Patent Publication Number: US-2009228137-A1

Title: Method for testing the fit or for testing the imbalance of a tool

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT Application No. PCT/EP2008/060868, filed Aug. 20, 2008, and also claims the benefit of German Application No. 10 2007 044 458.5, filed Sep. 10, 2007, both of which are incorporated herein by reference in their entirety and for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for testing the fit or for testing the imbalance of a tool exchangeably accommodated in a tool spindle which is mounted for rotation about a central longitudinal axis and is rotationally driven by a spindle drive motor. 
     Tool spindles are used in a machine tool. In order to achieve high machining accuracy, an inserted tool should run as true as possible. Deviations from running true may result from a non-aligning fit of the tool in the tool spindle due to contaminant or from various kinds of tool imbalances. Such irregularities are to be detected and rectified. 
     It is known from EP 0 881 032 A2 (corresponding to U.S. Pat. No. 6,059,702 A) to clean abutment surfaces with compressed air or with the coolant of the machine tool when inserting a tool. Moreover, it is possible to check the correct, i.e., aligning, fit of the tool, following insertion, by introducing compressed air and detecting the resulting decrease in pressure. However, a separate device for measuring the pressure is required for this. 
     The checking also takes a relatively long time as the drop in pressure to be analyzed usually only takes place slowly. 
     It is known from EP 1 745 884 A1 to test the aligning fit of a tool in a tool spindle using a light beam. 
     A device for detecting abnormal operation of a rotary tool having a plurality of edges is known from JP 61050758 A. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a simple and quick method for testing the fit or for testing the imbalance is provided. 
     In accordance with an embodiment of the invention, the tool is inserted on the tool spindle, the tool spindle is made to rotate with a specific rotational frequency f def  about the central longitudinal axis by means of the spindle drive motor, an actual value of a controlled drive current of the spindle drive motor is analyzed with respect to a frequency component with the rotational frequency f def , and it is determined whether the frequency component with the rotational frequency f def  lies within a threshold value range. 
     In the method according to the invention, the current ripple of the drive current of the spindle drive motor is analyzed. A check is carried out as to the extent to which the component with the specific rotational frequency (namely the frequency component with the specific rotational frequency of the tool spindle) is contained therein. This component is directly attributable to a non-aligning fit or a tool imbalance. A non-aligning fit or a tool imbalance is then recognizable from the analysis. 
     The method according to the invention can be carried out without any special additional hardware components. Furthermore, a detection of the actual value of the drive current can be used with the necessary evaluation algorithms. The additional apparatus expenditure is thereby minimized. 
     All of the work steps of the method according to the invention can be carried out quickly. An evaluation of the actual value of the drive current is possible within an extremely short time. In particular, there is no need to wait until measuring is possible again after dead times. 
     It is expedient for the tool spindle to be arranged on a machine tool. Testing the fit or testing the imbalance of a tool on a machine tool can thereby be carried out in a simple way. 
     In particular, a spindle controller controls the drive current of the spindle drive motor in order to maintain the rotational frequency. The method according to the invention can then be carried out in a simple way with minimized hardware expenditure. 
     In particular, the actual value of the drive current is detected at the spindle controller. The controlling of the current can thereby be analyzed in its frequency dependence and, in particular, with respect to its frequency component at the rotational frequency f def  in a simple way. 
     It may be provided that the spindle controller is arranged on the tool spindle and/or the spindle controller is arranged in a control device. The control device itself may be arranged on the tool spindle or it may be arranged remote from the tool spindle, for example, within a machine cladding of a machine tool. 
     It is expedient for comparative values for the component of the actual value of the drive current at the specific rotational frequency to be stored in a database. Prevailing irregularities can thereby be specified on the basis of comparative values in a simple way. In particular, known types of irregularity are stored in a database. A reference measurement may be carried out for a tool that has not yet been inserted so as to generate corresponding comparative values for the database. 
     In particular, when a tool is used for the first time, the frequency component of the actual value of the drive current with the rotational frequency f def  is detected at one or more rotational frequencies f def  and stored in the database. Comparative values are thereby generated for the corresponding tool. 
