Abstract:
A method and a device for facing surfaces of workpieces, in particular made of light metal alloys, in which a cutting tool is moved in a feed direction relative to the surface and removes material at a defined thickness by cutting, wherein the cutting tool is moved in a defined feed direction with one or more cutter bars oriented substantially parallel to the surface. The cutter bars are always set at an angle of &lt;90° but &gt;0° with respect to the feed direction. In this way, surfaces with high surface quality can be produced, in particular for workpieces made of light metal, with cost-effective machining parameters.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2013/003098, filed Oct. 15, 2013, which designated the United States and has been published as International Publication No. WO 2014/067619 A1 and which claims the priority of German Patent Application, Serial No. 10 2012 021 275.5, filed Oct. 29, 2012, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for facing surfaces of workpieces, in particular of light metal, and an apparatus for performing the method and a preferred application. 
     Conventional methods of facing surfaces of workpieces are milling with, for example, face cutters that are guided across the workpiece in defined feed directions, furthermore planing or slotting wherein the material is removed in steps which a linear feed motion or, for example, surface grinding. Milling, with which a high surface quality can be achieved, has proven particularly advantageous for workpieces made of light metal, for example, for flat surfaces of components for drive units of motor vehicles, which may possibly also have to exhibit a sealing function. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to provide a method of the generic type which allows with very reasonable processing times high surface qualities of the flat surfaces. Furthermore, a particularly suitable apparatus for carrying out the method is to be specified. 
     The solution of the object is attained with a method for facing a surface of a workpiece, with the steps providing a cutting tool having one or more cutter bars oriented substantially parallel to the surface of the workpiece, wherein the one or more cutter bars are adjusted at an angle &gt;0° and &lt;90° with respect to a defined feed direction; moving the cutting tool relative to the surface in the defined feed direction; and removing material with a defined thickness. A suitable apparatus disposed on a machine tool for facing a surface of a workpiece includes a cutting tool having a support body, a clamping device, and at least one substantially linear cutter bar attached to the cutting tool. The clamping device is constructed to be driven in a defined feed direction and with a rotary motion and optionally with a translational motion. The at least one cutter bar of the cutting tool is oriented substantially parallel to the surface and moved at an angle &gt;0° and &lt;90° with respect to the defined feed direction. 
     According to the invention, it is proposed to move the cutting tool with one or more cutter bars that are substantially aligned parallel to the surface in a defined feed direction, wherein the cutter bars are always set at an angle Pi of &lt;90°, but &gt;0° in relation to the feed direction. By this measure, a cutting path is generated at the cutter bars, which generates practically a combination of planing and slotting with an appropriate cutting angle at the cutter bars and milling with chip formation at an angle with respect to the feed direction. Surprising advantages are low heat generation even with dry machining, high feed rates, use of inexpensive cutting materials due to low cutting speeds, and high surface qualities. 
     According to another embodiment of the method, the angle Pi of the cutter bars relative to the feed direction can be changed continuously or locally during machining, thereby achieving, for example, better adjustments of the machining to the geometric properties of the workpiece or its surface areas. 
     It has also proven to be particularly advantageous to pivot the cutter bars of the cutting tool about a point A of the cutting tool during the feed motion, wherein the point A may be located at one of the end faces of the cutter bars, at the center of cutter bars of the cutting tool in the central axis of the clamping device, or between the end faces and the center of the cutter bars of the cutting tool. This provides another degree of freedom for adapting the cutting path, for example, to the material properties of the workpiece to be machined. 
     According to another advantageous embodiment of the method according to the invention, the controlled feed direction of the cutting tool may be controlled linearly, circularly, elliptically or otherwise nonlinearly, in order to allow further adaptation to individual geometric and material-related conditions and requirements. 
     According to a particularly preferred apparatus for performing the process on a machine tool with a clamping device (spindle) for a cutting tool that can be moved in a feed direction and in a rotation direction, the cutting tool may include a support body, a chuck (sleeve) and at least one substantially linear cutter bar affixed to the support body. The cutter bar may be a diamond bar, a hard metal bar or a bar made of a high alloy tool steel which is according to the method inclined with respect to the feed direction. 
     Alternatively, several staggered cutter bars may be arranged at the cutting tool, which are attached to separate clamping rails and adjustable in height and which are aligned for the cutting function to form a single, continuous cutter bar. 
