diff --git a/designs/round_duct_butterfly_damper/family.json b/designs/round_duct_butterfly_damper/family.json
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+{
+  "family": "round_duct_butterfly_damper",
+  "standard": "Ruskin MDRS25 / CDR94 (round manual balancing / control damper)",
+  "base_plane": "XY",
+  "description": "Round HVAC manual balancing (butterfly) damper in the closed position: a rolled sheet-metal frame spool, a circular blade filling the bore on a diametral through-shaft, and a shaft extension for the locking hand quadrant. Axle is round or square.",
+  "source": "Ruskin MDRS25 (light quadrant) + CDR94 (heavy control) round-damper datasheets; frame depth / blade / shaft ranges from their published size tables",
+  "contributor": "BenchCAD-org"
+}
diff --git a/designs/round_duct_butterfly_damper/part.py b/designs/round_duct_butterfly_damper/part.py
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+"""round_duct_butterfly_damper — the parametric part.
+
+A round HVAC manual balancing damper, drawn in the CLOSED position: a rolled
+sheet-metal spool (annular frame) that slips into round duct, a circular
+butterfly blade filling the bore, and a through-shaft on a diameter that carries
+the blade and extends out one side for the locking hand quadrant. The shaft is
+round, or a square axle (MDRS25 style). Airflow runs along the spool axis (Z).
+
+Interface + examples: docs/DESIGN_SPEC.md
+"""
+
+import cadquery as cq
+
+
+def build(duct_d, wall, frame_depth, blade_t, shaft_d, handle_ext, stub, square_shaft):
+    outer_r = duct_d / 2.0
+    bore_r = outer_r - wall
+
+    # frame spool: annular sheet-metal tube along Z, centred at z=0
+    result = cq.Workplane("XY").circle(outer_r).circle(bore_r).extrude(frame_depth / 2.0, both=True)
+
+    # butterfly blade: a disc filling the bore, centred at mid-depth (closed)
+    blade = cq.Workplane("XY").circle(bore_r).extrude(blade_t / 2.0, both=True)
+    result = result.union(blade)
+
+    # control shaft on a diameter (along X): bearing stub one side, handle
+    # extension the other. Round rod, or a square axle.
+    length = duct_d + stub + handle_ext
+    if square_shaft:
+        shaft = cq.Workplane("YZ").rect(shaft_d, shaft_d).extrude(length)
+    else:
+        shaft = cq.Workplane("YZ").circle(shaft_d / 2.0).extrude(length)
+    shaft = shaft.translate((-(outer_r + stub), 0.0, 0.0))
+    result = result.union(shaft)
+
+    return result
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diff --git a/designs/round_duct_butterfly_damper/spec.py b/designs/round_duct_butterfly_damper/spec.py
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+"""round_duct_butterfly_damper — the benchmark generator spec.
+
+A round manual balancing / butterfly volume damper. PARAM_SPEC declares each
+build() parameter; check() holds the engineering rules a reviewer audits. No
+parameters are coupled, so there is no refine(). Spec: docs/DESIGN_SPEC.md
+
+Sources:
+- Ruskin MDRS25 (light manual quadrant damper): 20 ga frame + blade (~0.9 mm),
+  3/8" (10 mm) square axle, 6" (152 mm) frame, Ø 4"-20" (102-508 mm).
+- Ruskin CDR94 (heavy round control damper): frame depth 152/229/305 mm, blade
+  1/4"-3/8" (6-10 mm), round axle 3/4"-3" (19-76 mm), Ø 102-1829 mm.
+  Frame depth / blade / shaft step UP with diameter across the size range.
