DE4244869C2 - Tool measuring method for machine spindle - Google Patents

Abstract

The tool (14) is moved between a spindle stock and a workpiece table. The relative position of the spindle stock to a reference point is measured by a displacement measurement system. The tool is moved in the direction of one of its coordinates to an optical measurement plane perpendicular to these coordinates. The measurement system produces a signal when the tool enters the plane, and the instantaneous spindle stock position is measured. The tool dimensions are then derived from the measured position and the relative position of the measurement plane to the reference point. The procedure is also conducted in a coordinate direction transverse to the longitudinal direction of the tool.

Description

The invention relates to a method for measuring a Tool in a spindle of a machine tool in which the tool by relative movement between one the spindle receiving headstock and a workpiece table will, where

• - the relative position of the headstock to a reference point is determined by means of a measuring system,
• - the tool in the direction of one of its coordinates essentially transverse to the coordinate optical measuring plane with assigned optical measuring system is delivered
• - The measuring system outputs a measurement signal, based on which determined whether the tool is immersed in the measuring plane,  
• - When the tool is immersed in the measuring plane, the current one relative position of the headstock measured as position measurement will, and
• - from the measured position value and from the relative position of the Measuring plane to the reference point the dimensions of the tool are calculated in the coordinate, where
• - The measuring signal of the measuring system with one of the measuring system recorded comparison value is compared to determine check whether the tool is immersed in the measuring plane.

Such a method is known from EP-B-00 98 930 known.

The known method is used for the tool of an NC Monitor machine tool for breakage or wear. To the tool will be delivered or withdrawn in the tool longitudinal direction by a cross to the feed direction the optical measurement plane of the tool. This Measuring plane is formed by a laser light barrier, whereby the interruption of the light barrier is determined by comparison and evaluated as a signal and the current relative position of the spindle holding the tool is measured.

In order to determine the length of the tool, you must also the relative position of the light barrier to the zero point of the Wegmeßsystemes be known, by means of which the position of the Spindle is determined.

By simply forming the difference between the relative Position of the light barrier and the current position of the spindle or the headstock can then calculate the length of the tool  will. By comparing the calculated length with a target length can then be concluded for wear or breakage.

To the light barrier consisting of an encoder and a sensor from contamination by cooling water, chips or other impurities to protect against inclinations, cover caps are provided. Furthermore are Blow nozzles available to clean the encoder and the sensor.

A disadvantage of the known method is that the accuracy the length depends on how accurate the relative location the light barrier is known. Does it happen, for example, during the Work processes of the machine tool for thermal deflections or other changes in position between the light barrier and the reference point of the measuring system is an exact one Length determination of the tool is no longer possible.

Furthermore, the length of the tool is determined by im Processing room impurities in the air affect pregnant because these impurities give a signal drop in the light barrier, which corresponds to the signal drop superimposed by a tool immersed in the beam is caused. In this way the measurement of the moment tanen relative position of the headstock with a large Uncertainty factor provided. This leads to a big one Measurement inaccuracy when determining the length of the tool.

However, the less precise the current length of the tool determines can become, the more uncertain are statements about breakage or wear, which are obtained with the known method should.

Furthermore, a method is known from EP-A-0 346 288, in which a tool partially into a wide laser beam  is driven to the diameter, the eccentricity and the Determine the roundness of the tool. A length measurement is not known from this publication.

Otherwise, this method has the same disadvantages as they already above in connection with that from EP-B-0 098 930 known methods have been described, in particular it is not intended for use with machine tools running.

Furthermore, WO 87/07550 discloses a method in which cutting edge completely with a laser beam and CCD cameras be measured.

DE-AS-22 31 776 discloses a device in which one elongated light source partially from one to be measured Object is shadowed so that depending on the geometry of the Object more or less light falls on a measuring sensor. Light from the same light source is also unobstructed mapped a reference sensor. A quotient of the Signals from both sensors allow influences of the Ambient light or fluctuations in operation Brightness of the light source has no effect on the measurement signal exercise.

