Therefore , it is necessary to acquire the cad model by conventional engineering or reverse engineering . reverse engineering of physical objects refers to the process of creating engineering design data ( including the cad model ) from existing parts by using many digitized technologies and cad technology 在没有产品cad模型和没有设计图纸或者设计图纸不完整的情况下,按照现有产品样件,利用各种数字化技术及cad技术重构其cad模型的过程,被称为实物反求工程。