Abstract: A method for forming a plurality of slits in a tube includes first, forming a plurality of grooves in the pipe by operating a grooving punch along the length of each of the slits to be made. Then, each slit is formed in the pipe by operating a hole-making punch. The punch has ends at an acute angle and the punch is recessed from the ends toward the center thereof. In view of the fact that each groove is formed beforehand in the surface of the pipe and that the corresponding slit is made while moving the hole-making punch slowly from the ends, a large load is not exerted on the pipe and therefore the pipe is not crushed, burrs are not generated and the cutting chip is removed.

Abstract: First a plurality of grooves is formed by operating a grooving punch along the length of each of slits to be made. Then, each slit is formed in the pipe by operating a hole-making punch having the ends at an acute angle and, recessed from the ends toward the center thereof. In view of the fact that each groove is formed beforehand in the surface of the pipe to be machined and that the corresponding slit is made while moving the cutting positions of the hole-making punch slowly from the ends, a large load is not exerted on the pipe at one tire and therefore the pipe is not crushed, burrs are not generated and the cutting chip is removed.

Abstract: A heat exchanger has plural flat tubes through which refrigerant flows, and a pair of header tanks disposed on each longitudinal ends of the flat tubes. Each of the flat tubes has a flow passage portion having plural flow passages through which refrigerant flows, and a non-flow passage portion disposed on both sides of the flow passage portion, having at least one non-flow passage through which no refrigerant flows. The flow passage portion is inserted into the header tank so that the flow passages communicate with the header tank, and the non-flow passage portion is exposed outside the header tank. Each of the flow passages has a circular-shaped cross-section while the non-flow passage has a polygonal-shaped cross-section, so that a wall thickness of the non-flow passage is made thinner than that of the flow passages. Therefore, weight of the flat tube is decreased, while sufficient strength thereof is maintained.