I created this simple dumbbell that may be 3D printed because I had unused weights and wanted to put them to use, without buying another set of dumbbells. This also has the advantage of being lightweight, when compared to the metallic ones, allowing for more flexibility in the lower range (e.g. lower minimum weight).

This model may be printed in two parts (without supports) and the nuts will hold it together well. If additional strength is desired, use glue between the halves and use the nuts to hold them together while the glue dries.
The threads are M30 and I suggest printing one file for each half ("half 1.stl" + “half 2.stl”), not the same file twice(2 x “half 1.stl”), because the threads must match, in order for the nut to work.

I have included the file with a complete dumbbell, just in case someone wants to print it in one go, but I don't recommend, since it will need a lot of supports.