The main aspect to be considered when stitching FRP semi-finished material is the handling of geometrically more complicated, usually threedimensional structures with simultaneous large spatial expansion. Technically the stitching of these types of structures is difficult if conventional stitch types are applied. For instance the two thread lockstitch requires access to the work piece from both sides. A double lock stitch head and a typical application for a double lock stitch are shown in figures 2 and 3.The existing limits of the well-known stitching techniques can be overcome by a technology where access to the work piece is only necessary from one side.

The ALTIN Nahtechnik has developed two separate stitching technologies, the one side stitching system, known as the OSS system and a tufting system.

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Figure 2 : Double Lock Stitch Head in Operation

The core of this system is a robot-guided special stitching head (see fig. 1). Its stitch formation mechanism is based on the principle of the simple chain stitch. This stitch is produced by two stitching tools. Both tools are manipulated from the top side of the work piece, so that it is not necessary to arrange any stitch formation element underneath the work piece. Only free space for penetration of the needles has to be taken into consideration.

The inserting angles of the stitching tools are 45° and 90°. Although the operational principle permits a variation of these angles within certain limits, these angles were determined to be optimal. The reason is that the stitching angle determines the final position of the stitching thread in the FRP structure.