Common Machining Processes FIGURE 8.1 Some examples of common machining processes.
Orthogonal Cutting FIGURE 8.2 Schematic illustration of a two-dimensional cutting process, or orthogonal cutting. (a) Orthogonal cutting with a well-defined shear plane, also known as the Merchant model; (b) Orthogonal cutting without a well-defined shear plane.
Chip Formation FIGURE 8.3 (a) Schematic illustration of the basic mechanism of chip formation in cutting. (b) Velocity diagram in the cutting zone.
Types of Chips FIGURE 8.4 Basic types of chips produced in metal cutting and their micrographs: (a) continuous chip with narrow, straight primary shear zone; (b) secondary shear zone at the tool-chip interface; (c) continuous chip with built-up edge; (d) segmented or nonhomogeneous chip; and (e) discontinuous chip. Source: After M.C. Shaw, P.K.Wright, and S. Kalpakjian. FIGURE 8.5 Shiny (burnished) surface on the tool side of a continuous chip produced in turning.
Hardness in Cutting Zone FIGURE 8.6 (a) Hardness distribution in the cutting zone for 3115 steel. Note that some regions in the built-up edge are as much as three times harder than the bulk workpiece. (b) Surface finish in turning 5130 steel with a built-up edge. (c) Surface finish on 1018 steel in face milling. Source: Courtesy of Metcut Research Associates, Inc.
Chip Breakers FIGURE 8.7 (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 on the rake face of cutting tools, acting as chip breakers. Most cutting tools now are inserts with built-in chipbreaker features. FIGURE 8.8 Various chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving radially outward from workpiece; and (d) chip hits tool shank and breaks off. Source: After G. Boothroyd.
Oblique Cutting FIGURE 8.9 (a) Schematic illustration of cutting with an oblique tool. (b) Top view, showing the inclination angle, i. (c)types of chips produced with different inclination angles.
Right-Hand Cutting Tool FIGURE 8.10 (a) Schematic illustration of a right-hand cutting tool for turning. Although these tools have traditionally been produced from solid tool-steel bars, they are now replaced by inserts of carbide or other tool materials of various shapes and sizes, as shown in (b).
Cutting Forces FIGURE 8.11 (a) Forces acting on a cutting tool in two-dimensional cutting. Note that the resultant forces, R, must be collinear to balance the forces. (b) Force circle to determine various forces acting in the cutting zone. Source: After M.E. Merchant. Cutting force Friction coefficient
Cutting Data TABLE 8.1 Data on orthogonal cutting of 4130 steel. TABLE 8.2 Data on orthogonal cutting of 9445 steel. FIGURE 8.12 Thrust force as a function of rake angle and feed in orthogonal cutting of AISI 1112 coldrolled steel. Note that at high rake angles, the thrust force is negative. A negative thrust force has important implications in the design of machine tools and in controlling the stability of the cutting process. Source: After S. Kobayashi and E.G.Thomsen.
Shear Force & Normal Force FIGURE 8.13 (a) Shear force and (b) normal force as a function of the area of the shear plane and the rake angle for 85-15 brass. Note that the shear stress in the shear plane is constant, regardless of the magnitude of the normal stress, indicating that the normal stress has no effect on the shear flow stress of the material. Source: After S. Kobayashi and E.G.Thomsen.
Shear Stress on Tool Face FIGURE 8.14 Schematic illustration of the distribution of normal and shear stresses at the tool-chip interface (rake face). Note that, whereas the normal stress increases continuously toward the tip of the tool, the shear stress reaches a maximum and remains at that value (a phenomenon known as sticking; see Section 4.4.1).
Shear-Angle Relationships FIGURE 8.15 (a) Comparison of experimental and theoretical shear-angle relationships. More recent analytical studies have resulted in better agreement with experimental data. (b) Relation between the shear angle and the friction angle for various alloys and cutting speeds. Source: After S. Kobayashi. Merchant [Eq. (8.20)] Mizuno [Eqs. (8.22)-(8.23] Shaffer [Eq. (8.21)]
Specific Energy TABLE 8.3 Approximate Specific-Energy Requirements in Machining Operations
Temperatures in Cutting FIGURE 8.1 Typical temperature distribution in the cutting zone. Note the severe temperature gradients within the tool and the chip, and that the workpiece is relatively cool. Source: After G.Vieregge. FIGURE 8.2 Temperature distribution in turning as a function of cutting speed: (a) flank temperature; (b) temperature along the tool-chip interface. Note that the rake-face temperature is higher than that at the flank surface. Source: After B.T. Chao and K.J.Trigger. FIGURE 8.18 Proportion of the heat generated in cutting transferred to the tool, workpiece, and chip as a function of the cutting speed. Note that most of the cutting energy is carried away by the chip (in the form of heat), particularly as speed increases.
