There are times where you have a small bottle that you'd like to refill from a larger bottle. While you can stick to old method of trying to very carefully balance the larger bottle over the smaller bottle I figured it's easier to just connect the two bottles together. I found an adapter here but it didn't screw on. Since the bottles are already unstable in this arrangement I figured I needed something that would more securely connect them.

This allows you to create an adapter of any size and thread size to connect any two bottles. You'll need to know the outer diameter of the threads and the number of threads per inch. I found the image below to be invaluable for this. Given a rough guess at an outer diameter and number of threads per inch I could select off of one of these charts to get the standard that it should match up with. Edit - that link vanished. Replaced with a couple of links of standard thread sizes.

Standard thread sizes
Metric thread sizes
Alternate

I've included the adapter pictured that connects a large Lubriderm bottle with a smaller one. The defaults for the OpenSCAD file print this adapter. I printed it without supports without any issues but I can't guarantee anything.

The OpenSCAD file uses the thread library from here which made this a breeze to create.