Method for hard finishing of a gear tooth or profile of a workpiece on a hard finishing machine

Abstract

The invention relates to a method for hard finishing a gear tooth or profile of a workpiece on a hard finishing machine, in which a tool machines a tooth flank or profile area of ​​the gear tooth or profile, wherein a tool spindle is driven by a drive motor at a predetermined speed. In order to carry out the hard finishing process in such a way that only minimal disruptive effects occur during the use of the gear tooth or profile, the invention relates to a method for hard finishing.The hard finishing process according to the invention comprises the following steps: a) Before engaging the tool with the workpiece: a1) Performing at least one measuring run in which the tool or workpiece spindle is driven at a changing speed while the resulting vibrations are measured by means of a vibration sensor; a2) Determining speeds or speed ranges at which, in comparison with other speeds or speed ranges, a strong and / or weak vibration excitation occurs (p. g , S k ) is present; b) Engaging the tool with the workpiece, whereby the drive of the tool or workpiece spindle is only carried out at such speeds at which, compared with other speeds, a weak vibration excitation (S k ) is present.

Description

[0001] The invention relates to a method for hard finishing a gear tooth or profile of a workpiece on a hard finishing machine, in which a tool machines a tooth flank or profile area of ​​the gear tooth or profile, wherein the workpiece is mounted on a workpiece spindle, wherein the tool is mounted on a tool spindle and wherein the tool spindle is driven by means of a drive motor at a predetermined speed.

[0002] Especially for noise-critical applications (particularly those in the field of electromobility), the teeth of a gear must be machined in such a way that, as far as possible, no integer orders or other waviness, which can lead to problems, are imprinted on the teeth during the hard finishing process. There are several causes for problematic waviness, including machine resonances, design-related resonances, excitation of these resonances by unfavorable speed ratios, or system-related problematic speeds due to the properties of the components used.

[0003] Hard finishing, and especially grinding, is typically performed at a constant cutting speed. This means the tool spindle must traverse various speed ranges critical to vibration, as its speed changes with the decreasing outer diameter of the tool (due to dressing), potentially leading to noise-inducing waviness on the gear teeth. Since, in gear machining using a grinding worm, the workpiece spindle speed is directly dependent on the tool speed (due to factors such as the number of threads on the grinding worm and the number of teeth on the workpiece), the workpiece axis also traverses critical speed ranges. Consequently, the corresponding speed ranges of both the tool spindle and the workpiece spindle are considerable.

[0004] Alternatively, a fixed, constant speed can be used, determined based on various criteria, and this speed is maintained throughout the entire service life or across the entire diameter of the grinding tool (especially the grinding worm). This has the disadvantage that the cutting speed changes constantly due to the decreasing tool size, which can have detrimental effects on the gear teeth.

[0005] The invention is based on the task of further developing a method of the type mentioned above in such a way that it is possible in a simple manner to carry out the hard finishing process, in particular the grinding of a gear, in such a way that only minimal disruptive effects occur that lead to vibrations when the gear or profile is used. The proposed method should, as far as possible, allow the necessary measures to be carried out automatically; that is, the machining process should be able to be adjusted to optimal conditions as automatically as possible.

[0006] The solution to this problem by the invention is characterized in that the hard finishing process comprises the following steps: a) Before the tool engages with the workpiece: a1) Performing at least one measurement run in which the tool spindle and / or the workpiece spindle is driven at a changing speed, while the resulting vibrations are measured at at least one location on the hard finishing machine using a vibration sensor; a2) Determination of speeds or speed ranges at which, compared with other speeds or speed ranges, a strong vibration excitation occurs (S g ) is present and / or at which, compared to other speeds or speed ranges, a weak vibration excitation (S) is observed. k ) is present; b) Engaging the tool with the workpiece, wherein the drive of the tool spindle and / or the workpiece spindle is carried out only or almost only at such speeds or in such speed ranges at which, compared with other speeds or speed ranges, a weak vibration excitation (S k ) is present.

[0007] Where the term "almost exclusively" refers to such speeds or speed ranges, this means that the speeds or speed ranges at which strong vibration excitation occurs are avoided as far as possible and are only relevant in connection with traversing the speeds or speed ranges in order to move from one speed or speed range with weak vibration excitation to another.

