12% cobalt. It was soon

discovered that molybdenum (smaller proportions) could be substituted for most of the

tungsten resulting in a more economical formulation which had better abrasion resistance

than the T series and undergoes less distortion during heat treatment.

Consequently about 95% of all HSS tools are made from M series grades. These

contain 5 - 10% molybdenum, 1.5 - 10% tungsten, 1 - 4% vanadium, 4% Chromium and

many grades contain 5 - 10% cobalt.

HSS tools are tough and suitable for interrupted cutting and are used to manufacture

tools of complex shape such as drills, reamers, taps, dies and gear cutters. Tools may also be

coated to improve wear resistance. HSS accounts for the largest tonnage of tool materials

currently used. Typical cutting speeds: 10 - 60 m/min.

Cast non-ferrous alloys:

Introduced in early 1915 by Ellwood Hynes. These materials have the following

principal elements with specified ranges, 40 - 50% cobalt, 15-35% chromium, 1-4% carbon

and 10 - 25% tungsten. These alloys are cast and ground to the desired shape, they are not as

tough as HSS and are sensitive to shock loading but resist shock better than carbides. It is

recommended for deep continuous rough cuts at relatively high feed rates and speeds as

much as twice those possible with HSS. They can retain harness up to 950 oC. It is not heat

treatable and has maximum hardness values of 55 - 64 Rc. These tools are used only in

special applications (formed tools).

Carbides:

Also known as cemented carbides or sintered carbides were introduced commercially

in 1930s and have high hardness over a wide range of temperatures, high thermal

conductivity, high Young's modulus making them effective tool and die materials for a range

of applications. The two groups used for machining are tungsten carbide and titanium

carbide; both types may be coated or uncoated. Tungsten carbide particles (1-5 m) are

bonded together in a cobalt matrix using powder metallurgy. The powder is pressed and

sintered to the required insert shape. A wide range of grades are available for different

applications. The proportion of cobalt (the usual matrix material) present has a significant

effect on the properties of carbide tools. 3 - 6% matrix of cobalt gives greater hardness while


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6 - 15% matrix of cobalt gives a greater toughness while decreasing the hardness, wear

resistance and strength. Tungsten carbide tools are commonly used for machining steels, cast

irons and abrasive non-ferrous materials. Titanium carbide has a higher wear resistance than

tungsten but is not as tough. With a nickel-molybdenum alloy as the matrix, Tic is suitable

for machining at higher speeds than those which can be used for tungsten carbide. Typical

cutting speeds are: 30 - 150 m/min or 100 - 250 when coated.

Cemented Carbides

Fig. 2.14: Magnified image of cemented carbide (courtesy: Kennametal Inc.)

The dark colored object is tungsten carbide and the light colored object is cobalt.

Composition / Grain Size vs. Properties

3 - 12% Cobalt and 1-5 m carbide grain size

Fig. 2.15: Grain size (courtesy: Kennametal Inc.)

Above fig. 2.15, shows microscopy images of the coarse grain size of about 5 m

and fine grain size of about 1 m. With increase in grain size and cobalt content resistance

decreases and toughness increases.

WC grain size

Coarse grained (5 m)Fine grained (1 m)

WC (tungsten carbide)

Co (Cobalt)


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Table 2.1: Grain size nomenclature (courtesy: Kennametal Inc.)

Grain Size Range Nomenclature

< 0.2 Nano

0.20.5 Ultrafine

0.50.8 Submicron

0.81.3 Fine

1.32.5 Medium

2.56.0 Coarse

> 6.0 Extra Coarse

Cermets:Developed in the 1960s, these typically contain 70% aluminum oxide and 30%

titanium carbide. Some formulation contains molybdenum carbide, niobium carbide and

tantalum carbide. Their performance is between those of carbides and ceramics and coatings

seem to offer few benefits. Typical cutting speeds: 150 - 350 m/min.

Ceramics:

Alumina Introduced in the early 1950s, two classes are used for cutting tools: fine

grained high purity aluminum oxide (Al2O3) and silicon nitride (Si3N4) are pressed into

insert tip shapes and sintered at high temperatures. Additions of titanium carbide and

zirconium oxide (ZrO2) may be made to improve properties. But while ZrO2 improves the

fracture toughness, it reduces the hardness and thermal conductivity. Silicon carbide (SiC)

whiskers may be added to give better toughness and improved thermal shock resistance. The

tips have high abrasion resistance and hot hardness and their superior chemical stability

compared to HSS and carbides means they are less likely to adhere to the metals during

cutting and consequently have a lower tendency to form a built up edge. Their main

weakness is low toughness and negative rake angles are often used to avoid chipping due to

their low tensile strengths. Stiff machine tools and work set ups should be used when

machining with ceramic tips as otherwise vibration is likely to lead to premature failure of

the tip. Typical cutting speeds: 150-650 m/min.


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Silicon Nitride:

In the 1970s a tool material based on silicon nitride was developed, these may also

contain aluminum oxide, yttrium oxide and titanium carbide. SiN has an affinity for iron and

is not suitable for machining steels. A specific type is 'Sialon', containing the elements:

silicon, aluminum, oxygen and nitrogen. This has higher thermal shock resistance than

silicon nitride and is recommended for machining cast irons and nickel based super alloys at

intermediate cutting speeds.

