Rhombic Enneacontahedron Post #2

Alright, here's the code. It is a total mess, but I knew I just wasn't going to get around to even starting to clean it up, so I just committed it before I forget about it (and likely struggle to locate it in the future).

https://github.com/eshira/polyhedra/tree/master/Rhombic-Enneacontahedron

I figured out the degree of freedom I forgot about. When you truncate, you can move new vertices in along old edges some amount, which you can choose. I split edges in three, moving each new vertex in by a third (see the diagram from wikipedia that I included in the previous post). Splitting in three makes sure you create regular hexagons for the soccer ball step (the truncated icosahedron). You don't have to create regular hexagons, and by modifying this you can make slim and broad rhombi, as shown in the images below.

This explains why I didn't match the values for the angles that I found on Wikipedia for the rhombic enneacontahedron. I suspect that most of these rhombic enneacontahedrons people make come from just a few instructional guides or paper folding guides online, which often refer to slim and broad rhombi, so probably they aren't making regular hexagons in the truncation step, which seems to yield very similar rhombi so that you can't easily call them broad or slim.

Extreme cases caused by changing parameter of truncation step seem to be less 'round'

Here's the final 3D printed assembly, after gluing in the magnets. The edge length on the edges the white and red tiles share are 2cm long, for reference

It would be fun to print the exaggerated versions with very broad and very slim rhombi at some point in the future.

Want to make your own? The code isn't super clean, so the steps at a high level:

• If you want to modify the truncation, you'll need to locate that section of the code, modify the appropriate line, and then get the code to spit out all the info you need on the angles and side lengths that are needed by the OpenSCAD script. There are a bunch of poorly commented areas where you can follow my pattern--I was working quickly and I just didn't make it easy to use yet. Sorry!
• If you keep the angles as in the OpenSCAD script, you'll make a shape like the one I made. You need to make sure the user controlled side length is the same for the tiles of type A and B. You also need to make sure you match the magnet info between those two types of tile.
• You can do this with just one magnet per edge if you want to save on magnets, though it won't be as strong and the tiles can pivot until locked into place by neighbors. You will need to go through my code because I didn't make it parametric either

Easiest way to do it? Keep everything exactly the same as my OpenSCAD code, render the STL and slice it for your 3D printer. Get at least 90*4 magnets, 3mm diameter by 2mm height, axially aligned. The magnet diameter and depth values in the script might still need to be adjusted to get a nice fit, depending on your 3D printer.

CRT and Magnets Exhibit

This Cathode Ray Tube + Magnets exhibit started off with a couple of CRT monitors that were gathering dust in storage. I was asked to consider putting them out in the main space of the makerspace. I decided I would only allow this if they did something. I set out to decide on what that something would be. It turned into a fun, easy, accessible to all ages exhibit that we now turn on for all the tours we lead through this makerspace. It is a great way to quickly and cheaply construct a meaningful interactive science exhibit to add to your collection.

Here's a document I put together to explain what's happening. I taped this to one of the TV antenna (the antenna is not used) so it stays front and center to the exhibit and people are encouraged to actually read it. Here's a lower quality image of the same document so you can see it embedded in the post.

Here's the bill of materials:

• CRT television. You may have to turn to eBay or Craigslist. These may only get harder to acquire with time.
• If the CRT has VHF/UHF inputs, you'll need a box like this one to convert the signal to composite video: https://www.amazon.com/dp/B0014KKV7W/
• For the camera, the backup camera is cheap and outputs a composite video signal over RCA connectors. https://www.ebay.com/itm/CMOS-Car-Rear-View-Reverse-Backup-Camera-Parking-Night-Vision-Waterproof-7-LED/291918612347
• If you'd like to place the camera elsewhere, you can get a cheap 2.4GHz transmitter like this one https://www.ebay.com/itm/2-4G-Wireless-Video-Transmitter-Receiver-Kit-for-Car-Rear-Backup-View-Camera/163041336550
• A powerful neodymium magnet. You want something strong enough to have an effect and a good size to be easy to handle. Something like 0.5 inch diameter and 0.5 inch height seems like a good size to me, but I'd suggest just seeing what's available and trying it. You can always stack up multiple smaller magnets.

