21st Century Manufacturing Episode 2 Part 5 ppt

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 suc

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 symbolC~>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 110 were of course mounted

on the side of PCBs and their precise dimensions were conveyed to 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 devices were 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 6.28 shows 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 electrmechan-ical constraints along with the analysis.The approach captured only the geometric aspects of the electrical design 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 symbolCe<>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

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 access to 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

iColor di~playconnttlor

Mounung holes Power connector

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6.8.9 Fabrication

F'lgme6.29 CAD solid model of the

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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 gatedesign, and parting plane specification Prototype molds and casings are shown

in Figure 6.30

F1pre 6.30Top: the aluminum mold halves; bottom; the injection molded plastic casings,

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 VlJDIOYbiplane.My othergrandfather, Browett Wright, was arailroadengineer; specifically he was a "knocker." He walked around the stockyards at Watford Junction,

on the outskirts of London, carrying a small hammer By lightly knocking on the wheelsandaxlesofwagons, and listeningtu theresulting "ring,"hiswell-trainedear could detecttheabsenceorpresenceofpotentially 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 Christmasconsumer season, theyeven did small-batch man-ufacturing runs for localsuppliers/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 such as a bench grinder, a small sheet-metal press, and a weldingser-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 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 potential investors Most likely, the company will need to go to a machine shop to get the prototype made For example, the first few Infopadcasings-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

7.1.3 The Tool and Die Shop-Machining and EDM

Metal cutting is more ubiquitous in industrial society than it may appear from the those used in cars and trucks, and many sheet metal products, for example those used

in steel furniture and filing cabinets, are formed indiesthat 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 thecutting 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" will be 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 sursur-face 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 of parts, 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 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-$7.5 to$2.5 billion for labor coststofixed machinery investments todisposable 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 shearit into smaller blanks that are then pressed into an ordinary soup can or filing cab-inet Thesheet-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 areextremely 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 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

odegrees 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 is also 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 similargeometry to metal cutting by planing However, this

is not altogether the best analogy because in wood cutting the physics of the process

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

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 thant.Thusris 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 forcesF NandF Rimportant? The answer is that they govern the life of the tool Large values ofF Rwill create high shear stresses and temperatures in this region A high value ofF Nwill 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 ofF Rwill 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

7.2.2Turning

The basic operation of turning (also called semiorthogonal cutting in the research cutting The work material is held in the chuck of a lathe and rotated The tool is held

tan oil = 1 ~~~~{[o:

Where r=i<1

Also noteF~,=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 whichthe uncut surface of the work passes the cutting edge of the tool, usually expressed in unitsofftlmin or m min-to Thefeed (f)

is the distance moved bythe tool in an axial direction at each revolution of the work Thedepth-of-au (w) is the thickness of metal removed fromthe 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

"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 fromII

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 inIIdifferent way.ntis is unfortu-nately confusing forIInew 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

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 adjustedbythe operator or programmer to achieve optimum cutting conditions The depth of cut is often fixedbythe initial size of the bar and the required sfze of the

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