GB2140951A - Apparatus and method for monitoring machining conditions of a machine tool - Google Patents

Abstract

Apparatus and a method for monitoring the machining conditions of a machine tool by the continual sequential measurement of a plurality of physical magnitudes characterising the machining process, and the combination of abnormal values which are detected being evaluated to indicate a condition identifying a tool breakage or process irregularities. For this purpose the ultrasonic emission from the machining process, the main and the feed drive powers are measured and sequentially evaluated with respect to its deviation from a predetermined nominal range for its distribution. If the nominal range of the measured value distribution is exceeded a signal is emitted and stored. The simultaneous occurrence of signals from all three measured values is emitted as a tool breakage signal and the simultaneous occurrence of two signals leads to the emission of an alarm signal relating to process irregularities. <IMAGE>

Description

SPECIFICATION Apparatus and method for monitoring machining conditions of a machine tool The invention relates to apparatus and a method for monitoring machining conditions of a machine tool.

Automatic machining with a programme control led machine tool having an automatic workpiece feed needs a continuous monitoring of the cutting process for disturbances and irregularities, and an independent control for tool breakages. The detection of disturbances and irregularities during the cutting process enables steps to be taken to prevent the disturbances. If a tool breakage is detected cutting should be discontinued immediately in order to prevent the considerable consequent damage arising from a tool breakage, such as the destruction of the workpiece surface and/or the destruction of components of the machine. Also, a tool exchange may be initiated in response to the tool breakage signal.

For machine tools which are used for machining different workpieces with different machining operations, the monitoring should as far as possible be independent of the different machining conditions.

It is best known from East German Patent Specification 105739 to deduce a tool breakage from a sudden reduction in cutting power or an increase of the cutting power above its normal value. This cutting power variation does not, however, occur in all cases of tool breakage. The types and patterns of tool breakages vary in a great many ways such that it is not possible using this known arrangement to detect all tool breakages which may take place and, in addition, machining processes leading to an increase in cutting power may be incorrectly interpreted as tool breakages.

It is known from West German Offenlegungsschrift 22 51 333 to form proportional values of the feed power with respect to the main cutting power and of the return power with respect to the main cutting power, from measured values of the machining power components. Two comparison operations are carried out in order to detect a tool breakage. In the first comparison operation the actual value of one machining power component is compared with an associated limit value. In the second comparison operation each of the proportional values is compared with a predetermined proportional value.

Although this method enables the detection of a plurality of tool breakages, the number of erroneous detections is too high to enable fully automatic operation of a machine tool with no operators. This is due to the fact that spurious individual pulses occur for example as a result of voids in the workpiece or in the case of rotary machining as a result of deviations from the cylindrical shape, which pulses are interpreted as tool breakage signals and may lead to an unnecessary interruption of the cutting process or stoppage of the machine tool.

Afurther method of differentiating between tool breakage and other interference effects is known from the W. German Offenlegungsschrift 30 43 827.

In addition to the comparison operations with the measured values of the cutting power components known from Offenlegungsschrift 22 51 333, the average values of these measured values and further difference values are calculated, and changes in the proportional values with respect to time are monitored. Although this enables erroneous decisions relating to a tool breakage to be decreased, the costs of the computer technology required for the calculation of quotients and the comparison operations, substantially increases the cost. Also, the processing operations required may increase the response time for a tool breakage to a point where the decision process for a tool breakage is too slow in relation to the onset of a tool breakage to prevent substantial damage.

All the known devices based on the measurement of the cutting powers requires a power measure ment sensor on the tool support. Each power sensor measurement is subject to elastic deformation in use, and these measuring sensors impair the rigidity of the machine tool with respect to the cutting engagement between the tool and the workpiece.

Other known devices (E. German Patent Specification 146501,W. German Offenlegungsschrift 21 25 426) avoid impairing the rigidity of the machine tool by detecting process irregularities in a manner which utilises measuring vibrations taking place at the cutting location, in the form of sonic signals conducted through solids or ultrasonic signals.

Evaluation of these signals enables a differentiation of the modifications of the machining process due to different cuases, such as, for example, material inhomogeneities, variations in chip shapes and also tool wear. However, it has not been possible to obtain an unambiguous tool breakage signal from these sonic signals as many factors lead to the production of vibrations even if the tool is in perfect condition.

The invention provides a method and an arrangement for monitoring machining conditions, by means of which non standard conditions such as a tool breakage may be reliably detected at a relatively low cost and may be differentiated from other process irregularities.

According to the invention there is provided apparatus for monitoring machining conditions of a machine tool, comprising means for providing signals representative of a plurality of different parameters which are each a characteristic of the machining process, one of said parameters comprising sonic emission by the process, means for comparing sequential values of said signals for each said parameter with a stored ranges of values therefor to provide an output when the signal falls outside the range, and means responsive to at least one predetermined occurrence of a combination of said outputs to provide a signal representative of a non-standard operating condition for the machine tool.

