diff --git a/FlatCAMObj.py b/FlatCAMObj.py
index 1d6e7754..6599f873 100644
--- a/FlatCAMObj.py
+++ b/FlatCAMObj.py
@@ -729,20 +729,20 @@ class FlatCAMExcellon(FlatCAMObj, Excellon):
                     exc_final.drills.append({"point": point, "tool": drill['tool']})
                 toolsrework=dict()
                 max_numeric_tool=0
-                for toolname in list(exc.tools.keys()):
+                for toolname in list(exc.tools.copy().keys()):
                     numeric_tool=int(toolname)
                     if numeric_tool>max_numeric_tool:
                         max_numeric_tool=numeric_tool
                     toolsrework[exc.tools[toolname]['C']]=toolname
 
                 #exc_final as last because names from final tools will be used
-                for toolname in list(exc_final.tools.keys()):
+                for toolname in list(exc_final.tools.copy().keys()):
                     numeric_tool=int(toolname)
                     if numeric_tool>max_numeric_tool:
                         max_numeric_tool=numeric_tool
                     toolsrework[exc_final.tools[toolname]['C']]=toolname
 
-                for toolvalues in list(toolsrework.keys()):
+                for toolvalues in list(toolsrework.copy().keys()):
                     if toolsrework[toolvalues] in exc_final.tools:
                         if exc_final.tools[toolsrework[toolvalues]]!={"C": toolvalues}:
                             exc_final.tools[str(max_numeric_tool+1)]={"C": toolvalues}