PURPOSE: To control slipping or rubbing on a contact part of a working roll and a back-up roll by arranging rolls having two different diameters in the longitudinal direction of the shaft in series as a 2nd roll and also making a 1st roll into a large diameter part and a small diameter part to correspond to the two rolls having different diameter.
CONSTITUTION: A tube to be worked 1 fed in a working roll 14 is applied with a vertical pressing force with a large diameter part 14A of the working roll being made in contact with a back-up roll 15. In this time, a working reaction force to incline the pressing direction caused by the plastic deforming amount of the longitudinal direction of the tube to be worked 1 is generated on the large diameter part 14A of the working roll, but it is made in contact with a small diameter part 14C of the working roll, the pressing direction of the working roll 14 is vertically acted against the longitudinal direction of the tube to be worked 1 with the back-up roll 16 having an outer diameter larger than the back-up roll 15. Accordingly, the working roll is not inclined with the working reaction force of the tube to be worked 1, spiral projecting parts 9a of groove cutting plug 9 is stably pressed into the inner wall of the tube to be worked 1.
SUZUKI YOSHIO
KAGAWA MANABU
YOKOMIZO KENJI
