Abstract:
A grinding center for the simultaneous grinding of a plurality of main and rod bearings and/or central and end-side sections of crankshafts includes first and second stations. Two main bearing grinding spindles, of which the first is movable only in the Z-direction and the second only insignificantly movable in the X-direction, are mounted on a common rod bearing-compound slide. In the final phase of grinding, a correction of variations in size between the two processed rod bearings occurs via a separated drive of the second rod bearing-grinding spindle in accordance with a size or roundness correction. The variations are detected by measuring devices. An inclined profiled grinding wheel is provided for the grinding of the end sections.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a grinding center for grinding crankshafts having main bearings and pin bearings, a plurality of main bearings and pin bearings as well as end-side surfaces, especially of a flange, being ground essentially simultaneously. 
         [0002]    Such grinding centers are used for rough-grinding and/or finish-grinding of large numbers of crankshafts. These are often crankshafts for four cylinder in-line engines in the automobile industry, in which engines each of two pin bearings are arranged at the same angular position with respect to the longitudinal axis of the crankshaft. These two pin bearings are ground simultaneously (time parallel) in order to increase productivity. One such procedure is described for instance in EP 1 044 764 A2 and EP 1 088 621 B1. 
         [0003]    Simultaneously grinding a plurality of bearings has been known for some time for the main bearings of crankshafts, e.g., from U.S. Pat. No. 3,487,588. The grinding spindle for the main bearings has a number of grinding wheels that is equal to the number of main bearings. The grinding wheels are disposed on a common axis. A more recent version of this can be found in DE 101 44 644 B4. 
         [0004]    In the grinding center for crankshafts in accordance with EP 1 044 674 A2, used for simultaneous grinding of two pin bearings on a crankshaft are a rough-grinding wheel and a finish-grinding wheel that are each securely mounted to discrete compound slide rests via the associated grinding spindle. The two compound slide rests can be moved independently of one another in the crankshaft longitudinal direction (Z direction) and can be adjusted relative to the crankshaft (X direction). Appropriately controlling the compound slide rests and the grinding spindles makes it possible to machine two pin bearings simultaneously in one clamping process, the one pin bearing being pre-ground and the other being finish-ground. The grinding process is monitored continuously via associated measuring devices. 
         [0005]    EP 1 088 621 B1 describes a method and an apparatus for simultaneously grinding at least two bearings on a crankshaft that is essentially the same as the grinding center depicted in EP 1 044 764 A2 in terms of design and operation. Each of these systems uses a discrete compound slide rest for each of the two grinding spindles used. Each of these compound slide rests requires separate activation for the entire grinding process and continuous monitoring and correction according to real-time data, acquired via measuring heads, about the roundness and dimensions of the ground bearing. Simply in terms of the machining two bearings, the design of the grinding center with two separate compound slide rests requires a great deal of space and significant complexity with regard to components and associated controls. 
         [0006]    EP 1 718 435 B1 describes a machine for machining workpieces in which machine grinding and/or turning apparatus are present. The grinding apparatus has a profiled grinding wheel that is inclined relative to the Z axis and by means of which the workpiece can be both face ground and also externally cylindrically ground. The tool is turned and ground in the same clamping. 
       SUMMARY OF THE INVENTION 
       [0007]    Proceeding from this prior art, it is the object of the invention to provide a grinding center for grinding crankshafts in which the design complexity and the space requirement are significantly reduced and with which simultaneous grinding of main bearings, pin bearings, and end-side surfaces is possible in a particularly rapid and efficient manner with high quality. 
         [0008]    In the inventive grinding center, the space requirement and structural complexity are reduced simply in that two stations for simultaneously grinding at least two bearings (at the same time) are combined to create one grinding center. In the first station, center sections of the pin-side and flange-side crankshaft end can be ground together with the main bearings, specifically on the face and/or in the diameter, and specifically by means of a profiled grinding wheel, the rotational axis of which is inclined relative to the Z axis of the workpiece; this grinding wheel is preferably arranged in the first station. Because all of the main bearings can be ground simultaneously in the first station, in contrast to the second station there is a time reserve that can be exploited. End-side surfaces, in particular those of the crankshaft flange, are ground time parallel, at least in part, to the main bearings and/or pin bearings. 
