diff --git a/CutList/Forms/MainForm.cs b/CutList/Forms/MainForm.cs
index 63832c1..71daa6c 100644
--- a/CutList/Forms/MainForm.cs
+++ b/CutList/Forms/MainForm.cs
@@ -41,8 +41,6 @@ namespace CutList.Forms
             toolbox = new Toolbox();
             cutMethodComboBox.DataSource = toolbox.Tools;
 
-            currentDocument = new Document();
-
 #if DEBUG
             loadExampleDataButton.Visible = true;
 #else
diff --git a/CutList/Services/CutListService.cs b/CutList/Services/CutListService.cs
index 3b0817c..e8c197c 100644
--- a/CutList/Services/CutListService.cs
+++ b/CutList/Services/CutListService.cs
@@ -27,11 +27,9 @@ namespace CutList.Services
                 var multiBins = ConvertToMultiBins(stockBins);
                 var binItems = ConvertToBinItems(parts);
 
-                var engine = new MultiBinEngine
-                {
-                    Spacing = cuttingTool.Kerf,
-                    Bins = multiBins
-                };
+                var engine = new MultiBinEngine();
+                engine.SetBins(multiBins);
+                engine.Spacing = cuttingTool.Kerf;
 
                 var packResult = engine.Pack(binItems);
                 return Result<SawCut.Nesting.Result>.Success(packResult);
diff --git a/SawCut/Nesting/MultiBinEngine.cs b/SawCut/Nesting/MultiBinEngine.cs
index bec2c34..e4ead14 100644
--- a/SawCut/Nesting/MultiBinEngine.cs
+++ b/SawCut/Nesting/MultiBinEngine.cs
@@ -15,15 +15,34 @@ namespace SawCut.Nesting
         public MultiBinEngine(IEngineFactory engineFactory)
         {
             _engineFactory = engineFactory ?? throw new ArgumentNullException(nameof(engineFactory));
+            _bins = new List<MultiBin>();
         }
 
-        public List<MultiBin> Bins { get; set; }
+        private readonly List<MultiBin> _bins;
+
+        /// <summary>
+        /// Gets the read-only collection of bins.
+        /// Use SetBins() to configure bins for packing.
+        /// </summary>
+        public IReadOnlyList<MultiBin> Bins => _bins.AsReadOnly();
+
+        /// <summary>
+        /// Sets the bins to use for packing.
+        /// </summary>
+        public void SetBins(IEnumerable<MultiBin> bins)
+        {
+            _bins.Clear();
+            if (bins != null)
+            {
+                _bins.AddRange(bins);
+            }
+        }
 
         public double Spacing { get; set; }
 
         public Result Pack(List<BinItem> items)
         {
-            var bins = Bins
+            var bins = _bins
                 .Where(b => b.Length > 0)
                 .OrderBy(b => b.Priority)
                 .ThenBy(b => b.Length)