Cubic Boron Nitride (CBN):

Introduced in the early 1960s, this is the second hardest material available after

diamond. CBN tools may be used either in the form of small solid tips or or as a 0.5 to 1 mm

thick layer of of polycrystalline boron nitride sintered onto a carbide substrate underpressure. In the latter case the carbide provides shock resistance and the cBN layer provides

very high wear resistance and cutting edge strength. Cubic boron nitride is the standard

choice for machining alloy and tool steels with a hardness of 50 Rc or higher.

Typical cutting speeds: 30 - 310 m/min.

Diamond:

The hardest known substance is diamond. Although single crystal diamond has been

used as a tool, they are brittle and need to be mounted at the correct crystal orientation to

obtain optimal tool life. Single crystal diamond tools have been mainly replaced by

polycrystalline diamond (PCD). This consists of very small synthetic crystals fused by a high

temperature high pressure process to a thickness of between 0.5 and 1mm and bonded to a

carbide substrate. The result is similar to CBN tools. The random orientation of the diamond

crystals prevents the propagation of cracks, improving toughness. Because of its reactivity,

PCD is not suitable for machining plain carbon steels or nickel, titanium and cobalt based

alloys. PCD is most suited to light uninterrupted finishing cuts at almost any speed and is

mainly used for very high speed machining of aluminum - silicon alloys, composites and

other non - metallic materials.

Typical cutting speeds: 200 - 2000 m/min.


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2.7.3 Desirable characteristics of Cutting tool material

Hot hardness: The hardness, strength, and wear resistance of the tool are maintained at the

temperatures encountered in machining operations. This ensures that the tool does not

undergo any plastic deformation and, thus, retains its shape and sharpness.

Toughness and impact strength (mechanical shock): Impact forces on the tool encountered

repeatedly in interrupted cutting operations (such milling, turning on a lathe, or due to

vibration and chatter during machining) do not chip or fracture the tool.

Thermal shock resistance: To withstand the rapid temperature cycling encountered in

interrupted cutting.

Wear resistance: An acceptable tool life is obtained before the tool has to be replaced.

Chemical stability and inertness: With respect to the material being machined, to avoid or

minimize any adverse reactions, adhesion, and toolchip diffusion that would contribute to

tool wear.

2.8 Coating for cutting tool materialsCoatings are frequently applied to carbide tool tips to improve tool life, productivity,

work piece surface finish. More than 65% of metal cutting inserts sold globally are coated.

There are two important Coating process

Chemical Vapor Deposition (CVD) Physical Vapor Deposition (PVD)

2.8.1 Chemical Vapor Deposition (CVD)

It is an atmosphere controlled process conducted at elevated temperatures (~1000 C)

in a CVD reactor. During this process, thin-film coatings are formed as the result of

reactions between various gaseous phases and the heated surface of substrates within the


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CVD reactor. As different gases are transported through the reactor, distinct coating layers

are formed on the tooling substrate. For example,

TiN is formed as a result of the following chemical reaction:

TiCl4 + N2 + H2 1000 C TiN + 4 HCl + H2.

Titanium carbide (TiC) is formed as the result of the following chemical reaction: TiCl4 +

CH4 + H2 1030 C TiC + 4 HCl + H2.

The final product of these reactions is a hard, wear-resistant coating that exhibits a

chemical and metallurgical bond to the substrate. CVD coatings provide excellent resistance

to the types of wear and galling typically seen during many metal-forming applications.

Fig. 2.16: Axial feed CVD (courtesy: Kennametal Inc.)

2.8.2 Physical Vapor Deposition (PVD)

Physical Vapor Deposition, or PVD, is a term used to describe a family of relatively

low temperature (500 C) vacuum coating processes that involve the generation of positively

charged ions through various methods. Reactive gases are introduced into the chamber to

Reactive gases Vacuum

Pump


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create various compounds. The positively charges ions are attracted to a negative bias given

to the tool substrates. This attraction results in a dense thin-film layer with an extremely

strong physical bond to the tool substrate

Features of PVD coatings

Both Monolayer and multilayer is possible Crack fee coating Fine grained & smoother than CVD coatings Compressive residual stress Can apply over the sharp edges Line-of-Sight processrequires tool fixture rotation

Fig. 2.17: Physical vapor deposition (courtesy: Kennametal Inc.)


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2.8.3 Properties of Coating

Chemical stability Improved hot hardness Microstructure Adhesion Coating thickness Residual stress (CVD-Tensile Stress, PVD-Compressive stress) Surface roughness Visual appearance

Table 2.2: Comparison between PVD and CVD Coating

PVD CVD

Full Name Physical Vapor Deposition Chemical Vapor Deposition

Process Temperature Low, 300 to 600 C High, 1000+ C

Coating Thickness 2 M to 8 M 2 M to 14 M

Material used

TiN, TiCN, TiAlN,TiB2,

TiN-TiAlN

High temperature

(~1000C) TiC, TiCN, TiN,

Al2O3, Diamond

multi-layers, nano-layer

coatings

Medium temperature

(~850C) TiCN, ZrCN

Plasma assisted CVD

(~600C) TiN, TiCN,

TiAlN

Multiple Layers No Yes

Applications Drilling, Milling Turning, Milling

Tools with Sharp Edges Threading, Grooving

Residual Stress Compressive Stress Tensile Stress


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2.9 Categories of Tool failure

Abrasive Wear

1) Flank Wear

Mechanical Failure

1) Chipping1a. Flank Chipping

1b. Rake Face Chipping

2) Depth of Cut notching3) Fracture

Heat Failure

1) Built up Edge1a. Rake Surface

1b. Flank Surface

2) Thermal Cracking3) Crater Wear4) Thermal Deformation

2.9.1 Abrasive Wear

Abrasive wear occurs as a result of the interaction between the work piece and the

cutting edge. This interaction results in the abrading away of relief on the flank of the tool.

