Two-piece mandrel assembly for deforming

A hole expansion mandrel assembly for use in expanding a pilot hole with a prescribed hole diameter through a work piece comprising:

• an elongate drive pin having a support central axis, and including a pulling section adjacent the leading end of said drive pin and an annular driving shoulder rearwardly of said pulling section, said pulling section having a first outside diameter a prescribed amount smaller than the hole diameter and said shoulder defining an annular driving face on that side facing the leading end of said drive pin having a second outside diameter a second prescribed amount larger than said first outside diameter;
• and,
• a seamless annular expansion collet defining a central passage therethrough about a collet central axis and an outside expansion surface about said collet central axis that tapers outwardly from the leading end of said collet, said central passage having a third inside diameter at least as large as said first diameter of said pulling section and smaller than said second outside diameter of said driving face, said expansion surface having a fourth minor outside diameter smaller than the hole diameter and a fifth major outside diameter larger than said second outside diameter of said driving face and larger than the hole diameter by a fifth prescribed amount, said collet further defining an annular abutting surface on the trailing end thereof, said collet slidably received over said pulling section with said abutting surface engaged by said driving face of said shoulder so that said pulling section can be inserted through the hole, leading end, first, until said expansion surface on said collet engages one side of the work piece about the hole and the pulling section can be gripped from the opposite side of the work piece to force the collet through the hole without significantly changing the size of said collet to expand the hole.