Ok, reading more of the documentation I’m getting a better appreciation of this. Nice work!
I like the idea of driving the DRV8825 directly from the gpio header on the Pi. And it’s a nice design that leaves the theta/rho carriage passive so no wires can get wound up.
@jeffeb3, the current “theta-stage” is 300mm in diameter, module 0.5, so 600 teeth. I didn’t think of printing the teeth yet… that might be an option. Would you be able to print that big? I’ll look at what printing capabilities I can access… might have to scale it down as you say.
I saw the (open) belt in the V1 engineering shop . I’ll look for a GT2 belt loop for the next generation just on the hope that it will take out some of the vibrations…
The bowls @Heath_H and I are so fascinated about are the ones Sisyphus use for their hardwood tables:
Thanks @jamiek!
I wanted to start as simple as possible and since running “real time” is not a requirement, running from the gpio pins of the raspi seemed OK. And I do think it is for now.
Having no moving wires that could wind up was kind of a given for me when I started. I just could not think of an good way to achieve this. Until I saw how Sisyphus does it 
All the kudos for this ingenious design goes to them.
No, but it would be easier to cnc a 300mm disc, with predrilled holes for some printed parts that had teeth. I’m just thinking aloud, but printing 4-6 arcs with teeth that mount to the disc would work. But this is just because I don’t have access to a laser cutter.
First pics with the WS2801 LED lightstrips that my friend Stephan prepared for sandtrails:
That is looking amazing! Very nice work overall on everything here
Thank you @Heath_H!
I’ve already got some really cool feedback and ideas for the next generation from you guys. Still hoping somebody will build one himself/herself so we can team up 
Quick question: I just read up the last 12 or so posts mentioning baking soda. This is what I use currently and it is beautifully white and smooth.
Sisyphus has a really fancy looking sand which seems to have more of the “flow” quality about it. My baking soda feels kind of “static” and “jagged” in comparison. Any chance anyone knows what sand they use and where to get something similar?
I feel a little hesitant buying replacement sand when I didn’t buy their table… 
I am also on the fence about baking soda. It does have incredible definition, but sometimes, it is distracting, and the detail is more accidental than intentional. Specifically if you do a wiper and then a spiral pattern, the peaks in the wiper show up a little.
I saw somewhere there is very fine sand at hobby stores. I don’t think any home depot type sand would work without some kind of sorting.
That’s exactly it!
I was at a DIY store today but the finest sand they had was still quite coarse. Which is obviously a super precise statement… I’ll keep looking. I’m sure someone has it.
Just tossing it out there, and it has potential drawbacks (high humidity would be bad), but what about using plaster or mortar/grout? Some plasters (especially those made for casting) are pretty damned fine. And somehow, I recall some grouts being particularly fine when dry (may still need some sifting).
Like I said, humidity could lead to… suboptimal performance. Especially if you use something like a dental plaster. That stuff’ll get rock hard (unlike plaster of paris, which gets a little harder than, say, chalk). But if you pack your box with dessicants and keep an eye on it, you could have something. Alternatively, if you get a really cool pattern, you could carefully mist/fog it, and let it set up!
This is what I love about brainstorming. Who will need 3D printers once we can do this? 
I’m using 50/50 baking soda and Kaytee critter bath powder. Seems to work well.
If you go to youtube and see my video for user jcmusix you can see my machine. It won’t let me post the link here so maybe the text below will help or just go to my user page and see the video.
watch?v=M7TPAl0VaxM
This is my underway sand table. I’m just beginning to figure out how to program it and would appreciate any suggestions.
For those who don’t like typing:
And nice work by the way. 
I’ve never heard of critter bath. What does it it do to the baking soda?
I think it’s basically just ash. Some pets like chinchillas can’t get their fur wet, it will start to mold, so they roll around in super fine sand or ash to clean out the oils in their fur.
Thanks for posting that!
Thanks! It will be more fun to watch when I get the programming figured out to make it do nicer patterns. It’s really easy to make one of these with a 3d printer for most of the parts. I cut the mounting boards on my small cnc machine using soft board but I may redo them with black or clear acrylic. I also changed the base mounts to a new design to make it run much quieter. These mounts use 1/4 inch rubber hose and really help. I ordered an rgb led programmable light strip to light up the inside as well.