+"""
+
+
+# ── 1. PARAM_SPEC ────────────────────────────────────────────────────────────
+PARAM_SPEC = {
+    "duct_d": dict(
+        desc="nominal round-duct / spool outer diameter",
+        unit="mm",
+        range={"easy": (120.0, 350.0), "medium": (110.0, 650.0), "hard": (110.0, 1000.0)},
+        source="Ruskin MDRS25/CDR94 round damper sizes (102-1829 mm range)",
+        askable=True,
+    ),
+    "wall": dict(
+        desc="rolled sheet-metal frame wall thickness",
+        unit="mm",
+        range={"easy": (0.9, 1.6), "medium": (0.9, 2.2), "hard": (0.9, 3.0)},
+        source="sheet-metal gauge (20 ga ~0.9 mm up); SMACNA round-duct gauge",
+        askable=True,
+    ),
+    "frame_depth": dict(
+        desc="axial length of the frame spool",
+        unit="mm",
+        range={"easy": (150.0, 175.0), "medium": (150.0, 240.0), "hard": (150.0, 320.0)},
+        source="Ruskin CDR94 frame depth 152 / 229 / 305 mm (steps with diameter)",
+        askable=True,
+    ),
+    "blade_t": dict(
+        desc="butterfly blade (disc) thickness",
+        unit="mm",
+        range={"easy": (1.5, 3.0), "medium": (1.5, 6.0), "hard": (1.5, 10.0)},
+        source="MDRS25 20 ga blade -> CDR94 1/4-3/8 in (1.5-10 mm)",
+        askable=True,
+    ),
+    "shaft_d": dict(
+        desc="control shaft / axle diameter (round rod, or square-bar side)",
+        unit="mm",
+        range={"easy": (12.0, 22.0), "medium": (10.0, 45.0), "hard": (10.0, 76.0)},
+        source="MDRS25 10 mm square axle; CDR94 round axle 19-76 mm",
+        askable=True,
+    ),
+    "handle_ext": dict(
+        desc="shaft extension beyond the frame on the hand-quadrant side",
+        unit="mm",
+        range={"easy": (45.0, 65.0), "medium": (40.0, 90.0), "hard": (40.0, 120.0)},
+        source="MDRS25 hand-quadrant standoff (~2-2.75 in) -> proportion",
+        askable=True,
+    ),
+    "stub": dict(
+        desc="shaft stub beyond the frame on the bearing side",
+        unit="mm",
+        range={"easy": (6.0, 14.0), "medium": (5.0, 22.0), "hard": (5.0, 32.0)},
+        source="proportion (bearing-side journal length)",
+        askable=True,
+    ),
+    "square_shaft": dict(
+        desc="axle is a square bar (1, MDRS25) vs a round rod (0, CDR94)",
+        unit="",
+        range={"easy": (0, 0), "medium": (0, 1), "hard": (0, 1)},
+        source="MDRS25 square axle vs CDR94 round axle (documented variants)",
+        choices={"easy": [0], "medium": [0, 1], "hard": [0, 1]},
+        feature=True,
+    ),
+}
+
+
+# ── 2. check ─────────────────────────────────────────────────────────────────
+def check(p: dict) -> list[str]:
+    """Engineering constraints (empty = valid). Each cites its rule."""
+    bad = []
+    d = p["duct_d"]
+
+    # frame is rolled sheet, not a thick pipe
+    if p["wall"] > d / 40.0:
+        bad.append("wall > D/40: frame is rolled sheet metal, not thick pipe (SMACNA gauge)")
+
+    # axle must not choke the bore — CDR94 shaft/diameter ratio stays small
+    if p["shaft_d"] > 0.20 * d:
+        bad.append("shaft_d > 0.20*D: axle too thick for the bore (CDR94 axle/diameter ratio)")
+
+    # blade must seat within the axial depth of the spool
+    if p["blade_t"] >= p["frame_depth"]:
+        bad.append("blade_t >= frame_depth: blade must seat inside the spool depth")
+
+    # the bore must stay open around the axle
+    if p["shaft_d"] >= d - 2.0 * p["wall"]:
+        bad.append("shaft_d >= bore: axle would span the whole bore, no blade area")
+
+    # hand-quadrant side extends past the bearing stub (quadrant standoff)
+    if p["handle_ext"] <= p["stub"]:
+        bad.append("handle_ext <= stub: hand-quadrant side must stand off past the bearing stub")
+
+    return bad