The main working part of cyclonic dust separators is the cone shaped chamber that creates a vortex as air is sucked into it. The air increases in velocity as it approaches the bottom of the cone. As Velocity increases, centrifugal force increases and separates the heavier dust from the air. The dust falls to the bottom of the chamber as it runs down the sides of the cone into a collection container.
Check out the image of a cyclonic separator for a better idea. (courtesy of http://www.thefullwiki.org/Cyclonic_separation)
There are already good designs on Instructables (http://www.instructables.com/id/Dust-Sniper-quiet-extractor-system/
) showing how to build a dust separator to use in a workshop, and i was about to follow their excellent instructions.
I noticed a PET soda bottle that had the right proportions of the cone needed to generate a vortex. Likewise i could use another PET bottle for the dust collection chamber. So, I figured i would save some time
not cutting out 2D patterns out of plastic AND recycle at the same time!
There was also the added benefit of the thread at the top of the bottle that could work as a sturdy attachment point to easily unscrew and empty the collection container.