Metal Cutting Lecture Notes (WSB610) PDF

Manufacturing Technology WSB610 Metal Cutting Radmehr...

Manufacturing Technology WSB610 Metal Cutting Radmehr P Monfared Mechanical, Electrical and Manufacturing Engineering Lecture Notes – Set 2 of 2 R.P.Monfared - Loughborough University  Lecture Notes – Set 2 Content: 1. An Introduction to Metal Cutting 2. Fundamentals of Cutting 3. Turning Operations 4. Milling Operations 5. Additional Machining Operations Further Reading ‐ Overview of Metal Cutting 6. Abrasive Machining Operations 7. Machining Centres 8. Cutting Tools 9. Non‐traditional Cutting Processes 10. Economical Aspects of Metal Cutting R.P.Monfared - Loughborough University  1 Loughborough University Wolfson School of Mechanical and Manufacturing Engineering Manufacturing Technology (WSA610, WSB610) Metal Cutting Part 6 – Abrasive Machining Operations Dr Radmehr P Monfared [email protected] TW233 1 Abrasive Machining Processes Abrasive machining is a material removal process that involves the use of abrasive cutting tools. Regardless of the form of the abrasive tool and machining operation, all abrasive operations can be considered as material removal processes with geometrically undefined cutting edges. Abrasive machining can be compared to the other machining operations with multipoint cutting tools. Each abrasive grain acts like a small single cutting tool with undefined geometry but usually with high negative rake angle. Basics facts An abrasive is a small, hard particle having sharp edges and an irregular shape. It is capable of removing small amounts of material from a surface through a cutting process that produces tiny chips. Abrasive machining processes are amongst the last operation performed on manufactured parts. Abrasive operations could be carried out manually or on a machine Ref. Valery Marinov, Kalpakjian, & N.R. DHAR Radmehr Monfared - Loughborough ©  2 Abrasive Cutting Processes Abrasive cutting tools include three major types according to the degree to which abrasive grains are constrained: 1) bonded abrasive tools: abrasive grains are closely packed into different shapes, the most common is the abrasive wheel. Grains are held together by bonding material. Abrasive machining process that use bonded abrasives include grinding, honing, superfinishing; 2) coated abrasive tools: abrasive grains are glued onto a flexible cloth, paper or resin backing. Coated abrasives are available in sheets, rolls, endless belts. Processes include abrasive belt grinding, abrasive wire cutting; 3) free abrasives: abrasive grains are not bonded or glued. Instead, they are introduced either in oil-based fluids (lapping, ultrasonic machining), or in water (abrasive water jet cutting) or air (abrasive jet machining), or contained in a semi-soft binder (buffing). The concept of undefined cutting edge in abrasive machining. Ref. Valery Marinov, & Kalpakjian Grinding Grinding is a material removal process in which abrasive particles arc contained in a bonded grinding wheel that operates at very high surface speeds. The grinding wheel is usually disk shaped and is precisely balanced for high rotational speeds. Typical grinding operations include: (a) cylindrical surfaces, (b) conical surfaces. (c) fillets on a shaft, (d) helical profiles, (e) concave shape, (f) cutting off or slotting with thin wheels, and (g) internal grinding. Ref. Valery Marinov, & Kalpakjian Radmehr Monfared - Loughborough ©  3 Grinding Wheels Grinding wheels consist of abrasive particles and bonding material. The bonding material holds the particles in place and establishes the shape and structure of the wheel. The way the abrasive grains, bonding material, and the air gaps are structured, determines the parameters of the grinding wheel, which are: abrasive material, grain size, bonding material, wheel grade, and wheel structure. A physical model of a grinding wheel showing its structure and wear and fracture patterns. Common types of grinding wheels made with conventional abrasives. Each wheel has a specific grinding face; grinding on other surfaces is not possible. Ref. Valery Marinov, & Kalpakjian Abrasive Material for Grinding Most common abrasive materials used in grinding operations are listed in the table below with their application. Abrasive material are much harder than conventional cutting tool materials. Their main characteristics are: hardness and friability. Friability is defined as the ability of abrasive grains to fracture into smaller pieces. This property gives abrasives their self-sharpening characteristics. Ref. Valery Marinov, & Kalpakjian Radmehr Monfared - Loughborough ©  4 Specifications of Grinding Wheels Grain size - The grain size of the abrasive particle is an important parameter in determining surface finish and material removal rate. Small grit sizes produce better finishes while larger grain sizes permit larger material removal rates. The abrasive grains are classified in a screen mesh procedure in which smaller grit sizes have larger numbers and vice versa. Grain sizes used in grinding wheels typically range between 6 and 600. Grit size 6 is very coarse and size 600 is very fine. Finer grit sizes up to 1000 are used in some finishing operations. Bonding Materials - Desirable properties of the bond material include strength, toughness, hardness, and temperature resistance. Bonding materials commonly used in grinding wheels include: 1. vitrified bond: vitrified bonding material consists of ceramic materials. Most grinding wheels use this bonding. They are strong and rigid, resistant to elevated temperatures, and relatively unaffected by cutting fluids; 2. rubber bond: rubber is the most flexible of the bonding materials. It is used as a bonding material in cut-off wheels; 3. resinoid bond: this bond is made of various thermosetting resin materials. They have very high strength and are used for rough grinding and cut-off operations; 4. metallic bond: usually bronze, are the common bond material for diamond and CBN grinding wheels. Diamond and CBN abrasive grains are bond material to only the outside periphery of the wheel, thus conserving the costly abrasive materials. Ref. Valery Marinov, & Kalpakjian Specifications of Grinding Wheels Wheel Grade - Wheel grades indicate the wheel bond strength. It is measured on a scale ranging from soft to hard. Soft wheels loose grains easily and are used for low material removal rates and grinding of hard materials. Harder grades are preferred for high productivity and grinding of relatively soft materials, Wheel Structure - indicates spacing of the abrasive grains in the wheel. It is measured on a scale that ranges from open to dense. Open structure means more pores and fewer grains per unit wheel volume. Standard Marking System for Aluminum- Oxide and Silicon- Carbide Bonded Abrasives Ref. Valery Marinov, & Kalpakjian Radmehr Monfared - Loughborough ©  5 Grinding Parameters Revolution Speed - Rotational speed of the grinding wheel. (N - rpm) Cutting Speed - The speed of the outer diameter of the wheel. (V – mm/min) Diameter - The wheel diameter (D - mm) Depth of cut - is called infeed and is defined as the distance between the machined and work surfaces. (d – mm) Width of cut - is called crossfeed and is defined as the distance that wheel feeds on to