     It may be provided that the actual value is detected several times. When generating comparative values when a tool is being used for the first time, a multiple actual value detection serves for data verification. When testing for fit or testing for imbalance of a tool that is to be inserted, a multiple actual value detection serves to correctly position the tool. For example, a corresponding analysis is carried out, and if the result is negative (the values lie outside the threshold value range), the tool is exchanged or reinserted. Another actual value detection is then carried out in order to check whether the tool is fitted correctly or there is now only an imbalance that is tolerable. 
     It is expedient for the tool to be inserted manually or by a mechanical tool changing device. In particular, a tool for workpiece machining is inserted mechanically. When determining comparative values for a tool that is to be newly inserted, the tool can be inserted manually or mechanically. 
     If the frequency component of the actual value of the drive current with the rotational frequency f def  lies outside the threshold value range, it is expedient for the tool to be exchanged and/or reinserted at least once. If an incorrect fit or an intolerable imbalance is recognized, action can then be taken to rectify the fault. Following exchange or reinsertion, another check is carried out by detecting the actual value, in order to test for correct fit or an imbalance. 
     It may be provided that a control device, in particular, of a machine tool emits a control signal if the frequency component of the actual value of the drive current with the rotational frequency f def  is not below the threshold value range and, in particular, is not below the threshold value range even after the tool has been exchanged several times and/or has been reinserted several times. For example, the control signal is a warning signal. 
     The following description of preferred embodiments serves in conjunction with the drawings to explain the invention in greater detail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       In order to assist the understanding of certain embodiments of the invention, reference will now be made to the appended drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  shows a front view of an embodiment of a machine tool with a three-dimensionally positionable and rotationally drivable tool spindle for accommodating a tool; 
         FIG. 2  shows a partial cross section of the tool spindle according to  FIG. 1  with an aligning fit of the installed tool; 
         FIG. 3  shows a partial cross section of the tool spindle according to  FIG. 1  with a non-aligning fit of the installed tool; 
         FIG. 4  shows a diagrammatic comparison of an aligning and a non-aligning fit, due to contaminant, of the non-rotating tool; 
         FIG. 5  shows a diagrammatic comparison of a tool rotation with an aligning and a non-aligning fit; 
         FIG. 6  shows a diagrammatic representation of an evaluation method in block diagram representation; and 
         FIG. 7  shows a schematic representation of a filtered signal of the actual value of the current relating to an unbalanced tool. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     The present invention now will be described more fully hereinafter. However, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. As used in this specification and the claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
     An embodiment of a machine tool shown in  FIG. 1  comprises a stand  1  which is formed by a frame and, seen in a horizontal z-direction, is rectangular, more particularly, approximately square. The stand  1  is formed by vertical side supports  2 ,  3  extending in a y-direction and by a horizontal upper cross beam  4  and a horizontal lower cross bream  5  which extend in an x-direction and join these side supports  2 ,  3 . The y-direction extends perpendicularly to the x-direction and is a vertical direction. The side supports  2 ,  3  and the cross beams  4 ,  5  are formed by hollow profiles and enclose an inside space  6  which, seen in a z-direction, is open at both ends, in particular, towards a work space  7 . The z-direction extends transversely to the x-direction and to the y-direction. The stand  1  is supported by an underframe  8  on a foundation or a foundation plate  9 . 
     An x-slide  10  which is also constructed in the manner of a frame is arranged so as to be displaceable in the x-direction on the end face of the stand  1  that faces the work space  7 . For this purpose, an x-guide rail  11 , on which the x-slide  10  is guided, is arranged on each of the cross breams  4 ,  5 . The x-slide  10  is driven by an x-motor  12  via an x-ball bearing spindle  13  extending in the x-direction and mounted in the side supports  2 ,  3  of the stand  1  or by a linear motor. 
     A y-slide  14  displaceable in the y-direction, i.e. vertically, is displaceably guided on the end face of the x-slide  10  that faces the work space  7 . For this purpose, a y-guide rail  15 , on which the y-slide  14  is displaceably guided, is arranged at each of the side regions of the frame-like x-slide  10 . The y-slide  14  is driven by a y-motor  16  mounted on the x-slide  10  via a y-ball bearing spindle  17  or by a linear motor. 
