EP0468385A2 - Method for the circumferential grinding of radial non-circular works - Google Patents

Abstract

A method is used for the circumferential grinding of radially non-circular workpieces, in particular for the grinding of cams (15) or polygons. The workpiece is rotated about a first axis (18), and a grinding wheel (11) is fed along a second axis (13) which encloses an angle of preferably 90 with the first axis (18). In a plurality of steps corresponding in each case to a revolution of the workpiece, material is removed from the surface of the workpiece (15), starting from a blank contour, along spiral paths of an engagement point (20) to form intermediate contours and finally to form a finished contour by the workpiece and the grinding wheel (11) being rotated or fed in a predetermined manner as a function of data sets. In each case after an intermediate contour is obtained, a new data set is called up for the following revolution of the workpiece. A predetermined absolute dimension (RGactual) of the workpiece is continuously measured, and a deviation ([2645]RG) from a desired value (RGdesired) is determined. The deviation ([2645]RG) is compared (40) with threshold values, and if the deviation ([2645]RG) falls short of the threshold values a predetermined number of steps are skipped. <IMAGE>

Description

The invention relates to a method for the peripheral grinding of radially non-circular workpieces, in which the workpiece is rotated about a first axis and a grinding wheel is fed along a second axis, which includes an angle of preferably 90 ° with the first axis, with the workpiece on Its surface, starting from a raw contour, is removed along spiral-like paths of an engagement point in a plurality of steps corresponding to one revolution of the workpiece in each case to intermediate contours and finally to a finished contour by rotating the workpiece and the grinding wheel in a predetermined manner as a function of data records are delivered, a new data record for the subsequent rotation of the workpiece being called up each time an intermediate contour is reached and in which a predetermined absolute dimension of the workpiece is continuously measured and a deviation from a target value is determined.

A method of the type mentioned above is known from EP-A 0 276 802.

Numerically controlled (CNC) grinding machines are used today for the peripheral grinding of radially non-circular workpieces, for example for the peripheral grinding of cams on a camshaft or polygon profiles. When machining the workpieces, the workpieces are clamped and rotated about their longitudinal axis (so-called C axis) by means of a controllable workpiece turning device. The grinding wheel, on the other hand, is arranged on a grinding carriage which can be advanced along a further axis (so-called X-axis) towards the axis of rotation of the workpiece. The C-axis and the X-axis are usually perpendicular to each other.

The desired circumferential profile of the workpiece, e.g. the cam shape or the polygon shape are now generated in that the workpiece is rotated around the C axis in predetermined angular increments as a function of one another and at the same time the grinding wheel is adjusted linearly along the X axis. These two mutually coordinated movements describe the desired profile of the workpiece in the so-called "path operation", while in the so-called "infeed operation" a further infeed movement is superimposed which corresponds to the desired material removal.

In order to move the C axis and the X axis in the manner described depending on the desired peripheral profile of the workpiece, data records are stored in the control of the grinding machine. These data records assign a certain linear setting of the X axis to each angular position of the C axis, the values of the data records being matched to one another in such a way that the desired circumferential profile is created, taking into account both the "rail operation" and the "delivery operation" ".

Workpieces of the type of interest here are delivered as blanks for carrying out such processes, i.e. as workpieces that still have a considerable allowance compared to the desired finished size, which must be removed with the respective process. However, since the overall measurement, i.e. the geometrical distance of the raw contour from the desired target contour, generally greater than the amount of material that can be removed in one machining step, the known methods are usually carried out in several steps in succession. For example, a few roughing steps with a relatively large infeed are usually carried out first, followed by a few finishing steps with a correspondingly smaller infeed, until the finished workpiece is finally extended, i.e. is clamped out of the workpiece holder.

As a result of this step-by-step machining of the workpiece from the raw contour to the target contour, the point of engagement of the grinding wheel on the workpiece is guided along a spiral path which begins at a first point of engagement of the grinding wheel on the raw contour of the still unprocessed workpiece and finally at an end point of the finished contour of the finished workpiece ends.