the workpiece at each grinding pass. (w – mm) Material Removal Rate (MRR) - Volume of material removed from workpiece per unit of time ( MRR – mm3/min) V = DN MRR = ( DN)wd Ref. Valery Marinov, Grinding Operations Grinding operations are carried out with a variety of wheel/part configurations. The basic type of grinding are: 1. surface grinding, 2. cylindrical grinding, 3. centerless grinding. Surface grinding Surface grinding is an abrasive machining process in which the grinding wheel removes material from the plain flat surfaces of the workpiece. Four common types of surface grinding with horizontal or vertical spindles, and with reciprocating linear motion or rotating motion of the workpiece. Ref. Valery Marinov, Radmehr Monfared - Loughborough ©  6 Grinding Operations Cylindrical grinding In this operation, the external or internal cylindrical surface of a workpiece is ground. In external cylindrical grinding (also centre-type grinding) the workpiece rotates and reciprocates along its axis, although for large and long workpiece the grinding wheel reciprocates. In internal cylindrical grinding, a small wheel grinds the inside diameter of the part. The workpiece is held in a rotating chuck in the headstock and the wheel rotates at very high rotational speed. In this operation, the workpiece rotates and the grinding wheel reciprocates. Ref. Valery Marinov, Grinding Operations Three types of feed motion are possible in cylindrical grinding according to the direction of feed: Traverse Feed grinding (also through feed grinding, cross-feeding) in which the relative feed motion is parallel to the spindle axis of rotation, Plunge grinding in which the grinding wheel is fed radially into the workpiece, and Combination of traverse and plunge grinding in which the grinding wheel is fed in angle to grind simultaneously the cylindrical part of the workpiece and the adjacent face. This methods provides a precise perpendicular mutual position of both surfaces. Traverse feed grinding Combination grinding Plunge grinding Ref. Valery Marinov, Radmehr Monfared - Loughborough ©  7 Grinding Operations Centreless grinding Centreless grinding is a process for continuously grinding cylindrical surfaces in which the workpiece is supported not by centres or chucks but by a rest blade. The workpiece is ground between two wheels. The larger wheel grinds, while the smaller wheel, which is tilted at an angle i, regulates the velocity of the axial movement of the workpiece. Centreless grinding can also be external or internal, traverse feed or plunge grinding. The most common type of centerless grinding is the external traverse feed grinding. This grinding operation is suitable for mass production. Ref. Valery Marinov, Grinding Operations Centreless grinding to improve roundness To improve roundness on precision parts such as bearings, shafts, spindles and turbines, special tools (part holders) may be used during the grinding process to accommodate the out of roundness of the parts while being grinded. External and Non-circular internal Grinders part Ref. roundness.net Radmehr Monfared - Loughborough ©  8 Grinding Operations Creep-feed grinding Grinding has traditionally been associated with small rates of material removal and finishing operations. However, grinding can also be used for large-scale metal removal operations similar to milling, and planing. In creep-feed grinding, the depth of cut is as much as 6 mm, and the workpiece speed is low. The wheels are mostly softer grade resin bonded with open structure to keep temperatures low. Creep-feed grinding can be economical for specific applications, such as grinding cavities, grooves, etc. Ref. Valery Marinov, & Kalpakjian Various Grinding Operations Plunge grinding on a cylindrical grinder with the wheel profiled Belt grinding to a stepped shape of turbine nozzle vanes Thread grinding by Grinding a non-cylindrical part on a CNC (a) traverse, and (b) plunge grinding. cylindrical grinder Ref. Valery Marinov, & Kalpakjian Radmehr Monfared - Loughborough ©  9 Dressing of Grinding Wheels Dressing is the process of conditioning worn grains on the surface of grinding wheels in order to produce sharp new grains. This is to avoid glazing (when chips fill the gap between grains and produce a shiny surface). Dressing is also used to produce required cutting profile on the grinding wheels. See Video Ref. Kalpakjian – Prentice Hall Honing Operation Honing is a finishing process performed by a honing tool, which contains a set of bonded abrasive sticks. The sticks are equally spaced about the periphery of the honing tool. They are held against the work surface with controlled light pressure, usually exercised by small springs. The honing tool is given a complex rotational and oscillatory axial motion, which combine to produce a crosshatched lay pattern of very low surface roughness. In addition to the surface finish of about 0.1 , crosshatched surface tends to retain lubrication during operation of the workpiece, thus contributing to its function and service life, such as combustion engine, hydraulic cylinders, gun barrels. Ref. Valery Marinov, & Kalpakjian Radmehr Monfared - Loughborough ©  10 Lapping Operation Lapping is a finishing operation that instead of a bonded abrasive tool, oil-based fluid suspension is used for material removal. The fluid, called a lapping compound, contains very small free abrasive grains (aluminium oxide and silicon carbide, with typical grit sizes between 300 and 600) which is applied between the workpiece and the lapping tool. The lapping tool is called a lap and is made of soft materials like copper, lead or wood. The lap has the reverse of the desired shape of the workpiece. To accomplish the process, the lap is pressed against the work and moved back and forth over the surface in a figure-eight or other motion pattern, subjecting all portions of the surface to the same action. Lapping is sometimes performed by hand, but lapping machines accomplish the process with greater consistency and efficiency. Ref. Valery Marinov, & Kalpakjian Lapping Operation The cutting mechanism in lapping is that the abrasives become embedded in the lap surface, and the cutting action is very similar to grinding, but a concurrent cutting action of the free abrasive particles in the fluid significantly improves the surface quality. Lapping is used to produce optical lenses, metallic bearing surfaces, and other parts requiring very good finishes and extreme accuracy. Ref. Valery Marinov Radmehr Monfared - Loughborough ©  11 Superfinishing Superfinishing is a finishing operation similar to honing, but it involves the use of a single abrasive stick. The reciprocating motion of the stick is performed at higher frequency and small amplitudes. Also, the grit size and pressures In superfinishing, the cutting action terminates by itself applied on the when a lubricant film is built up between the tool and abrasive stick are work surface. The result of these operating conditions is smaller. A cutting fluid mirror like finishes with surface roughness values around is used to cool the 0.01. Superfinishing can be used to finish flat and work surface and external cylindrical surfaces. wash away chips. Ref. Valery Marinov, & Kalpakjian Polishing and Buffing Polishing is a finishing