thus an especially exact and fast measurement can be carried out by means of the first measuring device 8 the friction welding device 1 according to fig1 preferably has a second measuring device 13 this measuring device may also be designed for measuring in a contactless manner and preferably be arranged permanently on the friction welding device 1 the second measuring device 13 may have any design preferably identical to the first measuring device 8 and have one or more sensors 23 especially a light sensor it may also emit a measuring beam 14 especially laser beam onto the front surface and the welding area 3 of the first workpiece part 2 the second measuring device 13 may also be designed and or arranged in an adjustable manner the second clamping device 7 is preferably displaceably mounted in the direction of the axis of rotation 19 it thus preferably forms the pressure generator for the plasticizing device and the pressing device as an alternative the second clamping device 7 may be arranged in a fixed manner while a displaceability of the first clamping device 6 or of both clamping devices is provided a controllable feeding device 21 is provided for this displaceable relative movement of the clamping device s 6 7 this feeding device may be designed e g as a hydraulic unit especially as a hydraulic cylinder and may be supported e g on the machine frame the friction welding device 1 preferably has a positioning device 15 which is arranged on.