Abstract:
A block piece for holding an optical lens to be worked in a machine tool, the block piece having a rotational axis  1.1 , a holding surface that can be made to face the lens and a bearing surface that is directed radially outwards and can be brought to bear in a workpiece spindle, wherein the holding surface has at least one recess  1.2   a  or a number of recesses  2.1 - 2.6 , which are distributed around the rotational axis  1.1 , have a diameter DA and serve for receiving bonding compound for adhesive bonding to the lens.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a one-part block piece for holding an optical lens to be worked in a machine tool, formed from a base body with an rotational axis, with a holding surface that can be made to face the lens and with a bearing surface, oriented radially outwards, and that can be brought to bear in a workpiece spindle, wherein the holding surface has at least one recess or several recesses distributed about the rotational axis or around the circumference, which have a diameter DA and serve for receiving bonding compound for adhesive bonding to the lens. The base body fulfills here the bearing function in the workpiece spindle and at the same time the total holding function for the lens or the spectacle glass. 
       BACKGROUND OF THE INVENTION 
       [0002]    A block piece is already known from DE 10 2004 023 036 A1 for the holding of an optical workpiece, especially a spectacle lens, so that it can be worked, comprising a base body, having an end surface against which the workpiece can be blocked by means of a temporarily deformable material, and a clamping surface, by which the workpiece blocked on the base body can be secured on the spindle of a processing machine. The base body is injection molded from plastic and provided with at least two recesses on its end surface to receive the bonding material, being disposed on either side of an imaginary plane that contains the center axis of the base body and whose boundary surface lying closest to the center axis of the base body forms an undercut. 
         [0003]    Moreover, DE 10 2004 023 036 A1 describes another block piece of metal, which is configured according to German standard DIN 58766 and serves for the use of a low-melting metal alloy as the bonding material. The base body has a central continuous borehole with a first and a second undercut, as well as two blind boreholes disposed inside the continuous borehole. In the blocked state of the spectacle lens (not shown), the continuous borehole is filled with the bonding material as far as the region of the second undercut Consequently, the spectacle lens is held by form-fitting to the block piece in both the axial and the circumferential direction. Thanks to the continuous borehole, the bonding material is accessible, so that an increased heat dissipation can be achieved. 
         [0004]    A block piece is known from DE 10 2007 007 161 A1 that is similar in configuration to the one described above, but is molded from a plastic such as a fiberglass reinforced polyamide. 
         [0005]    A block piece for an optical lens to be machined is known from DE 10 2004 021 703 A1. The block piece can be joined to the lens by a bonding material, while the block piece has a recess in which the bonding material can be secured by form-fitting and/or force-fitting to hold the lens. The recess is fashioned as a closed container for the bonding material, while the container borders on a receiving volume that can be filled with bonding material. 
         [0006]    Furthermore, two-part block pieces are also known from U.S. Pat. No. 3,383,808 B or JP 07/116950 A, which call for an intermediate piece provided with recesses in order to reduce the quantity of bonding compound, being sandwiched or cast between the block piece and the lens. 
         [0007]    Since the bonding material is heated for the purpose of filling and liquefaction, it needs to be cooled down after the filling and casting. The length of this cooldown is dependent on the compound used, so that the skilled person will use as little of the compound as possible. 
         [0008]    It is found with the use of block pieces that the bonding material detaches especially in the area of the center of the lens as a result of its hardening, because an uneven shrinkage process accompanies the cooling, corresponding to the cooling gradient. 
       SUMMARY OF THE INVENTION 
       [0009]    The problem of the invention is to configure and arrange a block piece so that a loosening of the bonding material from the lens is prevented. 
         [0010]    The problem is solved according to the invention in that at least three or four recesses are provided in the holding surface, distributed about the rotational axis or around the circumference, while the ratio of the diameter DA of the recess in the radial direction to the rotational axis to a diameter DL of the bearing surface is between 0.14 and 0.3 or between 0.21 and 0.27 or around 0.25 and/or the ratio of a depth TA of the recess in the axial direction to the rotational axis to a height H of the block piece is between 0.28 and 0.59 or between 0.41 and 0.56 or around 0.53 and/or the ratio of the diameter DA of the recess in the circumferential direction to the rotational axis to a width B 1  of the web ( 4 . 1 - 4 . 6 ) between the recess is between 0.5 and 3.5 or between 1 and 3.0 or between 1.5 and 2.5 or 2.1. 
