4.3 Design Iterations
We cover the mechanical design iterations of the finger/palm assembly and the arm assembly leading up to the final design.
Finger Assembly Iterations
Finger Version 1 (ideation)
From our prototype sketches, the motion is broken up into a finger that is actuated (rotated) via an offset slider block. Repeating this slider assembly multiple times results in the three fingers we need for our prototype. The actuator (another slider mechanism, referred to as “actuator” here) moves the slider-block from each slider assembly to extend the fingers. This actuator is positioned such that the finger sliders extend at different times to give the rock, paper, and scissors hand positions.
It was clear from this print that packaging two integrated slider cranks three different times would be difficult and size would be an issue especially if it would be cantilevered off of the arm assembly. In all of the following iterations, size of the palm would always be considered. The biggest thing that limited us from further discoveries on this iteration was that the orange ground shaft for the actuator mechanism broke off from the baseplate. The main take away was that future extrusions from the palm baseplate would need to be filleted to avoid stress concentrations.
Finger Version 2 (prototype)
For easier, planar parts such as the linkages, laser cutters were used to quickly iterate. However for parts that required variable tolerances, and more complex geometries, 3D printing was used. M3 bolts from the bins provided by the class were used for this prototype. This was the first prototype to successfully integrate the two slider cranks together.
Takeaways from the V1 finger assembly:
The sliders were made to be 3D printed slider blocks with a rectangular groove. This introduced a great deal of tolerance issues and misalignment potential in the sliders where they would “catch” on the surface roughness and require more forces to push the slider through.
Having three fingers actuated by the same “Link 2” from the actuator mechanism introduced a long shaft with 4 linkages cantilevered off the ground shaft. This not only broke our 3D printed ground shaft (to be replaced by a servo motor), but was an inefficient system that required each finger assembly to be checked for enough shaft length.
Friction is going to be a limiting factor in our assembly. In cranking this assembly by hand, it was clear that alignments, loose linkages on bolt shafts, and rubbing parts would require extra force to push through that a servo motor would likely struggle with
Finger Version 3 (intermediate)
For our second iteration of the assembly, the following changes were added:
Slider blocks were replaced with shafts and linear bearings. The linkages would be attached to a coupler that went over the linear bearing blocks.
In addition, the linkage system for the actuator was replaced with a gear train where each finger had its own inline slider-crank mechanism. The gears were adapted from gears found on McMaster. The “Link 2” in this slider crank was integrated into the gear train. In order to have the gear system on the same Z-axis plane as the linear bearing coupler, the gears needed to be elevated. A bracket was added to help mount the center of the gear however it interfered with the bolts needed to attach the other linkages to the gear.
A 360deg servo motor was added into this cad in an attempt to begin integrating electronics. The servo mounting holes were added to the palm baseplate however a slot to allow the servo body to protrude through was neglected.
(Not Pictured) Low friction PTFE washers were added between linkages and printed extrusions when they were mounted on the same bolt shaft to allow for easier movement between the parts while restricting movement of the linkages along the bolt axis. Locknuts on the end of each screw also helped with limiting the movement of the linkages along the bolts and resisting loosening during operations.
(Not pictured) linkages were cut from acrylic for aesthetic purposes while the gears were cut from wood to allow for some deformation and avoid fracture of an acrylic gear tooth.
Finger Version 4 (proto-final)
This design features our final palm baseplate part but outlines our iterations on the gear mounting. Initially a 6mm shaft was to be press fit into the palm baseplate however this fit was loose and we realized the torque being passed through the gears was non-negligible and that the shaft would needed to be fixed further. This led to the shafts being drilled and tapped for an M3 hole. Washers were placed on either end of the shaft to prevent the shaft from falling through the gear and baseplate holes. In addition, pipe cleaners were initially used as a soft standoff to keep the gears at their appropriate height but this was not rigid enough to fix them against the torque of the servo. This started the beginning of our gear misalignment issues.
Next, 3D printed standoffs were used to fix the height of the gears more rigidly however due to laser cutting tolerances and CAD issues, the smaller gear distance from the larger gear was incorrect and caused the smaller gear to not mesh with the larger gear. To fix this, a slot was drilled out from the mounting hole of the smaller gearshaft and larger washers were used to clamp the shaft down at its correct distance away from the larger gear.
We also had many issues regarding our transmission between the servo to the laser cut gear:
We initially printed a washer/coupler to press fit over the servo pinion gear and glue onto a laser cut gear. Within the first few seconds of testing, this coupler had been stripped and worn down to a ring around the pinion gear
We then decided to clamp the gear down to the servo using a M3 bolt into the servo motor shaft. However, the clamping force between the bolt head and servo shaft was insufficient and the servo only rotated the bolt instead of the bolt/gear stack.
The last iteration featured a locknut to clamp a face of the gear down instead of the servo shaft. This would also allow us to increase the clamping force significantly, which allowed us enough force to transmit the torque required to drive our fingers.
Arm Assembly Iterations
Arm Version 1 (ideation/prototype)
The main requirement for the arm was to produce a visible “arm pumping” motion so for the first iteration, the analysis was emphasized to produce a CAD that we could manufacture a small-scale prototype to find assembly interferences and movements for a larger-scale arm.
Arm Version 2 (intermediate)
To scale this arm assembly up and fix it to the table, a baseplate was added to replace the “Link 1/Ground Link” in the arm. A motor mount with a gear reduction was also added in an attempt to mechanically slow down the motor. Like the finger assembly, the linkages will be laser cut while the baseplate is 3D printed. There is a slot to allow the mechanism crank linkage to rotate fully instead of raising the mounting points and creating either a monolithic print or highly cantilevered mounts for the shaft and motor.
Takeaways from arm V2 assembly:
A gear reduction is not needed as PWM motor control can also achieve the same thing. Removing the gear reduction can also help with removing friction, tolerance issues, and failure modes in the assembly.
There is no clean way to house the Arduino at the moment, adding a slot would show intention for a finished product.