[0008] The measuring run is preferably one in which the rotational speed of the tool spindle and / or the workpiece spindle is increased from a minimum value, preferably zero, to a maximum value. However, it can also be one in which the rotational speed of the tool spindle and / or the workpiece spindle is decreased from a maximum value to a minimum value, preferably zero. Preferably, the increase or decrease of the rotational speed is linear. However, it is also possible to carry out the method with a predetermined rotational acceleration.

[0009] It has proven particularly effective to perform the measurement run during the machine's warm-up phase. However, other time periods or machining phases where the workpiece and tool are not engaged can also be used. It is especially preferred to perform the measurement run as part of a ramp-up test with predefined acceleration parameters.

[0010] When recording vibrations, those at the tool spindle and / or the workpiece spindle are preferably measured. However, other locations on the machine can also be used for this purpose.

[0011] The vibration acceleration is particularly preferred as the measured value. This can be done simply using known sensors.

[0012] Regarding the detection of the resulting vibrations (according to step a1 above), it is possible, as an alternative or additive method (in addition to detection using a conventional vibration sensor), to utilize drive signals from the machine that correlate with the vibration excitation. For example, currents and / or torques from drive motors can be measured and their behavior assessed, allowing conclusions to be drawn about particularly strong or weak vibration excitation. In this respect, drive signals from the control system of the machine tool axes can then be specifically accessed.

[0013] When considering which speeds or speed ranges should be classified as having strong vibration excitation on the one hand and as having weak vibration excitation on the other, a preferred definition may provide that (in connection with the execution of the above-mentioned step a2) an average value of the magnitude of the vibration excitation is calculated for a given time interval or speed range and is classified as speeds or speed ranges with strong vibration excitation (S g ) are defined as those that are a specified percentage above the mean value and / or as speeds or speed ranges with weak vibration excitation (S k) such values ​​are defined as those that lie a predetermined percentage below the mean value. For a defined evaluation range for time or rotational speed, a threshold value of, for example, 10%, 20%, 30%, or even 40% can be specified based on a mean value. This threshold must be exceeded above or below this value to indicate strong or weak vibration excitation, respectively.

[0014] The determination of rotational speeds or rotational speed ranges according to step a2 above is carried out according to a further development of the invention by subjecting the measured vibration to a frequency analysis and using the amplitude of the harmonics of the vibration as the basis for the assessment.

[0015] The machining (according to step b above) in the manner proposed according to the invention is preferably carried out only for the last machining operation of the gearing or profile, in particular for the last machining stroke.

[0016] Hard finishing is preferably a grinding or polishing process. In this case, the grinding is preferably carried out with a dressable grinding tool, in particular with a grinding worm, which is subjected to a number of dressing operations during the grinding of a large number of workpieces, whereby the outer diameter of the grinding tool is gradually reduced.

[0017] Thus, the invention aims to determine non-critical rotational speeds according to the proposed method for hard finishing, in particular for grinding, with which an optimal working result can be achieved.

[0018] The method has proven particularly effective in the field of gear teeth used in electromobility.

[0019] The proposed solution makes it possible to find the best possible compromise between constant spindle speed and constant cutting speed for noise-sensitive components, thus leveraging the advantages of both concepts. To achieve this, the optimal spindle speeds or speed ranges for the drive are determined and then implemented according to the proposed procedure.

[0020] Through the proposed analysis, particularly of the warm-up data or the start-up tests with adjusted acceleration parameters, various process parameters (especially the rotational speeds of the tool axis or workpiece axis) can be optimized and adjusted.

[0021] The process is then set up so that one or more constant rotational speeds are used in the last (and possibly also the penultimate) processing stage.

[0022] If multiple rotational speeds are necessary or useful, these can be selected in stages and depending on the screw diameter.

[0023] This results in a more stable machining process and the ability to set a noise-optimized process.

[0024] Furthermore, it is advantageous that the monitoring of the process (especially by means of order analysis) can be significantly improved, since the main cause for a dispersion of orders over the process, i.e. the changing rotational speed, is no longer present.