Finally, it is from the book by Hans Hart: "Introduction to the measuring technology ", VEB-Verlag Technik, Berlin, 1977, Pages 330-333, generally known devices before commissioning can be compared with normals.

Based on this, it is an object of the present invention to further develop the method mentioned at the beginning, that the disadvantages mentioned above are avoided. In particular, the measurement accuracy and the reproducibility should be  speed when measuring the tool. In addition, should The new process will help the machine tool can be operated even with greater accuracy.

This task is achieved in the method mentioned at the outset solved that in the comparison value of the current pollution degree of air in the work space is taken into account by only a drastic drop in the measurement signal compared to the Comparison value as immersion of the tool in the measuring plane is evaluated, and that before such a drastic signal drop taken comparison value is maintained.

This measure makes the one on which the invention is based Task completely solved. By comparing the measurement signal with a comparison value, namely those in the work area soiling of the machine tool, so to speak "hidden", because in the comparison value the current Pollution degree of the air in the work space thereby be seen that only a drastic change than immersion of the tool is taken into the measuring plane. Here will namely assumed that the change in intensity of the Laser beam through the Verun in the work area cleaning compared to the change in intensity due to the immersion of a tool is low.

The uncertainty when measuring the current relative position the headstock is drastically reduced, which in turn leads to a more precise measurement of the dimensions of the tool. This in turn leads to the machine tool having a higher Accuracy can be operated.  

It is preferred if before measuring the tool a measuring tool with defined dimensions is measured, and if from the measured position value for the measuring tool and the defined The relative position of the measuring plane is determined.

The advantage here is that the relative position of the measuring plane can be measured again at any time during the operation of the machine tool can, so that deflections of the machine are recognized and taken into account can be. Thermal expansion of the machine or other  Changes in position of the measuring plane are thus recognized and lead no longer lead to an incorrect calculation of the dimensions of the measurement Tool. So that both the measurement accuracy and the Reproducibility clearly in the inventive method elevated. Because now the current dimensions of the tool are more accurate can also be measured with the machine tool higher operational reliability and accuracy.

It is preferred if one before immersing the tool total measured value taken in the measurement plane, preferably the last one taken measurement value, is used as a comparison value.

The advantage here is that the comparison value is, so to speak, simultaneous can be determined with the measurement signal, so that even one rapidly changing contamination in the working area of the tool machine can have no influence on the measuring accuracy.

In particular, it is preferred if several of the above measures mentioned in the method according to the invention be performed.

This is advantageous because by combining the Measures mentioned above the measurement accuracy again can be increased. For example, for the determination the diameter or roundness of a tool a lot greater measurement accuracy is required than for the determination of the Length of a tool. So if with the invention Method, for example, the measurement accuracy by using a Comparison value and / or by using a measuring tool is increased to determine the relative position of the measuring plane, the diameter and the roundness of the tool determine even more precisely.  

But also when determining the length of a tool Combination of the measures "measuring tool" and "comparison value" the cumulative advantage that the measurement accuracy again is significantly increased.

All this leads to the dimensions of the tool being more accurate are known and thus the machine tool with higher resolution and operate with greater accuracy and reproducibility can.

It is further preferred if the comparison value is periodic is measured recurrently.

This measure is particularly advantageous if larger ones Contaminants, such as chips, fly through the work area, while the measurement is being made. These impurities can namely make the measuring system respond, although the tool has not yet been immersed in the measuring plane. Because the chips find their position in the work area relatively quickly change, their influence on the next measurement is not more available, so that such contaminants are out can be dazzled.

In a further development, it is preferred if the immersion of the tool in the measuring plane by measuring the difference is determined.

In a further development of the measure, the measurement signal with a comparison It is advantageous here to compare the value that the measured value and the comparison value can be determined at the same time, so that too rapid changes in the pollution of the work area be hidden.  

It is advantageous here if the optical measuring plane is replaced by a Light barrier, preferably a laser light barrier, is formed becomes.

