21st Century Manufacturing Episode 2 Part 9 ppt

Not recommended RecommendedFlgure 8.20 An example of design guides for soldering anrt hn17ing processes from Design Manufacturability Handbook edited by 1.. Quantita-tive scores are give

• Have screws and other fasteners been minimized and reduced to snap fits?

• Can everything be duneinallautomatic assembly machine?

• Can a base part, base plate, or central axle be used, to which everything else can be assembled? This helps to orient everything toward a central assembly theme

• Are subassemblies modular?

• Has group technology been used for the next part in the assembly process?

• Is the greatest value-adding task performed last? This is an important point in case something is damaged at the last minute

• Has the required assembly dexterity been minimized?

8.7.7 Design Checklist for Welding, Brazing, Soldering,

and Gluing

A brief description of joining methods is appropriate here KaJpakjian (1995) and DFAJDFM aspects

Welding processes: Intense heat from the electric arc of a "welding-stick," a con-trolled plasma are, or a "spot-welding" tool causes localized melting, mixing, and local resolidification of the surfaces of the two components being joined This

"micromelting/casting" needs to be done in a protective atmosphere; otherwise, the oxygen in the air forms local oxide deposits that damage the metallurgical integrity this atmosphere as it decomposes in the heat, thereby generating a covering shield

of inert gases

Brazing and soldering: A filler material is locally melted with a "soldering iron" (for soldered joints) or a flame (for a brazing operation) and made to flow between the but when the filler material resolidifies, a solid-state bond is created between each surface and the filler material The filler material may be conventional electrical solder (tin-lead alloys) or brazing compound (silver or copper alloys) Brazing gives

a higher strength than soldering

Gluing methods: Epoxy resins and acrylic glues provide a chemical bond between the two surfaces to be joined Clean surfaces devoid of grease and as much oxide as pos-sible are the ideal conditions Nevertheless, the bonds created are significantly lower are often susceptible over time to the ultraviolet rays in natural light.Jt is unwise to depend on a glued joint for long-term service

During CAD, designers aim to create component geometries that enhance the structural integrity of a formed joint Figure 8.20 shows some recommended joint stream manufacturing," the accessibility of a manually operated welding torch or

"spot-welding robot" should be considered For example, on an automobile assembly

Not recommended Recommended

Flgure 8.20 An example of design guides for soldering anrt hn17ing

processes (from Design Manufacturability Handbook edited by

1 G Bralla, © 1998 Reprinted by permission of the McGraw-Hill Companies.)

inal designer, the process planners, and the Iixturing engineers aU playa role in making the process easy or hard to execute, which in turn affects the resulting quality

is a main recommendation

8.7.8 Formal Methods of Scoring Assemblies

Formal schemes are now being used by corporations-Chrysler and Compaq are two notables-in a variety of industries to quantify the preceding lists as much as pos-sible Obviously, the best practices for all companies should increasingly include these assembly evaluations

8.7.8.1 Boothroyd and Dewhurst Method

The Boothroyd and Dewhurst evaluation method assigns scores to the following tasks:

• Parts count: this is simply counted, and where possible design changes are

made to reduce the number of parts

• Symmetry: axial symmetry is preferred and given the highest ranking.

• Size of parts: medium-sized parts that can be picked up easily by humans are

given the highest ranking Small read-write heads would be given a low

Heavy parts that might need hoists or human amplifiers to pick them up would also get a low ranking

• Shapes: smooth shapes that avoid tangling are given a high ranking

• Quanta of difficulty: finally, additional penalties are given to parts that are in

any way awkward, slippery, or easily damaged One way of calculating this ranking is to measure the time and level of skill needed to complete the task

8.7.8.2 Xerox Corporation

The Xerox scoring method is similar to that of Boothroyd and Dewhurst Quantita-tive scores are given in the following areas:

• Parts count (as before)

• Direction of assembly motion (as shown in Figure 8.19)

• Flxturing needs at each setup

• Fastening methods, with snap fits preferred over screws or joining methods 8.7.9Maintaining a System Perspeetiv& Closing Thoughts

In the design and prototyping of the ST Microelectronics' 'IouctsChtptv (the case study in Chapter 2) several DFMlDFA strategies were employed For example, snap aluminum molds more expensive to machine and more expensive to operate because

of the undercuts needed Thus, as a word of caution, the design team should certainly look at the "big picture." For the first run of 200 parts, it might not be worth the cost

of the snap fit But for millions of production parts, the extra time taken to machine the molds and also to operate the cores for the undercuts during molding will prob-ably payoff in "downstream assembly costs."

