Week 6: Computer-Controlled Machining

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Idea 1 / OS(ucks)B Can't Build Siege Weapons :(

I'm going to just straight-up say this - I dislike OSB with a passion after this week. My original plan consisted of constructing some kind of scaled-down Medieval siege weapon, for use in propelling my cat's toys with minimal effort required on my end. I would incorporate a button and a motor in the future electronics weeks to torque the mechanism, to further the purpose of satiating my laziness. I even came up with a comprehensive list of siege weapons and their mechanisms I could build! (see the Appendix for this section for ballistae, torsion catapults, trebuchet, and more!)

Unfortunately, my dreams were dashed as Anthony's expert MechE knowledge informed me that OSB would not fare well under the tensional and torsional forces :'(

Idea 2 / User-Oriented Cat-borrative Design

I went back to the drawing board, still wanting to dedicate one week for my cat, Atlas the Siberian. As I was looking around, I remembered that he has recently (see: past half year) been stealing the music stand I have set up next to my desk. Okay, well, it's partly my fault for encouraging him there by putting a towel on it, but still! I wanted to reclaim it, so I set out to build him a cat perch. However, as a Siberian Cat, he is a cool cat, and needs a cat perch unlike any other cat's. Thus, I set out to build him a tensegrity cat platform!

The first step involved measuring his regular "relaxed pose" lying position. I took this to be the longest edge, corner to corner, that I would need for the platform, as he's quite comfortable (and sometimes seems to enjoy) dangling or stretching his paws and tail over the edges. After all, if he doesn't fits, he won't sits.

I did a quick Google-fu to get some references for common designs, and ended up with a hexagon-shaped tensegrity structure to provide more tensional points of contact to stabilize it further. The center pieces were double-thick OSB, as was the base. In hindsight, I should've also made the platform double-thick in order to provide more grip for the self-tapping wood screw hooks I used for the tension wire. To prevent splinter injuries, the flat surfaces would all be wrapped in a 3D printed, translucent PLA case while the center supports will be sanded down smooth. I also imagined that the combination of translucent PLA with OSB underneath would be an interesting aesthetic.

'Ah, I see the constant tension and stress keeps it together, I can relate'

I 3D printed a small scale structure in order to test that the physics worked.

For this scale model, I used lengths of floss as the tension rope. Assembling it by myself, I didn't get the most evenly-flat platform, but proved that the physics worked! I discovered here that tension rope with any kind of stretch leads to instability through wobble, and planned to use 7-strand 550 paracord on the larger model.

After that, it was off to EDS mill my OSB pieces.

A bit of cleanup using rasps, files, and sandpaper was necessary, followed by the gluing of the double-thick pieces as well as the subassemblies together. This was left to cure overnight to ensure that those joints were strong and won't become the points of failure in use.

Using the aforementioned paracord, I clipped them to length using a technique where I first taped the cutting joint in order for the strands to not unravel.

Unfortunately, I discovered that I had forgotten to leave enough tolerance on the printed casings, so when attempting to attach them to the tensegrity perch, most of them snapped from being flexed :(

'I see your glass of wine on memory foam and raise you a cup of water on tensegrity' -Anthony

With the subassemblies cured, and with the casings largely unusable, I resolved to sand down the pieces as much as I could. All told, I went through countless sheets of sandpaper in various grits (including 220, 320, 400, and 550), over a 6-hour period.

The end result was OSB that was completely flat and smooth to the touch, although by that point I was cursing our lack of a belt sander, orbital sander, or even a sanding block!

With some assistance from Anthony in elevating and measuring, I attached and tensioned the paracord, then tidied up the slack paracord.

Tensegrity in Deployment

I discovered later, however, that in the exhaustion from the previous night, I had forgotten to load the platform while tensioning the paracord. Doing this generally helps prevent more flex and instability. I also received a suggestion from Dave Ludgin in our section to switch to steel cables, which have a component that allow you to incrementally tighten each length without needing to detach the lengths.

When it came to use, I ended up draping Atlas' towel over the top of the perch just to ensure he won't get splinters from the OSB. Although the wobbling panicked him when he first jumped on it, he appears to have warmed up to it (just a little, at least).

Admittedly, however, I appear to have failed in my original goal of moving him from my music stand to the perch - he's now claimed both as his, and uses the perch as a headrest instead :(

Appendix

https://www.thecollector.com/siege-warfare-powerful-medieval-weapons/
https://en.wikipedia.org/wiki/Ballista
https://www.youtube.com/watch?v=pkUpF7BXs3k
https://www.youtube.com/watch?v=Oovmd9_zFYA
https://www.youtube.com/watch?v=1751SAur6z0
https://www.youtube.com/watch?v=uL2b6jSr2lM
https://www.youtube.com/watch?v=hlC51FbnNSw
https://www.instructables.com/How-to-make-a-Model-Roman-Ballista-Torsion-powered/
https://www.popularmechanics.com/technology/a20494/build-a-little-ballista/
https://www.thingiverse.com/thing:23953
https://www.real-world-physics-problems.com/catapult-physics.html
Although a trebuchet and a torsion catapult had been constructed before, I was advised that they would not be be durable for a long time:
http://fab.cba.mit.edu/classes/863.19/EECS/people/bibekpandit/week6.html
http://archive.fabacademy.org/fabacademy2017/fablabveritas/students/80/portfolio3.html