CUTTING TOOL TECHNOLOGY

CUTTING TOOL TECHNOLOGY Tool Wear and failure Tool Materials Tool Geometry Cutting Fluids

Tool Wear and failure Cutting tools are subjected to high forces, elevated temperature and sliding; all these conditions induce wear. As a result of that, cutting tool wearing effects on the quality of machined surface and economics of machining operation. An additional factors are involved in tool wear: Cutting tool and workpiece material (their physical, mechanical and chemical properties). Cutting geometry Cutting fluids if used Processing parameters ( cutting speed, feed, and depth of cut). The types of wear on a tool depends on these variables. Two principal aspects: Tool material Tool geometry

Tool Life: Wear and Failure Cutting tools subjected to High forces High temperatures Sliding of the chip along the rake face Sliding of the tool along the freshly cut surface Induce tool wear Tool life Surface quality Dimensional accuracy Economics of cutting operations Two types of wear Flank and crater wear

Tool Wear There are three possible modes by which a cutting tool can fail in machining: Fracture failure: when excessive cutting force leading to brittle fracture . Temperature failure: when cutting temp- is too high. Gradual wears: loss of tool shape and cutting efficiency. Resulting in Gradual wearing of the cutting tool

Preferred Mode: Gradual Wear Gradual wear is preferred because it leads to the longest possible use of the tool Gradual wear occurs at two locations on a tool: Crater wear – occurs on rake face Flank wear – occurs on flank (side of tool)

Types of tool wear Diagram of worn cutting tool, showing the principal locations and types of wear that occur.

Types of tool wear Flank wears: from rubbing between newly work surface and the Flank (land )face. Crater wears: concave section, by the action of the chip sliding against the surface. Chipping of the cutting edge(catastrophic) Nose wear

Types of tool wear Figure : Crater wear, (above), and flank wear (right) on a cemented carbide tool, as seen through a toolmaker's microscope (photos by K. C. Keefe, Manufacturing Technology Lab, Lehigh University).

Tool Wear Zones – tool-chip interface – predominant at high speeds • Crater wear (crater) – tool-chip interface – predominant at high speeds • Flank wear (wear land) – tool-workpiece interface – predominant at low speeds

Tool / Chip Interface / Crater wear

Tool / work piece Interface / Flank wear

Flank wears Flank wear is generally attributed to: Sliding of the tool along the machined surface, causing adhesive and/or abrasive wear depending on the materials involved (material of workpiece and tool). Temperature rise, because of its adverse effects on the tool material properties.

AR is the real area of contact Adhesive Wear AR is the real area of contact

Tool wear as a function time

Effect of cutting speed on tool flank wear

Flank wears A tool-wear relationship was established by F. W. Taylor for cutting various steels as: Where: v = cutting speed; T = tool life [the time (in minutes) that it takes to develop a flank wear]; and n and C are parameters that depend on feed, depth of cut, work material, tooling material, and the tool life criterion used n is the slope of the plot C is the intercept on the speed axis at one minute tool life For ductile cast iron

Tool Life Curves (Taylor 1907) Log T (Tool Life) f1 Log V V- cutting speed T – the time that takes to develop a flank wear land of a certain dimensions n- constant depends on cutting conditions, Always, n > 0 C – constant (When T=1.0 min, V = C)

Taylor’s Equation for Tool Life VTn = C Tool-life curve Log-log curve T = (C/V)1/n LogT = 1/n logC – 1/n logV C Tool-life curves for a variety of cutting-tool materials. the slope of these curves is the exponent n in the Taylor tool-life equations and C is the cutting speed at T = 1 min.

Typical Values of n and C Tool material n C (m/min) C (ft/min) High speed steel: Non-steel work 0.125 120 350 Steel work 0.125 70 200 Cemented carbide Non-steel work 0.25 900 2700 Steel work 0.25 500 1500 Ceramic Steel work 0.6 3000 10,000

Crater wears The most significant factors affecting crater wears are : Temperature rise on the rake face The degree of chemical affinity between the tool and workpiece. The rake face is subjected to high levels of stress and temperatures, as well as sliding of the chip at relatively high speeds. The peak temperature can be on the order of 1373 K and the location of maximum depth of crater wear generally coincides with the location of maximum temperature at the tool-chip interface. The effect of temperature on crater wear has been described in terms of a diffusion mechanism [Diffusion is a thermal activated process] (that is, the movement of atoms across the tool-chip interface). Diffusion depends on the tool-workpiece material combination and on temperature, pressure and time. As these quantities increases, the diffusion rate increases