This section concludes with an interesting success story (Prentice, 1997) from

an extended "learning organization." During the redesign of a rather ordinary portable stereo, the question arose, What size should the outside plastic casing be? analysis, a great benefit was gained by adjusting the size so that a certain number of adjusting the size of an individual stereo it was possible to fit a greater number into what because the perfect tightness of fit prevented the cargo from shifting and becoming damaged It is perhaps unusual to be able to adjust a design based on a constraint so far away in the logistical chain, but the example does challenge an indi-vidual design engineer to think as broadly as possible in a learning organization 8.8 MANAGEMENT OF TECHNOLOGY

8.8.1 Integrated Product and Process Design

Economic pressures, particularly related to the quality of manufactured goods and time-to-market, are forcing designers to think not only in terms ofproduct design but also in terms ofintegrated product and process design and, finaUy,in terms of

deter-As a result of these three high-level needs, there is nowan even greater need fur comprehensive models-c-of the type introduced in Chapters 7 and 8-that pre-dict material behavior during a manufacturing process, the stresses and/or tempera-tures on associated tooling, and the final product integrity The overall goal is to enrich a CAD/CAM environment with:

• Physically accurate finite element analyses (PEA) and visualizations of the manufacturing process

• Access to process planning modules that allow detailed cost estimates For the polymer materials and moldmaking that have been the focus of this chapter, CAD/CAM-related URLs are given in Section 8.12

8.8.2 Databases and Expert Systems

In addition to the FEA methods in manufacturing, expert systems(Barr and Feigen-baum, 1981) continue to be valuable Expert systems formulate solutions to manu-facturing concerns that cannot be solved directly with quantitative analysis Since the early 1980s they have been useful in a wide variety of scheduling problems (see Adiga, 1993) Expertise is gathered by the formal questioning and recording process known asknowledge engineering.In this approach, engineers work with factory-floor personnel to compile records, taperecordings, and videotapes These build up a qual-itative model of the approaches needed for problem solving,

When carried out within a learning organization (see Chapter 2), it has been found that factory personnel and machinists react favorably to this approach-doc-umenting the kinds of problems that often arise with production machinery and, sim-ilarly, documenting setup andmonitoring procedures for individual machine tools (Wright and Bourne 1988) In the best situations the personnel are even flattered that theirskills are valued and worth capturing for subsequent generations The rules and qualitative knowledge of the experts are written down as a series of rules of the form "If then "The qualitative parameters in these fields might be nonquanti-tative datasuch as colors or approximate percentages

In other situations where manufacturing data is morequantitative, conventional relational databases or object-oriented databases are more useful (see Kamath, Pratt, and Mize, 1995) At a high level, such databases might describe the corporate history, meaning a history of the typical products, batch sizes, and general capabilities of the finn At a more medium level of abstraction, particular capabilities of the factory-floor machinery might be described, with achievable tolerances, operational costs, and avail-ability At the lowest level, the databases might contain carefully documented proce-dures for lithography and etching times, In any industry, the immediate availability of accurate manufacturing parameters for machinery setup and diagnosis is very valu-able, Such databases also facilitate incorporation of DFA and DFM data structures PDES/STEP has emerged as a worldwide scheme for developing a conunon informational framework for suchdatabases andCAD/CAM systems Its goal is to ensure that information on products and processes among different companies is compatible Now that so many large firms rely on subcontractors and outside sup-pliers to create their supply chain, the need for a common interchange format is more

