An employee was conducting preventative maintenance on an extrusion press and using an impact wrench to remove rivets on the shear cylinder. When the last rivet was removed, the stored energy in the cylinder released and dropped it onto the employee's hand. Their right thumb was caught between the cylinder and the casing of the rivet gun, resulting in crushing injuries. The employee sustained an amputation of the thumb without bone loss. The machine was locked out/tagged out at the time.