Abstract: This paper analyzes part's geometry and forces of unchamfer peg in hole in detail. The mathematical expressions of position and orientation error between unchamfer peg and hole in the situation of onepoint contact and two-point contact are calculated. The mechanical equations of resistance and jamming,as well as the resistance and jamming chart and the terms of avoided resistance and jamming are also given in this paper. The mathematical models and restrain terms given in this paper are the theoritical basis for forcemotion planning in robot assembly tasks.