Patent Publication Number: US-2007104551-A1

Title: Tool for trimming boreholes

Description:
The invention relates to a tool for trimming boreholes which, for example, end laterally in a cylindrical recess, according to the precharacterising part of claim  1 , as well as to a method for trimming such boreholes according to claim  37 .  
      Such a generic tool is known from the European patent application EP 1 362 659 A1 (application number 03011272.6-1262), published on 19.11.2003, to which the present application expressly refers, and whose content is expressly incorporated into the present application.  
      It has been shown that a tool of the type shown, for example, in FIGS. 20 to 22 of the European patent application EP 1 362 659 A1 is reliably able to neatly and gently remove the burr or residual chip which remains after metal-cutting processing at the point where a borehole leads to a recess, in that the cutting head that rotates in relation to the borehole, said cutting head having been inserted into the borehole so that it comes to rest radially within the location to be trimmed, by means of the device for generating a radial force is made to carry out an “orbital” i.e. a “wobbling” scraping movement or cutting movement along the outlet orifice.  
      In this arrangement the cutting edge of the cutting head, of which cutting edge there is at least one, is on a cycloid in relation to the internal surface of the borehole, which reliably prevents the occurrence of residual chip formation at some other position in the borehole.  
      However, the known tool can only be used optimally if the line of intersection of the outlet point between the borehole and the recess has a relatively short axial extension, which as a rule is the case when the axis of the borehole is essentially perpendicular on the internal surface of the recess, or—if the recess is also a cylindrical recess—when the diameter of the borehole is small in relation to the internal diameter of the recess, and when the axes of the borehole and the recess intersect at a right angle. This is the only way, during application of simple movement kinematics of the cutting head, to effectively preclude the cutting head—in a situation where the cutting edge, of which there is at least one, of said cutting head processes that position of the line of intersection which is closest to the chuck location of the tool—from leaving the inner borehole undamaged in the remaining region.  
      It is thus the object of the invention to improve the generic tool and the trimming method applied with said tool such that, while maintaining simple movement control of the tool, any desired lines of intersection between the borehole and the recess can be effectively trimmed without damaging or excessively scratching the internal surface of the borehole.  
      In relation to the tool, this object is met by the characteristics of claim  1 , while in relation to the method, said object is met by claim  37 .  
      The geometric design, according to the invention, of the cutting head, whose club shape or droplet shape has been modified such that said cutting head in the region of its largest outer diameter has a smooth closed surface, ensures that the cutting head, even if during its wobble-scrape movement carries out axial movement that is not specially coordinated with the line of intersection, in order to cover the entire line of intersection cannot damage the internal surface of the borehole, even if said cutting head processes the position of the line of intersection, which position is located closest to the chuck location of the tool. The cutting edge, of which there is at least one, can engage the line of intersection only where the smooth closed surface can project from the borehole. The tool according to the invention is thus particularly well suited to the processing of lines of intersection of outlet boreholes, where the axis of the boreholes is arranged at an acute angle, preferably at a very acute angle in relation to the internal surface or to the axis of the recess.  
      This results in an additional advantage in that the trimming process can be carried out more economically with the use of the tool according to the invention. The time required for trimming can be reduced because it is no longer necessary to switch off the rotary drive for the tool when the trimming process is completed before the tool is inserted into the next borehole. Due to the smooth closed surface in the region of the largest diameter of the cutting head, said cutting head cannot damage the borehole edge even if the tool is positioned comparatively inaccurately in relation to the borehole axis.  
      The tool according to the invention can be used both for trimming internal lines of intersection and for trimming external lines of intersection.  
      Advantageous improvements are the subject of the subordinate claims.  
      The radial force acting on the cutting head to achieve said cutting head&#39;s preferably controlled radial excursion can be generated in various ways.  
      Advantageous variants are the subject of the subordinate claims  2  to  7  and  8  to  10 .  
      A particularly simple construction is achieved with the improvements according to subordinate claims  2  to  7 . In these claims, a pressurised flow agent, which is present anyway in standard machining centres, for example a coolant and lubricant used in metal-cutting processing, is used for radially deflecting the cutting head so that it carries out the trimming function.  
