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Introduction ¾ What is machining? Removal of material from a workpiece • Cutting ○ Single or multipoint cutting tools. Each with a clearly defined tools shape • Abrasive processes Chapter 20 ○ Ex: grinding • Advanced machining processes Fundamentals of Cutting ○ Electrical, chemical, thermal, and hydrodynamic methods, and lasers. ¾ Advantages of machining Good dimensional accuracy May include internal and external geometrical features, sharp corners, and flatness Some surface characteristics and textures can’t be produced with other processes Economical for very small production runs Alexandra Schönning, Ph.D. Mechanical Engineering University of North Florida Figures by Manufacturing Engineering and Technology Kalpakijan and Schmid ¾ Disadvantages of machining Wastes material, require more energy, capital and labor Time: it takes longer to remove material than it does to form it. Typically expensive for large production runs Page 20-1 Page 20-2 Introduction Introduction: Chip Formation ¾ What are some cutting operations? ¾ Cutting processes cause ship formation ahead of the tool ¾ Some parameters effecting the cutting operation Turning • The workpiece is rotated and a cutting tool removes a layer of material as it moves in one direction Cutting-off Depth of cut Feed rate = distance the tool travels per unit revolution of the workpiece (for turning). Units: mm/rev, in/rev… • Cutting tool moves radially inward Slab milling • A rotating cutting tool removes a layer of material from the surface of the workpiece End milling Figure 20.1 Examples of cutting processes. • Rotating cutter travels along a certain depth in the workpiece to produce a cavity Page 20-3 Figure 20.3 Schematic illustration of a twodimensional cutting process, also called orthogonal cutting. Note that the tool shape and its angles, depth of cut, to, and the cutting speed, V, are all independent variables. Variables in cutting operations ¾ Independent variables Tool material, coatings, and tool condition Tool shape, surface finish, and sharpness Workpiece material, condition and temperature Cutting parameters, such as speed, feed, and depth of cut Cutting fluid Characteristics of the machine tool, such as stiffness and damping Workholding and fixturing Figure 20.2 Basic principle of the turning operations. Page 20-4 Factors Influencing Cutting Processes ¾ Dependent variables Type of chip produced Force and energy dissipated in the cutting process Temperature rise in the workpiece, the chip, and the tool Wear and failure of the tool Surface finish produced on the workpiece after machining Page 20-5 TABLE 20.1 Parameter Cutting speed, depth of cut, feed, cutting fluids Tool angles Continuous chip Built-up edge chip Discontinuous chip Temperature rise Tool wear Machinability Influence and interrelationship Forces, power, temperature rise, tool life, type of chip, surface finish. As above; influence on chip flow direction; resistance to tool chipping. Good surface finish; steady cutting forces; undesirable in automated machinery. Poor surface finish; thin stable edge can protect tool surfaces. Desirable for ease of chip disposal; fluctuating cutting forces; can affect surface finish and cause vibration and chatter. Influences tool life, particularly crater wear, and dimensional accuracy of workpiece; may cause thermal damage to workpiece surface. Influences surface finish, dimensional accuracy, temperature rise, forces and power. Related to tool life, surface finish, forces and power. Page 20-6 1 Mechanics of Chip Formation ¾ Orthogonal cutting: The tool edge is perpendicular to the movement of the tool ¾ Geometrical terms Types of chips produced in metal cutting Thickness of chip is always greater than the depth of the cut • Cutting ratio: r = to/tc (depth of cut/chip thickness), always less than 1 • Chip compression ratio: 1/r, always greater than unity Rake angle Relief angle Shear angle ¾ How are the chips formed? ¾ Why do