fineness=50;
holein=6;
od=8;
id=2.5; // 2.5mm = tap size for M3 x 0.5mm thread
or=od/2;
ir=id/2;
depth=10;

translate([depth*0+0, 0, 0])
bracket();
translate([depth*1+1, 0, 0])
bracket();
translate([depth*2+2, 0, 0])
bracket();
translate([depth*3+3, 0, 0])
bracket();

translate([0, depth*1+1, 0]) {
translate([depth*0+0, 0, 0])
bracket();
translate([depth*1+1, 0, 0])
bracket();
translate([depth*2+2, 0, 0])
bracket();
translate([depth*3+3, 0, 0])
bracket();
}

translate([0, -2+depth/2, 0]) cube(size=[depth*4, 0.1, 0.1]);
translate([0, -1+depth+depth/2, 0]) cube(size=[depth*4, 0.1, 0.1]);
translate([-2+depth/2, 0, 0]) cube(size=[0.1, depth*2, 0.1]);

module bracket(){
difference(){
hull(){
translate([holein,0,holein])
rotate(a = [-90, 0, 0])
socket();
translate([0,holein,holein])
rotate(a = [0, 90, 0])
socket();
translate([holein,holein,0])
socket();
translate(0.01,0.01,0.01)
cube(size = [holein, holein, holein], center = false);
}
translate([holein,0,holein])
rotate(a = [-90, 0, 0])
hole();
translate([0,holein,holein])
rotate(a = [0, 90, 0])
hole();
translate([holein,holein,0])
hole();
}
}

module socket() {
cylinder(h=depth, r1=or, r2=or, centre=false, $fn=fineness);
}

module hole() {
translate([0, 0, -1])
cylinder(h=depth+2, r1=ir, r2=ir, centre=false, $fn=fineness);
}