      In this deflection it is not only the pulse forces caused by the dynamic pressure of the flow agent in the region of the branch duct, but also the pulse forces caused by the deflection of the flow-agent flow that play a role so that the effective radial force remains well controllable.  
      By way of the pressure of the flow agent and/or the geometry of the tool shaft, radial deflection of the tool shaft and thus of the cutting head can be controlled within wide margins so that the radial play of the cutting head in the recess can also be specified comparatively inaccurately. As a consequence of this, the tool becomes more economical. Similarly, the control of the drive device in which the tool is held can be greatly simplified as a result of this because the tool can be positioned comparatively inaccurately in relation to the axis of the recess. The tool can thus be clamped in machines that work with relatively little precision. The tool is self-positioning as a result of its scraping movement on the internal circumference of the recess. It has been found that the operating principle according to the invention is applicable in relation to the entire spectrum of commonly used materials, i.e. steel, grey cast, right across to plastics.  
      Basically a single branch duct is sufficient in order to build up a pressure force in the region between the outlet orifice of said duct and the internal wall of the recess, which pressure force adequately deflects the tool in radial direction for at least one cutting edge to be effectively engaged.  
      A particularly effective manner of machining results if several branch ducts are provided. This modification further makes it possible to affix several cutting edges to the cutting head so that the required machining time can be further reduced. It is also possible for the branch ducts to be staggered in axial direction.  
      Experiments have shown that particularly advantageous results can be achieved with dimensions of the branch duct according to claim  3 .  
      By way of the length of the shaft the radial flexibility of the tool can easily be controlled, wherein there is an advantageous side effect in that a long shaft results in the tool being able to be used more universally, i.e. for trimming boreholes that end relatively deep in the interior of the recess.  
      The field of application preferably covers shaft lengths ranging from 5 to 1,000 mm.  
      In principle the branch duct can be aligned as desired; it can also be curved, for example helical in shape. Preferably, the branch duct, of which there is at least one, is straight, wherein it can be a borehole or an eroded recess. The latter case allows more flexibility in the design of the cross section of the duct.  
      If the cutting edge, of which there is at least one, is set at an angle to the axial plane of the tool, cutting conditions during trimming can be influenced in a targeted way so that working accuracy is enhanced.  
      Good results can be achieved with radial play according to claim  17 , wherein this play is coupled to the extent of working pressure of the flow agent.  
      A very simple alternative design of the device for generating a radial force forms part of claims  8  to  10 . In those claims an unbalanced mass of the tool is used for controlled radial deflection of the cutting head. By way of the rotary speed, the absolute extent of radial deflection can be controlled in a simple manner, which makes it possible to insert the cutting head into the recess or borehole, for example, at a relatively low rotary speed, and subsequently to sufficiently increase the rotary speed so that the desired trimming movement of the tool&#39;s cutter, of which cutter there is at least one, is generated. In this embodiment the design of the cutter head or of the cutters can be identical to that of the previously described variant.  
      A further option of influencing radial deflection consists of optimising the geometry of the tool shaft. With the improvement according to claim  13  the required radial flexibility of the shaft can be further improved.  
      With the improvement of claim  15  insertion of the tool is further simplified. The tool can in principle also be used to trim the entry opening of a borehole on the outside of a body or of a cylinder, wherein in this case the tool is either inserted into the borehole from the inside towards the outside, or the cutting head comprises a cutting edge on both sides of the smooth closed surface. A variant tailored to trimming of lines of intersection located on the inside is the subject of claim  15 . In this arrangement the cutting edge on the undercut side of the cutting head approaches the inside outlet opening of the borehole from the inside. In this process the wobble movement of the cutting head gradually scrapes regions of the borehole burr if it is not aligned in a plane that is perpendicular to the borehole axis, while the remaining regions of the inner wall of the borehole, which regions are axially offset in relation to the trimming position, are exposed to the smooth closed surface which, however, has no influence on the inner surface of the borehole.  
      There are practically no limitations relating to the selection of materials for the tool. Advantageous materials relating to the cutting head are stated in claim  18 , and relating to the shaft in claim  23 , wherein suitable coatings can, in particular, also be used in the embodiment according to claims  24  to  36 .  
      According to claim  6  there is a particular advantage in that in the tool the interface to the flow-agent connection is established with simple means.  
