diff --git a/src/components/three/ModelViewer.tsx b/src/components/three/ModelViewer.tsx
index 046c96c..f1363ab 100644
--- a/src/components/three/ModelViewer.tsx
+++ b/src/components/three/ModelViewer.tsx
@@ -454,10 +454,11 @@ function MeasureClickHandler() {
 
 function GridLayer() {
   const showGrid = useModelStore((s) => s.showGrid);
+  const gridY = useModelStore((s) => s.gridY);
   if (!showGrid) return null;
   return (
     <Grid
-      position={[0, -0.09, 0]}
+      position={[0, gridY, 0]}
       infiniteGrid
       cellSize={0.01}
       sectionSize={0.1}
@@ -471,6 +472,41 @@ function GridLayer() {
   );
 }
 
+/** Computes min Y of all visible models and updates store gridY when auto-follow is on. */
+function GridAutoFollower() {
+  const models = useModelStore((s) => s.models);
+  const { scene } = useThree();
+  useEffect(() => {
+    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);
+          }
+        }
+      });
+      if (has) {
+        useModelStore.setState({ gridY: box.min.y - 0.005 });
+      }
+    }, 100);
+    return () => clearTimeout(id);
+  }, [models, scene]);
+  return null;
+}
+
 export function ModelViewerCanvas({ url }: ModelViewerProps) {
   return (
     <Canvas
@@ -498,6 +534,7 @@ export function ModelViewerCanvas({ url }: ModelViewerProps) {
       <HoverDetector />
 
       <GridLayer />
+      <GridAutoFollower />
 
       <OrbitControls
         makeDefault
diff --git a/src/components/three/XRHudInWorld.tsx b/src/components/three/XRHudInWorld.tsx
index ce1921b..5c6bf98 100644
--- a/src/components/three/XRHudInWorld.tsx
+++ b/src/components/three/XRHudInWorld.tsx
@@ -309,6 +309,40 @@ function ToolsTab(p: XRHudInWorldProps) {
             onClick={() => setScaleRatio(preset)} />
         );
       })}
+
+      {/* Grid floor controls */}
+      <GridFloorRow />
+    </group>
+  );
+}
+
+function GridFloorRow() {
+  const { camera } = useThree();
+  const gridY = useModelStore((s) => s.gridY);
+  const gridAutoFollow = useModelStore((s) => s.gridAutoFollow);
+  const setGridAutoFollow = useModelStore((s) => s.setGridAutoFollow);
+  const setGridY = useModelStore((s) => s.setGridY);
+  const nudgeGridY = useModelStore((s) => s.nudgeGridY);
+  const placeAtFloor = () => {
+    // Headset eye Y minus avg standing eye height (1.6 m)
+    const floor = camera.position.y - 1.6;
+    setGridY(floor);
+  };
+  return (
+    <group>
+      <Text position={[-0.24, -0.15, 0]} fontSize={0.008} color="#94a3b8" anchorX="left">
+        Grid Y {(gridY * 1000).toFixed(0)}mm {gridAutoFollow ? '(auto)' : '(travado)'}
+      </Text>
+      <XR3DButton position={[-0.16, -0.175, 0]} size={[0.055, 0.022]} label="Auto"
+        active={gridAutoFollow} onClick={() => setGridAutoFollow(true)} />
+      <XR3DButton position={[-0.10, -0.175, 0]} size={[0.055, 0.022]} label="Travar"
+        active={!gridAutoFollow} onClick={() => setGridAutoFollow(false)} />
+      <XR3DButton position={[-0.04, -0.175, 0]} size={[0.04, 0.022]} label="−1cm"
+        onClick={() => nudgeGridY(-0.01)} />
+      <XR3DButton position={[0.005, -0.175, 0]} size={[0.04, 0.022]} label="+1cm"
+        onClick={() => nudgeGridY(0.01)} />
+      <XR3DButton position={[0.07, -0.175, 0]} size={[0.075, 0.022]} label="Pousar chão"
+        onClick={placeAtFloor} />
     </group>
   );
 }
diff --git a/src/pages/XRSession.tsx b/src/pages/XRSession.tsx
index 7abd1b7..0c69b64 100644
--- a/src/pages/XRSession.tsx
+++ b/src/pages/XRSession.tsx
@@ -359,10 +359,11 @@ function XRSnapHandler() {
 // ─── XR Grid ───────────────────────────────────────────
 function XRGrid() {
   const showGrid = useModelStore((s) => s.showGrid);
+  const gridY = useModelStore((s) => s.gridY);
   if (!showGrid) return null;
   return (
     <Grid
-      position={[0, -0.09, 0]}
+      position={[0, gridY, 0]}
       infiniteGrid
       cellSize={0.01}
       sectionSize={0.1}
@@ -376,6 +377,36 @@ function XRGrid() {
   );
 }
 
+/** Auto-follow grid to model bottom when enabled. */
+function XRGridAutoFollower() {
+  const models = useModelStore((s) => s.models);
+  const { scene } = useThree();
+  useEffect(() => {
+    if (!useModelStore.getState().gridAutoFollow) return;
+    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) {
+          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);
+          }
+        }
+      });
+      if (has) useModelStore.setState({ gridY: box.min.y - 0.005 });
+    }, 150);
+    return () => clearTimeout(id);
+  }, [models, scene]);
+  return null;
+}
+
 // ImageTrackingAnchor removed — replaced by XRHitTestPlacement
 
 // ─── XRSession Page ────────────────────────────────────
@@ -585,6 +616,7 @@ const XRSession = () => {
 
             <XRSnapHandler />
             <XRGrid />
+            <XRGridAutoFollower />
           </XR>
         </Canvas>
 
diff --git a/src/stores/useModelStore.ts b/src/stores/useModelStore.ts
index c4f771e..c9ce865 100644
--- a/src/stores/useModelStore.ts
+++ b/src/stores/useModelStore.ts
@@ -133,6 +133,15 @@ interface ModelStore {
   showGrid: boolean;
   setShowGrid: (show: boolean) => void;
 
+  /** World Y position of the grid plane (meters). */
+  gridY: number;
+  setGridY: (y: number) => void;
+  /** When true, grid follows the bottom of the loaded model automatically. */
+  gridAutoFollow: boolean;
+  setGridAutoFollow: (b: boolean) => void;
+  /** Nudge grid by delta meters and disable auto-follow. */
+  nudgeGridY: (delta: number) => void;
+
   wireframeColor: string;
   setWireframeColor: (color: string) => void;
 
@@ -308,6 +317,12 @@ export const useModelStore = create<ModelStore>((set, get) => ({
   showGrid: true,
   setShowGrid: (showGrid) => set({ showGrid }),
 
+  gridY: -0.09,
+  setGridY: (gridY) => set({ gridY, gridAutoFollow: false }),
+  gridAutoFollow: true,
+  setGridAutoFollow: (gridAutoFollow) => set({ gridAutoFollow }),
+  nudgeGridY: (delta) => set((s) => ({ gridY: s.gridY + delta, gridAutoFollow: false })),
+
   wireframeColor: '#8899aa',
   setWireframeColor: (wireframeColor) => set({ wireframeColor }),