21st Century Manufacturing Episode 2 Part 5 ppt

274 Computer Manufacturing Chap. 6 internal height for the electrical subsystems is 0.78 inch. Special features on the bottom case are the circular busses fur mounting the PCB, the through window for the battery set, and various mechanical dimensions and shapes of electrical switches. Such features needed to be compatible with the work of the electrical design team. 6.8.7 Coupling Constraints Originating in the Electrical Domain IC.>ml Other constraints originated on the "electrical side" and had to be accommodated on the "mechanical side." They are given the symbol C~>m' They were as follows: •The "z-height" of certain large devices on the PCBs were flagged and con- veyed to the mechanical designers for consideration. Often this exchange resulted in more favorable packaging and overall space saving. •Various switches,volume control knobs,and other 110were of coursemounted on the side of PCBs and their precise dimensions were conveyed to the mechanical team. Ultimately, this meant that they protruded through the plastic casing's boundary with user-friendly dimensions. These components were a power connector, a power switch,serial/parallel ports, audio 110jacks, a keyboard jack, two volume control knobs, and a connector for an attachable color display. •Certain devices,such as the power supply,required electromagnetic shielding. Once the sites for these deviceswere determined their dimensions were shared with other teams. •The locations and sizes of the mounting holes that attached the PCB to the bottom casing were also the product of a dialogue between electrical and mechanical designers. Figure 6.28shows the PCB layout of the InfoPad terminal with these electrical constraint features highlighted. To present these electrical constraints to mechan- ical designers in the DUCADE environment. electrical constraints along with the design were transformed into mechanical representation (3-D solid models) for analysis.The approach captured only the geometric aspects of the electrical design constraints. Methods for presenting other types of electrical constraints are still under development. 6.8.8 Resolving the Coupling and Constraints (C.<>m) After several iterations between the mechanical and electrical design teams, satis- factory compromises were achieved. TIley are given the symbol Ce<>m' The evi- dence in firms such as Hewlett-Packard and Sony is that, over many product revisions,these couplings can be further refined and the product gets more efficient and compact (Cole, 1999).In fact, an interesting exercise for students is to "dissect" several generations of the familiar Sony Walkman and see how the engineers have invented new ways of resolving the constraints (Ce<>m)' For a first prototype there 6.8 Case Study on Computer Manufacturing 275 Flpre 6.28 PCB layout and some electrical constraints. is a limit to how much can be achieved in the first few iterations. Nevertheless for the InfoPad the main constraints were resolved, leading-to the following new aspects of the "final" design: •The PCB design was changed to a customized "U shape." It provided a smaller form factor for the new specification and allowed accessto the selected battery set. • Some important electrical components were redesigned using different Ie packaging and different surface mounting techniques. 1bis reduced the device sizes and allowed them to fit into the smaller interior space of the mechanical casing. • The boundary features of the mechanical casing were changed for the added audio. keyboard jacks, two serial ports, and the color video unit. • The interior contour of the mechanical casing was modified according to the shape of the new PCB. Circular bosses were added and modified on the bottom casing for mounting the newly designed PCB. • Curved, spline features were added to the two sides of the terminal casing to improve the ergonomic and aesthetic design (Figure 6.29). Two volume controls i Color di~playconnttlor Mounung holes Power connector AUdio~ 276 6.8.9 Fabrication Computer Manufacturing Chap. 6 F'lgme6.29 CAD solid model ofthe Tl'IfnPlld The MOSIS service was used for the various ICs <www.mosls.org>,thePCBswere "fabbed" at a local bureau <http://sierraprotoexpress.com>. and the molds were fabricated on the CyberCut service <cybercut.berke1ey.edu>. The design of the molds for plastic injections involved the precise specification of taper angles for sep- arating the mold halves, shrink factors for different materials, core design, running gate design, and parting plane specification. Prototype molds and casings are shown in Figure 6.30. F1pre 6.30 Top: the aluminum mold halves; bottom; the