Self-drilling screw

A self-drilling screw comprising:

• a shank including a self-drilling portion, a head portion spaced from the self-drilling portion along a longitudinal axis, and a straight rod portion between the self-drilling portion and the head portion, with the self-drilling portion having a tip and two cutting edge portions extending toward the head portion from the tip, with the straight rod portion including a lower segment adjacent to the self-drilling portion and an upper segment adjacent to the head portion;
• a first thread spirally formed on the lower segment of the straight rod portion and including a plurality of thread convolutions, with each thread convolution having a lower cutting face facing the self-drilling portion, an upper cutting face facing the head portion, and a cutting line interconnecting the lower and upper cutting faces, with each of the plurality of thread convolutions provided with three to eight recess portions spaced in a circumferential direction of the straight portion, with each recess portion recessed inward from the cutting line in a radial direction of the straight rod portion and having a recess length in the circumferential direction of the straight portion, with a screw thread portion defined between two adjacent recess portions in the circumferential direction and having a screw thread length in the circumferential direction of the straight portion, with the screw thread length being 25% to 200% of the recess length, wherein the recess portions in the plurality of thread convolutions are arranged circumferentially offset with respect to the longitudinal axis forming a helical pattern in a same direction as the first thread;
• and
• a second thread spirally developed on the upper segment of the straight rod portion and having an outer diameter greater than an outer diameter of the first thread, with an upward spiral direction of the second thread being opposite to an upward spiral direction of the first thread,
• wherein the plurality of thread convolutions include a bottommost thread convolution directly adjacent the self-drilling portion, a topmost thread convolution adjacent the upper segment of the straight rod portion, and several middle thread convolutions between the bottommost and topmost thread convolutions,
• wherein when the self-drilling portion is driven into an article, chippings cut by the self-drilling portion are moved upward to the upper segment of the straight rod portion from the recess portions in the bottommost thread convolution through the recess portions in the middle thread convolutions and the recess portions in the topmost thread convolution.