Grinding gap geometry based on setup conditions and its effect on roundness generation is a complex phenomenon in through-feed centerless grinding process. The objective of this paper is to introduce a suitable solution for the use of simulation in through-feed centerless grinding, to assist in the selection of acceptable set-up conditions for centerless grinding and their effect on roundness quality. A qualitative assessment of geometrical influence parameters is carried out by means of a kinematics simulation based on a non-linear 3-dimensional model of the centerless grinding process. The novel aspect of the model analysis is the determination of the grinding gap with two new decisive parameters in centerless through-feed grinding and their effect on geometrical rounding stability. The ratio of the finishing to roughing length and ratio of the finishing to roughing material removal rate are crucial to minimize roundness errors.