operation to improve the surface finish by means of a polishing wheel made of fabrics or leather and rotating at high speed. The abrasive grains are glued to the outside surface of the polishing wheel. Polishing operations are often carried out manually. Buffing is a finishing operation similar to polishing, in which abrasive grains are not glued to the wheel but are contained in a buffing compound that is pressed into the outside surface of the buffing wheel while it rotates. As in polishing, the abrasive particles must be periodically replenished. Buffing is also usually done manually, although machines have been designed to perform the process automatically. Polishing is used to remove scratches and burrs and to smooth rough surfaces while buffing is used to provide attractive surfaces with high lustre. Ref. Valery Marinov, & Kalpakjian Radmehr Monfared - Loughborough ©  12 Surface Quality in Abrasive Operations See Video Ref. Valery Marinov Questions... Some questions to practice Various abrasive operations Grinding parameters Various grinding operations Application of honing operation Advantage of grinding operation Surface quality through abrasive operations Next Sessions Machining Centre Radmehr Monfared - Loughborough ©  13 Loughborough University Wolfson School of Mechanical and Manufacturing Engineering Manufacturing Technology (WSA610, WSB610) Metal Cutting Part 7 – Machining Centres Dr Radmehr P Monfared [email protected] TW233 Machining Centre Machining centres are designed to complete various cutting operations on one machine. Despite the complexity of conventional machining, each cutting process needs to be completed on individual machines, such as milling, turning, and drilling and parts need to be transferred from one machine to another, manually or by using transfer lines. Application of machining centre eliminates the need for significant amount of part transferring. Basics facts Machining centres are capable of operating many cutting operations by using multiple spindles, large pre-set tool storage, automatic tool and part changing systems, and computer controlled motions. The initial cost is high but cost per part could be very low. Suitable for low and medium volume production of parts. Suitable for production lines with variety of parts. Provides high dimensional accuracy, and requires less human intervention. Ref. Valery Marinov, Kalpakjian, & N.R. DHAR Radmehr Monfared - Loughborough ©  14 Example of Parts Examples of parts that can be machined on machining centers, using various processes such as turning, facing, milling, drilling, boring, reaming, and threading. Such parts would ordinarily require a variety of machine tools. Ref. Kalpakjian – Prentice Hall Types of Machining Centre Machining and Turning Centres are design in types of horizontal, vertical or combination of spindles. The maximum cutting dimensional capability of the machine is known as work envelop. Vertical spindles are suitable for flat surface and deep cavities because the cutting forces are transferred to the machine bed, thus better stiffness. Horizontal spindles are more suitable for larger parts locating on a pallet with multiple cutting processes on different part surfaces. Most turning centres are design with horizontal spindles. However, vertical machines are generally cheaper to build. Ref. Kalpakjian – Prentice Hall Radmehr Monfared - Loughborough ©  15 Types of Machining Centre Universal Machining Centres are equipped with several spindles and tool holders. Parts typically locate on a rotary tables that provides extra axes to the machine (rotation while machining). Pallets are used to automatically reposition parts or rotate them ( not when machining). Schematic illustration of a three-turret, two-spindle computer numerical controlled turning centre. Ref. Kalpakjian – Prentice Hall Gantry Machining Centre Gantry Machining Centres are designed for machining large and heavy parts. Work piece is typically fixed on the bed and the cutting head moves in several directions. Multiple cutting heads may be attached to the gantry and they could be controlled independently from each other. Typical operations on gantry machining centres are milling (horizontal or vertical), drilling and boring operations. Radmehr Monfared - Loughborough ©  16 Robotic Machining Robotic machining could be used as a very flexible cutting operations such as drilling and milling for parts that are not accessible (due to safety or size). Robotic machining is typically used for on-the-jig machining processes, where the parts cannot be brought to the machining centres. The accuracy of the advanced robotic cutters may reach to standard machining centres, but most robots lack stiffness required for machining. Multiple Axes Machining centres typically have 3 to 5 axes. A linear motion or a rotation is an axis if it is controlled simultaneously with the machine’s computer while cutting is in progress. There may be several motion or rotation of the same axis, such as multiple x axis with spindle feed and table move. Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Radmehr Monfared - Loughborough ©  17 Half Axis See Video Notion of 1/2 axis (e.g. 3.5 axes centre) refers to the 4th axis moving independent from other axes and not controlled automatically while cutting is in progress. Computer controlled rotary table provides 4th and 5th axes Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Machining Centres - Video Radmehr Monfared - Loughborough ©  18 Cutting Processes on Machining Centres A CNC machining centre has multiple horizontal and vertical spindles and several tool turrets. The motion of the tools/spindles are relative to the part movement and controlled by computer. A CNC machining centre can provide variable feed and variable spindle speed which are require to obtain constant cutting speed. Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Pallets Pallets transfer parts to the spindle and locate in correct position. Pallets usually equipped with part holding fixtures, design generically for a family of parts or specifically for a certain workpiece. Multiple parts could be held by the fixture. Machining centres may have pallet shuttle systems that allow two or more pallets be used. The second pallet may be used for loading new parts while the first one is being processed. A pallet pool or automatic guided vehicle (AGV) may be used to transfer pallets between machining centres. Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Radmehr Monfared - Loughborough ©  19 Pallets An automated guided vehicle (AGV) is a mobile robot that follows markers or wires on the floor, or uses vision or laser guides. AGV used in flexible manufacturing systems to handle material, pallets and parts within the shop floor. Automatic Tool Changing Machining centres require various tools to complete planned cutting operations. To change from one operation to another the cutting tool needs to be changed. This is done through the NC/CNC programme between cutting operations. An automatic tool changer is designed to exchange tool between tool storage drum or Tool Magazine and the machine spindle. Tool changer video Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Radmehr Monfared - Loughborough ©  20 Automatic Tool Changing Each tool’s specification (e.g. dimension and type) is registered in the tool magazine memory and will be selected automatically. Tools need to be measured accurately and positioned in the magazine in advance. See Video Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Touch Probe Touch probes used in machining centres for determining workpiece and tool positions and surfaces relative to the machine table or column. Touch probes are used to determine (a) the X-Y (horizontal) and Z (height) of a horizontal surface, (b) the position of the surface of a cutter (for instance, for cutter-diameter compensation), and (c) the length of a tool for tool-length offset. Ref. Kalpakjian – Prentice Hall Radmehr Monfared - Loughborough ©  21 NC Positioning Systems Positioning system is responsible to convert coordinates in NC/CNC programmes into the relative position between part and tool. Two basic type of motion control are used in NC positioning, (a) open system, (b) close loop. In open loop system the motion is not verified and the dimensional accuracy depends on the accuracy Digital-to-Analogue Converter (DAC) of the machine components and the Optical cutting forces. encoder In the closed loop system a continuous feed back verifies the exact movement of the work table as programmed. The open loop is cheaper and suitable for cutting small parts with minimal cutting forces. Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Recirculation Ball Screw Recirculation ball screw system is used in the table lead screw to transfer accurate movement to the table, eliminating the backlashes and oscillating movement caused by some servomotors. Recirculation ball system provides great motion Recirculation Ball accuracy and repeatability. This system enables Screw - Video much faster movement of the table in comparison to the conventional lead screws. Ref. Kalpakjian – M.Groover – J.Black – G.Tlusty Radmehr Monfared - Loughborough ©  22 Chip Collecting System Chips produced by machining centre need to be removed from the machine’s cabin and disposed rapidly. The chips need to be of the broken type. Continuous chips have to be avoided as they disturb the cutting operation, part and tool changing and cannot be disposed easily. Schematic illustration of a chip collecting system in a horizontal spindle machining center. The chips that fall by gravity are collected by the two horizontal conveyors at the bottom of the troughs. Chip Removal video Ref. Kalpakjian – Prentice Hall Characteristics of Machining centre They are capable of cutting variety of part sizes with complicated shapes and with high accuracy of up to ±0.0025 mm. Machining centres are versatile with up to 6 linear and rotational axes. Capable of rapid change over from one operation or part to another. They could be economical by eliminating the part transfer time and equipments and reducing the shop floor space occupied. The time required for part loading, tooling and gauging are reduced, thus improving the production lead-time and labour cost. Tool wear and post process gauging can be controlled automatically. Provides high repeatability and precision (less scrap). Their use can be justified in almost all volume of productions. Higher investment cost and skilled operator Ref. Valery Marinov, Radmehr Monfared - Loughborough ©  23 Questions… Which cutting operations could be used to produce these parts? Example… Which set of statements are correct? A* B C D Grinding should be used when work piece surface is uneven and rough, such as a forged part Milling and Grinding may be used when hard material, high quality surface and dimensional precision are required. Honing is typically used for external surface of cylinders to improve surface quality. Machining centre is NOT most suitable for very high volume production. Robotic machining provides higher accuracy than machining centres High cost of Broaching machine makes it suitable for low volume production of highly accurate parts. Milling and Grinding machines are used to produce Gears and Threads Threads can be cut on lathe machines and Grinding machines Radmehr Monfared - Loughborough ©  24 Questions... Some questions to practice Various types of machining centres Components of a machining centre Application of tool changers Application of pallet changers Advantage of machining centres in comparison with conventional operations Additional References: Manufacturing Process and Equipment – G. Tlusty Next Sessions Cutting-Tool Materials, Tool Wear, and Cutting Fluids Loughborough University Wolfson School of Mechanical and Manufacturing Engineering Manufacturing Technology (WSA610, WSB610) Metal Cutting Part 8 – Cutting Tools Dr Radmehr P Monfared [email protected] TW233 Radmehr Monfared - Loughborough ©  25 Cutting Tool for Machining Efficient metal cutting depends on the selection of proper cutting tools’ material and geometries. These two aspects along with the cutting parameters, directly influence on the productivity of the machining operation. The elements that influence the tool material selection include: Work material characteristics, hardness, chemical and metallurgical states, part characteristics such as geometry, accuracy, finish, and surface integrity, machine tool characteristics such as rigidity of machine and tool holder, cutting fluids and chip removal systems. Basic Characteristics To be harder than workpiece To maintain its hardness while cutting Resistance to thermal shocks Resistance to wear Resistance to impact (e.g. interrupted cutting process) To have chemical stability (inertness with the work material) Ref. DeGarmos, JTBlack – WILEY&SMe Solid Tools Solid tools are cutting tools with their body and shaft consist of one material (e.g. high speed steel - HSS). This tool type is sharpened by grinding to the geometry required for various forms of cuttings. The solid tools are advantageous because they can be sharpened (re-formed by grinding) and can take almost any complicated geometry required. These tools have typically lower quality in terms of physical characteristics and are usually cheaper. Ref. Mfg pro.1 - Cutting F.Klocke Radmehr Monfared - Loughborough ©  26 Tools with Inserts The cutting edge of a tool needs to have higher physical properties than the rest of a solid tool. Therefore it makes sense to attach cutting inserts to an insert holder. The inserts could be fixed to the holder (e.g. brazed) or could be clamped or screwed (i.e. they are changeable). Tools with clamped or screwed inserts have the advantage that several cutting edges can Tool Type Video be used on a single insert. If a cutting edge has reached the end of its service life, the insert is turned or twisted after releasing the attachment, thereby bringing a new cutting edge into action. The term “indexable insert” comes from this process (DIN ISO 1832). Ref. Mfg pro.1 - Cutting F.Klocke Standard Description for Inserts DIN ISO 1832 description for cutting inserts Ref. Mfg pro.1 - Cutting F.Klocke Radmehr Monfared - Loughborough ©  27 Standard Description for Inserts - continue Ref. Mfg pro.1 - Cutting F.Klocke Cutting Tool Characteristics The cutting tools are subjected to severe operating conditions such as high temperature and stress and therefore they need to have the following characteristics: 1. High hardness 