iter-png-3/o3 / tier-3-butt-hinge

```openscad
// Butt hinge (opened 180°)

// ---------- Parameters ----------
plate_len = 30;          // Y-length of each leaf
plate_w   = 25;          // X-width  of each leaf
plate_t   = 2;           // thickness (Z)

kn_seg    = 6;           // single knuckle length
kn_count  = 5;           // total knuckles
kn_od     = 8;           // knuckle outer Ø
kn_id     = 4.6;         // knuckle inner Ø (pin Ø 4 + 0.3 clearance)

pin_d     = 4;           // pin Ø
pin_len   = plate_len+2; // 32 mm  (1 mm over-hang each end)

// ---------- Rendering quality ----------
$fn = 64;

// ---------- Helper : cylinder along Y ----------
module cyl_y(h, r, center=false)
    rotate([90,0,0]) cylinder(h=h, r=r, center=center);

// ---------- Knuckle segment (hollow) ----------
module knuckle_seg()
difference(){
    cyl_y(kn_seg, kn_od/2);
    cyl_y(kn_seg + 0.2, kn_id/2);   // slightly longer for clean cut
}

// ---------- Countersunk M3 hole (6 mm Ø, 1 mm deep 90° taper + Ø3.2 through) ----------
module countersunk_hole(){
    union(){
        // 3.2 mm through hole (slightly longer than plate thickness)
        translate([0,0,-(plate_t/2+0.2)])
            cylinder(h=plate_t+0.4, r=3.2/2, $fn=32);
        // Conical countersink: Ø6 mm → Ø3.2 mm, depth 1 mm from top surface
        translate([0,0,plate_t/2-1])
            cylinder(h=1, r1=6/2, r2=3.2/2, $fn=64);
    }
}

// ---------- Leaf (left / right) ----------
module leaf(is_left=true){
    // X-offsets
    plate_x = is_left ? -(kn_od/2) - plate_w :  kn_od/2;          // plate anchor
    hole_x  = is_left ? -(kn_od/2) - plate_w/2 : kn_od/2 + plate_w/2; // hole line

    difference(){
        // Solid part (plate + knuckles)
        union(){
            // Plate
            translate([plate_x, 0, -plate_t/2])
                cube([plate_w, plate_len, plate_t]);

            // Knuckles (0-based indices)
            if(is_left){
                for(i=[0,2,4])
                    translate([0, i*kn_seg, 0]) knuckle_seg();
            } else {
                for(i=[1,3])
                    translate([0, i*kn_seg, 0]) knuckle_seg();
            }
        }

        // Screw holes – Y positions 7 mm, 15 mm, 23 mm  (8 mm pitch)
        for(y=[7,15,23])
            translate([hole_x, y, 0]) countersunk_hole();
    }
}

// ---------- Pin ----------
module pin(){
    // Shift –1 mm so it projects 1 mm beyond each end of the 30 mm knuckle stack
    translate([0,-1,0]) cyl_y(pin_len, pin_d/2, false);
}

// ---------- Assembly ----------
union(){
    leaf(true);   // left leaf (x < 0)
    leaf(false);  // right leaf (x > 0)
    pin();        // shared pin
}
```