$fn=50;


start = 10;
stop = 17;
step = 3;

for (cube_size = [ start:step:stop ] ){


difference(){
    cube(cube_size, center = true);
    
    cylinder(r=(cube_size-1)/2, h=cube_size+2, center = true);
    rotate([90,0,0])cylinder(r=(cube_size-1)/2, h=cube_size+2, center = true);
    rotate([0,90,0])cylinder(r=(cube_size-1)/2, h=cube_size+2, center = true); 
}
}
cube(10, center = true);