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Assembly modeling for product design and analysis
Most engineered products-from pencil sharpeners to aircraft engines-are assembled units. During product design and development, designers traditionaly consider not only functionality but also ease of manufacture of individual components and parts. However, little attention is given to those aspects of design that will facilitate assembly of parts. Assembly-related problems are typically discovered on the shop floor when its is either too late or too expensive to remedy them. Given the underemphasis on assemby design, this paper examines the current approach to teaching assembly design/drafting in the mechanic/ manufacturing engineering technology graphics curriculum. It presents an alternative approach in which the focus is shifted from assembly drafting to assembly design. Assembly modelers, which facilitate the construction, modification and analysis of complex assemblies, are a critical component in the assembly design process. These modules are found in many parametric solid modelers, including Pro/ENGINEER. Solid Works and Mechanic Desktop. Introduction Assembly modeling is currently very popular in industry. According to
an article in the Automotive Manufacturing & Production magazine published
in cooperation with the Society of Automotive Engineers (SAE), "whereas
in '88 it was parametric solid modeling; in '04 it is assembly modeling
and its applications that represents the newest trend in the world of
CAD/CAM."
Assembly Design and Analysis Assembly design and analysis are quite important in product development, especially since it is estimated that a full 50% of manufacturing costs are tied up in the assembly process.2 The greatest potential for increased productivity and significant reduction in production costs lies in the consideration of assembly requirements during the design stage of the product cycle.3 Designers, however, typically design for function, and to a lesser extent for manufacturing, but they rarely consider the assembly process. Consequently, assembly or production engineers are relied upon to solve assembly-related problems on the shop floor, an approach that is both time-consuming and costly. When evaluating alternate designs, ease of assembly is a key element in successful product development. Design considerations are often factored in during the product design stage with a Design for Assembly (DFA) tool. The most popular DFA method is a set of procedures developed by Boothroyd and Dewhurst.3 This method consists of two parts: (1) a catalog of generic part shapes and types, classified according to ease of feeding by parts feeders and ease of assembly by manual or automatic means, and (2) a source of rules, advice, or prompting questions concerning good DFA practice. In addition to DFA techniques, the following analytical procedures can help optimize assembly designs and are commonly found in assembly modelers: * Kinematic analysis - analysis of the motions of mechanisms * Dynamic analysis - analysis of the motions of mechanisms and the effects of mass * Tolerance analysis - determines the effects of individual part tolerances on the ease of assembly, the presence of tolerance stacks, and product performance * Finite element analysis - analysis of stress or strain and heat transfer * Mass properties analysis - computation of properties such as weight, center of gravity, and moments of inertia * Interference checking - determines if an interference or clearance exists between mating parts * Generation of exploded views - automatic generation of an exploded view of assembly * Generation of Bill of Materials (BOM) - automatic generation of an assembly parts list Current Pedagogical Approach to Assembly Design and Drafting The following analysis draws from the experiences of the authors and several professional colleagues in teaching engineering graphics. In addition, references [4] through [14] were reviewed, representing a wide spectrum of books on design/ drafting.4,5,6,7,8,9,10,11,12,13,14 The analysis revealed the following: 1. First, existing textbooks seem to focus almost exclusively on design/drafting of discrete parts. Almost all references devote more than 95% of the text to discrete parts rather than to assemblies. 2. Secondly, while all books include a chapter on manufacturing processes and their design/drafting-based implications, there was little if any treatment of assembly-- based considerations. 3. Lastly, what little treatment of assembly design/drafting in these texts focused exclusively on the creation of assembly drawings. In contrast, a review of recent manufacturing texts reveals that most devote either a full chapter or at least a portion of a chapter to assembly design issues, thereby high-- lighting the importance of this activity.15,16,17,18 The purpose of assembly drawings is to show how the parts fit together in the assembly and to suggest the function of the entire unit. Given the fact that these drawings are two-- dimensional representations of complex, three-dimensional, multipart assemblies, it is questionable how well these objectives are met. Focusing simply on how to create assembly drawings suggests that considerable effort and time is spent on clerical bookkeeping and detailing. An approach where the designer focuses on function and interconnected relationships, leaving the detailing and bookkeeping to an automated process, would be preferable. An