Abstract:
A device for measuring the rotational imbalance of a specimen, having a spindle unit with a spindle, which is embodied to hold the specimen and to allow it to rotate at a testing speed, a holder suspension by means of which the spindle unit is anchored to the machine base in pendulum fashion such that the imbalance forces occurring during the measurement operation are able to move the spindle unit back and forth in a predetermined measuring direction M, and a sensor arrangement that detects at least one imbalance parameter occurring in the measuring direction M during rotation of the spindle, wherein the spindle unit is supported by means of at least one auxiliary bearing that is only able to transmit forces in the direction of a normal N to the measuring direction M.

Description:
FIELD OF THE INVENTION 
     The invention relates to a device for measuring the rotational imbalance of an item such as a machine element or tool holder. 
     BACKGROUND OF THE INVENTION 
     It is not uncommon for the spindles of modern rotating machine tools such as drilling or milling machines to operate at very high speeds of 20,000 RPM and more. At these speeds, powerful centrifugal forces are generated with even a slight imbalance. These centrifugal forces not only strain the spindle bearings of the machine tool, but also shorten the service life of the tool and may also impair machining results. The tool holder (with or without a clamped tool) is therefore balanced in a balancing machine before being used in the machine tool. A typical representative of such a balancing machine is described in patent application WO 00/45983. 
     The central component of balancing machines of the type known from WO 00/45983 is the spindle to which the component to be balanced is fastened, which then brings the component to be balanced to the balancing speed. The spindle travels in a spindle holder and together with it, constitutes the so-called spindle unit. In modern balancing machines, the imbalance is determined directly in the spindle unit—the instantaneous forces occurring in a particular direction in the spindle unit are detected by means of a suitable sensor and constitute a measure for the magnitude and location of the imbalance. 
     In order to be able to generate the corresponding measurement signal as a function of the respective imbalance, the spindle unit in balancing machines of this kind is suspended in a special oscillating bearing. In the embodiment of WO 00/45983 described below as a representative example, this oscillating bearing is composed of two leaf springs embodied in the form of thin sheets of spring steel. Each of these leaf springs is fastened with its one end to the machine frame and with its other end to the outer circumference of the spindle unit. The two leaf springs are situated spaced apart from each other in a vertical plane extending radially to the spindle rotation axis. In this plane, they resist the action of tensile, compressive, and shear forces, but behave in an essentially flexible fashion in response to force components oriented perpendicular to this plane. The upper of the two leaf springs (in the above-mentioned vertical plane) is loaded with tensile force in the horizontal direction and shear force in the vertical direction while the lower leaf spring is loaded with compressive force in the horizontal direction and with shear force in the vertical direction. In this way, the spindle unit is secured in an elastically cantilevered fashion, thus permitting the occurrence and detection of those imbalance-induced movements of the spindle unit that permit conclusions to be drawn about the position and magnitude of the imbalance. 
     As shown in FIG. 3 of WO 00/45983, the rotating imbalance force sets the spindle unit essentially into a horizontally oriented pendulum motion around the oscillating bearing mounted at the 12 o&#39;clock position; the pendulum motion has only a very small amplitude that meets practically no resistance from the leaf springs. This results in an actuation of the force sensor protruding like a finger from the machine pedestal at the 3 o&#39;clock position. However, the two leaf springs behave rigidly in the vertical direction so that the spindle unit executes little or no relevant movements in the vertical direction under the influence of the imbalance-induced forces and also executes little or no relevant flexing in the vertical direction under the influence of a specimen that does not exceed the rated load. 
     The known embodiments—and in particular, embodiments that follow the embodiment principle or suspension principle known from WO 00/45983—have the problem that the bearing of the spindle unit is susceptible to shocks and overloading, i.e., can be damaged or at least disadvantageously influenced by a careless insertion of a specimen or by the insertion of a specimen that is too heavy. 
     The object of the invention is to eliminate this problem and to disclose a more rugged device that functions properly, even in cases involving heavy specimens. 