2. Hot hardness (maintaining hardness at high temperature) 3. Resistance to abrasion, wear due to severe sliding friction 4. Resistance to chipping of the cutting edge 5. High toughness (impact strength) 6. Strength to resist deformation 7. Chemical stability 8. Adequate thermal property 9. High elastic modulus (stiffness) 10. Correct geometry and surface finish Ref. DeGarmos, JTBlack – WILEY&SMe Radmehr Monfared - Loughborough ©  28 Properties of Cutting Tool Materials Properties Cutting tool Material Ref. DeGarmos, JTBlack – WILEY&SMe Coating the Cutting Edges No single cutting material have all required mechanical, thermal and chemical properties. For instance, material with chemical and thermal stability such as ceramics tend to be brittle with limited resistance to mechanical and thermal shocks. Therefore various layer of coatings are added to the tool. More than 75% of all carbide tools are coated by three different layers. Mechanical/Thermal Shock Triple coated carbide tools provide resistance to wear and plastic deformation in machining of steel, abrasive wear in cast iron, and build up edge Formation. Ref. DeGarmos, JTBlack – WILEY&SMe Radmehr Monfared - Loughborough ©  29 Coating the Tools Titanium carbide is the basic tool coat for strength and wear resistance. The second layer is aluminium oxide which is chemically stable at high temperature. The third layer is a tin layer of titanium nitride to reduce friction and avoid build up edge. Ref. DeGarmos, JTBlack – WILEY&SMe Cutting Temperature Control of the cutting temperature is important because : It affects the wear of the cutting tool. Cutting temperature is the primary factor affecting the cutting tool wear. It can induce thermal damage to the machined surface. High surface temperatures promote the process of oxidation of the machined surface. The oxidation layer has worse mechanical properties than the base material, which may result in shorter service life. It causes dimensional errors in the machined surface. Temperature expands the tool and therefore the position of the cutting tool edge shifts toward the machined surface, resulting in a dimensional error of about 0.01~0.02 mm. As a result, an additional shape error appears on the machined surface at the beginning of the cut as shown in the figure. Ref.: Valery Marinov, Radmehr Monfared - Loughborough ©  30 Cutting Temperature Almost all of energy dissipated in plastic deformation is converted into heat that in turn raises the temperature in the cutting zone. Since the heat generation is closely related to the plastic deformation and friction, three main sources of heat can be specified as below: Plastic deformation by shearing in the primary shear zone (heat source Q1) Plastic deformation by shearing and friction on the cutting face (heat source Q2) Friction between chip and tool on the tool flank (heat source Q3) Heat is mostly dissipated by, The discarded chip carries away about 60~80% of the total heat (q1) The workpiece acts as a heat sink drawing away 10~20% heat (q2) The cutting tool will also draw away ~10% heat (q3). If coolant is used in cutting, the heat drawn away by the chip can be as big as 90% of the total heat dissipated. Ref.: Valery Marinov, Cutting Temperature Measuring the temperature The mean temperature along the tool face is measured directly by means of different thermocouple techniques, or indirectly by measuring the infrared radiation, or examination of change in the tool material microstructure or microhardness induced by temperature. Some recent indirect methods are based on the examination of the temper colour of a chip, and on the use of thermosensitive paints. There are no simple reliable methods of measuring the temperature field. Therefore, predictive approaches must be relied on to obtain the mean cutting temperature and temperature field in the chip, tool and workpiece. Ref.: Valery Marinov, Radmehr Monfared - Loughborough ©  31 Cutting Temperature The temperature in metal cutting can be reduced by: a) application of cutting fluids (coolants), b) change in the cutting conditions by reduction of cutting speed and/or feed, c) selection of proper cutting tool geometry (positive tool orthogonal rake angle). Except using coolants, the simplest way to reduce the cutting temperature is to reduce the cutting speed and/or feed. The next diagrams show the dependencies between the mean cutting temperature and cutting conditions: Ref.: Valery Marinov, Cutting Fluids Cutting fluid (or coolant) is any liquid or gas that is applied to the cutting zone to improve cutting performance by reducing temperature and friction. A very few cutting operations are performed dry. Generally, it is essential that cutting fluids be applied to all machining operations. Cutting fluids serve three principle functions: to remove heat in cutting: the effective cooling action of the cutting fluid depends on the method of application, type of the cutting fluid, the fluid flow rate and pressure. The most effective cooling is provided by mist application combined with flooding. Application of fluids to the tool flank, especially under pressure, it ensures better cooling of the chips. to lubricate the chip-tool interface: cutting fluids penetrate the tool-chip interface improving lubrication between the chip and tool and reducing the friction forces and temperatures. to wash away chips: this action is applicable to small, discontinuous chips only. Special devices are subsequently needed to separate chips from cutting fluids. Ref.: Valery Marinov, Radmehr Monfared - Loughborough ©  32 Cutting Fluids Methods of application Manual application - Application of a fluid manually by the operator. It is not acceptable even in job-shop situations except for tapping and some other operations where cutting speeds are very low and friction is a problem not temperature. In this case, cutting fluids are used as lubricants. Flooding - In flooding, a steady stream of fluid is directed at the chip or tool-workpiece interface. Most machine tools are equipped with a recirculation system that incorporates filters for cleaning of cutting fluids. Mist applications - Fluid droplets suspended in air provide effective cooling by evaporation of the fluid. Mist application in general is not as effective as flooding, but can deliver cutting fluid to inaccessible areas that cannot be reached by conventional flooding. Ref.: Valery Marinov, Cutting Fluids Types of cutting fluids Cutting Oils Cutting oils are cutting fluids based on mineral or fatty oil mixtures. Chemical additives like sulphur improve oil lubricant capabilities. Areas of application depend on the properties of the particular oil but commonly, cutting oils are used for heavy cutting operations on tough steels (typically low cutting speed/temp. and high friction). Soluble Oils The most common, cheap and effective form of cutting fluids consisting of oil droplets suspended in water in a typical ratio water to oil 30:1. Emulsifying agents are also added to promote stability of emulsion. For heavy-duty work, extreme pressure additives are used. Oil emulsions are typically used for aluminium and cooper alloys. Chemical fluids These cutting fluids consists of chemical diluted in water. They possess good flushing and cooling abilities. Tend to