Wear at Low Speeds/High Temperature - Chemical Diffusion Diffusion Wear Wear at Low Speeds/High Temperature - Chemical Diffusion

Chipping Causes of chipping: Chipping is used to describe the breaking away of a piece from the cutting edge of the tool. The chipped pieces may be very small (microchipping or macrochipping), or they may involve relatively large fragments. Causes of chipping: Mechanical shock [ chipping by mechanical shock may occur in a region in the cutting tool where a small crack or defects already exists]- main cause of chipping Thermal fatigue [are typically caused by thermal cycling of the cutting tool resulting in thermal cracks, which are generally perpendicular to cutting edge]- main cause of chipping High rank angles can also contribute to chipping, because of the small included angle of the tool tip (a phenomenon similar to chipping of a very sharp pencil). Crater wear may also contribute to chipping, because it progresses toward the tool tip and weaken it, causing chipping

Tool Wear and failure Cutting Tool Materials The proper selection of cutting-tool materials is among the most important considerations in machining operation. In machining operation, the tool is subjected to: High temperatures High contact stresses Rubbing on the workpiece surface And the effects of chip climbing up the rake face of the tool

Cutting Tool Materials Tool failure modes identify the important properties that a tool material should possess A cutting tool must posses the following characteristics: Hardness- particular at elevated temperatures (hot hardness), so that the hardness and strength of cutting tool material are maintained at the temperature encountered in machining operation. Toughness- so that impact forces on the cutting tool in interrupted cutting operations such as milling or turning, do not chip or fracture the tool. Wear resistance- so that an acceptable tool life is obtained before the tool is replaced [hardness is the most important property to resist abrasive wear] Chemical stability- so that any adverse reactions that may contribute to tool wear are avoided or minimized

Cutting Tool Materials Several cutting-tool materials having a wide range of these characteristics

Hot Hardness Figure :Typical hot hardness relationships for selected tool materials. Plain carbon steel shows a rapid loss of hardness as temperature increases. High speed steel is substantially better, while cemented carbides and ceramics are significantly harder at elevated temperatures.

Cutting Tool Materials Tool materials are usually divided into the following categories in which they were developed and implemented: Carbon and medium – alloy steels High-speed steels Carbides Coated tools Alumina-based ceramics Cubic boron nitride Silicon-nitride-based ceramics diamond

Carbon alloy steels and medium alloy steel Carbon steels are the oldest of tool materials and have been used widely for drilling since the 1880s, with shorter tool life. Low alloy and medium alloy steels were developed later with longer tool life. These steels do not have sufficient hot hardness and wear resistance for machining at high cutting speeds where the temperature rises significantly. As from the figure As seen from the figure above, how rapidly the hardness of carbon steels decreases as the temperature increases. Consequently, the use of these steels is limited to very low-seed cutting operations

High-speed steels(HSS) High-speed- steel (HSS) tools are so named because they were developed to machine at high speeds than the carbon alloy steels. (produced in 1990s). Relatively High hardness compared to carbon steel, and wear resistance. Because of their high toughness and resistance to fracture, HSS are suitable for: For high rank angle tools (that is, small included angle) For use on machine tools that are subjected to vibration and chatters because of their low stiffness.

High-speed steels(HSS) There are two basic types of high-speed tools Molybdenum (M series)- contains up to 10% molybdenum, with chromium, vanadium, tungsten, and cobalt as alloying elements. Tungsten (T sereies)- contains 12 to 18% tungsten, with chromium, vanadium and cobalt as alloying elements The M series generally has higher abrasion resistance than the T series, undergoes less distortion during heat treatment, and less expensive. Consequently, 95% of HSS tools produced in the USA are made of M-series

Carbides The tool materials (carbon alloy steels and HSS) have significant limitations on characteristics such as strength and hardness, particularly hot hardness. Consequently, they cannot be used as effectively where high cutting speed, and hence high temperatures, are involved, and their tool life can be relatively short. Carbides, also known as cemented or sintered carbides, were introduced to meet the challenge of higher machining speeds

Cemented-Carbides – General Properties High hardness over a wide range of temperature (90 to 95 HRA). High elastic modulus ‑ 600 x 103 Mpa High thermal conductivity Low thermal expansion Good wear resistance Toughness lower than high speed steel, therefore, stuffiness of the machine is important and chatter can be detrimental. The two basic groups of carbides used for machining operations are Tungsten carbide Titanium carbides.