injection molded plastic casings, CHAPTER METAL-PRODUCTS MANUFACTURING 7.1 INTRODUCTION 7.1.1 The "Garag Shop at www.start·up-eompany.com The sepia photograph above, taken around 1917, shows William Woodland, one of my grandfathers. He was an aircraft mechanic. He is sitting in the cockpit of a Vickers VlJDIOY biplane. My other grandfather, Browett Wright, was a railroad engineer; specifically he was a "knocker." He walked around the stockyards at Watford Junction, 277 278 Metal-Products Manufacturing Chap. 7 on the outskirts of London, carrying a small hammer. By lightly knocking on the wheels and axles of wagons, and listening tu the resulting "ring," his well-trained ear could detect the absence or presence of potentially dangerous fatigue cracks. As a hobby, these men and their friends often set up small machine shops in their garages or basements where they would fabricate small personal projects. Sometimes, around the Christmas consumer season, they even did small-batch man- ufacturing runs for local suppliers/wholesalers. The garage shop allowed autonomy, custom fabrication, and reasonably good delivery times. A wide variety of projects could be accomplished with a well- equipped tool chest and just two main machines: a small lathe and a medium-sized three-axis milling machine. A drill press was an inexpensive addition that saved these men from setting up the entire mill just to drill a hole. If a few extras were added-c. such as a bench grinder, a small sheet-metal press, and a welding ser-e-then they were really in business. Many of today's famous figures in Silicon Valley and elsewhere also began their start-ups in a humble garage. It is most likely that many of these garages contained these basic tools in their early days. The most important point is this: these simple metal-cutting machines allow a range of parts to be made. The same set of cutting tools can be used to make a wide variety of parts of rather complex geometry. Machining is the most important metal- forming operation for this reason, despite the more glamorous appeal of FDM and other SFF processes. In the last few decades, machining has endured a bad reputa- tion for being wasteful and for being a little slow, but it always bounces back in fashion for these reasons of flexibility and good accuracy. 7.'.2 The Origin. of the Basic Machine Shop From these humble origins emerges the job shop or machine shop-really just a larger version of the garage shop but with bigger machines, more specialized machines, and perhaps several machinists. However, in the end it all boils down to flexibility and specialized service with reasonable delivery. Machining is a powerful way of producing the outer structures of weird proto- types that are just beginning the product development cycle or perhaps just entering the market: products located in the lower left comer of Figure 2.3. Also the job shop in a university or research laboratory is a place where machinists turn the new designs of mad inventors into reality. All over the world, unusual start-up companies are being formed: computer start-ups, biotech firms, in fact any new-product companies. At some point these companies will need a prototype, often to show the bank. for a loan or to impress potential investors. Most likely,the company will need to go to a machine shop to get the prototype made. For example, the first few Infopad casings-c-see Chapter 6- were readily machined with good tolerances of +1-0.002 inch (-50 microns). The fabrication time was approximately three days from the moment the design was fixed until a finished casing was fabricated by milling. 7.1 Introduction 279 7.1.3 The Tool and Die Shop-Machining and EDM Metal cutting is more ubiquitous in industrial society than it may appear from the preceding description of the traditional machine shop. All forgings, for example those used in cars and trucks, and many sheet metal products, for example those used in steel furniture and filing cabinets, are formed in dies that have been machined. In fact, most of today's electronic products are packaged in a plastic casing that has been injectio~olded into a die: cellular phones, computers, music systems, the Walkman, and all such products thus depend on the cutting of tools and dies for other processes that follow metal cutting. While the initial prototype of a new cellular phone might well be created with one of the newer rapid prototyping processes that emerged after 1987-stereolithography (SLA), selective laser sintering (SLS), or fused deposition modeling (FDM)-the final plastic products, made in batch sizes in the thousands or millions (see Chapter 8), will be injection-molded in a die that has been