cutting tool materials and cutting...

Cutting Tool Materials and Cutting Fluids

Dr. Mohammad Abuhaiba

HomeWork #2 22.37 obtain data on the thermal properties of various

commonly used cutting fluids. Identify those which are basically effective coolants and those which are basically effective lubricants.

Due Saturday 27/3/2010


Case Study 2 Contact several different suppliers of cutting tools, or

search their websites. Make a list of the costs of typical cutting tools as a function of various sizes, shapes, and features.

Due Saturday 27/3/2010


IntroductionCutting Tool Characteristics:

1. Maintaining hardness, strength, and wear resistance at elevated temperatures

2. Toughness3. Wear resistance4. Chemical stability

Tool Materials Categories:1. High-speed steels2. Cast-cobalt alloys3. Carbides4. Coated tools5. Alumina-based ceramics6. Cubic boron nitride7. Silicon-nitride-base ceramics8. Diamond9. Whisker-reinforced materials


IntroductionCutting Tool Material Hardnesses Table 22.1 and 22.2:

General Characteristics of Tool Materials

Table 22.3: General operating characterstics of cutting tool materials


HIGH SPEED STEELS Good wear resistance, relatively inexpensive Because of their toughness and high resistance to fracture,

HSS are especially suitable for: high +ve rake-angle tools interrupted cuts machine tools with low stiffness that are subjected to

vibration and chatter.

HSS tools are available in wrought, cast, and sintered forms They can be coated for improved performance HSS tools may also be subjected to:

surface treatments for improved hardness and wear resistance steam treatment at elevated temperatures to develop a black

oxide layer for improved performance


HIGH SPEED STEELS Two basic types of HSS:

1. Molybdenum (M series) Up to about 10% Mo, with Cr,Vn, W, Co as alloying

elements

2. Tungsten (T series) 12% -18% W, with Cr, Vn, and Co as alloying elements

M series generally has higher abrasion resistance than T series, undergoes less distortion during heat treating, and is less expensive

Example 22.1: List the major alloying elements in HSS and describe their effects in cutting tools


CAST-COBALT ALLOYS 38%-53% Co, 30%-33% Cr, and 10%-20%W High hardness, good wear resistance, can maintain

their hardness at elevated temperatures They are not as tough as HSS and are sensitive to

impact forces

Stellite Tools These alloys are cast and ground into relatively simple

tool shapes. used only for special applications that involve deep,

continuous roughing cuts at relatively high feeds and speeds, as much as twice the rates possible with HSS


CARBIDES Hardness over a wide range of temperatures. high elastic modulus and thermal conductivity. low thermal expansion.

Tungsten carbide (WC): Composite material consisting of WC particles

bonded together in a cobalt matrix Manufactured with powder-metallurgy techniques WC particles, 1-5 μm in size As Co content increases, the strength, hardness, and

wear resistance of WC decrease, while its toughness increases because of the higher toughness of cobalt


CARBIDESTitanium Carbide (TiC): Higher wear resistance than WC but is not as

tough

With a nickel-molybdenum alloy as the matrix, TiC is suitable for machining hard materials, mainly steels and cast irons, and for cutting at speeds higher than those for WC.


CARBIDES - Inserts Individual cutting tools

with several cutting points

A square insert has 8 cutting points

Figure 21.2: Typical carbide inserts with various shapes and chip-breaker features The holes in the inserts are

standardized for interchangeability


CARBIDES - Insert AttachmentFigure 21.3 Methods of attaching inserts to toolholders:

a. Clamping

b. Wing lockpins

c. Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws

d. Insert brazed on a tool shank


CARBIDES - Edge Insert Strength

Insert shape affects strength of cutting edge

To further improve edge strength and prevent chipping, all insert edges are usually honed, chamfered, or produced with a negative land.


CARBIDES – General notes

Stiffness of the machine tool is of major importance when using carbide tools

Light feeds, low speeds, and chatter are detrimental because they tend to damage the tool's cutting edge.

Light feeds, for example, concentrate the forces and temperature closer to the edges of the tool, increasing the tendency for the edges to chip off

Low cutting speeds tend to encourage cold welding of the chip to the tool

Cutting fluids, if used to minimize heating and cooling of the tool in interrupted cutting operations, should be applied continuously and in large quantities.


CARBIDES – Classification

Table 22.4: ISO classification of carbide cutting tools according to use

Table 22.5: Classification of Tungsten Carbides according to machining applications


COATED TOOLS Because of their unique properties, such as lower

friction and higher resistance to cracks and wear, coated tools can be used at high cutting speeds, reducing both the time required for machining operations and costs.

Coated tools can have tool lives 10 times longer than those of uncoated tools.


COATED TOOLSEffect of Coating Materials Because of their

unique properties, such as lower friction and higher resistance to cracks and wear, coated tools can be used at high cutting speeds, reducing both the time required for machining operations and costs.

Coated tools can have tool lives 10 times longer than those of uncoated tools.


COATED TOOLS - Coating MaterialsCoatings thickness of 2-15 μm, are applied on cutting tools and inserts by the following techniques:

1. Chemical-vapor deposition (CVD), including plasma-assisted CVD2. Physical-vapor deposition (PVD)

Coatings for cutting tools, as well as dies, should have the following general characteristics:

1. High hardness at elevated temperatures2. Chemical stability to the workpiece material3. Low bonding to the substrate to prevent flaking or spalling4. Little or no porosity

Honing of the cutting edges is an important procedure for the maintenance of coating strength; otherwise, the coating may peel or chip off at sharp edges


COATED TOOLS - Coating MaterialsTitanium Nitride coating (gold in color):

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

perform well at higher cutting speeds and feeds

Flank wear is significantly lower than that of uncoated tools

do not perform as well at low cutting speeds because the coating can be worn off by chip adhesion

Titanium Carbide coatings: on tungsten-carbide inserts have high flank-wear

resistance in machining abrasive materials


COATED TOOLS - Coating MaterialsCeramics Coatings:

Chemical inertness

Low thermal conductivity

Resistance to high temperature

Resistance to flank and crater wear

Most commonly used ceramic coating aluminum oxide (Al2O3). However oxide coating generally bond weakly to the substrate.


