21st Century Manufacturing Episode 4 doc

84 Product Design, ComputerAidedDesign (CAD),and Solid Modeling Chap. 3 How Step III anufacturin specifics 3.1a 3.1b Evaluation of requirements Relative According ~c:~~~~~ numerical Rank tor~~~7.:er class. ranking of ordering (1-10) c00~g)or requirements Easy to read 9 9 18 2 Easy to understand 10 9 19 1 Easy to carry/handle 6 7 13 5 Good aesthetics 4 8 12 5 Content relevant to cours 9 6 15 3 Reasonable price 7 7 14 4 Good resale value 5 5 10 8 Freedom from errors 5 6 11 7 Figure J.t The mapping process in QFD and Compton's (1997) example for a textbook (from Engineering Management by Compton, © 1997. Reprinted by permission of Prentice-Hall, Inc., Upper Saddle River, NJ). Figure 3,1b is Compton's hypothetical example for a textbook. This ranks the topics from "easy to understand" as number 1, down to "good resale value" as number 8.In another matrix, these customer requirements such as "easy to under- stand" can then be translated into specific product requirements such as "worked examples," "organization of material," "number of diagrams," and so on.Through a series of similar matrices the manufacturing actions needed to achieve the critical product characteristics are finally identified and launched into the process planning and production departments, For Compton's textbook example, the production department would select layouts, artistic style,and even the weight and dimensions of the book to reflect the desires of the customers. One can also imagine QFD applied to automobile design and fabrication. Sup- pose a consumer group typical ofVolvoowners-values crash resistance above all else. In the "product specifics" matrix (center of Figure 3.1a), the "what" down the Step I System concept specifics Step II Product specifics How How 3.4 The High-Level Engineering Phase of Design 85 left side might be "avoid crushing a door from a side impact," and the "how" along the top might be "an attainable stress level." In the "manufacturing specifics" the necessary sheet metal thickness and grade of steel might then emerge along the top. Sheet thickness and grade will also influence the final cost of the product in the man- ufacturing table. There isa drawback to the QFD method for the design of mass-produced prod- ucts such as standard automobiles. OFD assumes (a) the existence of distinct market sectors and (b) that everyone inside each sector has the same ordered set of prefer- ences. QFD for a group breaks down if the preferences of the group members are not the same (see Hazelrigg, 1996). For example, the group of consumers who value crash resistance above all else might well lean toward the marketing messages of Volvo. But if that group of people does not like the style, price, or color selections, the crash- resistance characteristic may not be high enough to boost sales. Having said this, OFD has been used successfully in situations where the customer group is small or where there is a limited range of product characteristics (Hauser and Clausing, 1988). 3.4.2 Axiomatic Design An alternative to OFD has been developed by Sub (1990) and colleagues. It also begins with customer needs. Three steps follow: identifying functional requirements (FR), design parameters (DP), and process variables (PV). In this regard, the map- ping shown in Figure 3.2 below is philosophically the same as Figure 3.1a for OFD. The idea is to map the qualitative desires of the customer to more concrete engineering terms. Functional requirements (FR) might well be the performance issues described for computers (amount of RAM and so forth) and automobiles (acceleration and the like) in the TQM discussion in Section 2.4.7. Design parame- ters COP)might then be the specific design parameters for a computer chip or a car engine needed to attain the FR. Process variables (PV) would then be the semicon- ductor or production-line technologies needed to create the design of the chip or the automobile engine. To make these mappings converge as the design team moves across Figure 3.2, Suh and colleagues propose two axioms: 1. Maintain the independence of the functional requirements (FRs). 