The invention accordingly provides for the monitoring of the machining conditions in a machine tool by the measurement of a plurality of physical magnitudes characterising the machine cutting process, which are evaluated with respect to a condition of the machining process identifying a tool breakage or process irregularities, in which, instead of the power measurement which impairs the rigidity of the machine, controlling parameters are introduced for the control of the machining process, which reliably enables the clear detection both of process irregularities and tool breakages without additional processing between the individual parameters.

In order that the invention may be more fully understood, an embodiment thereof will now be described by way of example firstly in broad outline and then in detail with reference to the single Figure of the accompanying drawing which is a block diagram of apparatus according to the invention.

Broadly, in machine tool monitoring systems the parameter of the machining process to be monitored should be substantially independent of different machining conditions and different materials used.

Also, if its takes a period of a few workpiece rotations to provide a clear tool breakage signal from the parameters, this is too slow for stopping cutting to prevent damage. Also, the system should be able to take into account, as far as possible, all the irregularities arising during the machining process, caused by voids, variations in chip shape, chip obstructions, workpiece shape errors and workpiece sections which are out of true and, additionally be able to eliminate single spurious error signals and spurious signals from periodically occurring disturbances (non-cylindricality) without having to follow the measured values ocurring through several workpiece rotations.

With a view to satisfying these criteria, the embodiment of the invention sequentially samples three different parameters of the machining process which respond differently to process irregularities. According to the invention, a tool breakage signal is only emitted if all three parameters simultaneously differ from a stored range of values for the parameters respectively.

We have found in tests that a tool breakage always produces an abrupt change in the values of all three of the measured parameters. However, process irregularities also lead to variations in the individual parameter measured values which cannot, on an individual basis, be differentiated from the measured value variations caused by a tool breakage.

These measured value variations caused by process irregularities do not, however, act in the same way on all three of the parameters measured. Thus, if all three of the measured parameter values concurrently deviate, this is a reliable indication of a tool breakage. In addition, deviations of one of the 3 measuring signals are assessed as random error signals and are suppressed so that no tool breakage signal is provided. If two of two measured values concurrently deviate for a predetermined time period, an alarm signal is emitted from which it is possible to infer a process irregularity or setting error which may be removed for example by an operator by modifying cutting values.The increase or reduction of cutting output parameters in the case of a continuously increasing or decreasing cutting depth, as for example in the case of taper turning from a cylindrical blank, is differentiated from process irregularities in that the absolute value of the parameter is not evaluated, but rather the deviation from a predetermined range is determined. For this purpose the distribution of each of the three measured values is calculated from several, for example ten, measured values, which distribution is then updated by the addition of each new measured value and the deletion of the earliest of the ten values. A slow increase or decrease of the absolute meaured value is not detected by the monitoring device if the predetermined range of distribution is maintained.

Knowledge of the absolute magnitudes of the parameters is therefore not necessary. As a result of this mathematical and statistical processing of the measured values and the calculation of the distribution relatively small variations in the measured value are compensated for, but sudden changes lead to a clear increase in the distribution. As a result of the choice of the distribution as a control parameter positive and negative variations of the measured values are taken into account identically.

We have found that the following parameters provide for a clear differentiation between process irregularities and a tool breakage:- ultrasonic emission, main drive power and feed drive power. A simultaneous abrupt variation of the measured values of all three parameters indicates a tool breakage. The concurrent variation of two of the three parameters for predetermined time indicates a process irregularity.

We have observed that workpiece voids cause an abrupt variation of the main and feed drive powers and therefore cause these to exceed their nominal distribution in a clear manner, whereas the ultrasonic emission responds less strongiy and does not therefore exceed its nominal distribution range.

Although the nominal range of the main and feed drive powers are in this situation exceeded simultaneously no alarm signal is emitted in this case as the additonally required time criterion for the presence of this condition is not present, since main and feed measured values, after passing the void, return to within their nominal distribution ranges within the predetermined time.

For variations in chip size, which are a further process irregularity, the ultrasonic emission exceeds the nominal distribution range, whereas the main and feed drive powers do not exceed their nominal ranges.

In the case of chip obstructions both the ultrasonic emission and the feed drive power exceed their nominal ranges, which may for example trigger an alarm signal if the process condition is not corrected over the predetermined time period. This process condition is, however, differentiated from a tool breakage in that the main drive output does not exceed its nominal range.

In the case of periodic variations of the measured output values caused by the non-circularity of one of the blanks subjected to rotary machining, the ultrasonic emission criterion is only subject to slight variations which do not exceed its nominal distribution range and which is therefore also clearly distinguished from a tool breakage.