8.8.3Economics of Large·Scale Manufacturing

Economically, the aims are to ensure a high-quality product and to reduce time-to-marketbyeliminating ambiguities and "rework" during CAM (Richmond, 1995) For example-as reportedbyHalpern (1998)-Grundig states that the dies for the front and back of its television casings cost approximately $300,000 each The average cost to make a single change to one of these is typically 10% of the original die cost, or $30,000 Evidently, integrated CAD/CAM systems of the type described

at the end of Chapter 6 are very important software tools for minimizing such rework during mold design, fabrication, and tryout

Most large-scale manufacturing operations (in either metal or plastic) are by

definition mature technologies that are well along the market adoption curve in Chapter 2 But customers deliberately choose these mature technologies because they are tried and true, giving reliable, predictable results These basic processes in thermoforming-may not have the glamour of stereollthography or selective laser sintering, but they remain central to many major industries and to the economy as

a whole

Nevertheless, to compete in global markets, all companies in these fields must apply creative methods and innovations These clearly include new CAD/CAM techniques that reduce time-to-market, the use of sensor-based automation at the shop-floor level to reduce labor costs, and quality assurance ufacturing flows will need to be broken down into modular segments Garment producers have considered such a change in order to address the custom tailoring

market The Economist (2000) argues that traditional manufacturers may well have to follow this example

8.9 GLOSSARY

8.9.1 Blow molding

Various kinds of blow molding allow plastic tubes and plastic sheets to be inflated ucts made by these methods

8.9.2Branching

In these polymers,the side branches lock into adjacent chains and provide additional interlocking and stiffness

8.9.3Cross Linking

In these polymers, additional elements link one chain to another The best example

With mechanical processing, such as extrusion or rolling, polymer chains can be folded into explicit structures to give the material more stiffness

8.9.5Design for Assembly

Design for assembly involves reducing the number of components, keeping the quality of the components high so that they can be easily assembled, simplifying fac-tory layout so that individual subcomponents come together easily, and ensuring that

as many operations as possible can be done in a vertical direction Vertical directions are shown in Figure 8.17

8.9.6Design Guides

A variety of heuristics that have been developed over time to aid the mapping from

a part design to a mold design Frequently a design guide relates to the elimination

of sink and distortions

8.9.7 Gate

The entrance to the mold cavity

8.9.8Glass Transftion Temperature

The glass transition temperature is approximately halfway between the glass plateau and the leathery plateau shown in Figure 8.1 Also, by extrapolating the two curves shown in Figure 8.2, the glass transition temperature is the intersection of glassy behavior and viscous behavior

8.9.9Ejectors

These are typically pins used at the end of the cycle to lift the part from the mold

8.9.10 Flash

If additional plastic is forced between the mold halves, because of a poor mold fit or wear, it is called flash In general this is to be avoided and may require additional hand finishing if excessive

8.9.11 Index of Strain-Hardening Sensitivity

Shown in Section 8.6, the strain-hardening sensitivity relates to the amount of strength increase with a given strain

8.9.12 Index of lime Sensitivity

Shown in Section 8.6, the time sensitivity is the relaxation-related property of the

8.9.13 Injection Molding

Injection of plastic into a cavity of desired shape The plastic is then cooled and ejected in its final form Most consumer products such as telephones, computer cas-ings, and CD players are injection molded

8.9.14 Packing

The phase of injection molding where the ram holds the liquid mold at pressure During this phase, approximately 10% more polymer is pumped into the mold cavity 8.9.15 Parlson

The dangling tube of plastic that is extruded into a heated mold for blow molding It

is subsequently pinched off at one end and inflated at the other during blow molding 8.9.16 Parting Plane

The separation plane of the two mold halves

8.9.17 Reciprocating-Screw Machine

The most used injection molding machine in industry: it combines the screwing action for the plasticization process and a ramming action for the injection process 8.9.18 Runners

In a multipart mold, the runners extend from the sprue to the individual gates of each part 8.9.19 Shrinkage

The amount of volume contraction of a polymer Usually this is 1 % t02% given the reciprocating-screw process

