This paper describes a mathematical model for calculating and analyzing the effects of some factors on the economics of the existing automatic assembly systems and those might be developed in the future. Results have been analyzed and compared with each other. Useful conclusions have been obtained and represent that the development of new grippers and feeders have very great benefit to the application of robots in the assembly systems. The conclusions of this work have good agreement with an experimental case study made to assemble a 24-piece gear box.