The method of the invention may be completed for different cutting conditions, such as arise for diffe rent machining contours or machining cuts on sections having different diameters, with assbciated nominal values of the individual measurement magnitudes which are ascertained experimentally and automatically by test cutting and stored, i.e. for individual programme units for different machining contours on a workpiece it is possible in each case to recall associated nominal values of the three measurement magnitudes.

The embodiment of the invention ensures, for an automatic machine tool, that the machine tool or the workpiece are not damaged by an undetected tool breakage and also that the machine tool is not shut down automatically if there is a process irregularity.

Further advantages of the invention are the detection of the measured values does not interfere with the transmission of cutting force in the machine tool and does not limit the working area of the machine tool. In addition a simple detection of the measured values of the main and feed drive powers is provided for. In addition, no relative calculations of the measured values are required, as a result of which the tool breakage signal is emitted with a very low time delay and the circuitry required is of a low degree of complexity.

The embodiment of the invention will now be described in detail with reference to the circuit diagram shown in the accompanying drawing.

A measured value receiver 1 (transducer) for the ultrasonic emission is connected on its output side to a gate 3 via an intermediate amplifier stage 2. In the same way a measured value receiver 4 for the main drive power is connected via amplifier 5 to a gate 6, and a measured value receiver 7 for the feed drive power is connected via an amplifier 8 to a gate 9. The control inputs for the gates 3, 6,9 are connected to respective outputs of a divider circuit 10 which operates the gates sequentially one after the other. The circuit 10 is controlled by a pulse generator 11. Analog signals from the receivers 1,4, 7 are thus multiplexed sequentially by operation of the gates 3,6,9 to analog-digital converter 12. The output of the converter 12 is applied to an evaluation circuit 13.Storage elements 14, 15, 16 associated respectively with each measured value receiver 1, 4, 7 are connected to the output of the evaluation circuit 13, the setting inputs S of these elements being connected to respective outputs of the divider circuit 10 which operate sequentially and may be called up synchronously with the respective gating inputs for the gates 3, 6, 9. A nominal value memory 17 is connected to the evaluation circuit 13, which memory comprises associated setting elements for each of the measured value receivers 1,4,7 and wherein the respective channel of the associated outputs of the divider circuit 10 may be called up.

The evaluation circuit 13 may also be activated by the pulse generator 11 via the divider circuit 10. The outputs of the memory elements 14, 15, 16 are connected as inputs to an AND gate 18 whose output is connected to the setting input Sofa memory 19 characterising the tool breakage. The outputs of two respective memory elements 14, 15 or 14, 16 or 15, 16 are connected to respectively associated AND gates 20, 21, or 22 whose outputs are connected to a common OR gate 23 whose output is applied via a timing element 24 to the setting input S of an alarm memory 25. The alarm memory 25 also has its release input provided for the transfer of the signal at the setting input S via a negation element 26 and the divider circuit 10 to the pulse generator 11.

The method of operation of the arrangement is as follows: The ultrasonic emission, main drive and feed drive power values are continuously detected by the corresponding measured value receivers 1,4,7 and are supplied to the respective gates 3, 6,9 via the associated amplifiers 2,5,8. By means of the pulse generator 11 and the divider circuit 10, the gates 3,6, 9 are switched successively in time so that the measured values are supplied successively via the analog-digital converter 12 to the evaluation circuit 13 for processing. The processing involves the calculation of the distribution of ten actual measured values and the comparison of this distribution of the measured values with a nominal range for the distribution predetermined by the nominal value memory 17.In accordance with the measured value called up for processing with the gate 3, 6, 9 from the measured value receivers 1,4, 7, the associated nominal value in the nominal value memory 17 is also called up by the pulse generator 11 via the divider circuit 10. The result of the evaluation takes the form either of digital "zero" signal if the stored range for the distribution has not been exceeded or a digital "erase" signal if the stored range for the distribution has been exceeded at the output of the evaluation circuit 13 and therefore at the memory elements 14, 15, 16. The associated memory element 14,15, 16 is actuated in each case by the pulse generator 11 via the divider circuit 10 so that the result produced in the case of an "erase" signal, i.e.

