diff --git a/Operation.cpp b/Operation.cpp
index 20c2559..f0db14e 100644
--- a/Operation.cpp
+++ b/Operation.cpp
@@ -643,6 +643,12 @@ Operation::ApproxWithArcsIfUseful( ICurveComposite* pCompo)
    int nCrvs = pCompo->GetCurveCount() ;
    if ( nCrvs < 5  ||  dLen > nCrvs * LIN_FEA_STD)
       return true ;
+  // limito l'approssimazione alle curve piane
+   Frame3d frRef ;
+   frRef.Set( ORIG, vtExtr) ;
+   BBox3d b3Crv ;
+   if ( ! pCompo->GetBBox( frRef, b3Crv)  ||  ! b3Crv.IsEpsilonZ( 100 * EPS_SMALL))
+      return true ;
   // calcolo approssimazione con archi (se possibile)
    PolyArc PA ;
    if ( ! pCompo->ApproxWithArcsEx( LIN_TOL_MID, ANG_TOL_STD_DEG, LIN_FEA_STD, PA))