         [0011]    This ensures that the quantity of bonding compound that can be brought into the center region of the block piece or lens is substantially increased; but this increased mass of bonding compound forms a so-called expansion or overrun reservoir, which prevents a detrimental loosening at the center due to shrinkage caused by the cooldown. When the center of the bonding compound cools down in the area of the zone of contact with the lens, the resulting shrinkage stresses cause an overrunning of the bonding compound present in the recesses, which is now at the end of the cooling chain. A loosening of the lens is prevented. 
         [0012]    It should be noted that the occurrence of shrinkage stresses is supported by the increased rate of cooling, so that the skilled person faced with the shortest possible process times will consider a cooling rate that is not too high. Given these circumstances, he will use as little bonding compound as possible, in order to achieve favorable process times. 
         [0013]    Since the at least four recesses are distributed about the circumference or in a direction about the rotational axis, i.e., at a spacing from each other, the stability of the block piece is also supported with respect to the clamping forces in the work spindle chuck. But this changes the ratio between the surface and the compound being taken up so much—as compared to an annular trench—that on the whole a relatively fast cooldown of this increased quantity of bonding compound occurs. 
         [0014]    The end stop surface for the work spindle, which bounds the bearing surface at the top, is critical to determining the height H of the block piece and the depth TA of the recess. The height S of the web is not a factor of consideration here. 
         [0015]    The recesses distributed about the axis or rotation or around the circumference can be arranged with the same or different radial distances from the rotational axis. To determine the radial distance, one uses a midpoint of the cross section of the particular recess opening situated in the holding surface. The further trend of the diameter of the recess in the axial direction has no influence on this. Thus, if the recess is tilted toward the rotational axis, or tapers conically or broadens out into a cone, this has no influence on determining the radial distance. 
         [0016]    The particular recess ensures a form-fitting coupling of the bonding compound to the block piece acting in the circumferential direction. 
         [0017]    The aforementioned relations of the parameters DA, DL, TA, H &amp; BL ensure a favorable relation between quantity or volume of bonding compound and its surface inside the particular recess. In this way a favorable cooldown rate is dictated, which fulfills the overrun function. 
         [0018]    It can also be advantageous for this if the diameter DA is at least 5 mm-8 mm and/or at least four, five, six or a larger number of recesses are distributed about the circumference or in the direction about the rotational axis. For a larger number, one will consider at first 7, 8, . . . , 23 or 24. With increased number or increased diameter comes a larger quantity of bonding compound. The same holds for the number of recesses, while the product of number and diameter is bounded by an upper limit due to the fixed size of the block piece, i.e., with increasing number of recesses the diameter achievable for each recess becomes smaller. Also with the size of the diameter it becomes easier to fill the hot bonding compound and detach or peel off the bonding compound from the block piece after the unblocking of the lens for the purpose of returning the block piece to the manufacturing process. Recesses with different diameters are also basically possible, each of them having the same or different radial distances from the rotational axis. Recesses are also provided in the shape of grooves, i.e., their diameter as measured in the radial direction is smaller or larger than their dimension in the circumferential direction. The cross section of the particular recess opening situated in the holding surface should also be used to determine the diameter. 
         [0019]    To determine the aforementioned relations of the diameter DA of the recess in the circumferential direction to the rotational axis to the width B 1  of the web in the circumferential direction to the rotational axis, one will use a mean value of all diameters DA and widths B 1 , regardless of possible differences in the case of the radial distances. 
         [0020]    Moreover, it can be advantageous for at least one recess to be configured as a borehole or continuous borehole and/or at least one recess to be configured as a blind hole. A borehole or a blind hole can be made quickly and easily. Basically, these geometry features can also be cast, although a finish machining expense can remain. The dimension of the core borehole is critical to determining the diameter DA of the recess or borehole. 
         [0021]    It can also be advantageous to provide a web with a width B 1  between every two recesses, preferably in the circumferential direction, the width B 1  being between 4 mm and 10 mm in dimension. As already mentioned, the heat dissipation that can be achieved increases with the available surface of the recess. Thus, if the recesses are not joined to each other, a web is ensured, which increases the available surface. The width B 1  of the web is also dimensioned according to the opening cross section of the recesses flanking it. Thus, the width B 1  measured on the holding surface is definitive. 
