I recommend starting with the 3D parts before you even order any electronics. The fit of these parts is exceedingly fussy and error-prone, and if you reach an impasse here you can shelve the project with minimum investment and heartache.
You’ll begin by tracking down a series of tubes:
These are 2-liter soda bottles in their larval form…before the blow-molding stage, but with the threads intact. Soda bottle preforms are watertight and near indestructible, making them popular with geocachers and schools needing kid-safe labware.
I found mine on eBay, but they also turn up on Amazon (search for “soda bottle preform” or “baby soda bottle”). The size and shape seems to be fairly standardized, but if they specifically mention “130 mm interior depth,” so much the better. Some places sell these one-off for just a few bucks apiece, or you can save by buying a set (using the spares for small parts storage…or getting started in geocaching).
I’ve started a thread in the Adafruit Forums to help in tracking down capsules that are known to work. Check the discussion, or add a note if you’ve found a winner.
Trouble finding preforms that fit the 3D parts? Not to worry! Some small soda or water bottles occasionally work. Look for one with the traditional coarse soda thread…many varieties of bottled water use a finer thread that won’t work for this project.
Clip or sand off any plastic “sprue” protruding from the end of the preforms. These are sharp and will hurt if caught in motion!
3D Printing
While waiting for your soda preforms to arrive, you can get a head start on the 3D printed parts. As mentioned, these are fussy and it may take a few tries to get right.
There are two pieces: a special bottle cap (with lanyard hole) and an insert that precisely fits inside the soda preform…we’ll be attaching electronics to this later. Print two of each — poi are usually spun in pairs — four parts total, but print just one part at a time. The prints come out cleaner (fewer strings), and it’s less frustrating restarting a small print job than having a whole complex bed-full-of-parts print job fail.
Both parts are designed to print without support.
Print the cap with 3 shells (rather than 2) for added durability. You may need to use a large brim for better bed adhesion, since it stands narrow-end-down.
The insert is about 130 mm tall and may not fit some smaller printers when standing upright. Printing sideways is fine but requires support material and additional cleaning up. For really small printers, angling it corner-to-corner can work (do you know the math puzzle about the long spear in the shipping box?)
After printing, clean up the parts with files and/or sandpaper. Be especially careful to remove any stabby protrusions in this slot in the insert:
A lithium-polymer battery will slide in there later. Puncturing LiPoly batteries is one way they can catch fire, hence the extra attention to removing plastic nubs here.
Test Fit
When your soda preforms arrive, check that the printed insert fits inside and the cap screws on securely. The insert should slide in and out easily. If there’s friction, smooth any protruding bits with sandpaper. When everything looks good, rinse off any remaining plastic dust and set aside until completely dry before adding electronics.
If sanding isn’t sufficient to make the insert fit your preforms, you can try tweaking the design in Autodesk 123D (.123dx model is included in the ZIP file).
Go Forward! Move Ahead!
So now, if everything successfully printed and fits together to your liking, you can proceed to order all the electronic parts.
Additionally, you’ll need a pair of handles and leashes for your poi. Originally I’d used paracord and hunted down some #8 nickel ball bearing fishing swivels (eBay), but you’ll find it easier and get better results ordering a ready-made “flowleash” pair from Flowtoys.com…they’re super comfortable, and the basic pair is only $10, or $20 for a pro version with stainless steel swivels. Their web site shows how to adjust the leashes and has lots of great poi spinning information (and if this whole 3D printing thing doesn’t work out for you, there’s some fine, if technologically simpler, LED poi available there).
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