diff --git a/CADability/BSpline.cs b/CADability/BSpline.cs
index 4404553e7..b1bb7a1ef 100644
--- a/CADability/BSpline.cs
+++ b/CADability/BSpline.cs
@@ -87,10 +87,40 @@ private GeoPoint[] GetCashedApproximation(double precision)
         {
             lock (lockApproximationRecalc)
             {
-                if (((precision > 0) && (approxPrecision > precision)) || approximation == null)
+                if (approximation == null || approxPrecision <= 0 || (precision > 0 && approxPrecision > precision))
                 {
                     approxPrecision = precision;
                     ICurve cv = (this as ICurve).Approximate(true, precision);
+                    
+#if DEBUG
+                    // Debug output to help diagnose approximation issues
+                    System.Diagnostics.Debug.WriteLine("=== BSpline Approximation Debug ===");
+                    System.Diagnostics.Debug.WriteLine($"Precision: {precision}");
+                    System.Diagnostics.Debug.WriteLine($"Poles count: {poles?.Length ?? 0}");
+                    System.Diagnostics.Debug.WriteLine($"Knots count: {knots?.Length ?? 0}");
+                    System.Diagnostics.Debug.WriteLine($"Degree: {degree}");
+                    System.Diagnostics.Debug.WriteLine($"Periodic: {periodic}");
+                    System.Diagnostics.Debug.WriteLine($"StartParam: {startParam}, EndParam: {endParam}");
+                    System.Diagnostics.Debug.WriteLine($"PlanarState: {(this as ICurve).GetPlanarState()}");
+                    System.Diagnostics.Debug.WriteLine($"Approximation result type: {cv?.GetType().Name ?? "null"}");
+                    if (cv is Line line)
+                    {
+                        System.Diagnostics.Debug.WriteLine($"  Line: Start={line.StartPoint}, End={line.EndPoint}");
+                        System.Diagnostics.Debug.WriteLine($"  Line Length: {line.Length}");
+                    }
+                    else if (cv is Polyline polyline)
+                    {
+                        System.Diagnostics.Debug.WriteLine($"  Polyline: {polyline.PointCount} points, Length={polyline.Length}");
+                    }
+                    else if (cv is Path path)
+                    {
+                        System.Diagnostics.Debug.WriteLine($"  Path: {path.CurveCount} curves, Length={path.Length}");
+                    }
+                    System.Diagnostics.Debug.WriteLine($"Original BSpline: Start={StartPoint}, End={EndPoint}");
+                    System.Diagnostics.Debug.WriteLine($"Original BSpline Length: {(this as ICurve).Length}");
+                    System.Diagnostics.Debug.WriteLine("=====================================");
+#endif
+                    
                     if (cv is Path)
                     {
                         Path path = (cv as Path);
@@ -516,6 +546,7 @@ private void InvalidateSecondaryData()
                 interdir = null;
                 interparam = null;
                 approximation = null;
+                approxPrecision = 0;
                 extent = BoundingCube.EmptyBoundingCube;
                 tetraederHull = null;
                 extrema = null;
@@ -3044,6 +3075,36 @@ CADability.Curve2D.ICurve2D ICurve.GetProjectedCurve(Plane p)
             }
             else
             {
+                // When poles become identical after projection, we need to filter them out
+                // and create a BSpline2D with the unique poles
+                List<GeoPoint2D> uniquePoles = new List<GeoPoint2D>();
+                List<double> uniqueWeights = new List<double>();
+                uniquePoles.Add(poles2d[0]);
+                uniqueWeights.Add(weights[0]);
+                for (int i = 1; i < poles2d.Length; i++)
+                {
+                    if (!Precision.IsEqual(poles2d[i], uniquePoles[uniquePoles.Count - 1]))
+                    {
+                        uniquePoles.Add(poles2d[i]);
+                        uniqueWeights.Add(weights[i]);
+                    }
+                }
+                
+                // If we have enough unique poles, try to create a BSpline2D
+                if (uniquePoles.Count >= degree + 1)
+                {
+                    try
+                    {
+                        BSpline2D bsp2d = new BSpline2D(uniquePoles.ToArray(), degree, this.periodic);
+                        return bsp2d;
+                    }
+                    catch
+                    {
+                        // Fall through to throughPoints approach
+                    }
+                }
+                
+                // Fallback: create through points
                 List<GeoPoint2D> throughPoints2d = new List<GeoPoint2D>();
                 if (ThroughPoints3dExist)
                 {
@@ -3161,8 +3222,8 @@ ICurve ICurve.Approximate(bool linesOnly, double maxError)
             {
                 Plane pl = (this as ICurve).GetPlane();
                 ICurve2D c2d = (this as ICurve).GetProjectedCurve(pl);
-                if (c2d == null || c2d.Length < maxError)
-                {   // zu kurze machen u.U. Probleme
+                if (c2d == null)
+                {   // projection failed, return degenerate line
                     Line line = Line.Construct();
                     line.SetTwoPoints((this as ICurve).StartPoint, (this as ICurve).EndPoint);
                     return line;
@@ -3214,6 +3275,17 @@ ICurve ICurve.Approximate(bool linesOnly, double maxError)
                         }
                     }
                     pos.Add(1.0);
+                    // Ensure we have enough sample points even if the curve appears linear
+                    // This prevents circular/curved splines from being simplified to lines
+                    if (pos.Count < 5)
+                    {
+                        pos.Clear();
+                        int minSamples = Math.Max(5, knots.Length);
+                        for (int i = 0; i < minSamples; ++i)
+                        {
+                            pos.Add((double)i / (double)(minSamples - 1));
+                        }
+                    }
                     return Curves.ApproximateLinear(this, pos.ToArray(), maxError);
                 }
                 else