         [0022]    Furthermore, it can be advantageous for the recess or borehole to have an undercut with respect to the direction of the rotational axis. An undercut can be formed in various ways. The undercuts known from the prior art are used with annular trenches and are thus fashioned in the form of an encircling groove. In the borehole configuration according to the invention, it is advantageous to use a thread inside the borehole and/or a tilting of the axis of the borehole relative to the rotational axis in order to form the undercut according to the invention. A tilting can be advantageous, since this can be realized already with the boring process, without the additional labor of cutting a thread. The tilting of the axis of the borehole occurs either in the circumferential direction, so that at least the depth of the borehole and/or the remaining wall thicknesses di, da can be preserved. A tilting to the rotational axis is also provided, which improves the form-fitting of the contracting bonding compound. Basically, the recess used to form the undercut can also have a conical cross section, i.e., it has an axially increasing diameter. The recess so produced can be either rotationally symmetrical or without rotational symmetry. Although the making of such an undercut involves a further machining step, it also affords much more flexibility in terms of the orientation and/or extent of the circumferential undercut, so that given space can be fully utilized. Thus, an undercut acting in the axial direction is provided, ensuring a form-fitting between the bonding compound and the block piece acting in the axial direction. 
         [0023]    The problem is also solved in that a shoulder broadening the holding surface is provided adjacent to the bearing surface, with an end stop surface placed opposite the holding surface and able to bear axially against the work spindle, the shoulder having an outer diameter D with D&gt;43 mm and the recess being disposed at least partly in the shoulder, while a diameter D 1  of the recess is at least 43 mm in size. The groove is preferably of circular or partially circular shape, so that it can be formed by a turning process. Although the diameter of the block piece is somewhat increased because of the shoulder, which somewhat restricts the lower limit for the size of the spectacle glasses being machined, the diameter according to the claim ensures an advantageously placed connection between the adhesive and the block piece. A detrimental loosening of the bonding compound from the lens is prevented at least in the margin region. An overrun reservoir is also produced in this way, which fulfills the overrun function. The adhesive as a whole cools down very quickly at the margin, so that the size relations mentioned above for claim  1  need not be applied. 
         [0024]    It can also be advantageous to dispose the recess inside the holding surface and/or inside a side surface of the shoulder. Both kinds of arrangement, i.e., orientation of the recess form a desirable undercut in relation to a direction parallel to the rotational axis, bringing about a form-fitting between the bonding compound and the block piece acting in an axial direction. Since thermoplastic bonding compound shrinks upon cooling, the recess situated on the outside in the side surface could justify a greater prestressing of the bonding compound. 
         [0025]    Moreover, it can be advantageous to provide one or more additional grooves inside the holding surface, forming an undercut, while the respective additional groove is tilted inward toward the rotational axis. While this increases substantially the quantity of bonding compound which can be introduced in the central region of the block piece or lens, this increased quantity of bonding compound forms a so-called expansion reservoir, which prevents a detrimental loosening at the center of the lens upon cooldown. If the core of the bonding compound cools down with a time delay, bonding compound can flow out from the grooves, so that detrimental shrinkage stresses are prevented in the region of the contact zone with the lens. The bonding compound inside the grooves also cools with a time delay on account of the increased mass. 
         [0026]    In connection with the configuration and arrangement of the invention, it can be advantageous for the additional groove to be circular or partly circular or spiral in shape. This leads to the same outcome, namely, an increasing of the available bonding compound over a partial region of the coupling zone between spectacle glass and block piece. 
         [0027]    Both the blind borehole and the grooves justify the common benefit that they are never quite completely filled with bonding compound when the bonding compound is applied. There remains an air cushion in the lower part of the recess, closed at the bottom. This air cushion plays on the one hand an insulating role, so that the increased reservoir of bonding compound according to the invention cools down more slowly and serves as an overrun reservoir. Furthermore, due to the pressing of the lens, an excess pressure is established in the recess, resulting in a prestressing of the bonding compound against the lens and counteracting a loosening process. 
