Abstract: Through synthesizing the menu script, user interface dialog and API programs, the parts model of CST (Cutting the Screw Thread) is automatically created in UG NX, using Visual C++ to call the functions of UG/OPEN API. The instance shows that the developing module has good correlation and can change exactly as the user inputs different parameters. The function of creating CST parts model in UG is achieved.

Key words: computer application, parametric model, second development of UG, CST, expressions