cut in metal with great precision and surface finish constraints. The machining of such dies, usually from highly alloyed steels, requires some of the most exacting precisions and surface qualities in metal-cutting technology. It prompts the need for new cutting tool designs, novel manufacturing software that can predict and correct for tool deflections and deleterious burrs, and new CAD/CAM procedures that incorporate the physics and knowledge bases of machining into the basic geometrical design of a component. At the same time it should be noted that if the die is a complex shape, a two-step macbining process may be needed. First, the "roughing cuts" will be done with conventional end milling as shown in Section 7.2.4. Second, the "finishing operations" willbe created by the elec- lrodischarge machining (EDM) process. In this process, a shaped carbon electrode slowly "sinks't into the metal mold. Electric arcing between the electrode and die sur- face takes place in a,dielectric bath. Material removal takes place by surface melting of the die surface and flushing the debris away with the dielectric fluid. 7.1.4 Full-Scale Production Using Machining Operations The previous sections focus on small-batch manufacturing operations that generate a small number ofparts, or even one-of-a-kind dies. In other sectors of industry,large- batch manufacturing is more the normal situation. Thus machining in all its forms- turning, milling, drilling, and the like-can also be seen as a large-scale manufacturing process. It supports mass-production manufacturing such as the auto and steel indus- tries, both positioned well along the market adoption curve in Figure 2.2. To summarize this economic importance, the cost of machining amounts to more than 15% of the value of all manufactured products in all industrialized countries. Metcut Research Associates in Cincinnati, Ohio, estimates that in the United States the annual labor and overhead costs of machining are about $300 billion per year (this excludes work materials and tools). U.S. consumption of new machine tools (CNC lathes, milling machines, etc.) is about $7.5 billion per year. Consumable cutting tool materials have U.S. sales of about $2 to $2.5 bil- lion per year. For comparison purposes, it is of interest to note a ratio of $300 to 280 Metal-Products Manufacturing Chap. 7 $7.5 to $2.5 billion for labor costs to fixed machinery investments to disposable cutting tools. 7.1.5 Full-Scale Production Using Other Metal-Processing Operations Sheet rolling is also a large-batch manufacturing process in which a rolling mill contin- uously produces flat strip in coils.Such strip is sold to a secondary producer, who will shear it into smaller blanks that are then pressed into an ordinary soup can or filing cab- inet. The sheet-metal forming of single discrete items, such as the hood of a car, is also a large-batch manufacturing process because many similar parts are produced on a con- tinuous basis from one very expensive die.This chapter considers these other examples of large-batch manufacturing, focusing as an example on sheet-metal forming. The machines and dies for all these processes are extremely expensive, so the analysis of the forces on the machines and dies is crucial. As managers of technology, a very valuable service is created if these force predictions lead to sensible machinery purchases-that is, machines that are powerful enough to deform the typical mate- rials and products being created by a company, but not overly powerful, hence wasting capability and investment costs. At the same time, the understanding of which factors affect quality assurance and the properties of the deformed material is equally important. 7.2 BASIC MACHINING OPERATIONS 7.2.1 Planing or Shaping A cutting tool moves through a steel block and removes a layer from its top surface (Figure 7.1). The discarded layer is called the chip. The mental image that seems to work well even in sunny California is that of a handheld snow shovel being pushed along a snow-covered sidewalk. The shovel is analogous to the tool, and the chip that rises up the face of the tool-actually called the rakeface-is analogous to the layer of snow. The chip then curls away from the face at some distance, called the chip-tool contact length, and falls away onto the surface being machined or to one side. If the shovel face made a perfect right angle with the sidewalk, the rake angle would be o degrees. Of course, a snow shovel is usually tilted back to a rake angle of about 20 or 30 degrees. In metal machining the rake angle for today's tools is often 6 degrees. It isalso quite common in metal machining for the rake face to be at a