An apparatus for manufacturing heat transfer tubes for use in evaporators and coolers for freezers, air conditioning units, refrigerators, and the like, with close machining tolerances. It forms cavities, with small openings communicating them with the outside, on the inside of a tube, through three steps of forming a multiplicity of alternate grooves and ribs in parallel on the inner surface of the tube, forming cuts in the concentrically directed crests of the ribs, and then forcing the crests sidewise so that the crest of each rib is bent down onto an intermediate part of the adjacent rib to form a cavity in between.