This measure also advantageously increases the Measuring accuracy at. A light barrier, preferably a laser light barrier, namely has a defined measuring beam with very small diameter, so that to interrupt the light beam only a very slight immersion of the tool in the measuring plane is required. It can also be provided that already a certain weakening of the light beam by the in this Beam penetrating tool - for example by 60% - as immersion of the Tool is recognized in the measuring plane. The resolution of the measurement systems is significantly increased in this way, so that in total together with the measurement can be carried out with greater accuracy.

In a development of the methods mentioned, it is preferred if they are carried out with the tool rotating and if the measuring system periodically delivers measured values.

This measure is particularly useful when determining roundness of the tool to be measured is advantageous, but contributes overall help that the inventive method by faster can be performed. There is no longer any need to wait until the tool has been braked to a standstill.

By periodically delivering the readings from one turn The tool can also easily adjust the roundness of the tool getting closed. An out-of-round tool is namely at tan potential approach to the measuring level temporarily into the measuring level dive and leave it temporarily so that the measured values in the rhythm of the rotating tool due to the one The roundness of the latter fluctuates periodically. The cognition  the lack of roundness of the tool increases the Operational reliability and machining accuracy of the tool machine.

Here it is further preferred if the speed of the tool is so adapted to the measuring period of the measuring system that the The speed and the measuring period are not synchronized with each other are.

Not synchronized here means that the Measuring period from the time for one revolution of the tool, preferably an integer multiple or one integer part of this time is different.

In this way it is achieved that during the entire measurement period Measured values from several points along the circumference of the tool be taken. In this way, even small deviations be detected in the roundness of the tool, which in turn the Measuring accuracy, reproducibility and processing accuracy increased.

Overall, it is preferred if the method is based on a Machine tool carried out with automatic tool change and between the tool changes the relative position of the Measuring level is determined.

The advantage here is that again and again during processing process by the machine tool a measuring tool can be used to determine the position of the measuring plane. Out Changes in the relative position of the measuring plane can then occur thermal deflections or other displacements in the Machine are closed. Then this realization will not only when determining the dimensions of each used Tool used, it can also be used for other  To change the machine setting parameters or to con troll.

Furthermore, it is for a machine with an automatic tool change is advantageous if the measuring plane is arranged in such a way that the respective tool when delivering to one immerses the workpiece in the measuring plane. That way "Dead times" between the individual processing steps are small held, because the spindle has to be changed first moved away from the workpiece and then again with the new tool be delivered. If the measurement plane is crossed, can the measuring of the tool during delivery, so to speak respectively. Please note that the measuring period of the Measuring system, compared to the delivery speed of the Tool must be sufficiently small. The more measured values namely are taken for each path element of the tool covered can, the finer this section can be divided. Is For example, a resolution of 0.1 mm is desired and will be Tool delivered at 10 mm per second, at least 100 readings per second can be taken to get the desired Achieve resolution of 0.1 mm.

An embodiment of the invention is shown in the drawing, Fig. 1 to 3, and is explained in more detail in the following description. FIGS. 4 and 5 relate to an alternative method, to take into account the actual degree of contamination of the air in the working space, which are not subject of the claims. Show it:

Figure 1 is a schematic representation of a machine tool in side view, on which the new method is carried out, the tool is not immersed in the measuring plane.

FIG. 2 shows an illustration like FIG. 1, but the tool is just immersed in the measuring plane;

Fig. 3 is a representation along the line III-III of Fig. 2;

Fig. 4 shows a second measuring system for the measuring plane, in a sectional view, in the view as in Fig. 3; and

Fig. 5 in a representation like Fig. 4, a third measuring system for the measuring plane.

Fig. 1 shows a side view schematic representation of a machine tool 10. The machine tool 10 comprises a tool table indicated at 11 for receiving a workpiece to be machined, which is not shown for the sake of clarity. Furthermore, a headstock 12 is provided, in which a spindle indicated at 13 is rotatably and driven mounted. A tool 14 facing the workpiece table 11 is seated in the spindle 13 .