we care what kind of chips are produced? ¾ The type of chip influences the surface finish of the workpiece and the overall cutting operation. ¾ Types of chips produced Continuous Built-up edge Serrated or segmented Discontinuous ¾ Two sides of the chip Shearing takes place along a shear zone Tool side • Below the shear plane the the workpiece is undeformed • A chip is formed above Figure 20.4 (a) Schematic illustration of the basic mechanism of chip shear plane formation in metal cutting. (b) Velocity diagram in the cutting zone. • In contact with the tool face (rake face) • Smooth surface Other surface • Jagged, rough appearance caused by the shearing mechanism See also Section 20.5.3. Source: M. E. Merchant. Page 20-7 Page 20-8 Continuous chips ¾ Usually formed with ductile materials at high speeds and /or rake angles ¾ Produce good surface finish ¾ Typically not desirable Built-up edge (BUE) chips (a) continuous chip with narrow, straight primary shear zone; ¾Layer of material from the workpiece is gradually deposited on the edge of the tool. ¾BUE becomes unstable and eventually breaks up Deposited randomly on the surface of the workpiece ¾BUE is one of the most common factors adversely affects the finished surface. ¾Somewhat desirable: thin layer protects the surface of the tool. ¾BUE formation is reduced by Tangled around the tool holder, the fixturing, and the workpiece Operation must be stopped to untangle/remove ¾ Problem solved by chip breakers changing machining parameters (cutting speed, feed, cutting fluids) Decreasing the depth of the cut Increasing the rake angle Using a sharp tool Using an effective cutting fluid Page 20-9 Serrated Chips Page 20-10 Discontinuous chips ¾Also called segmented or nonhomogeneous chips ¾Semicontinuous chips with zones of low and high shear strain ¾Chips have a saw-tooth like appearance ¾Observed in metals with Low thermal conductivity Strength that decreases sharply with temperaturewith Example: titanium ¾Segments that are firmly or loosely attached to each other ¾Form under conditions Brittle workpiece material Workpiece material with hard inclusions and impurities Very low or very high cutting speeds Low rake angle Lack of an effective cutting fluid Low stiffness of the machine tool ¾Forces vary during cutting because of the discontinuous nature of the chip formation Page 20-11 Page 20-12 2 Chip breakers Examples of Chips Produced in Turning ¾ Continuous chips are undesirable and a safety hazard. Figure 20.8 Various chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving away from workpiece; and (d) chip hits tool shank and breaks off. Source: G. Boothroyd, Fundamentals of Metal Machining and Machine Tools. Copyright ©1975; McGraw-Hill Publishing Company. Used with permission. Become entangled and interfere with cutting operation. ¾ Chips can be broken up with a chip breaker Traditionally: a piece of metal attached to the rake of the tool – bends the chip upward and breaks it. ¾ Soft materials Difficult to break using a chip breaker Instead: • machine in small increments and then pausing • Reverse the feed by a small increment Figure 20.7 (a) Schematic illustration of the action of a chip breaker. Note that the chip breaker decreases the radius of curvature of the chip. (b) Chip breaker clamped on the rake face of a cutting tool. (c) Grooves in cutting tools acting as chip breakers; see also Fig. 21.2. Page 20-13 Page 20-14 Mechanics of oblique cutting ¾ Orthagonal cutting: described previously Tool edge is perpendicular to the movement of the tool Chip slides up the face of the tool ¾ Oblique cutting An inclination angle is used to force the chip sideways Cutting Forces and Power ¾ Fc: cutting force ¾ Inserts: mounted on tool holders In the direction of the cutting speed May include tools for drilling, tapping, milling, planing, shaping, broaching, sawing, and filing ¾ Ft: thrust force In the direction normal to the cutting velocity ¾ F: friction force ¾ N: Normal force ¾ µ: coefficient of