The eccentricity of the punch and moving die with the number of strikes for v=5 mm/s is shown in Fig. 3. A displacement sensor was used to measure the movement of the moving die for ..=180°. Initially, the center axis of the punch and moving die is not the same. For n=1, the center axis of the moving die is 7 ไมโครเมตร away from the center axis of the punch. As the n is increases, the moving die moves and therefore, the gap between the center axis of the punch and die is steadily decreased. The punch and moving die reached the point of concentric after n=3 and above. The gap between the center axis of the punch and moving die was measured for less than 1 ไมโครเมตร at this point. For the further increase of number of strikes, the eccentricity of die to the punch remains stable on a low level.
This is shown by the pattern of variation of rollover depth for n=8 and above for the measured angle of มุม=0 and 180°, as in Fig. 4.