A Capstone Design Project: From Design Through Manufacture and Use

Transcription

1 A Capstone Design Project: From Design Through Manufacture and Use K. L. Devine & L. G. Reifschneider Department of Technology Illinois State University, Normal, IL ABSTRACT At Illinois State University, the addition of a new automation lab provides a unique opportunity for engineering graphics classes to design products and tooling for use in an automated manufacturing environment. This paper will describe a design project in which students enrolled in two design courses worked in teams to design a thermoformed tray boxinsert and the robotic end of arm tooling needed to automatically insert a product into the tray. During the project students employed four key graphic design technologies: inter-part assembly modeling, prototyping, simulation, and creation of CNC tool path from CAD data. I. Introduction The capstone design project presented in this paper follows the pedagogy implemented by other engineering educators (Wells, Goddard, & Mountain, 2001; Todd, 1993) in providing a learning experience that directly relates to the type of design activities seen in industry. Further, as with other senior design projects (Rodriguez, Choudhury, Keil, Ramrattan, & Ikonomov, 2006), extensive use of rapid prototyping enabled students to develop hardware solutions that facilitated the implementation of a sophisticated automation project within a single-semester time frame. During this capstone design project, students faced many real-world challenges as they worked to design, prototype, simulate, fabricate, and finally deploy products and tools that were used in an automated assembly line. The final steps involving fabrication and deployment required students to closely interact with the physical reality of the automation cell. The learning that comes out of this practical aspect of the project cannot be overstated and is viewed as essential to a well developed pedagogy in engineering technology (Feisel & Rosa, 2005.) Finally, the concurrent engineering skills practiced throughout this project are recognized as being important in industry (Bertozzi, Hebert, Rought, & Staniunas, 2007.) The automated assembly line packaged a box of chocolate candy to customer (visitor) specifications. Using a touch screen, visitors to the automation lab selected which type(s) of chocolate candy they wanted and entered their name. The robotic work cell then filled their order with no human intervention. Visitors left with a box of candy consisting of a box with a lid, plastic insert, six pieces of candy, and a printed label containing their name. II. Project Requirements Successful design involves working within constraints to achieve a design that meets customer needs. Customer needs are clearly defined at the onset of a design project and are the criteria that determine whether a certain design is successful, (Ulrich & Eppinger, 2000.) Constraints are factors the design must take as given. In this project there were four constraints: the candy to be presented, the box in which the package was to be contained, the model of

2 articulated arm robot in the automation lab, and the model of gripper used by the robot. There were two thrusts of design problems posed in the project: a thermoformed tray that would present an ordered display of the candies in the box and a set of end-ofarm tooling used in the automated cell that could manipulate the tray, the box, and the candies. The project requirements are summarized in Figure 1. TRAY Aesthetic Ergonomic Anti-nesting Registration Design Tasks Product Box Constraints Robot Gripper Customer Needs EOT DFM/DFA Manipulate product, tray & box Figure 1. Summary of project requirements. The customer needs defined in the project were of two types as well: those related to satisfying the end customer and those related to the manufacturing cell performing the automated assembly. The important needs of the end user, related to the tray, were aesthetics and ergonomics. The features of the tray that controlled the position of the candies also must permit easy access to the candies for removal. The important needs of the manufacturing cell involved both the endof-arm tooling and the tray. The design requirements of the end-of-arm tooling focused on functionality and ease of assembly. The tray should provide some means of repeatable placement for the robot and avoid locking together when fed to the automation cell. The remainder of this paper will present how four key graphic design technologies were employed by students in two classes to solve the design problems posed by this capstone design project. The paper will illustrate how the use of interpart modeling, prototyping, simulation, and CNC tool path generation work together to create solutions to non-trivial design problems. NX, a constraint-based parametric solid modeler, was used to create solid models, generate, and simulate CNC tool path. The rapid prototypes were built using a Dimension 3-D printer. The robot simulation was performed using Robot Studio from ABB. The remainder of this paper is presented as two tracks to highlight the continuity of the design process for the tray and the EOT. III. Tray Design The design requirements of the tray were two-fold: aesthetics for the end user and design for automated assembly. Ultimately, the tray should present the candies to the end customer in an aesthetic way that offers some ergonomic feature to facilitate getting the candy out of the box. Thermoforming requires unavoidable secondary processing to remove the extra sheet material around the perimeter of the formed product. Thus, critical to the automated assembly of the package, the tray must have a feature that registers the uncut tray to a shearing device to properly size the final cut sheet. In addition, stacks of the cut-to-size trays were going to be automatically fed to the robotic assembly line by means of a gravity-fed magazine. The stacking of the cut-to-size trays requires a design feature that prohibits nesting, or locking together of similar formed shapes. The registration feature would insure each tray is cut to the same size to guarantee they would fit inside the purchased box. Further, because the candies would be inserted into a tray that was already placed into a box, the repeatable cut size of the tray insert facilitates the automated insertion of candy pieces by the robot. As will be shown in the discussion of the prototyping, the best design incorporated finger spacers

