Cutting Tool Materials and Cutting Fluids by Dr. Oğuzhan YILMAZ

7/27/2019 Cutting Tool Materials and Cutting Fluids by Dr. Ouzhan YILMAZ

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Cutting-tool Materials & Cutting

Fluids

Dr. Ouzhan YILMAZ (Assoc.Prof.)

Room: 319

[email protected]


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Introduction

The selection of cutting tool materials is the most important factors in machiningoperations.

Cutting tools are generally recommended. There is no tool that fits exactly.

In the selection of cutting tool materials, the followings should be bear in mind:

Cutting tools are subject to high temperatures

High contact stresses

Rubbing along the tool-chip interface and machined surface

Cutting tool materials must possess the following characteristics:

- Hot Hardness: the cutting tool should maintain its hardness, strength and wearresistance at the high temperatures during machining.

- Toughness and impact strength: Cutting tools must resist the impact forces dueto interrupted cutting (milling or some turning operations) and forces due to

vibration and chatter without chipping or fracture.- Thermal shock resistance: Cutting tools should withstand the rapid temperature

cycling in interrupted cutting.


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Introduction

- Wear resistance: An acceptable tool life is

obtained before replacement is necessary.

- Chemical stability and inertness: Cutting

tool is to avoid or minimize any adverse

reactions, adhesions and tool-chip diffusion

that would contribute to tool wear.

Properties for determining desirable tool-

material characteristics:

Hardness and strength (wrt wp material)

Impact strength (interrupted cut, milling)

Melting temperature (tool material)

Physical properties (thermal conductivity,

thermal expansion)

Figure 1. Hot hardness variations

for different cutting tool materials


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Introduction

The cutting tool materials (listed according to developing and implementing order):

1. High-speed steels

2. Cast-cobalt alloys

3. Carbides

4. Alumina-based ceramics

5. Cubic-boron nitride

6. Silicon-nitride-based ceramics

7. Diamond

8. Whisker-reinforced materials and nano-materials

Carbon steels are the oldest tool materials and they have been used widely for drills,taps, broaches, and reamers.

They are inexpensive, easily sharpened, not have sufficient hot hardness, wear resistancefor high cutting speeds and hard materials. They are not preferred in modernmachining operations.


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Introduction


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Introduction


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Introduction


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High-speed Steel (HSS)

HSS tools were developed to machine at higher speeds.

HSS are the most highly alloyed of the tool steels.

They are hardened in various depth in order to have a good wear resistance.

Due to their toughness (high resistance to fracture), HSS are suitable for:

High positive rake angle tools

Interrupted cuts

Vibration and chatter due to low machine tool stiffness

Complex and single piece tools (drills, reamers, taps, and gear cutters)

HSS has lower hot hardness and theircutting speed are low compared to carbide tools.

The are two basic types of HSS:

- Molybdenum (M-series): %10Mo, Cr, V ,W and Co

- Tungsten (T-series): 12 to 18%W, Cr, V and CoM-series has higher abrasion resistance and less expensive. 95% HSS are M-series.

HSS are available in rolled, forged, cast, sintered (pm), coated, surface treated


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Cast-cobalt alloys

Composition: 53% Co, 30-33% Cr, and 10-20 W

They have good hardness and resist elevated temperatures.

They are not as tough as HSS and sensitive to impact forces.

They are used for special operations, involving deep, continuous roughing cuts at high

feeds and speeds.


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Carbides

Cemented and sintered carbides were introduced to meet the challenge for increasingly

higher cutting speed (not possible using with HSS and cast-cobalt tools)

Carbides have high hardness over a wide range of temperatures, high elastic modulus,

high thermal conductivity, and low thermal expansion.

Therefore, they are among the most important, versatile, and cost-effective tool and die

materials for a wide range of applications.

1. Tungsten Carbide (WC):

- Tungsten carbide particles bonded together in a cobalt matrix

- They are manufactured bypowder metallurgy technique (sintered)

- They are used for cutting steels, cast irons, and abrasive nonferrous materials.

Micro grain Carbides and Functionally Graded Carbides

2. Titanium Carbide (TiC): consists of a nickel-molybdenum matrix. It has higher wear

resistance than WC but not as tough.

TiC can machine hard materials(steels and cast irons) and at higher cutting speed than WC.


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Inserts

Inserts are individual cutting tools with several

cutting points.

A square insert has eight cutting points, and a

triangular insert has six.

Inserts are usually clamped on the tool holder with

various locking mechanisms.

Each inserts has a number of cutting points and after

one edge is worn, it is indexed (rotated in its holder)to make another cutting point available. Quick

insertion and removal features are available.

