diff --git a/VolTriZmapVolume.cpp b/VolTriZmapVolume.cpp
index f8a4722..917fa2a 100644
--- a/VolTriZmapVolume.cpp
+++ b/VolTriZmapVolume.cpp
@@ -2196,7 +2196,8 @@ VolZmap::CylBall_XYPerp( unsigned int nGrid, const Point3d & ptS, const Point3d
 
                      dSupZ = ( dDotUp - dX * vtP.x - dY * vtP.y) / vtP.z ;
 
-                     vtNmax = vtToolDir ^ vtMove ;
+                     Vector3d vtT = vtToolDir ^ vtMove ; 
+                     vtNmax = ( vtT.z < 0 ? vtT : - vtT) ;
                      vtNmax.Normalize() ;
                   }
                   else {
@@ -2223,7 +2224,8 @@ VolZmap::CylBall_XYPerp( unsigned int nGrid, const Point3d & ptS, const Point3d
                
                      dInfZ = ( dDotDw - dX * vtP.x - dY * vtP.y) / vtP.z ;
 
-                     vtNmin = - vtToolDir ^ vtMove ;
+                     Vector3d vtT = vtToolDir ^ vtMove ; 
+                     vtNmin = ( vtT.z < 0 ? - vtT : vtT) ;
                      vtNmin.Normalize() ;
                   }
                   else {