diff --git a/VolZmapGraphics.cpp b/VolZmapGraphics.cpp
index 45c868f..9ae7cff 100644
--- a/VolZmapGraphics.cpp
+++ b/VolZmapGraphics.cpp
@@ -1973,7 +1973,7 @@ VolZmap::ExtMarchingCubes( int nBlock, VoxelContainer& vVox) const
                               vtNlm.Normalize() ;
                               double dDotl = vtNlm * CompoVert[nComp][l].vtVec ;
                               double dDotm = vtNlm * CompoVert[nComp][m].vtVec ;
-                              if ( dDotl < - 0.85  ||  dDotm < - 0.85) {
+                              if ( ( dDotl < -0.01  &&  dDotm < -0.01)  ||  dDotl < -0.95  ||  dDotm < -0.95) {
                                  bOverTurning = true ;
                                  break ;
                               }