     SUMMARY OF THE INVENTION 
     Correspondingly, a spindle unit is provided with a spindle for the specimen; this spindle is anchored to the machine base in pendulum fashion by means of a holder suspension. Preferably, a bearing that permits a pivoting motion in one spatial direction is provided at a point on the circumference of the spindle unit, in a position local to this holder suspension. In many cases, this bearing simultaneously predetermines a definite home position of the spindle unit relative to the machine base. In any case, it permits the spindle unit to move back and forth in a predetermined measuring direction due to the imbalance forces occurring during the measurement operation; this back-and-forth motion is detected by means of sensors. In order to further reduce the effect of undesirable transverse forces on the sensors, at least one auxiliary bearing is provided to support the spindle unit. Aside from negligible friction forces, this auxiliary bearing is essentially able to transmit only forces oriented in the direction of a normal to the measuring direction. The auxiliary bearing significantly or completely relieves the above-mentioned pendulum support and the corresponding bearing from weight-related forces and in so doing, its inevitable friction forces and movement limitations have only a surprisingly small negative impact on the measurement precision. 
     It should be noted that the imbalance measuring device according to the invention does not absolutely have to be a “stand-alone machine,” but can also be a component of a multiuse machine such as a shrink-fitting device for shrink-fitting the tool into a tool holder, as described for example in WO 01/89758 A1. It is also advantageous to connect the imbalance measuring device to a presetting device for determining the reference length of a tool clamped in a tool holder. Finally, the imbalance measuring device can also be a component of the machine tool itself. 
     According to a preferred embodiment, the auxiliary bearing has only a single rolling element. In this way, it fulfills its task of deflecting the weight-induced loads, but allows the spindle unit a majority of its degrees of freedom and thus does not hinder the oscillations that are to be measured in order to ascertain the imbalance. 
     A particularly favorable embodiment is produced if the single or multiple rolling element(s) is/are situated between the holder elements in the holder suspension so that the holder elements rest against each other in a direction perpendicular to the measuring direction and the rolling element(s) permit the two holder elements to roll against each other in the measuring direction during operation. In such an embodiment, the pendulum bearing and the auxiliary bearing form a unit into which the sensor(s) and possible spring elements provided for prestressing purposes are preferably also integrated. This saves space and makes it possible to provide a replacement unit that in the event of a defect can be easily replaced and remedies all conceivable bearing and sensor malfunctions. 
     Another preferred embodiment has another auxiliary bearing in the form of a counter-bearing built into it, which is situated on the circumference of the spindle unit, essentially diametrically opposite the first auxiliary bearing, which is accommodated in the holder suspension. The counter-bearing includes at least one, preferably only one, rolling element. This achieves an even better support without any further perceptible limitation to the mobility of the spindle unit in the measuring direction, even with regard to possible wobbling motions of the spindle unit. 
     At any rate, this is the case when, as likewise preferably provided, two sensors are installed spaced apart from each other and the auxiliary bearing(s) is/are installed essentially midway between the two sensors, viewed in the direction of the spindle rotation axis; each of the two latter measures is also useful in and of itself. 
     Other advantages and functions of the invention ensue from the exemplary embodiments explained below in conjunction with the various figures, whose graphical disclosure content is essential to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an axial longitudinal section through a first exemplary embodiment according to the invention of a balancing machine for tool holders, viewed along a line  1 - 1  in  FIG. 2 ; however in  FIG. 1 , the one auxiliary bearing according to the invention with which this exemplary embodiment is equipped is situated behind the plane of the drawing and is therefore hidden from view. 
         FIG. 2  is an axial cross-section through the first exemplary embodiment of the balancing machine, viewed along a line II-II in  FIG. 1 ; however in this case, the auxiliary bearing according to the invention is situated underneath the plane of the drawing and is therefore hidden from view. 