slight negative angle of -6 degrees. This gives added strength to the delicate cutting edge. Wood planing has a similar geometry to metal cutting by planing. However, this is not altogether the best analogy because in wood cutting the physics of the process is different: the wood ahead of the sharp cutting edge is split, and a long crack runs ahead of the tool to make for rather modest cutting forces. In the machining of metals, although a ductile crack of microscopic proportions is obviously formed right down near the tip of the tool (otherwise the two surfaces would not separate), most of the work done is related to the shearing of metal in the shear plane. The shear plane is shown as OD in Figure 7.2. 7.2 Basic Machining Operations 2., Ftcure 7.1 Micrograph of chip formation. In Figure 7.2 and accompanying equations, the shear plane angle is related to the undeformed chip thickness t and the deformed chip thickness t e . It is common to define a chip thickness ratio (r == tJ(.). Since the chip slows down by frictional drag on the tool, tei:; always greater than t. Thus r is always less than unity. In the special case in which the rake angle, a, is set to zero, it can be seen that the tangent of the shear plane angle is just the undeformed chip thickness divided by the deformed chip thickness. lithe main cutting forces (F c and F T ) are measured with a force dynamometer, they can be resolved onto the all-important forces on the cutting edge of the tool. Why are these other forces F N and F R important? The answer is that they govern the life of the tool. Large values of F R will create high shear stresses and temperatures in this region. A high value of F N will be associated with a high normal pressure on the delicate cutting edge. Such high forces will create high friction and wear of the rake face. A large value of F R will also tend to lift the tool away from the surface being machined and make the surface finish irregular. It is therefore worth paying for lubricants and diamond-coated tools that minimize this force. 7.2.2 Turning The basic operation of turning (also called semiorthogonal cutting in the research laboratory) is also the one most commonly employed in experimental work on metal cutting. The work material is held in the chuck.of a lathe and rotated. The tool is held rigidly in a tool post and moved at a constant rate along the axis of the bar, cutting 282 Metal-Products Manufacturing Chap. 7 tan oil = 1 ~~~~{[o: Where r=i<1 Also note F~,=Frsino:+FTcos(l P/I P,cosQ;-FTsina Figure 7.2 Cutting forces during chip fonnation. away a layer of metal to form a cylinder or a surface of more complex profile. This is shown diagrammatically in Figure 7.3. The cutting speed (V) is the rate at which the uncut surface of the work passes the cutting edge of the tool, usually expressed in units offtlmin or m min-to The feed (f) is the distance moved by the tool in an axial direction at each revolution of the work. The depth-of-au (w) is the thickness of metal removed from the bar, measured in a radial direction.' The product of these three gives the rate of metal removal, a parameter often used in measuring the efficiency of a cutting operation. v f w = rate of removal (7.1) "nie feed rate (I) during turrung is also called the undcformed chip thickness (r).The depth-of- cut, W, in turning is also referred to as the undeformed chip width. Since machining has developed from II practitioner's viewpoint, the terminology is not really consistent from one operation to another. For example, in the diagrams for end milling, the term depth-ol-cut is used in II different way. ntis is unfortu- nately confusing for II new student of the field. Perhaps the best way to accommodate these inconsisten- cies is to always view the "slice" of material being removed as the "undeformed chip thickness" (r) and the direction normal to this (into the plane of the paper) as the "undefonned chip width" (w). 7,2 Basic Machining Operations 283 FIaure 7.3 Lathe turning showing a vertical cross section at top right and a detail of the insert geometry at bottom right, The dynamometer platform and the remote the£mowuple on the haltom of the insert are not used during today's production machining. However they are useful in the research laboratory for routine measurement of cutting forces and overaU temperature, The cutting speed and the feed are the two most important parameters that can be adjusted by the operator or programmer to achieve optimum cutting conditions. The depth of cut is often fixed by the initial size of the bar and the required sfze of the product. Workmate-rial Sh"Mpbnc>l-llgJe Deplhofci.!l,d Held in chuck Chip -PcedJ Dynamometer [...]