diff --git a/VolZmapVolume.cpp b/VolZmapVolume.cpp
index afd7c95..51867ce 100644
--- a/VolZmapVolume.cpp
+++ b/VolZmapVolume.cpp
@@ -4545,7 +4545,8 @@ VolZmap::CompCyl_Milling( int nGrid, const Point3d& ptS, const Point3d& ptE,
    double dOrtLen = vtMoveOrt.Len() ;
 
   // Parametri di contenimento
-   Vector3d vtPerp = vtToolDir ^ Z_AX ; vtPerp.Normalize() ;
+   Vector3d vtPerp = vtToolDir ^ Z_AX ;
+   bool bPerpOk = vtPerp.Normalize() ;
    if ( vtPerp * vtMove < 0)
       vtPerp.Invert() ;
    int nSkip = 0 ;
@@ -4568,12 +4569,14 @@ VolZmap::CompCyl_Milling( int nGrid, const Point3d& ptS, const Point3d& ptE,
          }
 
         // salto il controllo degli spilloni prima del piano di fianco verticale di inizio o dopo il piano di fianco verticale di fine
-         if ( ( ptC + Z_AX * m_dMinZ[nGrid] - ptI + vtPerp * dRad) * vtPerp < -EPS_SMALL  &&
+         if ( bPerpOk  &&
+              ( ptC + Z_AX * m_dMinZ[nGrid] - ptI + vtPerp * dRad) * vtPerp < -EPS_SMALL  &&
               ( ptC + Z_AX * m_dMaxZ[nGrid] - ptI + vtPerp * dRad) * vtPerp < -EPS_SMALL) {
             ++ nSkip ;
             continue ;
          }
-         if ( ( ptC + Z_AX * m_dMinZ[nGrid] - ptF - vtPerp * dRad) * vtPerp > EPS_SMALL  &&
+         if ( bPerpOk  &&
+              ( ptC + Z_AX * m_dMinZ[nGrid] - ptF - vtPerp * dRad) * vtPerp > EPS_SMALL  &&
               ( ptC + Z_AX * m_dMaxZ[nGrid] - ptF - vtPerp * dRad) * vtPerp > EPS_SMALL) {
             ++ nSkip ;
             continue ;