Patent Application
20160311077
The invention relates to a method for measuring and producing an outer contour of at least one region of a workpiece by grinding, as well as a grinding machine for carrying out said method.
In-process measurements are known as a manner of continuously measuring workpiece regions directly during machining, i.e., in particular, even during grinding, with corresponding adaptive control of the grinding process on the basis of the current measured workpiece dimensions.
In particular, measurement devices—for example, those of the companies Marposs S.p.A. or JENOPTIK Industrial Metrology Germany GmbH—are used when shaft parts and, in particular, bearing points on crankshafts are being ground.
DE 694 13 041 T2 also discloses a measurement sensor of the company Marposs S.p.A. for controlling linear sizes. That measuring device, disclosed in order to measure inner diameters of holes as well as outer diameters, has a movable sensor in the form of a spherical element, wherein an additional element that transfers deflections to the spherical element is provided. Therewith, the workpiece is measured with respect to the diameter thereof in a contact region on the outer or inner surface, which lies essentially in a plane perpendicular to the longitudinal direction of the component to be measured. With the known measuring device, the spherical element is in contact with an abutment surface over which the element is movable in the oblique direction, wherein the abutment surface is concave in the cross-section thereof, this concavity serving as a seat for the spherical element and guiding the same in the oblique direction. The measurement plane of the diameter to be measured is defined as a reference position.
DE 33 36 072 C2—which was also filed by the company Marposs S.p.A.—also describes a sensing device for measuring linear dimensions. Here, too, known sensing heads measure external dimensions as well as internal dimensions in one plane, perpendicular to the longitudinal axis of the finished workpiece section to be measured. There is no description of measurement of shape deviations, such as, for example, circularity defects.
The prospectus “MOVOLINE In-Prozess-Messtechnik” of the company Jenoptik describes an in-process measurement technique for measuring the larger dimensions of machined workpiece regions, including continuously measuring these dimensions in order to adaptively control the grinding process on the basis of the measured workpiece parameters, as well as optionally using these measurement devices in order to control the circularity, wherein the latter is measured at the end of the machining process (see the measurement systems DF500 or DF700, p. 15). With this known measurement system, there is also a description of working with two measuring heads in the sense of an in-process measurement in order to determine outer diameters. However, even here the shape dimensions are taken after completion of the grinding or after completion of a grinding process step, but are not used for adaptive control.
For the ever-increasing requirements for precision, especially in the grinding industry—for example, in the production of crankshafts, including bearings thereof—it is no longer only necessary to pay attention to having the greatest precision in achieving the required target dimensions in the smallest possible tolerance range, but rather it is also necessary to minimize shape deviations in, for example, the circularity of the workpiece region to be ground, in particular, a bearing point of a centric crankshaft bearing. This requirement applies especially in the manufacture of high-precision shaft sections.
The aforementioned known technical approaches have a problem in that the measurements of, in particular, the diameter of the workpiece regions to be ground preferably always take place in the middle of the grinding disc, which also corresponds to the middle of the bearing point to be ground or the workpiece region. The place of the measurement at a certain point is called a measurement track—that is, in the case described, the measurement track is located in the middle of the grinding disc in the axial direction, as seen over the grinding disc width. If, for example, lubrication holes are to be provided in the grinding region, or if the use of steady rests during grinding is intended, then the measurement track is also arranged off-center, i.e., it is measured off-center.
When the circularity or circularity defects are measured after the grinding with the known systems, then at least the current workpiece is not affected any further. The known and described measurement systems do not deliver sufficiently precise measurement results on the basis of which high-precision grinding results could be obtained if a workpiece region to be ground deviates from the cylindricity or if this region is to be deliberately ground into a cone, crown, or concavity, because the measurement values are only measured in a measurement track.
The present invention addresses the problem of providing a method and grinding machine by means of which both the dimensions and shape of a workpiece to be ground can be detected during the grinding, via in-process measurement, and the target shape can be adaptively corrected on the basis of these detected measurement values.
This problem is solved by a method having the features according to claim 1, and by a grinding machine having the features according to claim 13. Advantageous developments are defined by the respective dependent claims.
