Method for machining pre-toothed workpieces by means of a continuous generating grinding process and corresponding gear-cutting machine

Abstract

The invention relates to a method for machining pre-toothed workpieces by means of a continuous generating grinding process using a grinding worm wheel (100). The grinding worm wheel is dressed, and workpieces are machined using the dressed grinding worm. For each grinding stroke, the worm thread flanks have a contact zone (104, 104') in which the grinding worm wheel comes into machining engagement with the workpiece. Each contact zone has a quality-determining region, and the quality-determining regions of at least some of the contact zones are positioned so as to not overlap with the quality-determining regions of all other contact zones. A plurality of grinding strokes are carried out using each of the contact zones before the grinding worm wheel is dressed again. The invention also relates to a correspondingly designed gear-cutting machine.

Description

[0001] TITLE

[0002] Method for machining pre-toothed workpieces by continuous generating grinding and corresponding gear cutting machine

[0003] TECHNICAL FIELD

[0004] The present invention relates to a method for machining pre-toothed workpieces by continuous generating grinding and to a gear cutting machine which is designed to carry out such a method.

[0005] STATE OF THE ART

[0006] In gear manufacturing, the final hard finishing step is one of the most important steps in determining the quality of the gear. This process produces the geometry that will later mesh with the gear teeth. Continuous generating grinding is a frequently used process for hard finishing. Information on the fundamentals of continuous generating grinding can be found, for example, in H. Schriefer et al., "Continuous Generating Grinding of Gears," published by Reishauer AG, Wallisellen, June 2008, ISBN 978-3-033-01447-3.

[0007] In continuous generating grinding, a pre-toothed workpiece is machined in rolling engagement with a rotating grinding tool in the form of a helically profiled grinding wheel (grinding worm). The machining takes place in one or more grinding strokes. During each grinding stroke, the grinding worm is moved relative to the workpiece toothing along the workpiece axis (so-called axial feed motion), while the workpiece rotates around its workpiece axis in rolling engagement with the grinding worm.

[0008] Often, the workpiece is first machined in one or more roughing strokes, followed by one or more finishing strokes. The roughing and finishing strokes differ in their grinding performance: In the finishing strokes, a lower infeed and / or axial feed rate, and therefore a lower grinding performance, is usually selected than in the roughing strokes to ensure more precise compliance with quality specifications.

[0009] During each grinding stroke, a specific area of ​​the grinding worm comes into contact with the respective workpiece. Due to this machining action, the grinding worm in this area is subject to wear. To bring fresh, unused areas of the grinding worm back into contact with the workpiece, it is known to displace the grinding worm discontinuously or continuously along its worm axis relative to the workpiece (so-called "shifting" of the grinding worm). Various shift strategies are known for this purpose.

[0010] With "discontinuous shifting," all roughing strokes during machining of a workpiece are performed at the same shift position of the grinding worm. This is usually followed by a so-called shift jump to a shift position for finishing, and all finishing strokes are performed at this shift position. After finishing is complete, a so-called shift return to a new shift position for roughing the next workpiece occurs. The shift position is not changed during the roughing or finishing strokes. The operator usually selects the size of the shift jump and the shift return based on predefined formulas or empirical values.

[0011] With the "continuous shifting" technique predominantly used today, a shift movement is continuously superimposed during each roughing and finishing stroke. The selection of the shift amount per grinding stroke and the resulting shift feed rate is also based on predefined formulas or the operator's experience.

[0012] Dressable grinding worms are frequently used in generating grinding. Once the grinding worm has been shifted across its entire usable range, it is dressed again. Dressing tools, usually diamond-coated, are used for this purpose. During dressing, the outer diameter of the grinding wheel decreases. After a certain number of dressing processes, the grinding worm is worn out and must be replaced. The dressing tools also become increasingly worn with each dressing process. The number of dressing processes therefore affects not only the service life of the grinding worm, but also the service life of the dressing tools. The costs of the grinding worms and dressing tools therefore represent a substantial portion of the running costs. In addition, no workpieces can be machined during dressing, so each dressing process extends the cycle time.Overall, dressing represents both a significant cost and time factor. It is therefore generally desirable to design the shift strategy in such a way that as many workpieces as possible can be machined with the grinding worm while maintaining the required workpiece quality before the grinding worm has to be dressed again.

[0013] In practice, however, the grinding worm is often not used to its full potential. Many users are therefore keen to avoid manufacturing deviations on the finished workpieces at all costs. To this end, users often act with excessive caution, dressing the grinding worm after machining a relatively small number of workpieces, even though the grinding worm is actually still capable of producing further perfect workpieces. Optimal utilization of the grinding worm is also hampered by the fact that, with conventional shift strategies, the wear condition of the grinding worm often influences the resulting machining quality in a way that is difficult to predict. Therefore, a reliable prediction of when the grinding worm should be dressed is often impossible. Furthermore, with conventional shift strategies, the grinding worm is often worn unevenly across its width.As a result, dressing is often necessary as soon as just one area of ​​the grinding worm is worn, even though there are other areas that are not yet worn to the same extent.

[0014] PRESENTATION OF THE INVENTION

[0015] It is an object of the present invention to provide a method for machining pre-toothed workpieces by generating grinding, which enables better utilization of the grinding worm.

[0016] This object is achieved by a method according to claim 1. Further embodiments are specified in the dependent claims.

[0017] A method is therefore specified for machining pre-toothed workpieces by continuous generating grinding with a grinding worm rotating around a worm axis, the method comprising:

[0018] Dressing the grinding worm; and machining the workpieces after the grinding worm has been dressed, wherein each workpiece is machined with at least one grinding stroke, and wherein during each grinding stroke the grinding worm is moved relative to the respective workpiece along the workpiece axis, while the workpiece rotates about a workpiece axis in machining engagement with the grinding worm (axial feed movement).

[0019] Every workpiece has a workpiece toothing with (left and right) workpiece flanks. Each of these workpiece flanks has a working area. The "working area" refers to the area of ​​the workpiece flank that comes into contact with the flanks of a mating toothing during later use, thus defining the contact pattern of the toothing. The working area can be specified during the design of the toothing or determined by determining the contact pattern under specified load conditions. The process can include specifying the working area, e.g., using an input function with which the working area is defined in a suitable form, e.g., graphically in a representation of the respective workpiece flank. If necessary, the entire tool flank can also be defined as the working area.

[0020] During each grinding stroke, the workpiece flanks are machined with at least one worm thread flank of the grinding worm. Machining is typically performed with more than one worm thread flank, for example, in the case of a multi-threaded grinding worm and with conventional double-flank engagement, with the left and right flanks of each grinding worm thread. However, only one of these worm thread flanks is considered below.

[0021] For each grinding stroke, the area on the worm gear flank where the worm gear flank comes into contact with the workpiece during the respective grinding stroke is defined as the "contact zone." Each of these contact zones has an area that comes into contact with the useful areas of the workpiece flanks. This area is referred to below as the "quality-determining area" because it influences the workpiece quality in the relevant areas of the workpiece flanks, namely the useful areas. If the entire workpiece flank is considered the useful area, the quality-determining area extends over the entire contact zone, i.e., the contact zone and the quality-determining area are then identical.According to the invention, there are several contact zones on the worm thread flank, whose quality-determining areas are arranged essentially without overlap with all other contact zones, and with each of which a plurality of grinding strokes is performed before the grinding worm is dressed again. Contact zones whose quality-determining areas are arranged essentially without overlap with all other contact zones are referred to below as "decoupled contact zones."

[0022] There are therefore decoupled contact zones on the worm thread flank which are not just used for a single grinding stroke, but with which several grinding strokes can be carried out. The repeated use of the decoupled contact zones can result in particularly good utilization of the grinding worm. The grinding strokes carried out with a specific decoupled contact zone can, in particular, be grinding strokes when machining different workpieces, i.e. there can be decoupled contact zones with which two or more workpieces are machined. It can be provided that each of the decoupled contact zones is only used to carry out a very specific, selected grinding stroke per workpiece, e.g. the first, second, third, etc. grinding stroke of the respective workpiece.In particular, it can be provided that each grinding stroke is carried out with a different contact zone when machining an individual workpiece (e.g. first grinding stroke with a first contact zone, the second grinding stroke with a second contact zone, etc.).

[0023] A quality-determining area of ​​a contact zone is "essentially" arranged without overlap with another contact zone if it either does not overlap with the other contact zone at all, or the overlap is so small that the remaining overlap has no measurable influence on the resulting quality of the useful areas of the gearing manufactured with the contact zone in question. For example, a contact zone can still be considered decoupled if no more than 25% of the surface of the quality-determining area of ​​the contact zone overlaps with one or more other contact zones, preferably no more than 15%, particularly preferably no more than 5%.