This loss of relief is referred to as a wear land.

It depends on the hardness, elastic properties and Geometry of the two mating

surfaces.


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The larger the amount of elastic deformation a surface can sustain, the greater will be

its resistance to abrasion. A Brittle material like cast iron causes more of abrasion wear than

ductile steel.

It must also be noted that any material transferred from one surface to another which

is highly strain hardened could add to the abrasive wear. Further, the oxidation of the nascent

metal produces hard oxide particles which again contribute to the abrasive wear.

The width of the wear land is determined by the amount of contact between the

cutting edge and the work piece.

Fig. 2.18: wear land (courtesy: Kennametal Inc.)

Flank Wear

Fig. 2.19: Flank wear in insert (courtesy: Kennametal Inc.)

Flank: Is the Flat Surface of an insert perpendicular to the rake face

The cutting force normal to the direction of velocity keeps the tool pressed against the

wok piece. The friction between clearance face and the machined surface progressively

flattens the cutting edge. A flat wear land is produced on the clearance face extending from


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the cutting edge along the clearance face. As the length of the wear land increases friction

and heat generated in cutting increased and leads to further wear.

When the wear land reaches a critical value cutting becomes difficult. It leaves a

Burnished mark on the surface. More energy is required to remove the same amount of

material. Flank wear is mostly caused by abrasion of the flank and worsened by higher

temperatures caused at elevated speeds and cutting tool pressure.

Flank wear is the desired tool failure mechanism and it is the only mechanism that

can be predictable

Fig. 2.20: Flank and Crater wear on the tool clearance face (courtesy: Kennametal Inc.)

2.9.2 Mechanical Failures

Mechanical failures occurs from Insert wear caused by intense physical contact

between an insert and a work piece

Main Mechanical Failures are

1) Chipping 2) Notching 3) Fracture

Chipping

Tool wear results in the loss of small slivers from the cutting edge of the tool.

Chipping is also called frittering.

There are two Types of Chipping: 1) Flank Chipping 2) Rake Face Chipping

Flank Chipping or Mechanical Chipping

Mechanical Chipping occurs when small particles of the cutting edge are broken

away rather than being abraded away in abrasive wear.

Crater Wear

Flank Wear


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This happens when the mechanical load exceeds the strength of the cutting edge.

Mechanical chipping is common in operations having variable shock loads, such as

interrupted cuts. Chipping causes the cutting edge to be ragged altering both the rake face

and flank clearance. This ragged edge is less inefficient, causing forces and temperature to

increase, resulting in significantly reduced tool life.

Mechanical chipping is often the result of an unstable setup. i.e., a tool holder or

boring bar extended to far past the ideal length/diameter ratio, unsupported work pieces etc..,

Mechanical chipping is best identified by observing the size of the chip on both the

rake surface and the flank surface. The forces are normally exerted down onto the rake

surface producing a smaller chip on the rake surface and a larger chip on the flank surface

Fig. 2.21: Flank Chipping (mechanical chipping) (courtesy: Kennametal Inc.)

Rake Face Chipping: occurs due to Thermal Expansion and Radial Cutting Forces.

Chipping occurs when work pieces or cutting edge interface does not have adequate

clearance to facilitate an effective cut. This may be result of misapplication of a cutting tool

with inadequate clearance for the work pieces material being cut.


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Rake Surface Flank Surface Rake Face Chipping

Fig. 2.22: Rake face Chipping (observed on rake and flank surface) (courtesy: Kennametal

Inc.)

Depth Of Cutting Notching

Fig. 2.23: Depth of cut notching (courtesy: Kennametal Inc.)

It was described that the hardness of the chip and a thin layer of the machined surface

were significantly harder than the bulk material.


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It may be visualized that in turning, the tool will have its tip in the bulk of the material; but

at the distance equaling the depth of cut, the tool will be cutting through some significantly

harder material (the work hardened layer) causing a notch to appear on the flank face, called

the depth of cut notch.

Depending on the shape and geometry of the tool, the notch wear can be highly

influential on tool life or be completely insignificant compared with other modes of wear.

Effect

Localized failure at the depth of cut line. Localized Chipping and Localized Cratering

Typical with Stainless Steel, high temperature alloys a all work-hardening materials Typical when the work pieces have scale or a hardened surface.

Depth-of-Cut Notching can be minimized by following Methods

by CVD coatings by Cobalt enriched grades Increased lead angle (thins the chip reducing forces) Use tapered cuts

Fracture

Fig. 2.24: Failure due to fracture

Tool Fracture occurs when the tool is unable to support the cutting force over the

tool-chip contact area and results in loss of only a small part of tool. It is called as Chipping

or Breakage


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It is Common in interrupted cuts and in non-rigid setups.

Chipping and Breakage can be minimized by using

Tougher cutting tool material: Cobalt enriched grades higher cobalt TiC, & TaC grades

Stronger geometrya. By using Negative rake rather than the positive rakeb. Increasing Tool Nose

Maximize rigidity Reduced metal removal rate

2.9.3 Heat Related Failure

Below are Heat related Failures occurring in Cutting tool.

1) Built Up Edge 2) Thermal Cracking 3) Cratering 4) Thermal Deformation

Built-Up Edge

Fig. 2.25: Built up edge on insert (courtesy: Kennametal Inc.)