[0025] A further advantage is that potential problems in the machining process can be identified and resolved early on. The process-related, regularly occurring dressing intervals allow for cyclical monitoring. Additionally, idle times, for example due to a lack of parts, during which the machine warms up, can be used effectively by implementing the proposed procedure.

[0026] The drawing shows exemplary embodiments of the invention. Fig. Figure 1 schematically shows the ramp-up and ramp-down of the rotational speed of a tool spindle over time (upper part of the image) and the associated measured vibration acceleration, which was recorded by means of a sensor which is arranged on the tool spindle. Fig. Figure 2 shows an enlarged section of the measured vibration acceleration with an indication of the speed ranges with strong vibration excitation (S g ) and weak vibration excitation (S k ), Fig. 3. An example of an order analysis of a recorded vibration performed during warm-up before applying the proposed method (top left image), the resulting order analysis during the grinding process, where left bars before optimization and right bars after optimization are sketched for each order according to the proposed procedure (bottom left image), and an order analysis performed for the vibration during operation of the manufactured gear, where again left bars before optimization and right bars after optimization are sketched for each order according to the proposed procedure (bottom right image). Fig. 4. A flowchart illustrating the implementation of the proposed procedure, and Fig. 5 the curve at constant cutting speed and according to adapted steps according to the present invention, wherein in the upper part of the image the curve of the rotational speed of the tool axis is plotted against the diameter of a grinding screw and in the lower part of the image the curve of the cutting speed is plotted against the diameter of the grinding screw.

[0027] In Fig. Figure 1 in the upper part of the image shows how the rotational speed n of a tool spindle is linearly increased (from zero) to a maximum value during a test run, briefly held at maximum speed, and then reduced again (to zero). This ramping up and down can occur during the machine's warm-up phase.

[0028] The lower part of the image shows the vibration acceleration a corresponding to the respective rotational speed over time, which was recorded by an acceleration sensor located on the tool spindle.

[0029] The course of the recorded vibration acceleration is in Fig. Figure 2 shows an enlarged representation for the time range from 0 to approximately 70 seconds. In this representation, various areas have been marked that can be automatically determined by a computer: An evaluation range B is initially specified (which is freely configurable, but advantageously maps to the highest possible cutting speed – depending on the possible screw diameters and their specifications), for which an evaluation of the vibration acceleration profile was performed. It can be seen in the vibration acceleration profile over time (or its amplitude) that the vibration acceleration is subject to significant changes depending on the drive and resonance characteristics of the machine. If a mean value for the vibration acceleration is defined over evaluation range B, sections S g with strong vibration excitation as well as sections S k can be identified with weak vibration excitation. Fig. 2 is a number of such sections.

[0030] Accordingly, it follows that a weak vibration excitation of the machine or at least of the tool spindle occurs when the rotational speed is in the corresponding range S k comes to lie (the correspondence between the areas S k or S g and the rotational speed n results from Fig. 1).

[0031] The machine control system allows the corresponding speeds or speed ranges to be identified and stored.

[0032] During operation, i.e., while grinding a gear, only speeds for which weak vibration excitation has been identified are used. The grinding result improves accordingly.

[0033] In Fig. Figure 3 illustrates the result of the proposed procedure: The graph shown in the upper left illustrates an order analysis (order O) of the recorded vibration, which represents the amplitude A of the corresponding harmonics (order 1 to 40). The graph shows the curve before the inventive method is applied.

[0034] The corresponding order analysis for the grinding process is illustrated in the graphic below left. The amplitudes A for the individual harmonics are shown, indicated by the respective left-hand bars. The corresponding adjacent right-hand bars show the amplitudes of the respective orders that are present after carrying out the proposed procedure.

[0035] If the waviness of the profile and / or the flank line of the gear teeth is evaluated accordingly and the amplitudes A are plotted against the order, the representation shown below right results. Fig. 3. Again, pairs of bars are shown, with the leftmost bars representing the situation without the application of the proposed procedure and the rightmost bars representing the situation after the application of the proposed procedure.

[0036] The integration of the proposed method into a grinding process illustrates Fig. 4. Here is a flowchart that begins with a start-up test (data acquisition). This is followed by the determination of resonances, for which... Fig. 1. Reference is made to.

[0037] The next step is to determine or optimize the rotational speeds for machining; in this regard, see below. Fig. 2. Referenced.