         [0009]    When both stations are arranged with an axis orientation that is the same as the crankshafts to be ground, moving the crankshafts from one station to the other is also very simple. Moreover, arranging two grinding spindles for machining pin bearings on a common compound slide rest results in a number of advantages. These additional advantages are in particular simplification of the control of the grinding process and reducing the number of components and the space required. 
         [0010]    The control of the common grinding of two pin bearings occurs inventively in that the advance and monitoring/correction of the abrasion and concentricity of the ground bearing is initially accomplished only by controlling the movements of the common pin bearing compound slide rest. In this phase, the main grinding abrasion is for both pin bearings. It is only when the target dimensions have nearly been attained that the first grinding spindle and the second grinding spindle are controlled differently in terms of movement. The first pin bearing grinding spindle, which is rigidly connected to the pin bearing compound slide rest with regard to the adjusting direction (X direction) of the grinding wheels is further controlled by controlling the pin bearing compound slide rest according to measurement and roundness data obtained via a measuring device such that the required final target values are attained for the grinding process in question. 
         [0011]    The end-side flange of the crankshaft is preferably ground, specifically preferably finish-ground, time-parallel to the main bearings. The grinding wheel provided for this is profiled and inclined to the Z direction with regard to the axis of rotation such that the planar end surfaces and the cylindrical surfaces of the flange or pin can preferably be ground in one work step. 
         [0012]    The roundness values do not necessarily have to be measured at each pin bearing. These correction values can be determined according to a measurement in the control and can be stored for a certain number of crankshafts until another roundness measurement is performed. 
         [0013]    Although the advance of the second pin bearing grinding spindle in this phase does also follow the movement of the pin bearing compound slide rest, another movement component in the X direction overlays this movement. This additional movement component is a differential correction of dimensional and/or roundness deviations that occur on the two pin bearings that are being machined simultaneously. Such deviations can be caused for instance by different abrasion on the two grinding wheels. Another essential reason for this deviation is that the shafts warp slightly during the grinding process since stresses in the material can be released. In accordance with the invention, they are detected using continuous measurement of the dimensions and roundness of the two pin bearings, to which end each pin bearing is provided with corresponding measuring devices. 
         [0014]    The differences between the two pin bearings that are to be corrected are only minor in the final phase of grinding; experiments have shown that they are in the range of hundredths or thousandths of a millimeter. Therefore a very small adjustment range is adequate for the movement of the second pin bearing grinding spindle. This range advantageously should be about +/−0.2 mm. 
         [0015]    There is mutual adjustability between the two pin bearing grinding spindles in the axial direction on the compound slide rest. This makes it possible to adapt to different axial distances between the pairs of pin bearings that are to be ground, and also to adjust for different types of crankshafts. Axial adjustability is usefully included and automatically triggered in the machine control. In general the second pin bearing grinding spindle, which is already arranged to be adjustable in the radial direction, is also embodied to be axially adjustable, but the reverse design is also possible, wherein the second pin bearing grinding spindle is axially fixed on the pin bearing compound slide rest while the first pin bearing grinding spindle is used for axial displacement on the pin bearing compound slide rest. 
         [0016]    Preferably in embodying the invention one design of the drive is for moving the one (second) grinding spindle in the dimensional and roundness correction axis as an NC axis, since in this simple manner it is possible to integrate the CNC machine control. 
         [0017]    There is also an advantage in designing the grinding cell so that the planar sides of the cheeks of the crankshaft, which normally form the transition from the bearing to the actual cheek, are also ground in the first station during machining. The time T 1  can be used and adapted such that two pairs of pin bearings are machined in the corresponding time T 2 . 
         [0018]    The planar sides of the cheeks can be ground at the bearing points on the crankshafts either by displacing the main bearing compound slide rest in the Z direction or in that the main bearing grinding wheels are displaced axially on the main bearing grinding spindle;. However, it is also possible to displace the crankshaft in the axial direction relative to the main bearing grinding wheels. 