Built-up Edge is also called as Adhesion. This occurs due to welding between the tool

and chip (i.e. work material is deposited on the rake and flank face of the tool) at the

asperities and the subsequent breakage of the welds. When weld breaks it plucks away

material from the tool. We can expect that this wear will be inversely proportional to the


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hardness of the work material and directly proportional to the normal stress on the sliding

surface.

It is the product of the localized high temperature and extreme pressure at the tool and

chip interface.

It depends on the Normal face between the sliding surfaces and the apparent area of contact.

It is dependent upon the Relative hardness of the chip and tool.

Built-up edge is not stable and will slough off periodically, adhering to the chip or

passing through the tool and adhering to the machined surface.

Generally adhesion occurs on soft, gummy work pieces materials.

Rake Face

Fig. 2.26: Built up edge in Rake face (courtesy: Kennametal Inc.)

Welding of work pieces material to the rake face of the cutting tool Loss of effective geometry causes increases in cutting forces and eventual tool

breakage

Minimizing Built Up Edge

Using higher cutting speed- At high speeds, that is at high tool-chip interfacetemperatures, the welds between tool and chip would be predominantly temperature

welds. There is insufficient time for pressure welds to occur. Temperature welds

being soft will separate easily. No built up edge is formed. However there is small

amount of material plucked off from tool surface.


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PVD Coating by using materials like TiC, TiN: TiC and Tin have lesser affinity tosteel to form built-up edge. Moreover low wettability of these materials by ferrous

material reduces built-up edge formation. The edges are uniformly coated hence there

is less chance of adherence property

Polished edges: Adherence property is weaker at polished surfaces. Using Coolant: Coolant washes away built up material at earlier stages. by using positive rake : Area of contact is minimum. by Minimizing the flank wear

Flank Face Builtup Edge

This is normally associated with inadequate clearance angles under the cutting edge.Soft Springy materials tend to spring-back afterbeing cut and rub the flank of the tool.

Fig. 2.27: Built up edge in flank face (courtesy: Kennametal Inc.)

Thermal-Mechanical Cracking

Thermal Cracking

This thermal cracking is Evenly-spaced cracks perpendicular to the cutting edge

It is commonly observed in milling and interrupted cutting. Caused by variations in temperature in milling induce cyclic thermal shock as the

surface layer of tool repeatedly expands and contracts due to heating and cooling of

the edge


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Minimizing Thermal Cracking

Thermal Cracking can be minimized by following method:

Using Tougher, more thermal-shock-resistant tool material Use a grade with more TaC content Higher cobalt content carbide grade Avoid coolant if possible or assure a steady supply By reduced cutting speed.

Cratering

Fig. 2.28: Crater wear on insert (courtesy: Kennametal Inc.)

Cratering are Tool wear characterized by a concave depression in the rake face of the

cutting tool. Cratering is also called crater wear.

Cratering are Typical in machining carbon steels at elevated speeds. This are Caused by extreme heat & pressure of chip

Involves diffusion or dissolution of tool material into the chip

Minimizing Crater Wear

Crater Wear can be minimized by following methods: By Reduce Cutting speed (by reduced spindle speed) By using Higher TiC Content grade By Lower cobalt grade By Use of CVD coated grades - Al2O3 & TiC


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Thermal Deformation

Fig. 2.29: Thermal deformation (courtesy: Kennametal Inc.)

It is also called as plastic deformation and takes place as a result of combination of

high temperature and high pressures on the cutting edge.

When the cutting edge loses its hot hardness the forces created by the feed rate cause

the cutting edge to deform. The amount of thermal deformation is in direct proportion to the

depth of cut and feed rate.

It is typical in machining alloy steels at elevated speeds. Results in Bulging or blunting of the tool edge.

Minimizing Thermal Deformation

By Use of grades with higher TaC content By Use of grades with lower cobalt content By Using CVD coated grades - Al2O3 & TiC

2.10 Tool Life

The length of time that a cutting tool can function properly before it begins to fail

Taylors Tool life equation

Vc Tn= Ct (2.4)

Where, T is time in minutes,


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Ct is constant and varies with tool and work material, tool geometry

Exponent n determines the slope of the tool life curve and depends primarily on the tool

material

Vc is cutting speed in m/min

Some of the more common criteria for judging the end point of tool life are

1) Width of wear land i.e. occurrence of a certain width of wear land.2) Depth of crater wear i.e. occurrence of a certain depth of crater wear.3) Increase of cutting force, or power consumption, by a certain amount.4) Increase of radial force on the tool by a certain amount.5)

Increase of feed force by a certain amount.

6) Sudden change in finish and dimension of work piece.


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Chapter 3

LITERATURE REVIEW

3.1 Historical background

The history of cutting tools began during the industrial revolution in 1800 AD, but the

first cutting tool was cast using a crucible method in 1740. In 1868 R. mushet found by

adding tungsten we can increase hardness and tool life (air quenching). F.W. Taylor in

Pennsylvania did the most basic research in metal cutting between 1880-1905 and invented

High speed steel cutting tools. The initial development of cemented and sintered carbides

occurred in Germany in 1920s by osram study society for electrical lighting to replace

diamonds as a material for machining metal. Later the license was transferred to Krupp,

essen, germany at the end of 1925. In 1926 Krupp brings sintered carbide on to the market

under the name of WIDIA (in german acronym for Wie Diamant, means like diamond in

English).