It is known in such multi-step machining processes, for each step, i.e. for each revolution of the workpiece, to provide a specific data set, which in each case extends from an intermediate contour previously reached to the next intermediate contour. For this purpose, a data record is usually stored in the numerical control of the machine tool for each such step, so that these data records can be called up one after the other in a time-saving manner when machining the workpieces. Although it would also be possible to calculate the required data sets for each new processing step during the process, this would be too time-consuming with the means available today, which is why all data sets are usually defined beforehand and saved as fixed data in the control.

In real machining of non-circular workpieces, numerous interferences can now occur, which lead to machining errors. A first disturbing influence can result from the fact that a workpiece clamped at the ends, for example a long thin shaft, radially evades the grinding wheel which is in contact. Another disturbing influence consists in thermal length changes in components of the machine tool and also in the workpiece itself. Another disturbing influence results from dynamic tracking errors Learn if the grinding machine has to move relatively large masses (grinding slides) relatively quickly to perform a non-circular grinding process. Finally, there are also interference from wear of the grinding wheel, changes in position when clamping and unclamping, and the like.

The errors resulting from these interferences are divided into the so-called form errors and the so-called dimensional errors. In the case of shape errors, only the deviation from the ideally specified shape is taken into account, without absolute dimensions playing a role. In the case of the dimensional errors, however, only the absolute dimensions of certain characteristic points of the generated profile are checked, while the form accuracy is otherwise not taken into account.

In the previously conventional methods, such as those described in DE-Z "Werkstatt und Betrieb", 118 (1985), pages 443 to 448), a camshaft was first ground with predetermined data records for grinding cams of a camshaft. The camshaft was then unclamped from the grinder, measured at a different location, and the shape errors were determined. Correction values for the data sets were then derived from the determined shape errors of the real ground camshaft in order to be able to grind further camshafts with corrected data sets.

However, this conventional approach has several disadvantages. On the one hand, the dimensional errors mentioned are not taken into account, and on the other hand, the mentioned reclamping of the workpieces involves a considerable amount of time and the risk of further errors.

Another grinding method is now known from EP-A 0 276 802 mentioned at the outset, in which an out-of-round workpiece (camshaft) is measured by means of measuring probes during clamping in the grinding wheel in order to determine dimensional errors. In the known method, the first cam of the camshaft is first ground with predetermined data, then the dimensional errors are determined by means of the aforementioned measuring sensors, and the data records of the control of the grinding machine are corrected immediately so that all other cams of the camshaft are then ground with the corrected values can be.

This method thus has the essential advantage that a correction of the dimensional errors can be carried out at least for the other cams of the camshaft in one and the same clamping. However, there remains a certain disadvantage with the method mentioned in that the first cam was still ground with the uncorrected data records. In addition, this known method is not applicable to such non-circular workpieces in which only a single non-circular profile is provided on the workpiece. This is e.g. the case for polygon waves or polygon connections with outer or inner polygon.

The invention is therefore based on the object of developing a method of the type mentioned in such a way that machining errors are recognized, monitored and evaluated for the control of the machine tool from the first engagement of the grinding wheel on the workpiece, so that the ground workpieces are true to scale.

This object is achieved in that the deviation is compared with threshold values and that a predetermined number of steps are skipped when the threshold values are undershot.

The object underlying the invention is completely achieved in this way.

In contrast to the prior art, the method according to the invention uses a regulated method in which the dimensions of the workpiece are monitored by continuous measurement, in that a comparison with a setpoint takes place. If the target value provided for the respective machining or machining phase has been reached, the machining is stopped, with the result that a workpiece is always produced which has exactly the desired dimensions.

The method according to the invention therefore makes it possible for the first time to grind non-circular workpieces with precise dimensions even during the very first machining. But also for workpieces, e.g. Camshafts, in which the same non-circular profiles are to be ground at several points on the workpiece, it is possible with the method according to the invention, already at the first machining point, e.g. on the first cam to create an accurate profile.

