BENDING METHOD IN HOT BENDER AND HOT BENDER

PROBLEM TO BE SOLVED: To provide a method capable of controlling a higher precise wall thinning rate about small radius bending by a hot bender such as a high frequency bender. SOLUTION: An annular heating part wherein bending is generated is divided into plural areas in its circumferential direction, and a temperature distribution about these respective areas is formed. For this purpose, an angular position βof a neutral axis about bending is obtained using not only deformation resistance correlating with a heating temperature but also the variation of thickness, a thrust and a compressing force as an important matter. And th division of the area (three divisions of e.g. -90 deg. to θ2 , θ2 to θ3 and θ3 to +90 deg.) is executed in the β position so that the temperature distribution for maintaining the neutral axis is obtd., and the temperatures T1 , T2 , T3 in these respective areas are set. By this method, the neutral axis is supposed with high precision. Consequently, bending with higher precision is efficiently executed.