3.2 Overview

There are many researches done in field of Metal Cutting application. The importance

is being to reduce production cost by Enhancing the tool life and material removal rate.

This is possible by optimizing the 1) Machining Parameters like cutting speed, feed

and depths of cut (axial and radial). 2) By optimizing the Insert geometries like shape, cutting

edges, rake angles 3) By various coatings.

Mr. Milon D Selvam, research scholar at karpagam university has optimized the four

machining parameters i.e., number of passes, depth of cut, spindle speed and feed rate by

using CNC vertical machining center with fanuc control. Workpiece material was Mild steel,

processed using zinc coated carbide cutting tool inserts (diameter 25mm face milling cutter).

Optimization was done using taguchis L9 orthogonal array and was fine-tuned with genetic

algorithm. The optimum machining parameters were, number of Passes = 3, Depth of cut =

0.1162mm, Spindle speed = 1999 rpm, Feed rate = 497.7 mm/min. The surface roughness


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evaluated through taguchi technique is 0.975 m and 0.88 m. It is observed that all the four

parameters are predominantly contributing to the response[1].

In face milling of hardened steel (EN 90MnCrV8) the influence of cutting parameter

(i.e., cutting speed, feed rate and depth of cut) on cutting forces studied by Milenko sekulic

using taguchi method shows that among all the significant parameters, depth of cut is the

most significant parameter [2].

An experimental investigation made to find out optimum milling parameters for

machining EN8 steel using Seco R220.53-0125-09-8c tool holder with diameter 125mm face

milling cutter shows the optimum value for face milling is Cutting speed = 285 m/min, feed

rate = 0.27 mm/rev and depth of cut = 0.4 mm (for Surface roughness of 0.690 m). Alsoauthor concludes the cutting speed is statistically significant factors influencing the surface

roughness in milling process [3].

There is growing demand for superior quality production for its functional aspects, the

surface roughness here play a significant role. An Experimental investigation was conducted

by Nitin agarwal on effect of machining parameters on the surface quality of aluminum alloy

in CNC milling operation with HSS Tool. The Spindle speed, feed rate and depth of cut was

independent variable and surface roughness parameter is taken as dependent variable. The

speed considered was 800, 1000 and 1200 rpm. Feed range from 200 to 500 in steps of 100

mm/min. Depth of cut 0.25, 0.50 and 0.75 respectively. Experiment concluded that

1) The surface roughness could be efficiently calculated by using spindle speed, feedrate and axial depth of cut as the input variables.

2) Considering the individual parameters, depth of cut has been established as mostinfluencing parameter, followed by feed rate and spindle speed.

3) As the depth of cut influences the surface roughness considerably for a given feedrate, the increase in feed rate causes the surface roughness to increase. For lower

depth of cut, the feed rate increases with surface roughness [4].

Further Mathematical Relationship (1st order and 2nd order quadratic equations using

Design expert ver. 6.0) was developed between the tool life in end milling of hard material


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(AISI -D2) and the machining variable by using the experimental results of Response surface

Methodology (RSM). These model can be safely used to predict the tool life of machined

part of AISI D2 tool steel under the specified conditions, speed range = 40 80 m/min,

Depth of cut range is 0.52mm and feed range is 0.050.1 mm/tooth [5].

Another, study on the influence of cutting conditions cutting speed, feed velocity

and feed per tooth - on tool life and surface finish of the work piece in the face milling of flat

surfaces. Aiming to achieve this goal, several milling experiments were carried out with

different cutting speeds, feed velocities and feeds per tooth. In the first phase of the

experiments, cutting speed was varied without varying feed velocity, which caused a

variation in feed per tooth. In the second phase of the experiments, cutting speed and feed

velocity were varied in such a way that feed per tooth was kept constant. Tool flank wear andsurface roughness of the work piece were measured as cutting time elapsed. The main

conclusions of this work are that a) cutting speed has a strong influence on tool life,

regardless of whether feed velocity or feed per tooth varies and b) an increase in surface

roughness of the work piece is not closely related to an increase in wear of the primary

cutting edge [6].

One of the research studied on Performances of tool life and surface roughness on AISI

D2 Steel (58 HRc) using Indexable ball nose End mills employing carbide, cermet tools and

solid carbide ball nose end mills. Author carries out experiment to find Tool Wear

Mechanism (Chipping, Adhesion and attrition) with process parameters (Tool life and

Surface Roughness) by Taguchi and ANOVA Method shows that Best parameters found for

finish machining are Cutting speed 204 m\min, depth of cut = 0.2mm and width of cut =

0.2mm. He also suggests hard machining can potentially be an alternative to grinding and

EDM with a scope to improve productivity, increased flexibility decreased capital expenses

and reduced environmental waste [7].

Other experiment conducted on the Influence of the mechanical properties like

Tensile strength and hardness of the work piece material (DIN 42CrM04 (JUS C4732)

having tensile strength of 975 MPa, Hardness=265 BHN, Cutting tool Material HM P25,


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Vc=89.17 m/min, ap=1mm) have a significant influences the cutting force in face milling

and cutting energy in machining [8].

The Influence of radial depth or radial engagement is least touched subject. Shen yang of

Tianjin university did experimental investigation on Effect of radial depth on vibration and

surface roughness in face milling of austenitic stainless steel (AISI304) using Indexable

cemented carbide milling cutters (speed and feed were fixed). The results shows the

amplitude of vibration acceleration increased with the increasing radial depth up to 80mm,

also the vibration frequency varied with the radial depth. The minimum surface finish was

found radial depth was equaled to 40mm [9].