Assembly is just a question of plugging everything in to power and getting the signal in to the TV. Note that the backup camera requires a little work to plug in; it is designed to be hooked up to a 12V car battery connection point. You will need a 12VDC wall adapter if it does not come with one. You will need to make sure you have the red/positive connector and black/negative connector going to the right places. You will probably need to solder at least one connection, or use another strategy to connect the wires.

The magnet should be protected with something soft to avoid it hitting against metal and breaking or pinching fingers. I used two furniture feet and some masking tape. I also suggest putting it on a string so it doesn't wander off.

Cable management was the longest part of the project. I zip-tied all the extra length of cabling in back of the assembly. A single switch controls the extension cord to which the entire unit is plugged in. Note the CRTs make a high pitched noise that some people don't like to hear all day, so I don't leave it on all the time, since the main room of the makerspace is also a meeting and study space.

The cameras are taped to the top of the CRT and pointing at brightly colored pieces of paper. This is important because the magnet effect is not nearly as visible on black & white images.

An additional, optional modification I made to the back up cameras was the removal of the infrared lights that it contains in order to provide better visibility at night. The camera was very warm and when the IR lights activated, which sometimes happened if the exhibit experienced low light conditions, the image was washed out. I opened the unit and de-soldered the IR LEDs. My original plan involved putting gaffer's tape over the IR LEDs, but that seemed to make the camera heat up even more. There is a sensor inside the unit and in bright lighting the infrared LEDs are not activated, so this is not necessarily something that needs to be addressed for the exhibit to function.

It is also important to note that the backup camera is designed to mirror images. Note the sign in the image above is printed as a mirror image in order to show up correctly on the display. The backup distance overlay is another artifact of the choice to use a backup camera; I find it is fun and adds to the color distortion effect since it is displayed in bright colors.

Trapezohedra

Another set of magnetic tile polyhedra. This time, three different trapezohedra.

https://en.wikipedia.org/wiki/Trapezohedron

http://www.thingiverse.com/thing:2168615

https://github.com/eshira/polyhedra

I used this resource to define the kite and dihedral angles, but the hexagonal and heptagonal did not work out (and I believe the octagonal also has issues). Fixing them is on my to do list. I plan to write a script that actually generates them from the antriprisms.

Deltoidal Icositetrahedron

Another magnetic tile 3d printed polyhedron for the collection. This time, the deltoidal icositetrahedron.
http://www.thingiverse.com/thing:1604905
https://github.com/eshira/polyhedra/tree/master/Deltoidal-Icositetrahedron

Happy printing!

Rhombic Triacontahedron

Another magnetic tile 3d printed polyhedron for the collection. This time, the rhombic triacontahedron.

http://www.thingiverse.com/thing:1593892
https://github.com/eshira/polyhedra/tree/master/Rhombic-Triacontahedron

Happy printing!

Polyhedra Magnetic Tiles

My first submissions to Thingiverse:

Pentagonal Hexecontahedron Magnetic Tile

Pentagonal Icositetrahedron Magnetic Tile

Also on github @ https://github.com/eshira/polyhedra

Happy printing!

Bubinga Magnetic Box

Another CNC project -- some bubinga scrap turned into a box with a magnetic closure, rounded interior pocket, and painted turquoise inlay.


Clamped down to the CNC spoilboard.

One half already cut out and inlayed with magnets.

The box half directly from the machine after the first pass.

 Jig for milling out the other side. This is because the reach of the end mill / bit is limited, and also in order to mill out the hole for the inlay.

Pale red look before applying walnut oil.

The wood absorbs the walnut oil quite well so I keep reapplying, especially to rough spots like the cracks.

The magnet side.

Spring Break CNC Adventures

While the UW students were off on spring break, I took some time to work on old projects from "The Big Project List."

I found a scrap of plywood which was just the right size to mill out Curio Shelf Revision 1.

The pieces before assembly:

Due to a mistake in the lengths of the dividers, the shelf curves outward in the middle. Not a bad look, but not a look I'd keep in Revision 2.

Another project is a bamboo magnetic board with a french cleat in a V shape for the wall mount. First I did a quick mockup in OpenSCAD to visualize it. The small piece here would be mounted on the board. The large piece would be mounted on the wall. In the first image, the wall would be on the positive x-axis side, and the board on the -x axis side.

Here's the finished cleat. It doesn't look very neat however it is not visible when the board is mounted. Now it is easy to remove or replace the board on the wall. This means you can use it as a screw organizer that you store on the wall.

Here it is with some things attached to it.