Patent Publication Number: US-2023152079-A1

Title: Method and device for determining centers of a hollow shaft rotatably clamped as a workpiece in a machine tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase Application under 35 U.S.C 371 of International Application No. PCT/DE2021/000071, filed on Apr. 8, 2021, which claims the benefit of German Patent Application No. 10 2020 002 421.1, filed on Apr. 22, 2020. The entire disclosures of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     The disclosure relates to a technical solution for determining centers and the spatial course of these centers of a hollow shaft rotatably clamped as a workpiece in a machine tool, which is machined on its outer surface at least in sections. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     It is known to determine centers for bores in a workpiece and to determine the course of the respective bores or centers on the basis of two or more bore centers. For example, when machining longer hollow shafts to accommodate turbine blades, it is necessary to align the averaged axis of the inner cavity with the axis of rotation of the machine tool rotating the hollow shaft. For this purpose, the inner radius of a large number of points inside the hollow shaft is measured and then an alignment of the hollow shaft and the machine tool is realized by means of a mathematically fictitiously determined mean axis. If the inner contour of the bore is measured tactilely, problems often arise due to the length of the tactile measuring elements used and the vibrations that occur. For this reason, other measuring methods are increasingly being used instead of tactile methods. 
     In EP 2 527 084 A2, for example, it is proposed for the reduction of imbalances that hollow shafts to be machined are first optically scanned in sections and that centers of gravity or imbalances are calculated in sections on the basis of these optically recorded values. These points or sections are subsequently milled in a targeted manner in order to achieve largely optimum concentricity of the hollow shaft for subsequent use. 
     DE 199 58 373 A1 describes a method for reducing the eccentricity of the inner to the outer surface of a hollow workpiece rotatably clamped in a machine tool. Here, ultrasound is used to determine several measurement data dependent on the contour of the inner surface. These measurement data are used to calculate a nominal course of the outer surface, and the outer surface is subsequently machined according to the calculated nominal course in the same workpiece clamping as during the measurement. 
     A similar approach is described in EP 2 668 547 B1. Here, however, the measurement data of the contour of the inner surface determined with ultrasound are used for the calculation for a part of the outer surface, which is at least partially machined in a further process step. The workpiece is re-clamped over the partial surfaces generated in this way using clamping devices such as chucks and steady rests, and the other outer surfaces of the workpiece are subsequently machined. 
     By generating new clamping seats in this way for further machining, largely optimum machining of hollow shafts is possible. However, the use of ultrasound to measure the contours in the inner cavity of larger hollow shafts (e.g. with a length of more than 2 meters) is only possible to a limited extent. This is because the technical complexity of the equipment increases considerably and the measuring accuracy is reduced at the same time. Therefore, in addition to optical and ultrasound-based solutions, the fundamental aim is to modify other measurement methods known per se for such applications. 
     WO 2012/100 278 A1 describes a method for reducing the eccentricity of the inner to the outer surface of a hollow workpiece rotatably clamped in a machine tool. Here, a user-friendly and fast method for measuring the wall thickness of a workpiece with the aid of an ultrasonic measuring device is described. However, it remains unclear whether the procedure is carried out by contact (tactile) or with the aid of a carrier medium for sound transmission. It is described that the procedure is carried out on the outer surface in the circumferential direction or also preferably in the longitudinal direction of the workpiece. The ultrasonic measuring device is used to record measurement data with which at least one, in this case two, partial surfaces are generated, preferably by axis-parallel rotary milling. With the help of these partial surfaces, the workpiece is clamped in a new setup, but rotated by 180 degrees, which requires a manual operator action or automation and is not explicitly described. The wall thickness determined by the ultrasonic process is subject to the influence of the outer and inner contours and thus also of the clamping device used. Operator actions are required during the process. 