      With the improvement according to claim  7  the tool becomes an easily handled unit that can be inserted into commonly used tool-holding fixtures. In this arrangement the attachment- and fastening body at the same time forms the body for feeding-in the flow agent. This body is preferably in the shape of an elongated hollow cylinder which can even be glued to the shaft of the tool. When it comprises a suitable corrosion-resistant coating, this body can be made from ordinary steel because fixing to the tool-holding fixture can take place in that, by means of the flow-agent pressure that acts on the rear, the cylindrical body is pressed against a shoulder area in the tool-holding fixture.  
      When the effective cutting angle, or in the embodiment involving a milling cutter or a reamer, the tool back rake, is kept positive, for example ranging from 0 to 10°, preferably to 5°, the cutting edge can apply its metal-cutting effect already at relatively light radial pressure forces so that the flow-agent pressure can be kept lower.  
      The embodiment according to claim  21  results in a somewhat scraping effect of the cutting edge, of which there is at least one. The profile of the cutting edges is similar to that of a file, so that machining should be carried out with a higher flow-agent pressure when compared to the embodiment according to claim  20 .  
      If the cutting edge, of which there is at least one, is essentially helical in shape, this results in a particularly favourable cutter design for removing the burr.  
      Improving the tool according to claim  23  has advantages in particular if the shaft of the tool is extremely thin, for example in cases where the trimming procedure is to be carried out in the region of a borehole with a diameter of less than 1 mm that follows on from a comparatively deep borehole that is also of small diameter, for example up to approximately 4 mm. The material selection ensures that even with such a thin shaft design the tool remains sufficiently stable to precisely centre the cutting head even after repeated use. In this way the machining accuracy can be particularly well controlled. The cutting head itself can then be made from other materials and can, for example, be detachably affixed to the shaft of the tool.  
      It has been shown that the flow agent itself can be made of a gaseous medium, such as for example air, in order to generate the forces necessary to deflect the tool shaft. Of course any commonly applied coolants and lubricants can be used, including those used in reduced quantity lubrication techniques.  
      Preferably the device is operated at a flow-agent pressure ranging from 3 to 3,000 bar.  
      If the tool comprises an attachment- and fastening body according to claim  7 , it is advantageous if said fastening body is accommodated in the tool-holding fixture in the manner of a bayonet joint. A particular aspect of the present invention consists of the comparatively high flow-agent pressure to be used to fix the tool in the tool-holding fixture both axially and in circumferential direction. It has been shown that the cutting forces during trimming can easily be absorbed by the frictional force that arises when the attachment- and fastening body is pushed against a holding shoulder by the pressure of the flow agent. This is still further facilitated in that the diameter of the attachment- and fastening body can exceed the diameter of the cutting head. Such a design is described in the European patent application EP 1 362 659 A1.  
      The essential elements of the method, according to the invention, for trimming boreholes, for example boreholes that end laterally in an essentially cylindrical recess, are the subject of claim  37 .  
      The method of claim  38  is associated with a particular advantage in series machining of boreholes, where a multitude of boreholes have to be reliably trimmed in the shortest possible time. According to the invention the rotary drive of the tool does not have to be switched off after leaving a borehole and before the tool enters the next borehole.  
      Further advantageous embodiments form part of the remaining subordinate claims. 
    
    
      Below, several exemplary embodiments of the invention are explained in more detail with reference to diagrammatic drawings. The following are shown:  
       FIG. 1  shows a lateral view of a tool for trimming boreholes that end laterally in, for example, a cylindrical recess;  
       FIG. 2  shows the detail II from  FIG. 1 ;  
       FIG. 3  shows the partial section III-III from  FIG. 2 ;  
      FIGS.  4  to  6  are large-scale views of the tool according to FIGS.  1  to  3  in various operational phases of the machining process;  
       FIG. 7  shows a diagrammatic partial view of a variant of the tool according to FIGS.  1  to  3  with the accommodation and fixture in a tool-holding fixture being indicated;  
       FIG. 8  shows the view “VIII” of  FIG. 7 ;  
       FIGS. 9A  to  9 D show diagrammatic views of modified cutting heads of the tool;  
       FIG. 10  shows a diagrammatic view of a borehole that is to be trimmed in particularly inaccessible locations by means of a specially designed tool according to the invention;  
       FIG. 11  shows the detail “XI” from  FIG. 10 ; and  
       FIG. 12 , at a somewhat reduced scale when compared to that of  FIG. 11 , shows a tool with which the machining task according to  FIGS. 10 and 11  can be carried out. 
