A method of manufacturing an ultrasonic flow meter flow tube , wherein the flow tube comprises transducer holders , each transducer holder comprises an opening and a transducer seat for mounting of a transducer housing , and each transducer holder is forming one part of a pair of oppositely placed transducer holders , characterized in that the method comprises the following steps: - providing a transducer seat-machining tool for machining the transducer seat of one of the pair of transducer holders , the transducer seat-machining tool comprising a tool head mechanically connectable to a guide shaft , - mechanically connecting the tool head and the guide shaft , - mechanically guiding the tool head by a part of the transducer seat-machining tool engaging with the transducer holder opposite placed to the transducer holder to be machined, - machining the transducer seat of the transducer holder to be machined with the tool head ; each of the pair of transducer holders comprises a guide hole , and the tool head comprises a guide bushing , the method further comprising the following steps: - guiding the tool head of the transducer seat-machining tool , ensuring a surface of the transducer seat is perpendicular to a coaxial axis , by ∘ positioning the guide bushing in the guide hole of the transducer holder to be machined, and ∘ positioning the guide shaft or the guide bushing in a position engaging the guide hole of the opposite placed transducer holder . characterized in that the method comprises the following steps: - providing a transducer seat-machining tool for machining the transducer seat of one of the pair of transducer holders , the transducer seat-machining tool comprising a tool head mechanically connectable to a guide shaft , - mechanically connecting the tool head and the guide shaft , - mechanically guiding the tool head by a part of the transducer seat-machining tool engaging with the transducer holder opposite placed to the transducer holder to be machined, - machining the transducer seat of the transducer holder to be machined with the tool head ; each of the pair of transducer holders comprises a guide hole , and the tool head comprises a guide bushing , the method further comprising the following steps: - providing a transducer seat-machining tool for machining the transducer seat of one of the pair of transducer holders , the transducer seat-machining tool comprising a tool head mechanically connectable to a guide shaft , - mechanically connecting the tool head and the guide shaft , - mechanically guiding the tool head by a part of the transducer seat-machining tool engaging with the transducer holder opposite placed to the transducer holder to be machined, - machining the transducer seat of the transducer holder to be machined with the tool head ; each of the pair of transducer holders comprises a guide hole , and the tool head comprises a guide bushing , the method further comprising the following steps: - guiding the tool head of the transducer seat-machining tool , ensuring a surface of the transducer seat is perpendicular to a coaxial axis , by ∘ positioning the guide bushing in the guide hole of the transducer holder to be machined, and ∘ positioning the guide shaft or the guide bushing in a position engaging the guide hole of the opposite placed transducer holder . ∘ positioning the guide bushing in the guide hole of the transducer holder to be machined, and ∘ positioning the guide shaft or the guide bushing in a position engaging the guide hole of the opposite placed transducer holder . <EOS>