         [0019]    Particularly efficient operation of the grinding center results when the machining times T 1  and T 2  for the main bearings or pin bearings are coordinated with one another because then the two stations can be loaded and unloaded simultaneously and thus waiting times are eliminated. 
         [0020]    The pin chasing method is preferably used for grinding the pin bearing, this simplifying the bearing and drive of the crankshaft for machining the pin bearings. The main bearings ground in the first station can be used with nothing further for bearing the crankshaft in the second station, so that it is possible to attain a high level of precision when machining the pin bearings. Moreover, the inventive arrangement and control of the two pin bearing grinding spindles on only one compound slide rest means that there is only a single feed slide. Thus, a single feed slide causes the main movement of the two grinding wheels, specifically the pin chasing movement and the advance. This leads to significant simplification in the control compared to the prior art, since only one feed slide must be monitored and controlled during most of the machining. The control of the movement of the two grinding spindles, which is different in the final phase of the grinding, ensures that any deviations between the two pin bearings are detected and compensated so that ultimately both pin bearings are ground to the target dimensions. 
         [0021]    The clamping and rotational drive of the crankshafts via specially embodied main bearing headstocks and pin bearing headstocks or corresponding tailstocks permits the grinding center to be employed in a particularly flexible manner. Clamping the crankshaft with the option of rotation about the main bearing longitudinal axis or about the pin bearing longitudinal axis permits selection between normal grinding or pin chasing grinding for the pin bearing grinding. 
         [0022]    Continuous measurement of the dimensions and roundness of the bearing being machined permits real-time detection of and extremely precise correction to the grinding. 
         [0023]    The grinding wheel for grinding the flange is preferably arranged opposite the side of the crankshaft on which the grinding wheels for the pin bearings and main bearings are arranged. However, in accordance with another preferred embodiment, it is also possible for all of the grinding wheels to be arranged on one side of the crankshaft. The grinding wheel for grinding the flange and/or the pin is either provided in the first station for grinding the main bearings or in the second station for grinding the pin bearings or in each of the two stations. 
         [0024]    Naturally, in addition to four-cylinder crankshafts other crankshafts can also be ground using an inventive grinding center if they have two pin bearings attached to the crankshaft in the same angular position. Likewise, it is possible to machine camshafts if they have at least two main bearings and two cams arranged in the same angular position. 
         [0025]    The invention also relates to a method for grinding the main bearings and pin bearings and/or center sections of crankshafts. 
         [0026]    In the inventive method, the end-side surfaces of the flange or pin of the crankshaft are ground, at least in part, at the same time as their main bearings and/or pin bearings. 
         [0027]    The grinding center and the method in accordance with the invention are explained in greater detail in the following using the exemplary embodiments depicted in the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a schematic top view of a grinding center according to the invention that is embodied as a grinding cell; 
           [0029]      FIG. 2  is a schematic top view of the first station of the grinding cell, which is used for machining the main bearings of a crankshaft; 
           [0030]      FIG. 3  is a schematic top view of the second station of the grinding cell, which is employed for machining the pin bearings; 
           [0031]      FIG. 4  depicts the clamping of the crankshaft in the first station of the grinding cell; 
           [0032]      FIG. 5  depicts details of the clamping of the crankshaft in the second station of the grinding cell; 
           [0033]      FIG. 6  depicts the arrangement of a device for measuring the dimensions and roundness of a bearing to be machined in the second station; 
           [0034]      FIG. 7  is a section through a grinding cell according to the invention along the section VII-VII in  FIG. 1 ; 
           [0035]      FIG. 8  is a schematic elevation of the first station of the grinding cell having a profiled grinding wheel, for the flange, arranged opposite the main bearing grinding wheels; 
           [0036]      FIG. 9  is an elevation in accordance with  FIG. 8 , but with the profiled grinding wheel arranged on the side of the main bearing grinding wheels; 
           [0037]      FIG. 10  is a schematic elevation of the second station of the grinding cell having a profiled grinding wheel, for the flange, arranged opposite the pin bearing grinding wheels; 
           [0038]      FIG. 11  is a partial section along the section line XI-XI in  FIG. 5 ; and 
           [0039]      FIG. 12  is a partial section along the section line XII-XII in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]      FIG. 1  is a top view of a grinding center embodied as a grinding cell  1 . This grinding cell has a common machine bed  2  on which are arranged two stations  3 ,  4  for machining crankshafts  22  by grinding. The stations  3 ,  4  have a common grinding table  5  on which holding apparatus and drives for each of the crankshafts  22  are present. The grinding cell also normally has a machine cover and loading and unloading apparatus for feeding in and removing the crankshafts  22  and for transporting them from the first station  3  to the second station  4 . These are not shown in  FIG. 1 , however, nor is the CNC control device with input keyboard or hydraulic and/or pneumatic supply devices shown. 
