Soda Bottle Cap for 1/4 NPT Pneumatic Adapter

Last crawled date: 4 years, 2 months ago

This is a cap for a soda bottle that allows you to tap female 1/4 NPT threads in the top for a pneumatic fitting, allowing the bottle to be used as a compressed air tank. I used a 2 liter Pepsi bottle. Because Pepsi is clearly the superior soda. Also works with 1 liter bottles. Not recommended for use on water bottles, if the threads even fit.
SODA BOTTLES ARE NOT DESIGNED TO BE USED AS COMPRESSED AIR TANKS, USE AT YOUR OWN RISK. MAKE SURE ANY BOTTLE YOU USE IS USED FOR CARBONATED BEVERAGES SO YOU KNOW THE BOTTLE CAN WITHSTAND SOME PRESSURE. I DO NOT KNOW IF WATER BOTTLES WILL WORK. OVERPRESSURIZED SODA BOTTLES THAT POP CAN HURT YOU, AND IF THE CAP FAILS THE CAP AND BOTTLE CAN SHOOT OFF SOMEWHERE AND HIT SOMEONE. I BELIEVE THIS DESIGN SHOULD BE SAFE AND HAVE TESTED IT TO 60 PSI MYSELF AT THE TIME OF WRITING, BUT I MAKE NO GUARANTEE THAT YOUR PRINTER WILL PRODUCE A STRONG AND SAFE PART.
I've used it up to 60psi and it held pressure overnight when I tested it, but supposedly 2L bottles will start to fail around 120-150 psi (I make no guarantees that your bottle will do the same). I purposefully made the hole a little bit small, drilled it to the proper size with a drill bit, and then tapped it with a 1/4 NPT tap. Might have worked if I printed it to the right diameter, but getting an airtight seal can be a pain in the ass and it takes like 30 seconds to drill it out. The solidworks file is included if you want to change the diameter of the hole to eliminate the drilling or use a different sized fitting.
MAKE SURE YOU USE AN NPT TAP FOR THE PNEUMATIC FITTING. MAKE SURE THE TAP ENTERS FROM THE TOP OF THE CAP AND EXITS ON THE THREADED SIDE. NPT threads are slightly tapered to improve sealing, and they don't work as well backwards. If you're having trouble getting the fitting into the threaded hole in the cap, try tapping further up the tap to widen the taper in the hole. A normal tap for screws is not the same. I found this tap at Ace Hardware for about 7 or 8 bucks, but couldn't find one in stock at the Home Depot or Lowes. You can also get it on McMaster for like 20 bucks.
I used a 1-1/8 x 7/8 x 1/8 O-ring in the cap to seal against the bottle. I could not get a good seal without the oring. I got this from Blain's Farm and Fleet, but McMaster should have them as well. Might be able to get it at another hardware store.
I modified an existing CAD model of a soda bottle cap found here: https://grabcad.com/library/water-bottle-cap-and-neck-pco-1881-1. 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 made by this guy.
If you want a quick test to see if you're leaking air and where, fill a container up with water, put some air in the bottle, and stick the cap in the water to see bubbles from the leak.