    
    
      In  FIG. 1  the reference character  10  shows a preferably rotationally symmetric finishing tool that is, for example, rotary driven, in an embodiment as a trimming tool, with which it is possible, in a particularly economical way and particularly reliably, to trim the radial inner ends, i.e. the region of the line of intersection  16 , of boreholes  12  which at an acute angle PHI end laterally in an essentially cylindrical recess  14  in a workpiece  18 . However, it should be pointed out that the tool can also be static, and instead, or in addition, the workpiece can be made to rotate. Furthermore, the tool can also be used for trimming outlet orifices on the, for example, external cylindrical surface of the workpiece.  
      The tool comprises a cutting head  22  on a shaft  20 , which cutting head has at least one cutting edge  21 —in the example shown it has a plurality of helical cutting edges that are evenly distributed around the circumference—which cutting edges  21  can carry out metal-cutting processing. Preferably, the cutting head comprises a plurality of cutting edges  21 , which at least in sections extend in axial direction, as shown in  FIG. 2 .  
      The tool comprises an interior flow-agent duct  24 , from which in the region of the shaft  20  at least one branch duct  26  emanates. This branch duct  26  is arranged such that with its outlet orifice  28  it comes to rest at a predefined radial spacing AR (shown enlarged in  FIG. 2 ) in relation to the internal surface of the borehole  12  when the cutting head  22  of the tool has been inserted into the borehole  12  until the cutting edges  21  in the region of the cutting head  22  completely overlap the line of intersection  16 , as shown in  FIG. 5 .  
      As shown in  FIGS. 2 and 3  the cutting edges  21  are distributed around the entire circumference so that the outlet orifice  28  is at circumferential spacing to at least one cutting edge  21 , for example to the diametrically opposed cutting edge.  
      FIGS.  1  to  3  further show that the diameter DS of the cutting head  22  has been selected such that it can be inserted with radial play SR into the borehole  12 . The radial play is preferably up to several tenths of millimetres, e.g. ranging from 0.1 mm to 5 mm.  
      A special feature of the tool consists of the tool being tailored specifically for trimming lines of intersection  16  that have a relatively long axial length EA ( FIG. 1 ), which is for example the case when the axis A 14  of the borehole  14  is arranged at an acute angle PHI in relation to the axis A 12  of the borehole  12 .  
      The cutting head  22  conically widens, starting from the shaft  20 , up to a region  29  of the largest diameter, which region follows on from the region of the cutting edges  21 . The region of largest diameter  29  has a smooth closed surface. The axial length is variable; in  FIG. 3  it is designated A 29 .  
      A round tip section  40  follows on from the region  29 , which tip section  40  is also smooth, i.e. without any cutting edges or without other machining profiles.  
      The cutting head  22  is thus essentially in the form of a droplet.  
      The tool according to FIGS.  1  to  3  thus has a cutting edge design such that a positive effective cutting angle or tool back rake RSW is formed on the cutting edge  21 . In this way a cutting function is imparted to the cutting edge  21 . However, it is also possible to design the angle RSW so that it is negative.  
      Axial and rotatory fastening of the tool in a tool-holding fixture takes place in the manner of a bayonet joint. On its end facing away from the cutting head  22 , the shaft  22  comprises an attachment- and fastening body  44  by means of which the tool can be fastened so as to be torsionally rigid and non-slidable. This body is essentially rectangular in shape and interacts with an undercut recess (not shown in detail) in the tool-holding fixture, which recess is designed in the manner of a bayonet joint.  
      With this design of the tool the following working principle with the effects described below with reference to FIGS.  4  to  6  can be implemented.  
      In order to implement the rotary drive the tool  10  is accommodated in a tool-holding fixture so as to be torsionally rigid and non-slidable. The tool-holding fixture is associated with a rotary drive (not shown in detail), a feed drive and a flow-agent pressure source.  
      However, the feed and/or,the rotary drive can also be provided for the workpiece  18 . Furthermore, an additional rotary drive and/or feed device can be provided for the workpiece  18 .  