COATED TOOLS - Coating MaterialsMultiphase Coatings:

Carbide tools with 2 or 3 layers of such coatings.

Particularly effective in machining cast irons and steels.

Typical applications of multiple-coated tools:

High-speed, continuous cutting: TiC/Al2O3.

Heavy-duty, continuous cutting: TiC/Al2O3/TiN.

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


COATED TOOLS - Coating MaterialsMultiphase Coatings: Figure 21.7

Multiphase coatings on TiC substrate. 3 alternating layers

of Al2O3 are separated by very thin layers of TiN

Inserts with as many as 13 layers of coatings

Coating thickness: 2 to 10 μm.


COATED TOOLS - Coating MaterialsMultiphase Coatings:

Functions of coatings:

1. TiN: low friction

2. Al2O3: high thermal stability

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

4. A thin carbide substrate: high fracture tougness

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


COATED TOOLS - Coating MaterialsDiamond-Coated Tools:

Thin films are deposited on substrates with PVD and CVD techniques.

Thick films are obtained by growing a large sheet of pure diamond, which is then laser cut to shape and brazed to a carbide shank.

Diamond-coated tools are particularly effective in machining nonferrous and abrasive materials, such as Al alloys containing Si, fiber-reinforced and metal-matrix composite materials, and graphite.


ALUMINA-BASED CERAMICS Consist primarily of fine-grained, high-purity Al2O3. They are cold-pressed into

insert shapes under high pressure and sintered at high temp; the end product is referred to as white, or cold-pressed, ceramics.

Additions of TiC and ZrO help improve toughness and thermal-shock resistance. Alumina-based ceramic tools have very high abrasion resistance and hot

hardness. More stable than HSS and carbides, so they have less tendency to adhere to metals

during cutting leading to lower tendency to form a BUE. Consequently, in cutting cast irons and steels, good surface finish is obtained with

ceramic tools. Ceramics lack toughness, and their use may result in premature tool failure by

chipping or catastrophic failure. Effective in high-speed, uninterrupted cutting operations. -ve rake angles are preferred in order to avoid chipping. Tool failure can be reduced by increasing stiffness & damping capacity of machine

tools, mountings, & workholding devices, thus reducing vibration and chatter.


CUBIC BORON NITRIDE (CBN) made by bonding 0.5-1-mm

layer of polycrystalline CBN to a carbide substrate by sintering under pressure

CBN tools are also made in small sizes without a substrate

Figure 21.9 Construction of a polycrystalline CBNor a diamond layer on a TiC insert


CUBIC BORON NITRIDE (CBN) Because CBN tools are brittle, stiffness of machine tool and

fixturing is important in order to avoid vibration and chatter

to avoid cracking due to thermal shock, machining should generally be performed dry, particularly in interrupted cutting operations such as milling.

Figure 21.10 Inserts with polycrystalline CBN tips (top row) and solid polycrystalline CBN inserts (bottom row)


SILICON-NITRIDE BASED CERAMICS Consist of SiN with various additions of Al2O3, yttrium

oxide, and TiC Toughness, hot hardness, and good thermal-shock

resistance.

An example of a SiN-base material is sialon, composed of : Si, Al, On, and N It has higher thermal-shock resistance than silicon

nitride recommended for machining cast irons and nickel-

based super-alloys at intermediate cutting speeds

Because of chemical affinity to iron, SiN-based tools are not suitable for machining steels


DIAMOND Low friction High wear resistance Ability to maintain sharp edge Used when good surface finish and dimensional

accuracy are req. (soft non-ferrous & abrasive non-metallic materials)

Low rack angles are generally used > strong cutting edge

Used at high speed Most reasonable for light uninterrupted finishing cut Diamond is not recomm for mach plain carbon steels

or titanium, because of its strong chem. Affinity


CUTTING FLUIDSFunction:

Reduce friction & wear, improving tool life & surface finish

Reduce forces and energy consumption

Cool the cutting zone, reducing workpiece temp and thermal distortion

Wash away chips

Protect machined surface from envir onmental corrosion

Situation in which cutting fluid is harmful: Interrupted cutting operations

May cause the chip to become curlier, thus concentrating the stresses closer to the tool tip, so concentrate the heat closer to the tool tip which reduces tool life


CUTTING FLUIDSTypes of cutting fluids

Oils Emulsions Semisynthetics Synthetics

Methods of application1. Flooding

Flow rates = 10L/min for single point tools to 225L/min per cutter for multiple tooth cutters

2. Mist fluid is supplied to inaccessible areas better visibility of the workpiece effective with water based fluids & in grinding operations at air pressures of 70 kPa-600kPa requires venting limited cooling capacity

3. high pressure systems 5.5MPa-35MPa acts as a chip breaker

4. Through the cutting tool system


CUTTING FLUIDSFigure 21.11 Schematic illustration of proper methods of applying cutting fluids in various machining operations:

a. Turning

b. Milling

c. Thread grinding

d. Drilling


CUTTING FLUIDSEffects of cutting fluids

on workpiece material

on machine tool

biological and environmental effects


cutting tool materials and cutting...

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