In a preferred development of the method according to the invention, a first plurality of roughing steps and a subsequent second plurality of finishing steps are provided, and all further roughing steps are skipped if the threshold value is undershot during a roughing step.

This measure has the advantage that when machining in several phases (roughing / finishing), during the course of one of the several machining phases, the attainment of an intermediate target value is monitored in order to immediately set the end of this machining phase.

Correspondingly, in a plurality of finishing steps and a subsequent extension step when the threshold value is undershot during a finishing step, all further finishing steps can be skipped.

The method according to the invention can thus also be used in subsequent machining phases or when machining workpieces that are machined with only one type of machining.

It is further preferred within the scope of the present invention if a finite amount of delivery is specified during some of the plurality of steps and the amount is dimensioned differently for successive steps. In particular, the amount of delivery for successive steps can decrease.

This measure has the advantage that, depending on the circumferential profile, material or type of grinding wheel, the oversize to be removed can be individually determined for successive processing steps.

• Bei einer ersten Variante eines Ausführungsbeispiels der Erfindung wird eine Mehrzahl von Schritten vorgesehen, die größer ist als die im ungünstigsten Fall für das jeweilige Werkstück erforderliche Anzahl, und es werden bei Unterschreiten des Schwellwertes die noch nicht abgearbeiteten Schritte übersprungen.

In order to be able to skip the above-mentioned predetermined number of steps within the scope of the present invention, there are two possibilities, for example:

• In a first variant of an exemplary embodiment of the invention, a plurality of steps are provided which are greater than the number required for the respective workpiece in the worst case, and the steps which have not yet been processed are skipped if the threshold value is undershot.

As an alternative to this, however, it is also possible to work with a loop by providing a plurality of steps in which the last steps are formed by repeating a specific step and this repetition is terminated when the threshold value is undershot.

While in the former case there is the advantage that each new processing step to be processed can be set individually, in the second case the control may be simplified.

Further advantages result from the description and the attached drawing.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own without departing from the scope of the present invention.

• Fig. 1 ein äußerst schematisiertes Schaubild einer Schleifmaschine zur Durchführung des erfindungsgemäßen Verfahrens;
• Fig. 2 ein Diagramm zur Erläuterung einer im Rahmen des vorliegenden Verfahrens eingesetzten Regelung;
• Fig. 3 eine Seitenansicht eines Nockens einer Nockenwelle zur Veranschaulichung eines Werkstücks, wie es mit dem erfindungsgemäßen Verfahren vorteilhaft bearbeitet werden kann;
• Fig. 4 eine Darstellung, ähnlich Fig. 3, jedoch für den Fall eines Polygon-Profils;
• Fig. 5 ein weiteres Diagramm zur Erläuterung des erfindungsgemäßen Verfahrens;
• Fig. 6 ein Blockschaltbild zur Erläuterung der Modifikation von Datensätzen;
• Fig. 7 ein Flußdiagramm zur weiteren Erläuterung des erfindungsgemäßen Verfahrens;
• Fig. 8 einen Ausschnitt aus dem Flußdiagramm der Fig. 7 zur Erläuterung weiterer Einzelheiten des erfindungsgemäßen Verfahrens.

Embodiments of the invention are shown in the drawing and are explained in more detail in the following description. Show it:

• Figure 1 is an extremely schematic diagram of a grinding machine for performing the method according to the invention.
• 2 shows a diagram for explaining a regulation used in the context of the present method;
• 3 shows a side view of a cam of a camshaft to illustrate a workpiece as it can advantageously be processed using the method according to the invention;
• FIG. 4 shows a representation similar to FIG. 3, but for the case of a polygon profile;
• 5 shows a further diagram to explain the method according to the invention;
• 6 is a block diagram to explain the modification of data records;
• 7 shows a flow chart for further explanation of the method according to the invention;
• FIG. 8 shows a detail from the flow chart of FIG. 7 to explain further details of the method according to the invention.