     According to a very advantageous development of the invention, at least one collection channel for chips produced during machining may be arranged in the cutting tool upstream of the cutter bars. This ensures that there are no adverse effects on the surface to be machined caused by squashing already removed chips. In addition, chips are prevented from entering the component. 
     For this purpose, flushing channels may be provided in the cutting tool by which cutting chips can be removed from the collection channel following the machining process, for example by using compressed air. Accordingly, the chips are held in the cutting tool until the machining operation or at least one machining cycle has been completed, whereafter and only then are the cutting chips flushed out. 
     For wet processing of the surfaces of the workpiece, coolant channels for supplying to the cutter bars a lubricating and/or coolant, for example an emulsion, may be provided in the cutting tool. An MQL medium may be supplied for minimum quantity lubrication (MQL). 
     Lastly, a particularly preferred use of the device is for processing of flat surfaces of cylinder heads, engine blocks and/or gearbox housings of drive units for motor vehicles made of light metal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Several embodiments of the invention will be explained in more detail below with reference to the accompanying schematic drawings, which show in: 
         FIG. 1  a three-dimensional view on a cutting tool designed as a monoblock, with a support body, a massive sleeve as a clamping device and a continuous cutter bar; 
         FIG. 2  a schematic diagram of the orientation of the cutting tool with the cutter bar according to  FIG. 1  along three processing paths, wherein the cutter bar is in each case pivoted about a point A; 
         FIG. 3  the processing of the flat surface on a cylinder head for internal combustion engines in several machining steps using the cutting tool of  FIG. 1 ; 
         FIGS. 4 and 5  respective views taken along the sectional plane I-I of  FIG. 3 , based on which additional machining steps for facing of the sealing surface of the cylinder head are explained; 
         FIG. 6  a cutting tool alternative to  FIG. 1  with several staggered cutter bars; and 
         FIG. 7  the cutting tool of  FIG. 6  in a partial side view, showing the collection channels for chips located upstream of the cutter bars. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A short cutter bar  16  may be a diamond bar; alternatively, the cutter bar  16  may be a hard metal bar or a bar made of high-alloy tool steel (high-speed steel) and is affixed to the support body  12  by a clamping rail  18  and a plurality of screws  20 . In another embodiment, the clamping rail  18  may also be soldered to the support body  12 . 
     The cross-sectional contour of the cutter bar  16  is shaped similar to a planing or slotting tool with an unillustrated cutting angle of the blade. 
     The cutting tool  10  clamped on an unillustrated machine tool can be moved in a rotational and translational fashion along a feed direction s for facing surfaces of workpieces, as described in  FIGS. 2 to 4 . 
     As shown in  FIG. 2 , the cutting tool  10  can be pivoted, under guidance by the machine tool in path  1 , about a pivot point A at an angle Pi of about 45° with respect to the feed direction s and then linearly fed in the feed direction s, wherein facing takes place, with the depth of the cutter bar  16  adjusted according to the surface dimension. 
     The pivot point A for path  1  is positioned at one end face of the cutter bar  16 . The pivot angle Pi may optionally be varied during machining from 0° to 90° with respect to the feed direction s, for example, for creating machining paths having a variable width. 
     The pivot point A of the cutting tool  10  for path  2  is located in the center of the linear cutter bar  16 , wherein the angle Pi can again be varied for adaptation to geometrical and/or material-specific situations. 
     Path  3  shows a pivot point A of the cutting tool  10  which is located between the end-side pivot point A (path  1 ) and its central position (path  2 ) and which can also be changed during processing as needed (see inserted arrows). 
     Paths  1  to  3  show a preferred linear feed motion s. However, this feed motion can also be controlled by the machine tool and be nonlinear or circular, elliptical or the like, and among other things be determined by the selected position of the pivot point A. 
       FIG. 3  shows schematically facing a surface (sealing surface  21 ) of a cylinder head  22  that faces a cylinder crankcase of an internal combustion engine and is made of light metal or an aluminum alloy, wherein facing is performed with the machine tool and the cutting tool  10  in several machining steps. 
     The cutting tool  10  is here moved by the machine tool with a rotationally and translationally controlled motion into the position  10   a , wherein the cutter bar  16  is aligned at an angle in accordance with path  1  of  FIG. 2 ; the height of the cutting tool  10  is then adjusted and the cutting tool  10  is finally fed along the feed direction s 1  parallel to the surface to be produced, with a defined width of the surface being machined in each case. The pivot angle Pi about the point A can be kept constant or optionally varied, as described above. 