Terminology in Turning FIGURE 8.19 Terminology used in a turning operation on a lathe, where f is the feed (in mm/rev or in./rev) and d is the depth of cut. Note that feed in turning is equivalent to the depth of cut in orthogonal cutting (see Fig. 8.2), and the depth of cut in turning is equivalent to the width of cut in orthogonal cutting. See also Fig. 8.42.
Tool Wear Taylor tool life equation: TABLE 8.4 Range of n values for various cutting tools. FIGURE 8.20 Examples of wear in cutting tools. (a) Flank wear; (b) crater wear; (c) chipped cutting edge; (d) thermal cracking on rake face; (e) flank wear and built-up edge; (f) catastrophic failure (fracture). Source: Courtesy of Kennametal, Inc.
Effect of Workpiece on Tool Life FIGURE 8.21 Effect of workpiece microstructure on tool life in turning. Tool life is given in terms of the time (in minutes) required to reach a flank wear land of a specified dimension. (a) Ductile cast iron; (b) steels, with identical hardness. Note in both figures the rapid decrease in tool life as the cutting speed increases.
Tool-Life Curves FIGURE 8.22 (a) Tool-life curves for a variety of cutting-tool materials. The negative inverse of the slope of these curves is the exponent n in tool-life equations. (b) Relationship between measured temperature during cutting and tool life (flank wear). Note that high cutting temperatures severely reduce tool life. See also Eq. (8.30). Source: After H.Takeyama and Y. Murata.
Tool Wear FIGURE 8.23 Relationship between craterwear rate and average tool-chip interface temperature in turning: (a) high-speed-steel tool; (b) C1 carbide; (c) C5 carbide. Note that crater wear increases rapidly within a narrow range of temperature. Source: After K.J. Trigger and B.T. Chao. TABLE 8.5 Allowable average wear lands for cutting tools in various operations. FIGURE 8.23 Interface of chip (left) and rake face of cutting tool (right) and crater wear in cutting AISI 1004 steel at 3 m/s (585 ft/min). Discoloration of the tool indicates the presence of high temperature (loss of temper). Note how the crater-wear pattern coincides with the discoloration pattern. Compare this pattern with the temperature distribution shown in Fig. 8.16. Source: Courtesy of P.K. Wright.
Acoustic Emission and Wear FIGURE 8.25 Relationship between mean flank wear, maximum crater wear, and acoustic emission (noise generated during cutting) as a function of machining time. This technique has been developed as a means for continuously and indirectly monitoring wear rate in various cutting processes without interrupting the operation. Source: After M.S. Lan and D.A. Dornfeld.
Surface Finish FIGURE 8.26 Range of surface roughnesses obtained in various machining processes. Note the wide range within each group, especially in turning and boring. (See also Fig. 9.27).
Surfaces in Machining FIGURE 8.27 Surfaces produced on steel in machining, as observed with a scanning electron microscope: (a) turned surface, and (b) surface produced by shaping. Source: J.T. Black and S. Ramalingam. FIGURE 8.28 Schematic illustration of a dull tool in orthogonal cutting (exaggerated). Note that at small depths of cut, the rake angle can effectively become negative. In such cases, the tool may simply ride over the workpiece surface, burnishing it, instead of cutting.
Inclusions in Free-Machining Steels FIGURE 8.29 Photomicrographs showing various types of inclusions in low-carbon, resulfurized freemachining steels. (a) Manganese-sulfide inclusions in AISI 1215 steel. (b) Manganese-sulfide inclusions and glassy manganese-silicate-type oxide (dark) in AISI 1215 steel. (c) Manganese sulfide with lead particles as tails in AISI 12L14 steel. Source: Courtesy of Ispat Inland Inc.
Hardness of Cutting Tools FIGURE 8.30 Hardness of various cutting-tool materials as a function of temperature (hot hardness). The wide range in each group of tool materials results from the variety of compositions and treatments available for that group.
Tool Materials TABLE 8.6 Typical range of properties of various tool materials.
Properties of Tungsten-Carbide Tools FIGURE 8.31 Effect of cobalt content in tungsten-carbide tools on mechanical properties. Note that hardness is directly related to compressive strength (see Section 2.6.8) and hence, inversely to wear [see Eq. (4.6)].
Inserts FIGURE 8.32 Methods of mounting inserts on toolholders: (a) clamping, and (b) wing lockpins. (c) Examples of inserts mounted using threadless lockpins, which are secured with side screws. Source: Courtesy of Valenite.
