Reverse Engineering     Physical Model to CAD model.   CAD models are often unavailable or unusable for parts, which must be duplicated or modified. This is a particular problem for long life cycle systems for which spare part inventories have been exhausted and original suppliers are unable or unwilling to provide custom manufacturing runs of spare parts at affordable prices and in a timely manner. For many parts, either the CAD system was not used in the original design or the documentation on the original design is otherwise inadequate or unavailable. For a variety of reasons, CAD models, even when they exist, may not be sufficient to support modification or manufacturing using modern methods. Finally shop floor changes to the original design may mean that the original CAD models no longer accurately reflect the geometry of the part. Reverse engineering techniques can be used to create CAD models of a part based on sensed data acquired using three dimensional position digitization techniques.