     After traversing the path s 1 , the cutting tool  10  is moved, as indicated by dashed lines, to the position  10   b  and traverses the second half of the surface (as evident, with a defined overlap) in an opposite feed motion s 2 , with the cutter bar  16  once more being oriented at an angle with respect to the feed direction. 
     When the surfaces have greater widths, this process can be performed in loops or in a meander pattern until the entire surface of the cylinder head  22  is faced. If applicable, a single machining operation may be sufficient for surfaces having smaller widths. 
       FIGS. 4 and 5  describe additional processing steps for facing the sealing surface  21  of the cylinder head  22 .  FIG. 4  shows in a side view along the section plane I-I the sealing surface  21  subsequent to facing according to the above description of  FIG. 3 . Accordingly, the cutting tool  10  was moved along mutually overlapping machining paths s 1 , s 2  across the sealing surface  21  of the cylinder head  22 . However, the surfaces  23 ,  25  of the machining paths s 1 , s 2  are not, as desired, aligned flat with respect to each other. Rather, the surfaces  23  converge at a transition edge  27  along in a wedge-shape or obtuse manner. 
     One such disadvantageous contour of the surfaces  23  results especially with older processing devices whose cutting tool  10  deviates, due to tolerances, by an angular displacement from an adjusted 90° position which allows planar surface machining. The tolerance-induced angular displacement of the cutting tool  10  corresponds to the wedge angle α shown in  FIG. 4 , with which the two surfaces  23  converge at the transition edge  27 . In  FIG. 4 , the wedge angles α of the surfaces  23  are shown excessively large for sake of clarity. 
     In order to achieve a substantially flat sealing surface  21  of the cylinder head  22  in spite of the tolerance-induced angular displacement of the cutting tool  10 , the cutting tool  10  is moved across the sealing surface  21  along an additional machining path s 3  illustrated in  FIG. 5 . The machining path s 3  is located between the first and second machining path s 1  and s 2 . As a result, material is removed in the region of the transition edge  27 , where the two surfaces  23  converge with a wedge shape, resulting in a more uniform surface contour compared to  FIG. 4 . The material removal is indicated in  FIG. 5  by hatching. 
       FIGS. 6 and 7  show an alternative cutting tool  24 , which is described only to the extent as it differs substantially from the embodiment of  FIG. 1 . Functionally identical parts are provided with the identical numerals. 
     As shown in  FIG. 6 , the cutting tool  24  has a sleeve  14  as clamping means to the machine tool and a support body  12 , in which several, shorter cutter bars  26  are provided in a staggered arrangement. 
     The cutter bars  26  are each height-adjustable and secured on the support body  12  by cassettes  28  and clamping rails  30  using screws generally designated with  20  so as to form overall a perfectly flat cutter bar  26 . 
     Is also important in that the cutter bars  26  overlap in length in the feed direction s and are staggered (arranged one behind the other, see also  FIG. 6 ) with a short distance to each other. Each of the cutter bars  26  may alternatively or additionally be clamped slightly obliquely with respect to a linear feed direction s. 
       FIG. 7  also shows collection channels  32  formed on the clamping rails  28  and positioned in the feed direction s upstream of the cutter bars  26 , which accommodate chips formed during the face-machining and thus keep the chips away from the cutting process. 
     The collection channels  32  are connected to unillustrated flushing channels of the cutting tool  10 , wherein the ablated chips can be blown off after a respective machining cycle via which the collection channels  32 , preferably by using compressed air. 
     Furthermore, unillustrated cooling channels can be provided in the cutting tool  24  (or  10 ) when face-machining with a medium that lubricates and cools the cutter bars  26  (or  16 ) or with an emulsion, with the cooling channels conveying the emulsion to the cutter bars  26  and  16  via a corresponding conveying device. 
     Instead of the cylinder head  22  described with reference to  FIG. 3 , other parts made of light metal, in particular mass-produced parts for drive assemblies of motor vehicles, can be face-machined as described above; for example, sealing or connecting surfaces of cylinder crankcase housings, gearboxes, etc. 
     The cutter bars  16 ,  26  may be designed in the context of possible feed forces and cutting performance to have a length from 0 mm to 200 mm.