Add latches to the lid

Makefile

@@ -1,5 +1,5 @@
 SCAD    = wargame_box.scad
-PARTS   = box lid insert
+PARTS   = box lid insert latch
 STLS    = $(addprefix stl/,$(addsuffix .stl,$(PARTS)))
 
 all: $(STLS)

README.md

@@ -1,12 +1,13 @@
 # Wargame Accessory Box & Dice Tray
 
 A parametric OpenSCAD box for carrying tabletop wargaming accessories.
-Three printable parts:
+Four printable parts:
 
 | Part     | Purpose                                                            |
 |----------|--------------------------------------------------------------------|
 | `box`    | Open tray. With the insert removed it doubles as a dice tray.      |
-| `lid`    | Fully detachable friction-fit lid (no hinge to get in the way).    |
+| `lid`    | Fully detachable lid, held shut by snap latches hinged on filament pins (or a classic friction fit). |
+| `latch`  | One-piece snap latch — print two per latch position.               |
 | `insert` | Removable compartment tray for tokens, measuring tapes, etc.       |
 
 ![Assembly](docs/assembly.png)
@@ -19,14 +20,50 @@ the free space in your backpack — they are **outer** dimensions and
 measured space exactly.
 
 Pick what to render with the `part` variable (or the customizer):
-`assembly` (exploded preview), `box`, `lid`, `insert`, `print`.
+`assembly` (exploded preview), `box`, `lid`, `insert`, `latch`, `print`.
 
 Or export everything from the command line:
 
 ```sh
-make            # exports stl/box.stl, stl/lid.stl, stl/insert.stl
+make            # exports stl/box.stl, stl/lid.stl, stl/insert.stl, stl/latch.stl
 ```
 
+## The latch lid
+
+By default (`lid_style = "latch"`) the lid is held shut by snap
+latches that hinge on short pieces of filament:
+
+* The **lid** carries pairs of lugs along its front and back edges.
+* Each **latch** is a single printed part with a pivot barrel that
+  fits between a lug pair. Its window snaps over a chamfered catch
+  bar on the box wall; pull the flared tip outward to open.
+* To assemble one latch: cut a ~23 mm piece of 1.75 mm filament,
+  slide it through lug → latch barrel → lug, and fix it with a drop
+  of glue **on the outer lug holes only** — the lug holes are sized
+  tight and the barrel hole loose, so the latch keeps pivoting
+  freely. Trim the pin flush.
+
+Tuning knobs (all per side unless noted):
+
+* `latches_per_side` — how many latches on each long wall
+  (default 2, evenly spread).
+* `catch_proud` — how far the catch bar sticks out = how hard the
+  snap is (default 1.4 mm).
+* `latch_play` — vertical play between latch and catch bar
+  (default 0.1 mm). Use a small negative value to preload the lid
+  shut.
+* `filament_d` — hinge pin diameter, in case you want to use
+  2.85 mm filament or a piece of wire instead.
+
+Note: lugs and latches stick out about 7 mm beyond the box on the
+front and back, so allow `total_y + 14` of space in the backpack.
+With latches doing the holding, a looser lip (`lid_clearance` around
+0.3–0.4) makes the lid pleasant to take off.
+
+Prefer the original hinge-free push-fit lid? Set
+`lid_style = "friction"` and you get the old behaviour, including
+the thumb grooves for prying the lid off.
+
 ## Configuring the insert layout
 
 Compartments are defined by the `layout` list, one entry per row
@@ -55,10 +92,13 @@ of the automatic height and print two with different layouts.
 ## How the parts work together
 
 * The **lid** is a flat plate with a lip that plugs into the box opening.
-  `lid_clearance` (default 0.2 mm per side) controls the friction fit —
+  With the latch lid the lip just aligns the lid (and keeps tokens in
-  print a test fit and adjust for your printer/material. Shallow thumb
+  their compartments); with `lid_style = "friction"` it carries the
-  grooves on the box's front and back walls let you pry the lid off; they
+  whole fit. `lid_clearance` (default 0.2 mm per side) controls how
-  do **not** pierce the wall, so the dice tray stays fully closed.
+  snug it is — print a test fit and adjust for your printer/material.
+  In friction mode, shallow thumb grooves on the box's front and back
+  walls let you pry the lid off; they do **not** pierce the wall, so
+  the dice tray stays fully closed.
 * The **insert** sits below the lid's lip, so the lip also keeps loose
   tokens from jumping compartments in the bag. Finger notches on its
   short walls let you lift it straight out; then the empty box is your
@@ -68,7 +108,9 @@ of the automatic height and print two with different layouts.
 ## Printing notes
 