         FIG. 3  is a perspective side view of the first exemplary embodiment of the balancing machine, showing the one auxiliary bearing according to the invention, which is situated between the two holder elements of the holder suspension of the spindle unit. 
         FIG. 4  is a perspective side view of a second exemplary embodiment of the balancing machine, showing the first auxiliary bearing according to the invention, which is situated between the two holder elements of the holder suspension of the spindle unit, and a view of the second auxiliary bearing situated on the side diametrically opposite the holder suspension. 
         FIG. 5  is an axial longitudinal section through the second exemplary embodiment of a balancing machine. 
         FIG. 6  is a perspective side view of a third exemplary embodiment of the balancing machine, which is distinguished by the fact that the balancing machine is equipped with four auxiliary bearings situated spaced apart from one another by 90° around the circumference of the spindle unit. 
         FIG. 7  is a top view of the third exemplary embodiment of the balancing machine shown in  FIG. 6 . 
         FIG. 8  is a side view of a fourth exemplary embodiment of the balancing machine, which is distinguished by the fact that the balancing machine is equipped with a bearing ring that largely encompasses the circumference of the spindle unit. 
         FIG. 9  is an axial cross-section through the fourth exemplary embodiment of the balancing machine in  FIG. 7 . 
         FIG. 10  is a schematic side view of the spindle unit. 
         FIG. 11  is a detail view of the bearing cage of the ball bearing ring. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The balancing machine, general views of which are shown in  FIGS. 1 and 2 , has a housing  1  serving as a machine base. To make the balancing machine insensitive to vibrations and to give it optimal steadiness, the housing  1  is composed of a heavy material such as concrete or the like. In a chamber  3  that is accessible from above, the housing accommodates a spindle unit  7  that is driven by an electric motor  5 . The spindle unit  7  has a rotating spindle  11  whose rotation axis  9  is preferably oriented vertically; the vertical orientation of the spindle prevents the force of gravity from influencing the measurement result in any way. 
     At its upper end, the spindle is equipped with an operationally replaceable coupling adapter  13  that has a receiving opening centered on the rotation axis  9 . This receiving opening is used for connecting a standard specimen, labeled  17 , that is to be balanced. The specimen can be a tool holder, for example in the conventional steep taper design, or can be a hollow shaft taper holder (HSK holder) or also another rotor. The spindle  11  is embodied in the form of a hollow spindle. It contains an actuating unit  19 , which uses a collet chuck  21  to hold the tool holder  17  in the coupling adapter  13  during the imbalance measurement. The coupling adapter  13  is fastened to the spindle  11  by means of screws  23  and is replaceable i.e. can be adapted to the type of tool holder to be measured. 
     The spindle  11  is supported without play by two ball bearings  25 ,  27  that are spaced axially apart from each other in a hollow cylindrical spindle holder  29 ; axial bearing play is compensated for by prestressing springs  31  and a spring nut  33  encompassing the spindle  11 . 
     The electric motor  5  is situated axially parallel to the rotation axis  9 , next to the spindle unit  7  and, together with the spindle holder  29 , is flange-mounted onto the same side of the connecting yoke  35 . An endless belt drive  47  produces the drive connection between the electric motor  5  and the spindle  11 , i.e., between their belt pulleys. 
     In this exemplary embodiment, the angle of the imbalance vector to be measured is determined directly at the spindle  11  and not at the electric motor as in conventional imbalance measuring devices—but this does not limit the invention to this approach. 
     The structural unit comprised of the electric motor  5  and spindle unit  7  in this exemplary embodiment is secured to the housing or machine base  1  by means of a holder suspension  49  detachably fastened to the spindle holder  29 . In this exemplary embodiment, the holder suspension  49  includes two essentially plate-shaped holder elements  51 ,  53 . The first holder element  53  is fixed relative to the spindle unit  7 —in the present case by being fastened to the spindle holder  29 , possibly also through the use of an intermediate piece. The second holder element  51  is fixed relative to the machine base  1 ; here, too, it is immaterial whether this holder element is fastened directly to the machine base or is fastened to an intermediate piece that is in turn correspondingly fastened to the machine base. 