     Located on the y-slide  14  is a tool spindle unit constructed as z-slide  18 . This unit comprises a housing-like sleeve  19  which is displaceably guided on z-guide rails  20  mounted in the y-slide  14 . The displacement in the z-direction takes place by means of a motor (not shown in the drawings). Arranged in a rotationally fixed manner in the sleeve  19  and immovably in the z-direction is a tool housing  21  of substantially circular cross section, in which, in turn, the actual tool spindle  22  is mounted so as to be rotationally drivable about a central longitudinal axis  23  extending in the z-direction. 
     Mounted in the work space  7  in front of the stand  1  on the foundation plate  9  is a workpiece carrier bed  24  on which is supported a workpiece carrier  25  constructed in the manner of a bridge. Arranged on the workpiece carrier  25  is a B-rotary table  26  which is drivable for rotation about a vertical B-axis of rotation  28 , i.e., running parallel to the y-direction, by a B-rotary motor  27  mounted on the workpiece carrier  25 . Mounted on the B-rotary table  26  is a workpiece carrier  29  which is able to accommodate a workpiece  30  that is to be machined. 
     To the extent to which the machine tool has so far been described, it is, in principle, known and in common use (cf. for example, EP 0 617 244 B1). 
     The tool spindle  22  is constructed as a hollow shaft which is rotatably mounted by means of roller bearings  34  in the tool spindle housing  21  ( FIG. 2 ). It is driven by a spindle drive motor  35 . The right-hand side of  FIG. 2  shows a portion of the spindle drive motor  35 , namely the rotor stacks  26  rotationally fixedly connected to the tool spindle  22 , and the stator winding heads  37  rotationally fixedly arranged in the tool spindle housing  21 . 
     At its free end facing the work space  7 , the tool spindle  22  is provided with a receptacle  38  tapering conically from the outside inwards, into which is inserted a hollow shaft cone  39  of a tool  40  to be accommodated therein. The tool also has an abutment surface  41  which extends radially in relation to the central longitudinal axis  23  and rests against an end face  42  of the tool spindle  22  that extends radially in relation to the axis  23  when the tool  40  is aligned in the tool spindle  22 . 
     Arranged in the tool spindle  22  constructed as a hollow shaft is a tension rod  43  by means of which a collet chuck  44  is actuated, which engages in the hollow shaft cone  39 . The collet chuck  44  comprises individual clamping elements  45  which, when the tension rod  43  moves into the tool spindle  22 , are pushed outwards by a spreading cone  46  and engage behind corresponding projections  47  in the hollow shaft cone  39 , whereby the tool  40  is clamped to the tool spindle  22 . Such a configuration of a tool spindle  22  including an actuating unit for the tension rod  43  is known and quite common in practice. 
     The tool  40  is correctly installed in the tool spindle  22 , namely such that the central longitudinal axis  48  of the tool  40  is in coaxial alignment with the central longitudinal axis  23  of the tool spindle  22  ( FIG. 2 ). However, it may also be that the axis  48  is not in alignment with the axis  23 . This is the case, for example, when a contaminant  49 , for example, in the form of a metal chip or the like, is located between the abutment surface  41  and the end face  42 . In such a case, the tool  40  executes a wobbling movement relative to the tool spindle  22  during rotational drive of the tool spindle  42 . This is shown in  FIG. 3  for the configuration of the tool spindle  22  according to  FIG. 2 .  FIG. 4  shows in dash-dot lines such a non-aligning installation of the tool  40  compared to the aligning position shown in solid lines.  FIG. 5  shows in dash-dot lines the untrue rotation of the tool  40  compared to the aligning tool  40  shown in solid lines. Apart from the contaminant  49  an imbalance in the tool  40  may also give rise to such a wobbling movement. Both causes are summarized in the following by the term irregularity. 
     In the following a method is described, by means of which an untrue rotation of the tool  40  is detected and evaluated, whereby it is checked whether the tool  40  is aligned in the tool spindle  22  and/or whether it has an imbalance which is not shown in more detail in the Figures. 
