METHOD AND DEVICE FOR CONTROLLING A MILLING MACHINE

Abstract

In a method for controlling a milling machine, a milling operation is able to be monitored on the basis of a temporally variable parameter of a numerical control in that the parameter is monitored for the exceeding or undershooting of a limit value during the milling operation. The method includes filtering out the tooth-meshing frequency in the time characteristic of the parameter (Iqnom).

Claims

1. A method for controlling a milling machine, comprising: monitoring a milling operation with the aid of a temporally variable parameter of a numerical control; monitoring the parameter for exceeding or undershooting a limit value during the milling operation; and filtering out a tooth-meshing frequency of a milling tool in a time characteristic of the parameter.

2. The method according to claim 1, further comprising processing the parameter, measured in a fixed clock rate, by a scanning-rate converter, the scanning-rate converter outputting the parameter with a variable clock pulse for further processing.

3. The method according to claim 2, wherein the variable clock pulse corresponds to a multiple of the tooth-meshing frequency of the milling tool, the tooth-meshing frequency corresponding to a product of a rate of rotation of the milling tool and a number of cutting blades of the milling tool.

4. The method according to claim 3, wherein the variable clock pulse corresponds to (a) five to twenty times and/or (b) ten times the tooth-meshing frequency of the milling tool.

5. The method according to claim 3, wherein the tooth-meshing frequency is filtered out of the parameter output with a variable clock rate by averaging, by a moving-average filter, across a number of values that corresponds to the multiple of the tooth-meshing frequency.

6. The method according to claim 5, further comprising integrating the parameter in the moving-average filter in an integration stage and subsequently differentiating the parameter in a differentiation stage.

7. The method according to claim 6, wherein the differentiation stage includes a shift register having a number of register slots that corresponds to the multiple of the tooth-meshing frequency, through which consecutive parameter values are shifted at a time delay.

8. The method according to claim 7, wherein the shift register includes ten register slots.

9. The method according to claim 5, further comprising converting the parameter at an input of the moving-average filter from a floating point number to an integer number, and converting the parameter at an output of the moving-average filter from an integer number to a floating point number.

10. The method according to claim 5, further comprising triggering a warning and/or a termination of the milling operation in accordance with a comparison of the parameter output at an output of the moving-average filter with a limit value.

11. A numerical control for controlling a milling operation on a machine tool, comprising: a monitor unit adapted to monitor a temporally variable parameter of the numerical control with the aid of a limit value; and an averaging filter adapted to filter out a tooth-meshing frequency from a time characteristic of the parameter.

12. The numerical control according to claim 11, wherein the averaging filter includes a moving-average filter having an integration stage and a downstream differentiation stage.

13. The numerical control according to claim 12, wherein the differentiation stage includes a shift register having a number of register slots that corresponds to a multiple of the tooth-meshing frequency, the shift registers adapted to store consecutive parameter values.

14. The numerical control according to claim 13, wherein the shaft register includes ten register slots.

15. The numerical control according to claim 11, further comprising a scanning-rate converter connected upstream from the averaging filter adapted to output, at a variable scanning rate, the parameter, supplied at a fixed scanning rate.

16. The numerical control according to claim 15, wherein the variable scanning rate corresponds to a multiple of the tooth-meshing frequency.

17. The numerical control according to claim 16, wherein the variable scanning rate corresponds to (a) five to twenty and/or (b) ten times a tooth-meshing frequency of the milling tool, the tooth-meshing frequency corresponding to a product of a rate of rotation of the milling tool and a number of cutting blades of the milling tool.

18. A numerical control for controlling a milling operation on a machine tool, comprising: Monitoring means for monitoring a temporally variable parameter of the numerical control with the aid of a limit value; and averaging filter means for filtering out a tooth-meshing frequency from a time characteristic of the parameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 schematically illustrates the interaction of the scanning-rate converter and the moving-average filter.

[0026] FIG. 2 schematically illustrates a scanning-rate converter.

[0027] FIG. 3 schematically illustrates a moving-average filter.

[0028] FIG. 4 illustrates effect of the filtering on the measuring signal of the parameter.

DETAILED DESCRIPTION

[0029] FIG. 1 schematically illustrates the interaction of a scanning-rate converter 1 and a moving-average filter 3, which may be provided as a software module in numerical control NC of a milling machine, for example.

[0030] The number n of cutting blades of a tool, stored in a tool table within numerical control NC, for example, and instantaneous rate of rotation DF of the spindle are forwarded to an arithmetic unit 2. By multiplying the two values, arithmetic unit 2 calculates instantaneous teeth-meshing frequency FZ and outputs it to scanning-rate converter 1.

