Abstract The normal contact forces in a vibratory finishing machine were measured and compared with the resulting changes in surface roughness and hardness of two aluminum alloys, AA1100-O and AA6061-T6. The principal variables were the media size, degree of lubrication and the duration of the vibratory finishing. The changes in hardness and roughness were found to depend mostly on the lubrication condition, the media roughness, and the size of the media, since these influenced the interaction between the media and the workpiece, and hence the extent of plastic surface deformation per impact. The impact force parameters such as the average force, maximum force, and impulse, however, did not vary appreciably amongst the three media for dry and water-wet conditions. Thus, the differences observed in hardness and roughness were due to smaller scale differences in the impact contact conditions. On average, a sensing disk with a diameter approximately equal to that of the media was in contact with media for approximately 30% of the total finishing time. This was consistent with videotaped observations showing that the media was loosely-packed as it flowed past the workpiece, with relatively large gaps in the packing near the workpiece surface.