A blank tube 30 is supported on a grooved mandrel 24 and relative rotation between the tube and disc rollers 54 causes a plurality of vertical helical fins to be formed in the outer wall of the tube, and a plurality of helical ribs 33 to be formed in the internal wall of the tube. Further engagement between the finned tube and disc rollers 56 causes the vertical fins to be bent over in incremental steps to form subsurface channels. The disc rollers 56 have spacers therebetween and each has an angled contact face which varies in inclination along the length of the disc roller cluster. IMAGE
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