C-shaped frames of the above mentioned kind are previously known and are used for joining of for instance car body components, frame works, train cars, air crafts, etc. They are made as I-shaped section in high tensile steel since high requirements are put on the stiffness and the structural strength of the frame. Particularly when working operations, such as e.g. riveting, is in question, large opening forces arise on the shanks of the C-shaped frame. The frame is also to be exposed to a large number of working cycles and a minimum of 2 millions cycles is a demand. The C-frame must be able to operate on deeply situated parts in certain situations, i.e, be able to reach a working area which is situated e.g. 1 meter inside an outer limitation. At large scale production in the automotive industry the C-shaped tool holder is being handled by a robot, which means that the weight of the C-frame should be so low that the total weight of the system is less than the allowed operation weight of the robot. At manual operation it is also desired to minimize the mass of the system.
Using the robots of today, one has reached and in some cases also exceeded the weight limit, but the market demands both larger and more stable tool holders with maintained or even lowered weight. The investments in robots are so high that it must be possible to use existing ones even for new and more extensive working operations.
Tests have been made to produce C-shaped frames from composite materials as solid models but the load strains in the intersections in the form, i.a., of shear and tear stresses in the glue joints will become so high, that these tests could not be performed. A typical steel C-frame, having an inner depth of 850 mm, a gap between the ends of the shanks of 400 mm and a calculated load in the form of opposed directed forces of 53 kN, has a weight of about 150 kg at a maximal permitted deflection of 7 mm between the ends of the shanks. This weight together with the weight of the necessary tool equipment exceed the carrying and operating capability of the robot and are therefore not acceptable.