     As is particularly clear from  FIGS. 2 and 3 , the two holder elements  51 ,  53  are fastened spaced a predetermined distance apart from each other by a plurality of spacers—here embodied in the form of leaf spring elements  55 . In the present case, a total of four leaf spring elements are used. Two of the leaf spring elements are situated one above the other on the one side of the two holder elements and are situated in a common plane parallel to the rotation axis  9 , see  FIG. 3  in particular. This plane is referred to below as the main plane of the leaf spring elements. Two additional leaf spring elements, arranged in corresponding fashion, are situated on the opposite side of the holder elements in another main plane. 
     The leaf spring elements  55  are essentially rigid in opposition to force components that act on them in the direction of their main plane. However, the leaf spring elements only offer a negligible bending resistance to the force components that act on them in the direction normal to their main plane. 
     The holder elements  51 ,  55  are thus secured to each other, spaced apart by an essentially constant distance, but when influenced by imbalance-induced forces, can move relative to each other in a direction that essentially corresponds to the normal to the main plane of the leaf springs—this direction is also referred to below as the measuring direction. Strictly speaking, a certain—even if only slight—rotating motion of the holder elements relative to each other is also possible in this case. Such a rotating motion can occur, for example, due to the fact that the two leaf springs situated in the lower region are momentarily deflected more powerfully than the two upper leaf springs. 
     It should be generally noted that the movements that the respective imbalance induces in the spindle unit are slight. In tools that are intended for operating speeds of 20,000 RPM or more, these movements typically lie in the range of ± a few hundredths of a millimeter up to a few tenths of a millimeter. 
     This movement of the holder elements  51 ,  53  is measured so as to obtain a signal that enables a conclusion to be drawn about the magnitude and location of the imbalance. To this end, a plurality of sensors, preferably two of them, are situated between the holder elements in the measuring direction. Since only small movements are to be detected, force transducers are used, usually of the piezo-electronic type. Sensors that detect the instantaneous speed and/or acceleration of the holder elements relative to each other are conceivable, but remain theoretical with such small movements. 
     As is clear from  FIGS. 2 and 3 , one of the sensors is situated between the holder elements  51 ,  53  close to the upper end of the spindle unit  7  while the other of the two sensors is situated between the holder elements  51 ,  53  close to the lower end of the spindle unit  7 . Such a sensor arrangement makes it possible to measure imbalances that are unevenly distributed along the rotation axis and therefore result in wobbling motions of the spindle rotation axis  9 . 
     As is particularly clear from  FIG. 2 , each of the holder elements is provided with a projection  57 ,  59  for each of the sensors. Between each pair of these projections  57 ,  59 , a sensor  61  is mounted, which records the force that these two projections exert on each other under the influence of the imbalance. 
     As is clear from  FIG. 2 , the force sensors  61  rest by means of contact ball bearings  63  against the projections  57 ,  59  associated with them in order to isolate the sensors as much as possible from the influence of potential transverse forces that can corrupt measurement results. The mounting of the sensors on the holder elements and the clamping of the holder elements relative to each other will be described in greater detail further below. 
     Despite the above-mentioned contact ball bearings  63 , however, the occurrence of transverse forces on the sensors  61  must be avoided as much as possible; it is therefore necessary to prevent the plate-shaped holder elements  51 ,  53  from moving transversely relative to each other, i.e., in the direction of a normal N to the measuring direction M (in the vertical direction in the present case), even with only a slight amplitude. This can occur, for example, when a heavy specimen is inserted into the spindle unit or is dropped. In the extreme case, this can result in damage to the holder suspension  49 . 
