PURPOSE: To detect easily the generation of thickness deviation due to the off- center of a plug by detecting a thrust load acting on the rolling plug of mill during rolling, and judging the generating condition of the thickness deviation of a rolling blank pipe basing on the detected results.
CONSTITUTION: A thrust load is detected by a load cell 9 incorporated in a thrust block, and its signal is amplified by an amplifier 10, then the load signal is analyzed by a signal analyzing device 11 to display the presence or absence of the generation of thickness deviation and the degree of generation. The thrust load signal amplified by the amplifier 10 is displayed by the wave form as shown by (A) or (B). (A) shows a wave form at the time of no off-center of plug, (B) shows that of the off-center. A temporary increase of the load during rolling is caused by an unstationary rolling at the time of biting by rolls. The device 11 obtains the amount of load by signal analyzing.