form more stable emulsions but may have harmful effects to the skin. Ref.: Valery Marinov, Radmehr Monfared - Loughborough ©  33 Tool Life and Tool Wear Tool life is measured with the actual time of satisfactory machining, or, volume of products or work material removal before failure. Cutting speed is the most important factor in Parameters which affect the tool life include: tool wear. As cutting speed increases, wear cutting conditions (speed , feed , depth of cut) rate also increases, so the same wear ratio is reached in less time, i.e., tool life decreases cutting tool geometry (tool rake angle) with cutting speed. However, higher cutting properties of work material speed may mean more product in a unit of properties of the tool material time, which may reflect a longer tool life. This is important when calculating the Tool breakage and tool wear are the main production costs. factor to reduce the tool life, Wear zones Gradual wear occurs at three principal location on a cutting tool. Accordingly, three main types of tool wear can be distinguished as, crater wear flank wear corner wear Ref.: Valery Marinov, Tool Wear Crater wear: consists of a concave section on the tool face formed by the action of chip sliding on the surface. Crater wear affects the mechanics of the process increasing the actual rake angle of the cutting tool and consequently, making cutting easier. At the same time, the crater wear weakens the tool wedge and increases the possibility for tool breakage. In general, crater wear is of a relatively small concern as long as it does not extend to the edge of the tool. Flank wear: occurs on the tool flank as a result of friction between the machined surface of the workpiece and the tool flank. Flank wear appears in the form of “wear land” and is measured by the width of this wear land, Flank wear affects to the great extend the mechanics of cutting. Cutting forces increase significantly with flank wear. Ref.: Valery Marinov, Radmehr Monfared - Loughborough ©  34 Tool Wear Corner wear: occurs on the tool corner. Can be considered as part of the wear land and respectively flank wear since there is no distinguished boundary between the corner wear and flank wear land. The corner wear is considered as a separate wear type because of its importance for the precision of machining. Corner wear actually shortens the cutting tool thus increasing gradually the dimension of machined surface and introducing a significant dimensional error in machining, which can reach values of about 0.03~0.05 mm. Other common types of wear: Chipping – Unpredictable tool failure – could cause by impacts in interrupted cutting processes. Build-up Edge – Deposit of work piece material adhering to the surface of the tool. Occurs in cutting more ductile material. Deformation – Occurs in very high temperature causing the tip of tool deform microscopically. See tool wear Thermal cracking – Occurs when rapid cool/hot cycle temperature applies to the tool. Video Ref.: Valery Marinov, Tool Wear Control Wear control The rate of tool wear strongly depends on the cutting temperature, work and tool material, and stresses and impact at the cutting contact points. The figure shows the influencing factors on the rate of tool wear. Ref.: Valery Marinov, Radmehr Monfared - Loughborough ©  35 Tool Selection Manufacturing design considerations for cutting tool selection Ref. Valery Marinov, High-Speed Dry Cutting High Dry high-speed machining has been used in industry through the development of high performance carbide and diamond tools with thermally protective hard surface coating. Turning and milling operations are the easiest to convert to dry machining as the cutting edges in these operations are exposed and chips leave the cutting zone quickly and therefore dissipate heat. However in drilling operations, chips are not flushed out without the use coolant system. Using suction mechanism in encouraged for chip removal. Tooling cost will increase and this has to be optimised based on cost saving due to the reduction in cutting time. Significant saving on machining time, Saving on coolant management system, Vacuum and mist coolant systems (jet spray of air and drops of coolant) may be used, Both feedrate and cutting speed should be increased No coolant high speed cutting video Radmehr Monfared - Loughborough ©  36 Questions... Some questions to practice Various parameters while selecting tool Impact of temperature on cutting tool Influence of cutting fluids on machining operation Additional References: Materials and Processes in Manufacturing by JT Black & RA Kohser Next Sessions Non-Traditional Cutting Processes Loughborough University Wolfson School of Mechanical and Manufacturing Engineering Manufacturing Technology (WSA610, WSB610) Metal Cutting Part 9 – Non-traditional Cutting Processes Dr Radmehr P Monfared [email protected] TW233 Radmehr Monfared - Loughborough ©  37 Non-traditional Cutting Processes There are situations in manufacturing processes that traditional cutting methods through chip formation or abrasion are not possible or not economical. For example for material with very high hardness or flexibility, workpieces with complex shapes, high surface finish and dimensional accuracy, and sensitive material to thermal shocks caused by machining. Basic Characteristics The source of cutting force in non-traditional methods are typically different from applying cutting force from tool to material. Typically there are no contact with workpiece, and therefore minimised friction. Complex shapes can be machined, as the cutting is not limited to rotary and reciprocating motions. More expensive machinery and tools and high skilled labour required. Should be used when traditional methods are not possible or economical. Ref.: Valery Marinov, Kalpakjian Classification of Non-traditional Processes The non-traditional processes are often classified according to the principle form of energy used. They include: Mechanical processes: the mechanical energy differs from the action of the conventional cutting tool. Examples include ultrasonic machining and jet machining; Electrochemical processes: using electrochemical energy to remove material. Examples include electrochemical machining, and electrochemical grinding; Thermal energy processes: use thermal energy generated by the conversion of electrical energy to shape or cut the workpiece. Examples include electric discharge processes, electron beam machining, laser beam machining, and plasma arc cutting; Chemical machining: chemicals selectively remove material from portions of the workpiece, while other portions of the surface are mask protected. Ref.: Valery Marinov Radmehr Monfared - Loughborough ©  38 Ultrasonic Machining Ultrasonic Machining is a non-traditional process, in which abrasives contained in a slurry flows around the work by a tool oscillating at low amplitude (25-100 ) and high frequency (15-30 KHz). The basic process is that tool is pushed against the work with a constant force. A constant stream of abrasive slurry passes between the tool and the work (gap is 25-40 ) to create abrasion and carry away chips. Limitations of the ultrasonic machining include very low material removal rate, extensive tool wear, small depth of holes and cavities. However, the