03 - Manufacturing & Assembly

We designed our mechanism to be built with easy repeatability since we were going to make multiple prototypes in addition to the final product. After learning from the slider-crank project, we wanted to avoid machining complex parts since it can be a long and cumbersome process. Additionally, any mistakes that would need to be fixed would consume more time than what we allocated for assembling each prototype.

Our first prototype was scaled down to a 1:4 size, and was laser cut out of acrylic because the material was readily available and laser cutting is a quick and simple process. We used simple screws and nuts as the pin joints. This allowed us to visualize the motion of the mechanism early on so we could analyze its behavior to design a more elegant final product.

Our second prototype was a 1:1 scale (same as final model) so any issues encountered with manufacturing and assembly would be noted and tackled prior to final product completion. Since our mechanism has many customized parts, most pieces were 3D-printed with ABS plastic in high resolution to ensure better strength such as the fixed pieces on the shank link and shoe cover. The shank link was chosen to be a pre-made aluminum rail, again, to reduce the time to manufacture the entire mechanism. Machining a smooth railing requires a reasonable amount of time to ensure that contact surfaces with the slider provide minimal friction. All other linkages were cut from wood because we were more concerned about the motion of the 1:1 scale of the mechanism rather than any friction or the aesthetics of the wood. PVC piping was cut into cuffs to fix the mechanism to the shank and ankle while some velcro straps were added to provide better stability. The spring used in this iteration was too stiff and did not deflect enough to give the user feedback, so a new one was purchased for the final product.

Learning from the second prototype’s assembly, our final prototype for the slider was 3D-printed, but roller bearings and a rotary rod were used to contact the aluminum rail and create the sliding motion. This was to drastically reduce surface friction between the railing and the slider which created functional issues in the second prototype. The final prototype’s shoe cover was designed to have a longer bottom so that the rubber strips below it had more surface to grip onto the floor. Many of the same parts from the second prototype were used in the final such as the customized PVC cuffs and some 3D printed hinges. The foam layer inside the PVC cuffs for comfort was trimmed from the second prototype to the final so the user had a more sensitive feedback response. Aside from the slider, the main change from the second iteration was replacing the wood with acetyl bars (machined and cut on the drop saw) and swapping out the spring for a more compliant response.

Figure 10: Acrylic link for slider from Prototype 1.

Figure 11: Linkages made from wood and aluminum railing with PVC in Prototype 2.

Motion of slider-crank from Prototype 2.

Figure 12: Assembly of spring encasement for final prototype.