diff --git a/src/components/three/ModelViewer.tsx b/src/components/three/ModelViewer.tsx
index dc0b8df..2ff0ced 100644
--- a/src/components/three/ModelViewer.tsx
+++ b/src/components/three/ModelViewer.tsx
@@ -1035,38 +1035,41 @@ function GridLayer() {
   );
 }
 
-/** Computes min Y of all visible models and updates store gridY when auto-follow is on. */
+/** Continuously aligns the grid Y to the bottom of all visible models so the
+ *  piece always looks like it is sitting on the grid — including after
+ *  calibration, fine-tuning, repositioning or scale changes. */
 function GridAutoFollower() {
-  const models = useModelStore((s) => s.models);
   const { scene } = useThree();
-  useEffect(() => {
+  const lastY = useRef<number>(Number.NaN);
+  const acc = useRef(0);
+  useFrame((_, dt) => {
     const auto = useModelStore.getState().gridAutoFollow;
     if (!auto) return;
-    // Defer to next tick so models have rendered/positioned
-    const id = setTimeout(() => {
-      if (!useModelStore.getState().gridAutoFollow) return;
-      const box = new THREE.Box3();
-      let has = false;
-      scene.traverse((obj) => {
-        if (obj instanceof THREE.Mesh && obj.geometry) {
-          // Skip helpers (markers, rings)
-          if (obj.geometry instanceof THREE.SphereGeometry) return;
-          if (obj.geometry instanceof THREE.RingGeometry) return;
-          if (obj.userData.__edgeLine) return;
-          obj.updateWorldMatrix(true, false);
-          const b = new THREE.Box3().setFromObject(obj);
-          if (isFinite(b.min.y)) {
-            if (!has) { box.copy(b); has = true; }
-            else box.union(b);
-          }
+    // Throttle to ~10 Hz to keep this cheap.
+    acc.current += dt;
+    if (acc.current < 0.1) return;
+    acc.current = 0;
+    const box = new THREE.Box3();
+    let has = false;
+    scene.traverse((obj) => {
+      if (obj instanceof THREE.Mesh && obj.geometry) {
+        if (obj.geometry instanceof THREE.SphereGeometry) return;
+        if (obj.geometry instanceof THREE.RingGeometry) return;
+        if (obj.userData.__edgeLine) return;
+        const b = new THREE.Box3().setFromObject(obj);
+        if (isFinite(b.min.y)) {
+          if (!has) { box.copy(b); has = true; }
+          else box.union(b);
         }
-      });
-      if (has) {
-        useModelStore.setState({ gridY: box.min.y - 0.005 });
       }
-    }, 100);
-    return () => clearTimeout(id);
-  }, [models, scene]);
+    });
+    if (!has) return;
+    const target = box.min.y - 0.005;
+    if (!Number.isFinite(lastY.current) || Math.abs(target - lastY.current) > 1e-4) {
+      lastY.current = target;
+      useModelStore.setState({ gridY: target });
+    }
+  });
   return null;
 }