Further, Multi-Layer Hard Coating on Cutting Tools also enhances the tool life.Studies are also done comparing the performance of different Titanium based coatings like

Titanium nitride (TiN) Titanium carbon nitride (TiCN) and Titanium Alumina nitride

(TiALN) using Taguchi method shows that TiCN hard coating has best performance (Tool

Life) among above on AISI 1045 Carbon Steel in Face Milling operation [10].

In comparison with coating performance produced by PVD (TiN-TiALN) and CVD

(TiN-Al203-TiCN) process on carbide insert for face milling operation on TC6 (Difficult to

cut, Titanium machining) work piece under the dry condition, several tool life test and tool

wear experiments were conducted using 5-axis machining center. The effect of varying

cutting speeds on cutting forces, surface roughness and chip formation was investigated.

Surface roughness had small rise by increasing cutting speed from 50 to 140 m/min. Also

increase in feed rates, keeping constant speed and depth of cut shows increase in surface

roughness. With the force fluctuation due to increase in cutting speed shows increased

surface roughness. Chip formation and its morphology are the important features of metal

cutting process. When cutting speed increases from 50 to 140 m/min the chip deformed from

curling to ribbon shape.

It was noticed that crater wear and fracture were major types of rake wear found in common.

In milling process, the rake face of cutting tool can produce dramatic friction, high

temperature and high pressure. When cutting edge is in contact with the chip or work piece


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or both, the plastic flow occurs on surface material which in detrimental to the abrasive

resistance of cutting tool.

Further from SEM investigation it is observed that the coatings on cutting edges were

removed, this is because of continual impact between rake face of cutting tool and the work

piece in milling process. Another reason is friction between coating layer and chip [11].

An study conducted to develop an optimization technique to determine the

coefficients of the extended Taylor tool life equation in milling. The best set of cutting

conditions that yield the fastest convergence for the coefficients of the extended Taylor tool

life equation and associated confidence intervals for the coefficients was determined. This

was done by obtaining the minimum ratio (NC) between maximum and minimum singular

values of the sensitivity matrix of tool life related to variation of machine parameters. Theycompared their technique to the commonly used fractional factorial technique used to

determine the coefficients of the Taylor tool life equation during dry face milling of AISI

1045 rolled steel (mean hardness of 197 HB) with triple

TiN/TiC/TiN coated carbide inserts (ISO P45-M35 class). The mean percentage error

and standard deviation between tool life values was higher for the fractional factorial

technique compared to the optimization procedure. The same study was repeated for AISI

304 stainless. However, it was found that mean percentage error between tool life estimates

obtained for AISI 304 stainless steel was 46 % compared to 10 % for AISI 1045 steel. It was

found that irregular flank wear patterns and variations in work piece material composition in

the case of AISI 304 stainless steel caused more variation in tool life estimates compared to

AISI 1045 steel [12].

Other study on effect of wear for honed radius edges shows that increase in edge

radius tends to increase in wear rate, especially at the initial cut in wear phase. The uncut

chip thickness is less than or equal to the edge radius, forces actually decreases substantially

with flank wear until most of the edge radius has been worn out [13].

Finite element method based simulation is attracting researchers for the better

understanding of the chip formation mechanism, heat generation in the metal cutting zones,


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tool-chip interfacial frictional characteristics. Prediction of temperature and stress

distribution plays a vital role in enhancing the tool life.

Study on FEM Simulation of Edge rounded insert for machining AISI 1045 steel by

using dynamic explicit arbitrary lagrangian eulerian method yields results that are highly

essential in predicting residual stresses, temperature and other property on machined surface

[14].

Study on tool chip interfacial friction properties by using analysis of machining was

carried on by several temperature models for calculating the average temperatures at primary

and secondary deformation zones and present comparisons with the experimental data

obtained for AISI 1045 steel through assessment of machining models activity. The proposed

methodology was utilized to measure forces and chip thickness obtained through a basicorthogonal cutting test. This conveniently determined the work material flow stress at the

primary deformation zone and interfacial friction characteristics along the tool rake face [15].


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Chapter 4

METHODOLOGY

Metal cutting operation, Face milling, cutting tool materials, Different mode of

cutting tool failure, parameters affecting the face milling operation, optimization methods

were studied by literature survey.

Observations were made on different milling operation and strategies used in

Kennametal metal cutting lab and production shops. Manufacturing process of tool holders,

Tungsten carbide cutting tools were documented.

Going through different catalog and journals, literature we got to know that there was

not many experiments conducted in optimizing the radial depth of cut (Ae) to achieve better

tool life and Material removal rate. Most of the experiments conducted to improve tool life

were by optimizing the

a). machining parameters like Feed, Speed and Axial depth of cut

b). by optimizing the cutting tool geometry

c). by varying different types of coatings

This made us to conduct an experiment to achieve optimum tool life and reduce

production cost by optimizing the machining parameters considering radial depth of cut.

4.1 Design of Experiment

The Four variable used for the design of experiments are cutting speed, feed, axial

depth of cut and radial depth of cut. The experiment is carried under dry condition.

The minimum cutting speed (Vc) of 170 m/min is taken as constant, the effective feed

per tooth (fz) or chip load is 0.16 mm and axial depth of cut (Ap) is kept constant to 3mm

throughout the experimentation. The only parameter varied is Radial depth of cut which

contributes more to material removal rate. The radial depth of cut (Ae) is varied at 20%, 50%

and 80% of cutting diameter and its influence on power consumption, surface roughness

(Ra), Material removal rate and tool wear.


