Soda Bottle Cap for 1/4 NPT Pneumatic Adapter

thingiverse

I used a 2-liter Pepsi bottle as the base for this project because it's clearly the best soda available. The cap allows you to tap female 1/4 NPT threads into the top for a pneumatic fitting, effectively turning the bottle into a compressed air tank. You can also use a 1-liter bottle, but don't try using one with water in it - if the threads even fit. Using a soda bottle as a compressed air tank is not recommended and comes with risks. Make sure any bottle you choose was originally used for carbonated beverages, or it may not be able to withstand pressure. I've tested this design myself up to 60 PSI, but don't assume your printer will produce a safe part. I personally tested the cap to 60psi overnight without issues, but some sources claim that 2L bottles start failing around 120-150 psi. Don't rely on these numbers - each bottle is different. I made the hole slightly smaller than necessary by drilling it with a bit and then tapping it with a 1/4 NPT tap. If you print it to the right diameter, you might not need to drill it out, but getting an airtight seal can be frustrating. The Solidworks file is included if you want to adjust the hole size to eliminate the drilling process or use a different fitting. Make sure you use an NPT tap for the pneumatic fitting and enter from the top of the cap, not the bottom. NPT threads are slightly tapered to improve sealing, but they don't work as well when installed backwards. If you're having trouble getting the fitting into the threaded hole, try tapping further up the tap to widen the taper in the hole. A normal tap won't work - this is a specialized tool. I found mine at Ace Hardware for about 7 or 8 bucks, but couldn't find one in stock at Home Depot or Lowe's. To seal against the bottle, I used a 1-1/8 x 7/8 x 1/8 O-ring in the cap. Without it, you won't get a good seal - trust me on this one. You can find these O-rings at Blain's Farm and Fleet or McMaster. I modified an existing CAD model of a soda bottle cap found online to create this project. All I did was turn it into a hexagon, add some thickness to the top, and put a hole in the middle. The threads and basic cap were created by someone else. If you want to test your cap for leaks, fill a container with water, put some air in the bottle, and then stick the cap in the water - any bubbles will indicate where the leak is coming from.