Carbide inserts are available in a variety of shapes,

such as square, triangle, diamond and round.

The strength of the cutting edge of an insert dependson its shape.

The smaller the included angle, the lower the

strength of the edge.

Typical inserts with chip-breaker

features.

Figure 3 Methods of attaching inserts to toolholders:

(a) Clamping, and (b) Wing lockpins. (c)Examples of

inserts attached to toolholders with threadless

lockpins, which are secured with side screws


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Inserts

The smaller the included angle, the lower the strength of the edge.

Figure 4 Relative edge strength and tendency for

chipping and breaking of insets with various

shapes. Strength refers to the cutting edge shown

by the included angles.

Figure 5 Edge preparation of inserts

to improve edge strength

Chip-breaker features on inserts are for the

purpose of;

(a) Controlling chip flow during machining

(b) Eliminating long chips

(c) Reducing vibration and heat generated

The selection of a particular chip-breakerfeature depends on the feed, depth of cut,

workpiece material, type of chip produced


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Classification of Carbides Carbides grades are classified using the letters P,M and Kfor a range of applications.


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Classification of Carbides


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Coated Tools Coatings on cutting tools have such

properties:

Lower friction

Higher adhesion

Higher resistance to wear and cracking

Acting as a diffusion barrier

Higher hot hardness and impact resistance

Coated tools can have lives 10 times longer

than those of uncoated tools, allowing for

high cutting speeds and thus reducing both

the time requiring for machining operations

andproduction costs.

Coated tools are used in 40% to 80% of all

machining operations (turning, milling,drilling)

Decreasing machining time also leads to

decreasing machining cost.

Figure Development of cutting tool materials

and improvement cutting speed year-by-year.


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Coating materials and coating methods

Commonly used coating materials are:

Titanium nitride (TiN)

Titanium carbide (TiC)

Titanium carbonitride (TiCN)

Aluminum oxide (Al2O3)

These coatings generally in the thickness range from 2 to 15 m, are applied on

cutting tools and inserts by two-techniques:

(1) Chemical-vapor deposition (CVD)(2) Physical-vapor deposition (PVD)

The CVD process is the most commonly used method for carbide tools with

multiphase and ceramic coatings.

PVD-coated carbides with TiN coatings have higher cutting edge strength, lowerfriction and lower tendency to form built-up edge.


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Coating materials and coating methods

Coatings forcutting tools and dies should have the following general characteristics:

High hardness at elevated temperatures

Chemical stability and inertness to the workpiece materials

Low thermal conductivity to prevent temp rise in the substrate

Compatibility and good bonding to the substrate

Little or no porosity in the coating, to maintain its integrity and strength

The effectiveness of coatings is enhanced by the hardness, toughness and high thermal

conductivity of the substrate (carbide or HSS)

Titanium-nitride Coatings (gold color): It provides low friction coefficient, high

hardness, resistance to high temp and good adhesion to the substrate.

They improve life of HSS and carbide, drill bits and cutter.

They perform well at higher cutting speeds and feeds.


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Coating materials and coating methods Titanium-carbide coatings: are on tungsten carbide inserts which have high-flank

wear resistance in machining abrasive materials.

Ceramic coatings: Because of their chemical inertness, low thermal conductivity,resistance to high temperature, and resistance to flank and crater wear. The mostcommonly used ceramics is aluminum oxide (Al2O3)

Multiphase coatings: Carbide tools are now available with two or three layers of suchcoatings and particularly effective in machining cast irons and steels.

For example, one could first deposit TiC over the substrate, followed by Al2O3 and then

TiN. The first layer should bond well, the outer layer should resist wear and have lowthermal conductivity and intermediate layer should bond well and be compatible bothlayers.

Typical applications of multiple-coated tools are as follows:

1. High-speed, continuous cutting: TiC/ Al2O3

2. Heavy-duty, continuous coating: TiC/ Al2O3 /TiN

3. Light, interrupted cutting: TiC/TiC+TiN/TiN

The thickness of these layers is on the order of2 to 10m. Thinner coatings increasehardness with decreasing grain size. Thin layers are harder than thick layers.


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Coating materials and coating methods A typical multi-phase-coated carbide tool may consists of the following layers, starting

from the top, along with their primary function:

1. TiN: low friction2. Al2O3 : high thermal stability

3. TiCN: fiber reinforced with a good balance of resistance to flank wear and crater wear,

particularly for interrupted cutting.