     The tool machine comprises a control device  52  which controls the movement and positioning of the tool spindle  22  in the x-direction and y-direction and possibly in the z-direction and the movement and positioning of the workpiece  30  relative to the stand  1 . The control device  52  may also comprise an evaluating device. Furthermore, a spindle controller  54  is provided, which controls a drive current of the spindle drive motor  35 , in order to rotate the tool spindle about the central longitudinal axis  23  with a specific rotational frequency f def . 
     The spindle controller  54  may be integrated into the control device  52  or, for example, arranged on the tool spindle  22 . 
     An untrue rotation of the tool  40  on the tool spindle  42  is recognized as follows in accordance with the invention. After a tool change, the tool spindle  22  with the tool  40  inserted therein is set in rotation. The newly inserted tool  40  may be inserted manually or preferably by an automatic tool changing device. After the desired rotational frequency f def  (i.e., the set rotational frequency) is reached and prior to engagement of the tool  40  on the workpiece  30 , the actual value of the drive current of the spindle drive motor  35  is evaluated. The current is detected. This is indicated diagrammatically by the block with reference numeral  56  in  FIG. 6 . Furthermore, the associated actual rotational frequency f def  is detected. This is indicated by reference numeral  58  in  FIG. 6 . 
     A frequency analysis is then carried out on the associated current signal which is plotted against time in  FIG. 7 . For example, a frequency spectrum is determined using fast Fourier transformation (FFT). This is indicated by reference numeral  60  in  FIG. 6 . This is followed by an evaluation, for example, in the control device  52 . The evaluation is indicated by reference numeral  62  in  FIG. 6 . 
     When carrying out the evaluation while the tool spindle  22  is rotated at the specific rotational frequency f def , the actual value of the drive current of the spindle drive motor  35  is analyzed with respect to its frequency component with the rotational frequency f def . This frequency component is a direct measure of an untrue running and, in particular, an imbalance. 
     The actual value of the drive current may contain components with other frequencies. In particular, it contains components with a multiple of the rotational frequency f def , which are due to the spindle drive motor comprising a discrete number of pairs of poles. 
     The frequency component of the drive current, which is controlled by the spindle controller  54 , with the specific rotational frequency, is determined and compared with reference values at the control device  52 . 
     The reference values are stored in a database  64 . A check is carried out as to whether the deviation of the actual value of the drive current with the component with the specific rotational frequency lies within a threshold value range or not. If the deviation lies within the threshold value range, then the tool change is classified as successful and machining of the workpiece can take place. 
     If a deviation outside the threshold value range is determined, the tool is then exchanged or the tool  40  is inserted again. This step or these steps may possibly be repeated several times. 
     If it is ascertained that the threshold value range is exceeded and, for example, even after reinserting the tool  40  once or several times, the threshold value range is still exceeded, the control device  52  then emits a control signal which initiates corresponding actions. For example, the control signal is a warning signal which shuts down the machine tool. The warning signal may, for example, also be to the effect that insertion of another tool is requested. 
     By virtue of the solution according to the invention, the current ripple ( FIG. 7 ) at the actual value of the drive current of the spindle drive motor is analyzed with respect to a component with the specific rotational frequency. An aligning or non-aligning fit of the tool  40  or a tolerable or intolerable tool imbalance is concluded from the analysis. 
     As a rule, no additional hardware components are required for the method according to the invention. The means provided in any case for moving and positioning the tool spindle  22  may be used. Furthermore, the actual value of the drive current of the spindle drive motor  35  can be detected in a simple way. 
     The additional apparatus expenditure for performing the method according to the invention for testing for fit and testing for imbalance is, therefore, minimized. 
     Furthermore, the work steps for performing the method according to the invention can be carried out quickly. An evaluation of the actual value of the drive current is possible within a very short time. There is essentially no dead time or the like during which measuring is not possible. 
     To determine the reference values stored in the database  64 , a tool  40  is inserted when used for the first time, the tool spindle  22  is made to rotate about the central longitudinal axis  23  and in a similar way to that described hereinabove, the component of the actual value of the drive current with the specific rotational frequency is detected and stored in the database  64 . These steps may possibly be carried out several times (at least twice) for data verification. 
     The tool is then exchanged. Such reference measurements may be carried out for different tools  40 . 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing description. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.