Abstract:
A system for measuring an internal dimension, such as the radius, of a cylindrical cavity over a determined length, particularly in a hollow shaft belonging to a turbomachine. The system includes a measurement module mounted removably on a support that has translational mobility inside and along the axis of the cavity, the module including a measurement component for measuring the dimension and delivering signals corresponding to the measured values and incorporating a recording and storage for recording and storing the values. More particularly, the module includes a mechanical sensor component able to move between a retracted position and an extended position for sensing the internal surface of the cavity.

Description:
The present invention relates to the field of turbomachines and is aimed at a means for measuring the internal dimensions of a hollow shaft. 
   BACKGROUND OF THE INVENTION 
   In a turbomachine such as an aeronautical gas turbine engine, the shafts connecting the various compressor and turbine disks are principal component parts revolving at high speed and capable of withstanding high loadings while at the same time needing to remain relatively light in weight. Their manufacture involves boring a longitudinal cavity in the forging, the exterior surface of which has already been machined. After this boring operation, the internal dimensions of the shaft need to be checked in order in particular to be able to make any corrections that might be needed in order to confine the wall thickness to a certain tolerance band. 
   DESCRIPTION OF THE PRIOR ART 
   The conventional means known to the applicant for performing dimensional checks on a bored hole are time-consuming and limited to a partial check, of the order of ten or so measurement points on a shaft 1500 mm long. One known means is to perform the check using a machine of the three-dimensional measurement type. These means involve in particular taking impressions in order to allow measurements to be performed in regions which have radii of curvature or gradients that are difficult to access, and this is a complex and lengthy operation. 
   SUMMARY OF THE INVENTION 
   The applicant has set itself the objective of creating a measurement system capable of checking the dimensions inside a shaft after the longitudinal cavity has been bored out. 
   Another objective is to be able to perform the checking operations while the workpiece is still in place on the machine tool, without having to move it to a dedicated measurement installation. 
   Another objective is to be able to take the measurements at a higher speed than before and over a large number of points. 
   Another objective is to allow realignment with respect to the initial bore during the finishing rework operations. 
   It has been possible to achieve these objectives using a system for measuring an internal dimension of a cylindrical cavity over a determined length, such as in a turbomachine hollow shaft, which system comprises a measurement module mounted removably on a support that has translational mobility inside and along the axis of the cavity, the module comprising a measurement component for measuring the dimension and delivering signals corresponding to the measured values and incorporating a recording and storage means for recording and storing said values. 
   For preference, the module comprises a measurement component of the mechanical sensor type able to move between a retracted position and an extended position for sensing the internal surface of the cavity. This arrangement allows the module to be installed precisely inside the cavity without having to allow for a risk of damaging the measurement component. 
   According to another feature, the component is placed in position by control means, the power supply of which is built into said measurement module. 
   More specifically, when the cavity is the result of the boring-out of a cylindrical metal workpiece using machining, the mobile support is the arbor that acts as a support for the boring head of said machine tool. 
   According to another feature, when the machine tool has an automatic control station, the latter comprises a control means for controlling the position of the arbor axially and/or angularly, and a control means for controlling the measurement component. For preference, control signals are transmitted between the control station and the measurement module wirelessly, for instance using infrared. 
   According to another feature, the system comprises a data reading means for reading the data recorded in the module. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features will emerge from the following description of a nonlimiting embodiment of the method of the invention, this description being accompanied by the attached figures in which: 
       FIG. 1  depicts a measurement module according to the invention, placed inside a hollow shaft of a turbine in a first position, 
       FIG. 2  shows the module inside the turbine shaft of this example, in position at the distal end of the shaft, 
       FIG. 3  shows, in greater detail, the part of the measurement module that has the sensing measurement component, 
       FIG. 4  schematically depicts the control of the measurement system. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As can be seen in  FIGS. 1 to 4 , the workpiece, the internal dimensions of which are to be checked in this instance, is a turbine shaft. The shaft  1  has been manufactured by forging a lump of metal to give it the desired exterior shape. Next, the workpiece has been externally machined. Finally it has been placed on a support  2  and bored out by machining from one end  11  which is the proximal end, as far as its opposite end  14 , termed the distal end. The machine tool  3  comprises an arbor  31  rotated about itself and driven in a translational movement along an axis XX by a drive unit  32  provided with suitable drive means. These elements are depicted schematically in  FIG. 4 . 
   In order to bore the shaft  1 , the arbor  31  is equipped at its distal end with an appropriate tool. This is not depicted as the boring operation has been completed. 
     FIGS. 1 and 2  show the geometry of the shaft  1  in cross section. The diameter of the central cavity is not constant between the two ends  11  and  14 . This diameter is constant over a first, entry, portion  11 ; it widens gradually over a second portion  12  that forms a frustoconical internal surface, then remains constant over a third portion  13 , which is the longest portion. It then narrows and continues with a more complex shape over a final portion  14 . 
   The exterior shape is substantially cylindrical with a flanged portion  15  near the entry  11 . This flange is used to connect the shaft to the turbine rotor, not shown. 
   The system of the invention has been developed in order to check the surface of the internal cavity essentially along the second and third portions  12  and  13  to which access is reduced. 
   The system of the invention comprises a measurement module  40  of tubular shape made in two parts  41  and  42 . The distal part  41  has a diameter slightly smaller than that of the proximal part  42 . The adjective “proximal” denotes the part closest to the drive system  32 . “Distal” denotes the part furthest from the drive system  32 . 