[0024] To move from one decoupled contact zone to another decoupled contact zone, the grinding worm is shifted between two grinding strokes relative to the machining position of the workpieces along a shift direction parallel to the worm axis, similar to the previously explained shift jump and shift reversal in conventional shift strategies. However, in conventional shift strategies, the shift jump and shift reversal are designed such that the quality-determining area of ​​each contact zone at least partially overlaps with one or more other previously or subsequently used contact zones. The use of decoupled contact zones, on the other hand, enables better control of the machining process, since the wear condition of the grinding worm in the quality-determining areas of the decoupled contact zones is not influenced by the wear condition of previously used, overlapping contact zones.

[0025] The proposed method does not preclude the possibility that, in addition to the decoupled contact zones, further contact zones may exist on the worm thread flank that overlap with one another, as in conventional shift strategies, and that only some of the grinding strokes per workpiece are performed with decoupled contact zones, while one or more other grinding strokes are performed with non-decoupled contact zones. For example, it is conceivable to perform only the roughing strokes with decoupled contact zones, while the contact zones of the finishing strokes overlap, or vice versa. However, preferably, all contact zones used on the worm thread flank are decoupled contact zones. In particular, preferably, all grinding strokes during the machining of all workpieces are performed exclusively with decoupled contact zones before the grinding worm is dressed again.

[0026] The proposed method also does not preclude the possibility of having one or more decoupled contact zones on a worm thread flank that serve only to execute a single grinding stroke on a single workpiece, e.g., the first roughing stroke or the last finishing stroke on exactly one workpiece. This can be useful if particularly controlled machining conditions are desired for the respective grinding stroke. However, each of the decoupled contact zones is preferably used for multiple grinding strokes, and in particular for machining multiple workpieces, before the grinding worm is dressed again.

[0027] In preferred embodiments, not only are the quality-determining regions of the decoupled contact zones arranged substantially without overlap with all other contact zones, but the decoupled contact zones are arranged substantially completely without overlap with all other contact zones, i.e. they either do not overlap with other contact zones at all, or they overlap with other contact zones only to such an extent that this has no measurable influence on the quality of the tooth flanks machined with these contact zones, both inside and outside the useful areas.

[0028] For each decoupled contact zone, the number of grinding strokes performed with this contact zone before the grinding worm is redressed can be predefined, or this determination can be made automatically during machining of the workpieces based on measured values. Surprisingly, this number can be very high, amounting to at least 10, at least 20, or even at least 50, without any loss of quality in the finished workpieces. Even 100 or more grinding strokes per contact zone are possible before the grinding worm is redressed. Overall, this allows many times more workpieces to be machined before the grinding worm is redressed compared to conventional shift strategies.This allows the lifetime of the grinding worm and the dressing tools to be massively increased compared to conventional shift strategies, and as a result, the costs of the generating grinding process can be significantly reduced.

[0029] To automatically determine the number of grinding strokes per decoupled contact zone, it can be provided that a wear parameter is determined during the grinding strokes, which characterizes the wear state of the grinding worm in the respective contact zone. And that, depending on the wear parameter, a decision is automatically made as to whether a machining operation should be carried out again in the same contact zone for an immediately following or later grinding stroke, or whether this contact zone should no longer be used for further grinding strokes before the grinding worm is dressed again. With this procedure, each decoupled contact zone is used for grinding strokes until the wear parameter indicates that this contact zone is worn. This allows the number of workpieces machined between two dressing processes to be optimized.

[0030] The wear parameter can, in particular, be an intensity measure for a grinding intensity during the respective grinding stroke, in particular a maximum, integral, mean value, or a low-pass filtered spectral component of a performance indicator determined during the respective grinding stroke. The performance indicator can, in particular, be the power and / or current consumption of a tool spindle used to drive the grinding worm to rotate. This is based on the consideration that with increasing wear at a given infeed, less and less grinding worm material is available for the machining process and, in addition, the surface properties of the grinding worm flank change. Overall, this is expressed in a decreasing grinding intensity. However, the wear parameter does not necessarily have to be based on the grinding intensity.For example, a wear parameter can also be determined on the basis of a spectral analysis of structure-borne sound signals recorded during the respective grinding stroke.

[0031] The decision as to whether a contact zone is worn can be made, in particular, based on a comparison of the wear parameter with a threshold value. However, more complex decision criteria are also conceivable, e.g., a decision that additionally considers a corresponding wear parameter during another grinding stroke on the same workpiece. For example, a decision can be made that a contact zone used for a roughing stroke of the respective workpiece is considered worn and will no longer be used for further grinding strokes if, on the one hand, the above-mentioned intensity measure during the roughing stroke falls below a first threshold value and, on the other hand, the intensity measure determined for a subsequent finishing stroke exceeds a second threshold value.Instead of simply comparing only the current value of the wear parameter with the threshold value, it is also possible to consider a method that better distinguishes between real trends and random fluctuations, e.g. by monitoring how many of the last N grinding strokes carried out with the contact zone in question exceeded or fell below the threshold value and making the decision on this basis, or by performing a (linear or non-linear) regression analysis of the wear parameter and comparing at least one value of the regression function determined in this way with the threshold value.

[0032] Preferably, for each workpiece during a grinding stroke in which the workpiece in question is machined with one of the decoupled contact zones, a shift movement is also performed simultaneously with the axial feed movement, i.e., the grinding worm is continuously shifted relative to the workpiece both along the axial feed direction and along the shift direction (so-called diagonal movement). As a result, during the machining operation in the respective grinding stroke, the grinding worm covers a total of a specific shift path along the shift direction and a specific axial stroke path along the axial feed direction relative to the workpiece. The ratio of shift path to axial stroke path is referred to as the "diagonal ratio." For a cylindrical grinding worm and a cylindrical workpiece, the contact zone on the worm thread flank without shift movement corresponds to a single contact path.The contact path is the area on the worm thread flank in which the worm thread flank comes into cutting engagement with the workpiece during a grinding stroke, provided there is no axial feed movement or shifting movement. Each contact path has a conical helical shape on the grinding worm flank. Since the contact between the grinding worm and the workpiece is not exactly point-like, but occurs over a certain cutting width, the contact path has a finite width. When a shifting movement occurs, each contact zone no longer corresponds to a single contact path, but is composed of a multitude of partially overlapping contact paths shifted relative to one another along the tool width direction.

[0033] If a shift movement is performed during a grinding stroke, generally only a portion of the contact paths of a contact zone will come into contact with the useful area of ​​the workpiece flank, and these contact paths will generally not come into contact with the useful area along their entire length. Therefore, generally only a portion of the contact paths along a portion of the respective contact path form the quality-determining area of ​​the contact zone. If, on the other hand, no shift movement is performed during the grinding stroke, each contact zone with a cylindrical workpiece and cylindrical tool corresponds to exactly one contact path. In this case, a central section of the contact path usually forms the quality-determining area of ​​the contact zone. If, on the other hand, the entire workpiece flank is considered the useful area, all contact paths along their entire length form the quality-determining area of ​​the contact zone, i.e.the entire contact zone becomes quality-determining.

[0034] Preferably, the grinding worm in the decoupled contact zones either has no modification of the worm flight flank at all, or it has a modification of the worm flight flank in the decoupled contact zones which, when viewed along a contact path, does not change during the shift movement within the contact zone from contact path to contact path along the tool width direction. The modification of the grinding worm in the decoupled contact zones can, in particular, be a pure profile modification, i.e. a modification of the grinding worm profile which does not change in the width direction of the grinding worm across the decoupled contact zones. In particular, it can be a profile angle modification, profile crowning, profile waviness, tip relief and / or root relief.As a result, despite the diagonal movement on the workpiece flanks, either no modification occurs at all, or a pure profile modification occurs on the workpiece flanks that does not change along the width direction of the workpiece. In the present process, the diagonal movement is therefore preferably not used to transfer width-variable modifications of the worm thread flanks to the workpiece flanks in order to create a workpiece flank modified in the workpiece width direction. The choice of the diagonal ratio is therefore made purely from technological perspectives, such as the service life of the grinding worm, the workpiece quality, and / or the roughness structure of the workpiece flanks (as in so-called "low noise shifting," where the diagonal grinding process is used to specifically influence the roughness structure, see, for example,US6379217B1), but not from the point of view that a width-variable geometric modification of the workpiece flanks is to be created.