[0038] The grinding process is then carried out at the appropriate speeds or speed ranges, whereby speeds that cause strong vibration are avoided.

[0039] This allows for further monitoring of the process and, if necessary, another warm-up (without the tool engaging the workpiece) to repeat the determination of resonances and to redefine or optimize rotational speeds for machining, especially if feedback from process monitoring or from the test benches (EOL, single-flank / double-flank rolling test bench, tactile waviness analysis) gives cause for doing so.

[0040] Furthermore, in the event of a shortage of parts, i.e., if no parts to be ground are available and the machine is therefore not currently in use, another warm-up can be performed to repeat the determination of resonances and to redefine or optimize speeds for machining; i.e., a new measurement run is initiated.

[0041] In Fig. Figure 5 shows the rotational speed n of a grinding screw mounted on a tool spindle, plotted against the grinding screw diameter D; the lower figure shows the cutting speed v against the grinding screw diameter D. Both graphs depict the curve of a constant cutting speed (const) as well as that of adjusted speed steps.

[0042] It can be seen that while the adjustment (adapted steps to) does not achieve a constant cutting speed, it allows for targeted operation only at those speeds that result in low vibration excitation (see speed ranges S). k in Fig. 2) Nevertheless, the required cutting speed can be maintained even with a decreasing screw diameter.

[0043] This allows for improved grinding, resulting in a gear tooth that exhibits a lower tendency to excite vibrations during subsequent operation.

[0044] The inventive approach therefore consists in the fact that in the last shaping machining stage (normally the finishing stroke and / or a polishing stroke) a constant cutting speed is no longer used, but a constant rotational speed is maintained.

[0045] The rotational speed does not need to be kept constant across the entire diameter of the grinding worm; a stepwise change in speed is also possible, as shown in Fig. 5 is shown.

[0046] To determine the appropriate rotational speeds, for example, various speed ramps with reduced acceleration can be driven during the setup process or the adjustment of the process parameters to check where the resonances lie.

[0047] These speed ramps can also occur during a warm-up phase. They can be initiated automatically or started by a machine operator, particularly if anomalies are detected during process monitoring.

[0048] In addition to various speed ramps, near-process warm-up and / or near-process ramp-up can also be used. Here, machining is simulated with near-process machining parameters without any intervention of the workpiece or tool. This can be particularly useful for verifying whether the selected speeds also deliver satisfactory results under these conditions.

[0049] Furthermore, analyses from the machining process, values ​​from test benches (single or double flank, EOL) or gravel analyses can be used as input variables or evaluation variables to adapt the process.

[0050] The information obtained from the measurement data can also be used for automated or manual troubleshooting and diagnosis.

[0051] Before the grinding cycle starts (for example, after a changeover), all relevant axes are examined for their resonances. This can be done in particular by increasing the speed of the workpiece spindle and / or the tool spindle, as mentioned above.

[0052] Once the speeds or speed levels are set, a near-process warm-up can be used. During a near-process warm-up, all axes and speeds are in the positions they will be in during grinding. The resonances can shift due to different axis positions, changes in mass (for example, due to a change in the screw diameter), etc. Therefore, checking the resonances according to the proposed procedure can also be carried out, for example, while waiting for workpieces.

[0053] The start-up tests can take a relatively long time, therefore it is advisable to provide a short warm-up run for verification purposes if parts are lacking.

[0054] The basic concept here is always that a noticeable vibration during idling is always correlated with a noticeable vibration during operation. Furthermore, a noticeable vibration during operation leads to problematic waviness on the gear teeth.

[0055] However, since it is difficult to distinguish which axis is responsible for the vibration during the grinding process, the warm-up process, in which the workpiece and tool are not engaged, is better suited for the proposed analysis.

[0056] Not only a shortage of parts, but also conspicuous workpieces can necessitate a warm-up run with corresponding analysis. These conspicuousnesses can be identified, for example, from the results of process monitoring, and in particular as a result of a performed order analysis.

[0057] Results from various test benches (EOL, single- or double-sided test bench or gravel analysis) may also trigger a repeat execution of the proposed procedure.

[0058] Based on a knowledge database and uniquely determined resonances (fingerprint), a specific rotational speed can be set once.