     Another technical solution for machining a workpiece is described in EP 2 572 826 A1. Here, a tool is used which takes into account the deviation of the actual rotational axis from the nominal rotational axis of a workpiece by feeding the tool with the aid of sensor measurement data. The sensor measurement data are recorded by at least two co-rotating measuring means of known shape. These measuring means are each attached to the end face of the full workpiece and each have an annular and each have an at least partially spherical known shape. Measurement data are recorded by several inductively operating sensors, with the aid of which existing form and position deviations are improved or reduced by machining with a geometrically determined cutting edge in order to meet the highest geometric requirements. The machining process used is not described further. The process requires workpiece preparation on the workpiece face with the measuring equipment to be used. The sensors are aligned with the workpiece through an opening in the clamping device or starting from a tailstock. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     It is the aim of the disclosure to create a new technical solution for determining centers and the spatial course of these centers of a hollow shaft rotatably clamped as a workpiece in a machine tool using methods other than ultrasound. 
     This aim is achieved in terms of process technology in that first of all the radial distances between the sensor and the workpiece contour are detected without contact over at least one full angle of the workpiece contour at at least two defined axial positions in defined angular relative positions between the axis of rotation of the sensor and the axis of rotation of the hollow shaft using at least one sensor operating according to the eddy current principle. Then, by calculating any constant radius to the detected radial distances, a vector is formed from polar coordinates with the values angle and radius with radial distances, which is converted into Cartesian coordinates. A geometric workpiece center point assignable to the corresponding axial position of the hollow shaft is calculated via averaging. Then, from at least two such calculated workpiece center points at different axial positions, a center axis lying in space is calculated by a regression analysis, which approximates the workpiece center points. Subsequently, starting from the center axis along the axis of rotation of the hollow shaft, any number of diameters concentric to the center axis is calculated, with which new clamping seats for the hollow shaft are machined, which redefine the axis of rotation of the hollow shaft concentrically to the center axis. In this case, the radial distances at at least two defined axial positions along the axis of rotation of the hollow shaft can be recorded with a sensor one after the other in time. Alternatively, it is also possible for these radial distances to be detected with at least two sensors simultaneously or one after the other in time. 
     To carry out the method, a device is proposed which is arranged in a fixed or exchangeable manner in the working space of a machine tool with the aid of a holder on a tool carrier and carries one or more sensors operating according to the eddy current principle in the same or definedly different alignment and design and can be freely positioned in the working space via at least one machine axis of the tool carrier. Advantageous embodiments are the subject of subclaims, which are explained in more detail in an embodiment example. 
     With the technical solution according to the disclosure, a method and a device become available which enable a non-contact measurement in a large measuring range by applying the eddy current principle. Thus, by means of a distance measurement by eddy current technique, the bore centers are measured. Due to the calculation with a virtual diameter, the centers are determined independently of the concrete diameter, whereby inner contours are measured. The vibration-reducing components ensure high measuring accuracy even for long workpieces to be measured. As a result, (spatially) precisely positioned clamping seats and steady rest seats can be created on hollow shafts and similar workpieces, which are used for a new clamping of the workpiece in an optimal spatial orientation for subsequent machining. Thus, a technical solution is created for the generation of clamping seats for a new clamping of a workpiece rotatably clamped in a machine tool with a free contour section in the interior of the workpiece, which is machined on its outer surface at least in sections, wherein the center axis is calculated with the proposed method and the new clamping seats are manufactured with the proposed device. The measuring process as well as the subsequent machining process for producing the new clamping seats is carried out in an unmanned process. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
       Exemplary embodiments are explained below with reference to the drawings, wherein: 
         FIG.  1   a    shows the basic structure with representation of the operative connection of the assemblies during the measurement; 
         FIG.  1   b    shows a representation of the effective connection of the assemblies during the milling of new clamping seats concentric to the calculated center axis; 
         FIG.  1   c    shows a representation of the effective connection of the assemblies during chucking of the hollow shaft; 
         FIG.  1   d    shows a representation of the effective connection of the assemblies during the milling and turning of new clamping seats for further machining; 
         FIG.  1   e    shows a representation of the effective connection of the assemblies in a new clamping situation; 
         FIG.  2    shows the basic structure of the device for measured value acquisition; 
         FIG.  3    shows a representation of the vibration reducing components; and 
         FIG.  4    shows the structure of the device in a modified version. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     The technical device arrangement shown in the drawings is designed for carrying out a method for determining centers and the spatial course of these centers of a workpiece rotatably clamped in a machine tool with a free contour section in the interior, preferably a hollow shaft, which is machined on its outer surface at least in sections. 