4. A thin-carbide substrate: high fracture toughness

5. A thick-carbide substrate: hard and resistant to plastic deformation at high temperatures.

Figure 7 Multiphase coatings on a tungsten-carbide

substrate. Three alternating layers of aluminum

oxide are separated by very thin layers of titanium

nitride. Inserts with as many as thirteen layers of

coatings have been made. Coating thicknesses are

typically in the range of 2 to 10 m.


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Coating materials and coating methodsDiamond Coatings: Polycrystalline diamond is being used widely as a coating for cutting

tools, particularly on tungsten-carbide and silicon-nitride inserts.

They are effective in machining nonferrous metals, abrasive materials such asaluminum alloys containing silicon, fiber-reinforced and metal-matrix composite

materials, and graphite.

Diamond-coated inserts: Thin films are deposited on substrates through PVD or CVD

techniques.

Thick films are obtained by growing a large sheet ofpure diamond, which is then lasercut to shape and brazed to a carbide insert.

Multilayer nanocrystal diamond coatings are used to give strength to the coating.

Good adherence of the diamond film to the substrate and minimize the difference in

thermal expansion between diamond and substrate materials.


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Coating materials and coating methodsMiscellaneous Coating Materials:

- Titanium carbonitride (TiCN) and Titanium-aluminum nitride (TiAlN) are effective in

cutting stainless steels.- TiCN is harder and tougher than TiN and used on carbides and HSS.

- TiAlN is effective in machining aerospace alloys

- Chromium carbide (CrC) is used for cutting softer metals that tend to adhere to the

cutting tool, such as aluminum, copper and titanium

- Zirconium Nitride (ZrN)- Hafnium Nitride (HfN)


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Alumina-based Ceramics Ceramic tool materials consist primarily offine-grained, high purity aluminum oxide.

They are cold pressed into insert shapes underhigh pressure and sintered at high

temperatures. Additions oftitanium carbide and zirconium oxide help improve properties such as

toughness and thermal shock resistance.

Alumina-based ceramic tools have very high abrasion resistance and hot hardness.

Chemically, the are more stable than HSS and carbides, so they have less tendency to

adhere to metals and lower tendency to form a build-up edge. Ceramics lack toughness, and their use may result in premature tool failure by

chipping or in catastrophic failure.

Ceramic inserts are effective in high-speed, uninterrupted cutting operations.

Ceramic tool shape and set-up are important. Negative rake angles (large included

angles) generally are preferred in order to avoid chipping due to the poor tensilestrength of ceramics.


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Cerments (ceramic and metal) They consist ofceramic particles in a metallic matrix.

A typical cermet consists of70% aluminum oxide and 30% titanium carbide; other

cermets contain molybdenum carbide, niobium carbide, and tantalum carbide.

Cermets have chemical stability and resistance to built-up edge formation.

Brittleness and high cost are limitations.

Figure 8 Ranges of properties for

various groups of tool materials.


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Cubic Boron Nitride

Cubic boron nitride (cBN) is the hardest material available.

cBN is made by bonding a 0.5 to 1 mm layer of polycrystalline cubic boron nitride to a

carbide substrate by sintering under high pressure and high temperatures.

While the carbide provides shock resistance the cBN layerprovides very high wear

resistance and cutting edge strength

At elevated temperatures, cBN is chemically inert to iron and nickel (no wear due to

diffusion)

Because cBN tools are brittle, stiffness of the machine tool and fixturing is importantto avoid vibration and chatter.

Figure 9 Construction of a

polycrystalline cubic boron nitride

or a diamond layer on a tungsten-

carbide insert

Figure 10 Inserts with polycrystalline

cubic boron nitride tips (top row) and

solid polycrystalline cBN inserts (bottom

row).

Di d


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Diamond

Of all known materials, the hardest substance is diamond.

As a cutting tool, it has highly desirable properties, such as low friction, high wear

resistance, and the ability to maintain a sharp cutting edge.

Diamond is used when a good surface finish and dimensional accuracy are required,

particularly with soft nonferrous alloys and abrasive nonmetallic and metallic

materials.

Synthetic or industrial diamonds are widely used.

Because diamond is brittle, tool shape and sharpness are important. Low rake angles are generally used to provide strong cutting edge.

Wear may occur through microchipping (caused by thermal stresses and oxidation) and

through transformation to carbon.

Diamond tools can be used satisfactorily at almost any speed, but are most suitable for

light, uninterrupted finishing cuts. Diamond is not recommended for machining plain-carbon steels or titanium, nickel,

and cobalt-based alloys.

Diamond is also used as an abrasive in grinding and polishing operations and coatings.