Insert Strength FIGURE 8.33 Relative edge strength and tendency for chipping and breaking of inserts with various shapes. Strength refers to that of the cutting edge shown by the included angles. Source: Courtesy of Kennametal, Inc. FIGURE 8.34 Edge preparations for inserts to improve edge strength. Source: Courtesy of Kennametal, Inc.
Historical Tool Improvement FIGURE 8.35 Relative time required to machine with various cutting-tool materials, with indication of the year the tool materials were introduced. Note that, within one century, machining time has been reduced by two orders of magnitude. Source:After Sandvik Coromant.
Coated Tools FIGURE 8.36 Wear patterns on high-speed-steel uncoated and titanium-nitride-coated cutting tools. Note that flank wear is lower for the coated tool. FIGURE 8.37 Multiphase coatings on a tungsten-carbide substrate. Three alternating layers of aluminum oxide are separated by very thin layers of titanium nitride. Inserts with as many as 13 layers of coatings have been made. Coating thicknesses are typically in the range of 2 to 10 µm. Source: Courtesy of Kennametal, Inc.
Properties of Cutting Tool Materials FIGURE 8.38 Ranges of properties for various groups of cutting-tool materials. (See also Tables 8.1 through 8.5.) FIGURE 8.39 Construction of polycrystalline cubicboron-nitride or diamond layer on a tungsten-carbide insert.
Characteristics of Machining TABLE 8.7 General characteristics of machining processes.
Lathe Operations FIGURE 8.40 Variety of machining operations that can be performed on a lathe.
Tool Angles FIGURE 8.41 Designations and symbols for a right-hand cutting tool. The designation right hand means that the tool travels from right to left, as shown in Fig. 8.19. TABLE 8.8 General recommendations for tool angles in turning.
Turning Operations FIGURE 8.42 (a) Schematic illustration of a turning operation, showing depth of cut, d, and feed, f. Cutting speed is the surface speed of the workpiece at the tool tip. (b) Forces acting on a cutting tool in turning. Fc is the cutting force; Ft is the thrust or feed force (in the direction of feed); and Fr is the radial force that tends to push the tool away from the workpiece being machined. Compare this figure with Fig. 8.11 for a two-dimensional cutting operation.
Cutting Speeds for Turning FIGURE 8.43 The range of applicable cutting speeds and feeds for a variety of cutting-tool materials. TABLE 8.9 Approximate Ranges of Recommended Cutting Speeds for Turning Operations
Lathe FIGURE 8.44 General view of a typical lathe, showing various major components. Source: Courtesy of Heidenreich & Harbeck.
CNC Lathe FIGURE 8.45 (a) A computer-numerical-control lathe, with two turrets; these machines have higher power and spindle speed than other lathes in order to take advantage of advanced cutting tools with enhanced properties; (b) a typical turret equipped with ten cutting tools, some of which are powered.
Typical CNC Parts FIGURE 8.46 Typical parts made on computer-numerical-control machine tools.
Typical Production Rates TABLE 8.10 Typical production rates for various cutting operations.
Boring Mill FIGURE 8.47 Schematic illustration of the components of a vertical boring mill.
Drills FIGURE 8.48 Two common types of drills: (a) Chisel-point drill. The function of the pair of margins is to provide a bearing surface for the drill against walls of the hole as it penetrates into the workpiece. Drills with four margins (double-margin) are available for improved drill guidance and accuracy. Drills with chip-breaker features are also available. (b) Crankshaft drills. These drills have good centering ability, and because chips tend to break up easily, they are suitable for producing deep holes. FIGURE 8.49 Various types of drills and drilling operations.
Speeds and Feeds in Drilling TABLE 8.11 General recommendations for speeds and feeds in drilling.
Reamers and Taps FIGURE 8.50 Terminology for a helical reamer. FIGURE 8.51 (a) Terminology for a tap; (b) illustration of tapping of steel nuts in high production.
Typical Machined Parts FIGURE 8.52 Typical parts and shapes produced by the machining processes described in Section 8.10.
Conventional and Climb Milling FIGURE 8.53 (a) Illustration showing the difference between conventional milling and climb milling. (b) Slab-milling operation, showing depth of cut, d; feed per tooth, f; chip depth of cut, tc and workpiece speed, v. (c) Schematic illustration of cutter travel distance, lc, to reach full depth of cut.
Face Milling FIGURE 8.54 Face-milling operation showing (a) action of an insert in face milling; (b) climb milling; (c) conventional milling; (d) dimensions in face milling. Terminology for a face- FIGURE 8.55 milling cutter.