In Fig. 1, 10 designates an extremely schematically illustrated grinding machine. The grinding machine 10 comprises a grinding wheel 11, which rotates in a direction indicated by 12 about an axis, not shown in FIG. 1. The grinding wheel 11 is adjustable along a linear first axis 13, the so-called X axis. For this purpose, the grinding wheel 11 is arranged on a grinding carriage, not shown in FIG. 1, which can be displaced in a conventional manner in the direction of the first axis 13, likewise likewise not shown in FIG. 1 for the sake of clarity.

A cam 15 is shown in FIG. 1 as an example of a radially out-of-round workpiece, as can be machined using the method according to the invention. The cam 15 has a base circle portion 16, i.e. an area with a constant radius, and also an elevation section 17, an area in which the cam 15 is radially out of round.

The cam 15 is part of a camshaft that is clamped about its longitudinal axis in a second axis 18 of the grinding machine 10. The second axis 18 is an axis of rotation, as indicated by an arrow in FIG. 1. In practice it is referred to as the C axis.

When the grinding wheel 11 processes the cam 15, it engages at an engagement point 20 on the circumference of the cam 15. The "engagement point" is of course to be understood as a linear contact of the grinding wheel 11 on the cam 15 perpendicular to the plane of the drawing in FIG. 1.

The second axis 18 is usually perpendicular to the first axis 13, but the two axes can also form a finite angle include another size.

To get the desired perimeter profile, e.g. of the cam 15, the cam 15 is rotated about the second axis 18 in predetermined angular steps, and the grinding wheel 11 is simultaneously reciprocated along the first axis 13 in a predetermined manner. In this way, the desired profile is described while the engagement point 20 is moving, and at the same time the required delivery is set.

In this respect, the grinding machine 10 of FIG. 1 corresponds to the prior art, as was initially recognized.

The grinding machine 10 also has a length measuring device 25, which is arranged in a fixed position in the vicinity of the cam 15 and works during the machining process. The length measuring device 25 has two measuring beaks 26, 27 which, in the illustration in FIG. 1, rest against the cam 15 from above and from below. The measuring beaks 26, 27 are able to follow the cam shape, as indicated by double arrows in FIG. 1. They each measure the current radius R. In the position of the cam 15 shown in FIG. 1, the upper measuring beak 26, for example, measures a value Ri which almost corresponds to the maximum survey value of the cam 15, while the lower measuring beak 27 measures a value R ₂ , which equals the base circle radius R _{G of} the cam 15 in the base circle section 16.

The measured values determined by the measuring beaks 26, 27 are forwarded from the outputs of the length measuring device 25 to a minimum selection 30. The minimum selection 30 only forwards the smaller of the two measured values R ₁ or R ₂ . Since the base circle section 16 sweeps over a circumferential angle of more than 180 °, at least one of the measuring beaks 26 or 27 bears against the base circle section 16, which also represents the area of the minimum radius.

Accordingly, there is always a value R _Gist at the output of the minimum selection 30, which corresponds to the respective actual value of the base circle radius R _G.

In a comparator 35 connected downstream of the minimum selection 30, this value R _Gist is now compared with a target value R _Gsoll , which is supplied to the comparator 35 by a controller via a terminal 36. The comparator 35 thus determines the deviation of the actual value R _Gist from the target value R _Gsoll , and the resulting deviation is designated AR _G in FIG. 1. The deviation AR _G is now fed to a threshold value stage 40 connected downstream of the comparator 35.

For an explanation of the threshold level 40, reference may be made here to FIG. 2.

2 shows the deviation AR _G over time t during a machining process.

As can be easily seen, the deviation AR _{G of} the actual value R _Gist from the target value R _{Gsoll of} the desired finished contour _decreases with progressive processing, ie progressive time t, as shown with a curve 50 in FIG. 2. The course 50 first reaches a point 51 and then a point 52 in the course of the machining. The point 51 lies on a dividing line 53 which separates a roughing area 54 from a finishing area 55, the machining steps of the roughing in FIG. 2 and in the following figures with SR and the finishing with SL are characterized.