         [0028]    It is advantageous to provide at least one groove with a width B 2  opposite the holding surface, the groove being provided at the height of the web in relation to the circumferential direction. Accordingly, the groove is arranged between two recesses, so that in the present case at hand, where both the groove and the recess are conically tapered, an increased depth of the recesses adjacent to the groove is possible. If the groove were in the axial direction relative to the recess, the depth of the recess and thus the quantity of bonding compound that could be taken up would have to be much smaller. 
         [0029]    It can be important to the present invention for the groove to be configured as a keyway or indexing groove. Since this type of block pieces is standardized, especially with respect to the configuration of the outer surfaces and the aforementioned grooves, an adapting of the architecture of the recesses to the given space circumstances is important. 
         [0030]    In connection with the configuration and arrangement of the invention, it can be advantageous for the width B 2  to be larger than the width B 1 . This ensures a narrower web and a correspondingly large borehole. Since the groove is broader than the web, the aforementioned central arrangement between the boreholes is realized in order to avoid an axial overcutting of the groove. 
         [0031]    It can furthermore be advantageous to provide at least one recess opposite the holding surface in relation to the direction of the rotational axis and to configure the holding surface free of passages in relation to the recess. Thus, there is no need to seal off the recess or a passageway when blocking the lens or when filling in the bonding compound. The surface areas available in this way that make contact with the filled-in bonding compound ensure a good cooldown for the latter. Furthermore, the block piece can be held with a suction cup in the central region, for lack of a passageway. 
         [0032]    It can also be advantageous for an end face of the holding surface to be curved in configuration. This ensures an adapting to the convex shape of the lens being taken up and secured and thus an optimal thickness of the bonding compound being introduced. The magnitude of the radius of curvature of the end face is preferably oriented to the radii of curvature of the lenses being fastened or an average value for these. Thus, with the exception of the recesses, a uniform thickness of the bonding compound over the radius can be assured. The exceptions from this uniform thickness formed by the recesses ensure, as already described, a relatively slow cooling overrun reservoir to avoid detachment effects at the lens. 
         [0033]    Moreover, it can be of advantage for the block piece to be metal. The block pieces without passages in the prior art are made of plastic, while the metal block pieces have a passageway in the center. This passageway is omitted in order to increase the contact surface with the bonding compound. The metal configuration, furthermore, ensures sufficient strength in the context of the recesses of the invention. In the area of the central recess, the action of a pushrod placed there to loosen the block piece from its seat is very good, because this region is likewise made of metal. Metal furthermore enables a good cleaning and is also much more robust than plastic. 
         [0034]    In a system according to the invention consisting of a block piece as described above with a lens held against it in sheetlike fashion by means of bonding compound, the adhesion between the lens and the bonding compound is also assured in the area of the center. The bonding compound, regardless of its diameter DV, forms a sheetlike intermediate layer between the block piece and the lens placed at a distance from the block piece. 
         [0035]    Preferably, the diameter DV of the bonding compound is greater than the diameter DL of the bearing surface and greater than the diameter D of the block piece. Thus, adhesion is assured between the lens and the bonding compound beyond the diameter D of the block piece in the marginal region of the lens or the spectacle glass. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    Further benefits and details of the invention are explained in the patent claims and in the specification and shown in the figures. There are shown: 
           [0037]      FIG. 1   a  a top view; 
           [0038]      FIG. 1   b  a bottom view; 
           [0039]      FIG. 2  a sectional representation according to view A-A; 
           [0040]      FIG. 3  a sectional representation according to view B-B; 
           [0041]      FIG. 4  a perspective view from the bottom with blocked lens; 
           [0042]      FIG. 5  a sectional representation of another sample embodiment according to  FIG. 3 ; 
           [0043]      FIG. 6   a  another sample embodiment in the view according to  FIG. 3 ; 
           [0044]      FIG. 6   b  a sample embodiment per  FIG. 6   a  in the view according to  FIG. 1   a;    
           [0045]      FIG. 7  another sample embodiment in the view according to  FIG. 6   b.    
