Machining Processes TABLE 8.7 General characteristics of machining processes.

A. Turning (Lathe) Operations FIGURE 8.40 Various cutting operations that can be performed on a lathe. Common features: 1.Produce round surfaces 2.The workpiece turns at N (rpm) and the tool has the feed motion.

Cutting Parameters for Turning 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. F c is the cutting force; F t is the thrust or feed force (in the direction of feed); and F r is the radial force that tends to push the tool away from the workpiece being machined. Compare this figure with Fig for a two-dimensional cutting operation.

Cutting Parameters for Turning – Close Up FIGURE 8.19 Terminology used in a turning operation on a lathe, where f is the feed (in./rev or mm/rev) and d is the depth of cut. Note that feed in turning is equivalent to the depth of cut in orthogonal cutting (Fig. 8.2), and the depth of cut in turning is equivalent to the turning is equivalent to the width of cut in orthogonal cutting. See also Fig

Cutting Parameters - Listing Cutting Process Parameters: –Cutting Speed V (in/min) V= .D.N –Depth of Cut d (in) –Feed Rate f (in/min) or f (in/rev) f (in/min) = f (in/rev)*N(rev/min) The selection of cutting parameters depends on: –Workpiece material –Cutting tool material –Surface finish (desired) –Dimensional accuracy (desired) –Machine tool capacity

Force and Power Calculations Cutting velocity = V (in/min) = . D (in). N(rpm) Minimum Cutting Velocity = V minimum = . D o. N(rpm) Maximum Cutting Velocity = V maximum = . D f. N(rpm) Average Cutting Velocity = V average = . (D o +D f )/2. N(rpm) Material Removal Rate = MRR (in 3 /min) = . D avg. d. f (in/rev). N(rpm) ORMRR (in 3 /min) = . D avg. d. f (in/min) Cutting Power = HP = MRR (in 3 /min). u t (hp.min/in 3 ) : u t (hp.min/in 3 ) is the material specific energy (Table 8.3) Also, Cutting Power = F c (lbf)* V (in/min) / Torque (lbf.in) = Power (hp) * ((in.lbf/min)/hp) / 2  N Cutting Time = t (minutes) = l(in) / [f(in/rev).N(rpm)] = l(in) / f(in/min)

Range of Cutting Speeds FIGURE 8.43 The range of applicable cutting speeds and fees for a variety of tool materials. Source: Valenite, Inc. Notice that Ceramic tools has the highest cutting speed applications with low- moderate feeds, while Carbide tools have a wide range of feed applications at relatively low cutting speeds.

Cutting Speeds in Turning TABLE 8.8 Approximate range of recommended cutting speeds for turning operations.

Components of a Lathe FIGURE 8.44 Schematic illustration of the components of a lathe. Source: Courtesy of Heidenreich & Harbeck.

B. Milling Operations (Horizontal & Vertical) 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: G. Boothroyd, Fundamentals of Machining and Machine Tools.

Typical Parts Made by Milling FIGURE 8.52 Typical parts and shapes produced by the cutting processes described in Section 8.9.

1. Slab Milling (Horizontal) FIGURE 8.53 (a) Schematic illustration of conventional milling and climb milling. (b) Slab-milling operation, showing depth of cut, d; feed per tooth, f; chip depth of cut, t c ; and workpiece speed, v. (c) Schematic illustration of cutter travel distance to reach full depth of cut. Two Types: Conventional (Up) and Climb (Down) Millings. Up milling is preferred when the workpiece has a hard surface. Down Milling is preferred when the workpiece is difficult to clamp.

Slab Milling Analyses Cutting velocity = V (in/min) = . D (in). N(rpm) where D(in) and N(rpm) are the cutter’s diameter and rotational speed. Let the number of cutting edges (teeth) = (n) and the width of cut = w (in). Material Removal Rate = MRR (in 3 /min) = w. d. f (in/tooth). n (teeth). N(rpm) OR MRR (in 3 /min) = w. d. f (in/min) where f (in/min) = f (in/tooth). n (teeth). N(rpm) Cutting Power = HP = MRR (in 3 /min). u t (hp.min/in 3 ) : u t (hp.min/in 3 ) is the material specific energy (Table 8.3) Torque (lbf.in) = Power (hp) * ((in.lbf/min)/hp) / 2  N Cutting Time = t (minutes) = (l + l c )/ f(in/min) : l c = (D.d) 1/2

Other Horizontal Milling Operations FIGURE 8.58 Cutters for (a) straddle milling and (b) form milling.

2. Face Milling (Vertical) 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. The width of cut, w, is not necessarily the same as the cutter radius. Source: Courtesy of The Ingersoll Cutting Tool Company.

Face-Milling: Four Inserts (teeth) Cutter FIGURE 8.55 Terminology for a face-milling cutter.

Face Milling: Entry and Exit Examples FIGURE 8.57 (a) Relative position of the cutter and insert as it first engages the workpiece in face milling, (b) insert positions toward the end of cut, and (c) examples of exit angles of insert, showing desirable (positive are negative angle) and undesirable (zero angle) positions. In all figures, the cutter spindle is perpendicular to the page.

