Asterisk Drive Pods
Created by Enoch and Ali
99040E
Vestavia Hills, Alabama
Introduction
A big challenge on designing our robots chassis this year was finding a way to fit an asterisk
drive while also allowing for enough space to intake disks.
Our goal was to fit 4” wheels so we could go at our desired speed without additional gearing.
However, because any holonomic drive needs to angle the wheels it was difficult to get the c
channels for the center wheel and the angled wheels to line up without much complexity.
In the end, we went with a design that mounted the c channels for the outer and inner
wheels separately which worked decently. However, it left a lot on the table in terms of simplicity.
Current chassis design
To create a more elegant solution we decided to make asterisk drive pods that would allow
us to make the middle wheels 3 holes wide, keep the installation simple and strong, and
minimize the space it takes up.
Designing
To design the parts we decided to use Fusion360 (Version 2.0.15050 x86_64) because it
makes collaborating and working across multiple computers seamless- a must have for a team
project. We also like having the history feature that allows us to quickly experiment with ideas
and go back easily if they don’t work.
We settled on the following features to guide our work:
●
Must fit on a 155x130x75mm 3d printer
●
Hotswap- we want to be able to quickly switch out motors and eliminate the wasted
space used by the motor cap
●
3d printed bearing- simplifies assembly
●
Fit up to 4” wheels- our desired size
●
Connects at two points with 3 holes (~1.5”) inbetween
To create it we started by creating a representation of the chassis using a vex parts
library so we have something to mount it to
Next we position a 4” wheel around it and created a basic shape by drawing a 2d sketch
of the inner and outer sides from the bottom and extruding them upwards. We also added a hole
where the bearing would be.
Rough Draft
Next was the most challenging task: creating a mount for the motor. Because we
decided to opt out of a cap, we needed to incorporate one into the print. To accomplish this we
added a motor and the extra width it needed onto the part and created planes from the various
flat surfaces on it and did an extrude cut to cut away from material where the motor was. On
some of them we also projected geometry onto the plane if it was needed. To check if we cut out
enough material we used section views to see if the two parts were overlapping. Lastly we
added cutouts to fit zip ties to attach the motor with, a robust method that is used across our
competition.
Development of motor mount
While we had created a part that met most of our goals, it was not optimized for 3d
printing because it could not fit on our printer without needing far too many supports, and it was
too weak. To fix this issue we decided to split it into two parts: a motor side and a bearing side,
and we would also reinforce the bearing side because it was the thinnest and most vulnerable
during matches.
In order to split the part in two we copied it and deleted the other half from each side. We
then created a straight attachment point where we designed a screw hole by importing a screw
and creating a spot for the head to side and the nut to slot in. We also beefed up the bearing
side by making it thicker.
Splitting the part in two
To finalize the design we added some fillets, rounds, and chamfers to smooth the part
out. We also had to cut some corners on the motor side because it was a little bit out of size. To
resolve the issue we literally cut some corners by using a sketch and extrude, and we also
removed two .25” chunks from the outer mounting hole to allow us to save build plate space by
putting spacers there instead, something that we predicted would have no impact on the
structural integrity.
Final pictures of the cad
Manufacturing
We decided to 3D print the parts using a resin 3d printer because we were eager to try it
out and wanted to see how good the results were.
The biggest issue we encountered was poor adhesion to the build plate and between
layers, which resulted in many prints failing prematurely. Through this we got better at leveling
and cleaning the build plate which reduced the failure rate.
A failed print
Curing is an important stage in resin prints because the resin needs to fully solidify and
harden to firstly be at their full strength but secondly non-toxic enough to handle without gloves.
For curing we used a water curing method where we let the part sit in the sun in water for
around two hours. The water lets the uv light disperse evenly around the part, making it a
powerful method for curing prints.
Our curing setup
After we had created and cured the motor and bearing sides we assembled them. All of
the parts fit together well except the bearing hole and the gap for the nut were a little too small.
To fix the bearing we drilled it out until the axle spun in it smoothly. To fix the nut we used a
slightly smaller nut like the one in our cad set, but we also could have filed the sides to make the
space bigger.
Image of assembled design
Reflection
While the designing part of the process was something we are familiar with, it still gave
us chances to improve our skills in designing parts to be 3D printed and making them fit in tight
constraints. However, the main learning part of the process was in resin 3D printing. We started
the project with virtually no experience but with some dedication we were able to use it well
enough to create good parts.