      When the borehole  12  in the radial inner outlet region is to be trimmed, the tool  10  is first moved to the borehole  12  (position according to  FIG. 4 ). Due to the radial play SR positioning can be relatively inaccurate, which makes it possible to use relatively inaccurate machines. Furthermore, because the region  29  of the cutting head, i.e. the region of largest diameter, comprises a smooth closed surface, the cutting edges  21  can damage neither the outlet  17  nor the internal surface of the borehole  12 , even if the tool is inserted into the borehole  12  with the rotary drive running.  
      The tool  10  is then inserted sufficiently far into the borehole  12  (or a corresponding kinematically inverse movement ensures a corresponding relative position) for the outlet position, i.e. the line of intersection  16  with the diagrammatically indicated residual chip or burr  18 G, to be reached. This position is shown in  FIG. 5 .  
      At the latest when the front-most cutting edge  21  has reached this position, flow agent, for example water or some other tool coolant and lubricant, or a gaseous flow agent, is fed to the internal flow-agent duct  24  at relatively high pressure of between 3 and 3,000 bar. Thus, interaction with the interior circumferential wall of the borehole  14  results in corresponding dynamic pressure in the region of the outlet orifice  28 , of which there is at least one. In addition, due to the pulse resulting from the deflection of flow agent, a radial excursion force acts on the cutting head  22 , which is subjected to eccentric orbital movement. The cutting edges thus move on a cycloid.  
      If several outlet orifices  28  are provided, they are unevenly distributed on the circumference, such that the sum of the dynamic pressure forces generated in the region of the outlet orifices  28  between the cutting head  22  and the interior wall of the borehole can deflect the shaft  20  in radial direction so that the cutting edge that is situated opposite the resulting dynamic pressure force contacts the burr  18 G that is to be machined, wherein such contact occurs at the line of intersection  16 , thus cutting or scraping along said line of intersection  16 .  
      In other words, at this point in time the tool makes an orbital movement that is superimposed on the rotary movement, with the radius of the orbital movement resulting from the play of the cutting head as shown in  FIG. 5 .  
      The branch ducts  28 , which can also be axially staggered, have, for example, a diameter i.e. an inside diameter ranging from 0.1 to 5 mm.  
      The above description clearly shows that with the pressures of the flow agent as stated, the dynamic pressure forces are sufficient to deflect the flexible shaft  20  to an adequate extent. By means of the length of the shaft, which length can range from 5 to 1,000 mm, the elastic deformation can be controlled.  
       FIG. 2  shows that the branch ducts  26  are of a straight-line design. These ducts can be formed by a borehole or by an eroded recess.  
       FIG. 5  shows that the tool first removes the burr  18 G that is furthest away from the tool-holding fixture (not shown). The burr  18 GN is not necessarily reached by the cutting edges.  
      It is only when the tool is gradually withdrawn in axial direction V (compare  FIG. 5 ) while the supply of flow agent is kept up that the cutting edges  21  come into close enough proximity to the burr  18 GN so as to remove said burr  18 GN. This phase is shown in  FIG. 6 . The diagram shows that in this phase the cutting edges  21  can contact the burr  18 GN as a result of springy deflection of the shaft  20 , but that contact between the cutting edges and the remaining internal surface of the borehole  12  is prevented because it is only the region  29  that contacts said internal surface. However, the region is smooth, i.e. it is not designed to have a metal-cutting or scraping effect, so that the quality of the internal surface remains undiminished.  
      The tool can be made from wear-resistant steel, high-speed steel (HSS, HSSE, HSSEBM), hard metal, ceramics or cermet and can comprise a suitable commonly applied coating.  
      Below, there is a description as to how the tool can be fastened to a tool-holding fixture so as to be torsionally rigid and non-slidable. To this effect reference is made to  FIGS. 7 and 8 , in which a variant of the tool according to  FIG. 1  to  3  is indicated.  
      In  FIG. 7  an attachment and fastening body, designated  44  in FIGS.  1  to  3 , which attachment and fastening body is formed in one piece with the shaft  20 , is designed as a glued-on cylindrical sleeve  144 . Otherwise the tool  110  corresponds to the tool  10 .  
