Abstract: An orbital tool assembly for forming rivet heads has a housing attached to a driven member rotatable upon a first axis, the housing having a bore along a second axis at an acute angle to the first axis. A head having an axis coincident with the second axis and including a fluid pressure chamber is retained within the housing with bearings interposed. A coaxial driver on the head has a plurality of bores parallel to the second axis in communication with the pressure chamber, and mounts a reciprocal peening tool holder in each bore, supporting an axial peening tool. An anti-rotation arm at one end extends at right angles to the driver and is secured thereto with its other end engaging a bracket, the arm being free for rocking movements in a plane passing through the first and second axes. Rotation of the head oscillates the peening tools in the rocking plane, the peening tools being in operative engagement with unheaded rivets.