2. Minimize the information content of the design. The first axiom aims to create a design where the functional elements are decoupled. For example, Suh analyzes avariety of designs for an injection molding machine (see ~ 'P" User needs Fixure 3.Z The framework for axiomatic design (from The Principles of Design by Nam P.Suh, © 1990 by Oxford University Press.Jnc.). Consumer Functional Physical Process 86 Product Design, Computer Aided Design (CADl, and Solid Modeling Chap. 3 Suh, 1990, 72-78). In some designs, three functional requirements (FRs)-the melting rate of the plastic, the flow rate, and the pressure rise in the extruder-arc all affected byjust one design parameter (DP). the rotational speed of the screw (the reader might glance ahead to Chapter 8 to see the screw-machine operation). This coupled design is criticized because it will be impossible to regulate any of the three functional requirements (FRs) independently. As a result, alternative designs are explored where individual functional requirements (FRs) can be controlled inde- pendently of a screw mechanism. The mathematical way to create such independence is to set up a "design matrix," A, where (FR) = A (DP).The matrix elements define the nature of the rela- tionship between each of the functional requirements (FR;) and each design param- eter (DP j ). The individual elements of the matrix are given by A ~ 8FR, (3.1) 'J SDP j A "decoupled'' product design will be achieved if A is a diagonal matrix. In other words, A should he a r.quare matrix. and its nonzero elements should appear only on the main diagonal. There should be zeros elsewhere to exclude any potential cou- pling. Various designs can thus be analyzed. The aim is to arrive at a situation where any specific function of the device is related to one and only one design parameter. Speaking colloquially, if one were to "tweak" that design parameter, it should only influence that one functional requirement and not cascade into other functions of the device. Intuitively, the second axiom is quickly grasped. It advises the designer to create simple subcomponents and devices. In fact this is also one of the key ideas in the Boothroyd and Dewhurst (1999) DFM/A software described in Section 3.5.2. They advocate simplifying the shape of individual components in an assembly and simplifying the fit of one component with another, for example, using press fits where possible rather than screws.This is further described in Chapter 8 with reference to the redesign of IBM's ProPrinter. 3.5 THE ANALYTICAL PHASE OF DESIGN Soon after the conceptual design and the high-level analysis are in place, detail designs should be pinned down. Although these can be done with simple "desktop CAD/drawing systems," most of today's CAD programs encourage their users to add software modules to their basic CAD package. These include, but are not limited to, (a) constraint-based design and parametric modeling, (b) design for manufacture/ assembly/environment (DFMlAIE) scoring, and (c) finite element analysis (FEA). 3.5.1 Constraint-Based Design and Parametric Modeling Rather than build up a model with specific dimensions, it is often useful to create constraints between certain features or lines. Consider the simple cover plate to a regular domestic light switch. Using the U.S.standard dimensions as an example, the 3.5 The Analytical Phase of Design 87 outer dimensions of the plate are usually 112 X 68 millimeters (4.5 X 2.72 inches). But in fact, these dimensions do not have to be constrained; for example, in toy stores one can find light switch covers where the outer shape is a popular cartoon character. However, in the United States, there is a central rectangular slot for the actual light switch plus two holes for the screws that attach the plate to a standard switch box. The dimensions of the central slot and the positions of the screw holes must always be constrained to fit the switch box. In summary, an artistic redesign of a light switch cover can thus be wildly imaginative on the outside contours-but it must be strictly constrained on these inner, more engineering-like dimensions. Another example from Shah and Mantyla (1995) is shown in Figure 3.3: • Line 3 is parallel to line 5. • Line 2 is a circular are, tangent to lines 1 and 3. • Line 4 is oriented at 0:. to line 3. • Line 1 is horizontal and length b. • Line 5is perpendicular to 1 and length a. A more sophisticated development of such constraints is parametric modeling. This procedure allows fast scaling of an object into unique variations from one original. Consider a simple aluminum can for soft drinks (figure 3.4). Rather than specify that the dimensions are 120 millimeters (4.75 inches) high and 62 millimeters (2.5 inches) in diameter the ratio 4.7512.5 == 1.9 might be used. The height and diam- eter are then described in proportion to each other without specific dimensions being used. Then the model can be scaled up or down without having to numerically respecify each of the new dimensions. Figure 3.3 Example parametric definitions (from Parameter and Feature-Based CAD/CAM, Jami Shah and M. Mantyla, © 1995.Reprinted by permission of John Wiley & Sons. Inc.). 