      Those components in the embodiment according to  FIG. 7 , which components correspond to the components of the tool according to FIGS.  1  to  3 , have corresponding reference characters that are prefixed by “1”.  
      The sleeve  144  is made of ordinary steel which preferably comprises a corrosion protection coating. In addition to gluing, a headless screw (not shown) can be used which connects the sleeve  144  to the shaft  120  in a positive-locking manner.  
      The designation  146  refers to a chamfer by means of which a fluid-proof connection to the flow-agent source is established.  
      The special feature of the embodiment according to  FIGS. 7 and 8  consists of the flow-agent pressure being able to be used for holding the tool in a rotationally and axially secure manner in the tool-holding fixture  130 .  
      To this effect a locking plate  150 , which in the face of the tool-holding fixture  130  can be radially slid against a spring  148 , is used, which locking plate  150  comprises a keyhole opening  152 . When the locking plate  150  with activation button  151  is slid downwards against the force of the spring  148  in  FIG. 8 , the larger circular borehole in the locking plate is aligned with a cylindrical recess  154  in the tool-holding fixture  130  so that the tool can be inserted from the front into the tool-holding fixture. As soon as a shoulder  156  of the sleeve  144  moves behind the slide plane of the locking plate  150 , the latter can slide upwards as a result of the action of the spring  148  until it abuts against a pin  158 . In this process the slot-shaped section of the keyhole opening  152  slides along the outer circumference of the shaft  120 . The sleeve  144  is thus, trapped behind the locking plate.  
      If accordingly, as indicated by the arrows in  FIG. 7 , the flow-agent pressure acts on the rear of the sleeve  144 , the sleeve with the hatched area  162  is pressed against the rear of the locking plate. This compression force is adequate to provide rotational securing of the tool, all the more so since the cutting edges of the tool do not have to cut thick chips.  
      It has already been mentioned above that the flow-agent pressure should be increased to relatively high levels in order to ensure adequate radial deflection of the tool shaft. The pressure generation device should be in a position to generate flow-agent pressure ranging from 30 to 3,000 bar. For particular designs of the tool shaft and/or the clearance fit between the tool and the tool-holding fixture, pressures of 3 bar can, however, already be adequate.  
      Preferably the relative rotary speed between the tool and the workpiece is kept within the range of 100 and 50,000 rpm, wherein a cutting speed ranging from 20 to 300 m/min is selected.  
      Instead of using a flow-agent-activated device to generated a circumferential radial force, it is also possible to provide an unbalanced mass attached to the shaft. This unbalanced mass can be designed to be in one piece with the tool, or instead it can be designed to be a separate component on the tool, which component is preferably attached so that its position can be changed.  
      The shaft, too, can comprise a high-strength material, e.g. a hard material, a hard metal, a cermet material or a composite material, such as for example a carbon-fibre-reinforced plastic material, with the elasticity of the shaft being such that the radial deflections of the cutting head and thus of the shaft, which radial deflections occur during the trimming process, occur exclusively in the elastic deformation region.  
      At least in regions the tool comprises a coating, preferably in an embodiment as a hard material coating.  
      The hard material coating comprises, for example, diamond, preferably nanocrystalline diamond, made of TiN or (Ti, Al)N, a multilayer coating or a coating comprising nitrides with the metal components Cr, Ti and Al and preferably a small percentage of elements for grain refinement, wherein the Cr content is 30 to 65%, preferably 30 to 60%, particularly preferably 40 to 60%, the Al content is 15 to 35%, preferably 17 to 25%, and the Ti content is 16 to 40%, preferably 16 to 35%, particularly preferably 24 to 35%, in each case in relation to all metal atoms in the entire coating.  
      The structure of the entire coating can comprise a homogeneous mixed phase.  
      The structure of the entire coating has several individual layers that are homogeneous per se, which alternately comprise on the one hand (Ti x Al y Y z )N, wherein x=0.38 to 0.5, and y=0.48 to 0.6, and z=0 to 0.04, and on the other hand CrN, wherein preferably the uppermost layer of the wear-resistant coating is formed by the CrN coating.  
      An alternative coating essentially comprises nitrides with the metal components Cr, Ti and Al and a small percentage of elements (κ) for grain refinement, with the following composition: 
      a Cr content exceeding 65%, preferably ranging from 66 to 70%;     an Al content of 10 to 23%; and     a Ti content of 10 to 25%, 
 
 in each instance relating to all metal atoms in the entire coating. 