assembly modeler facilitates this type of automated process. Assembly Modeling Assembly modelers can be defined as advanced geometric modelers in which the data structure is extended to allow representation and manipulation of hierarchical relationships and mating conditions.19 Geometric modeling systems, whether they are wireframe, surface or solid, have been used mainly to design or model an individual part rather than for the assembly of parts.20 Their data structure is designed to store and manipulate geometric data of individual parts only. Such systems, therefore, facilitate the analysis of individual parts and components. Assembly modelers, on the other hand, generate assembly-based data. Critical components of assembly modelers are defined as follows. BASE PART An assembly is created by adding parts and components to a base or root (parent) part using assembly constraints. The base part is that part in the assembly to which most other components are attached. For example, in the assembly of an automobile, the chassis would be considered the base part. DEGREES OF FREEDOM (DOF) Degrees of freedom are independent movements a body is capable of achieving. In the general case, a body has six degrees of freedom: three translational and three rotational movements. When a part is added to the base part or subassembly, some of its degrees of freedom are eliminated. MATING CONDITIONS (OR ASSEMBLY CONSTRAINTS) Assembly constraints define how components or subassemblies fit together. Constraints are used to align and orient parts in the assembly model with respect to each other.21 HIERARCHICAL RELATIONSHIPS When creating an assembly, the base part is likened to the root of a tree, such as the directory structure used in operating systems. As parts and components are added to the base, these parts are attached to the appropriate node in the branched structure, as shown in figure 2. Tree-like structures result in parent-child relationships. In such relationships, when a parent feature is modified the children may be modified as well. Traditional solid modelers can be used to create assemblies, though not assembly models, by using geometric transformations such as "move" and "rotate" to stack parts and components. In this case, the CAD database would not be robust enough to address functional issues such as whether parts A and B of an assembly should or should not move relative to each other. Further, if they may move, should such movement be translational or rotational? This type of information is necessary to conduct various analyses on the assembly, for example, a kinematic analysis of a gearbox. In CAD systems that offer assembly modeling capabilities, the geometric modeler acts as an input to the assembly modeler, as shown in figure 3. Designers use the geometric modeler to create and analyze individual parts and components, then use the assembly modeler to create and analyze the assembly. The process of creating an assembly model involves the following steps. First, the parts and components used in the assembly are individually modeled in the geometric modeler and saved as separate files. Next, the assembly modeler is invoked and a name is given to the current assembly. A base part is selected from among the previously created parts and loaded into the assembly modeler. The other members of the assembly are then added to the base part using assembly constraints. The order in which the assembly is generated implicitly defines the hierarchical relationships among the parts. This process of building the assembly model greatly promotes the engineer's understanding of the design. Since the designer must define specific motions of each part and the sequence in which component\s are added to the base, they are more likely to understand how parts fit together as well as realize the purposes of the assembly. The designer can also graphically move any component in the assembly and observe the resulting motion in the rest of the assembled model. Once a design has been completed, the assembly modeler can be queried to provide information on interferences and clearances between parts. The modeler can also compute mass properties for the entire assembly. Lastly, the modeler can automatically create exploded views, a bill of materials, and an assembly drawing. Thus, the designer can focus on creative processes, leaving many of the tedious clerical tasks to the system. Assembly Modeling and the Manufacturing Assembly Process Many of the features in an assembly modeler help optimize the manufacturing assembly process. For instance, the ability of the assembly modeler to furnish information on interferences and clearances between mating parts is particularly useful. Such information would enable the designer to eliminate an interference between two mating parts where it is impractical to provide for an interference based on physical assembly requirements. This activity can be accomplished within the modeling program, thereby averting any productivity loss that might occur when interferences are detected on the shop floor. Also, a knowledge of mass properties for the entire assembly, particularly the center of gravity, may permit the designer to redesign the assembly based on equilibrium and stability considerations. In the absence of such information, the presence of an elevated center of gravity and the attendant instability would only be detected after physical assembly on the shop floor. Three- dimensional exploded