this is a protection cap for an inductive or capacitive 18mm Sensor.
You need following libary
http://dkprojects.net/openscad-threads/
for changes with the openscad file

You should print it with ABS. Printing without Support.
You should turn it upside down for printing.

i added V2 it has a cutout for the thread and 0.1 more space.
also the heigth is 4mm more to get more material on the screw of the sensor.