Carbides Tungsten carbide (WC) - is a composite material consisting of tungsten-carbide particles bonded together in a cobalt matrix (also known as cemented carbide). The a mount of cobalt significantly affects the properties of tungsten carbide. As the cobalt content increases, strength, hardness and wear resistance decreases. Tungsten-carbide tools are generally used for machining steels, cast irons, and abrasive nonferrous materials, and have largely replaced HSS tools wear

Carbides Titanium carbide (TiC) - is a composite material consisting of titanium-carbide particles bonded together in a cobalt matrix. Has higher wear resistance with a nickel-molybdenum alloy as the matrix. Tic is suitable for machining hard materials, mainly steels and cast irons, and for machining at higher speeds than those for tungsten carbides

Coated tools A variety of materials can be used as coating over : High speed steel (HSS) And carbide tools. Because of their unique properties, coated tools can be used at high cutting speeds, thus reducing the time required for machining, hence costs. Coated tool can improve tool life by as much as 10 times of uncoated tools From the figure below, the machining time has been reduced by a factor of more than 100 since 1900.

Coated tools Commonly used Coating materials include: Titanium nitride (TiN) Titanium carbide (TiC) Titanium carbonitride (TiCN) Aluminum oxide (Al2O3). Ceramic coating Coating thickness range of 2 – 10 µm. Coatings are applied by two methods Chemical- vapor deposition (CVD): is the most commonly used coating application method for carbide tools with multiple phases and ceramic coating. Physical-vapor deposition (PVD): used for carbide tools with Titanium nitride (TiN) coatings. The coated carbides with TiN coatings have higher cutting-edge strength, less friction, lower tendency to form a built-up edge, and are smoother and more uniform thickness (2-4 µm).

Multiphase coating Photomicrograph of cross section of multiple coatings on cemented carbide tool (photo courtesy of Kennametal Inc.)

Coated tools The most recent technology for multiple coatings, is medium-temperature chemical-vapor deposition (MTCVD), it provides higher resistance to crack propagation than do CVD coatings. Coatings should have the following characteristics: High hardness at elevated temperature. Chemical stability and inertness to workpiece material. Low thermal conductivity Good bonding to the substrate, to prevent flanking Liitle or no porosity

Alumina-base ceramics Alumina-base ceramics, consists primarily of fine grained, high purity aluminum oxide Al2O3. They are pressed into insert shapes under high pressure and at room temperature, then sintered at high temperature with no binder. Alumina-base ceramic tools have: Very high hot hardness over a wide range of temperature. Very high abrasion resistance Chemically, they are more stable than HSS and carbides; thus they have less of tendency to adhere to metals during machining and hence lower tendency to form built-up edge. consequently, good surface finish is obtained

Alumina-base ceramics Not recommended for heavy interrupted cuts (e.g. rough milling) due to low toughness (ceramics lack toughness), which can result in premature tool failure by chipping or fracture. The shape and setup of ceramic tools are also important: Smaller rake angles, and hence larger include angles, are generally preferred in order to avoid chipping. The occurrence of tool failure can be reduced by increasing the stiffness and damping capacity of machine tools and workholding devices, thus reducing vibration and chatter.

Cubic Boron Nitride Next to diamond, cubic boron nitride (cBN) is the hardest material presently available. The cBN cutting tools are made by bonding a 0.5 to 1mm layer of polycrystalline cubic boron nitride to a carbide substrate by sintering under high pressure. While carbide provide good toughness, the cBN layer provides very high wear resistance and cutting-edge strength. At elevated temperature, cBN is chemically inert to iron and nickel, and its resistance to oxidation. It is therefore suitable for machining hardened ferrous and high-temperature alloys. Because cBN tools are brittle, stuffiness and damping capacity of machine tool and fixturing device are important to avoid vibration and chatter.

Diamond The hardest substance of all know materials is diamond. Diamond is a crystalline form of carbon, and single crystal. As cutting tool, it has : Low tool-chip friction High wear resistance And ability to maintain a sharp cutting edge. Diamond tools can be used satisfactorily at almost any speed but are suitable mostly for light, uninterrupted cuts It is used when very fine surface finish and dimensional accuracy are required, particularly with abrasive nonmetallic materials and soft nonferrous alloys.