views generated by the assembly modeler can help designers verify whether obvious violations of common DFA guidelines are present, such as absence of chamfers on mating parts. All of these analyses can be achieved within the framework of the assembly modeler. Additionally, the assembly model may be imported into third-party programs that can perform kinematic, dynamic, or tolerance analysis. Tolerance analysis is of great relevance to the physical assembly process. With the input of the assembly model and other user-supplied information such as individual part tolerances, tolerance analysis programs can check the assembly for the presence of tolerance stacks. Tolerance stacks are undesirable situations in which acceptable tolerances on individual parts combine to produce an unacceptable overall dimensional relationship, thereby resulting in a malfunctioning or nonfunctioning assembly. Stacks are usually discovered during physical assembly, at which point any remedial procedures become expensive in terms of time and cost. Tolerance analysis programs can help the user eliminate or significantly reduce the likelihood of stacks. Based on the results of the tolerance analysis, assembly designs may be optimized by modifying individual part tolerances. Note, however, that tolerance modifications have cost implications; in general, tighter tolerances increase production costs. Engineering handbooks contain tolerance charts that indicate the range of tolerances achieved by manufacturing processes such as turning, milling, and grinding.22 Designers use these tables as a guide for rationally assigning part tolerances and selecting manufacturing processes. Case Study A case study was conducted at Southwest Texas State University to illustrate the assembly modeling process. The study involved the design of a butterfly valve assembly that consisted of a valve body, butterfly-type disk, a stem, a notched positioning plate, and a handle. In function, butterfly valves provide a positive shutoff and are used as a throttling valve set in any position from fully open to fully closed. Parametric Technology's Pro/ENGINEER Mechanical Computer Aided Design (MCAD) software was used to model the assembly. The individual components were created as separate geometric models in the part mode and saved as ".PRT" files. Next, the assembly modeling mode was invoked and the valve body was declared the base part. After specifying assembly constraints, the assembly was built by adding the remaining components to the base part. The completed assembly model was then saved as an ".ASM" file. An exploded version of the assembly model is shown in figure 4. Next, the assembly was analyzed to evaluate its manufacturability. The model was checked for the presence of any unwarranted interferences and clearances, then exploded to check for any violations of DFA guidelines. Lastly, an assembly drawing was generated from the completed model. At this point, the assembly model could be imported into third-party software for further analysis such as tolerance analysis. Finally, stereolithography (STL) files of each component were generated for the rapid prototyping (RP) process. The RP machine used in this case study was a Helisys 1015 machine that uses a layer-subtractive method to build prototypes. In this process, a laser beam cuts layers of paper that have been glued together with a heated roller on a build platform. The STL files for each part in the assembly were loaded into LOM Slice, a proprietary control software developed by Helisys for their RP machine. This software constructs a slice file from the STL file that is then used by the laser to cut layers of paper. LOM Slice was also used to set process parameters such as cutting speed, laser power, heater speed, and platform speed. To save time and maximize platform space utilization, the smaller part files were merged together, allowing the system to build several parts at the same time. The valve body, the largest of the five parts, took approximately 24 hours to build and consisted of 922 layers of paper. After the parts were built on the RP machine, they were removed from the build platform and decubed. A variety of fit and function types of analyses can then be performed on the physical prototype of the assembly, shown in figure 5. Thus, assembly modeling facilitates the assembly design and analysis processes. Conclusion Assembly modeling is an extension of geometric modeling that facilitates the construction, modification, and analysis of complex assemblies. Parts and components are added to an assembly by specifying mating conditions or assembly constraints. The resulting assembly model can then support various types of analysis such as design for assembly, kinematic analysis, dynamic analysis, and tolerance analysis. Assembly modelers permit a battery of virtual engineering tests to be conducted on an assembly prior to manufacture, helping to lower costs and optimize the overall design and manufacturing process. The following instructional guidelines are recommended: 1. Due emphasis must be placed on assembly modeling and design in graphics courses. In addition, students should be exposed to some common DFA guidelines. 