[0035] To achieve the desired decoupling of the contact zones, the following procedure can be used: Assume that the grinding worm assumes a first shift position relative to the workpiece along the shift direction at the end of the machining operation of a first grinding stroke, and that it assumes a second shift position at the beginning of the machining operation of a subsequent (but not necessarily immediately subsequent) second grinding stroke. Then, it is preferred that there is a distance between the first shift position and the second shift position that corresponds at least to the width of a contact path formed on a flank of the grinding worm during the first grinding stroke, measured along the worm axis, and that the grinding worm assumes exclusively shift positions that lie outside the range between the first shift position and the second shift position during all grinding strokes.In this way, it can be achieved that the contact zones used in the grinding strokes mentioned are completely free of overlap.

[0036] The width of the contact path is determined by the progression of the cutting width along the contact path. The cutting width can vary along the contact path because the engagement conditions change along the contact path. It can also vary during a grinding stroke, for example, because several teeth of the workpiece are machined with the same contact path and the workpiece allowance can vary from tooth to tooth due to pitch errors in the workpiece gearing.It is therefore advisable to determine the cutting width profile along the contact path when removing a nominal allowance corresponding to the maximum expected allowance. From this, determine a nominal width of the contact path during the first grinding stroke. Select the distance between the first shift position and the second shift position so that this distance corresponds to at least this width, measured along the worm axis. In addition to the nominal allowance, the calculation of the cutting width takes into account the macrogeometry of the workpiece (in particular, module, number of teeth, tooth shape), the macrogeometry of the grinding worm (in particular, module, number of threads, pressure angle), and the machining parameters (in particular, the infeed and the axial feed per workpiece revolution).

[0037] Expressed numerically, it is preferred if the following relationship is satisfied for the distance between the first shift position and the second shift position:

[0038] Ay > a p 0 / sin a , where a p 0 is the (nominal or real) width of the contact path, measured perpendicular to the contact path, and where a is the pressure angle of the grinding worm in the axial section of the grinding worm. The width of the contact path perpendicular to the contact path corresponds to a good approximation of the width of the contact path measured in the profile height direction of the grinding worm in the axial section of the grinding worm, and therefore a p 0Alternatively, this width can also be used. For a grinding worm with different pressure angles on the right and left worm thread flanks, the values ​​of Ay for the right and left worm thread flanks may differ. In this case, the larger of these values ​​is preferred.

[0039] In some embodiments, a performance indicator characterizing a current grinding performance is continuously measured for each workpiece at least during machining with the decoupled contact zones, and at least one grinding parameter is controlled as a function of the performance indicator. By decoupling the contact zones in which the machining intervention takes place during the selected grinding stroke, such control becomes considerably more reliable than if contact zones are used whose quality-determining regions overlap with other contact zones, because the wear state of the grinding worm in the other contact zones does not influence the control in the respective quality-determining region. In particular, the axial feed rate (i.e., the speed of the axial feed movement) can be controlled as a grinding parameter.In this case, the at least one grinding parameter is preferably controlled in such a way that during the respective grinding stroke the performance indicator follows a target curve or does not exceed a predetermined maximum value.

[0040] In order to reduce the dependence of the performance indicator on process parameters such as tool diameter, workpiece diameter and module or infeed and thus to facilitate the specification of a target curve or a maximum value and to improve the control, it can be provided that a normalization operation is carried out when determining the performance indicator in order to normalize the performance indicator. The normalization operation depends on at least one process parameter, wherein the process parameter is at least one geometric parameter of the tool, at least one geometric parameter of the workpiece and / or at least one setting parameter of the machine tool. The normalization operation is carried out in such a way that the normalized performance indicator depends less strongly on the at least one process parameter than without the normalization operation.

[0041] As soon as at least one of the mentioned process parameters changes, the normalization operation is preferably recalculated. Recalculating the normalization operation may, in particular, involve applying a model that describes an expected dependence of the performance indicator on the process parameters, in particular a model of a process force or process performance.

[0042] For further considerations regarding the normalization operation, reference is made to publication W02021048027A1, the contents of which are incorporated in their entirety into the present disclosure by reference.

[0043] Furthermore, a gear cutting machine for machining pre-toothed workpieces by continuous generating grinding is specified, which is specifically designed to carry out the methods disclosed above.The gear cutting machine comprises: a tool spindle configured to clamp a grinding worm thereon in order to drive the grinding worm to rotate about a tool axis; a workpiece spindle configured to clamp a pre-toothed workpiece thereon in order to drive the pre-toothed workpiece to rotate about a workpiece axis; an axial slide configured to generate an axial feed movement of the tool spindle relative to the workpiece spindle along an axial feed direction having a component parallel to the workpiece axis; a shift slide configured to generate a shift movement along a shift direction parallel to the worm axis; and a machine control configured to carry out one of the methods disclosed above.

[0044] BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Preferred embodiments of the invention are described below with reference to the drawings, which are for illustrative purposes only and are not to be construed as limiting. In the drawings:

[0046] Figure 1 schematically illustrates a grinding worm in engagement with a gear-shaped workpiece;

[0047] Figure 2 illustrates the temporal progression of the power consumption of the

[0048] Tool spindle that drives the grinding worm during a grinding stroke;

[0049] Figure 3A illustrates, by way of example, the grinding intensity during roughing when machining a plurality of workpieces using a conventional shift strategy;

[0050] Figure 3B illustrates the grinding intensity during finishing when machining a plurality of workpieces using the conventional shift strategy;

[0051] Figure 4 illustrates a worm gear flank with a contact pad shown on it;

[0052] Figure 5 illustrates a section of a contact path with an associated instantaneous contact area;

[0053] Figure 6 illustrates a worm gear flank with two contact zones that partially overlap;

[0054] Figure 7 illustrates a worm gear flank with two contact zones that do not overlap;

[0055] Figure 8 illustrates a virtual face gear with a contact path;

[0056] Figure 9 illustrates contact zones on a virtual face gear using a first shift strategy; Figure 10 illustrates contact zones on a virtual face gear using a second shift strategy;

[0057] Figure 11A illustrates, by way of example, the grinding intensity during roughing when machining a plurality of workpieces using the same roughing contact zone repeatedly;

[0058] Figure 11 B illustrates, by way of example, the grinding intensity during finishing when machining a plurality of workpieces using the same finishing contact zone repeatedly;

[0059] Figure 12 shows a diagram in which the grinding intensity during roughing (vertical axis) and finishing (horizontal axis) is plotted for a variety of workpieces;

[0060] Figure 13A illustrates, by way of example, the time course of the power consumption of the tool spindle when the axial feed rate is controlled in such a way that the power consumption does not exceed a maximum value;

[0061] Figure 13B illustrates, by way of example, the corresponding temporal course of the axial feed rate;

[0062] Figure 14A illustrates, by way of example, the time course of the power consumption of the tool spindle when the axial feed rate is controlled in such a way that the power consumption follows a target curve;

[0063] Figure 14B illustrates, by way of example, the corresponding temporal course of the axial feed rate;

[0064] Figure 15A illustrates an example of a workpiece flank with a useful area;

[0065] Figure 15B illustrates an example of a worm gear flank with a quality-determining area; and

[0066] Figure 16 illustrates a generating grinding machine.

[0067] DESCRIPTION OF PREFERRED EMBODIMENTS

[0068] Kinematics of continuous generating grinding

[0069] Figure 1 illustrates the kinematics of continuous generating grinding. A grinding worm (i.e., a helically profiled grinding wheel) 100 rotates about a worm axis B. The grinding worm meshes with a gear-shaped workpiece 200 having workpiece teeth 201 in the manner of a helical gear. The workpiece 200 rotates about a workpiece axis C. During a grinding stroke, the grinding worm 100 is moved relative to the workpiece 200 along an axial feed direction Z, which runs parallel or inclined to the workpiece axis, preferably across the entire width of the workpiece teeth 201 (so-called axial feed movement). The position of the grinding worm 100 relative to the workpiece 200 along a radial feed direction X determines the amount of material removed during the grinding stroke.In order to bring new, unused areas of the grinding worm 100 into engagement with the workpiece 200, the grinding worm is continuously or discontinuously displaced relative to the workpiece 200 along a shift direction Y, which runs parallel to the worm axis B (so-called tangential shift movement).

[0070] All of the aforementioned linear movements (feed, axial feed movement, shift movement) are to be understood as relative movements, which can be generated either by a corresponding movement of the grinding worm in space and / or by a movement of the workpiece in space.

[0071] Machining of workpiece 200 is often performed in multiple grinding strokes, but can also be performed in a single grinding stroke. When multiple grinding strokes are performed, they often involve one or more roughing strokes followed by one or more finishing strokes. The roughing strokes are generally performed with a larger infeed in the X direction than the finishing strokes. In the following, the index r is used for roughing strokes, and the index f for finishing strokes.

[0072] For the following considerations, it is assumed that both the workpiece and the grinding worm have a cylindrical basic shape. However, these considerations can also be applied to situations in which the workpiece and / or the grinding worm have a different basic shape, e.g., to situations in which a conical workpiece is being machined or in which a conical, barrel-shaped, or globoid-shaped grinding worm is used. The workpiece toothing can be straight or helical.