... cannot from "N" block dimensions 01 05 OOTlM6 x -2. 00y-.75zOsJOOOM3 NI0 0 15 020 N 25 Gl x7 .50 F50 GOT2M6 x-l.Oy .26 z-.49 s1700M3 Ul.x6 .26 F50 N30 N 35 y-l.76 x- .26 y .26 OOx-l.Oy . 25 z- .50 s2000 Gl x6 . 25 F70 y-1. 75 x- . 25 y . 25 G0T3M6 G81 xl.0y-. 75 [z-. 156 RO} FlOs4000M3 N40 0 45 N50 N 55 N60 N 65 070 N 75 N80 0 85 N90 x4.0 N 95 GO T4M6 081 »i.uy-. 75 F15s3000M3 x4.0 Ntoo M2 [z- .57 5 ROJ GO ", rapid travel; M6 '" call... diameter changes Feed may be as low as 0.0 1 25 mrn (0.00 05 inch) per revolution and with very heavy cutting up to 2. 5 mm (0.1 inch) per revolution Depth of cut may vary from o over part of the cycle to over 25 mrn (1 inch) It is possible to remove metal at a rate of more than 1,600 em' (100 inches") per minute, but such a rate would be very uncommon, and 80 to 160 em' (5 to 10 inches") per minute would normally... +A- 0:) (A ~ + A - 0:) sinecos (~ 7.11 Merchant's force circle (7 .2) (7.3) 29 2 Metal-Products Manufacturing Chap 7 where t = undeformed w k = the shear A = chip thickness = undeformed the friction Differentiating chip width yield the first equation dFc _ d$ - strength angle twkcos(;\ ~$COS2 of the metal on the tool's with respect - a)cos (24 J ($ + being to the shear + machined rake face plane angle gives... long-run injection molding might need a 22 .5 kilowatt (30 horsepower) spindle Such a machine will probably have a correspondingly larger bed and tool changer and cost on the order of $ 25 0,000 In industry, too often educated guesses are used to make such purchasing decisions Formal analysis of the type shown here is recommended 7.3 Controlling the Machining Process 29 3 Having predicted Fe for the typical... a physical component is critical to the success of quickly producing a quality part Ideally, the interchange from a design fonnat to a manufacturing format should preserve all the design information, plus it should be easy to Metal-Products Manufacturing 28 8 Chap 7 L fE-=-(i)-= =-=-=-3 =-=-(i)==L'·-=1 i Figure 7.9 Test part to be milled for G and M example Above: photograph of machined block Below:... typical milling cutters have a number of teeth (cutting edges), which may vary from 2 to over 100 The new surface is generated as each tooth cuts away an arc-shaped segment, the thickness of which is the feed or tooth load Feeds are usually light, not often greater than 0 . 25 mm (0.01 inch) per tooth, and frequently less than 0. 0 25 nun (0.001 inch) per tooth However, because of the large number of teeth, the... ooord.;F = feed rate (inJmin) Call tool 1 '2 into spindle -1" offclearance;y "" .26 and ;:=-.49 = rough cut Unearcut!ox=6 .26 ;createsfirstshoulder Stays in linear teeo (01) Rapid travel to starting point for finish cut Finish cut Tool 3 spot drill 081 drill code canned cycle-down to depth and rapid retract Other hole done (081) DrillT4 RO is asubdatum,lOO mils (.1") above part End of program Notes: Please note...Metal-Products Manufacturing 28 4 Chap.7 Cutting speed is usually between 3 and 20 0 m min -1 (10 and 600 ft/min) However, in modem machining high-speed aluminum machining, speeds alloys The rotational may be as high as 3 ,50 0 m min-1 when speed (rpm) of the spindle is usually con- stant during a single operation, so that... of Dr James Stori) = widtb of cut, DOC = 7 .2 Basic Machining 28 7 Operations (a) Ideal geometry (b) Form error (c) Surface roughness F1gue 7.7 Effect of tool deflection on form error and surface roughness [courtesy of DrJames Stori} Ideal geometry Form error Surface roughness F1gure 7.8 Deviations in form and surface quality (courtesy of Dr James Stori) 7 .2. 5 Feature Creation by Milling Using "G and... programmed tool) as early as 1 955 -1960 APT is a higher level language than G and M codes that treats each line in Figure 7.9 above as an "object." Methods based on APT are still used for machine tool programming in today's factories and again they compile into G and M codes However, many other high-level programming systems that break down 7.3 Controlling TABLE the Machining 7.1 28 9 Process G and M Program . OOTlM6 05 x -2. 00y 75zOsJOOOM3 NI0 Gl x7 .50 F50 0 15 GOT2M6 020 x-l.Oy .26 z 49 s1700M3 N 25 Ul.x6 .26 F50 N30 y-l.76 N 35 x 26 N40 y .26 0 45 OOx-l.Oy . 25 z 50 s2000 N50 Gl x6 . 25 F70 N 55 y-1. 75 N60 x 25 N 65 y . 25 070. x6 . 25 F70 N 55 y-1. 75 N60 x 25 N 65 y . 25 070 G0T3M6 N 75 G81 xl.0y 75 [z 156 RO} FlOs4000M3 N80 x4.0 0 85 GOT4M6 N90 081 »i.u y 75 [z 57 5 ROJ F15s3000M3 N 95 x4.0 Ntoo M2 GO ", rapid travel; M6 '". as low as 0.0 1 25 mrn (0.00 05 inch) per revolution and with very heavy cutting up to 2. 5 mm (0.1 inch) per revolution. Depth of cut may vary from o over part of the cycle to over 25 mrn (1 inch).