     According to  FIG.  1   a   , the workpiece, which is designed as a hollow shaft  4 , is clamped with an end face section in a clamping means designed as a chuck  2 . In addition, the outer surface of the hollow shaft  4  is supported in at least one further clamping means. In the example according to  FIG.  1   a   , however, the support is not provided in just one further clamping means, but in two further clamping means designed as steady rests  5 . In this supported position, the hollow shaft  4  rotates around the workpiece rotation axis  1 . The direction of rotation is shown with a stylized arrow. 
       FIG.  1   a    also shows that the workpiece contour  13  is not concentric. Rather, the outer contour and the inner contour of the hollow shaft  4  run eccentrically to each other. To ensure that correct machining is nevertheless possible, centers are first determined for the hollow interior of the hollow shaft  4 . 
     A device  9  shown on the right in  FIG.  1   a    is provided for this purpose, the basic structure of which is again shown separately in  FIG.  2   . The device  9  is arranged in the working area of a machine tool, which is not shown in more detail here, with the aid of a holder  14  on a tool carrier  8 . Both a fixed and an exchangeable arrangement are possible, with the exchangeable variant being advantageous due to the possible use of cassettes. The machine axes of the tool carrier  8  are shown with two stylized arrows  10 . 
     At least one sensor  6  operating according to the eddy current principle is arranged on the device  9 . The axis of rotation referred to the sensor  6  is marked with the reference sign  12 . However, the sensor  6  itself does not rotate about this axis of rotation  12 . If the device  9  is equipped with several sensors  6 , sensors  6  of the same or different design can be provided for this purpose. Likewise, the multiple sensors  6  can be arranged in the same or defined different orientation. Regardless of the specific number, design and orientation, each sensor  6  can be freely positioned in the working area of the machine tool via at least one machine axis  10  of the tool carrier  8 . 
     In the embodiment shown, only one sensor  6  is provided. With this sensor  6 , which operates according to the eddy current principle, the radial distances between the sensor  6  and the workpiece contour  13  are detected at at least two defined axial positions  7 .  FIG.  1   a    shows three positions in this respect, whereby the sensor  6  is located here at the middle axial position  7  as an example. The contactless detection of the radial distances takes place in defined angular relative positions between the axis of rotation  12  of the sensor  6  and the axis of rotation  1  of the workpiece  4  over at least one full angle of the workpiece contour  13 . 
     If only one sensor  6  is used, the radial distances are detected at at least two defined axial positions  7  along the axis of rotation  1  of the hollow shaft  4  in succession. If, on the other hand, several sensors  6  are present, the radial distances are also recorded either in succession or, advantageously, simultaneously. 
     A qualitatively good recording of measured values is achieved if additional vibration-reducing components are assigned to the device  9 . According to  FIG.  3   , several disk-like components  15  are structurally integrated in the interior of the device  9  for this purpose. These components  15  each have an elastic component in the circumferential direction which is radially braced with the device  9 . The vibration-reducing components  15  also have centrally and/or eccentrically arranged openings  16  for axial feed-through and attachment of cables and lines. Furthermore, it is provided that at least one vibration-reducing component  15  is rigidly connected to at least one adjacent vibration-reducing component  15  and is attached to the holder  14  to the tool carrier  8 . As a result, the at least one sensor  6  rotating about the axis of rotation  12  performs very precise movements without deflections affecting the measurement. 