2. The best method for including assembly modeling and analysis in the ET curriculum should be defined. Since parametric solid modelers are becoming standard tools in mechanical and manufacturing environments, many institutions use programs such as Pro/ENGINEER, Solid Works, or Mechanical Desktop for instruction. Many manufacturing and mechanical ET programs require at least two graphics courses. In the first of these, which is often a freshman- level course, the principles of parametric solid modeling typically are introduced. Incorporating assembly modelers into this course would change the emphasis from the creation of orthographic views to the generation of such views from the model. Institutions could retain a certain subset of descriptive geometry and the creation of orthographic projections simply to help students understand basic principles. The first course would focus mostly on individual part design and assembly. In the second course, which typically focuses on machine design and drafting, assembly modeling should be used as the means for completing assignments and projects that involve assemblies rather than individual parts. Course emphasis would shift from teaching students how two-- dimensional assembly drawings are created to demonstrating how assembly drawings are generated from assembly models. Lastly, in a manufacturing processes class, students could be exposed to the applications of assembly modeling in manufacturing. Such a shift in the pedagogical focus will require students to reason about issues such as the functionality, interconnectivity, and manufacturability of the assemblies they are designing. Acknowledgment This activity was funded in part by a grant from the National Science Foundation Division of Undergraduate Education and the state of Texas. "Assembly design and analysis are quite important in product development, especially since it is estimated that a full 50% of manufacturing costs are tied up in the assembly process." References 1. Vasilash, 0. S. "Autofact '97: A Quick Look at Some of the Latest Offerings for the Digital Manufacturing World." Automotive Manufacturing and Production 110, no. 1 (January 19981- A%A7 2. Bedworth, D. D., M. R. Henderson, and P. M. Wolfe. Computer- Integrated Design and Manufacturing. New York: McGraw-Hill, Inc., 1991. 3. Boothroyd, G. Assembly Automation and Product Design. New York: Marcel Dekker, Inc., 1992. 4. Bertoline, G. R., E. N. Wiebe, C. L. Miller, and L. 0. Nasman. Technical Graphics Communication. Chicago: Irwin, Inc., 1996. 5. Bertoline, G. R., E. N. Wiebe, C. L. Miller, and L. 0. Nasman. Engineering Graphics Communication. Burr Ridge, Ill.: Irwin, Inc., 1995. 6. Bethune, J. D. Engineering Graphics With Auto CAD. Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1995. 7. Brown, W. C. Drafting for Industry. South Holland, Ill.: The Goodheart-Wilcox Company, Inc., 1990. 8. Earle, J. H. Engineering \Design Graphics. Upper Saddle River, N.J.: Prentice Hall, 2000. 9. Earle, J. H. Graphics for Engineers. 5th ed. Upper Saddle River, N.J.: Prentice Hall, 2000. 10. Gicsecke, F. E., A. Mitchell, H. C. Spencer, I. L. Hill, J. T. Dygdon, J. E. Novak, and S. Lockhart, S. Engineering Graphics. Englewood Cliffs, N.J.: Prentice-Hall, Inc., 2000. 11. Gicsecke, F. E., A. Mitchell, H. C. Spencer, I. L. Hill, J. T. Dygdon, J. E. Novak, and S. Lockhart. Technical Drawing. Englewood Cliffs, N.J.: Prentice-Hall, Inc., 2000. 12. Gicsecke, F. E., A. Mitchell, H. C. Spencer, I. L. Hill, J. T. Dygdon, J. E. Novak, and S. Lockhart. Modern Graphics Communication. Englewood Cliffs, N.J.: Prentice-Hall, Inc., 2000. 13. Kersten, L. Technical Drawing with AutoCAD. New York: McGraw- Hill, Inc., 1991. 14. Sorby, S. A., K. J. Manner, and B. J. Baartmans. 3-D Visualization for Engineering Graphics. Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1998. 15. ElWabil, S. D. Process and Design for Manufacturing. Boston: PWS Publishing Company, 1998. 16. Kalpakjian, S., and S. R. Schmid. Manufacturing Engineering and Technology. Upper Saddle River, N.J.: Prentice Hall, Inc., 2001. 17. Tlusty, J. Manufacturing Processes and Equipment. Upper Saddle River, N.J.: Prentice Hall, 2000. 18. Wright, P. K. 21st Century Manufacturing. Upper Saddle River, N.J.: Prentice Hall, 2001. 19. Zeid, 1. CAD/CAM Theory and Practice. New York: McGrawHill, Inc., 1991. 20. Lee, K. Principles of CAD/CAM/CAE Systems. Reading, Mass.: Addison-Wesley Longman, Inc., 1999. 21. Howell, S. K. Mechanical Desktop: Parametric Solid and Assembly Modeling. Boston: PWS Publishing Company, 1998. 22. American National Standard. Preferred Metric Limits and FitsANSI B4. 2 - 1978. New York: The American Society of Mechanical Engineers, 1984. Dr. Sriraman is an associate professor of Technology and the program coordinator of the Manufacturing Engineering program at Southwest Texas State University (SWT). He has presented at and published in several ASEE conferences and journals. He has also secured numerous grants from sources such as the NSF and SME. Dr. Sriraman is also the faculty advisor to the SME student chapter at SWT. His teaching and research interests include the areas of CAD, rapid prototyping, quality assurance, automation, and manufacturing systems. Dr. DeLeon is an associate professor of Technology and the assistant dean of the University College at SWT. He has presented at and published in several NAIT and ASEE conferences and publications. He has secured several internal and external grants to help update technology laboratories and curriculum. His teaching and research interests are in the areas of CAD, rapid prototyping, quality assurance, and industrial safety.
Source: Journal of Engineering Technology |
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