The headstock 12 can be moved in the three indicated coordinates X, Y and Z relative to the workpiece table. To determine the relative position of the headstock in the Z direction, a displacement measuring system 15 is provided with a Z scale indicated at 16 . A reference point for the Z direction is indicated at 17 , which corresponds to the X / Y plane in the selected coordinate system.

To determine the relative position of the headstock 12 in the Z direction, a reference plane is fixed on the headstock 12 , which corresponds to the transition between the spindle 13 and tool 14 . This reference plane is indicated in FIG. 1 by a measuring arrow 19 which points to the scale 16 . The distance between the reference point 17 and the measuring arrow 19 indicates the relative position of the headstock 12 .

An optical measuring system 21 is arranged on the workpiece table 11 , which in the present case is a light barrier 22 . The light barrier 22 includes a transmitter 23 , which is designed here as a laser light source 24 .

There is also a receiver 25 which is designed as a photocell 26 . An evaluation unit 27 is assigned to the receiver 25 .

The laser light source 24 outputs a laser beam 28 which is directed onto the photocell 26 . An optical measuring plane 29 is spanned by the laser beam 28 and runs parallel to the X / Y plane in FIG. 1.

The measuring plane 29 is arranged at a distance Rz from the reference point 17 . This distance Rz is either known as a machine parameter, but it can also be calculated using the method according to the invention. This will be described below.

While in Fig. 1, the tool 14 is not immersed in the measuring plane 29, is the case in Fig. 2, in which the measuring tool 14 dips just in the measuring plane 29 and interrupts the laser beam 28. The laser beam 28 will either no longer hit the photocell 26 at all, or only with reduced beam power, which is indicated by the broken line of the laser beam 28 'in FIG. 2 to the left of the tool 14 .

The interruption of the laser beam 28 or the reduction in its intensity is recognized by the evaluation unit 27 . The evaluation unit 27 then queries the current relative position Pzm of the headstock 12 in relation to the reference point 17 .

In the position of the headstock 12 shown in FIG. 2, the geometric relationship between the relative position of the measuring plane 29 and the headstock 12 is such that the length l of the tool 14 results from the difference between Rz and Pzm. The length l of the tool 14 is defined so that it speaks the distance from the tool tip to the reference plane indicated by the measuring arrow 19 ent on the headstock.

Between the positions shown in Fig. 1 and Fig. 2 of the spindle stock has been moved in the Z-direction 12. The measuring system 21 is now designed so that the evaluation unit 27 queries the photocell 26 with a measuring period which is matched to the speed of the headstock 12 in the Z direction and to the desired resolution. The faster the headstock 12 is moved and the higher the desired resolution, the smaller the measuring period must be. The higher the travel of the headstock 12 per unit of time, the more measured values have to be taken per unit of time. If the infeed speed is, for example, 10 mm per second and the measuring accuracy is 0.1 mm, at least 100 measured values per second must be taken. By increasing the number of measured values per unit of time, the measuring accuracy can also be increased.

It has now been shown that the relative position Rz of the measuring plane 29 relative to the reference point 17 changes during the operating time of the machine tool due to thermal deflections or other changes in position. Since this also influences the measuring accuracy, provision is now made to measure the relative position Rz with the aid of a measuring tool 31 . For this purpose, instead of a tool 14, a measuring tool 31 is clamped in the spindle 13 , the length l of which is known.

The above-described method for measuring a tool is now carried out for the measuring tool 31 , but the relative position Rz of the measuring plane 29 must be determined. From the geometric relationship shown in FIG. 2 it follows that Rz corresponds to the sum of the current relative position Pzm and the length l.

The insertion of a measuring tool can be carried out again and again between the individual tool changes, in particular in machine tools with automatic tool change during the machining of a workpiece with different work, so that the position of the measuring plane 29 is checked over the entire machining time. This not only increases the accuracy when measuring the tool used, it can also detect changes in position within the machine and take them into account accordingly.

