Forming defects, roll and warp, in hemming of mild steel sheets with flat surface-convex edge geometry are experimentally investigated. The objectives in this study were to develop fundamental knowledge of the effects of process parameters on the final hem quality (defects including roll-in/out, recoil and warp without wrinkling and hem-out) and to develop improved process and tool design rules for flanging and hemming based on experimental findings. The experiments were designed using half a fractional factorial array with four 2-level factors replicated three times. All main effects were determined as well as non-alias 2-level interactions. Results of the experiments were summarized for the use of process and die designers. (C) 2004 Elsevier B. V. All rights reserved.