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Fig. 4.1: Flow chart of Experimentation

Developing a milling

strategy for optimum

tool life

Finite Element Analysis

Selection of Cuttin Tool

Selection of work piece

Pre-processor

Work piece modeling

Enter Process parameter

Simulation

Post Process

Experimentation

Prepare insert (Edge honing)

Hc Test, Check Honed

Radius, Laser Marking,

Edge check.

Lab Test (Machining)

Results and Discussion

Conclusion


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Chapter 5

EXPERIMENTATION

The lab experimentation was conducted in Kennametals metal cutting lab and Finite

element analysis was carried on Third Wave AdvantEdge 2D at CAE team, Kennametal.

5.1 Selection of Cutting tools

Manufacturer: Widia

Cutter: M690

Diameter: 63mm

Insert: SDMT 1204 PDR-MH

Coating: TN7535

Suitable for machining C45 (AISI 1045) carbon steel.

Table 5.1:, Insert ISO Designation

S D M T

ShapeClearanceangle Tolerance Features

Square 15 degree 0.06 mm

12 04 PDR MH

Size ThicknessPositiveDegree,Right Hand

Positive geometry and

stable cutting edges,problem free machiningof a wide range ofmaterials12.7 mm 4 mm


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Fig.5.1: Selection of milling insert for M690 cutter. (From widia catalog pg. 278)


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Table 5.2: Insert Grade Description (From widia catalog pg. 327)

Table 5.3: Selection of feed per tooth and Cutting speed (From widia catalog pg. 327)

Select P-2 for C45 (AISI 1045) material

Feed per tooth (fz, mm) = 0.1 to 0.3

Cutting Speed (Vc, m/min) = 165 to 250

Insert Grade Grade Description

TN 7535HC-P35

Coated Carbide Insert,MT-CVD/CVD processwith TicN-Al203-TiNmultilayer coating forLight and Mediummachining for steels andnodular cast iron

fz

Vc


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5.2 Work piece

Table 5.4: Work piece material specification

5.2.1 Dimensions of the work pieces

Fig.5.2: Work piece (modeled and drafted in NX6)

Material Size: 150 x 150 x 300 mm

Work piece Material C45 steel (DIN)

Equivalent AISI 1045 steel

Hardness 201 BHN

Chemical composition

C 0.43

Si0.4

(maximum)

Mn 0.5

Cr+Mo+Ni0.63

(maximum)


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5.2.2 Work piece Hardness Test

(a)

(b) (c)

Fig.5.3: a) Equo tip Portable hardness tester b) Hardness measuring c) Ball indenter d)

Measured hardness value (courtesy: Kennametal Inc.)

(d)

Hardness was checked in two different places using portable equo tip instrument, the

brinells hardness value was observed to be in range of 183 to 201 BHN. We consider the

higher hardness number i.e. 201 BHN


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5.3 Vertical Milling Center Specification

Fig.5.4: Vertical Machining Center (from Mazak catalog)

Table 5.5: Machine specification

Manufacturer Mazak

Model FJV-200

Type Vertical Machining Center

Control Mazatrol 640M

Maximum Power 22 KW

Maximum RPM 12000 rpm

Maximum Travel X= 550mm, Y= 400mm, Z= 400mmWork holding Machine Vice

ATC 40

Table specification Width= 450mm, Length = 800mm


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5.4 Pre-Metal Cutting Test

5.4.1 Edge preparation

Edge preparation type: Radius (Honed)

Table 5.6: Edge round value check

Insert 1 Radius in mm

Edge-1 0.054

Edge-2 0.0548

Edge-3 0.058

Edge-4 0.059

Insert 2 Radius in mm

Edge-1 0.0572Edge-2 0.0616

Edge-3 0.0606

Edge-4 0.0592

Fig.5.5: Edge Hone radius measurement unit (courtesy: Kennametal Inc.)


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Fig. 5.6: Honed radius measuring setup (courtesy: Kennametal Inc.)

Insert hone radius is checked in Kennametal facility to ensure that the manufactured

value lies within limits. All Insert were well within limit.

5.4.2 Weight of the Insert

Fig.5.7: Insert weight (courtesy: Kennametal Inc.)

Initial Insert weight was measured using precision weighing machine, the measured

weight was observed to be 6.991 gms.

5.4.3 Coercive Field Strength

Coercive field strength Hc is the necessary force required to completely demagnetize

a magnet, Higher the number the better the magnetism property.

Coercivity is usually measured in oersted or ampere/meter units and is denoted by Hc.

Coercive field strength is also called as the magnetic field strength

Our specimen (SDMT1204-PDR-MH) has Coercive field strength (Hc)=147.3 oe.

Insert

Measuring Probe

Insert


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Coercive force is within the range.

Fig. 5.8: Foerster koerzmat HCJ meter (Coercive field strength measurement device)

5.4.4 Laser Marking of Insert

Fig.5.9: Laser Marking Machine (courtesy: Kennametal Inc.)

Laser marking is done to identify the cutting edges.

Insert


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Assembling Insert (SDMT1204 PDR-MH) to milling tool holder (M690)

Fig.5.10: Assembly of insert with cutter (courtesy: Kennametal Inc.)

Assembly of the insert to the cutter is done using the specified hardware parts. One

more insert is used as backup insert to avoid accident due to failure of primary insert.