[0031] The measured value of parameter Iqnom utilized for the monitoring is forwarded to the scanning-rate converter 1, i.e., in a fixed clock pulse Tf. In the example, the setpoint current for the drive of the spindle rotating the milling tool, which is regulated by numerical control NC and to a certain degree is proportional to instantaneous spindle load, is utilized as parameter Iqnom. Ideally, setpoint current Iqnom corresponds to the measured actual current for the spindle drive. Since the meshing of each tooth of the rotating milling tool with the workpiece decelerates the spindle, control NC responds by a rise in setpoint current Iqnom, as illustrated as unfiltered signal a of FIG. 4.

[0032] Scanning-rate converter 1 forwards this parameter Iqnom at a variable clock pulse Tv. Variable clock pulse Tv is selected such that it corresponds to a whole-number multiple of tooth-meshing frequency FZ. For example, a tenfold tooth-meshing frequency FZ as the variable clock pulse Tv may be utilized and may provide an appropriate compromise between low circuitry outlay (or computational power) and excellent damping of tooth-meshing frequency FZ in the subsequently resulting signal. A range from five to twenty times the tooth-meshing frequency FZ may be similarly utilized.

[0033] Parameter Iqnom, thusly converted in its scanning rate, is forwarded to a moving-average filter 3, which averages a certain number of the most recently transmitted values of parameter Iqnom, i.e., a number that corresponds to the previously used multiple of tooth-meshing frequency FZ. Thus, in the example, averaging across the most recent ten values takes place, and parameter Iqnom thusly averaged is output to a monitoring unit 4. The fact that tooth-meshing frequency FZ has been damped considerably in this averaged signal is apparent from signal b of FIG. 4, especially when comparing it with unfiltered signal a.

[0034] FIG. 2 schematically illustrates an exemplary embodiment of a scanning-rate converter 1. Other circuits, e.g., decimation filters, cascaded integrator-comb filters, and circuits that are able to convert a signal to a desired scanning rate may be provided, as well.

[0035] With the aid of a delay element 1.1, a linear amplifier 1.2, and two summing points 1.3, scanning-rate converter 1 makes it possible to output parameter Iqnom at the output of scanning-rate converter 1 with a variable clock pulse Tv that corresponds to ten times the tooth-meshing frequency ZF. Typical values are a tooth-meshing frequency of 60 Hz (at n=6 teeth and a spindle rate of rotation DF=600 rotations per minute, corresponding to 10 Hz), from which a variable clock pulse Tv of 600 Hz results.

[0036] Fixed clock pulse Tf, at which parameter Iqnom is initially detected, is in the range from 3.3 kHz to 20 kHz, and preferably at 5 kHz, for example.

[0037] FIG. 3 schematically illustrates an exemplary embodiment of a moving-average filter 3 to which parameter Iqnom is supplied at a variable clock pulse Tv.

[0038] In order to avoid errors in the subsequent calculation, the circuit illustrated in FIG. 3 uses integer values for its calculation. As a result, parameter Iqnom is first converted in a converter 3.4 from a floating point number into an integer number. This is followed by an integration stage 3.1, which integrates parameter Iqnom. Differentiation stage 3.2 that follows differentiates the thusly obtained value again, this being done in an especially uncomplicated manner by shifting the values through a shift register having register slots 3.3, whose number substantially corresponds to the selected multiple of tooth-meshing frequency FZ. In the example, ten register slots 3.3 are therefore required. It is then sufficient to generate the difference of the values upstream and downstream from the shift register and to output them as filtered parameter Iqnom.

[0039] There exists a certain time delay between individual register slots 3.3, which is constant at a constant tooth-meshing frequency FZ. In an effort to obtain an excellent filter effect even at an accelerating rotary motion of the tool, i.e., at a varying tooth-meshing frequency FZ, the individual delay times are selected to differ. It should apply at all times that the sum of all delay times corresponds to the time required for a single spindle rotation, divided by the number n of teeth of the tool.

[0040] After a renewed conversion from an integer number into a floating point number in a further converter 3.5, the filtered value of parameter Iqnom is able to be forwarded to a monitoring unit 4, which carries out monitoring for an exceeding or undershooting of limit values and is able to carry out a switch-off reaction or output a warning as the case may be.

[0041] Due to the filtering of tooth-meshing frequency FZ in the time characteristic of parameter Iqnom, which can be seen by comparing signals a and b illustrated in FIG. 4, monitoring of the processing operation that is considerably less susceptible to interference is able to be carried out using filtered signal b. The method and the device described herein are suitable for monitoring numerically controlled machine tools in which a rotating tool having one or more cutting blades processes a workpiece, the cutting blades periodically engaging with the workpiece. Such machine tools are also referred to as milling machines.