FIG. 3 shows a top view of the arrangement from FIG. 2, a second optical measurement plane 32 , which lies in the X / Z plane. The measuring plane 32 is also spanned by the laser beam 28 .

Furthermore, a path measuring system 33 for the Y direction is shown, which comprises a u reference point 34 and a Y scale 35 . With the help of the displacement measuring system 33 , the relative position Py of the headstock 12 is measured in the Y direction.

Furthermore, a measuring device 36 is indicated, which is intended to display the measurement signals output by the evaluation unit 27 . Such a display is of course not necessary, it is only intended to represent the measurement signal as such.

Furthermore, the evaluation unit 27 has an output 37 , at which it is displayed whether a tool 14 or a measuring tool 31 is immersed in the measuring plane 29 or 32 .

In Fig. 3, a measurement arrow is indicated at 38 , which runs through the axis of rotation of the tool 14 . The distance between the measuring arrow 38 and the reference point 34 indicates the relative position Py of the headstock 12 in the Y direction. A position measuring system corresponding to the position measuring system 33 is also provided for the X direction, but is not shown for reasons of clarity.

In the 14 'position of the tool 14 , the evaluation unit 27 outputs a signal at its output 37 , which indicates that the tool 14 is immersed in the measuring plane 32 . Now the current relative position Pym of the spindle stock 12 or the measuring arrow 38 is measured and determined by difference formation with the relative position Ry of the laser beam 28 in the X / Z plane of the radius of the tool 14 .

If a measuring tool 31 whose radius r is known is used again, the relative position Ry of the measuring plane 32 can be calculated using the current position Pym.

This means that there can also be displacements within the machine tool can be recognized in the Y direction. By another optical Measuring system that is aligned in the Y direction can correspond appropriate measurements can also be made for the X direction.

In the following it must be considered with regard to the measuring accuracy that oil-polluted air is present in the working area of the machine tool, which air is additionally contaminated with small impurities in the form of chips and other abrasion. These impurities drift around in the working area of the machine tool 10 and cause the laser beam 28 to strike the photocell 26 with different intensity. In order to prevent a signal drop due to such impurities from causing the tool 14 to be immersed in the measuring plane 32 or 29 , the current measuring signal from the evaluation unit or the photocell 26 is compared with a reference value. This reference value is taken periodically and corresponds in the simplest embodiment to the last measured value. This means that the current measured value is compared in each case with the last measured value taken, and that it is concluded from the comparison whether a tool 14 plunges into the measuring plane 32 . It is assumed here that the change in intensity of the laser beam due to the impurities present in the work space is small compared to the change in intensity due to the immersion of a tool 14 . However, if a drastic drop in intensity, for example by 60%, is detected, the last measurement value taken is retained as a comparison value for all subsequent measurements. In this way, the effects of the impurities can be masked out, so to speak.

Table 1 will now be used to describe how the roundness of a tool 14 can also be determined using the method described so far. In table 1, five different states during the immersion of a tool 14 in the measurement plane 32 are shown in the left column. The respective associated measurement signal of the evaluation unit 27 is indicated in the middle column, U0 being the reference value.

The right-hand column shows the output 37 of the evaluation unit 27 , a plus sign denoting the immersion of a tool.

In the top line, the tool 14 is still so far away from the measuring plane 32 that, despite the out-of-roundness of the tool 14, which is clearly shown in Table 1, the laser beam 28 is never interrupted. The measurement signal here is, for example, 30 mV. The output 37 does not output a signal.

In the second line, the tool 14 has moved tangentially to the measuring plane 32 , but has not yet been immersed in the measuring plane 32 . Due to contamination in the work area, the measurement signal has now dropped to 27 mV, but output 37 does not yet indicate a submerged tool.