[0059] As previously mentioned, it is advantageous if process monitoring includes or is based on an order analysis of the recorded vibrations. The varying rotational speeds caused by a grinding process at a constant cutting speed can lead to increased excitation of machine resonances (which can shift, for example, due to a decreasing screw diameter). Similarly, statistical analysis methods can be used much more effectively within process monitoring because crucial boundary conditions (workpiece and tool rotational speeds) can be kept constant. Therefore, "normal distributions" can be assumed when analyzing the orders; however, at a constant cutting speed during the finishing stroke, the boundary conditions are sometimes significantly altered due to the varying rotational speed, making a classic statistical analysis of all workpieces difficult to perform.

Claims

[1] Method for hard finishing of a gear tooth or profile of a workpiece on a hard finishing machine, wherein a tool machines a tooth flank or profile area of ​​the gear tooth or profile, wherein the workpiece is mounted on a workpiece spindle, wherein the tool is mounted on a tool spindle and wherein the tool spindle is driven by a drive motor at a predetermined speed, characterized by that the hard finishing process includes the following steps: a) Before the tool engages with the workpiece: a1) Performing at least one measurement run in which the tool spindle and / or the workpiece spindle is driven at a changing speed, while the resulting vibrations are measured at at least one location on the hard finishing machine using a vibration sensor; a2) Determination of speeds or speed ranges at which, compared with other speeds or speed ranges, a strong vibration excitation occurs (S g ) is present and / or at which, compared to other speeds or speed ranges, a weak vibration excitation (S) is observed. k ) is present; b) Engaging the tool with the workpiece, wherein the drive of the tool spindle and / or the workpiece spindle is carried out only or almost only at such speeds or in such speed ranges at which, compared with other speeds or speed ranges, a weak vibration excitation (S k ) is present. [2] Method according to claim 1, characterized by that the measuring run is one in which the rotational speed of the tool spindle and / or the workpiece spindle is increased from a minimum value, preferably from zero, to a maximum value. [3] Method according to claim 1, characterized bythat the measuring run is one in which the rotational speed of the tool spindle and / or the workpiece spindle is reduced from a maximum value to a minimum value, preferably from zero. [4] Method according to claim 2 or 3, characterized by that the increase or decrease in rotational speed is linear. [5] Method according to any one of claims 1 to 4, characterized by that the measurement run takes place during a warm-up phase of the machine. [6] Method according to any one of claims 1 to 5, characterized by that the vibrations are measured at the tool spindle and / or at the workpiece spindle. [7] Method according to any one of claims 1 to 6, characterized by , that in step a1 of claim 1 the vibration acceleration is recorded as a measured value. [8] Method according to any one of claims 1 to 6, characterized by, that in step a1 of claim 1, drive signals of the machine are recorded as measured values, for which there is a correlation with the vibration excitation, in particular currents and torques of drive motors. [9] Method according to any one of claims 1 to 8, characterized by , that the determination of rotational speeds or rotational speed ranges according to step a2 of claim 1 is carried out such that for a given time range or rotational speed range an average value of the magnitude of the vibration excitation is formed and is expressed as rotational speeds or as rotational speed ranges with strong vibration excitation (S g ) are defined as those that are a specified percentage above the mean value and / or as speeds or speed ranges with weak vibration excitation (S k ) such values ​​are defined as those that lie a predetermined percentage below the mean. [10] Method according to any one of claims 1 to 9, characterized by, that the determination of rotational speeds or rotational speed ranges according to step a2 of claim 1 is carried out by subjecting the measured vibration to a frequency analysis and using the amplitude of the harmonics of the vibration as the basis for the assessment. [11] Method according to any one of claims 1 to 10, characterized by , that the machining according to step b) of claim 1 is carried out only for the last machining operation of the gearing or profile, in particular for the last machining stroke. [12] Method according to any one of claims 1 to 11, characterized by that hard finishing is a grinding or polishing process. [13] Method according to claim 12, characterized by, that the grinding is carried out with a dressable grinding tool, in particular with a grinding worm, which is subjected to a number of dressing operations during the grinding of a large number of workpieces, in which the outer diameter of the grinding tool is gradually reduced.

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