 * No supports needed for any part. Print all parts flat side down
-  (the lid prints plate-down, lip up).
+  (the lid prints plate-down, lip and lugs up). The latch prints
+  standing on its flat side — that gives a clean pivot hole and puts
+  the layer lines along the strip, where the snap stress is.
 * PLA or PETG, 2–3 perimeters, ~10 % infill (the parts are mostly walls).
 * If the lid is too tight/loose, tune `lid_clearance` in steps of 0.05 mm.
   Same for the insert with `insert_clearance` (looser is fine there —

wargame_box.scad

@@ -1,9 +1,11 @@
 // ============================================================
 // Wargame Accessory Box & Dice Tray
 // ------------------------------------------------------------
-// Three printable parts:
+// Printable parts:
 //   * box    - open tray, doubles as a dice tray
-//   * lid    - fully detachable friction-fit lid
+//   * lid    - fully detachable lid, held shut by snap latches
+//              hinged on filament pins (or classic friction fit)
+//   * latch  - one-piece snap latch (print 2 per latch position)
 //   * insert - removable compartment tray (configurable layout)
 //
 // Set `part` below (or via customizer / -D on the CLI) to
@@ -12,7 +14,7 @@
 // ============================================================
 
 /* [What to render] */
-part = "assembly"; // [assembly, box, lid, insert, print]
+part = "assembly"; // [assembly, box, lid, insert, latch, print]
 
 /* [Outer dimensions (must fit the backpack)] */
 // Total outer width (X), including walls
@@ -39,16 +41,39 @@ tray_chamfer = 6;
 // Thickness of the flat lid plate
 lid_top = 2.4;
 // Height of the lip that plugs into the box opening
-lip_h = 10;
+lip_h = 5;
 // Lip wall thickness
 lip_t = 2.0;
 // Gap per side between lip and box wall.
 // Tune for your printer: smaller = tighter friction fit.
+// With lid_style = "latch" the latches do the holding, so a
+// looser 0.3 - 0.4 makes the lid easy to take off.
 lid_clearance = 0.20;
 // Thumb groove on the box's outer walls to pry the lid off
-// (does NOT pierce the wall, dice tray stays closed)
+// (friction lid only; latches replace it)
 thumb_grooves = true;
 
+/* [Latches] */
+// How the lid is held shut. "latch": snap latches hinged on
+// short pieces of filament (see README). "friction": original
+// push fit.
+lid_style = "latch"; // [latch, friction]
+// Latches per long (front/back) wall, spread out evenly
+latches_per_side = 2;
+// Diameter of the filament used as hinge pin
+filament_d = 1.75;
+// Latch strip width
+latch_w = 12;
+// Latch strip thickness (thinner = softer snap)
+latch_t = 2.0;
+// Catch bar distance below the box rim
+catch_drop = 14;
+// Catch bar protrusion from the wall = snap engagement
+catch_proud = 1.4;
+// Vertical play between latch window and catch bar.
+// Negative values preload the lid shut.
+latch_play = 0.1;
+
 /* [Insert] */
 // Gap per side between insert and box wall
 insert_clearance = 0.40;
@@ -103,6 +128,36 @@ ins_h = insert_h_override > 0
 lip_x = inner_x - 2 * lid_clearance;    // lid lip outer size
 lip_y = inner_y - 2 * lid_clearance;
 
+// --- latch hinge internals ---
+lug_w         = 5;                    // lid lug width (each side)
+hinge_gap     = 0.4;                  // side play lug <-> latch
+pin_boss_r    = filament_d / 2 + 2.3; // lug / barrel outer radius
+pin_standoff  = pin_boss_r + 0.4;     // pin axis to wall face
+lug_hole_d    = filament_d + 0.15;    // tight: pin glued into lugs
+barrel_hole_d = filament_d + 0.35;    // loose: latch pivots freely
+latch_gap     = 0.4;                  // latch strip to wall face
+bar_h         = 2.8;                  // catch bar height
+bar_rise      = 1.2;                  // straight part below chamfer
+window_rail   = 3;                    // strip rails beside window
+window_w      = latch_w - 2 * window_rail;
+bar_len       = window_w - 0.8;
+grip_l        = 6;                    // thumb flare below window
+
+z_pin     = box_h + lid_top - pin_boss_r;  // hinge axis, lid closed
+z_bar_top = box_h - catch_drop;
+// latch window, measured down the strip from the pin axis
+win_t1    = z_pin - z_bar_top - 0.4;
+win_t2    = z_pin - z_bar_top + bar_h + latch_play;
+latch_len = win_t2 + 4 + grip_l;           // pin axis to latch tip
+
+function latch_pos_x() =
+    latches_per_side <= 0 ? []
+    : [for (i = [1 : latches_per_side])
+          total_x * i / (latches_per_side + 1)];
+
+assert(lid_style != "latch" || z_pin - latch_len > 2,
+       "box too shallow for the latch: reduce catch_drop");
+
 // ------------------------------------------------------------
 // Helpers
 // ------------------------------------------------------------
@@ -141,7 +196,7 @@ module box() {
             }
             if (tray_chamfer > 0) tray_chamfers();
         }
-        if (thumb_grooves)
+        if (thumb_grooves && lid_style == "friction")
             for (m = [0, 1])
                 translate([total_x / 2,
                            m == 0 ? -(14 - groove_depth)
@@ -149,6 +204,25 @@ module box() {
                            box_h - 16])
                     cylinder(h = 20, d = 28, $fn = 48);
     }
+    if (lid_style == "latch")
+        for (x0 = latch_pos_x()) {
+            translate([x0, 0, z_bar_top]) catch_bar();
+            translate([x0, total_y, z_bar_top])
+                mirror([0, 1, 0]) catch_bar();
+        }
+}
+
+// catch bar on the outer wall: origin on the wall face with the
+// bar top at z = 0, protruding in -y. Chamfered on top so the
+// latch rides over it, flat below so the latch can't cam out.
+module catch_bar() {
+    translate([-bar_len / 2, 0, 0])
+        rotate([90, 0, 90])
+            linear_extrude(bar_len)
+                polygon([[0, 0],
+                         [-catch_proud, -bar_h + bar_rise],
+                         [-catch_proud, -bar_h],
+                         [0, -bar_h]]);
 }
 
