$fn=100;
hull()
{
translate([2,2,0]) sphere(r=1,center=true);
translate([-2,2,0]) sphere(r=1,center=true);
translate([2,-2,0]) sphere(r=1,center=true);
translate([-2,-2,0]) sphere(r=1,center=true);
}
For screws:
For a big list of various libraries:
Look at bolts, "Round anything", screws, etc.
Adventures in Computing / tom@mmto.org