In the third line, the tool 14 has rotated by approximately 90 °, so that the laser beam 28 is now completely interrupted. The measurement signal is now only 10 mV. The evaluation unit 27 recognizes this drastic signal drop and takes the last measurement signal (from the second line) of 27 mV as a comparison value. Furthermore, the immersion of a tool 14 is displayed at the outlet 37 .

In the fourth line, the distance between the tool 14 and the measuring plane 32 has not changed, but due to the rotation of the non-circular tool, the laser beam 28 is no longer interrupted. The photocell accordingly delivers a measurement signal of 28 mV, which is compared with the comparison value U0 of 27 mV. The evaluation unit recognizes from the comparison that no tool is immersed anymore.

In the last line of Table 1, the tool 14 has rotated through a further 90 ° and is again immersed in the laser beam 28 . The measurement signal is now 12 mV, the deviation from the value from the third line being caused by atmospheric changes in the working area of the machine tool. By comparing the 12 mV and U0 of 27 mV, the evaluation unit 17 again recognizes the immersion of a tool 14 and indicates this at the output 37 .

It should now be mentioned here that the number of measured values per unit of time and the number of revolutions of the tool per unit of time are coordinated with one another in such a way that no synchronization takes place. This means that the measuring period is different from the time for one revolution of the tool, preferably even from an integral multiple or an integral part of this time. In other words, the measuring system detects the tool 14 for each measured value in a different rotation state, so that the out-of-roundness can be reliably recognized.

Of course, Table 1 gives only a brief extract from the entire measurement sequence. The feed speed of the tool 14 in the Y direction is so low compared to the speed of the tool 14 and the number of measured values per unit of time that the out-of-roundness is measured virtually stationary.

Because the measured value is compared in each case with a previously taken comparison value in order to conclude that a tool 14 is immersed in the measuring plane 32 or 29 , no special measures are provided here for protecting the transmitter 23 or the receiver 25 . Soiling is automatically hidden, so to speak.

In FIG. 4 is a further optical measuring system 39 is shown as it can be used in the machine tool according to FIGS. 1 to 3.

The measuring system 39 comprises, as it were, a further system 40 which is connected in parallel to the original measuring system 21 . For this purpose, the laser beam 28 with a beam splitter 41 and a deflecting mirror 42 is divided into two parallel laser beams 28 'and 28 ''. With the help of a further deflection mirror 43 and a deflection mirror 45 mirrored on one side, the laser beam 28 '' is also directed onto the photocell 26 .

In the beam path of both laser beams 28 'and 28 ''photo closures or shutters 47 and 48 are provided, which are switched via control lines 49 and 50 by the evaluation unit 27 .

The evaluation unit 27 switches the shutters 47 and 48 in push-pull, so that the photocell 26 either measures the laser beam 28 'or the laser beam 28 ''. Since the tool 14 is only immersed in the laser beam 28 ', the laser beam 28 ''thus remains unaffected by this, the comparison measurement takes place between two measured values taken almost simultaneously. It is particularly advantageous that only a single laser light source 24 and only a single photo cell 26 are required. Contamination of the air in the working area of the machine tool can also be hidden in this way.

Another possibility of masking out impurities is shown in FIG. 5. The measuring system 51 shown here also has a second path 52 , for the second laser beam 28 '', however, a separate photo cell 53 is provided. Via the connecting line 54 , the photocell 53 is also read out by the evaluation unit 27 , which carries out a difference measurement between the photocells 26 and 53 . In this way, the two parallel laser beams 28 'and 28 ''are read out and compared at the same time, so that the immersion of the tool 14 can be measured regardless of contamination.

All of the methods described so far for eliminating the effects of impurities in the work space obviously increase the measuring accuracy of the system. This is because not only the complete interruption of the laser beam 28 'can be detected by an immersed tool 14 , but also a slight weakening of the laser beam 28 ' by an only partially immersed tool 14 is possible. On the other hand, since the laser beam 28 used has only a very small diameter, the relative position Pym or Pzm of the headstock 12 can be determined precisely to a fraction of the diameter of the laser beam and thus to a fraction of a millimeter. In connection with the high resolution of the measuring system, this leads to the fact that not only a very precise measurement of a tool 14 used in the machine tool is possible both in length and in diameter, and the roundness of a rotating tool can also be determined quickly.

