A radial compression mechanism for compressing devices such as stents and prosthetic heart valves, the mechanism comprising: a) a base (30, 30'); b) a plurality of compression dies arranged in a circular array about a central axis; each comprising: i) an inward facing working surface; and ii) an outer surface; wherein said plurality of compression dies form a central cylindrical cavity, defined by the working surfaces and an outer perimeter along the outer surfaces of the plurality of compression dies; c) a string wrapped two or more revolutions about the outer perimeter of the plurality of compression dies that contacts each of the outer surfaces of the plurality of compression dies; wherein the plurality of compression dies are coupled to said base (30, 30') and configured to move in unison from an open position, wherein the central cylindrical cavity has an open cavity diameter, radially inward to a closed position wherein the cylindrical cavity has a closed cavity diameter; wherein pulling of the string forces the plurality of compression dies to move in unison from said open position to said closed position; and wherein said open cavity diameter is larger than said closed cavity diameter; wherein the cylindrical cavity has a substantially continuous surface having no more than a 500 μm gap between adjacent die working surfaces from said open and closed position. a) a base (30, 30'); b) a plurality of compression dies arranged in a circular array about a central axis; each comprising: i) an inward facing working surface; and ii) an outer surface; wherein said plurality of compression dies form a central cylindrical cavity, defined by the working surfaces and an outer perimeter along the outer surfaces of the plurality of compression dies; i) an inward facing working surface; and ii) an outer surface; c) a string wrapped two or more revolutions about the outer perimeter of the plurality of compression dies that contacts each of the outer surfaces of the plurality of compression dies; wherein the plurality of compression dies are coupled to said base (30, 30') and configured to move in unison from an open position, wherein the central cylindrical cavity has an open cavity diameter, radially inward to a closed position wherein the cylindrical cavity has a closed cavity diameter; wherein pulling of the string forces the plurality of compression dies to move in unison from said open position to said closed position; and wherein said open cavity diameter is larger than said closed cavity diameter; wherein the cylindrical cavity has a substantially continuous surface having no more than a 500 μm gap between adjacent die working surfaces from said open and closed position. wherein the plurality of compression dies are coupled to said base (30, 30') and configured to move in unison from an open position, wherein the central cylindrical cavity has an open cavity diameter, radially inward to a closed position wherein the cylindrical cavity has a closed cavity diameter; wherein pulling of the string forces the plurality of compression dies to move in unison from said open position to said closed position; and wherein said open cavity diameter is larger than said closed cavity diameter; wherein the cylindrical cavity has a substantially continuous surface having no more than a 500 μm gap between adjacent die working surfaces from said open and closed position.