Abstract: A method and apparatus for interlocking at least one first tube (10) having a first diametric spacing between opposite points of the tube wall with a second tube (4) having a second diametric spacing between opposite points of the tube wall which is larger than the first diametric spacing. The first tube is loaded over a mandrel (50) having a punching end (54), and disposed into a holding block (30) having a holding cavity (32) conforming closely to the outer surface of the first tube. The second tube is disposed over a receiving block (40), which has a receiving cavity conforming closely to the outer surface of the first tube. The punching end of the mandrel is driven through the second tube and into the cavity in the receiving block, followed by the first tube. When the first tube reaches a lower limit of the receiving cavity, the first tube is compressed axially.