     The device  9  can be further designed to achieve a largely optimum adaptation for specific application requirements in each case. For example, the device  9  can be arranged on an extendable sensor carrier for hollow shafts  4  with long cavities. Furthermore, the energy required to operate the device  9  can be supplied either contactlessly or by cable. In this case, a contactless energy supply is designed, for example, as an inductive power supply. Furthermore, the measurement data acquired by means of the device  9  can be transmitted contactlessly or by cable to a computing unit with which the measurement data can be transmitted to a further computing unit of a control system of the associated machine tool. 
     As soon as the radial distances between sensor  6  and workpiece contour  13  have been detected, a vector of polar coordinates with the values angle and radius with radial distances is formed by calculating any constant radius with the detected radial distances. This vector is converted into Cartesian coordinates and a geometric workpiece center point assignable to the corresponding axial position  7  of the hollow shaft  4  is calculated by averaging. Then, from at least two calculated workpiece center points at different axial positions  7 , a center axis  3  lying in space is calculated by a regression analysis, which approximates the workpiece center points. Subsequently, starting from the center axis  3  along the axis of rotation  1  of the hollow shaft  4 , any number of diameters concentric to the center axis  3  are calculated. 
     With the diameters calculated in this way, new clamping seats are machined for the hollow shaft  4 , which redefine the axis of rotation  1  of the hollow shaft  4  concentrically to the center axis  3 . This process step is shown in  FIG.  1   b   . The hollow shaft  4  continues to be clamped with a front end section in the clamping means designed as chuck  2  and rotates about the axis of rotation  1 . In addition, the hollow shaft  4  is still supported with its outer surface in the steady rest  5  shown on the right, but no longer in the steady rest  5  shown on the left. The new clamping seats  24  are rotationally milled concentrically to the calculated center axis  3  orthogonally. Preferably, each new clamping seat  24  is created with a turning/drilling/milling unit  20  by a respective associated milling tool  21 . The traversing axes of the turning/drilling/milling units  20  are each shown with two stylized arrows  19 . 
     After the new clamping seats  24  are machined, the chucking of the hollow shaft  4  takes place. This process step is shown in  FIG.  1   c   , where the arrow marked “A” stylizes the chucking of the hollow shaft  4  and a simultaneous process so that new clamping seats can be milled. For this purpose, the tool carrier  8  comprises a rotatable clamping means  17 , which is designed to be selectively drivable and lockable. The clamping means  17  is designed for an inner or outer receptacle of the hollow shaft  4  and can be actuated manually or automatically. The clamping means  17  is freely positionable in the working space of the machine tool via at least one machine axis  10  of the tool carrier  8 .  FIG.  4    shows this equipment design again separately. 
     According to  FIG.  1   c   , the hollow shaft  4  is released from the clamping means designed as chuck  2  and is supported on the new clamping seats via the steady rest  5  shown on the left and the clamping means  17  on the tool carrier  8 . The clamping device  17  rotates about a rotation axis  18  so that the hollow shaft  4  also executes a rotating movement. In this clamping situation, the workpiece rotation axis  1 , center axis  3  and clamping device rotation axis  18  are congruent with each other. 
     This is followed by milling and turning of new clamping seats  24  for further machining. This process step is shown in  FIG.  1   d   . The hollow shaft  4  is further released from the clamping means designed as chuck  2  and supported via the steady rest  5  shown on the left and the clamping means  17 . The turning/drilling/milling units  20  already explained for  FIG.  1   b    are used for machining. Instead of two milling tools  21 , one milling tool and one turning tool  22  are now used, for example. 
       FIG.  1   e    shows the clamping situation after milling and turning the new clamping seats  24 . The hollow shaft  4  is now clamped again in the clamping means designed as chuck  2  and rotates about the workpiece rotation axis  1 , which is congruent with the calculated center axis  3 . In addition, the hollow shaft  4  is supported with its outer surface in the steady rest  5  shown on the right, but not in the steady rest  5  shown on the left. 
     The foregoing description of the embodiment has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.