   

      The coating preferably comprises two layers, wherein the lower layer is formed by a thicker (TiAlCrκ)N base coating in a composition as a homogeneous mixed phase that is covered by a thinner CrN covering coating as the upper layer. Preferably, yttrium is used as an element (κ) for grain refinement, wherein the percentage of the total metal content of the coating is below 1 at %, preferably up to approximately 0.5 at %.  
      Finally, according to another alternative, the hard material coating can essentially comprise nitrides with the metal components Cr, Ti and Al, and preferably with a small percentage of elements (κ) for grain refinement, with a structure as a double-layer coating, wherein the lower layer ( ) is formed by a thicker (TiAlCr)N base coating or (TiAlCrκ)N base coating in a composition as a homogeneous mixed phase that is covered by a thinner CrN covering coating as the upper layer, wherein the base coating comprises 
      a Cr content exceeding 30%, preferably 30 to 65%;     an Al content of 15 to 35%, preferably 17 to 25%; and     a Ti content of 16 to 40%, preferably 16 to 35%, particularly preferably 24 to 35%, 
 
 in each instance relating to all metal atoms in the entire coating. 
   

      The overall thickness of the layer should be between 1 and 7 μm.  
      If a thicker base coating and a covering coating are used, the thickness of the lower coating should be between 1 and 6 μm and the thickness of the thinner covering coating should be between 0.15 to 0.6 μm.  
      Preferably the coating is deposited by means of cathodic arc vapour deposition or magnetron sputtering, and the surface of the tool, which surface carries the wear-resistant coating, is preferably subjected to substrate cleaning by means of plasma-supported etching using inert gas ions, preferably Ar ions.  
      The above description makes it clear that the method for trimming the lines of intersection makes do with simple axial movement of the tool  10 , irrespective of the length of the axial extension EA ( FIG. 1 ) of the line of intersection. It is sufficient to move the tool slowly from the position according to  FIG. 5  to the position according to  FIG. 6 . The droplet-shaped design of the cutting head automatically ensures that the cutting edges  21  do not touch the internal surface of the borehole.  
      Of course, the method can also be designed such that during the trimming procedure the cutting head is moved several times to and fro between the positions shown in  FIGS. 5 and 6 , a procedure which can also take place so as to match the gradient of the line of intersection.  
      In relation to the geometry of the cutting head, too, the invention is not limited to the embodiments presented above. Examples for common and sensible embodiments of the cutting head are shown in  FIGS. 9A  to  9 D, which embodiments as far as their shape and cutter design are concerned are guided by the designs of hard metal burrs, for example of the company August Rüggeberg GmbH &amp; Co. KG, PFERD-Werkzeuge, 51709 Marienheide.  
      All the embodiments of  FIGS. 9A  to  9 D share a common characteristic in that the respective cutting edge section  222 ,  322 ,  422  and  522  ends by a predetermined dimension MA in front of the region  229 ,  329 ,  429 ,  529  with the largest outer diameter.  
      In the variant according to  FIG. 9A  or  9 C the cutting edge section comprises a tooth arrangement in the manner of a micro burr, while the embodiments according to  FIGS. 9B and 9D  comprise coarser cutting edges. The diagrams show that the axial length of the cutting edge section can be varied within wide limits, as can the axial length of the region  229  to  529 . Similarly, the alignment of the cutting edges, namely helical according to  FIG. 9B  or axially according to  FIG. 9D , can be selected as required, e.g. depending on the material to be cut. The tip of the cutting head can not only be of cylindrical shape, but also of flame shape, spherical shape, sphero-cylindrical shape, arch-pointed shape, conical pointed shape, arch-round shape or disc shape.  
      With reference to FIGS.  10  to  12  an exemplary embodiment of the invention is explained by means of which it becomes possible to effectively trim extremely small boreholes that are difficult to access. To simplify description, with this embodiment too, those components that correspond to the previously described variants have similar reference characters, which are, however, prefixed by “9”.  