[0073] Figure 2 shows, as an example, the time profile of the power consumption P(t) of a tool spindle, which drives the grinding worm 100 to rotate, during a grinding stroke. Rapid fluctuations in power consumption, e.g., due to the constantly changing contact conditions between the grinding worm and the workpiece, are not taken into account in this illustration. In practice, such rapid fluctuations can be filtered out by suitable filtering, e.g., with a low-pass filter.

[0074] The power consumption initially increases continuously as the worm enters the gear teeth, then levels off and decreases continuously again during the run-out. The maximum power consumption is referred to below as grinding intensity I. s designated.

[0075] The power consumption of the tool spindle is an example of a performance indicator that characterizes the instantaneous grinding performance. Instead of power consumption, the current drawn by the tool spindle can also serve as a performance indicator, which ultimately corresponds to a torque measurement and thus approximately a measurement of the cutting force. At constant voltage, the current drawn is proportional to the power, or at constant speed, the torque is proportional to the power. Therefore, the current can also at least approximately characterize the grinding performance. The grinding intensity I sis a scalar quantity calculated from the temporal progression of the performance indicator and characterizes the grinding performance during a grinding stroke. Instead of the maximum of the performance indicator, another scalar quantity can also be used as the grinding intensity, e.g., the integral or the (possibly appropriately weighted) temporal mean of the performance indicator.

[0076] The grinding performance depends on a number of parameters, in particular on the infeed between the grinding worm and the workpiece along the radial infeed direction, on the speed of the axial feed movement (axial feed rate), on the speeds of the tool and workpiece spindles, on the geometry of the grinding worm, on the geometry of the workpiece and on the tribological conditions in contact (lubricant, surface quality of the worm and material, as well as materials).

[0077] However, grinding performance is also influenced by the wear condition of the grinding worm. For example, a partially worn grinding worm is not able to remove the same amount of material during one grinding stroke as a freshly dressed grinding worm. As a result, increasing wear of the grinding worm leads to a decrease in cutting performance. For given grinding parameters, the grinding intensity I s thus serve as an indicator of the wear condition of the grinding worm in the currently used area of ​​the grinding worm.

[0078] Grinding intensity when grinding with conventional

[0079] Conventional shift strategies usually result in the first workpieces being machined after dressing with grinding worm areas that are subject to increasing wear from workpiece to workpiece until an equilibrium is reached where the wear state no longer changes from workpiece to workpiece. This will be explained in more detail for a conventional shift strategy using Figures 3A and 3B.

[0080] For this purpose, it is initially assumed that each workpiece is machined with a single roughing stroke and a single finishing stroke, with continuous shifting occurring during both the roughing and finishing strokes. It is also assumed that between the roughing and finishing strokes, a shift jump occurs into a finishing section of the grinding worm that has not yet been used for roughing, and that after finishing, a shift return occurs exactly at the point where the previous roughing stroke ended. This corresponds to a conventional shift strategy, as known from the state of the art and frequently used in continuous generating grinding.

[0081] Figure 3A shows schematically the grinding intensity I for such a shift strategy r (n) during roughing when machining a plurality of workpieces n, in Figure 3B the grinding intensity I f(n) during finishing, where the grinding worm was freshly dressed before machining the first workpiece. Due to increasing wear in the roughing area, the grinding intensity gradually decreases during roughing of the first workpieces until it reaches a substantially constant value. During finishing, the opposite is true: The grinding intensity initially increases during machining of the first workpieces, as correspondingly more material is removed in the finishing stroke due to the lower material removal in the roughing stroke, and then levels off.

[0082] The reason for this behavior will be examined in more detail below. on the grinding worm

[0083] During the helical generating motion between the grinding worm and the workpiece, at least one flank of a grinding worm thread (worm thread flank) and at least one flank of the workpiece toothing (workpiece flank) are in approximately point contact. The contact point between the worm thread flank and the workpiece flank continuously moves over the respective worm thread flank during the machining of an individual workpiece flank due to the rotation of the grinding worm 100 and the workpiece 200. The sequence of contact points on the grinding worm is referred to below as the contact path. Figure 4 illustrates such a contact path 102 on an individual worm thread flank 101. The contact path 102 has a conical helical shape. The resulting sequence of contact points on the workpiece flank is referred to below as the contact track for ease of differentiation.

[0084] However, when a finite amount of stock is removed, the contact between grinding worm 100 and workpiece 200 is not exactly point-like. Rather, a finitely extended contact area exists at any given time, in which the grinding worm penetrates the workpiece flank subject to stock removal. Figure 5 schematically illustrates a section of a contact path 102 with a contact area 103 on the worm thread flank. The contact area 103 generally has an elongated shape along the contact path 102 and is asymmetrical across the contact path.

[0085] The extent of the contact area 103 transverse to the direction along which the contact path runs is referred to below as the cutting width a p The cutting width a pdepends, among other things, on the macrogeometry of the workpiece, the macrogeometry of the grinding worm, the removed stock on the workpiece flank, and the axial feed per workpiece revolution. It can vary along the contact path. For example, with an involute gear, the engagement ratios change along the involute, which also affects the cutting width. As a result, the contact path width can vary along its length. Contact zones with continuous shift motion

[0086] Without shift movement, the grinding worm performs the cutting work exclusively along a single contact path 102. When a continuous shift movement is initiated, this contact path is continuously screwed along the worm thread flank and thus axially to the grinding worm during the course of a grinding stroke, so that the contact path moves over the worm thread flank 101.

[0087] In this way, during a grinding stroke, a total area of ​​the worm gear flank engages the workpiece, which is referred to as the "contact zone." The contact zone that engages the workpiece during a single grinding stroke essentially has a conical helical shape, similar to a single contact path, but due to the superimposed shift movement, it is wider than a single contact path. Without shift movement, the contact zone corresponds to a single contact path.

[0088] Figure 6 shows, in a highly schematic form and not to scale, two contact zones 104, 104' on a worm thread flank 101. These contact zones engage during the machining of successive workpieces during a selected grinding stroke (here specifically during the roughing stroke) when proceeding according to the conventional shift strategy described above. With such a conventional shift strategy, the roughing stroke of the subsequent workpiece begins exactly at the point in the Y direction where the roughing stroke of the previous workpiece ended. In this case, the two contact zones 104, 104' overlap due to the finite width of the contact paths. The width of the overlap area approximately corresponds to the width of a single contact path, i.e., the cutting width a p , when roughing.

[0089] In practice, this means that the wear condition of the grinding worm in the contact zone 104 influences the machining conditions in the subsequent grinding stroke, which is carried out with the overlapping contact zone 104'.

[0090] Variable grinding intensity with conventional shift strategies

[0091] Based on the above considerations, it is clear why the grinding intensity with conventional shift strategies is initially not constant from workpiece to workpiece, even when all grinding parameters remain unchanged. The reason for this behavior is that the contact zones used in the machining of a subsequent workpiece partially overlap with the contact zones used in the machining of the previous workpiece. With conventional shift strategies, the amount by which the grinding worm is shifted during each roughing or finishing stroke is usually significantly smaller than the cutting width measured along the worm axis. Figuratively speaking, this means that the grinding worm gradually builds up wear after dressing: the first workpiece is machined with a contact zone that has not yet experienced any wear.The second workpiece is machined with a partially overlapping contact zone, with a larger portion of this contact zone having already been used to machine the first workpiece and a smaller portion having not yet experienced any wear. For the third workpiece, part of the contact zone has already been used to machine two previous workpieces, another part has only been used to machine a single previous workpiece, and another part has not yet experienced any wear, and so on. Each subsequent workpiece is thus machined with a contact zone increasingly subject to wear until equilibrium is reached and the wear level of the worm over one grinding stroke is the same in each contact zone as in the previous contact zone.

[0092] Increasing wear in the contact zone during roughing is reflected in a decreasing roughing grinding intensity I r, because the cutting performance decreases with increasing wear. This leads to an increasing finishing grinding intensity, because more cutting work must be performed during finishing. This corresponds exactly to the observed behavior shown in Figures 3A and 3B.

[0093] The above considerations can easily be applied to situations in which more than one roughing and / or finishing stroke or only a single grinding stroke is used.

[0094] Grinding with decoupled contact zones

[0095] To prevent the wear condition of a specific contact zone from being influenced by the wear condition of other, partially overlapping contact zones, it is advisable to decouple the contact zones from each other so that they no longer overlap. With the shift strategy described above, this can be achieved, for example, by reducing the shift offset.