     According to the invention, an auxiliary bearing  73 ,  75 ,  77  is provided in the gap between the two holder elements to prevent this from occurring. This auxiliary bearing includes a ball bearing  73  via which the holder element  53  associated with the spindle unit  7  is supported against the holder element  51  associated with the machine base, see  FIG. 3 . For this purpose, the holder element  53  associated with the spindle unit is provided with a support  77  that protrudes into the gap, by means of which it presses onto the ball bearing  73  from above. The ball bearing in turn rests against a base support  75  that belongs to the other holder element  51  and protrudes into the gap. 
     This ball bearing  73  permits the two holder elements  51 ,  53  to roll against each other essentially unhindered in the measuring direction. But in a direction perpendicular to the measuring direction, in this case namely in the vertical direction, the two holder elements  51 ,  53  are connected via the ball bearing  73  and the leaf spring elements  55  to an intrinsically almost rigid holder suspension  49 . 
     The auxiliary bearing or more precisely the ball bearing  73  is arranged so that it is situated essentially halfway between the upper and lower sensor. Because of this arrangement, the auxiliary bearing does not hinder the function of any of the sensors, even if the influence of corresponding imbalances causes the spindle unit to execute a kind of minimal pivoting motion around the ball bearing  73  in which the upper part of the holder element  53 , for example, moves in the measuring direction and the lower part of the holder element moves in the opposite measuring direction. 
     The ball bearing  73  is secured in captive fashion in that its sides, which are not situated in the flow of force, engage with the required amount of play in a small hollow that is provided for this purpose in each of the holder elements close to the support  77  and close to the base support  75 . These will be described in greater detail further below. 
     This embodiment can be implemented very simply and without incurring any appreciable costs. Ball bearings are standard components that can be obtained at a low cost. Also, it makes no difference in terms of cost to provide the support  77  and the base support  75  as additional equipment to the two holder elements  51 ,  53 , which are present anyway. 
     A decisive advantage is the fact that the holder suspension  49 , which is embodied according to the invention and accommodates the ball bearing between the holder plates  51 ,  53 , constitutes a structural unit that is intrinsically closed to a large extent and only has a few definite interfaces with the surrounding components. This unit can be installed with ease, without having to take into account any other bearings. This can be referred to as a so-called “cartridge embodiment”. 
     The advantage of such an intrinsically closed unit comes into play primarily when the balancing machine must be repaired on site. This is true because particularly when the two holder elements  51 ,  53  are provided with a dovetail guide or a similar indexing, which predetermines their precise position on the spindle unit and machine base, respectively, even when being reinstalled on them, the holder suspension  49  can be easily installed and removed or replaced on site without requiring readjustment of the machine. The problems affecting the leaf springs, support bearing, or sensors and causing the machine to malfunction can then be reliably and quickly remedied by replacing the entire holder suspension  49 . It is thus possible to precisely repair the defective holder suspension again at the plant. 
     Another advantage lies in the fact that the contour of the flange surfaces of the cartridge-like holder suspension  49  is not influenced by the ball bearing  73  accommodated in the gap between the holder plates  51 ,  53 . As a result of this fact, it is also suitable to use the holder suspension  49  according to the invention for purposes of retrofitting and as a replacement for previously installed holder suspensions identical to it, which are mounted on the flange. 
     It should be noted that despite the presence of the auxiliary bearing  73 ,  75 ,  77 , the leaf springs  55  are generally still mounted on the two holder plates in order for the leaf springs to provide additional securing in the vertical direction as well. Theoretically, however, it would also be conceivable for the leaf springs to now be connected to the holder plates so that they articulate in the vertical direction. It is thus possible, where necessary, for all vertical forces to be deflected via the ball bearing  73 . 
     As mentioned above, the holder suspension  49  according to the invention has still other advantageous features that are of interest specifically for a “cartridge embodiment” and should therefore be described in greater detail now. 
     One of the holder elements (e.g., the holder element  51 , see  FIG. 2 ), is provided with another projection  65  in addition to the projection  57  so that the projection  59  of the other holder element  53  is situated between these two projections  57  and  65 . An elastic prestressing element  67  is clamped between the projections  65  and  59 , and provides for a certain amount of prestressing of the sensor  61 . Adjusting screws  69 ,  71  situated in the projections  57  and  65  at opposite ends from each other in the measuring direction make it possible to adjust the position of the sensor  61  and to adjust the prestressing force of the prestressing element  67 . 