4 ParaUel(line 3.line 5) • Line B parallel to line 5 •Line2 acirculararc,tangenttolines! and 3 •Line4 orientedatatoline3 •Line 1 horizontal and length b •Line 5 perpendicular to 1aod length a 88 Product Design, Computer Aided Design (CAD), and Solid Modeling Chap. 3 Constantl/d Figure 3.4 Parametric scaling of a soft drink can. 3.5.2 Design for Assembly, Manufacturing, and the Environment {DFA/M/EI DFNMIE software packages give manufacturing and environmental feedback to the designer or design team. Ideally, designs can be modified early in the design process to improve manufacturability and decrease environmental impact. New techniques to analyze a particular design for its manufacturability include the design for manufacturing and assembly (DMFA) software tools by Boothroyd and Dewhurst (1999).Theirs is a commercialized product that designers can access by CD or the Web.A suite of tools is available that contain, for example, DFM software for machining, DFM software for sheet metalworking, DFE software to assess envi- ronmental impacts,and their best-known DFA module for evaluating assembly. The DFA for assembly module involves two key ideas: •The quality of individual subcomponents must be high. Also, their number must be reduced as much as feasible. •Assembly operations must be as simple as possible. For example, factory lay- outs should be orderly, the shape of individual components should be simple, design features should simplify the assembly of one component with another, and assembly operations should not fight gravity. Their DFM for machining module "Machining for Windows" assistsa designer with the following issues: developing operation and process plans, obtaining cost esti- mates at the earliest stages of conceptual design, developing quotations, and plan- ning for production. 3.5.3 Analysis and Decision-Based Design Finite element analysis (PEA) allows the optimization of material use and perfor- mance in a quantitative and automatic manner, both of which are fundamental to the detail design process.In many casesthis detailed engineering analysis provides tech- nical innovations, but it rarely influences the original high-level concept. 3.5 The Analytical Phase of Design 89 Despite the apparent rigor of FEA calculations, they should be interpreted with caution. In particular, the cummun "safety Iactors" that have been developed over many decades for design work should still be applied. This is because there is uncertainty in the engineering materials that are used today, and also in the boundary conditions that are used for the finite element modeL Siddall's (1970) simple diagram succinctlycaptures thisconcept of "uncertainty" as it relates to decision- based engineering design (Hazelrigg, 1996). In general the designer does not have the luxury of measured, or a priori known, values of stress due to loading; instead the designer can only calculate stress based on a probability function. Similarly, given today's steelmaking and other production methods, the designer does not know pre- cise values for the yield strength and similar properties; again there is a probability function. The diagram shows the great dangers that can arise when the "tails" of the two probability functions overlap in the "failure zone." A key question arises: What does a designer do to address the "failure zone"? The answer critically depends on the preferences, or values, of the designer. For rou- tine consumer products the goal of a design is to make money, and more is better (Hazelrigg, 1996). With "making money" as the main objective, the designer might choose mate- rial properties in such a way that "an occasional failure is acceptable"; for example, 3.4 parts per million might be acceptable (Figure 2.15). In such circumstances, the consumer might be temporarily disappointed by a product failure. But if a fast, cour- teous warranty procedure is put in place by the company, then in the end the design utility, u, will be acceptable. In other words, a consumer product will not be "overde- signed and overcostly" from choosing material properties that are "oversafe'' so that no failures will occur. By sharp contrast, for the aerospace and aircraft industry, a designer should reevaluate the goal of making money and balance it with "stability and reliability." In the extreme case of a nuclear weapon, mentioned at the beginning of Chapter 2, society demands "reliability" rather than "cost effectiveness" as the main design goal. In Figure 3.5, this means that the two curves should move apart on the x axis leaving no possibility for a failure zone. Figure3.5 A frequency distribution for predicting the strength ofa design. It shows the inevitable spread in material properties on the right and loading stresses on the left (courtesy of James Siddall,1970) Stress due to loading Yield stress of material Stress 90 Product Design, Computer Aided Design (CADl,and Solid Modeling Chap. 3 With these thoughts in mind the designer can now begin to create a specific design with specific dimensions. 