[0096] This is illustrated by way of example in Figure 7, which shows two contact zones 104, 104' that engage during the roughing stroke when machining successive workpieces if the shift recess is suitably reduced. The contact zones 104, 104' now extend side by side across the worm thread flank 101 without overlap, i.e., they are decoupled from each other.

[0097] The amount by which the shift recess must be reduced in order to decouple the contact zones 104, 104' in the example of Figure ? can be clearly derived by a geometric consideration.

[0098] For this purpose, a reference profile in the axial section of the grinding worm is first considered, which can be brought into engagement with the gear teeth of the grinding worm and the workpiece produced with it in such a way that these gear teeth roll on it without deviation. Figure 8 schematically shows such a reference profile 110. The left flanks of the reference profile 110 have a profile angle α. An engagement path for the left flanks can be drawn above the reference profile. The engagement path runs perpendicular to the left flanks of the reference profile. The profile angle α corresponds to the pressure angle in the axial section of the grinding worm. However, instead of the engagement path, Figure 8 shows a band with the width a pThis band symbolizes the contact path. At the points where the contact path intersects the respective left flank, the contact path has the width a p , measured perpendicular to the direction along which the contact path runs. Since the contact path on the grinding worm flank has a conical helical shape and thus runs predominantly in the circumferential direction of the grinding worm, the width of the contact path, measured perpendicular to the contact path direction, corresponds in good approximation to its width measured in the vertical direction of the respective left flank. Measured along the worm axis, however, the width of the belt is approximately a p / sin a. This value is referred to below as the "width of the contact path, measured along the grinding worm axis." A similar calculation can also be made for the right-hand flanks.

[0099] This means that if two consecutive contact zones are not to overlap, there must be a distance of at least Ay > a between the shift position of the grinding worm at the end of the machining operation carried out with the first contact zone and the shift position at the beginning of the machining operation carried out with the second contact zone. p / sm a, measured along the screw axis.

[0100] The above consideration is indeed simplified, since it does not take into account, for example, the screw pitch, a cutting width a that changes between the head and foot area of ​​the screw thread p and possible modifications of the screw flanks are taken into account, but it provides a practical approximate solution and can be easily refined accordingly if necessary.

[0101] Calculating the cutting width

[0102] The cutting width a pcan be calculated approximately analytically or determined numerically by a penetration calculation. One possibility for analytical calculation is disclosed in W02021048027A1. According to this document, a p approximately calculated from the technology parameters and gearing parameters as follows:

[0103] Here, a Pimax a maximum expected cutting width at a given rolling length and at a given nominal allowance for large axial feeds, l k the contact length of the chip zone and s z the axial feed per revolution of the workpiece.

[0104] The maximum cutting width can be calculated as follows:

[0105] Here, L y the rolling length and q t the nominal measurement.

[0106] For the contact length l k the following analytical relationship can be derived: Here r denotes pSS eq the equivalent grinding worm radius. The equivalent grinding worm radius r pSS eq results from the real grinding worm radius r pSS as follows: r p ss,eq = r P ss / Sin a

[0107] Here, a denotes the pressure angle of the grinding worm gear in axial section.

[0108] As a result, it is possible to adjust the cutting width a p along a contact path for each machining situation. Instead of the analytical calculation, a penetration calculation can also be performed to determine the cutting width a p along a contact path.

[0109] As explained above, the cutting width can vary along the contact path. To achieve reliable decoupling of the contact zones even with variable cutting widths, it is advisable to proceed as follows: The nominal allowance is the maximum expected allowance on all teeth of the gearing that is removed from the tooth flanks at a given infeed. The cutting width a p along a contact path for this nominal dimension. The width a p 0 of the contact path, measured in the profile height direction of the worm thread flank, the maximum value of the cutting width a p along the contact path. The width of the contact path, measured along the screw axis, is then determined by dividing this value by sin:

[0110] Ay > a p 0 / sin a. Contact zones

[0111] If each workpiece is not machined with a single stroke, but with at least one roughing stroke and at least one finishing stroke, various shift strategies are conceivable that take the above considerations into account and can achieve decoupling of the contact zones. Two possible shift strategies are explained below as examples.

[0112] A first shift strategy is illustrated in Figure 9. This figure, similar to Figure 8, shows contact zones on a reference profile. However, to maintain clarity, the reference profile is not shown here, and the shape and course of the contact zones are shown significantly compressed compared to Figure 8. The width of the grinding worm is given by b ss Figure 9 can also be understood as a representation of a (unfolded) worm gear flank 101, in which the horizontal axis represents the width position y ssalong the grinding worm axis and the vertical axis the height position h ss or indicates the pitch path along the worm thread flank.

[0113] With this shift strategy, the grinding worm is divided into a roughing area 111 and a finishing area 112, as with a conventional shift strategy. When machining the first workpiece, a roughing stroke is performed first. During the roughing stroke, shifting occurs continuously in the Y-direction. During the roughing operation, the contact path moves along the grinding worm axis. As a result, the roughing stroke occurs with a roughing contact zone whose width, measured along the grinding worm axis or in the Y-direction, corresponds to the amount of the shift feed during the roughing operation plus the width of a single contact path. The amount of the shift feed during the roughing operation is shown in the figure as y. rthe width of a single contact path along the grinding worm axis is called Ay r . Overall, roughing uses a roughing contact zone with a width y r + Ay r , measured along the screw axis.

[0114] After the end of the roughing operation of the first workpiece in the roughing area 111, a shift jump into the finishing area 112 takes place by a (positive) amount Ay sl to the point where the finishing stroke of the first workpiece begins. Continuous shifting also occurs during the finishing stroke, whereby the shift speed can be the same or different from the roughing stroke. The total shift feed during the finishing stroke is y f, the width of a single contact path along the screw axis during finishing Ay^. As a rule, the width of the contact path during finishing will be smaller than during roughing due to the smaller infeed. Overall, a finishing contact zone with a width of y is thus created during finishing. f + Ay^, measured along the screw axis.

[0115] After the finishing stroke, a shift return occurs before the roughing stroke of the second workpiece begins. Unlike conventional shift strategies, however, the shift return now occurs exactly at the same point where the roughing stroke of the first workpiece began. The same roughing contact zone used for the roughing stroke of the first workpiece is now used a second time for the roughing stroke of the second workpiece. The finishing stroke of the second workpiece is also executed with the same finishing contact zone used for the finishing stroke of the first workpiece.

[0116] These contact zones are now used for machining additional workpieces until one of these contact zones is worn out. After the finishing stroke of the last workpiece machined with these contact zones, a reduced shift return by a (negative) amount Ay occurs. s2. This amount is chosen so that: Ay$i + Xr + Ay s2 > Ay r In this way, the roughing contact zone of the next workpiece is decoupled from the roughing contact zone of the previous workpiece.

[0117] A second shift strategy is illustrated in Figure 10. In the shift strategy outlined here, a first group of workpieces is machined with a grinding worm area 121, a second group of workpieces with an adjacent grinding worm area 122, etc. For each workpiece, a roughing stroke with a roughing contact zone is performed, followed by a finishing stroke with a finishing contact zone.

[0118] To prevent the roughing contact zone and the finishing contact zone of a workpiece from overlapping, an additional shift movement (shift jump) is performed between the end of the roughing cut and the beginning of the finishing cut of each workpiece. The amount Ay slThis additional shift movement is chosen to be sufficiently large that the first contact path at the beginning of the finishing operation just no longer overlaps with the last contact path at the end of the roughing operation. This is the case when the worm is moved at least by the amount Ay between the end of the roughing operation and the beginning of the subsequent finishing operation. r is shifted. The contact zones of the roughing stroke and the finishing stroke are thus decoupled from each other.

[0119] After the end of the finishing operation for the last workpiece of the first group, another additional shift movement is carried out by an amount y s2which is at least Ay^ before the roughing operation of the following first workpiece of the second group begins. This decouples the roughing contact zone for the workpieces of the second group from the finishing contact zone for the workpieces of the first group. When machining the workpieces of the second group, the procedure is then the same as for machining the workpieces of the first group. This decouples all roughing and finishing contact zones. If more than one individual roughing stroke and / or more than one individual finishing stroke are carried out, the corresponding contact zones are preferably also decoupled from one another. If only a single machining stroke is carried out per workpiece, the procedure can also be analogous.

[0120] Of course, many other shift strategies with decoupled contact zones, each of which carries out multiple grinding strokes, are possible. Not all contact zones need to be used for the same number of grinding strokes. For example, the finishing contact zones can be used for a significantly different number of grinding strokes than the roughing contact zones, meaning that the change from one roughing contact zone to another roughing contact zone can occur independently of the change between the finishing contact zones. A change to another contact zone can also occur regardless of whether the contact zone in question is already worn.For example, if there are three roughing contact zones, a first workpiece can be machined with the first roughing contact zone, a second workpiece with the second roughing contact zone, and a third workpiece with the third roughing contact zone, before a fourth workpiece is again machined with the first roughing contact zone, a fifth workpiece with the second roughing contact zone, and a sixth workpiece with the sixth roughing contact zone, etc. In this way, all roughing contact zones always have a similar state of wear.