Face Milling Analyses Cutting velocity = V (in/min) = . D (in). N(rpm) where D(in) and N(rpm) are the cutter’s diameter and rotational speed. Let the number of inserts (teeth) = (n) and the width of cut = w (in). Material Removal Rate = MRR (in 3 /min) = w. d. f (in/tooth). n (teeth). N(rpm) OR MRR (in 3 /min) = w. d. f (in/min) where f (in/min) = f (in/tooth). n (teeth). N(rpm) Cutting Power = HP = MRR (in 3 /min). u t (hp.min/in 3 ) : u t (hp.min/in 3 ) is the material specific energy (Table 8.3) Torque (lbf.in) = Power (hp) * ((in.lbf/min)/hp) / 2  N Cutting Time = t (minutes) = (l + 2l c )/ f(in/min) : l c = D/2

Cutting Speeds in Milling TABLE 8.11 Approximate range of recommended cutting speeds for milling operations.

3. Drilling Operations FIGURE 8.49 Various types of drills and drilling operations.

Typical Speeds and Feeds in Drilling TABLE 8.10 General recommendations for speeds and feeds in drilling.

4. Broaching Operations FIGURE 8.61 (a) Cutting action of a broach, showing various features. (b) Terminology for a broach. The total depth of cut is equal to the product of the depth of cut per tooth by the number of teeth. Advantages: good surface finish/accuracy, high production rate, and complex internal/external geometries. Disadvantages: expensive tooling and high power machine tools (hydraulic). Can be performed using horizontal or vertical broaching machine tools (push or pull).

Examples of Internal and Surface Broaching FIGURE 8.60 (a) Typical parts made by internal broaching. (b) Parts made by surface broaching. The heavy lines indicate broached surfaces. Source: General Broach and Engineering Company. FIGURE 8.62 Terminology for a pull-type internal broach used for enlarging long holes.

Chatter and Vibration FIGURE 8.69 Chatter marks (right of center of photograph) on the surface of a turned part. Source: General Electric Company. Chatter results in: -poor surface finish -loss of dimensional accuracy -premature tool failure -damage to machine tool -noise

Two types of chatter: A.Forced Vibration: periodic applied force caused by the machine tool motor, gears, pumps, …etc. Can be reduced by increasing the machine stiffness. B.Self-Excited Vibration: caused by the cutting process itself; such as the interaction between the workpiece/tool/chips and the machine tool. For example, a rough surface on the workpiece causes fluctuations in the depth of cut, which in turn causes fluctuations in cutting forces, which causes tool chatter, which causes rough surface finish……etc. Can be reduced by changing the cutting parameters as well as increasing the machine tool stiffness/damping.

Damping (Rate of Decay) FIGURE 8.70 Relative damping capacity of gray cast iron and epoxy-granite composite material. The vertical scale is the amplitude of vibration, and the Horizontal scale is time. Source: Cincinnati Milacron, Inc. Sources for Damping: A. Workpiece Material

FIGURE 8.71 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 will be. Source: J. Peters. Sources for Damping: B. Machine-Tool Joints (number of components) C. External Dampers (shock absorbers)

Chatter (vibration) is higher at: - higher cutting forces - higher workpiece hardness - discontinuous chips - higher tool wear - dry machining - worn machine tool ways (or power transmission) Chatter can also be reduced by: - minimize tool overhang - modify tool/cutting geometry (angles) - change process parameters (speed, feed, depth) - increase machine stiffness/damping

Machining Economics As with most engineering problems we want to get the highest return, with the minimum investment. In this case we want to minimize costs, while increasing cutting speeds (to minimize cutting time). EFFICIENCY will be the key term - it suggests that good quality parts are produced at reasonable cost. Cost is a primarily affected by, 1.Tool life/cost 2.Power consumed 3.Machining time The production throughput is primarily affected by, 1.accuracy including dimensions and surface finish 2.MRR (metal removal rate)

Machining Economics…continued The factors that can be modified to optimize the process are: 1.cutting velocity (biggest effect) 2.feed and depth 3.workpiece material (only if is optional) 4.tool material/shape (hence cost) 5.cutting fluid There are two basic optimization criteria (or trade off): 1.Minimum cost - exemplified by low speeds (hence longer tool life), and low MRR (hence low production throughput) 2.Maximum production rates - exemplified by high speeds (hence short tool life), and high production cost. There are many factors in addition to these, but these are the most commonly considered

Cost and Time/Piece in Machining FIGURE 8.72 Graphs showing (a) cost per piece and (b) time per piece in machining. Note the optimum speeds for both cost and time. The range between the two optimum speeds is known as the high-efficiency machining range. Minimum Cost Criteria 1.Machining cost decreases with increasing V 2.Tool Cost increases with increasing V 3.Tool change cost increases with increasing V 4.Nonproductive costs (overheads) is independent of cutting speed V. 5.Total Cost = Summation of costs 1-4 has a minimum value (optimum) at a specific V=V min =V(minimum$)