3.6 THE DETAILED PHASE OF DESIGN In the "old days,"the creative designers would pass their drawings down to the drafting office,where pencil and paper were used to document the detailed, fonnal drawings with appropriate tolerances. With the advent of CAD stations,it was obvious that much of this pencil-and-paper work could be replaced with pull-down computer menus and the drag- and-drop positioning from the mouse. The detailed CAD methods that are available fall into several categories, including wire frame methods and solid-geometry techniques. Chapter 3 presents some tutorials on these basicmethods. CAD drawings are created step by step to show the construction techniques. However, reading these tutorials, or buying the book Teach Yourself AutoCAD, is no substitute for going to a CAD workstation, log- gingin, and persevering through the tedious little commands that eventually create a nice isometric viewand the three principalorthogonal viewsofafairlystandard object.Detailed design usingCAD is like driving a car:no amount of tutorials in high schooldrivers' edu- cation classescan prepare you for the actual experience of cornering, even at 10mph. For detail design (as opposed to conceptual design) the utility of traditional CAD programs as a communication tool is undeniable. Drawings remain consistent, are repeatable, and conform to standards. Designers and draftspersons can compare component designs to check for clearance, flushness, perpendicularity, and other fit- related qualities. Although this process is often tedious, it is a great improvement over manual methods. CAD has also been useful in articulating precise details so that designers can better understand their implications during each design iteration. Most contemporary CAD tools have both wire frame and solid modeling capa- bilities. Solid modeling provides a sense of form usually lacking in wire frames. Solid modeling is a more intuitive system, and the process of constructing a model also provides insights into the manufacture (particularly the machining) of parts and assemblies. Solid modeling by both constructive solid geometry (CSG) and destruc- tive solid geometry (OSG) is described later in this chapter. 3.7 THREE TUTORIALS: AN OVERVIEW The object pictured inFigure 3.6is an example of a prototype of a joystick for the user of a virtual reality (VR) environment. The joystick has a somewhat heavy mass as its base. As the mass is moved in three-dimensional space, accelerometers in the base adjust the virtual environment appropriately. Information is transferred to the computer via a par~ allelport. With the exception of the handle, the device is fabricated from components with rather simple geometries. The joystick will now be designed with three different CAD methods: • Wire frame using the AutoCAD package (Section 3.8) • Constructive solid geometry (CSG) using the AutoCAD package (Section 3.10) •Destructive solid geometry (DSG) using the SolidWorks package (Section 3.11) 3.8 First Tutorial: Wire Frame Construction 91 F1pre 3.6 Virtual reality joystick (from a student group led by Ryan Inouye). The three tutorials show the characteristics of each CAD technique. In addition the case study at the end of the chapter shows the features of the SDRC package and some aspects of parametric based design.' 3.8 FIRST TUTORIAL: WIRE FRAME CONSTRUCTION Simple wire frame CAD systems use basic mathematical and computer graphic tech- nologies.wire frame programs begin by allowing users to choose points from a local or relative reference frame (the choice of the local frame is often arbitrary). These points are then mapped onto a global reference frame.Finally,lines are drawn between points. The final image is therefore a connection of lines that may be hard to view clearly. For example, (a) lines that should be hidden, perhaps representing the back of an object, will remain visible during construction, and (b) no helpful shading of the front face will be possible. A quick glance ahead to the figures depicting wire frame constructions shows these difficulties with visualization. IThe use ofAutoCAD,SolidWorks,and SDRC isnot intended to be an endorsement of these prod- ucts or a deliberate exclusion of the other CAD products listed in the URLs at the end of the chapter. Fur- thermore, all CAD packages are capable of wire frame, CSG, and DSG. Even the cheaper SyslemScan do some parametric design.A variety of CAD systems was chosen for the chapter to deliberately show some product mix rather than choosing jut one. Handle Top Base Back