In order to separate the areas 54, 55 from one another, a threshold value AR _{GSR is} stored in the comparator 40, while the end of the finishing area 55 is characterized by a threshold value AR _GSL , which is preferably zero.

If the course 50 of the deviation Δ R _{G reaches} the first point 51, that is to say the end of the roughing area 54 has been reached, the threshold value stage 40 generates a first signal Si, while when the point 52 is reached the end of the finishing area 55 turns on corresponding signal S _{2 is} generated.

The signals S ₁ and S ₂ are passed from the output of the threshold stage 40 to an input of a programmable logic controller 41, which in turn controls a numerical control device 42 of the grinding machine 10. The numerical control device 42 is connected to data outputs 43 and 44 for the movement units of the X-axis, ie the first axis 13, and the C-axis, ie the second axis 18.

The operation of the signals S ₁ and S ₂ on the control unit 42 will be described in more detail below with reference to FIGS. 5 to 9.

Fig. 3 shows an enlarged view of the cam 15 in side view. The cam 15 is shown in the unprocessed state, so that a raw contour is present on the circumference with 60. 61 denotes an intermediate contour that is generated as an intermediate result in the course of the grinding process. 62 finally denotes a finished contour, i.e. the contour of the finished cam with the desired dimensions.

It is understood that the representation of FIG. 3 and also that of the following FIG. 4 is to be understood only extremely schematically and that the dimensions given are exaggerated for the purpose of illustration. It is further understood that a large number of intermediate contours 61 are present between the raw contour 60 and the finished contour 62, of which only one is shown for the sake of clarity.

In FIG. 3, 63 denotes a starting point at which the grinding wheel attacks the blank shown for the first time, as symbolized by an arrow 64. Starting from the starting point 63 the current engagement point, which was designated by 20 in FIG. 1, follows a spiral-like path 65 which moves away from the raw contour 60 with increasing infeed and approaches the first intermediate contour 61 in order to reach an intermediate point 66 there. The intermediate point 66 has a radial distance from the starting point 63, which corresponds to the oversize between the raw contour 60 and the first intermediate contour 65.

After repeating such spiral tracks 65 several times, the finished contour 62 is finally reached.

Usually, for machining a cam 15, first several such steps (spiral-like tracks 65) are carried out in the roughing operation with a relatively large infeed and then a further several steps in the finishing operation with a correspondingly smaller infeed.

Fig. 4 shows corresponding relationships in the case of a polygon profile 70, as it is e.g. is used for torque connections between shafts and hubs or spindles and tools.

In FIG. 4, 71 denotes a raw contour, 72 an intermediate contour and 73 a finished contour. The grinding wheel begins its processing at a starting point 74, as symbolized by an arrow 75, and then again follows a spiral path 76 to an intermediate point 77 on the intermediate contour 72.

Apart from the different shape of the workpiece, the conditions correspond to those of FIG. 3.

FIG. 5 shows a diagram which shows the dependence of the infeed A X set for successive steps on the time t during a machining process.

A staircase curve 80 can be clearly seen in FIG. 5, which means that the infeed from processing step to processing step, i.e. is changed in stages from revolution to revolution of the workpiece. However, "step by step" is to be understood in such a way that the infeed during a processing step, i.e. during one revolution of the workpiece, can only be adjusted to the extent that the amount of infeed desired for the machining step over a relatively short period of time, i.e. the rotation of the workpiece is set over a very small angular range. So it is e.g. in the case of cam grinding, it is known to set the entire infeed by moving the grinding wheel 11 when the grinding wheel is in engagement with the base circle section 16 of the cam 15.

On the other hand, as shown in dashed lines in FIG. 5, the infeed can be set continuously or quasi-continuously, in which case a continuous calculation of the infeed increments with the coordinates of the profile to be generated must then be carried out over the entire circumference of the workpiece.