      The borehole  912  to be trimmed is a borehole of, for example, 0.7 mm diameter and a length L of, for example, 6 to 7 mm, wherein this borehole continues on from a deep-hole borehole  970  which also has a small diameter DT of, for example, up to 4 mm and a depth TT of, for example, 80 mm.  FIG. 11  shows the constellation in the region of the borehole  912  at a scale M of 10:1.  
      The dot-dash line shows the tip region of the trimming tool  910  whose cutting head  922  is inserted into the borehole  912  such that the outlet edge  916  can be trimmed.  
       FIG. 12  shows the tool  910  true to scale, namely at a scale M of approximately 5:1.  
      A shaft  920  follows on from a chuck section  944 , with the length LS of said shaft  920  corresponding at least to the dimension TT of the borehole  970 , and with the diameter DS of said shaft  920  being selected such that the shaft  920  can be accommodated with predetermined radial play SR in the borehole  970 .  FIG. 12  shows in dot-dash lines of the borehole  970  the position allocation between the borehole  970  and the tool  910  that has been inserted in the borehole for the purpose of carrying out the trimming process.  
      The shaft  920  again comprises an inner borehole  924  by way of which it is possible to feed pressure agent from the chuck section  944 . Reference character  926  designates a radial duct whose outlet orifice faces the internal wall of the borehole  970  at a predefined spacing.  
      On the end facing away from the body  944 , the shaft  920  carries a so-called trimming lance  974 , which at the end of a pin  976  carries, the actual cutting head  922 . The diameter D 929  of the cutting head is slightly smaller than the diameter D 912  of the borehole  912 . As is also shown in  FIG. 12  the trimming lance  974  is detachably attached to the tool shaft  920 , for example screwed to said tool shaft  920  so that the inner borehole  924  is closed off.  
      The description of the tool shows that when the inner borehole  924  is subjected to pressure, radial deflection of the shaft  920  and thus of the cutting head  922  can be caused as a result of the circumferentially uneven distribution of the radial boreholes  926 , by means of which deflection the trimming process can be carried out. The region  978  of the borehole  912  can be trimmed in the same manner as position  916 . To this effect the cutting head can also comprise a cutting edge design on the other side of the region  929 .  
      Designing the tool according to  FIG. 12  makes it possible to use different materials for the sections  944 , for the shaft  920  and for the actual trimming lance  974  with the cutting head  922 . Since the shaft  920  in comparison to its diameter DS has a very long axial length, it has been shown to be advantageous to produce this shaft from a high-strength material whose elasticity has been selected such that the radial deflections that occur during trimming are situated exclusively in the elastic deformation region of the material. Suitable materials include hard materials, such as for example hard metals or cermets, as well as composite materials, such as for example carbon-fibre reinforced plastic composite materials.  
      Of course the shape of the cutting head  922  is not limited to the geometric shapes shown. Instead, any common geometric shape can be used, wherein the design of the cutters can also be varied within a wide range. The length L 976  of the pin  976  is selected depending on the axial length of the borehole  912 .  
      In relation to the design of the radial borehole  926  there is also wide scope for its design or variation according to size, position and number, as has also been described in the exemplary embodiments described above.  
      The tool according to  FIG. 12  can of course also be stimulated to carry out the movements required for the trimming process by means of an unbalanced mass integrated in the tool.  
      Of course, deviations from the embodiments described are possible without leaving the fundamental idea on which the invention is based.  
      For example, several internal flow-agent ducts can be provided.  
      If the tool is used for trimming several boreholes that are staggered in axial direction, it is advantageous to carry out flow-agent supply to the tool with increased pressure only when the cutting head reaches the vicinity of the borehole outlet to be trimmed.  
      The invention thus provides a tool for trimming lines of intersection on the ends of boreholes, such as boreholes that end laterally in a cylindrical recess, for example. Said tool has a cutting head which is arranged on a shaft that has at least one cutting edge that extends in the axial direction, at least in sections, and carries out a machining process by a relative rotational movement between the tool and the workpiece. The tool according to the invention is provided with a device for generating a radial force, by which means the cutting head can be radially deflected in the rotational movement thereof in a preferably controlled manner, said cutting head having a diameter that is selected such that it can be introduced into the borehole with radial play. The cutting head is essentially in the form of a droplet and has a smooth closed surface in the region of the largest outer diameter thereof.