How to replace my kitchen sink basket with no lock nut?
this looks like a chinese unit one of my granite suppliers used to use.  they are the biggest piece of junk on the planet.  if i remember correctly, the small screw in the middle acts as a retainer for the whole unit.  you have unscrew it from above, and it holds the whole thing together.  the problem we had was that the screw got locked into the metal insert thats in the lower plastic section and just seizes right up.  our solution was a heat gun.  just melt the lower bowl and keep pulling it out in chunks until you can get a grinder in and cut the bolt.  then throw everything away and start again, using a quality metal strainer assembly