3 in the tray that served the dual purpose of ease of access to the candy and anti-nesting of stacked trays. Apart from the product design problems solved in the project, the application of the aforementioned four key graphic design technologies used to design and manufacture the tray insert follows. Interpart assembly modeling is a powerful modeling technology that allows geometric information from one part to be automatically incorporated in another part. The generation of the formed tray shape followed directly from the surface of the thermoforming mold through a process in NX called Geometry Wave-Linking. This modeling technique mimics the actual thermoforming process where a heated softened plastic sheet is formed onto an existing mold surface. The wave linking process begins by having a solid model of the mold and an empty part file that will become the tray model. In the case of this project, the mold surface was created by modeling tapered pockets on the face of a block. The surface of the mold that would contact plastic is wave-linked into the empty part file of the tray as a sheet body. The wave-linked sheet body becomes a solid body by offsetting the sheet by a value equal to the thickness of the formed plastic sheet. Figure 2 illustrates the formed sheet part that results from the wave-linking of the surface data from the mold to the tray file. The final result of this interpart modeling is a model of a thermoformed tray that was created from a model of a mold. The power of the wave-linking technique is that it creates associative geometry. Consequently, if the mold design changes, such as the depth of the tapered pockets or the position of a pocket, the wave-linked sheet body in the tray part file automatically updates. The interpart assembly modeling allowed rapid design iterations for improvement of aesthetics, ergonomics, and the location of registration features. Tray Tray: thickened (offset) sheet. Sheet: wave-linked surface from mold. Mold Figure 2. Wave-linked tray from mold surface. Physical prototyping followed from a series of design iterations done in NX. Fused deposition models (FDM prototypes) of several designs were made to evaluate how well actual formed trays worked for trimming and ergonomics. The FDM prototypes were built with vent holes already incorporated in the model. Due to the relatively low pressures involved in thermoforming, the FDM prototype is strong enough to function as a mold. Figure 3 shows two designs that were evaluated. The finger pocket design in Figure 3B was deemed better for holding the candies in place. Further, if these pockets were designed to avoid symmetry they could eliminate the undesired nesting of stacked trays as demonstrated in Figure 4. The antinesting function was necessary because the trays were to be automatically fed to the assembly line through a gravity-fed magazine.

4 CNC machining. Deep, narrow pockets and excessively small fillets are examples of mold geometry that may be difficult or impossible to machine. Simulation highlights these problem areas as well as other problems such as fixture and tooling interference. Figure 5 illustrates the NX simulation of the CNC toolpath during a roughing operation of one of the thermoforming molds. Figure 3A Finger pockets Design A Design B Registration features Figure 3B Figure 3. Alternative FDM prototype molds. CNC programs of the two mold designs were created using the manufacturing module of NX. Several aspects of the mold design made the machining process complex. For example, the mold pockets had tapered walls and edge blends that required careful planning to machine properly. Additionally, because the vent holes were only in diameter, they required larger holes to be drilled from the opposing side of the mold, commonly referred to as back-drilling. Simulation of the CNC tool path was performed in NX. Simulation of tool path was important because some mold geometry, although modeled in CAD and created as a rapid prototype, may not be suitable for Anti-nested stacked trays with anti-symmetric finger pockets: Design A, Design B, Design A Figure 4. Anti-nested trays for automatic feeding when loaded A, B, A, etc. Finally, the CNC toolpath generated in NX was used to machine the molds. Figure 6 shows the actual machining of a mold. Four molds were machined to create a multi-cavity thermoforming mold of two A and two B designed trays. The fours molds then mounted into a compound mold to improve production efficiency as over 1,000 trays were made and used in the automated assembly project.

