Chuck master jaw and false jaw construction

A jaw construction for a chuck body that is rotatable about an axis extending in an axial direction, comprising a master jaw connected to the body for movement in a radial direction with respect to the axis and having an axial outer side with a fixed mushroom-shaped axial extension having a shank with a curved inner stop surface and an outer locking head of larger radial dimension than said shank, said extension having a radially extending relief shoulder defined between said shank and said locking head at opposite radial ends of said extension and around said shank, a false jaw having an axial inner wall engaged with said axial outer side and having a recess shaped to receive said extension by insertion in the axial direction and into which said extension extends when said false jaw is engaged with said master jaw, said recess of said false jaw having a curved mating surface and an inner groove into which said locking head engages with said curved stop surface engaging on said curved mating surface, said shoulder on one radial end of said extension engaging over a radially extending side of the groove in a crescent-shaped area extending perpendicular to the radial and axial directions of the chuck body, anti-rotation means engaged with said false jaw for preventing rotation thereof with respect to said master jaw, and radial fixing means engaged between said extension and said false jaw in said recess of said false jaw to hold said false jaw in a fixed radial position on said main jaw.