tool never contacts the workpiece and the grinding pressure is very low, which make the process suitable for hard and brittle materials. Ultrasonic Video - Bullentech.com Ref.: Valery Marinov Ultrasonic Machining The ultrasonic machining process can be used to cut through holes and blind holes of round or irregular cross-sections. The process is best suited to poorly conducting, hard and brittle materials like glass, ceramics, carbides, and semiconductors. There is small heat and stress generated in the process. The critical parameters to control the process are the tool frequency, amplitude and material, abrasive grit size and material, feed force, slurry concentration and viscosity. The acoustic head (similar to the spindle head in milling machine) provides a static constant force, as well as the high frequency vibration. The tools are produced of tough but ductile metals such as a variety of stainless steel. Abrasive slurry consists of a mixture of liquid (water is the most common but oils or glycerol are also used). The common types of abrasive materials include boron carbide, silicon carbide. See Video Ref.: Valery Marinov Radmehr Monfared - Loughborough ©  39 Electric Discharge Machining (EDM) In electric discharge processes, the work material is removed by a series of sparks that cause localised melting and evaporation of the material on the work surface. This process also called spark-erosion machining. A basic EDM system consist of shaped tool (electrode) and the workpiece connected to a DC power supply and placed in a dielectric (non conducting) fluid. When potential electric charge difference between tool and workpiece is high enough, spark discharges through the fluid and remove a small amount of metal from the workpiece surface. The capacitor discharge is repeated at rate of 50 to 500 kHz, with voltage of up to 380 V and current of up to 500 Amps. See Video Ref.: Valery Marinov, Kalpakjian Electric Discharge Machining (EDM) The EDM process is based on melting temperature at the cutting points, and therefore some very hard materials can be machined in this way. Clearly only metallic (conductive) based material can be processed. Electrodes are made of highly conductive material such as copper, tungsten and graphite. The tools are made by various (non)traditional methods depending on the complexity of the electrode geometry. These include machining, casting, powder metallurgy. Tool wear is an important factor in EDM process and has direct impact on the dimensional accuracy. The electrode made of higher melting point (e.g. graphite) will have the minimum wear. The dielectric fluid is used to : provide an electrical insulation until the capacitor is fully charged and electric potential is high enough provide a mean for carrying out the removed material from the workpiece to cool down the cutting environment Ref.: Valery Marinov, Kalpakjian Radmehr Monfared - Loughborough ©  40 Advanced EDM Processes Electric Discharge Machining (EDM) process is used in various applications in industries, including manufacturing of die cavities for large automobile body panels and parts of small plastic injection moulds. Material removal rate (MRR) for EDM process varies from 2 to 400 mm3/min. In a rough cutting process, removed material may recast to the surface and therefore may produce low surface finish. In this case a finish process (very low MRR) requires to provide high surface finish. Step cavities and internal cavities Some Design Considerations: can be produced by controlled Electrode shape has direct impact on the cost of machining. movement of the workpiece Deep slots and narrow opening to be avoided. and also rotation of the Not economical for very fine surface finish. specially purposed Bulk of material removal should be done by normal machining. electrodes. Ref. Kalpakjian – Printice Hall Wired EDM Process Wired EDM or electrical-discharge wire cutting is a special form of EDM that uses a small diameter wire as the electrode to cut narrow shapes in workpieces. The workpiece is fed continuously and slowly passes the wire in order to achieve the desired cutting path. While cutting, the wire is continuously advanced between a supply spool and a take-up spool to present a fresh electrode of constant diameter to the work. This helps to maintain dimensional accuracy. The dielectric fluid is supplied by nozzles directed at the tool and workpiece or the workpiece is submerged in a dielectric bath. See Video Ref.: Valery Marinov, Kalpakjian Radmehr Monfared - Loughborough ©  41 Wired EDM Process - Video Wired EDM cutting for tight tolerance machining Wired EDM Process Wire diameters range from 0.08 to 0.30 mm and made of brass, copper or tungsten. The wire is usually used once and is typically inexpensive. It travels in constant speed (0.1 to 9 m/min). The Wire EDM is suitable for many application including cutting dies, cams, etc. from sheet metal. Modern wired EDM machines are equipped with multi-axis wire cutting facility. The final surface quality varies depending on the speed of wire and the feed rate. The wired EDM machines are typically expensive. Ref.: Valery Marinov, Kalpakjian Radmehr Monfared - Loughborough ©  42 Water-Jet Machining Water jet cutting uses a high-pressure, high velocity stream of fluid directed at the work surface to cause slotting of the material. The speed of liquid is typically many times faster than sound (sound speed is about 343 m/s or 768 mph). Water is the most common fluid used, but additives such as alcohols, oil products and glycerol are also used to improve the fluid characteristics. The fluid is pressurised at 150-1000 Mpa (1 megapascal is 10 bars) to produce jet velocities of 540-1400 m/s. The fluid flow rate is typically from 0.5 to 2.5 l/min. Depending on the material, the thickness of the workpiece can increase up to 25 mm for steel (or even higher for softer material). Ref.: Valery Marinov, Kalpakjian Water-Jet Machining WJM is suitable for cutting through ductile (rather than brittle material). Typical work materials involve soft metals, paper, cloth, wood, leather, rubber, plastics, and frozen food. WJM is an efficient and clean process and is widely used in automotive industry to cut the dashboard material and some of the body panels. It is also used in food industry for cutting and slicing food. Surface finish depends on the feed rate and water pressure, but generally is not recommended for machining parts with high quality surface and very high dimensional accuracy. Advantage of this process include: No heat is produced Cut can be started at any point on the workpiece (no drilling required). The burr produced is minimal It is an environmentally safe process (not necessarily greener process considering the energy used to provide high pressure water) See Video Ref.: Valery Marinov, Kalpakjian Radmehr Monfared - Loughborough ©  43 Abrasive Water-Jet Machining In Abrasive Water Jet Cutting, a narrow, focused, water jet is mixed with abrasive particles such as silicon carbide and aluminium oxide. This jet is sprayed with very high pressures resulting in high velocities that cut through most materials. The presence of abrasive particles in the water jet reduces cutting forces and enables cutting of thick and hard materials (steel plates over 80mm