     Each of the two sensors  61  and the resilient prestressing element associated with it is thus supported against the opposing holder element  53  so that the projection  59  of this holder element  53  transmits the forces exerted on it during operation to the other holder element  51  via the prestressing element  67  or via the force sensor  61  (if a speed or acceleration sensor is used instead of a force sensor, then the sensor is connected in parallel with another spring element, which is not shown here). This ensures that the holder elements are always coupled to each other directly and the leaf spring elements  55  are not stressed, i.e., are not appreciably used to transmit force between the holder elements in the measuring direction. 
     It should also be noted that the leaf spring elements  55  are associated in pairs with the force sensors  61  and are situated opposite each other, likewise in pairs, in the measuring direction. Otherwise, the prestressing elements  67  are supported in articulating fashion in order to avoid transverse forces between bearing points, as shown in  FIG. 2 . 
     The force sensors  61  situated spaced apart from each other at the upper and lower end of the spindle unit  7  are preferably supported in opposite directions on the two holder elements  51 ,  53 . As shown by  FIG. 2  for the upper force sensor  61 , it is supported relative to the rotation axis  9  in clockwise fashion against the projection  59  of the holder element  53  associated with the spindle and in counterclockwise fashion against the projection  57  of the holder element  51  associated with the housing. By contrast, the lower force sensor  61  is supported in clockwise fashion against the holder element  51  associated with the housing and in counterclockwise fashion against the holder element  53  associated with the spindle. The advantage of this arrangement is the fact that with a tilting movement of the spindle  11 , both of the force sensors  61  are either loaded with pressure in the same direction or relieved of pressure in the same direction. As a result, characteristic curve differences of the force sensors that depend on the force direction do not affect the measurement result. 
       FIG. 4  shows a second exemplary embodiment of the invention. Provided that nothing to the contrary is stated below, this second exemplary embodiment corresponds to the first exemplary embodiment and therefore the description given above also applies to the second exemplary embodiment. 
     The difference in this second exemplary embodiment of the invention is the fact that in addition to the holder suspension  49  with the integrated bearing  73 ,  75 ,  77 , another auxiliary bearing in the form of a counter-bearing  79  is used, which is as a rule situated diametrically opposite the holder suspension  49 , in fact at a height that essentially corresponds to halfway between the upper and lower sensor  61 . For this purpose, the spindle unit has a counter-bearing plate  81  mounted on it, which presses via another ball bearing  73  against a counter-bearing plate  83  associated with the machine frame  1 , thus preventing a bending moment, which acts on the holder suspension  49 , from occurring in the spindle unit  7 . In the region of the ball seat, the two counter-bearing plates  81 ,  83  are in principle embodied in precisely the same way as the two holder elements  51 ,  53  at corresponding locations. 
     The design of this exemplary embodiment is very favorable, particularly when the spindle unit  7  is very heavy and/or the motor  5  and spindle unit  7  are combined as a block to form a single unit. Specifically with this embodiment type, it is also possible either for the leaf springs to be embodied as very thin and therefore very flexible and/or for them to be flange-mounted to the holder plates in an articulating fashion in the vertical direction. A particularly thin embodiment of the leaf springs promotes reaction precision. 
       FIG. 5  shows an axial section through the second exemplary embodiment shown in  FIG. 4 . This sectional view clearly demonstrates how the ball bearings  73  are held in their proper position—respective recesses, bores, or hollows  85  are provided on the two holder plates  51 ,  53  and on the two counter-bearing plates  81 ,  83  and accommodate the flanks of the balls  73  situated outside the direct flow of force. Stated more precisely, each hollow  85  accommodates a plastic insert  87  that encompasses the flank of the ball  73  oriented toward it with the required amount of play, thus holding the ball bearing  73  without preventing it from being able to move as required. 
