Eagle Claw Manufacturing Plan

Ossur Prosthetics

From: Peter K. Rubin (Senior Mechanical Engineer)
To: Jeremy Schneider (Product Manager)
Subject: Manufacturing system for Eagle Claw prosthetic rock-climbing leg

Foreword

     During the past year, our engineering team has researched, designed, and prototyped the Eagle Claw, a prosthetic rock-climbing leg for above-knee amputees. Many amputee rock climbers have difficulty climbing with their standard prostheses, which are often heavy, bulky, and poorly shaped for rock climbing. This temporary, task-specific replacement prosthetic leg is comprised of a rigid, lightweight aluminum pylon connected to an aluminum foot with rubber gripping surfaces oriented to optimize the user’s ability to grip rock ledges.

      We have developed a manufacturing plan for the Eagle Claw that is optimized to ensure efficient production of a high-quality product. This memo outlines our proposed manufacturing system and describes each manufacturing operation in detail. In order for us to meet our production deadlines, we ask that you review and approve our manufacturing plan by the end of this month.

Summary

     Many of the parts required for the Eagle Claw do not need to be specially manufactured. The pylon and prosthetic adapters can be supplied internally by Ossur’s current manufacturing system, while non-specialized parts such as rock climbing rubber, nuts, bolts, and adhesives can be acquired from external suppliers. Only the foot itself will need to be specially manufactured for the Eagle Claw project. The manufacturing system we propose is fully automated, with the exception of final assembly. Each foot will be plasma cut from plate aluminum and bent using a hydraulic press. Ribs will then be TIG welded onto the bottom of the foot. Finishing and hard anodizing will be outsourced, and final assembly will be completed in-house.

Introduction

     The following sections describe the sequence of operations required to manufacture the Eagle Claw. For each operation, we discuss the relevant processes, machinery, tooling, and materials handling equipment. Ossur’s marketing department has informed us that approximately 10,000 Eagle Claws are to be manufactured, and we have chosen our manufacturing plan accordingly.

Plasma Cutting the Foot

     The main piece of the foot is cut from 0.25” thick 5052 aluminum to the shape specified by the template in Appendix A. This alloy of aluminum was chosen because it is inexpensive, is ductile, and has a high strength-to-weight ratio. During the early stages of this project, prototypes of this part were made by hand. A rough shape was cut with a band saw, and the final shape was cut with a 3/4” end mill as the part was strap-clamped to a rotary table mounted on a Bridgeport mill.

     For large-scale manufacturing, however, a far more efficient cutting approach is required. After considering various cutting methods, including CNC milling, sheet metal stamping, water jet cutting, flame cutting, C02 laser cutting, and plasma cutting, we determined that plasma cutting is the best choice for this procedure. It is fast, suitable for cutting aluminum, and relatively inexpensive. See Appendix B for a summary of our analysis of metal cutting processes.

     The foot template is cut from ¼” by 36” by 60” plates of aluminum, which are fed by conveyor into the plasma cutter. Each sheet makes approximately 50 feet and 150 ribs (to be described later). As the feet are cut by the plasma arc and separated from the sheet, they fall onto another conveyor where they are transported to the hydraulic press to be bent. Cut ribs are redirected directly to the welding machine via a different conveyor belt. Excess sheet is ejected from the plasma cutter and recycled.

Bending the Foot

     Each foot that comes from the plasma cutter slides into alignment onto a convex steel buck (see Appendix C). A 50-ton hydraulic press pushes the corresponding concave mold onto the flat foot, causing it to deform into the appropriate curved shape. Because 5052 is particularly ductile, this process will not compromise the strength of the material. The foot is removed from the hydraulic press by a robotic arm and flipped upside down onto a computer-controlled TIG welding machine.

Welding Ribs onto the Foot

     Structural ribs are added along each toe to the bottom of the foot in order to increase the strength of the foot without significantly adding to its weight. These ribs were cut earlier by the plasma cutter to precisely match the shape of the curved foot, and are now positioned against the bottom of the foot by a robotic arm.

     The ribs are welded to the foot using a computer-controlled TIG (tungsten inert gas) welding machine. To increase the speed of this process, two electrodes will be used simultaneously and symmetrically, one on each side of the foot. Each electrode should apply 300 amperes of AC current with a 3/16” electrode, and 3/16” aluminum filler rod should be added to the welds as needed. It is imperative that the welds fully penetrate the material to ensure maximum strength. The rib position and welding pattern is depicted in Appendix D.

     When welding is completed, the completed feet are transferred to a conveyor to gradually cool. At this point, the parts are visually inspected for flaws and recycled if they are defective. Acceptable parts are packaged and transported to be finished.

Finishing the Foot

     The finishing process is subcontracted to PK Selective, a company that specializes in industrial anodizing. The feet are first polished to remove burrs and scratches, and are then hard anodized in a variety of colors. Although hard anodizing is more expensive than standard anodizing, it will result in a much thicker, harder, and scratch-resistant outer shell, which is necessary for a rock climbing foot that will regularly collide with sharp rocks. A hard anodization thickness of 0.003” is sufficient to prevent scratching.

Assembling the Leg

     Each finished foot that returns from PK Selective is ready to be incorporated into a leg assembly. A list of parts needed for each assembly is attached (see Appendix E). Each Eagle Claw is assembled by hand. First, the four-hole adapter is attached to the foot with bolts, which are secured with acorn nuts and tightened with a handheld power bolt driver. The lower and upper and tube clamps are secured to the pylon and tightened using the bolt driver. Finally, the lower tube clamp is positioned over the pyramid protruding from the four-hole adapter, and the two parts are tightened using the bolt driver. This last connection could be postponed to permit the Eagle Claws to be shipped in two parts to save space and to be assembled by the user’s prosthetist. See Appendix F for a diagram and rendering of the assembly at this stage.

     Finally, rubber strips are secured to the tip of the heel and the three toes using quick-set epoxy. These rubber strips provide the user of the Eagle Claw with an enhanced surface to grip rocks. The Eagle Claw is now fully assembled and ready to distribute. The assembly of each Eagle Claw should take no longer than one minute per foot.

Conclusion

     Using the manufacturing plan described above, the Eagle Claw rock-climbing leg can be mass produced according to our design specifications cheaply and efficiently. This proposed plan uses human labor only at the final assembly stage, when doing so is cost efficient, and subcontracts only the finishing stage of production, which we are incapable of performing in-house. We are confident that this manufacturing system will be successful, and we hope that you approve this plan by the end of the month in order to expedite the production of the Eagle Claw.