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0046]      FIG. 4  shows a one-part block piece  1  in the form of a base body  1  with a lens  8  blocked by means of a bonding compound  7  in perspective view from the bottom. Starting from a holding surface  2  of the base body  1  (not visible), the bonding compound  7  extends both in the axial and in the radial direction, so that the lens  8  is supported as a whole by means of a diameter DV of around 53 mm to 68 mm. The diameter DV can also be increased to the dimension of the lens  8  being accommodated so as to ensure the largest possible and maximum support during the machining process. 
         [0047]    The block piece  1  or base body  1  has a basic annular shape and is produced generally by cutting from metal. Starting from the holding surface  2  with a diameter D, against which the lens  8  is secured by means of bonding compound  7 , the block piece  1  has a cylindrical bearing surface  3  with a diameter DL at the outer circumference side, by means of which the block piece  1  with the blocked lens  8  can be clamped in a work spindle chuck (not shown). The bearing surface  3  at the front end terminates across a shoulder  1 . 2  with an end stop surface  1 . 5  disposed at the spindle side, whose diameter D is larger than the diameter DL of the bearing surface  3 . At the rear end, the bearing surface  3  is adjoined by a conical centering surface  1 . 3 , which is bounded by a flat end face  1 . 4  at the rear side. 
         [0048]    In the end face  1 . 4  there are provided a central conically shaped recess  6  as well as two keyways  5 . 1 ,  5 . 2  disposed diametrically opposite each other with respect to an rotational axis  1 . 1 . The depth of the keyways  5 . 1 ,  5 . 2  corresponds almost to the height Hz of the centering surface  1 . 3 . The keyways  5 . 1 ,  5 . 2  have an essentially V shape, terminating at the inside with a radius R. Furthermore, an indexing groove  5 . 3  is provided in the annular end face  1 . 4 , arranged with an offset relative to the keyways  5 . 1 ,  5 . 2  in the circumferential direction, ensuring the desired angular orientation of the block piece  1  in the work spindle chuck. 
         [0049]    According to  FIGS. 1   a  and  3 , the holding surface  2  of the block piece  1  has a central depression  9 , extending almost across the entire diameter D, having a depth t of around 0.5 mm. This depth t can vary depending on the curvature of the lens  8  being blocked. Inside the holding surface  2  so formed there are provided six recesses distributed about the circumference or in the direction about the rotational axis  1 . 1  in the form of boreholes  2 . 1 - 2 . 6 . Between every two boreholes  2 . 1 - 2 . 6  there is formed a web  4 . 1 - 4 . 6 , having a width B 1  of around 5 mm. According to  FIG. 2 , a diameter DA of the recess or borehole  2 . 1  is 10 mm. Therefore, one gets a ratio of width B 1  to diameter DA of around 0.5. 
         [0050]    The borehole  2 . 1 - 2 . 6  has a depth TA of around 8.5 mm. So that the holding surface  2  with the depression  9  and the recesses  2 . 1 - 2 . 6  is configured free of openings with regard to the recess  6  or the centering surface  1 . 3 , the boreholes  2 . 1 - 2 . 6  are limited in their depth TA. The respective borehole  2 . 1 - 2 . 6  is arranged relatively close in the region of the bearing surface  3  of the block piece  1 . A remaining wall thickness da between the respective borehole  2 . 1 - 2 . 6  and the bearing surface  3  is around 1 mm. Between the respective  2 . 1 - 2 . 6  and the rear-side recess  6  there remains a wall thickness di of a good 0.5 mm. 
         [0051]    The cross section shape of the block piece  1  shown in  FIG. 2  is generally standardized. The bearing surface  3  already mentioned above has a diameter DL of 43 mm. This is bounded at the top by the shoulder  1 . 2 , while the lower end of the block piece  1  adjoins the centering surface  1 . 3 . The block piece  1  has an overall height of 19 mm, which is composed of the height H of 17 mm of the standardized part of the block piece  1  up to the end stop surface  1 . 5  and a height S of 2 mm for the shoulder  1 . 2 . The height S of the shoulder  1 . 2  can vary. The end stop surface  1 . 5 , however, determines the relative position to the work spindle and thus constitutes a nearly fixed quantity. Coaxially to the rotational axis  1 . 1 , in the rear side of the block piece  1 , the aforementioned recess  6  is provided, having the shape of a truncated cone and no flow connection to the holding surface  2 . In combination with the aforementioned centering surface  1 . 3 , the block piece  1  thus has a circumferentially encircling centering section Z of trapezoidal shape in cross section, bounded by the end face  1 . 4 . 