thick can be cut). The velocity of the stream is up to 900 m/s, about 2.5 times the speed of sound. This process is particularly suitable for heat-sensitive materials that cannot be processed in traditional way. The application of abrasive water jet is limited in size and shape of the parts. For instance the smallest hole made in this method is 3mm. With multi-axis CNC controlled machine complex 3D parts can be produced with relatively high surface finish. Ref.: Valery Marinov, Kalpakjian Abrasive Jet Machining In Abrasive Jet Machining, fine abrasive particles (typically ~0.025mm) are accelerated in a gas stream (commonly air) towards the work surface. As the particles impact the work surface, they cause small fractures, and the gas stream carries both the abrasive particles and the fractured (wear) particles away. The nozzle could be fed automatically or be hand held. No sharp edges or fine holes can be produced in this method. The pressure of the gas for hand held machining is between 2-9 bars. Ref.: Valery Marinov, Kalpakjian Radmehr Monfared - Loughborough ©  44 Chemical Machining Chemical Machining (CM) was developed based on the observation that certain chemicals attack metals and remove small amount of material from the surface by using chemical dissolutions. Chemicals are usually acids and alkaline solutions. Chemical machining is one of the oldest of Workpiece surface is controlled non-traditional machining processes, and by removable layer of maskant which has been used to engrave metals protects part of the surface from reacting with chemical. and hard stones. The chemical machining is suitable for low production manufacturing. It has low tooling and equipment cost, but material removal rate is also low. Ref. Kalpakjian – Printice Hall Chemical Machining Chemical machining processes include chemical milling and chemical blanking. Chemical Milling process refers to removing material from a surface and is used for creating shallow cavities on various metals. It is widely used in aerospace industry to remove layers of material from panels for design or weight reductions. Chemical blanking is used to cut complex shapes through thin metals. It is widely used in electronic industry to produce PCB’s. Ref. Kalpakjian – Printice Hall Radmehr Monfared - Loughborough ©  45 Laser Cutting Laser Cutting or Laser Beam Machining (LBM) is a technology in which a beam of highly coherent light called a Laser is directed towards a small area of the work piece to cause melting and evaporating part of the workpiece. Laser cutting is typically used for cutting material, drilling or engraving surface of workpieces. It can be used to perform precision micro- machining on most material including metal, ceramic, silicon, diamond, and graphite. The rays of a laser beam are monochromatic and Mirror parallel, so it can be focused to a very small diameter and can produce energy as high as100 MW per mm2. It is especially suited for making accurately placed holes Semi transparent with diameter as low Mirror as 0.005 mm. Lens workpiece Laser - Light Amplification by Stimulated Emission of Radiation Ref. IIT Kharagpur , Wikipedia Laser Cutting How it works - In a very simplified view, if sufficient energy is applied to an atom, the electrons can leave what is called the ground-state energy level and go to an excited level. The level of excitation depends on the amount of energy that is applied to the atom via heat, light, or electricity. Once an electron moves to a higher-energy orbit, it eventually wants to return to the ground state. When it does, it releases its energy as a photon - a particle of light. A laser is a device that controls the way that energised atoms release photons. A flash tube injects light to the ruby rod to excite the atoms. Some atoms emit photons and some move back and forth in direction of the tube. Monochromatic light leaves the tube to a lens to be highly focused on the work piece. Ref. IIT Kharagpur , HowStuffWorks Radmehr Monfared - Loughborough ©  46 Laser Cutting The most commonly used laser type in machining use CO2 and Ruby for lasing material. In a CO2 laser, a mixture of CO2, N2 and He continuously circulate through the gas tube. Such continuous recirculation of gas is done to minimize consumption of gases. CO2 acts as the main lasing medium whereas Nitrogen helps in sustaining the gas plasma. Helium on the other hand helps in cooling the gases. Laser cutting video The efficiency of laser machining is usually 5 to 15% and therefore may not be most cost effective process for high volume production. Laser cutting is suitable for thin and tough/hard material, which have complex shapes and difficult for work holding. It produces hazardous x rays and requires further safety preparations. Material removal rates are usually very low and cost of machining equipment is very high. The laser unit typically attaches on a computer controlled machine. Distortion to the material due to the heat is minimised. Laser consumes a very significant power to operate. Ref. IIT Kharagpur , Questions... Some questions to practice Various types of cutting energy in non-traditional machining Concept of ultrasonic machining Application chemical milling in industry Application EDM Advantage of water-jet machining process Next Session £ Economical Factors in Metal Cutting Radmehr Monfared - Loughborough ©  47 Loughborough University Wolfson School of Mechanical and Manufacturing Engineering Manufacturing Technology (WSA610, WSB610) Metal Cutting Part 10 – Economical Aspects of Metal Cutting Dr Radmehr P Monfared [email protected] TW233 Economical Aspects Economical aspects of manufacturing are important consideration in design of manufacturing processes. Regardless of how well a product meets design specification, it has to meet the economical and market criteria in order to remain competitive. General trend in manufacturing operations is to produce Better Cheaper and Faster products. Note: this hand out only focus on the cost factors related to the metal cutting. Basic Concept Some of the contributing factors to the cost of a product are material, design specifications, manufacturing processes, labour and overhead costs Cost of a product determines its marketability and customer acceptance Design should be kept simple when possible Quality should be optimised (higher quality is not necessarily better) Machining processes should be selected according to the design, quality, volume, production rate, and cost. Ref. Kalpakjian – Prentice Hall Radmehr Monfared - Loughborough ©  48 Cost factors for Metal Cutting Processes Some of the determining factors on the manufacturing costs include: Product requirements, such as mechanical/physical/chemical characteristics Production processes (e.g. metal cutting processes or casting) Product volume Rate of production (i.e. volume per unit of time) Expected product service life Environmental factors Regulations Some of the cost elements in manufacturing productions include: Running cost of production facilities (e.g. machines, tools, inspection) Running cost of manufacturing organisation (e.g. estate, utilities, admin) Investment cost and overheads Labour costs Cost of raw and semi finished material Part handling and inspections Rejected and reworked parts Trainings Cost f