Device at tool holder (10) of the kind comprising an essentially C-shaped frame preferably having a long inner range, which frame at its free shank ends carries said tool, for instance a riveting, a glueing or a welding unit. The holder (10) comprises a truss or frame work, which includes an outer and an inner C-shaped frame (11, 23) and a number of slewing brackets (27 32) connected to these. The frames (11, 23) and the slewing brackets (27 32) are arranged to form a number of connected triangular sections, which connection points (18 22, 37) are designed as intersections or joints. The outer C-shaped frame (11) is arranged only to support thrust forces and the inner frame (23) thrust and tensile forces and the slewing brackets (27 32) substantially tensile forces.