Engineers and designers may notice an increased productivity in their work using the API (Application Programming Interface) of a general purpose CAD system. Using this API, macros as well as complete applications can be easily developed. The current paper presents an application, called Cylin3Der, that makes use of these capabilities. The application deals with the automation of the design process of a pneumatic double acting cylinder. The aim is to provide to the user a simple form that will act as a starting point for the cylinder design process, based on a set of parameters that follow the relevant ISO 15552 standard. The design process is supported by a group of tools incorporated. These tools deal with: the creation of the components (solid models) of the cylinder, the assembly of the finished product, rendering and finally the creation of the various related documents.