8.9.20 Snap Fit

Projections molded into a part that deflect to provide mechanical fastening with other parts

8.9.21 Sprue

The runway between the injection machine's nozzle and the runners or the gate 8.9.22 Thermoforming

In this process, plastic sheets are clamped around the edge, heated, and inflated with air pressure The dome can be free-formed or formed against a mold to create sur-face impressions

8.9.23 Thermoplastic Polymers

Polymers that undergo reversible changes between the glassy, leathery, viscous,

8.9.24 Thermosetting Polymers

Polymers that undergo irreversible changes from the liquid to solid state, often by adding other chemicals such as epoxy resins

8.9.25 Undercuts

"Sideways" recesses or projections of the molded part that prevent its removal from the mold along the parting direction They can be accommodated by specialized mold design such as sliders

8.9.26 Young's Modulus

Young's modulus defines the stiffness of a material and is given by stress divided by strain in the elastic region

6.10 REFERENCES

Adiga,S.1993 Object-oriented software for manufacturing systems London; Chapman Hall Barr ,A., and E.A- Feigenbaum 1981 The handbook of artificial intelligence: Volumes 1-3 Los

Altos, CA: William Kaufmann

Beitz,w.,and K Grote, 1997 Dubbe! tascnenoucn fUr den mascninenbau {Pocket book for mechanical engineering). Berlin: Springer-Verlag

Boothroyd, G., and P Dewhurst 1983 Design and assembly handbook. Amherst: University

of Massachusetts

Boothroyd, G., P Dewhurst, and W Knight ]994 Product design for manufacture and assembly. New York: Marcel Dekker

Bcrrus, M., and 1 Zysman 1997 Globalization with borders: The rise of wintelism as the future of industrial competition Industry and Innovation 4 (2) Also see Wintelism and the changing lerms of global competition: Prototype of the future Work in Progress from Berkeley Roundtable on International Economy (BRIE)

Bralla,1 G., ed 1998 Design for manufacturabillty handbook, 2d ed New York: McGraw-HilI Dewhurst, P., and G Boothroyd 1987 Design for assembly in action Assembly Engineering

Economist: 2000 All yours (April1): 57-58

GE Plastics 2000 GE engineering thermoplastics design guide Pittsfield, MA: General Elec-tric Company Also see bttp:l/www.~pla8tics.com

Glanvill,A B., and E N Denton 1965 Injection mold design fundamentals. New York: Indus-trial Press

Halpern, M 1998 Pushingthe design envelope with CAE Mechanical Engineering Magazine,

November,66 71

Hollis, R L., and A Quaid 1995 An architecture ror agile assembly.In Proceedings Of the American Society of Precision Engineers' 10th Annual Meeting, Austin, TX.

Kalpakjian, S 1995 Manufacturing engineering and technology. Menlo Park, CA: Addison Wesley See in particular Chapters 27~30

Kamath, M., 1 Pratt, and 1 Mize 1995 A comprehensive modeling and analysis environment for manufacturing systems.In 4th Industria! Engineering Research Conference, Proceedings,

759-768.Also seebttp:llwww.okstate.edulcodm,

Magrab, E B 1997 Integrated product and process design and development. Boca Raton and

McCrum, N G., C P Buckley, and C B BucknalJ 1997 Principles of polymer engineering.

Oxford and New York: Oxford Science Publications

Mcloughlin, 1 R., and A V Tobolsky 1952 The viscoelastic behavior of polymethyl-methacrylate Journal of Colloidal Science 7: 555-568.

Niebel, B W.,A B Draper, and R.A Wysk.1989 Modern manufacturing process engineering.