 module tray_chamfers() {
@@ -203,6 +277,77 @@ module lid() {
             rbox(lip_x - 2 * lip_t, lip_y - 2 * lip_t,
                  lip_h + 1, max(inner_r - lip_t, 0.1));
     }
+    if (lid_style == "latch")
+        for (x0 = latch_pos_x()) {
+            translate([x0, 0, 0]) lug_pair();
+            translate([x0, total_y, 0]) mirror([0, 1, 0]) lug_pair();
+        }
+}
+
+// ------------------------------------------------------------
+// Latch hardware
+// ------------------------------------------------------------
+// One-piece snap latch hinged on a short piece of filament: the
+// lid carries two lugs, the latch barrel sits between them, and
+// the filament pin is glued into the (tighter) lug holes only,
+// so the latch pivots freely. The window in the strip snaps
+// over the catch bar on the box wall.
+
+// one lid lug, centered on x = 0 at the lid edge y = 0, hanging
+// outward in -y. Modeled in lid print orientation, so the pin
+// boss rests flat on the bed.
+module lid_lug() {
+    difference() {
+        hull() {
+            translate([-lug_w / 2, -pin_standoff, pin_boss_r])
+                rotate([0, 90, 0]) cylinder(r = pin_boss_r, h = lug_w);
+            translate([-lug_w / 2, 0, 0]) cube([lug_w, 2, lid_top]);
+        }
+        translate([-lug_w / 2 - 1, -pin_standoff, pin_boss_r])
+            rotate([0, 90, 0]) cylinder(d = lug_hole_d, h = lug_w + 2);
+    }
+}
+
+module lug_pair() {
+    off = latch_w / 2 + hinge_gap + lug_w / 2;
+    for (sx = [-1, 1]) translate([sx * off, 0, 0]) lid_lug();
+}
+
+// modeled flat: x = down the strip from the pin axis, y = away
+// from the box wall, z = width. Prints standing on that flat
+// face -> clean pivot hole and layer lines along the strip.
+module latch() {
+    u0 = latch_gap;             // inner face (toward the wall)
+    u1 = latch_gap + latch_t;   // outer face
+    flare = 2.5;
+    difference() {
+        linear_extrude(latch_w) {
+            // pivot barrel
+            translate([0, pin_standoff]) circle(r = pin_boss_r);
+            polygon([
+                [0, u0],
+                [latch_len - 2, u0],
+                [latch_len, u0 + 1.4],       // tip chamfer, rides
+                [latch_len, u1 + flare],     // over the catch bar
+                [latch_len - 2, u1 + flare], // thumb flare to open
+                [win_t2 + 4, u1],
+                [0, u1]
+            ]);
+        }
+        // pivot hole
+        translate([0, pin_standoff, -1])
+            cylinder(d = barrel_hole_d, h = latch_w + 2);
+        // window that snaps over the catch bar
+        translate([win_t1, -1, window_rail])
+            cube([win_t2 - win_t1, u1 + flare + 2, window_w]);
+    }
+}
+
+// latch in closed/engaged position on the front wall (y = 0)
+module placed_latch(x0) {
+    translate([x0, 0, z_pin])
+        rotate([0, 0, 180]) rotate([0, 90, 0])
+            translate([0, 0, -latch_w / 2]) latch();
 }
 
 // ------------------------------------------------------------
@@ -263,14 +408,29 @@ module assembly() {
     color("LightGreen", 0.7)
         translate([0, total_y, box_h + ins_h + 25])
             rotate([180, 0, 0]) translate([0, 0, -lid_top]) lid();
+    // latches hang from the floating lid's lugs
+    if (lid_style == "latch")
+        color("Tomato")
+            translate([0, 0, ins_h + 25])
+                for (x0 = latch_pos_x()) {
+                    placed_latch(x0);
+                    translate([0, total_y, 0])
+                        mirror([0, 1, 0]) placed_latch(x0);
+                }
 }
 
 if (part == "box")      box();
 else if (part == "lid")    lid();
 else if (part == "insert") insert();
+else if (part == "latch")  latch();
 else if (part == "print") {
     box();
     translate([total_x + 15, 0, 0]) lid();
     translate([0, total_y + 15, 0]) insert();
+    if (lid_style == "latch" && latches_per_side > 0)
+        for (i = [0 : 2 * latches_per_side - 1])
+            translate([total_x + 15 + pin_boss_r,
+                       total_y + 15 + i * (2 * pin_boss_r + 4), 0])
+                latch();
 }
 else assembly();