Riveting method expanding a work counterbore

In the method of securing a rivet slug to work containing a bore, a substantially cylindrical first counterbore that opens to one side of the work, and a second counterbore which is frusto-conical and tapers forwardly between the first counterbore and the bore, the steps that include,

• a. inserting the slug to extend within the bore and counterbores, and
• b. deforming the slug axially and radially to fill the bore and counterbores and to permanently expand the first counterbore, said deformation being carried out to radially expand that portion of the slug within the first counterbore to substantially greater extent than that portion of the slug within said bore,
• c. the slug prior to said deformation having a shoulder axially facing the second counterbore, and said insertion step being carried out to locate said shoulder against the second counterbore, and to locate opposite ends of the slug projecting outwardly from opposite sides of the work, prior to said deformation.