         [0052]    As already described, two keyways  5 . 1 ,  5 . 2  are provided inside the centering section Z, which start from the lower end face  1 . 4  and taper with respect to the direction of the rotational axis  1 . 1 . Moreover, the indexing groove  5 . 3  is provided within the centering section Z. 
         [0053]    In order to ensure an undercut  2 . 8 , the borehole  2 . 1  per  FIG. 2 , for example, has a thread oriented coaxially to a borehole axis a, which is configured at least across a partial height of the depth TA. All parts or only one part, such as every second borehole  2 . 1 - 2 . 6 , can be configured with such a thread. Alternatively or additionally, an undercut  2 . 8 ′ can be formed by a tilting of the borehole axis a relative to the rotational axis  1 . 1 , as shown in  FIG. 1   a . The ends of the borehole axis a marked with “+” on the opening cross section, on the one hand, and at the lowest point of the recess  2 . 1 ,  2 . 4 , on the other hand, have an offset V relative to each other, which according to the sample embodiment of  FIG. 1   a  (recess  2 . 1 ) extends in the circumferential direction and according to the sample embodiment of  FIG. 1   a  (recess  2 . 4 ) in the radial direction to the rotational axis  1 . 1 . Thus, the depth TA of the borehole  2 . 1 - 2 . 6  and the wall thicknesses di, da can at least be preserved or enlarged. It is possible to provide all boreholes  2 . 1 - 2 . 6  or only some, such as two opposite ones or every second borehole  2 . 1 - 2 . 6  with the offset V in the circumferential direction and/or in the radial direction (recess  2 . 2 ). When using the offset V in the circumferential direction for several boreholes, it is advantageous for the respective offset V to be oriented opposite with respect to a circumferential direction (recesses  2 . 1 ,  2 . 2 ). The offset V can also be oriented in the same direction, especially if it is combined with an offset V in the circumferential direction. If all boreholes  2 . 1 - 2 . 6  are provided with an offset V, an opposite offset provides a form fit acting in the axial direction regardless of a relative movement between the bonding compound and the holding surface in the circumferential direction. 
         [0054]    In  FIG. 1   b , the boreholes  2 . 1 - 2 . 6  are positioned such to the two keyways  5 . 1 ,  5 . 2  with respect to the circumferential direction that the respective keyway  5 . 1 ,  5 . 2  is disposed between two boreholes  2 . 1 - 2 . 6  each time, and therefore in the region of a web  4 . 1 ,  4 . 2 . In  FIG. 3 , which shows the sectional representation of the view B-B of  FIG. 1   b , there is no opening between the respective keyway  5 . 1 ,  5 . 2  and the borehole  2 . 2 ,  2 . 6 . The end face  2 . 7  of the holding surface  2  is flat in configuration. 
         [0055]    The keyway  5 . 1 ,  5 . 2  per  FIG. 1   b  has a width B 2  of around 10 mm and tapers to a width of just 5 mm, terminating in a radius R of 2.5 mm. 
         [0056]    The end face  2 . 7 ′ can also be curved if needed, or be provided with a radius of curvature R 2 , so that an adapting to the shape of the lens  8  being taken up is assured. The magnitude of the radius of curvature R 2  of the end face  2 . 7 ′ varies in the range of 70 mm and 90 mm. It is preferably oriented to the radii of curvature of the lenses  8  being secured or an average value thereof. 
         [0057]    According to the sample embodiment of  FIG. 5  (right side), in the region of the shoulder  1 . 2  there is provided an undercut  1 . 2   a  acting in the axial direction, in the form of an encircling groove  1 . 2   a  with an axial depth t 2  and an aperture angle α. The groove  1 . 2   a  is provided in the shoulder  1 . 2  in the region of the largest diameter D of the block piece  1 , in order to assure the largest possible force of adhesion of the adhesive (not shown) to the block piece  1 , especially when bringing up a tool against a margin of a lens being machined, not shown here. The depth t 2  of the groove  1 . 2   a  increases in the axial direction to the rotational axis  1 . 1 . Therefore, the undercut  1 . 2   a  has a bottom surface  1 . 2   b  sloping toward the outside. The aperture angle α is between 25° and 45°, preferably 35°. The groove  2 . 1   a  is tilted outwardly opposite the rotational axis  1 . 1  and is provided alternatively or additionally to the recesses ( 2 . 1 - 2 . 6 ) shown in  FIG. 1   a - 2  with the borehole axis a. 