Cutting Mechanics FIGURE 8.56 The effect of lead angle on the undeformed chip thickness in face milling. Note that as the lead angle increases, the undeformed chip thickness (and hence the thickness of the chip) decreases, but the length of contact (and hence the width of the chip) increases. Note that the insert must be sufficiently large to accommodate the increase in contact length. FIGURE 8.57 (a) Relative position of the cutter and the insert as it first engages the workpiece in face milling, (b) insert positions at entry and exit near the end of cut, and (c) examples of exit angles of the insert, showing desirable (positive or negative angle) and undesirable (zero angle) positions. In all figures, the cutter spindle is perpendicular to the page.
Milling Operations FIGURE 8.58 Cutters for (a) straddle milling; (b) form milling; (c) slotting; and (d) slitting operations. TABLE 8.12 Approximate range of recommended cutting speeds for milling operations.
Milling Machines FIGURE 8.59 (a) Schematic illustration of a horizontal-spindle column-and-knee-type milling machine. (b) Schematic illustration of a vertical-spindle column-and-knee-type milling machine. Source: After G. Boothroyd.
Broaching FIGURE 8.60 (a) Typical parts finished by internal broaching. (b) Parts finished by surface broaching. The heavy lines indicate broached surfaces; (c) a vertical broaching machine. Source: (a) and (b) Courtesy of General Broach and Engineering Company, (c) Courtesy of Ty Miles, Inc.
Broaches FIGURE 8.61 (a) Cutting action of a broach, showing various features. (b) Terminology for a broach. FIGURE 8.62 Terminology for a pull-type internal broach, typically used for enlarging long holes.
Saws and Saw Teeth FIGURE 8.63 (a) Terminology for saw teeth. (b) Types of saw teeth, staggered to provide clearance for the saw blade to prevent binding during sawing. FIGURE 8.64 (a) High-speed-steel teeth welded on a steel blade. (b) Carbide inserts brazed to blade teeth.
Gear Manufacture FIGURE 8.65 (a) Schematic illustration of gear generating with a pinion-shaped gear cutter. (b) Schematic illustration of gear generating in a gear shaper, using a pinionshaped cutter; note that the cutter reciprocates vertically. (c) Gear generating with a rack-shaped cutter. (d) Three views of gear cutting with a hob. Source: After E.P. DeGarmo.
Machining Centers FIGURE 8.66 A horizontal-spindle machining center, equipped with an automatic tool changer. Tool magazines in such machines can store as many as 200 cutting tools, each with its own holder. Source: Courtesy of Cincinnati Machine. FIGURE 8.67 Schematic illustration of a computer numerical-controlled turning center. Note that the machine has two spindle heads and three turret heads, making the machine tool very flexible in its capabilities. Source: Courtesy of Hitachi Seiki Co., Ltd.
Reconfigurable Machines FIGURE 8.68 Schematic illustration of a reconfigurable modular machining center, capable of accommodating workpieces of different shapes and sizes, and requiring different machining operations on their various surfaces. Source: After Y. Koren.
Reconfigurable Machining Center FIGURE 8.69 Schematic illustration of assembly of different components of a reconfigurable machining center. Source: After Y. Koren.
Machining of Bearing Races FIGURE 8.70 Sequences involved in machining outer bearing races on a turning center.
Hexapod FIGURE 8.71 (a) A hexapod machine tool, showing its major components. (b) Closeup view of the cutting tool and its head in a hexapod machining center. Source: National Institute of Standards and Technology.
Chatter & Vibration FIGURE 8.72 Chatter marks (right of center of photograph) on the surface of a turned part. Source: Courtesy of General Electric Company. FIGURE 8.73 Relative damping capacity of (a) gray cast iron and (b) epoxy-granite composite material. The vertical scale is the amplitude of vibration and the horizontal scale is time. FIGURE 8.74 Damping of vibrations as a function of the number of components on a lathe. Joints dissipate energy; thus, the greater the number of joints, the higher the damping. Source: After J. Peters.
Machining Economics FIGURE 8.75 Qualitative plots showing (a) cost per piece, and (b) time per piece in machining. Note that there is an optimum cutting speed for both cost and time, respectively. The range between the two optimum speeds is known as the highefficiency machining range.
Case Study: Ping Golf Putters FIGURE 8.76 (a) The Ping Anser golf putter; (b) CAD model of rough machining of the putter outer surface; (c) rough machining on a vertical machining center; (d) machining of the lettering in a vertical machining center; the operation was paused to take the photo, as normally the cutting zone is flooded with a coolant; Source: Courtesy of Ping Golf, Inc.