5 again shows areas 54 'and 55' for the roughing SR and the finishing SL. It can also be seen that the amount of the infeed AX set for a processing step is not constant. Preferably, the infeed desired for successive machining steps is set ever smaller and naturally considerably larger during roughing than during finishing. 5 shows as an example an infeed amount A, X for the first machining step, ie the first rotation of the workpiece, a smaller infeed amount A ₄ X for the fourth machining step, still during the roughing SR, and finally a much smaller infeed amount Δ oX which already affects a processing step during the SL finishing.

FIG. 6 shows in a schematic form how data records are generated for successive processing steps.

In Fig. 6, 85 denotes a profile memory that contains a so-called basic profile. This basic profile can be stored in Cartesian coordinates, in polar coordinates or in the coordinates of the two axes 13, 18. If necessary, a coordinate transformation can be carried out between these various coordinates by methods known per se.

If we now consider in FIG. 6 the case in which the basic profile is stored in the profile memory 85 in the coordinates C and X of the two axes 13, 18 of the grinding machine 10, an infeed A X for successive machining steps can be stored in an infeed memory 86.

Using a logical link 87, the basic profile stored in the profile memory 85 can now be converted for each of the successive processing steps, so that a second profile memory 88 is created in which modified profiles C ^* , X ^{* are} stored. In the simplest case, this takes place in that the C coordinates are adopted unchanged from the first profile memory 85, while the X coordinates are only changed additively by the desired delivery amount AX for the respective processing step.

At the end of the process symbolized in FIG. 6, there are as many data records in the further profile memory 88 as correspond to the desired steps for the present grinding method.

7 now shows a flow diagram 90 for explaining the method according to the invention.

In Fig. 90, 91/1 ..., 91/4, 91/5 ..., 91 / n denote blocks which correspond to the individual machining steps or data records C, X in the roughing area 54 ". 91 / In contrast, n + 1 ..., 91 / n + 3, 91 / n + 4 ..., 91 / n + m denote blocks or data records for the machining steps in the finishing area 55 ".

At the end of the finishing area 95 "there is a block, designated 92, which symbolizes the movement of the workpiece out of the grinding machine 10.

In the variant of the method according to the invention shown in FIG. 7, the numbers n and m for the roughing and finishing steps are so large that they are larger than the number of machining steps required for the respective workpiece in the worst case. In other words, this means that if one would process all n roughing steps, all m finishing steps, even under the most unfavorable conditions, a workpiece would always be produced whose final dimensions are smaller than the desired ones.

In the context of the method according to the invention, however, the absolute dimensions of the workpiece are now continuously measured and monitored after each processing step in the manner shown in FIG. 1.

If it turns out that after a certain number of roughing steps or finishing steps the final dimension specified in the respective area 53 "or 55" has been reached, the signals S ₁ and S _{2 are} generated.

In the example shown in FIG. 7, after the fourth roughing step 91/4 in the threshold value stage 40 it may have been recognized that the deviation AR _G from the final dimension of the finished contour has just reached or fallen below a predetermined threshold value AR _GSR , as in FIG 2 was indicated by item 51. Accordingly, the signal S _{1 is} generated by the threshold value stage 40.

The signal S ₁ in the programmable logic controller 41 and the subsequent CNC control unit 42 has the consequence that a jump 93 takes place in the flow diagram 90 of FIG. 7, which immediately leads to the end of the process sequence after the fourth roughing step 91/4 has been processed Roughing area 51 "continues, that is to say skips the further roughing steps 91/5 ..., 91 / n which are provided per se. The process is then immediately continued with the finishing steps 91 / n + 1 ....

If, for example, it is now recognized after the third finishing step 91 / n + 3 that the deviation Δ R _{G of} the base circle radius from the target dimension R _{Gsoll of} the finished contour only deviates by the threshold value Δ R _GSL , which is preferably 0, then in 7 generates the second signal S ₂ , with the result that the second jump 94 shown in FIG. 7 is carried out at the end of the finishing area 55 ". The further finishing steps 91 / n + 4 ..., This eliminates 91 / n + m and the finished workpiece is immediately extended.