         [0058]    According to the sample embodiment of  FIG. 5  (left side), an undercut  1 . 2   a  in the form of an encircling groove is provided in a side surface  1 . 2   c  of the shoulder  1 . 2 , disposed outwardly in the radial direction. The undercut  1 . 2   a  has a triangular cross section, dictated by the shape of the grooving tool. Alternatively, arc-shaped, half-round or rectangular cross section shapes are also provided for the undercut  1 . 2   a.    
         [0059]    The two variants of the groove  1 . 2   a  according to the left and right side of  FIG. 5  are provided alternatively or additionally in the holding surface  2 . 
         [0060]    According to the sample embodiment of  FIGS. 6   a  and  6   b , alternatively to the boreholes there are provided several concentric recesses  2 . 1 - 2 . 6 , configured as an additional groove. The respective groove  2 . 1 - 2 . 6  or an axis of symmetry a of the cross section Q are inclined or tilted with respect to the rotational axis  1 . 1 , so that they form an undercut  2 . 8 ′ with respect to the axial direction. Furthermore, the respective additional groove  2 . 1 - 2 . 6  is tilted inwardly to the rotational axis  1 . 1 . The number of grooves  2 . 1 - 2 . 6  can be varied however desired. It is also provided to have only one or two grooves. 
         [0061]    The grooves  2 . 1 - 2 . 6  are also provided when using a curved end face  2 . 7 ′ (shown by broken line). 
         [0062]    Alternatively to the concentric grooves  2 . 1 - 2 . 6 , in  FIG. 7  a spiral groove is provided, preferably only the inner grooves  2 . 2 - 2 . 6  being configured as a unified spiral groove  2 . 2 , while the groove  2 . 1  provided in the region of the largest diameter D of the block piece  1  is circular in shape. 
         [0063]    All sample embodiments of  FIGS. 5 ,  6   a  and  7  have in common that at least one undercut  1 . 2   a  or groove  2 . 1  is situated at the very outside, in a diameter region between 80% and 100% of the diameter D of the block piece  1 . 
         [0064]    The embodiments of the grooves  1 . 2   a  per  FIG. 5  on the one hand and the grooves  2 . 1 - 2 . 6  per  FIGS. 6   a  and  7  are provided alternatively or additionally. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           1  block piece, base body 
           1 . 1  rotational axis 
           1 . 2  shoulder 
           1 . 2   a  undercut, groove 
           1 . 2   b  bottom surface 
           1 . 2   c  side surface 
           1 . 3  centering surface 
           1 . 4  end face 
           1 . 5  end stop surface 
           2  holding surface 
           2 . 1  recess, borehole, groove 
           2 . 1 ′ recess, borehole, groove 
           2 . 2  recess, borehole, groove 
           2 . 2 ′ recess, borehole, groove 
           2 . 3  recess, borehole, groove 
           2 . 4  recess, borehole, groove 
           2 . 5  recess, borehole, groove 
           2 . 6  recess, borehole, groove 
           2 . 7  end face 
           2 . 7 ′ end face 
           2 . 8  undercut 
           2 . 8 ′ undercut 
           3  bearing surface 
           4 . 1  web 
           4 . 2  web 
           4 . 3  web 
           4 . 4  web 
           4 . 5  web 
           4 . 6  web 
           5 . 1  keyway, groove 
           5 . 2  keyway, groove 
           5 . 3  indexing groove 
           6  recess 
           7  bonding compound 
           8  lens 
           9  depression 
         a borehole axis, axis of symmetry 
         B 1  width 
         B 2  width 
         da wall thickness 
         di wall thickness 
         D diameter 
         DA diameter 
         DL diameter 
         DV diameter of bonding compound 
         H height 
         Hz height 
         Q cross section 
         R radius 
         R 2  radius of 2.7′ 
         S height of shoulder 
         t depth 
         t 2  depth of 1.2a 
         TA depth 
         V offset 
         Z centering section