         [0041]    The first station  3  for the grinding cell  1 , which is depicted individually in  FIG. 2 , is for grinding the main bearings  23  of the crankshafts  22 . To promote understanding, the most important functional parts in the first station  3  therefore have “main bearing” added to their identifiers. The main bearings  23  ( FIG. 4 ) are ground by means of a plurality of main bearing grinding wheels  10  that are arranged on a main bearing grinding spindle  9 . The main bearing grinding spindle  9  itself is attached to a main bearing compound slide rest  6  that can be moved, CNC controlled, in the Z direction, which corresponds to the crankshaft longitudinal axis  29 , and in the X direction, which permits an adjustment perpendicular to the crankshaft longitudinal axis  29 . Guide or slide tracks on which the main bearing compound slide rest  6  is moved in the Z direction are not shown because they are covered by covers  16 . The crankshaft  22  to be machined is clamped between a main bearing workpiece headstock  7  and a main bearing tailstock  8 , as is shown in greater detail in  FIG. 4 , and in accordance with the depiction in  FIG. 2  is caused to rotate by the main bearing headstock  7 . At least two main bearings  23  on the crankshaft  22  are rough-ground or finish-ground simultaneously in the first station  3 , a time T 1  being required for this. 
         [0042]    The second station  4  in the grinding cell  1 , which is depicted individually in  FIG. 3 , is employed for machining the pin bearings  24  through  27  on the crankshaft  22 , two pin bearings  24  through  27  that are disposed in the same angular position with respect to the crankshaft longitudinal axis  29  being ground simultaneously. The time required for grinding all four pin bearings  24  through  27  is T 2 . To promote understanding, the most important functional parts of the second station  4  therefore have “pin bearing” added to their identifiers. 
         [0043]    The crankshaft  22  to be ground is also clamped centrally in the second station  4 , i.e. the common longitudinal axis of the clamping devices on both sides is the same as the longitudinal axis  29  of the crankshaft  22 , which is defined by its main bearings  23 . As can be seen from  FIGS. 3 and 5 , in the second station  4  the crankshaft  22  is clamped at its exteriorly disposed main bearings  23 , which have been ground in the first station  3 . This produces a precise reference for the pin bearings  24  through  27  to the main bearings  23  of the crankshaft. 
         [0044]    In accordance with  FIG. 3 , pin bearing workpiece headstocks  12 ,  13  are provided on both sides of the crankshaft  22  for clamping. The chucks  31  for these pin bearing workpiece headstocks  12 ,  13  are provided with supports and each is driven by the C 1  or C 2  axis, which rotate absolutely synchronously. However, in the second station  4  the crankshaft  22  can also be received between tips and is then driven by a pin bearing workpiece headstock  12 , at least only on one side, the chuck of which is provided with floating clamping jaws  33  and effects an equalizing, radially no-clearance rotary drive. The crankshaft  22  is then aligned by centering it on the centering tips. 
         [0045]    The manner in which the crankshaft  22  is received in the second station can be varied and optimized according to the individual circumstances. In both stations  3  and  4 , the crankshaft  22  can be supported by one or a plurality of self-centering steadies. 
