John Brunhaver and Samantha Brunhaver
We started brainstorming by talking about what we wanted to get out of this assignment. We wanted to use the Replicator, to see if it was something we would interested in buying in the future. We also wanted to divide our object into multiple parts so that we could get equal experience working in Google Sketchup. Additionally, John wanted something that could be spray painted and that related to his background as a military kid; Sam wanted something that would be interesting to draw and talk about in class. Based on all of this, we decided to make a tank.
Our first step was to look at other tanks that people had posted to Thingiverse and Shapeways, to find a style that we both liked. We decided to use this photo as our inspiration.
Then we worked out the exact details and dimensions of our tank at the whiteboard. We decided to model the tank in six parts so that all of the details were facing upward and parallel to the z. That meant, two wheel tracks, two chassis halves, a turret, and a barrel. Since we were using the Replicator, we scaled the parts to fit in a bread box-sized envelope.
Then we drew the parts in Sketchup. One thing we both discovered is that drawing an individual part does not take long, but figuring out the best way to draw it does. When we were finished, we combined all of our parts into one file and converted it to STL.
After our model was complete, we went to the lab to print. Here, we learned a few lessons:
(1) Even though we created and exported our model to STL in inches, our model was tiny when we opened it in ReplicatorG. The first way we tried to fix this was by clicking on “fill print bed”, which made the model about 30 times larger. However, the printer had a difficult time laying the raft, we think either because the raft was too large or because it was trying unsuccessfully to create rafts for each part. In any case, the extruder became jammed with plastic so we cancelled the job. When we scaled the model down by about half, it finally printed correctly.
(2) Whenever we tried printing the turret or barrel, the gcode “failed”. Through troubleshooting, we eventually decided on these rules of thumb. One, don’t try extruding objects from curved surfaces; instead, use fans. Two, don’t try printing circular rods with length in xy; make the cross-sectional polygonal. (E.g., our barrel rod is actually octagonal.)
(3) In very small models, some small details are (lightly) printed while others are left out. If you look closely enough, you can see the hubs in the middle of the wheels in the wheel track. But our name across the top of turret was dropped completely.
Our final model took about 1.5 hours to print. One side effect from printing in six pieces is that every piece had a “bad” side where it was connected to the raft. We smoothed these sides down with exacto knives and sandpaper. Afterwards, our wheel tracks fit perfectly in the lower half of the chassis, and our barrel fit in the turret. What didn’t fit were the top and bottom halves of the chassis, since they both bowed a little bit. We put a thin layer of putty between the two to even this out.
Next, we spray painted the chassis, turret, and barrel, first with a base and then with color. Then we assembled the entire model, using superglue on the critical joints, i.e., the wheels to the chassis, the turret to the chassis, and the barrel to the turret.
Here is the finished result:
For this assignment, we also had to pair with another group and create a story about our objects:
Here is a haiku instead:
Tommy is a tank
He joined the army to help
He is very very small