5 Figure 5. Simulation of milling Design A mold. The use of an ATC allows the robot to use more than one tool to complete a task. During use, the robot automatically detaches one tool and picks up another. To successfully change tools, each of the tools must be placed accurately in a storage device that is within the working envelope of the robot. Because human operators manually load tools into the storage devices, fool-proofing features were included to help ensure the tool was loaded correctly. Figure 7 shows the finished EOT with student-designed mounting plate, gripper jaws, and tool storage device. ATC tool clamp attached to robot ATC tool holder Figure 6. Machining of Design A mold design. IV. End of Arm Tooling Design End of arm tooling (EOT) are physical tools that are attached to industrial robots to allow the robot to complete meaningful tasks. Welding torches, grippers, and resistance spot welding tools are common examples of EOT. The design requirements of the EOT in this capstone project focused on ease of tool manufacture (assembly) and the functional requirements of the automation cell. Students were first given information regarding the relevant design constraints. Specific constraints involved the work envelope of the robots, dimensions and mounting options of the purchased pneumatic gripper actuators, and the mounting requirements for purchased automatic tool changers (ATC) that would be used in the automated cell. Student designed mounting plates Student designed 3-jaw gripper Figure 7. Completed EOT. Student designed storage device CAD models of the products (candy, plastic trays, etc.) that the robots were required to move were given to the students. Because the EOT being designed by the students would also be used in future automation projects, some additional part handling requirements were included in this project (i.e. manipulate cylindrical and block shapes). Highlights of the EOT project requirements were: Robot must pick up candy pieces using vacuum and also pick up cylindrical and block shaped parts using 2-jaw and 3-jaw grippers.

6 Mounting plates must allow three purchased tools (2-jaw and 3-jaw grippers, and vacuum cup) to be attached to two ATC tool holders. Tool storage devices must be capable of storing two tools used with the ATC. They must include fool proofing features and must accurately locate the tools for robots to pickup. Tool storage devices must be positioned within the work envelope of the robot without intruding on the workspace required for the automated candy assembly project. Because the EOT design effort involved many purchased components, students first downloaded STEP models from vendor websites. The downloaded files were imported into NX and positioned relative to one another using standard assembly modeling techniques. Non-associative interpart modeling techniques were used to extract critical geometry from the imported models, thus allowing students to design new tooling components without the need to recreate existing geometry. This process of creating new component parts within the context of a partially completed assembly model is often referred to as top-down assembly modeling. NX uses variables called expressions to define dimensional feature parameters. By default, the scope of expressions is limited to a single part file; however NX has an interpart expressions function that allows expressions in one part to be used in any number of parts within an assembly. Students used interpart expressions to control critical design dimensions such as gripper jaw dimensions, material thicknesses, and distance between jaws. By using interpart expressions, students were able to make modifications in several component part files by only editing one expression in the top-level assembly file. Robot reach studies were performed using Robot Studio, a solids-based robot simulation program developed by the robot manufacturer ABB. Benefits of using robotic simulation software include collision detection and the ability to conduct reach studies to evaluate various work cell layouts. The robot simulation allowed students to assess the EOT designs using accurate robot kinematics and work envelopes. CAD models of the EOT designs were imported in to the simulation program and digitally attached to a robot. Models of the tool storage device and products to be handled were also imported and placed within the work cell. The virtual robot was then programmed to determine the extent to which the robot/eot assembly could complete tasks such as picking up the products and placing the EOT into the storage device. The EOT storage device was positioned at several locations within the work cell to evaluate possible locations for the storage device to be placed once built. Figure 8 shows the virtual robot placing one of the end of arm tools into the storage device. Figure 8. Robot simulation to evaluate work cell layout for reach and clearance of EOT components. As with the tray products, rapid prototyping was used to evaluate EOT designs. FDM models of EOT designs were assembled to the purchased components