New York: McGraw-Hill

Prentice, 8.1977 Re-engineering the logistics of grain handling:The container revolution In

Managing enterprises: Stakeholders, engineering, logistics, and achievement, 297-305 London: Mechanical Engineering Publications Limited

Pye, R G W 1983 Injection mold design London: Godwin

Richmond, 0.1995 Concurrent design of products and their manufacturing processes based upon models of evolving physicoeconomic state In Simulation of mate rials processing: Theory, methods, and applications, editedby Shen and Dawson, 153-155 Rotterdam: Balkema

Vrabe, K., and P K.Wright. 1997 Partingdirections and partingplanes for the CAD/CAM of plastic injection molds Paperpresented at the ASMEDesign Technical Conference, Sacra-rnento,CA

Wright,P K.,and D A Boume.1988 Manufacturing intelligence Reading,MA:Addison Wesley

8." BIBLIOGRAPHY

Modern Plastics Encyclopedia. New York.: McGraw-Hill Published annually

8.'2 URLS OF INTEREST

For mold design: www.cmold.com

General design withpolymers: www.IDESINC.com

Bayer polymers division: http://www.bayerus.comfpolymersl

Magics: http://www.materialise.comf

GE plastics: http://www.ge.comlplasticsl

Society of Plastics Engineers: http://www.4spe.orgi

Trading networks: www.iprocure.com, www.memx.com, andwww.commerceone.com

8.'3 CASE STUDY ON ASSEMBLY

This case study invites the reader to think about how much investment in automa-tion is needed to assemble a product Batch size is a main consideration Product revision is another: if designs change quickly, it may be difficult to justify automation

if low-cost labor is available

Referring to Figure 2.6, one helpful guide is to consider whether a company's current and future products and typical batch sizes are suited to (a) manual assembly, (b) human-assisted computerized assembly, (c) flexible robotic assembly, or (d) hard automation withless need for reprogrammability,

Manual assembly:This type of craftsmanship will dominate for one-of-a-kind

pany.At the same time, manual assembly is likely to be the best choice for high-volume clothing and shoe manufacture Since styles change quickly, the economics favor the use

of intensely human assembly in countries that offer low wage rates For example, shoe manufacturing in such countries is likely to be done more or less entirely by hand, by people sitting at simple gluing and sewing machines,or standing at simple transfer lines.3

Human-assisted computerized assembly: This is typically seen in U.S., Japanese, and

other internal finishes on the car In this work, human dexterity is needed to care-fully manipulate subcomponents into their proper places This situation describes more of a middle ground of automation The ClM system is installed to orchestrate

the line flow and the delivery of subcomponents, but human workers are very much

(PCB) assembly firms, many of which are subcontractors to the brand-name com-puter companies These are the new service industries for the comcom-puter industry, flow lines, but a noticeable amount of human interaction is needed to load machines, monitor progress, and step in if there is a problem The economics in this industry seems to justify U.S.-based assembly operations, perhaps because the batch sizes are smaller and communications between design and subcontractors are enhanced by proximity These speciality PCB assembly firms are also able to buy large quantities

of electronic devices in bulk and thus achieve economies of scale

Flexible robotic assembly: Further along the spectrum, all the leading automobile

com-panies in the United States and Japan have installed medium-cost robots and ClM sys-tems to spot-weld and paint cars The large batch sizes, heavy and/or unpleasant tasks, and a willingness to invest for the long haul have justified the investment in elM A tour of today's standard automobile line reveals that almost no shop personnel are ever, that a great deal of personnel are needed to participate in final assembly.)

Hard automation: In Chapter 2 it was emphasized that for extremely large batch sizes,

it might even be economical to revert to noncomputerized machines Speaking collo-veyor lines pump out the same product day in, day out As stated, this is often referred

to as fixed or hard automation In such factories, some basic computer controls and sensors are needed for monitoring and control, but reprogramming is not needed Chapter 6 reviewed the increasing miniaturization of disc-drive components and how difficult it is becoming to assemble them by hand with microscopes and tweezers What are the considerations for automation? In the final analysis, will automating disc-drive assembly payoff? The batch sizes are large, but are they large sufficient reliability Perhaps it is not worth risking a huge investment in automated

3U is a disheartening fact, but in today's civilization, some people are pleased to leave a rural envi-ronment (0 earn only $100 a month in an industrial setting, while others spend more than $100 at a