The clamping members are urged together by the same fluid pressure; and to this end, one of the clamping members is mushroom-shaped with the head of the mushroom comprising...http://www.google.com/patents/US3890819?utm_source=gb-gplus-sharePatent US3890819 - Apparatus for deforming sheet metal blanks by fluid pressureAdvanced Patent SearchPublication numberUS3890819 APublication typeGrantPublicati