Front 92 Product Design, Computer Aided Design (CAOl, and Solid Modeling Chap. 3 Although wire frame models lack the advantages of solid representations, they are nevertheless useful.The inner workings of wire frame programs are also simple to understand and thus adapt for special user-generated purposes. Also, the com- puter's calculations mirror those commonly found in linear algebra, dynamics, and robotics classes. The fact that most wire frame programs require less powerful computers has until very recently been the most compelling reason for their use. However, the inevitable progress in semiconductor design and manufacturing (see Chapter 5 and associated figures) and the availability of cheaper, more powerful machines now bring solid modeling tools to the average desktop machine. For illustrative purposes, consider the base of the joystick. By selecting a number of desired lengths for the base, a series of lines can be developed to outline the object. In CAD constructions of symmetrical objects it is quite common to draw only half of the object and then mirror it across a central plane. 'This general approach willbe used here. The advantage is that the overall time taken is less,and both sides of an object are identical from a symmetry viewpoint. Figure 3.7 is the right half,plan view of the base.The halfwaypoint of the line segment that represents the back of the base willbe the mirroring plane origin, 8. Therefore, point a isat coor- dinate x = 0, y = 0, Z = 0, or more simply (0,0,0).The line beginning at point 8 was drawn with the line command. The line travels from the middle of the back bottom edge of the base to the end of that edge.The line then travels toward the front of the base for a short distance, turns to make the angle along the right-side bottom edge of the base to r,and finally moves across the front bottom edge. Figures 3.7 and 3.8 provide the results of some of these initial suggestions on how big the base of the object should be. Figure 3.7 is the top view of the external edges of half of the base. Figure 3.8 is an isometric view of the same lines.It provides insights into the three-dimensional nature of the lines. The viewpoint in Figure 3.8 was accomplished with the vpoint command in AutoCAD. The setting for the viewpoint is (1,-1,1),or positive in the x direction, neg- f Front of joystick base Mirroring plane .> Right side of base in plan view Back of joystick a(O,O,O) Flpn 3.7 Top view of one-half of the base. The line segment contains point a in its center. 3.8 First Tutorial: Wire Frame Construction Figure 3JI Isometric view of one-half of the base (viewing from the back of the joystick where the wires can be seen in the earlier photograph). ative in the y direction, and positive in the Z direction, with each axis held to 1:1pro- portions (i.e., isometric). While making drawings, it is a good idea to experiment with the overall view by changing the viewpoint proportions to see the effects of fore- shortening. The line starting at point b was drawn in a similar manner according to the dimensions for the top surface of the object. All points on the b line have a constant, z dimension above the points on the a line. The lines originating at c and d follow the same x-y profile. Each of these lines has its own, constant z coordinate. Points a, b, e, and d were specified with Cartesian coordinates. The choice of coordinates was dictated by the desired object size and ease of calculation. Thepoints at c and d were first determined with Cartesian coordinates. However, the lines that originate at e and d, going along the angled portion and the front edge, have been determined using polar coordinates. Points e and f were determined by a fixed dis- tance and an angle from points cand II.Temporary construction lines establish all the points for the shape df and ceobut are later removed. They are there temporarily to validate the correct shape of the slanting surface along ref. The outline of the base exterior begins to become clear in Figure 3.9. The mirror command was used to duplicate the objects across the y-z plane at x = O.Figure 3.9 contains some over- hangs and extraneous line segments. The overhangs can be trimmed with the trim command to leave segments ceoelf,ij, and gb as the required edges. This is done by first clicking on thc excess segments jl, hk, I'm,and en as the objects to trim. The erase command then leaves segments ee, df, gb, Ij, ej, and bf. The outline of the object is revealed by constructing lines between points 0 and hob andj,j andq,p andf,fand e,and e and r,all with the line command. For example, lines along the z direction (between e and f,j and h, ) at the intersection of planes complete the general outline of the object. Figure 3.10 presents the results thus far. With the general outline of the object described, the generation of inner details and cavities becomes the-next significant step. 93 Left side to be mirrored Right side of base Bock [...]