diff --git a/src/components/ViewerControls.tsx b/src/components/ViewerControls.tsx
index b7eb4cf..741304d 100644
--- a/src/components/ViewerControls.tsx
+++ b/src/components/ViewerControls.tsx
@@ -36,6 +36,7 @@ export function ViewerControls() {
     selectedElementKeys, hiddenElementKeys, isolatedElementKeys,
     hideSelectedElements, isolateSelectedElements, showAllElements, clearElementSelection,
     cameraMode, setCameraMode,
+    gridCalibMode, setGridCalibMode, setGridAutoFollow,
   } = useModelStore();
   const activeModel = models.find(m => m.id === activeModelId);
   const hasSelection = selectedElementKeys.size > 0;
@@ -106,6 +107,30 @@ export function ViewerControls() {
         <span className="font-mono text-xs">Grid</span>
       </Button>
 
+      {/* Grid calibration: click a face on the piece to anchor the grid there */}
+      <Button
+        variant={gridCalibMode ? 'default' : 'outline'}
+        size="sm"
+        className="gap-2 h-9"
+        disabled={!activeModelId}
+        onClick={() => {
+          const next = !gridCalibMode;
+          setGridCalibMode(next);
+          if (next) {
+            setGridAutoFollow(false);
+            toast.info('Calibrar Grid: clique numa face da peça');
+          } else {
+            toast.dismiss();
+          }
+        }}
+        title="Posiciona o grid na altura de uma face da peça"
+      >
+        <LayoutGrid className="h-3.5 w-3.5" />
+        <span className="font-mono text-xs">
+          {gridCalibMode ? 'Clique na face…' : 'Calibrar Grid'}
+        </span>
+      </Button>
+
       {/* Scale presets */}
       <ScaleSelector />
 
diff --git a/src/components/three/ModelViewer.tsx b/src/components/three/ModelViewer.tsx
index 2ff0ced..992c984 100644
--- a/src/components/three/ModelViewer.tsx
+++ b/src/components/three/ModelViewer.tsx
@@ -5,7 +5,7 @@ import { Loader2 } from 'lucide-react';
 import * as THREE from 'three';
 import { useModelStore, type SceneModel } from '@/stores/useModelStore';
 import { findNearestVertex, detectHoleAtFace, findNearestEdgeSegment, detectCircularEdgeAtPoint } from './SmartMeasure';
-import { registerModelLocalGroup, unregisterModelLocalGroup, getModelWorldScaleFactor } from '@/lib/modelTransforms';
+import { registerModelLocalGroup, unregisterModelLocalGroup, getModelWorldScaleFactor, getAllModelLocalGroups } from '@/lib/modelTransforms';
 import { parseIFCtoThree } from '@/lib/convertIFC';
 import { GLTFLoader } from 'three/examples/jsm/loaders/GLTFLoader.js';
 import { mainCameraRef, mainControlsRef, viewAnim, calibration, pushModelFaceNormal, computeCalibrationQuaternion, subscribeCalibration } from './viewCubeBus';
@@ -1035,34 +1035,39 @@ function GridLayer() {
   );
 }
 
-/** 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. */
+/** Continuously aligns the grid Y to the bottom of all registered model
+ *  groups (not the entire scene), so the piece always sits on the grid —
+ *  including after calibration, fine-tuning or repositioning. */
 function GridAutoFollower() {
-  const { scene } = useThree();
   const lastY = useRef<number>(Number.NaN);
   const acc = useRef(0);
   useFrame((_, dt) => {
     const auto = useModelStore.getState().gridAutoFollow;
     if (!auto) return;
-    // Throttle to ~10 Hz to keep this cheap.
     acc.current += dt;
     if (acc.current < 0.1) return;
     acc.current = 0;
+    const groups = getAllModelLocalGroups();
+    if (groups.length === 0) return;
     const box = new THREE.Box3();
     let has = false;
-    scene.traverse((obj) => {
-      if (obj instanceof THREE.Mesh && obj.geometry) {
+    const tmp = new THREE.Box3();
+    for (const g of groups) {
+      g.updateWorldMatrix(true, true);
+      tmp.makeEmpty();
+      g.traverse((obj) => {
+        if (!(obj instanceof THREE.Mesh) || !obj.geometry) return;
         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 (isFinite(b.min.y)) tmp.union(b);
+      });
+      if (!tmp.isEmpty() && isFinite(tmp.min.y)) {
+        if (!has) { box.copy(tmp); has = true; }
+        else box.union(tmp);
       }
-    });
+    }
     if (!has) return;
     const target = box.min.y - 0.005;
     if (!Number.isFinite(lastY.current) || Math.abs(target - lastY.current) > 1e-4) {
@@ -1073,6 +1078,43 @@ function GridAutoFollower() {
   return null;
 }
 
