Patent ID: 8302448

Claim:
A method for cold expansion of a hole formed in a work piece, the method comprising: providing a body comprising at least a cylindrically shaped part having an inner hole in an axial direction of the body, wherein the body further comprises along a circumferential direction of the body a plurality of members which are movable in a radial direction of the body, such that an outer diameter of the cylindrically shaped part of the body can be changed; introducing the body into the hole formed in the work piece; inserting a core into the inner hole of the cylindrically shaped part, wherein at least one of the core or the inner hole comprises a portion having a changing diameter and the inner hole and the core get into contact with respect to each other upon insertion of the core, such that in dependency of an insertion depth of the core into the inner hole in the axial direction the members move in the radial direction outwardly and the outer diameter of the cylindrically shaped part of the body increases and, accordingly, acts on an inner circumference of the hole formed in the work piece so that a boundary zone of the hole is compressed, which leads to a compression or hardening and, accordingly, to an introduction of compressive stress in the boundary zone, wherein the inner hole and the core have a polygonal cross section in a direction perpendicular to a longitudinal axis of the core, and wherein the hole formed in the work piece is expanded over an entire thickness of the work piece in which the hole is formed.