[0121] Another conceivable variant involves a pool of equally wide, decoupled contact zones. Each of these contact zones initially serves as a finishing contact zone during one or more grinding strokes. As soon as the contact zone in question no longer delivers sufficient finishing quality (which can be determined, for example, by the grinding intensity), it is subsequently used as a roughing contact zone for one or more roughing strokes.

[0122] Considerations for choosing grinding parameters

[0123] When machining with decoupled contact zones, it is advantageous to select grinding parameters (especially the radial infeed and the axial feed rate) that are not too aggressive, so that the wear of the grinding worm per grinding stroke remains low in the respective contact zone. It is also advantageous to select a relatively large ratio between the shift travel and the axial stroke (i.e., the diagonal ratio) for each of the contact zones, so that the wear per grinding stroke is distributed over a relatively large contact zone.

[0124] This makes it possible to grind a surprisingly large number of workpieces using the same contact zone without unacceptable quality losses. For example, it has been demonstrated that several dozen, sometimes even more than 100, grinding strokes can be performed per contact zone without any loss of quality. The service life of the grinding worm can thus be increased several times over compared to conventional shift strategies. This more than compensates for the possibly slightly reduced productivity due to the more conservative grinding parameters selected.

[0125] Grinding intensity during repeated grinding with the same contact zone

[0126] Figures 11A and 11B schematically show the progression of the grinding intensity when using a single roughing and finishing contact zone in one of the shift strategies outlined above.

[0127] Since the roughing contact zone is used for machining multiple workpieces, it wears increasingly. This increasing wear is reflected in the roughing grinding intensity I in Figure 11A. r decreases continuously from workpiece to workpiece without reaching a substantially constant value, as was the case with a conventional shift strategy in Fig. 3A. The increasing wear in the roughing contact zone means that increasingly more cutting work must be performed during finishing. As a result, the finishing grinding intensity If increases continuously from workpiece to workpiece in Figure 11B, again without reaching a substantially constant value.

[0128] This behavior, unlike a conventional shift strategy, allows for automated decisions about when to switch to a fresh, unused roughing or finishing contact zone. In one embodiment, for example, a lower threshold roughing intensity I r thr be defined. As soon as this intensity is undershot (or as soon as this intensity has been undershot for a certain number of workpieces or a suitable running average of the intensity across several workpieces has been undershot), the workpiece changes to a fresh roughing contact zone. Accordingly, an upper threshold finishing intensity If thrdefined. As soon as this threshold is exceeded (or has been exceeded for a certain number of workpieces, or a suitable running average intensity value across multiple workpieces has been exceeded), the switch to a fresh finishing contact zone occurs. Alternatively, the switch to a fresh finishing contact zone can occur whenever a switch to a new roughing contact zone also occurs. The threshold values ​​I r>thr , I f>thr can, for example, be determined empirically by measurements on a grinding worm known to be worn.

[0129] Instead of grinding intensity, any other wear indicator that correlates with the wear condition of the grinding worm can be considered. Accordingly, threshold values ​​for this wear indicator can be defined for roughing or finishing. If these values ​​are exceeded, the machine switches to the next contact zone. For example, a low-frequency (e.g., low-pass filtered) spectral component of the standardized performance indicator can also serve as a wear indicator.

[0130] As soon as all roughing and finishing contact zones are worn, the grinding worm is redressed. This ensures optimal utilization of the grinding worm, as it is only dressed when it must be considered worn based on objective criteria.

[0131] Switch to fresh contact zone based on more complex criteria

[0132] In the example shown in Figures 11A and 11B, the decision as to when a particular contact zone is worn was made based on a comparison of a single scalar wear indicator with a threshold value. Alternatively, the decision can also be made based on more complex criteria. An example is illustrated in Figure 12.

[0133] This figure shows schematically the roughing intensity I r (vertical axis) and the finishing intensity I f (horizontal axis) for a large number of workpieces, each machined with a roughing stroke and a finishing stroke, with all roughing strokes being carried out with the same roughing contact zone and all finishing strokes being carried out with the same finishing contact zone.

[0134] Initially, the roughing intensity is relatively high and the finishing intensity relatively low, as derived above. As the number of workpieces increases, the roughing intensity decreases and the finishing intensity increases (direction of the arrow in Figure 12). When the roughing contact zone is worn, the roughing intensity falls below a threshold value I r thr and the finishing intensity exceeds a threshold I ftthr The decision to switch to a fresh, unused roughing or finishing contact zone can now be made taking both thresholds into account. A switch can occur, for example, if the following two conditions are met simultaneously across a certain number of workpieces: I r < I rithr , / y > If thr This corresponds to the area designated as "critical" in Fig. 12.

[0135] Here, too, any other wear indicator can be considered instead of grinding intensity. Grinding intensity during roughing or finishing is therefore just one example of a wear indicator during roughing or finishing.

[0136] Standardization of the measured grinding performance

[0137] The performance indicator or the grinding intensity calculated from it can be appropriately standardized to improve the comparability of the performance indicator for different conditions.

[0138] The normalization operation is preferably based on a model that describes the expected dependence of the process performance on geometric parameters of the grinding worm and the workpiece, as well as on the setting parameters of the generating grinding machine. The process performance model can, in particular, be based on a force model that describes the expected dependence of the cutting force acting at the contact point between the grinding worm and the workpiece on geometric parameters of the finishing tool, geometric parameters of the workpiece, and setting parameters of the finishing machine.

[0139] The normalization operation can, for example, involve multiplying the measured values ​​of the performance indicator or derived quantities by a normalization factor. However, more complex normalization operations are also conceivable. The normalization factor can, in particular, be an inverse intensity quantity calculated using the process performance model for the specific processing situation, or a quantity derived from it. For example, the process performance model used in W02021048027A1 can be used.

[0140] Preferably, the normalization factor is recalculated after each dressing operation, and the same normalization factor is applied to all values ​​of the performance indicator or a variable calculated therefrom that were determined between two dressing operations.

[0141] Control of the axial feed rate based on the grinding power

[0142] If the grinding power is too high, production deviations may occur, e.g., shape deviations and / or thermal damage to the edge zone of the workpiece flanks (so-called grinding burn). There is also a risk of damage to the grinding worm, e.g., by chipping on one or more worm threads. To prevent this, the grinding process can be controlled. Specifically, at least one grinding parameter, in particular the axial feed rate v z (t) can be controlled in such a way that a specified control strategy is maintained. Various control strategies can be used. Two possible control strategies are explained below using Figures 13A and 13B, and Figures 14A and 14B, respectively. However, other control strategies are also conceivable.

[0143] Figure 13A illustrates, as an example, the temporal progression of a performance indicator in the form of the power consumption P(t) of the tool spindle during a grinding stroke, when the axial feed rate is controlled such that the (preferably standardized) performance indicator does not exceed a maximum value ("maximum value control"). Without control of the grinding process, in this example, the power consumption would exceed a predetermined maximum value P max at least temporarily (dotted line). To avoid this, the axial feed rate v z(t) must be controlled so that this maximum value is not exceeded. As soon as the grinding power approaches the maximum value, the feed rate is reduced due to the control system so that the grinding power does not exceed the maximum value. Any known control principle can be used for this purpose, e.g., PID control. The resulting curve of the feed rate v z (t) is illustrated in Figure 13B.

[0144] The example in Figure 14A illustrates the temporal progression of the power consumption of the tool spindle when the axial feed rate is controlled such that the power consumption at least approximately follows a target curve ("target curve control"). The target curve can, for example, describe the expected power consumption based on a process power model under ideal conditions assuming a certain stock allowance distribution. For example, the process power model mentioned above can be used to calculate the target curve. Deviations from the target curve can arise, for example, if a workpiece has a stock allowance distribution that deviates from the assumed stock allowance distribution; however, other factors can also play a role, such as the hardness distribution and the current wear condition of the grinding worm.The dotted line shows the power consumption curve as it would occur with a constant axial feed rate. In this example, this curve deviates significantly from the target curve. For example, the power consumption is lower at the beginning during the run-in phase than in the target curve, but significantly higher later on. The control system compensates for these deviations by increasing the feed rate when the power consumption deviates negatively from the target curve and decreasing it when the deviation is positive. This is illustrated as an example in Fig. 14B.