+/** When gridCalibMode is on, the next click on the model sets the grid Y to
+ *  the world Y of the clicked surface point and disables auto-follow. */
+function GridCalibrationHandler() {
+  const { camera, scene, gl } = useThree();
+  const gridCalibMode = useModelStore((s) => s.gridCalibMode);
+  const raycaster = useMemo(() => new THREE.Raycaster(), []);
+  const mouse = useMemo(() => new THREE.Vector2(), []);
+  useEffect(() => {
+    if (!gridCalibMode) return;
+    const canvas = gl.domElement;
+    const prevCursor = canvas.style.cursor;
+    canvas.style.cursor = 'crosshair';
+    const onPointerUp = (ev: PointerEvent) => {
+      if (ev.button !== 0) return;
+      const rect = canvas.getBoundingClientRect();
+      mouse.x = ((ev.clientX - rect.left) / rect.width) * 2 - 1;
+      mouse.y = -((ev.clientY - rect.top) / rect.height) * 2 + 1;
+      raycaster.setFromCamera(mouse, camera);
+      const hits = raycaster.intersectObjects(scene.children, true);
+      const hit = hits.find((h) => isPickableModelMesh(h.object));
+      if (!hit) return;
+      useModelStore.setState({
+        gridY: hit.point.y - 0.001,
+        gridAutoFollow: false,
+        gridCalibMode: false,
+        showGrid: true,
+      });
+    };
+    canvas.addEventListener('pointerup', onPointerUp);
+    return () => {
+      canvas.removeEventListener('pointerup', onPointerUp);
+      canvas.style.cursor = prevCursor;
+    };
+  }, [gridCalibMode, camera, scene, gl, raycaster, mouse]);
+  return null;
+}
+
 /** Mouse drag handler for desktop "Posicionar" mode: translates/rotates the active model. */
 function PositionDragHandler() {
   const { camera, gl, scene } = useThree();
@@ -1259,6 +1301,7 @@ export function ModelViewerCanvas({ url }: ModelViewerProps) {
 
       <GridLayer />
       <GridAutoFollower />
+      <GridCalibrationHandler />
 
       <PositionDragHandler />
       <SelectionHandler />
diff --git a/src/lib/modelTransforms.ts b/src/lib/modelTransforms.ts
index c3ef211..c69f135 100644
--- a/src/lib/modelTransforms.ts
+++ b/src/lib/modelTransforms.ts
@@ -22,6 +22,10 @@ export function getModelLocalGroup(modelId: string | null | undefined): THREE.Ob
   return localGroups.get(modelId);
 }
 
+export function getAllModelLocalGroups(): THREE.Object3D[] {
+  return Array.from(localGroups.values());
+}
+
 export function worldToModelLocal(
   modelId: string | null | undefined,
   p: { x: number; y: number; z: number },
diff --git a/src/stores/useModelStore.ts b/src/stores/useModelStore.ts
index 82680f3..ba96846 100644
--- a/src/stores/useModelStore.ts
+++ b/src/stores/useModelStore.ts
@@ -257,6 +257,9 @@ interface ModelStore {
   setGridAutoFollow: (b: boolean) => void;
   /** Nudge grid by delta meters and disable auto-follow. */
   nudgeGridY: (delta: number) => void;
+  /** When true, the next click on a model face sets grid Y to that point. */
+  gridCalibMode: boolean;
+  setGridCalibMode: (b: boolean) => void;
 
   wireframeColor: string;
   setWireframeColor: (color: string) => void;
@@ -535,6 +538,8 @@ export const useModelStore = create<ModelStore>((set, get) => ({
   gridAutoFollow: true,
   setGridAutoFollow: (gridAutoFollow) => set({ gridAutoFollow }),
   nudgeGridY: (delta) => set((s) => ({ gridY: s.gridY + delta, gridAutoFollow: false })),
+  gridCalibMode: false,
+  setGridCalibMode: (gridCalibMode) => set({ gridCalibMode }),
 
   wireframeColor: '#00f3ff',
   setWireframeColor: (wireframeColor) => set({ wireframeColor }),