Diamond Because diamond is brittle, tool shape and sharpness are important: Low rake angles and large included angles are normally used to provide a strong cutting edge. Wear of diamond tools may occur by microchipping (caused by thermal stresses) and transformation to carbon (caused by the heat generated during cutting)

Tool Geometry Two categories: Single point tools Multiple cutting edge tools

Tool Geometry Figure 23.9 Three ways of holding and presenting the cutting edge for a single‑point tool: (a) solid tool, typical of HSS; (b) brazed insert, one way of holding a cemented carbide insert; and (c) mechanically clamped insert, used for cemented carbides, ceramics, and other very hard tool materials.

Tool Geometry Solid Tool High speed steel (HSS) and carbon steel cutting tools can be shaped in one piece and grounded to various geometries. However, after the cutting edge wears and becomes dull, the tool has to be removed from its holder and reground, which is a time consuming process. The need for a more efficient method led to the development of inserts.

Tool Geometry Inserts are individual cutting tools with a number of cutting edges and in various shapes. Thus, a square insert has eight cutting edges, and a triangular has six cutting edges. Inserts are available with a wide variety of chip-breaker features for controlling chip flow and reducing vibration and heat generated

Tool Geometry Inserts are available with various locking mechanisms which are usually clamped on the tool shank: Clamped : is the preferred methods because after one cutting edge is worn, it is indexed (rotate in it is holder) so that another edge can be used Wing lock pins Brazed: is less frequently used, because of the difference in thermal expansion between the insert and tool-shank materials. Mechanically Clamping insert Mechanically Wing lockpins insert Brazed inserts

Tool Geometry inserts The strength of the cutting edge of an insert depends on its shape; the smaller the included angle of the edge, the lower is its strength

Tool Geometry inserts In order to further improve edge strength and prevent chipping, inserts are usually chamfered (honed). Most inserts are chamfered to a radius of about 0.025 mm. Figure : Common insert shapes: (a) round, (b) square, (c) rhombus with two 80 point angles, (d) hexagon with three 80 point angles, (e) triangle (equilateral), (f) rhombus with two 55 point angles, (g) rhombus with two 35 point angles. Also shown are typical features of the geometry.

Tool Geometry inserts A collection of metal cutting inserts made of various materials (photo courtesy of Kennametal Inc.).

Tool Geometry Turning

Tool Geometry Boring

Tool Geometry Twist Drills By far the most common cutting tools for hole‑making Usually made of high speed steel Figure : Standard geometry of a twist drill.

Tool Geometry Face Milling Cutter Teeth cut on side and periphery of the cutter Figure : Tool geometry elements of a four‑tooth face milling cutter: (a) side view and (b) bottom view.

Cutting Fluids Also called lubricants and coolants. Cutting fluids are used extensively in machining operation to improve cutting performance via : Cooling the cutting zone, thus reducing workoiece temperature and distortion, and improving tool life [Heat generation at shear and friction zones]. Easier handling of work part Reducing friction and wear, hence improving tool life and surface finish [Friction at tool‑chip and tool‑work interfaces]. Reducing forces and energy consumption. Wash a way chips. Protect the newly machined surfaces from environmental attack.

Cutting Fluid Functions Cutting fluids can be classified according to function: Coolants - designed to reduce effects of heat in machining Lubricants - designed to reduce tool‑chip and tool‑work friction

Coolants Water used as base in coolant‑type cutting fluids Most effective at high cutting speeds where heat generation and high temperatures are problems Most effective on tool materials that are most subjected to temperature failures.

Lubricants Usually oil‑based fluids Most effective at lower cutting speeds Also reduce temperature in the operation

Cutting Fluids-deficiencies There are situations in which the use of cutting fluids can be detrimental: In an interrupted cutting operations, such as milling, the cooling action of the cutting fluid increases the extent of alternate heating and cooling ( thermal cycling) to which the cutter teeth are subjected. This condition can lead to thermal cracks (thermal fatigue) Cutting fluids may also cause the chip to become more curled, thus concentrating the stresses on the tool closer to the tool tip and reducing tool life.

Cutting Fluids- cost effectiveness Cutting fluids can present biological and environmental hazards that require proper recycling and disposal, thus adding to cost of the machining operation. The use and application of cutting fluids can also be a significant item in machining operations costs. For these reasons, dry cutting, or dry machining, has become an increasingly important approach in which no coolant or lubricant is used in cutting operation Even though this approach would suggest that higher temperatures and more rapid tool wear would occur. Dry cutting has been associated with high-speed machining, because of the fact that higher cutting speeds transfer a greater amount of heat from cutting tool to chip, which is a naturel strategy for reducing the need for a coolant