[0145] These control strategies can also be modified and / or combined. For example, in Fig. 14B, a maximum feed rate can be defined that must not be exceeded, or the feed rate can only be controlled during the phase of the grinding stroke in which a specified minimum power consumption value is exceeded. Many other modifications to the control strategy are conceivable.

[0146] With decoupled contact zones, such strategies for controlling the grinding process can be used with far greater predictability and reliability than when overlapping contact zones are used. As long as the contact zones overlap, the wear condition of the grinding worm varies over the course of each individual grinding stroke. As a result, the grinding performance over the course of each grinding stroke is strongly influenced by the changing wear of the grinding worm, i.e. the control system reacts strongly to the wear of the grinding worm and is therefore unable to optimally fulfill its actual task. Due to the decoupling, the control is now primarily influenced by the properties of the workpiece, in particular by its changing allowance, and possibly by inhomogeneities in the grinding worm and the wear condition in the relevant contact zone itself, but no longer by changing wear conditions in other contact zones.

[0147] The decoupling of the contact zones paves the way for intelligent control of the grinding process.

[0148] Consideration of the useful area of ​​the gear to be manufactured

[0149] Typically, when a finished workpiece is used in a gearbox, not the entire left and right flanks of the workpiece engage with a mating toothing, but only a specific area of ​​the respective flank. This area is referred to in this document as the "usable area." It can be determined, for example, by measuring the contact pattern of the toothing under real operating conditions.

[0150] Fig. 15A illustrates, by way of example, a workpiece flank 210 of a gear with the associated useful area 211. The useful area 211 does not extend over the entire width of the workpiece flank in the workpiece width direction z. ws. It also does not extend over the entire height of the workpiece flank in the workpiece height direction h ws .

[0151] In this case, one goal of workpiece machining may be to achieve the highest possible gear quality within the working area, while the quality of the gear outside the working area may be of secondary importance. In this case, it is advantageous to determine the area of ​​the contact zones on the worm gear flank that come into contact with the working area during gear machining and thus determine quality.

[0152] Fig. 15B shows, by way of example, such a quality-determining region 105 of a contact zone 104 on a worm thread flank 101. The quality-determining region can be determined, for example, by a machining simulation, as is well known to those skilled in the art, or by machining tests.

[0153] The advantages explained above can also be achieved if, instead of the entire contact zone, only the quality-determining area of ​​the contact zone is arranged without overlap with all other contact zones. In this case, the wear condition in the quality-determining area is not influenced by the wear condition of all other contact zones. A contact zone in which only the quality-determining area is arranged without overlap with all other contact zones can therefore also be considered a "decoupled" contact zone, and all of the above considerations apply analogously to such decoupled contact zones. Structure of a generating grinding machine

[0154] Figure 16 shows an example of a gear grinding machine 1 suitable for performing the methods described above. This gear grinding machine is also referred to below as the "machine."

[0155] The machine 1 has a machine bed 11 on which a tool holder 12 is guided so as to be displaceable along the radial feed direction X. The tool holder 12 carries an axial slide 13 which is guided so as to be displaceable relative to the tool holder 12 along the axial feed direction Z. A grinding head 14 is mounted on the axial slide 13 and can be pivoted about a pivot axis running parallel to the X direction (the so-called A axis) in order to adapt to the helix angle of the gear to be machined. The grinding head 14, in turn, carries a shift slide on which a tool spindle 15 can be displaced relative to the grinding head 14 along the shift direction Y. The grinding worm 100 is clamped on the tool spindle 15. The grinding worm 100 is driven by the tool spindle 15 to rotate about a tool axis B.

[0156] The machine bed 11 further supports a pivotable workpiece carrier 20 in the form of a turret, which can be pivoted about a pivot axis C3 between at least three positions. Two identical workpiece spindles are mounted diametrically opposite one another on the workpiece carrier 20, of which only one workpiece spindle 21 is visible in Figure 16. A workpiece can be clamped onto each of the workpiece spindles and driven to rotate about a workpiece axis C1 or C2. The workpiece spindle 21 visible in Figure 16 is in a machining position in which a workpiece 200 clamped thereon can be machined with the grinding worm 100. The other workpiece spindle, offset by 180° and not visible in Figure 16, is in a workpiece change position in which a finished workpiece can be removed from this spindle and a new blank can be clamped onto it.A dressing device 30 is mounted offset by 90° to the workpiece spindles.

[0157] Machine 1 thus has a multitude of moving components, such as slides or spindles, which are controlled by corresponding drives. These drives are often referred to in the technical world as "NC axes," "machine axes," or simply "axes." This term sometimes also includes the components driven by the drives, such as slides or spindles.

[0158] Machine 1 further comprises a plurality of sensors. By way of example, only two sensors 18 and 19 are schematically indicated in Figure 1. Sensor 18 is a vibration sensor for detecting vibrations of the housing of grinding spindle 15. Sensor 19 is a position sensor for detecting the position of axial slide 13 relative to tool carrier 12 along the Z direction. In addition, machine 1 comprises a plurality of other sensors. These sensors include, in particular, further position sensors for detecting the actual position of a linear axis, rotation angle sensors for detecting the rotational position of a rotary axis, current sensors for detecting the drive current of a rotary axis, and further vibration sensors for detecting vibrations of a driven component.

[0159] All driven axes of machine 1 are digitally controlled by a machine control system 40. The machine control system 40 comprises several axis modules 41, a control computer 42, and an operator panel 43. The control computer 42 receives operator commands from the operator panel 43 as well as sensor signals from various sensors of the machine 1 and calculates control commands for the axis modules 41 from these. It also outputs operating parameters to the operator panel 43 for display. The axis modules 41 each provide control signals for a machine axis at their outputs. In this way, all of the aforementioned movements can be controlled, in particular radial infeed, axial feed movement, and shift feed movement.

[0160] A monitoring device 44 is connected to the control computer 42.

[0161] The monitoring device 44 can be a separate hardware unit assigned to the machine 1. It can be connected to the control computer 42 via a known interface, e.g., via the well-known Profinet standard, or via a network, e.g., via the Internet. It can be physically part of the machine 1 or it can be located spatially remote from the machine 1.

[0162] During operation of the machine, the monitoring device 44 receives a large number of different measurement data from the control computer 42. Among the measurement data received by the control computer are sensor data that were recorded directly by the control computer 42 and data that the control computer 42 reads from the axis modules 41, e.g. data that describe the target positions of the various machine axes and the target current consumption in the axis modules.

[0163] The monitoring device 44 can optionally have its own analog and / or digital sensor inputs to directly receive sensor data from additional sensors as measurement data. The additional sensors are typically sensors that are not directly required for controlling the actual machining process, e.g., acceleration sensors for detecting vibrations or temperature sensors.

[0164] The monitoring device 44 can alternatively also be implemented as a software component of the machine control 40, which is executed, for example, on a processor of the control computer 42, or it can be designed as a software component of the service server 45 described in more detail below.

[0165] The monitoring device 44 communicates directly or via the Internet and a web server 47 with the service server 45. The service server 45, in turn, communicates with a database server 46 containing the database DB. These servers can be located remotely from machine 1. The servers do not need to be a single physical unit. In particular, the servers can be implemented as virtual units in the so-called "cloud."

[0166] The service server 45 communicates with a mobile device 48 via the web server 47. The device 48 can, in particular, run a web browser with which the received data and its analysis are visualized. The device does not need to meet any special computing power requirements. For example, the device can be a desktop computer, a notebook computer, a tablet computer, a mobile phone, etc.

[0167] Processing of workpieces

[0168] For the sake of completeness, the following describes how workpieces are typically machined using machine 1. In order to machine an as yet unmachined workpiece (blank) 200, the workpiece 200 is clamped by an automatic workpiece changer onto the workpiece spindle located in the workpiece change position. The workpiece change takes place in parallel with the machining of another workpiece on the other workpiece spindle located in the machining position. Once the newly machined workpiece 200 is clamped and machining of the other workpiece is complete, the workpiece carrier 20 is pivoted 180° about the C3 axis so that the spindle with the newly machined workpiece 200 moves into the machining position. Before and / or during the pivoting process, a centering operation is carried out using the associated centering probe.For this purpose, the workpiece spindle 21 is rotated, and the position of the tooth gaps of the workpiece 200 is measured using a centering probe. The pitch angle is determined on this basis.

[0169] When the workpiece spindle carrying the workpiece 200 to be machined has reached the machining position, the workpiece 200 is brought into collision-free engagement with the grinding worm 100 by moving the tool carrier 12 along the X-axis. The workpiece 200 is then machined by the grinding worm 100 in rolling engagement. This is achieved through one or more grinding strokes, for example, one or more roughing strokes followed by one or more finishing strokes. During each grinding stroke, the grinding worm 100 is continuously advanced along the Z-axis relative to the workpiece 200 with a constant or variable radial X-feed (so-called axial stroke). During the machining stroke, the tool spindle 15 can be slowly and continuously shifted along the shift axis Y. Alternatively or additionally, a shift jump can occur between two grinding strokes.

