Cutting Tool Materials and Cutting Fluids
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
Introduction Cutting Tool Characteristics: 1. Maintaining hardness, strength, and wear resistance at elevated temperatures 2. Toughness 3. Wear resistance 4. Chemical stability Tool Materials Categories: 1. High-speed steels 2. Cast-cobalt alloys 3. Carbides 4. Coated tools 5. Alumina-based ceramics 6. Cubic boron nitride 7. Silicon-nitride-base ceramics 8. Diamond 9. Whisker-reinforced materials
Introduction Cutting 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
CARBIDES Titanium 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 Attachment Figure 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 TOOLS Effect 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 Materials Coatings thickness of 2-15 μm, are applied on cutting tools and inserts by the following techniques: 1. Chemical-vapor deposition (CVD), including plasma-assisted CVD 2. Physical-vapor deposition (PVD) Coatings for cutting tools, as well as dies, should have the following general characteristics: 1. High hardness at elevated temperatures 2. Chemical stability to the workpiece material 3. Low bonding to the substrate to prevent flaking or spalling 4. 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 Materials Titanium 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 Materials Ceramics Coatings: Chemical inertness Low thermal conductivity Resistance to high temperature Resistance to flank and crater wear Most commonly used ceramic coating aluminum oxide (Al 2 O 3 ). However oxide coating generally bond weakly to the substrate.
COATED TOOLS - Coating Materials Multiphase 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/Al 2 O 3. Heavy-duty, continuous cutting: TiC/Al 2 O 3 /TiN. Light, interrupted cutting: TiC/TiC + TiN/TiN.
COATED TOOLS - Coating Materials Multiphase Coatings: Figure 21.7 Multiphase coatings on TiC substrate. 3 alternating layers of Al 2 O 3 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 Materials Multiphase 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 Materials Diamond-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 metalmatrix composite materials, and graphite.
ALUMINA-BASED CERAMICS Consist primarily of fine-grained, high-purity Al 2 O 3. 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 Al 2 O 3, 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 nickelbased 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 nonmetallic 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 FLUIDS Function: 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 FLUIDS Types of cutting fluids Oils Emulsions Semisynthetics Synthetics Methods of application 1. 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 FLUIDS Figure 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 FLUIDS Effects of cutting fluids on workpiece material on machine tool biological and environmental effects