         [0046]    Provided in the second station is a pin bearing compound slide  11  that can be moved in the direction of the axes Z 2  and X 2 , which are perpendicular to one another, and thus can be moved parallel to the crankshaft longitudinal axis  29  and perpendicular thereto. The pin bearing compound slide  11  supports a first pin bearing grinding spindle  14  and a second pin bearing grinding spindle  15 . The first pin bearing grinding spindle  14  is securely connected hereby to the pin bearing compound slide  11  in the direction perpendicular to the crankshaft longitudinal axis  29 . In contrast, the second pin bearing grinding spindle  15  is arranged movable in the direction perpendicular to the crankshaft longitudinal axis  29  on the pin bearing compound slide  11 . Its movement is controlled based on a dimensional or roundness error that is obtained from an in-process measurement during grinding. To this end, in-process measuring heads  19  for a measuring device  20  ( FIG. 6 ) continuously measure the diameter of the pin bearings  24 ,  27  or  25 ,  26 , which are ground in pairs, during the grinding. 
         [0047]    Each of the two pin bearing grinding spindles  14 ,  15  supports a pin bearing grinding wheel  17 ,  18  whose axial distance from one another must be equal to the distance between the pin bearings  14  through  17  that are to be ground in pairs. To this end, the two pin bearing grinding spindles  14 ,  15  must be movable relative to one another axially on the pin bearing compound slide rest  11 , that is, in the direction of the rotational axis of their pin bearing grinding wheels  17 ,  18 . The axial distance between the pin bearing grinding spindles and pin bearing grinding wheels must be adjusted every time a different type of crankshaft is to be ground or when a specific crankshaft that has a pair of pin bearings with a different distance between them is to be ground next. To this extent the change in the distance must be included in the entire control of the grinding process. The first pin bearing grinding spindle  14  or the second pin bearing grinding spindle  15  can be arranged displaceable in the direction of its longitudinal axis on the pin bearing compound slide rest  11 . 
         [0048]      FIGS. 5 ,  11  and  12  provide a particularly clear depiction of the particularity of crankshafts  22  for four cylinder in-line engines: the two outer pin bearings  24  and  27  have the same angular position with respect to the rotational and longitudinal axis  29  of the crankshaft  22 , as do the two interior pin bearings  25  and  26 , the angular position of the two pairs of pin bearings  24 ,  27  and  25 ,  26  differing from one another. 
         [0049]    This attribute is used for operating the inventive grinding center in an economic manner. Specifically, the two pin bearings  24 ,  27  and  25 ,  26  are each ground simultaneously using the two pin bearing grinding wheels  17  and  18 , the term “simultaneously” also having the same meaning as the grinding terms “time-parallel” or “at the same time”. In any case, what is meant is that the grinding process unfolds in approximately the same time, but not that it must be ended at exactly the same point in time. The second pin bearing is frequently not finish-ground until after the first pin bearing, in that e.g. a dressing amount of 0.02 mm is to be removed. 
         [0050]      FIG. 6  depicts the arrangement of a measuring device  20  for continuously measuring the roundness and dimensions of a pin bearing in the second station  4   
         [0051]    by means of a measuring head  19 . During grinding, the measuring head  19  is positioned against pin bearing  24 - 27  that is to be monitored and continuously generates signals regarding the dimensions and/or roundness of the pin bearing  24 - 27 , which signals are evaluated by the CNC control and used to generate control commands for the drives for the pin bearing compound slide  11  and/or the dimensions and roundness correction axis  44 . The position of the measuring device  20  indicated by the broken lines is a retracted position that the measuring device  20  assumes for instance during a dressing process and/or when the parts of the pin bearing grinding wheels  17 ,  18  are being handled. 
         [0052]      FIG. 7  depicts a schematic side elevation of the first station  3  in the grinding cell  1  in accordance with the section VII-VII in  FIG. 1 . 
