Many of the processes used for fabricating and assembling aircraft structures have become automated. This is the case with the installation of fasteners in aircraft wing panels wherein a multifunction machine accomplishes the task of drilling holes and installing the fasteners. One such machine, known as the "Gemcor," is manufactured by the General Electro-Mechanical Corporation of Buffalo, New York. The "Gemcor" utilizes a transfer head that selectively positions tools mounted thereon over the wing panels to accomplish the sequence of steps required to install a fastener. In a typical installation operation, the transfer head positions a motorized drill over the wing and a hole is drilled through the wing skin. The transfer head then removes the motorized drill and engages a rivet tool with a large hydraulically-operated cylinder on the machine. In operation, the rivet tool first picks up a rivet and the hydraulic cylinder then causes the rivet tool to push the rivet into the hole. A bucking cylinder is positioned on the reverse side of the wing to form the shop head on the rivet. Finally, the transfer head disengages the rivet tool from the hydraulic cylinder and positions a shaver over the rivet tool to trim the rivet head on the top side of the wing.
Although the "Gemcor" machine functions well to install rivets, it is unsuitable for installing bolts and other similar fasteners where coldworking of the hole is required to resist localized stress concentrations. Briefly, coldworking is the process of plastically deforming metal at a temperature below its annealing point in order to increase its hardness and tensile strength. Several methods can be used for coldworking the fastener holes. One method pushes a shaft having an enlarged diameter portion (commonly referred to as a mandrel) through a hole from one directions to radially expand the hole and compress the region surrounding it to thereby achieve the desired strain hardening. Another method involves lining the hole with a sleeve and pulling a mandrel through the sleeve to expand and thereby coldwork the hole. Yet another method includes inserting a collapsible mandrel (also known as a split mandrel) in collapsed form through the fastener hole, expanding the mandrel, and then pulling the mandrel back through the hole to coldwork the hole.
Attempts to use a solid mandrel coldworking tool on the Gemcor have been unsuccessful. The actuating mechanism of the Gemcor is located on the top side of the wing panel, and thus, the mandrel must first be pushed through the fastener hole from the top side of the wing panel and then pulled back through the hole. This leaves a protruding ridge of metal on the reverse side of the hole that causes premature weakening of the metal around the hole when the wing panel is under stress.
The use of a sleeved mandrel has also proven unsatisfactory because of the difficulty in automatically placing sleeves over the mandrel prior to coldworking and then removing the sleeve out of the hole after coldworking. In addition, the metal sleeves are costly to supply in large numbers.
While the split mandrel coldworking tool is ideally suited for this application, there is no known mechanism that permits it to be used on the Gemcor machine. The present invention solves this long standing problem and fulfills a significant need in the industry.