[0170] At the same time as the workpiece is being machined, the finished workpiece is removed from the other workpiece spindle and another blank is clamped onto this spindle.

[0171] If, after machining a certain number of workpieces, the grinding worm 100 has become so worn that it is too blunt and / or the flank geometry is too inaccurate, the grinding worm is dressed. To do this, the workpiece carrier 20 is pivoted by ±90° so that the dressing device 30 is positioned opposite the grinding worm 100. The grinding worm 100 is then dressed with the dressing tool 33. LIST OF REFERENCE SYMBOLS

[0172] 100 grinding worm

[0173] 101 worm gear flank

[0174] 102 Contact path

[0175] 103 Contact area

[0176] 104 Contact zone

[0177] 104' contact zone

[0178] 105 quality-determining area

[0179] 110 Reference profile

[0180] 111 Grinding worm area for roughing

[0181] 112 Grinding worm area for finishing

[0182] 121 Grinding worm area for first workpieces

[0183] 122 Grinding worm area for second workpieces

[0184] 200 workpieces

[0185] 201 Workpiece gearing

[0186] 210 Flank of the workpiece toothing

[0187] 211 usable area

[0188] I Generating grinding machine

[0189] II Machine bed

[0190] 12 tool carriers

[0191] 13 axial slides

[0192] 14 Grinding head

[0193] 15 tool spindle

[0194] 18 Vibration sensor

[0195] 19 Position sensor

[0196] 20 workpiece carriers

[0197] 21 Workpiece spindle

[0198] 41 Axle module

[0199] 42 tax calculators

[0200] 43 Control panel

[0201] 44 Monitoring device

[0202] 45 Service server 46 Database

[0203] 47 web servers

[0204] 48 mobile devices

[0205] B Worm axis

[0206] C Workpiece axis

[0207] X radial feed direction

[0208] Y shift direction

[0209] Z axial feed direction b ws Workpiece width b ss Grinding worm width a p Cutting width y r Shift amount during roughing yy Shift amount during finishing

[0210] Ay r Width of a contact path along the screw axis during roughing

[0211] A y Width of a contact path along the screw axis during finishing

[0212] Ay slAmount of the shift jump between roughing and finishing

[0213] Ay s2 Amount of the shift (back) jump between finishing and next roughing

[0214] P Power consumption of the grinding spindle

[0215] P max Maximum power consumption

[0216] I s Grinding intensity

[0217] I r Grinding intensity during roughing

[0218] If grinding intensity during finishing

[0219] I r thr Threshold value of grinding intensity during roughing

[0220] Jf.thr Threshold value of grinding intensity during finishing t Time n Number of workpieces au arbitrary units

Claims

PATENT CLAIMS 1. A method for machining pre-toothed workpieces (200) by continuous generating grinding with a grinding worm (100) rotating about a worm axis (B), the method comprising: Dressing the grinding worm (100); and Machining the workpieces (200) after the grinding worm (100) has been dressed, wherein each workpiece (200) is machined with at least one grinding stroke, wherein during each grinding stroke the grinding worm (100) is moved relative to the respective workpiece (200) along a workpiece axis (C), while the workpiece (200) rotates about the workpiece axis (C) in a machining engagement with the grinding worm (100), wherein each workpiece (200) has a workpiece toothing (201) with workpiece flanks (210), wherein during each grinding stroke the workpiece flanks (210) are machined with at least one worm thread flank (101) of the grinding worm (100), wherein on the worm thread flank (101) there is a contact zone (104, 104'), in which the worm thread flank (101) is in contact with the workpiece flanks during the respective grinding stroke (210), wherein on each workpiece flank (210) there is a useful area (211), and wherein the contact zone (104,104') has a quality-determining region (105) which comes into contact with the useful regions (211) of the workpiece flanks (210), characterized in that there are several decoupled contact zones (104, 104') on the worm thread flank (101), the quality-determining regions (105) of which are arranged substantially without overlap with all other contact zones (104, 104') of the worm thread flank (101) and with which a plurality of grinding strokes are carried out before the grinding worm (100) is dressed again.

2. Method according to claim 1, wherein a predetermined number of grinding strokes are carried out with each of the decoupled contact zones before the grinding worm (100) is dressed again.

3. The method according to claim 2, wherein the predetermined number is at least 3, preferably at least 5, in particular at least 10.

4. Method according to claim 1, wherein for each of the decoupled contact zones a wear characteristic (7 r ; I f ) is determined, which characterizes a wear condition of the grinding worm in the respective contact zone (104, 104'), and wherein, depending on the wear parameter ( / r ; If) a decision is automatically made as to whether a machining intervention is to be carried out again in the same contact zone (104, 104') for a subsequent grinding stroke or whether this contact zone (104, 104') is no longer used for further grinding strokes before the grinding worm is dressed again.

5. The method according to claim 4, wherein the wear parameter ( / r ; I f ) is an intensity measure for a grinding intensity, in particular a maximum, integral, mean or low-pass filtered spectral component of a power indicator (P(t)) for an instantaneous grinding power.

6. Method according to claim 4 or 5, wherein the decision is made based on a comparison of the wear parameter ( / r ; I f ) or a value derived therefrom with a threshold value (I r)thr ', If it hr) is made.

7. Method according to one of the preceding claims, wherein a plurality of workpieces (200) are machined with each of the decoupled contact zones (104, 104') before the grinding worm (100) is dressed again.

8. The method according to claim 7, wherein with each of the decoupled contact zones (104, 104') exclusively one selected grinding stroke per workpiece is carried out, in particular a roughing stroke or a finishing stroke.

9. Method according to one of the preceding claims, wherein all contact zones (104, 104') used on the grinding worm (200) are arranged relative to one another in such a way that their quality-determining regions are arranged substantially without overlap with all other contact zones (104, 104').

10. Method according to one of the preceding claims, wherein for each Workpiece (200) during a grinding stroke in which the workpiece (200) in question is machined with one of the decoupled contact zones (104, 104'), the grinding worm (100) is continuously displaced relative to the workpiece (200) both along an axial feed direction (Z), which has a component parallel to the workpiece axis (C), and along the shift direction (Y), so that the grinding worm (200) covers a total of a shift path along the shift direction (Y) and a stroke path along the axial feed direction (Z) relative to the workpiece (100) during the machining intervention in the grinding stroke in question.

11. Method according to one of the preceding claims, wherein the grinding worm in at least a part of the decoupled contact zones (104, 104') does not have a modification of the worm flight flank (101) which is different along different contact paths within the respective contact zone.

12. Method according to one of the preceding claims, wherein the grinding worm assumes a first shift position at the end of the machining engagement of a first grinding stroke, wherein the grinding worm assumes a second shift position at the beginning of the machining engagement of a subsequent second grinding stroke, wherein between the first shift position and the second shift position there is a distance (Ay) which corresponds to at least a width of a contact path on a grinding worm flank (101), measured along the worm axis (B), and wherein the grinding worm (100) exclusively assumes shift positions (Y) which lie outside the range between the first shift position and the second shift position during all grinding strokes.

13. The method according to claim 12, wherein the following relationship is satisfied for the distance (Ay) between the first shift position and the second shift position: Ay > a p 0 / sin a , where a p 0denotes the width of the contact path, measured perpendicular to the contact path course, and where a denotes an angle of pressure of the grinding worm (100) in axial section.

14. Method according to one of the preceding claims, wherein during processing with the decoupled contact zones (104, 104') a performance indicator (P(t)), which characterizes an instantaneous grinding performance, is measured and at least one grinding parameter is controlled as a function of the performance indicator (P(t)).

15. A gear cutting machine for machining pre-toothed workpieces (200) by continuous generating grinding, comprising: a tool spindle (15) designed to clamp a grinding worm (100) thereon in order to drive the grinding worm (100) to rotate about a worm axis (B); a workpiece spindle (21) designed to clamp a pre-toothed workpiece (23) thereon in order to drive the pre-toothed workpiece (23) to rotate about a workpiece axis (C); an axial slide (13) designed to generate an axial feed movement of the tool spindle (15) relative to the workpiece spindle (21) along an axial feed direction (Z) having a component parallel to the workpiece axis (C); a shift slide which is designed to generate a shift movement of the tool spindle (15) relative to the workpiece spindle (21) along a shift direction (Y) running parallel to the screw axis (B);a dressing device (30); and a machine control (40), characterized in that the machine control (40) is designed to carry out the method of one of the preceding claims.

Images