... Christoph M Hoffman, © 1989.Morgan Kaufmann Publlshers.) ~:C(h"'k) c d A 1 -~ -~~ a : 3.1 Vertices Verte" 1 Defining 3 b D(bottom) 2 TABLE f B the Pyramid with Vertex Edge Edg 2 3 a b 1 f 2 3 3 4 \ 4 2 4 a Winged-Edge Face and edge Lon Right A " A Data " Pred Pre' , Fllges for right fac face Succ Succ d e B D C D D B d Structure Edge_,f"rleftfac~ c b C A A A A a Succ b f a f B B p", d e d b d f b... direction Product Design, Computer 1 04 Aided Design (CAD), and Solid Modeling Chap 3 FlJ:Uft3.2iJ Lshaped bracker rfrorn C~om~tric •• Solid Mod~Ung:A" nd Introduction by Christoph M Hoffman, © 1989 Morgan Kaufmann Publishers.) FiJ:Uft3.21 BooleantreeofCSG operations and individual features from the previous figure ~~c, / :tAra71ate (1) Box (1 ,4, 8) 3.9.5 x-trailate Box (8 ,4, 1) z-cylinder(l,l) Feature-Based... namely, where a cylinder intersects a material surface The dimension is known from the distance ab in the earlier figures.Thus in Figure 3. 14, the upper smaller circles are coincident with the top surface, meaning they are through-holes for the screw threads Figure 3. 14 also illustrates that circle y intersects one of the vertical surfaces of the rectangular pocket Circle y was modified by trimming the... insights into manufacturability provided the designer is sympathetic to the manufacturing operations Process planning, fixturing, and orientations also become much clearer This is because the Boolean nature of CSG requires that features be added and subtracted in a logical manner If done well, the ordering of the features can anticipate manufacturing For communications, CSG models can be rendered: a process... more "artsy," slanted faces at the front In Figures 3. 24 and 3.25 the individual objects are shown in the appropriate locations for solid subtraction The former is the wire frame rendition, while the latter is the solid Figure 3.26 is the final part Rendering can then be initiated with the render command Be sure to compare Figure 3.26 with Figure 3. 14 108 Product Design, Computer Aided Design (CAD),... gratefully acknowledged In the wire frame tutorial (Section 3.8), the person sitting at the CAD system's user interface mostly clicks on points and connects them with lines to create the joystick in Figure 3. 14 By contrast, in the solid modeling tutorials, the person sitting at the CAD system's user interface constructs the joystick by adding, subtracting, or intersecting individual bodies (Section 3.10) or... succeeding one Table 3.1 gives other data for the pyramid This is how the data are stored in the computer for compactness and accessibility It also captures the geometric relationships in a brief manner 3.9 .4 Constructive Solid Geometry (CSGI The general techniques for solid modeling in CAD/CAM were developed during the 1970s,responding to an obvious need in industry to move beyond the ambiguities of wire... 94 Product Design, Computer Aided Design (CAD), and Solid Modeling Chap 3 Ilemporaryoonstructionlines(ceanddf) Figure 3.9 Outline of the base exterior before the trimming process F1gure3.10 Outline of the... half Such manual operations are a major disadvantage of wire frame modeling Ambiguous drawings can result if they are not attended to, which may result in costly mistakes during manufacture Figure 3. 14 is now completed to the extent required for this illustration Screw holes for the base plate and a hole for the parallel port cable have been omitted here, but are eventually required To provide greater... fact correspond to manufactured features The designer in this case calls upon a more restricted menu of features, such as holes and pockets, which correspond to the physical actions of a "downstream" manufacturing process A menu that includes holes and pockets will correspond to machining." This idea sets the stage for the destructive solid geometry (DSG)-5ection 3.II-further on in this chapter First, . understand 10 9 19 1 Easy to carry/handle 6 7 13 5 Good aesthetics 4 8 12 5 Content relevant to cours 9 6 15 3 Reasonable price 7 7 14 4 Good resale value 5 5 10 8 Freedom from errors 5 6 11 7 Figure J.t The. can for soft drinks (figure 3 .4) . Rather than specify that the dimensions are 120 millimeters (4. 75 inches) high and 62 millimeters (2.5 inches) in diameter the ratio 4. 7512.5 == 1.9 might be used 3 Constantl/d Figure 3 .4 Parametric scaling of a soft drink can. 3.5.2 Design for Assembly, Manufacturing, and the Environment {DFA/M/EI DFNMIE software packages give manufacturing and environmental