According to the invention, with the method, a target outer contour of at least one region of a workpiece—in particular, a crankshaft—is measured with regard to dimensions and shape, and also produced with regard to dimensions and shape through longitudinal or plunge grinding by means of a grinding disc on a grinding center, with a computerized numerical control (CNC) system. Therewith, first, an actual contour on the workpiece or workpiece region is measured. A measurement device detects the measurement values of the dimensions and the shape, namely, in at least two measurement planes that are spaced apart from one another, extend transversely to the longitudinal extension of the respective workpiece region, and are located in the grinding disc engagement region. The at least two measurement planes are produced by a relative movement of the workpiece region and the measurement device in the Z axis direction, relative to the movement of the grinding disc in the direction of the Z-axis thereof. This means that on the one hand, the measurement device can be moved on the workpiece region to be ground in the axial direction of the longitudinal extension thereof, namely, when the grinding disc is fixed, but also means on the other hand that it is also possible for the measurement device to be fixed and for the workpiece to be moved relative to the measurement device. The grinding disc itself may then be moved in the Z-axis direction along the workpiece region to be ground; it is also possible, however, to use a grinding disc of such a width that the entire workpiece region to be ground can be ground for the purpose of plunge grinding, without movement of the grinding disc in the Z-axis direction thereof. The measurement values of the dimensions and shape of the ground workpiece region on the at least two measurement planes are transmitted to the CNC system. This CNC system is controlled on the basis of these measurement values in such a manner that any deviations from the target contour that may be present—namely, with regard to dimensions and shape—are corrected, and the target contour of the workpiece region in question is ground adaptively on the basis of the measurement values that were acquired for the particular measurement planes of a workpiece region. Adaptive grinding should be understood here to mean that for the purpose of in-process measurement, both the dimensions and the shape of the workpiece region to be ground are measured continuously or at intervals, and input into the control device, wherein the control device is configured so as to be adaptively adjustable on the basis of these measurement values, both with regard to the dimensions and with regard to the shape, such as, for example, the circularity of the workpiece section to be ground. This ensures that the quality of the workpiece region to be ground with regard to dimensions and also shape—in particular, circularity—is significantly better than what can be produced with the known grinding and measurement methods.
Thus, with the method according to the invention, the measurement track is adjusted during the grinding over the grinding disc width in the axial direction so that the entire outer contour can be detected during the grinding and so that the measurement values corresponding thereto can be inputted into the control device in order to advance the grinding disc such that the shape deviations can thus be continuously corrected, i.e., automatically compensated.
The method according to the invention is especially applicable to pin-chasing grinding, which is used to grind, in particular, the pin bearings of a crankshaft. The pin bearing can now be ground first in the context of in-process measurement with regard to the diameter and shape of the bearing, as well as with regard to shape tolerances and the shape—for example, cylindricity, conicity, or deviations therefrom—or a crowned or concave shape of the respective bearing journal, namely, as measured over the bearing width. In order to achieve the most precise target contour possible, adaptive grinding realized in a plurality of measurement tracks on the basis of the acquired measurement values is also used when the pin bearings are being ground.
In a preferred embodiment, in which the measurement device moves in the Z-axis direction relative to the grinding workpiece, the measurement device is thus automatically displaced in relation to the width of the grinding disc, i.e., in relation to the geometric longitudinal axis of the workpiece to be ground. The number of the measurement tracks or measurement planes to be used on the workpiece to be ground depends on the required precision and also on the target shape of the outer contour to be measured.
Preferably, deviation from the shape—such as circularity, cylindricity, conicity, crowning, and/or concavity—is measured through two measurement planes spaced apart as widely as possible on the workpiece region; further preferably, the measurement planes are adjusted steplessly over the entire measurement region. This is advantageous in that the number of the measurement planes to be measured, or the distance thereof from one another, can be optionally set for any measurement task and for any target contour. In order to reliably determine the crowning or concavity on shaft sections, measurements in at least three measurement planes are provided.
Further preferably, the measurement device is stationary on the grinding spindle head, relative thereto in the X-direction, and is arranged so as to be displaceable in the Z-direction relative thereto; the grinding spindle head is also displaceable in the Z-axis direction, such that here, too, the respectively desired measurement planes or measurement tracks can be individually and steplessly adjusted in accordance with the precision and the target outer contour to be ground.
Preferably, the measurement device is moved by means of an electric drive, which is preferably controlled in a freely programmable manner. Freely programmable control endows the measurement device—and, therewith, the flexibility of the method according to the invention—with a high degree of freedom, forming the basis for use on the most diverse target outer contours to be ground.
Preferably, however, it is also possible for the measurement device to be hydraulically or pneumatically moved in the Z-direction. The use of a hydraulic or pneumatic drive device for moving the measurement device or the use of a freely programmable electric drive depends on the respective purpose of use, and on the available budget for the machine on which the method according to the invention is realized.
