figuring out what to say:
- once we settled on the four bar linkage design, we began construction of a simple prototype
- The rigid body holding the shovel was made of two linkages with a 3D printed shovel designed to be attached to the end
- So although the mechanism was functionally a 4-bar linkage, we made 6 linkages for the initial prototype
- For each prototype, we modeled the whole assembly as much as possible in CAD
- The linkages (including the ground linkage that held it all together) were laser cut out of acrylic and the bearings were then inserted into them
- For all of the bearings, we tried our best to have them press-fit into the linkages. However, due to the finnicky and sometimes inconsistent tolerances of the laser cutter as a result of factors such as the software trace feature and the laser diameter, the fit wasn't always perfect.
- Even with the same hole diameter for every cut, some of the bearing holes came out with a perfect press fit while others were too loose. Whenever the holes were slightly too loose, we would add a small dot of hot glue to the hole before inserting the bearing. This would force it to stay in place.
- In addition to the linkages, we laser cut a few circular spacers to add to the linkages' stack where they were necessary to prevent the linkages from crossing or colliding while moving
- We encountered an emergency complication for our first prototype, when we discovered the night before it was due that the steel shaft we were planning on using for the joints was made of hardened steel and would be impossible to cut on any of the equipment we had access too.
- With no other option, we resorted to using hot glue sticks for the first prototype's shafts.
- Because we had yet to finalize what the support structure would look like, we attached the initial prototype to a cardboard box full of dirt using dowels and hot glue.
- Overall, the first prototype was of pretty low construction quality due to the limited time it was constructed in before the deadline and the issue we encountered with the shaft last minute.
- However, it allowed us to test the digging motion and acted as a proof of concept for the main linkage design
- We took the feedback we received on our initial prototype into consideration as we started on the next build
- This next prototype evolved and changed over time as new parts were added to it and modified until it eventually became our final product
- Trying to avoid the issue we had with the uncuttable steel shaft, we ordered new 6mm ID bearings and new aluminum 6mm OD shaft
- However, when they arrived in the mail, we were dismayed to find that the shafts would not fit in the bearings without the use of a vice
- To get around this issue and make prototyping easier, we purchased 1/8" diameter wooden dowels and sanded them down to a press fit in the bearings
- While the wooden dowels were not as sturdy and reliable as metal shafts would have been, they provided us with the versatility and customizability that we desperately needed at this stage of the process because they could be easily cut and sanded down to the size we needed.
- However, when they arrived in the mail, we were dismayed to find that the shafts would not fit in the bearings without the use of a vice
- Linkages and supports were designed in CAD and laser cut out of plywood.
- Plywood was chosen at this stage for is low cost and customizability.
- We were designing a lot of features as we went at this point, so we wanted the option to be able to screw into it, sand it, and add holes as we found it to be necessary
- The tradeoff was that this material was quite rough, creating friction between joints that served to slow the mechanism down
- We ran into the same issue with cutting the bearing holes as before: the hole sizes just weren't consistent when cut by the laser.
- Some holes would press-fit the bearings perfectly, while others had to be held in place by hot glue
- The three linkages that used to work together to hold the shovel were then redesigned into one linkage, as was advised by our TA during the prototype demo
- Plywood was chosen at this stage for is low cost and customizability.
- At this stage, we began thinking of how we would time the seed dropping and tried to incorporate it into our existing design
- Seeing the way the shovel linkage rotated during it's motion inspired us to use the orientation of the mechanism to our advantage
- Inspired by those little mazes that you flip around and tilt to navigate a metal ball through them, we designed and 3D printed a plastic "maze" that would fit inside the linkage that held the shovel
- While at the top of it's rotation, the entry hole would be pointed upwards for the seed to be dropped inside
- Then, after the shovel scoops away the dirt, the linkage will rotate naturally and the seed would roll downwards out the exit hole
- this would consistently drop it at the point we wanted (between the digging out and replacing of the dirt)
- We unfortunately had to 3D print the maze twice, because a small error while modeling it in CAD resulted in the holes for the shaft in the feature not being aligned properly. We remade the model, double-checked that it would fit by adding it to the prototype assembly with all the proper mates, and then reprinted it.
- With all the linkages and the maze printed and cut, we redesigned the shovel head slightly to fit the new design. Now it and the maze would be sandwiched between two plywood laser cut layers.
- With the linkages and maze all designed, we assembled them all together using the bearings we ordered and the wooden dowels.
- The next step was then to incorporate the motor. To do this, we needed a motor mount that could mount on to the ground linkage and hold the motor in place.
- We designed this in CAD, going off of the current assembly and designing it to attach to what we already had.
- We then 3D printed it
- We found that the motor mount worked great for holding the motor in place, however the motor was still free to rotate within the circular mount, which became a problem.
- To fix this, an additional part was designed that would slot onto the existing motor mount. It had holes for two tiny m2 screws that would fit into holes on the motor and prevent the motor from being able to rotate inside the mount
- We designed this in CAD, going off of the current assembly and designing it to attach to what we already had.
- With that issue sorted, we needed a way to snugly attach the motor to the grounded input link, so that the motor could spin it
- We did this with a 3D printed part that was then glued into the bearing hole on that linkage
- We had a couple of iterations of this because we found that unless the part fit very tightly on the motor, the motor would slip inside of it and wouldn't be able to move the mechanism
- We finally solved this issue by making the hole for the motor D-shaped like the motor shaft and printing it with zero tolerance so that it would be press-fit
- Still, even with a perfect initial fit, we still found that over time continued use began to wear down the soft 3D printed plastic, making the fit looser and looser. So perhaps once the right fit was found, we should have switched to a less soft material than plastic.
- After the motor was added to the assembly and we fixed the issued with the motor slipping, we started working on our stretch goal of getting the whole mechanism to move every time a cycle was completed.
- Using some 3D models for bevel gears we found online, and 3D models of regular gears that we generated online and then modified to fit what we needed, we modeled a gear assembly that would be able to translate the constant input motion of the motor into intermittent rotation of the mechanism's wheels.