A double-sided machine tool, in particular a double-sided grinding machine, comprising a machine for determining the position of a working surface of a working disc of the double-sided machine tool wherein the double-sided machine tool has two working discs (12, 14) which between working surfaces facing to each other form a working gap for the double-sided processing of flat work pieces and of which at least one can be rotationally driven, wherein the device comprises an optical measurement device with a radiation source, an optical detector device and an analysis device which are designed to be arranged outside of the working gap, and the double-sided machine tool comprises a control device which, on the basis of the position of the working surface (16, 18) determined by the analysis device, adjusts the position of the working surface (16, 18) and/or the width of the working gap to a constant value, characterized in that - the device has a beam guiding device which is designed to be at least partially guided into the working gap, wherein the beam guiding device has an exit opening located in the working gap for optical radiation generated by the radiation source when said beam guiding device has been guided into the working gap, and wherein the beam guiding device guides the optical radiation proceeding from the radiation source first essentially parallel to the working surfaces (16, 18) into the working gap, deflecting said radiation at a measuring site via at least one deflecting surface to a working surface (16, 18), receives radiation reflected from the working surface (16, 18) and guides it back from the working gap out to the optical detector device, wherein the analysis device is designed to determine the position of the working surface (16, 18) by means of a detection result of the detector device, - the beam guiding device has a fluid inlet connected to a fluid supply, with which at least one measurement area permeated by the optical radiation after exiting the exit opening of the beam guiding device in the working gap can be flushed. - the device has a beam guiding device which is designed to be at least partially guided into the working gap, wherein the beam guiding device has an exit opening located in the working gap for optical radiation generated by the radiation source when said beam guiding device has been guided into the working gap, and wherein the beam guiding device guides the optical radiation proceeding from the radiation source first essentially parallel to the working surfaces (16, 18) into the working gap, deflecting said radiation at a measuring site via at least one deflecting surface to a working surface (16, 18), receives radiation reflected from the working surface (16, 18) and guides it back from the working gap out to the optical detector device, wherein the analysis device is designed to determine the position of the working surface (16, 18) by means of a detection result of the detector device, - the beam guiding device has a fluid inlet connected to a fluid supply, with which at least one measurement area permeated by the optical radiation after exiting the exit opening of the beam guiding device in the working gap can be flushed.