friction ¾ Shaving Shaving: similar to shave wood using a planer Used to improve surface finish and dimensional accuracy Typically from 0.5 to 2 F R ⋅ sin ( β ) N R⋅ cos ( β ) Fn Fs µ Figure 20.11 Forces acting on a cutting tool in two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces. Fc⋅ sin ( φ) + Ft⋅ cos ( φ) Fc⋅ cos ( φ) − Ft⋅ sin ( φ) F N Ft + Fc⋅ tan ( α ) Fc − Ft⋅ tan ( α ) Page 20-15 Thrust Force and Power Page 20-16 Temperature in Cutting ¾ Thrust force can cause deflections in the tool holder, work holding device, and the machine tool Acts downward ¾ Power Power=Fc*v Dissipated mainly in the shear zone • Power for shearing = Fs*vs Dissipated also through friction • Power for friction = F*vc Cutting ratio = r = to/tc (=depth of cut / chip thickness)=vc/v ¾ Example In an orthagonal cutting operation, depth of cut to = 0.005 in, cutting speed v= 400 ft/min, rake angle α = 10o, and the width of cut = 0.25 in. It is observed that chip thickness tc = 0.009 in, Fc = 125 lb, and Ft = 50 lb. Calculate what percentage of the total energy goes into overcoming friction at the tool-chip interface. Solution: Percentage is: friction energy/total energy = Fvc/Fcv = F*r / Fc r = to/tc = 5 / 9 =0.555 F=R*sin(β) Fc = R*cos(β- α) R = [(Ft2+Fc2)]1/2 = [(502+1252)]1/2 = 315 lb Fc = R*cos(β- α) Æ 125 = 135*cos(β-10) • Gives: β = 32o ¾ Why is knowledge of the temperature rise important? Excessive temperature adversely affects the strength, hardness, and wear resistance of the cutting tool Difficult to control dimensional accuracy Thermal damage to the machined surface Distortion of the machine ¾ Main source of heat generation: ¾ What does the temperature depend on? Specific heat Thermal conductivity Depth of cut Type of cutting fluid ¾ How is temperature measured? Thermocouples embedded in the tool and/or workpiece Infrared radiation Thermal emf (elctromotive force) Shear zone Tool-chip interface F=R*sin(β) Æ F=135*sin(32) = 71.5 lb Percentage: F*r / Fc = 71.5*0.555 / 125 = 0.32 Æ 32% Page 20-17 Page 20-18 3 Tool life: wear and failure Tool life: wear and failure ¾ Cutting tools are subject to ¾Chipping High localized stresses High temperatures Sliding of the chip along the rake of the face Sliding of the tool along the freshly cut surface Breaking away of small pieces from the cutting edge of the tool Unlike wear, which is a gradual process, chipping results in a sudden loss of tool material and a corresponding change in shape ¾ This effects Tool life, quality of the machined surface, dimensional accuracy, economics of cutting operations ¾ Types of wear Flank wear • On the relief face of the tool • Due to rubbing of the tool against the machined surface and due to high temperatures Crater wear • Occurs on the rake face of the tool • Due to ○ temperature at the tool-chip interface ○ Chemical affinity between the tool and the workpiece Page 20-19 Page 20-20 Surface finish an surface integrity ¾ Surface finish influences Dimensional accuracy Mechanical properties ¾ Surface integrity Properties such as fatigue life and corrosion resistance Influenced by Machinability ¾ Negative effective rake angle Due to rounded tool edge (dull tool) Tool will ride over the surface without removing any material ¾ Defined using the factors: Surface finish and integrity of the machined part Tool life obtained Force and power requirements Chip control ¾ In manufacturing plants tool life and surface roughness are considered the most important factors in machinability ¾ In steels: • Temperatures generated during processing • Residual stresses • Metallurgical (phase) transformations • Surface plastic deformation, tearing, and cracking Machinability has been improved by adding sulfur and lead • Sulfur: ○ Act as stress raisers in the primary shear zone ○ Chips break up easily and are small ¾ BUE (built-up edge) • Lead: Has the greatest influence on the surface finish ○ Lead melts at low temperatures ○ Low shear strength Æ acts as a liquid lubricant Page 20-21 Page 20-22 4