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Whisker-reinforced Materials and Nanomaterials

In developing new tool materials, the followings are concered:

- High fracture toughness

- Resistance to thermal shock

- Cutting-edge streghth

- Creep resistance

- Hot hardness

Whiskers as reinforcing fibers in composite cutting tool materials

Examples of whisker-reinforced cutting tools include:

(a) silicon-nitride based tools

(b) Aluminum-oxide based tools

Suitable nanomaterials are carbides and ceramics.Nanomaterials are applied as a thin coating and able to machine at higher speeds

T l C d R di i i f T l


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Tool Costs and Reconditioning of Tools

Tool costs vary widely, depending on the tool material, size, shape, chip breaker

features and quality.

Tooling costs in machining have been estimated to be on the order of2 to 4 % of themanufacturing costs.

Cutting tools can be reconditioned by resharpening them, using tool and cutter

grinders with special fixtures.

Reconditioning of coated tools is also done by recoating them.


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Cutting Fluids Cutting fluids have been used extensively in machining operations to achieve the

following results:

(a) Reduce friction and wear, thus improving tool life and the surface finish(b) Cool the cutting zone, thus improving tool life and reducing the temperature and

thermal distortion of the wp

(c) Reduce forces and energy consumption

(d) Flush away the chips from the cutting zone

(e) Protect the machined surface from environmental corrosion.

Cutting fluid could be a coolant, a lubricant or both.

The effectiveness of cutting fluid depends on:

(1) Type of machining operation

(2) Tool and workpiece materials

(3) Cutting speed

(4) Method of application


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Cutting Fluids Water: excellent coolant, but not a lubricant. Results oxidation.

Cutting fluids are used for turning, milling, drilling, gear cutting, thread cutting,

tapping, and internal broaching. In some cases, cutting fluids may cause: curly chip which lead to heat concentration

on tool tip, reducing tool life, thermal cycling (interrupted cutting) and thermal

cracks.

Cutting fluid has a capillary action that seeping the tool-chip interface.

T f C tti Fl id


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Types of Cutting Fluids

Four general types of cutting fluids are commonly used in machining operations:

(1) Oils: mineral, animal, vegetable, compounded, and synthetic oils, typically used for

low-speed operations where temp rise is not significant.(2) Emulsions (soluble oils): a mixture of oil and water and additives, generally used for

high-speed operations. Water makes it effective coolant. Oil eliminates the oxidation

and supply lubrication.

(3) Semisynthetics are chemical emulsions containing little mineral oil, diluted in water,

and with additives that reduce the size of oil particles, making them more effective.(4) Synthetics: are chemicals with additives, diluted in water and containing no oil.

Methods of Cutting fluid Application


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Methods of Cutting-fluid Application

There are four basic methods of cutting fluid applications in machining:

(1) Flooding: flow rates are from 10L/min for single-point tools to 225L/min per cutter

for multiple-tooth cutters as in milling. For drilling and milling, flushing pressure is700 to 14000 kPa.

(2) Mist: cooling of inaccessible areas. It is effective with water-based fluids at air

pressures from 70 to 600kPa.

(3) High-pressure systems: Delivering cutting fluid via specially designed nozzles that

aim a powerful jet of fluid to the zone, into the clearance or relief face of the tool.

Pressure is 5.5 to 35MPa. Used in high-speed applications and CNC machines

(4) Through the cutting tool system: Cutting fluids is passed through the passages

produced in the cutting tool.

The aim is to supply fluid into cutting zone for specific machining operations

(a) gun drilling: a long small hole through the body of the drill

(b) boring bars: a long hole through the shank (tool holder)

Flooding applications


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Flooding applicationsFigure 11 Schematic illustration of

proper methods of applying

cutting fluids in various

machining operations: (a) turning,(b) milling, (c) thread grinding,

and (d) drilling

Mist Cooling

Effects of Cutting Fluids


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Effects of Cutting Fluids

The selection of a cutting fluid, the followings should also be considered:

Workpiece material and machines tool

Biological considerations

The environments

Cutting fluids containing sulfurshould not be used with nickel-based alloys

Fluids containing chlorine should not be used with titanium

Machined parts should be cleaned and washed to remove any cutting fluid residue.

Since additional process can easily be applied, such as coating, painting, welding,

brazing etc. See surface cleaning section 34.16

Operator health concern due to mist, fumes, smoke and odors of cutting fluids.

Cutting fluid also environmental effects after used

Proper techniques should be used forfiltering cutting fluids to be reused

Disposal of cutting fluids must obey environmental laws of the government.

Cutting Tool Materials and Cutting Fluids by Dr. Oğuzhan YILMAZ

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