feat(extrude): symmetric extrude using part expression

- Uses ISOGRID_RIB_sandbox_N_thk expression for thickness
- Creates expression if missing, uses existing if present
- Symmetric extrude: ±thk/2 from sketch plane
- Fallback to literal value if expression fails

tools/adaptive-isogrid/src/nx/import_profile.py

@@ -455,7 +455,8 @@ def _delete_feature(session: Any, feature: Any, lister: Any, label: str = "featu
 
 
 def _extrude_sketch(part: Any, sketch: Any, thickness: float,
-                    normal: List[float], extrude_name: str, lister: Any):
+                    normal: List[float], extrude_name: str,
+                    sandbox_id: str, lister: Any):
     """
     Extrude a sketch using the standard NXOpen pattern:
       - Create ExtrudeBuilder with null feature
@@ -615,11 +616,33 @@ def _extrude_sketch(part: Any, sketch: Any, thickness: float,
         builder.Direction = direction
         lister.WriteLine("[extrude] Step 4: Direction set")
 
-        # Step 5: Set limits
+        # Step 5: Set limits — symmetric extrude using part expression
-        builder.Limits.StartExtend.Value.RightHandSide = "0.0"
+        # Use the pre-defined expression ISOGRID_RIB_sandbox_N_thk
-        builder.Limits.EndExtend.Value.RightHandSide = str(float(thickness))
+        thk_expr_name = f"ISOGRID_RIB_{sandbox_id}_thk"
+        half_expr = f"{thk_expr_name}/2"
+
+        # Ensure the expression exists (create if missing)
+        try:
+            expr = None
+            for e in part.Expressions:
+                if e.Name == thk_expr_name:
+                    expr = e
+                    break
+            if expr is None:
+                part.Expressions.CreateExpression("Number", f"{thk_expr_name}={thickness}")
+                lister.WriteLine(f"[extrude] Created expression: {thk_expr_name}={thickness}")
+            else:
+                lister.WriteLine(f"[extrude] Using existing expression: {thk_expr_name}={expr.Value}")
+        except Exception as exc:
+            lister.WriteLine(f"[extrude] Expression warning: {exc}")
+            half_expr = str(float(thickness) / 2.0)
+
+        # Symmetric extrude: ±thk/2 from sketch plane
+        builder.Limits.StartExtend.Value.RightHandSide = half_expr
+        builder.Limits.EndExtend.Value.RightHandSide = half_expr
+
         builder.BooleanOperation.Type = NXOpen.GeometricUtilities.BooleanOperation.BooleanType.Create
-        lister.WriteLine("[extrude] Step 5: Limits set (0 → {})".format(thickness))
+        lister.WriteLine(f"[extrude] Step 5: Symmetric limits ±{half_expr}")
 
         # Step 6: Commit
         extrude_obj = builder.Commit()
@@ -849,7 +872,8 @@ def main():
         extrude_name = EXTRUDE_NAME_PREFIX + sandbox_id
 
         extrude_feat = _extrude_sketch(
-            work_part, sketch, rib_thickness, normal, extrude_name, lister
+            work_part, sketch, rib_thickness, normal, extrude_name,
+            sandbox_id, lister
         )
         if extrude_feat is not None:
             lister.WriteLine(f"[import] ✓ Sketch + Extrude complete for {sandbox_id}")