Preferably, measurements are taken during the grinding, as is the case with in-process measurement. Preferably, such in-process measurement takes place during the finish grinding. However, it is also possible that the advance of the grinding disc is interrupted for the purpose of measurement, and the grinding process continues after successful measurement, wherein the grinding disc remains in the hold position thereof until the measurement process is complete. It is also possible to first acquire the measurement values in the at least two measurement planes after the finish grinding, assess the overall measured contour of the workpiece, and take the results into account when grinding the next workpiece, optionally then with a correction for the contour incorporated into the control, by means of the CNC of the grinding disc.
Often, in particular, for bearing journals, it is required that the target outer contour deviate slightly from an ideal cylindrical shape. Generally, such shape deviation is determined by the intended use of the component, in terms of load and lubrication.
With such relatively low deviation from cylindricity, this deviation is produced by tilting the grinding disc in a horizontal plane about a CNC-controlled axis. The horizontal plane then runs horizontal to the central axis of the workpiece. With the method according to the invention, in such a case, measurements are taken in such a number of measurement planes in the longitudinal extension of the workpiece region to be ground that the target outer shape can be determined with the required high precision, and correspondingly the grinding disc is controlled via CNC thereof in order to produce this target outer shape with regard to the advance thereof onto the workpiece region. The target shape of the workpiece region is generally ground by a grinding program entered into the CNC system, wherein the grinding program is adaptively adjusted as a result of the measurement of the target outer shape, which means that corrections or correction functions are entered into the grinding program so as to make it possible to further reduce defects that would otherwise arise or overlap during the grinding.
Preferably, it is also possible to produce the target shape of the workpiece region to be ground, by means of a grinding disc that has been previously dressed so as to correspond to the desired target shape, the workpiece region being ground in a corrected manner by again dressing the grinding disc. This means that the method according to the invention can also be used with a dressing wheel, so that even regular high-precision dressing of the grinding disc makes it possible to achieve corresponding precision with regard to dimensions and shape on the workpiece region to be ground, in a manner that represents a significant improvement or increase with regard to the precision relative to the prior art.
The method according to the invention thus makes it possible not only to exactly measure the cylindricity, conicity, or crowned or concave shape of a bearing, in particular, of a crankshaft over the bearing width on the grinding machine during the grinding, but also to directly correct same by targeted, adaptive intervention and correction via the grinding program. With the known methods, it has been necessary to first externally measure the crankshaft for this purpose. On the finish-ground workpiece, these shape deviations could also no longer be corrected without the bearing point then being ground, for example, so small that the crankshaft has to be rejected.
This disadvantage is all the more at play when the crankshafts have large dimensions, which is often the case with crankshafts for truck engines or stationary diesel engine aggregates. In particular, when large crankshafts are being ground, the requirements for the cycle time when the crankshafts are produced are not as critical as with smaller components.
This makes it possible to increase the number of measurements that can be performed in a plurality of measurement planes, precisely as in the present invention, which does indeed increase the machining times slightly, but also contributes to significantly increasing the quality of the finished component. Nonetheless, the price of these, in particular, large crankshafts is already relatively high after prefabrication, and amounts to several hundred or several thousand euros. The method according to the invention thus is ever more important with more expensive and elaborate production of the raw components in the machining steps before the grinding. In particular, this applies to the production of special crankshafts in small batch sizes.
According to the preferred embodiments of the method according to the invention, the components to be ground can be given high quality and tight dimensional and shape tolerance by:
According to another aspect of the present invention, a grinding machine according to the invention is provided, on which the method according to any of claims 1 to 12 is carried out. This grinding machine according to the invention comprises a measurement device by means of which the dimensions and shape—such as diameter or circularity—of workpiece regions of a workpiece—in particular, a crankshaft—around a center are measured and produced with a central longitudinal axis. This grinding machine comprises a grinding disc that is mounted in a grinding spindle head and grinds with simultaneous advance in the direction of the X-axis thereof. An “X-axis” typically refers to the movement of the grinding disc, preferably at right angles, relative to the longitudinal extension of the workpiece region to be ground. The measurement device associated with the grinding machine according to the invention is arranged on the grinding spindle head and configured so that a sensor can be pivoted to bear onto the workpiece region, wherein the measurement device, the sensor implementing the actual measurement, or the sensing element forms measurement planes that are arranged transversely to the longitudinal axis of the workpiece region, and that can be arranged at any position in the direction of the workpiece longitudinal central axis in a manner corresponding to the movement of the measurement device or the sensor in this direction, for the purpose of measurement. It shall be readily understood that it is also possible that the measurement device is fixedly arranged, whereas a workpiece spindle head covering the workpiece can be moved in the Z-direction. Such a grinding machine according to the invention makes it possible to measure the ground workpiece regions during the grinding, namely, with regard to the dimensions and shape thereof, and simultaneously to adaptively—i.e., correctively—influence the advance of the grinding disc—i.e., the X-axis advancement thereof in the event of any deviations from the target contour that may be present. This significantly improves the precision of the ground workpiece.