   The diameter of the tubular part  41  allows it to pass through the aperture of the portion  14  of the shaft. The diameter of the part  42  allows the module  40  to be introduced via the portion  11  of the shaft. Here it exceeds the aperture of the portion  14 . When housed inside the shaft, the axis of the module  40  coincides with the axis XX of the shaft. A securing means  421  has the function of removably mounting the module  40  on the arbor  31  of the machine  3 . This means is shaped in such a way as to fit onto the arbor  31  in the same way as the tool for which the module is substituted. 
   In the intermediate zone between the parts  41  and  42  the module comprises a sensing measurement component  43  in the form of a lever shaped as a right-angled bracket, articulated about an axis perpendicular to the axis XX. One leg  431  of the bracket is provided at its free end with a sensing ball; the other leg  432  is connected to an electrically-operated tension actuator  44  working in opposition with a tension spring  45 . In order to bring the sensing component  43  into the retracted position the actuator is made to apply tension against the force of the spring  45 . When the actuator is released, the spring pulls on the leg  432  and this causes the leg  431  with the sensing ball to leave the tubular envelope of the module  40 . 
   A rule  46  is also mechanically connected to the leg  432 . Its purpose is to make a precise note of the movement of the component  43 . To this end, it is electrically connected to a data storage and recording means  461 . The Heidenhain company provide one example of this kind of rule. 
   The tubular part  41  contains an electrical accumulator cell  47  to power the actuator  44  when this actuator is activated. Its free end is provided with an interface  48  for receiving infrared signals, converting the infrared signals into electrical signals for controlling the actuator, and electrical signals for recording the value recorded by the rule  46 , respectively. 
   The module comprises an electrical connection means  49  for connection to a corresponding external means via which the accumulator cell  47  can be recharged and the data recorded in the data storage means  461  transferred to an external reading means. 
     FIG. 4  shows the diagram for control of the system. The control station  100  of the machine tool is connected to the shaft drive system in order to set both the axial position of this shaft  31  and its angular position about the axis XX. The station  100  is also connected to an infrared control component  110  which is positioned along the axis XX facing the interface  48 . 
   The system also comprises a support  120  for the measurement module  40 , situated close to the machining and checking installation, on which it can be positioned with its connection means  49  engaged with a corresponding connection means  122  belonging to the support  120  to allow, on the one hand, recharging of the accumulator cell  47  and, on the other hand, recovery of the data present in the data storage means  461 . 
   This support  120  is connected to a calculating means, such as a personal computer  130 , equipped with data processing software. 
   As can be seen in the figure, it is possible, according to the results of the analysis of the values, to envision transmitting a machining command to the control station  100  in order to make the necessary corrections to the machining. This corrective action can be performed because the shaft is present on the machine tool. This is one advantage of the system. 
   The way the system works is as follows. 
   Once the shaft  1  has been fully bored out, the boring head is extracted from the shaft  31  and the boring tool is dismantled from it. The turbine shaft  1  thus machined remains in place on the support  2 , so that its internal dimensions can be checked and recorded according to the invention. The tool is replaced by the measurement module  40  of the invention. 
   As can be seen in the figure, the device comprises a bed  10  forming a support for the workpiece  1 . The workpiece  1  is supported at one end by a steady  2  into which it is slid and in which it is positioned precisely in space. The workpiece is supported in the lengthwise direction. 
   The measurement module  40  has been mounted on the shaft  31  of the boring head in place of the boring tool. At the opposite end of the bed to the boring head drive means, and in the continuation of the axis XX there is the infrared control system  110  for controlling the components contained in the measurement head. 
   The measurement module  40  thus receives the control signals from the control unit  100 . 
   When the module is in place on the boring head, the device is started up by infrared remote control. Everything is operated by the control station  100 . The first step consists in calibrating the device along the abscissa X, along the axis XX, and along the ordinate Z, perpendicular to the axis XX. 
   The station  100  controls the drive unit  32  in such a way as to bring the module  40  into position at the distal end as in  FIG. 2 , at the maximum depth. 
   Through the infrared control  110  and the interface  48 , the measurement component  43  is extended. This operation is performed by cutting off the electrical power supply to the actuator, the tension spring  45  then causing the leg  431  to pivot out of the module  40 . It comes to bear via the ball against the internal wall of the shaft. An ordinate&#39;s measurement Z on the rule  46  corresponds to this position of the component  43 . Furthermore, the axial position X is known. 
   The program begins the series of measurements by making the module  40  move in the direction of the proximal end  11  over successive determined points axially separated for example by 0.1 mm, and at different angular positions, for example four angular positions (at 0°, 90°, 180° and 270°). At these defined points, the Z-value supplied by the rule  46  is recorded. When the series of measurements has been completed, the module  40  is withdrawn from the shaft and placed on the support  120  to gather all the Z-data stored in the memories  461 . These data are transmitted to the computer  130  where they are analyzed. On the strength of this information, the operator can take the necessary steps, for example can use the control station  100 , which also comprises the numerical control means for controlling the machining, to command the external machining needed to correct the wall thickness. 
   In other words, in order to work, the system requires dedicated software to control and exploit the data recorded in the module  40 . This module  40  is run by a software which receives commands from the unit  100  by infrared; the unit  100  also controls the synchronizing of the clock cards between the arbor  31  and the measurement head  40 . 
   Data processing is performed away from the machine tool by a personal computer  130  which compares the readings from the unit  40  with the theoretical model. This comparison provides a map  131  of the recorded dimensions, indicating the value of dimensions that are out of tolerance. This record also acts as a frame of reference  132  for balancing the workpiece during finishing operations, chiefly for machining the exterior shape which needs to be concentric with the readings from the module  40 .