All of the measures described so far significantly increase the accuracy when measuring a tool. Furthermore, the new method offers the advantage that changes in position within the machine tool can also be detected if a measuring tool is measured, the dimensions of which are defined and known. The values obtained in this way can in turn be used when the next machining tools 14 are measured.

Claims (10)

1. Method for measuring a tool ( 14 ) in a spindle ( 13 ) of a machine tool ( 10 ), in which the tool ( 14 ) by relative movement between a spindle stock ( 12 ) receiving the spindle ( 13 ) and a workpiece table ( 11 ) is delivered, the relative position (Py, Pz) of the headstock ( 12 ) to a reference point ( 34 , 17 ) being determined by means of a position measuring system ( 33 , 15 ), the tool in the direction of one of its coordinates (Y, Z) in If the optical measuring plane ( 32 , 29 ), which is essentially transverse to the coordinate (Y, Z), is supplied with an assigned optical measuring system ( 21 , 39 , 51 ), the measuring system ( 21 , 39 , 51 ) outputs a measuring signal, which is used to determine whether the tool ( 14 ) plunges into the measuring plane ( 32 , 29 ), when the tool ( 14 ) plunges into the measuring plane ( 32 , 29 ) the current relative position (Pym, Pzm) of the headstock ( 12 ) as a measured position value (Pym, Pzm) is measured, and from the measured position value (Pym, Pzm) and from the relative position (Ry, Rz) of the measuring plane ( 32 , 29 ) to the reference point ( 34 , 17 ) the dimensions (r, l) of the tool ( 14 ) in the coordinate (Y, Z) are calculated, the measuring signal of the measuring system ( 21 , 39 , 51 ) being compared with a comparison value (U0) recorded by the measuring system ( 21 , 39 , 51 ) in order to determine whether the tool ( 14 ) is immersed in the measuring plane, thereby characterized in that in the comparison value (U0) the current degree of pollution of the air in the work space is taken into account by only a drastic signal drop of the measurement signal compared to the comparison value (U0) is evaluated as immersion of the tool in the measuring plane, and before such a drastic signal drop taken comparison value is maintained. 2. The method according to claim 1, characterized in that the comparison value (U0) is measured periodically becomes. 3. The method according to claim 1 or 2, characterized in that the measurement value last taken before immersing the tool ( 14 ) in the measuring plane ( 32 , 29 ) is used as a comparison value (U0). 4. The method according to any one of claims 1 to 3, characterized in that the immersion of the tool ( 14 ) in the measuring plane ( 32 , 29 ) is determined by a differential measurement. 5. The method according to any one of claims 1 to 4, characterized in that the optical measuring plane ( 32 , 29 ) by a light barrier ( 22 ), preferably a laser light barrier, is formed. 6. The method according to any one of claims 1 to 5, characterized in that it is performed with a rotating tool ( 14 ). 7. The method according to any one of claims 1 to 6, characterized in that the measuring system ( 21 , 39 , 51 ) periodically delivers measured values. 8. The method according to claims 6 and 7, characterized records that the speed of the tool to the Measuring period of the measuring system is adapted to the speed and the measuring period are not synchronized with each other.   9. The method according to any one of claims 1 to 8, characterized in that a measuring tool ( 31 ) with defined dimensions (r, l) is measured before measuring the tool ( 14 ), and that from the measured position value (Pym, Pzm) the relative position (Ry, Rz) of the measuring plane ( 32 , 29 ) is determined for the measuring tool ( 31 ) and the defined dimensions (r, l). 10. The method according to any one of claims 1 to 9, characterized in that it is carried out on a machine tool ( 10 ) with automatic tool change, and that the relative position (Ry, Rz) between the tool changes is determined.