the exceeding of the nominal area of the distribution, sets the memory element and in the case of the "zero" signal, i.e. the nominal area of the distribution has not been exceeded, resets the memory element. The outputs of the memory elements 14, 15,16 are provided statically at the inputs of the AND gate 18 such that if the nominal ranges for the distribution of all three measured values is simultaneously exceeded, the tool breakage signal is emitted. If the nominal range for the distribution of two of the three measured values is exceeded, the timing element 24 is controlled via the respective AND gates 20,21 or 22 and the OR gate 23. If a monostable multivibrator is used as the timing element 24, the signal is only supplied to the setting input S of the alarm memory 25 if the signal from the OR gate 23 is present after the relaxation time. The transfer of the setting input S into the alarm memory 25 takes place, for the purposes of clear separation of the activation signals for the measured value processing, at different pulse time points, i.e. in the pulse rest as a result of a negated pulse signal. In the meantime, the memory elements 14,15, which are reset as a result of the fact that the nominal range of the distribution is no longer exceeded, return the monostable multivibratorto its initial state. It is therefore ensured that if the nominal range is exceeded on a short-term basis this does not lead immediately to the emission of an alarm signal and is only displayed as a process irregularity if present in an unchanged manner for more than a predetermined time period set by the monostable 24. Both the alarm memory 25 and the memory 19 for displaying a tool breakage are returned to their initial states by external erase signals L after the disturbances have been removed. In the case of the alarm memory 25 this takes place at the commencement of each new cut. In the case of the memory 19 the erasure takes place after exchange of the broken tool.

Claims (17)

1. Apparatus for monitoring machining conditions of a machine tool, comprising means for providing signals representative of a plurality of different parameters which are each a characteristic of the machining process, one of said parameters comprising sonic emission by the process, means for comparing sequential values of said signals for each said parameter with a stored range of values therefor to provide an output when the signal falls outside the range, and means responsive to at least one predetermined occurrence of a combination of said outputs to provide a signal representative of a non-standard operating condition for the machine tool.

2. Apparatus according to claim 1 including for each said parameter, means for accumulating a predetermined number of the sequential values of the parameter representative signals and for comparing the distribution of the accumulated values with said stored range to provide said output signal with said distribution extends outside said range.

3. Apparatus according to claim 2 wherein the accumulating means accumulates a running total of ten of said sequential values.

4. Apparatus according to claim 1,2or3,where- in signals representative of three of said parameters are provided.

5. Apparatus according to claim 4 including means responsive to the concurrent occurrence of outputs from the comparing means for three of the parameters, to provide a tool damage signal representative of said non-standard operating condition.

6. Apparatus according to claim 4 or 5 including means responsive to the concurrent occurrence for a predetermined time of outputs from the comparing means for two of said parameters, to provide an alarm signal representative of said non-standard operating condition.

7. Apparatus according to claim 6 including an AND gate means for detecting the concurrent occurrence of said outputs, and monostable means responsive to the output of the AND gate to define said predetermined time.

8. Apparatus according to claim 7 inlcuding three AND gates each arranged to detect a respective concurrent occurrence of a respective different two of said outputs, and an OR gate to direct the outputs of the AND gates to the monostable means.

9. Apparatus according to any preceding claim wherein said parameter signal providing means comprises analog electrical transducers for the parameters respectively, an analog to digital converter, and means for sequentially gating samples of each of the analog signals from the transducers to the converter.

10. Apparatus according to any preceding claim wherein said parameters include the machine tool main drive power and feed drive power.

11. Apparatus for monitoring machining conditions of a machine tool, substantially as hereinbefore described with reference to the accompanying drawings.

12. A machine tool provided with apparatus according to any preceding claim.

13. A method of monitoring the machining conditions of a machine tool by the continuous measurement of a plurality of physical magnitudes characterising the machining process, which are evaluated with respect to a condition of the machining process identifying a tool breakage of process irregularities, wherein the ultrasonic emission of the machining process and two characteristic magnitudes of the machining output are measured as physical magnitudes and the measured values are periodially examined, wherein the measured values of each of the physical magnitudes is evaluated with respect to its deviation from a predetermined nominal region of its distribution in such a way that a signal is emitted if the nominal region of the distribution is exceeded, which signal is stored until the subsequent examination, wherein the simultaneous occurrence of the signals of all three physical magnitudes is emitted as a tool breakage signal and the simultaneous occurrence of two signals, after a time period determined from the cutting time, is emitted as an alarm signal for process irregularities.

14. A method as claimed in claim 13, wherein the characteristic magnitudes of the machining output are the main drive power and the feed drive power.

15. A method as claimed in claim 13 or 14, wherein the distribution of the previous ten measured values is used as the basis for the decision relating to whether the nominal area of the distribution of the measured values has been exceeded.

16. A method of monitoring machining conditions of a machine tool, substantially as hereinbefore described with reference to the accompanying drawing.

17. Apparatus for monitoring machining conditions of a machine tool, comprising means for providing signals representative of a plurality of different parameters which are each characteristic of the machining process, means for each said parameter arranged to accumulate a number of the values of the signals, means for comparing the distribution of said values with a stored range to provide an output signal when the distribution extends outside the range, and means responsive to at least one predetermined occurrence of a combination of said outputs to provide a signal representative of a non-standard operating condition forthe machine tool.