diff --git a/src/lib/modelTransforms.ts b/src/lib/modelTransforms.ts
new file mode 100644
index 0000000..58d8807
--- /dev/null
+++ b/src/lib/modelTransforms.ts
@@ -0,0 +1,35 @@
+import * as THREE from 'three';
+
+/**
+ * Registry of the innermost local-frame group of each model in the scene.
+ * Measurements are stored in this local frame so they follow the model
+ * when the user repositions/rotates it via the "Posicionar" tool.
+ */
+const localGroups = new Map<string, THREE.Object3D>();
+
+export function registerModelLocalGroup(modelId: string, obj: THREE.Object3D) {
+  localGroups.set(modelId, obj);
+}
+
+export function unregisterModelLocalGroup(modelId: string, obj?: THREE.Object3D) {
+  // Only delete if the registered object matches (avoid race between unmount/mount of the same id)
+  const current = localGroups.get(modelId);
+  if (!obj || current === obj) localGroups.delete(modelId);
+}
+
+export function getModelLocalGroup(modelId: string | null | undefined): THREE.Object3D | undefined {
+  if (!modelId) return undefined;
+  return localGroups.get(modelId);
+}
+
+export function worldToModelLocal(
+  modelId: string | null | undefined,
+  p: { x: number; y: number; z: number },
+): { x: number; y: number; z: number } | null {
+  const g = getModelLocalGroup(modelId);
+  if (!g) return null;
+  g.updateWorldMatrix(true, false);
+  const v = new THREE.Vector3(p.x, p.y, p.z);
+  g.worldToLocal(v);
+  return { x: v.x, y: v.y, z: v.z };
+}