Abstract: Study on crack causes of the large L /D ratio case mandrel. The measures of avoid crack have been constituted. The better effects have been obtained. The leading conclusions are enumerated as follows. They are favorable to reduce the mandrel's weight and to improve the mandrel's rigidity that the mandrel's framework adopts the truss structure,and the parts and the assemblies adopt the hollowly structure of rib and lightweight material. That use improving plaster on the mandrel's surface,the ability of the mandrel avoiding transmutation is toned up; the cracks do not appear easily.

Key words: large L/D ratio, case, mandrel, prevent crack, technology