Fig. 5.11: Mitutoyo Surface roughness testing device (tally surf) (courtesy: Kennametal Inc.)

Device specification

Measuring range : 12.5mm

Measuring speed : 0.25 mm/s

Traversing direction : Backward Detector

Detecting method : Skid measurement (differential inductance)


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5.4.5 Initial Images

Fig. 5.12: Optical Microscope (courtesy: Kennametal Inc.)

The Ram opticals Sprint MVP 200 optical microscope is used to measure wear.

Optical microscope comes with motorized precision XYZ stages, high resolution zoom

optics, color metrology camera, LED coaxial light and Measure-X metrology software.

Fig. 5.13: Optical measuring setup to measure wear (courtesy: Kennametal Inc.)

The magnification used for measurement is 50x.


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Insert 1. Initial images of Edge-1

Facet (crater face) Flank face

Nose Radius Rake Face

Initial images of Edge-2



Fig. 5.14: Insert1- Initial images of edge -1 and edge-2 (magnification 50x) (courtesy:

Kennametal Inc.)


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Insert 2, Initial images of Edge-2



Initial images of Edge-4



Fig. 5.15: Insert2- Initial images of edge -2 and edge-4 (magnification 50x) (courtesy:

Kennametal Inc.)

Prior to running the experiment on machining, each cutting insert was wiped down

with cleaning agent and Initial images are captured in optical microscope to compare the

image with subsequent images and also to ensure that the edges are free from defects.


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Chapter 6

FINITE ELEMENT ANALYSIS

Finite element method based modeling and simulation is attracting researchers for

better understanding. We have used Third wave AdvantEdge software tool for Finite element

analysis purpose.

6.1 Third wave AdvantEdge

Third wave AdvantEdge from Minneapolis is a special program written for

machining simulations. This is developed based on the dynamic explicit lagrangian

formulation. It is the ideal tool for the companies that manufacture and design cutting tools

for the metalworking industry. The model is built by selecting the type of machining

operation and defining the necessary process parameters. Since we are focusing on face

milling operation the process parameter are feed, spindle speed, axial depth of cut, radial

depth of cut, length of cut and initial temperature of the work piece.

The model created by Third Wave AdvantEdge is also thermo-mechanically coupled.

In Third Wave AdvantEdge, a staggered procedure is adopted for the purpose of coupling the

thermal and mechanical equations. Geometrically identical meshes for the thermal and

mechanical models are used. Mechanical and thermal computations are staggered, assuming

the constant Temperature during the mechanical step and constant heat generation during the

thermal step.

A mechanical step is taken first based on the current distribution of temperature, and

the heat generated is computed from plastic working and frictional heat generation. The heat

thus computed is transferred to the thermal mesh and the temperatures are recomputed by

recourse to the forward-Euler algorithm. The resulting temperatures are transferred to themechanical mesh and incorporated into the thermal-softening model, which completes one

time stepping cycle.

Certain assumptions are made to simulate the complex procedure of metal cutting with

FEM as listed below. These assumptions are used to define the problem to be solved as well

as to apply the boundary and loading conditions:


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a) The cutting speed is constant.b) The cutting velocity vector is normal to the cutting edge.c) The work piece material is a homogeneous polycrystalline, isotropic and

incompressible solid.

d) The work piece is set at a reference temperature of 20 C at the beginning of thesimulation.

e) The machine tool is perfectly rigid and no influence of machine tool dynamics onmachining is considered.

f) The friction is constant at tool-chip interaction and tool-work piece interaction.

The model created by Third Wave AdvantEdge is also thermo-mechanically coupled.

In Third Wave AdvantEdge, a staggered procedure is adopted for the purpose of coupling thethermal and mechanical equations. Geometrically identical meshes for the thermal and

mechanical models are used. Mechanical and thermal computations are staggered, assuming

the constant Temperature during the mechanical step and constant heat generation during the

thermal step.

A mechanical step is taken first based on the current distribution of temperature, and the heat

generated is computed from plastic working and frictional heat generation. The heat thus

computed is transferred to the thermal mesh and the temperatures are recomputed by

recourse to the forward-Euler algorithm. The resulting temperatures are transferred to the

mechanical mesh and incorporated into the thermal-softening model, which completes one

time stepping cycle.

Certain assumptions are made to simulate the complex procedure of metal cutting with

FEM as listed below. These assumptions are used to define the problem to be solved as well

as to apply the boundary and loading conditions:

g) The cutting speed is constant.h) The width of cut is larger than the feed (plane strain condition), and both are constant.i) The cutting velocity vector is normal to the cutting edge.j) The work piece material is a homogeneous polycrystalline, isotropic, and

incompressible solid.

k) The work piece is set at a reference temperature of 20 C at the beginning of thesimulation.


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l) The machine tool is perfectly rigid and no influence of machine tool dynamics onmachining is considered.

m)The friction is constant at tool-chip interaction and tool-work piece interaction.

The finite deformation formulation used in Third Wave AdvantEdge incorporates the

hybrid triangular elements for spatial discretization. The element has three corners. The

separation of nodes, thus forming the chip from the work piece during a cutting simulation, is

achieved by continuous re-meshing. Therefore, during the metal-cutting process, the work

piece material is allowed to flow around the cutting tool edge and when the elements in the

vicinity become distorted, Third Wave AdvantEdg

Documents

DEVELOPMENT OF MILLING STRATEGY FOR OPTIMUM TOOL LIFE AND PRODUCTION COST.pdf