Method of fabricating a heat exchanger tube

A method for the manufacture of a metallic heat transfer tube, the steps comprising:

• a) supporting a plain tube on a mandrel oriented inside thereof;
• b) roll forming helically extending and radially outwardly extending fins so that flank surfaces on mutually adjacent fins are axially spaced from one another to define a primary groove therebetween having a bottom wall laterally bordered by roots providing a transition surface between the bottom wall and the respective flank surfaces;
• and
• c) roll forming the mutually facing flank surfaces and the roots bordering the bottom wall into a reformed bottom wall having a reformed juncture with the mutually adjacent fins and at least one re-entrant secondary groove having a width less than an axial spacing between the junctures between the reformed bottom wall and the mutually adjacent fins.