Machining Processes TABLE 8.7 General characteristics of machining processes. Process C haracteristi cs Com mercial t olerances (±mm ) Turning Turning and f acing operati ons on all types of m aterials;requires skill ed l abor; low production rate, butm edium to high w it h turretlathes and au tomati c m achines, req uiring l ess-skill ed l abor. Fine:0.05-0.13 R ough:0.13 Skiving: 0 .025-0.05 Boring Internal surfaces or pro fil es,w it h characteristics simil arto turning;stiff ness of bo ri ng bar important t o avoid chatt er. 0.025 Dril ling R ound ho les of va ri ous sizes and depths; req uires bori ng and ream ing for improved a ccuracy; high production rate; l aborskill requir ed depends on ho le location and accuracy specifi ed. 0.075 Mill ing Variety ofshapes involving contours, f lat su rfaces, and slots; w ide variety oftooling; versatil e; l ow to m edium producti on rate;requir es skill ed l abor. 0.13-0.25 Planing Flatsurfaces an d straightcontour profil es on l arge surfaces;suitable forlow -quantity production;labor skill required depends on pa rt sh ape. 0.08-0.13 Shaping Flatsurfaces an d straightcontour profil es on re lati vely sma ll w orkpieces;suitable forlow -quantit y production; laborskill required dep ends on partshape. 0.05-0.13 Broaching External and internal flatsurfaces, s lots and contours wit h good surface fi nish; co stly tooling; h igh production rate; l aborskill requir ed depends on pa rt shape. 0.025-0.15 Sawing Straightand con tourcuts on flats orstructural shapes; notsuit able f orhard m aterials unless saw has carbide teeth or i s coated w it h diam ond;low producti on rate; requires only l ow l aborskill. 0.8
Transcript
Machining Processes
TABLE 8.7 General characteristics of machining processes.
Process Characteristics Commercial tolerances(±mm)
Turning Turning and facing operations on all types ofmaterials; requires skilled labor; low production rate,but medium to high with turret lathes and automaticmachines, requiring less-skilled labor.
Fine: 0.05-0.13Rough: 0.13Skiving: 0.025-0.05
Boring Internal surfaces or profiles, with characteristicssimilar to turning; stiffness of boring bar important toavoid chatter.
0.025
Drilling Round holes of various sizes and depths; requiresboring and reaming for improved accuracy; highproduction rate; labor skill required depends on holelocation and accuracy specified.
0.075
Milling Variety of shapes involving contours, flat surfaces,and slots; wide variety of tooling; versatile; low tomedium production rate; requires skilled labor.
0.13-0.25
Planing Flat surfaces and straight contour profiles on largesurfaces; suitable for low-quantity production; laborskill required depends on part shape.
0.08-0.13
Shaping Flat surfaces and straight contour profiles on relativelysmall workpieces; suitable for low-quantity production;labor skill required depends on part shape.
0.05-0.13
Broaching External and internal flat surfaces, slots and contourswith good surface finish; costly tooling; highproduction rate; labor skill required depends on partshape.
0.025-0.15
Sawing Straight and contour cuts on flats or structural shapes;not suitable for hard materials unless saw has carbideteeth or is coated with diamond; low production rate;requires only low labor skill.
0.8
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. 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 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. 8.42.
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)
Maximum Cutting Velocity = Vmaximum = . Df. N(rpm)
Average Cutting Velocity = Vaverage = . (Do+Df)/2. N(rpm)
Material Removal Rate = MRR (in3/min) = . Davg . d . f (in/rev) . N(rpm)
OR MRR (in3/min) = . Davg . d . f (in/min)
Cutting Power = HP = MRR (in3/min) . ut (hp.min/in3)
: ut (hp.min/in3) is the material specific energy (Table 8.3)
Also, Cutting Power = Fc (lbf)* V (in/min) / 396000
Torque (lbf.in) = Power (hp) * 396000 ((in.lbf/min)/hp) / 2N
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.
Note: (a) These speeds are for carbides and ceramic cutting tools. Speeds for high-speed steeltool are lower than indicated. The higher ranges are for coated carbides and cermets. Speeds fordiamond tools are significantly higher than those indicated.(b) Depths of cut, d, are generally in the range of 0.5-12 mm (0.02-0.5 in.)(c) Feeds, f, are generally in the range of 0.15-1 mm/rev (0.006-0.040 in./rev).
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, tc; 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 (in3/min) = w . d . f (in/tooth) . n (teeth) . N(rpm)OR MRR (in3/min) = w . d . f (in/min)where f (in/min) = f (in/tooth) . n (teeth) . N(rpm)
Cutting Power = HP = MRR (in3/min) . ut (hp.min/in3)
: ut (hp.min/in3) is the material specific energy (Table 8.3)
Torque (lbf.in) = Power (hp) * 396000 ((in.lbf/min)/hp) / 2N
Cutting Time = t (minutes) = (l + lc)/ f(in/min) : lc = (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 (in3/min) = w . d . f (in/tooth) . n (teeth) . N(rpm)OR MRR (in3/min) = w . d . f (in/min)where f (in/min) = f (in/tooth) . n (teeth) . N(rpm)
Cutting Power = HP = MRR (in3/min) . ut (hp.min/in3)
: ut (hp.min/in3) is the material specific energy (Table 8.3)
Torque (lbf.in) = Power (hp) * 396000 ((in.lbf/min)/hp) / 2N
Cutting Time = t (minutes) = (l + 2lc)/ f(in/min) : lc = D/2
Cutting Speeds in Milling
TABLE 8.11 Approximate range of recommended cutting speeds for milling operations.
CUTTING SPEEDWORKPIECE MATERIALm/min ft/min
Aluminum alloysvast iron, grayCopper alloysHigh-temperature alloysSteelsStainless steelsThermoplastics and thermosetsTitanium alloys
Note: (a) These speeds are for carbides, ceramic, cermets, and diamond cutting tools. Speeds forhigh-speed steel tools are lower than indicated.(b) Depths of cut, d, are generally in the range of 1 mm-8 mm (0.04 in.-0.3 in).(c) Feeds per tooth, f, are generally in the range of 0.08 mm/rev-0.46 mm/rev (0.003 in./rev -0.018 in./rev).
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.
Note: As hole depth increases, speeds and feeds should be reduced. Selection of speeds andfeeds also depends on the specific surface finish required.
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.
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/cost2.Power consumed3.Machining time
The production throughput is primarily affected by,1.accuracy including dimensions and surface finish2.MRR (metal removal rate)
Machining Economics…continuedThe factors that can be modified to optimize the process are:
1. cutting velocity (biggest effect)2. feed and depth3. 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=Vmin=V(minimum$)