This initiation of the jumps 93 and 94 is shown again in FIG. 8 on the basis of an enlarged detail of the flow diagram 90 in the roughing area 54 ″.

It can be seen that after processing a roughing step 91 / i with the data record C ₁ , X ₁ in a block 97, the current deviation Δ i R _G from the target value of the base circle radius R _Gsoll , which is present at the output of the comparator 35, is called up .

A decision block 98 now compares whether this deviation Δ i R _{G is} even greater than the target value Δ R _GSR . If so, go to the following roughing step 91 / i + 1. If this is not the case, ie the threshold value ΔR _{GSR has} already been reached, the jump 93 to the first finishing step 91 / n is initiated.

It is understood that the flow diagram 90 shown in FIGS. 7 and 8 is only one example of several possibilities. For example, instead of a very large number of blocks 91, which is greater than the maximum required number, a smaller, limited number of blocks can also be provided, which in the end is designed like a loop with any number of repetitions of the last step in each case. This last step in each case would have to be provided with a relatively small delivery amount and processed until the comparator 35 with the subsequent threshold value stage 40 recognizes that a limit value has been reached, in order then to suppress further repetitions.

Claims (7)

1. A method for the circumferential grinding of radially non-circular workpieces (15; 70), in which the workpiece (15; 70) is rotated about a first axis (18) and a grinding wheel (11) along a second axis (13) with the the first axis (18) encloses an angle of preferably 90 °, the workpiece (15; 70) on its surface, starting from a raw contour (60; 71) along spiral paths (65; 76) of an engagement point (20 ) in a plurality of steps corresponding to one revolution of the workpiece (15; 70) to intermediate contours (61; 72) and finally to a finished contour (62; 73) by removing the workpiece (15; 70) and the grinding wheel (11 ) depending on data records in preparation are rotated or delivered in the correct manner, a new data record for the subsequent rotation of the workpiece being called up each time an intermediate contour (61; 72) is reached, and in which a predetermined absolute dimension (R _Gist ) of the workpiece (15; 70) is continuous measured and a deviation (Δ R _G ) from a target value (R _Gsoll ) is determined, characterized in that the deviation (Δ R _G ) is compared with threshold values (Δ R _GSR , Δ R _GSL ) and that when the value _{falls below} the Threshold values (ΔR _GSR , AR _GSL ) are skipped a predetermined number of steps. 2. The method according to claim 1, characterized in that a first plurality of roughing steps (91 / 1-91 / n) and a subsequent second plurality of finishing steps (91 / n + 1-91 / n + m) are provided and that if a threshold value (ΔR _GSR ) is undershot during a roughing step (91/4), all further roughing steps (91 / 5-91 / n) are skipped. 3. The method according to claim 1 or 2, characterized in that a plurality of finishing steps (91 / n + 1-91 / n + m) and a subsequent extension step (92) are provided, and that when the threshold value (Δ R _GSL ) during a finishing step (91 / n + 3) all other finishing steps (91 / n + 4-91 / n + m) are skipped. 4. The method according to one or more of claims 1 to 3, characterized in that a finite amount of an infeed (Δ X) is predetermined during some of the plurality of steps, and that the amount is dimensioned differently for successive steps. 5. The method according to claim 4, characterized in that the amount of delivery (Δ X) decreases for successive steps. 6. The method according to one or more of claims 1 to 5, characterized in that a plurality of steps is provided which is greater than the number required for the respective workpiece (15; 70) in the worst case, and that when the threshold values are undershot (Δ R _GSR , AR _GSL ) the steps that have not yet been processed are skipped. 7. The method according to one or more of claims 1 to 5, characterized in that a plurality of steps is provided, in which the last steps are formed by repeating a specific step, and that when the threshold values are undershot (Δ R _GSR , Δ R _GSL ) a repeat is canceled.

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