7 in order to identify assembly problems that needed to be resolved. This was especially helpful with the interface plate between the ATC and the 3-jaw gripper because there were many fasteners of various types required in a relatively small area. The prototypes were physically mounted to the robots, as shown in Figure 9, in order to verify that the robot and EOT could perform the required tasks that were previously simulated. Finally, the prototyped EOT was placed into an FDM model of the tool storage device to assess the positioning and fool-proofing features of the design. students to evaluate clamp-tool clearances in several critical locations. Screen-captured images of the CNC simulation were also used to prepare setup documents for the CNC machine operator. Figure 10 shows a screen image of the simulation. Figure 11 shows the part being machined on a 3-axis CNC machining center. Storage device Mounting plate Mounting plate Figure 10. Simulation of EOT milling. 2-jaw gripper Figure 9. Prototype EOT affixed to robot. CNC tool path generation for the EOT was created using the manufacturing module of NX. Several milling fixtures were designed and fabricated to aid in the machining of the parts. In order to conserve material, the mounting plate and tool storage device were nested together and cut from one part blank. Unfortunately, the nesting of these two parts into one part blank added to the complexity of the required mill fixture and CNC program. Simulation was especially helpful for this CNC program because it allowed Figure 11. Machining of EOT on 3-axis CNC machining center. V. Conclusion The design of the tray proved successful by the measures identified at the onset of the project. The tray promotes the academic program in an attractive way and it provides a means to easily remove the candies.

8 Further, the two designs of finger pockets used to facilitate removal of the candies, when stacked in alternating order, provides a means to prohibit nesting of stacked trays. The tray also had enough flat areas to permit picking and placing by the robots. Finally, the tray incorporated registration features to allow for repeatable and accurate trimming of the formed trays. Likewise, the EOT designs met all the functional requirements of the project. They were readily assembled and completed all the required tasks of the automated assembly work cell. The tray design, coupled with the end of arm tooling design, made possible the completely automated assembly of over 1,000 custom boxes of candy as shown in Figure 12. design project that went beyond CAD modeling and simulation to ultimately solve a real world problem. A key design lesson learned by students involved in the project was the importance of designing to meet the customer needs. They realized, too, that there are two types of customers: the end user of the product and the intermediate users involved in making the product for the customer. VI. References Bertozzi, N., Hebert, C., Rought, J., & Staniunas, C. (2007). Implementation of a Three-Semester Concurrent Engineering Design Sequence for Lower- Division Engineering Students. Engineering Design Graphics Journal, 71(1), Feisel, L. D., & Rosa, A. J., (2005). The Role of the Laboratory in Undergraduate Engineering Education. Journal of Engineering Education, 94(1), Rodriguez, J, Choudhury, A., Keil, M., Ramrattan, S., & Ikonomov, P. (2006). Applications of Rapid Prototyping for Engineering Design Projects, Proceedings of the 2006 American Society for Engineering Education Annual Conference and Exposition. Todd, R. H., Sorenson, C. D., and Magleby, S. P. (1993). Designing a Senior Capstone Course to Satisfy Industrial Customers. Journal of Engineering Education, 82(2), Figure 12. Automatically packaged candies (box lid and custom printed label not shown). Students reacted favorably to the project because it involved creating working products from designs they developed. They learned from first-hand experience what it means to pass from the design of a part to a functioning component in a larger process. Design students had authentic design problems to solve as their work was essential to the successful completion of an automation class project. Further, the experience gained during the project had great value for students interviewing for jobs because they could discuss a Ulrich, K. T., & Eppinger, S. D. (2000). Product Design and Development, 2 nd Ed. Boston, MA: McGraw-Hill. Wells, R.L., Goddard, D.L., & Mountain, J.R. (2001). Integrating the Product Realization Process into a Mechanical Engineering Curriculum using Desktop Manufacturing Equipment. Proceedings of the 2001 American Society for Engineering Education Annual Conference and Exposition.