         [0053]    At the beginning of the pin bearing grinding in the second station  4 , the mutual axial distance between the two pin bearing grinding wheels  17 ,  18  is adjusted, for instance, to the distance between the pin bearings  24  and  27 . Then grinding of these pin bearings  24 ,  27  begins with the pin chasing method that is CNC controlled. For this, first the two pin bearing grinding spindles  14 ,  15  are moved together perpendicular to the crankshaft longitudinal axis  29 . The second pin bearing grinding spindle  15  remains stationary relative to the pin bearing compound slide rest  11 . This applies both to the rough-grinding phase and the finish-grinding phase. However, the diameter just attained for each of the pin bearings  24 ,  27  is measured during grinding and its roundness is determined. As the finished dimensions are neared in the finish-grinding phase, the movement by the second grinding spindle  15  is decoupled from that of the pin bearing compound slide rest  11 . The pin bearing compound slide rest  11  is moved according to the measurement on the pin bearing  24  in the sense of a dimension or roundness correction axis  44 , the final dimensions and the required roundness of the pin bearing  24  finally being attained by means of the first pin bearing grinding spindle  14 . The second pin bearing grinding spindle  27  simultaneously performs correction movements with respect to the pin bearing compound slide  11  according to the separate measurement on the pin bearing  27  if the measurements for the pin bearing  27  differ from those for the pin bearing  24 . These differences result from the continuous measurement for both pin bearings  24  and  27 . The computer for the machine control analyzes the measurement results and provides corresponding correction and control signals for the drive for the second pin bearing grinding spindle  15 . 
         [0054]    Naturally, the second pin bearing grinding spindle  15  only needs to be slightly movable in the direction of the X axis with respect to the pin bearing compound slide rest  11 . An advantageous displacement path, in practice, can be, for instance, in the range of +/−0.2 mm. The grinding center can be adjusted such that the grinding time T 1  is equal to the grinding time T 2 . Two of the main bearings  23  are then ground in approximately the same time as a pair  24 ,  27  or  25 ,  26  of the pin bearings. 
         [0055]    Then the pin bearing compound slide rest  11  is withdrawn, the distance between the two pin bearing grinding spindles  14 ,  15  is adjusted to the distance between the center pin bearings  25 ,  26 , and the grinding cycle starts over. 
         [0056]      FIG. 8  provides a simplified schematic drawing of the first station in the grinding cell, in which drawing the main bearings  23  on the crankshaft  22  are undergoing multilayer grinding by means of main bearing grinding wheels  10 . In the first station  3  the main bearing grinding wheels  10  grind the main bearings  23 . If the planar surfaces of the cheeks of the crankshaft  22  that have the main bearing pins are to be ground, the spindle with the main bearing grinding wheels is moved axially relative to the crankshaft  22 . However, it is also possible for the crankshaft  22  to be moved along its rotational axis relative to the main grinding wheels  10 . A profiled grinding wheel  45  is arranged opposite the main bearing grinding wheels  10  on a spindle  46  that is inclined relative to the Z axis, i.e. to the spindle axis of the main bearing grinding wheels  10 . The grinding wheel  45  is profiled such that and its angle to the Z axis is arranged such that the flat end faces and also the cylindrical surfaces of the flange  47  on the crankshaft  22  can be ground simultaneously. The grinding wheel  45  can be adjusted along the adjusting axis X. 
         [0057]      FIG. 9  is an elevation in accordance with  FIG. 8  in which, in contrast to the arrangement in accordance with  FIG. 8 , the profiled grinding wheel  45  with its spindle  46  is arranged on the same side of the crankshaft  22  as the main bearing grinding wheels  10 . The end-side surfaces  48 , specifically the flat end faces and the cylindrical surfaces of the flange, are ground in one work step using the profiled grinding wheel  45 , it being possible to adjust the profiled grinding wheel  45  along its adjusting axis X. 
         [0058]    In accordance with this embodiment, the main bearing grinding wheels  10  are arranged on a common spindle and grind the main bearings between the cheeks  49  of the crankshaft  22 . 
         [0059]      FIG. 10  is a schematic elevation of the second station  4  of the grinding cell having a profiled grinding wheel  45  arranged opposite the pin bearing grinding wheels  17 ,  18  for grinding the cylindrical and flat surfaces  48  of the flange  47  on the crankshaft  22 . The profiled grinding wheel  45  with its spindle  46  can be adjusted along its adjusting axis X and grinds the flange  47  in one work step. The profiled grinding wheel  45  is arranged opposite the pin bearing grinding wheels  17 , 18  in order to avoid any collision of the grinding wheels and in order to obtain simultaneous machining of the surfaces to be machined. The pin bearing grinding wheels  17 ,  18  with their spindles  14 ,  15  grind each pin bearing between the cheeks  49  using the pin chasing grinding method.