Preferably, the measurement device has (or the sensor thereof is in the form of) two measurement surfaces arranged in the manner of a prism. During measurement, these measurement surfaces each contact the workpiece region at the contact region in a defined distance from one another. The measurement surfaces are therewith arranged on the legs of the prism, one measurement surface being provided on each leg. The actual sensing element for measurement is arranged in the middle part of the prism between the measurement surfaces. The measurement device is displaced to the contact region by means of a hydraulic, pneumatic, or electric drive. Preferably, this entails a CNC-controlled measurement device that is arranged on the grinding spindle head so as to be able to realize a defined contact position, and thus highly-precise measurement.
The grinding disc used to grind the workpiece region preferably has a width that corresponds approximately to the length of the workpiece region. With such a constellation or such a wide grinding disc, the grinding disc is advanced and thereupon grinds the workpiece region to be ground essentially by plunge grinding, without the grinding disc needing to be displaced in the direction of the Z-axis thereof in order to grind the respective shaft section.
According to another embodiment, the grinding disc is configured with a width that is smaller than the axial length of the workpiece region to be ground, wherein in such a case, the grinding disc carries out longitudinal grinding along the axis of rotation thereof over the axial longitudinal direction of the workpiece region to be ground, and is thus moved along the Z-axis thereof during the grinding.
Further preferably, the grinding machine comprises a measurement device configured such that the measurement planes of the respective workpiece region—in particular, a pin bearing journal, on which measurements are being taken—make it possible to determine a conical, crowned, or concave shape of the workpiece region and produce said shape on the basis of the measurement values.
More advantages, possible uses, and specific embodiments shall now be explained in greater detail, with reference to the accompanying drawings.
The pivotable measurement system 1 illustrated in
The measurement device 1 is arranged fixedly on the grinding spindle head with regard to the X-axis thereof, so that when the grinding disc 5 is moved with the grinding spindle head 4 along the X-direction of the measurement device 1, the measurement device is also moved along.
In a conventional manner, the grinding disc 5 is advanced over the X-axis thereof, which is also CNC-controlled, against the pin bearing journal 2 to be ground. The Z-axis of the grinding spindle head 4 may either be arranged under the X-axis—in which case a cross slide construction (not shown) is preferably provided—or under the grinding table, in which case the grinding table is moved with the corresponding grinding table structures, such as a workpiece spindle head and tailstock (both not shown). These two embodiments are quite common in the construction of grinding machines.
According to the invention, it is important that a relative movement in the direction of the Z-axis or ZM-axis is provided between the workpiece—i.e., the crankshaft 3—and the grinding disc 5. This causes the measurement device 1 to take measurements in different measurement planes, so that the component to be measured can be precisely measured in a plurality of planes along the axis thereof, and also the complete target outer contour 10 can be measured, which has not been the case thus far with measurement devices and systems according to the prior art.
It is thus evident in
The CNC-controlled ZM-axis is independent of the CNC-controlled Z-axis, and therefore, during the grinding, the measurement device 1 can automatically adjust, in the ZM-axis, the measurement plane on the pin bearing journal 2 being ground in parallel to the axis direction of the grinding disc 5 on the pin bearing journal 2. The measurement device 1 according to the invention thus makes it possible, even during the grinding, to conduct the measurements on the bearing point being ground—i.e., during the continuous grinding process, i.e., an in-process measurement method—with regard to the cylindrical shape, conicity, crown, or concavity, and to correct the advances of the grinding disc 5 through the grinding program during the grinding.
Thus, high-precision bearing points are produced with the method according to the invention, because the results of the in-process measurement with regard to dimensions and shape of the bearing point to be measured are inputted to the control device, and a corrected target outer contour 10 is produced on the basis of these measurement values. This results in a significantly higher quality of the ground workpiece regions, i.e., the bearing points of the crankshaft.
The shape of a pin bearing journal 2 may also be crowned or concave, for load-related reasons or, for example, for lubrication-related reasons. This is depicted in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2013 226 733.9 | Dec 2013 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/078469 | 12/18/2014 | WO | 00 |