       FIGS. 6 and 7  show a third exemplary embodiment of the invention. Provided that nothing to the contrary is stated here, this third exemplary embodiment corresponds to the first and second exemplary embodiments and therefore the descriptions given above also apply to the third exemplary embodiment. 
     This third exemplary embodiment differs from the first and second exemplary embodiments by the fact that not just a single auxiliary bearing, but several auxiliary bearings in the form of counter-bearings  79  are used. To be specific, a total of three counter-bearings  79  are used here, which are situated offset from one another by (essentially) 90° in the circumference direction, as shown in  FIG. 7 . Each of these counter-bearings  79  corresponds to the counter-bearing described in connection with the second exemplary embodiment, thus minimizing parts complexity. This achieves a four-point support that holds the spindle unit very rigidly in position perpendicular to the measuring direction M (the vertical direction in this specific case), which is advantageous when balancing very heavy specimens but is disadvantageous to the extent that it hinders the tilting of the spindle unit as a result of which it becomes more difficult to draw conclusions about the position of the imbalance in the axial direction of the specimen. 
     Furthermore,  FIG. 7  shows quite clearly how the holder suspension  49  and counter-bearings  79  are each affixed to the spindle unit  7  by means of a respective dovetail guide  104  and as a result can be removed from the spindle unit  7  and reinstalled on it in a reproducible fashion with regard to their precise position. 
     It should be noted that in all of the previously described exemplary embodiments, the ball bearings  73  of the counter-bearings  79  in particular, can be alternatively embodied in the form of rollers instead of balls. These optional rollers are then situated so as to permit a rolling action in the measuring direction M, thus offering no hindrance to the mobility of the spindle unit in the measuring direction. 
     Purely from a patent law standpoint, it should be noted at this point in that as an equivalent to a support on rollers, it would naturally also be conceivable to provide a support on rods, which extend with their longitudinal axes parallel to the spindle rotation axis and which are embodied as flexible in the measuring direction or are connected in articulating fashion in the measuring direction and therefore support the spindle unit (only) in the vertical direction, while permitting oscillations in the measuring direction M. 
       FIGS. 8 through 10  show a fourth high-precision exemplary embodiment of the invention. 
     In this case, the spindle unit  7  is supported not only at certain points by roller elements or by one or more balls  73 , but is instead supported over a large portion of its circumference. For this purpose, the spindle unit  7  or its spindle holder  49  is provided with a support ring  89 . This support ring  89  is functionally associated with a counterpart ring embodied in the form of the bridge  91  in this exemplary embodiment. The bridge  91  is provided with a fork section  93  at its one end and is provided with a support tab  95  at its other end. The fork section  93  embraces and is fastened to the holder plate  53  associated with the machine frame  1 . The support tab  95  rests, possibly with the interposition of a support piece  97 , against the machine frame  1 . The spindle unit  7  is inserted into the central opening  99  of the bridge  91 , extends through it, and is supported with its support ring  89  on the bridge  91  via a ball bearing ring  101 . The ball bearing ring  101  is composed of a number of ball bearings  73 , which are held in position by a conventionally embodied cage  103 . 
     In this way, the spindle unit  7  is precisely supported along its circumference by an arc that spans an angle of 220° to 270°, which naturally provides for an optimum level of precision. 
     The bridge  91  that is fastened to the machine frame  1  at both ends can be simultaneously used to stabilize the machine frame  1  itself. In some cases, this provides some compensation for the additional material cost for the bridge. 
     For the sake of completeness, it should be noted that the holder arrangement including the sensors in this fourth exemplary embodiment can correspond to the holder arrangement  49  of the first exemplary embodiment. A bearing arrangement encompassing such a large amount of the spindle unit can in certain cases make it possible to alternatively use a conventional holder arrangement, i.e. a holder arrangement  49  without an integrated auxiliary bearing or ball bearing.