PURPOSE: To manufacture a tube having an inside spiral groove high in dimensional accuracy by using a skew rolling mill having a plug provided with a spiral groove, carrying out skew hot rolling of a metallic tube stock, removing the oxide and carrying out skew cold rolling again.
CONSTITUTION: Suitable skew angles to rollers 1, 2, 3 and crossed axis angles between them are set respectively and while they are rotated and driven in the same direction, heated metallic tube stocks 7 are supplied, bit into the rollers 1, 2, 3 and at the same time a plug 4 is inserted into a core hole 7a. The metallic tube stock 7 is charged by the pressure of the rollers 1, 2, 3 in the spiral groove 4a to make a metallic tube stock 8 having an inside spiral groove. After the bend of this stock is straightened by a straightening machine and the metallic oxide is removed by pickling and shot peening, a skew cold rolling finish is performed again. The inside spiral grooved metallic tube having a spiral groove high in dimensional accuracy and good in shape can be manufac tured efficiently.
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