GB2051362A

GB2051362A - Monitoring tool life

Info

Publication number: GB2051362A
Application number: GB7917910A
Authority: GB
Original assignee: PRODUCTION ENG RESEARCH ASS OG
Current assignee: PRODUCTION ENG RESEARCH ASS OG
Priority date: 1979-05-23
Filing date: 1979-05-23
Publication date: 1981-01-14

Abstract

In monitoring the operating condition of a power driven tool, a preselected frequency of vibration of the tool is monitored through a vibration sensor (12) and a bandpass filter (20) and converted to a d.c. voltage which varies with the amplitude of the monitored frequency of vibration. The d.c. voltage is compared with two different preset levels in comparators (46, 48) to generate a warning signal and inhibit drive to the tool respectively when the levels are exceeded. <IMAGE>

Description

SPECIFICATION Apparatus for monitoring tool life The present invention relates to apparatus for monitoring the operating condition of cutting tools such as are used in, for example lathes, drilling machines and machining centres.

It is generally impossible to predict accurately the useful life of a tool such as a high speed steel twist drill which is used under a given set of operating conditions, e.g. cutting speed and feed rate, but it is of course essential to ensure that a tool is withdrawn from use before total failure becomes a distinct possibility. An undetected broken tool in, for example an automatic lathe, can cause catastrophic damage to the workpiece, tooling and also to the machine itself. In a typical example an automated machine might attempt to tap a thread in a hole which is not present because a twist drill normally used to drill such holes is broken.

Naturally, great reliance is placed upon a machine operator to monitor the condition of a tool in use and decide when it should be replaced.

A consequence of this is that tools tend to be discarded well before the end of their useful life resulting in unnecessarily long machining times and poor tool utilisation.

In automated and semi-automated machines there is an even greater tendency to "under-run" tools because of the even more severe consequences of undetected tool failure.

Systems for monitoring the life of a tool and stopping a machine when its tool breaks have been proposed. Such systems detect the absence of the whole or part of a tool but are of very limited value since they are insensitive to the presence of a slightly damaged tool having for example a chipped cutting edge. In addition, such systems quite often do not respond quickly enough to tool breakage.

The present invention seeks to provide an improved system for monitoring the operating condition of a tool.

Accordingly the present invention provides a system for monitoring the operating condition of a power driven tool comprising means for generating a signal representative of a frequency of vibration of the tool, means for comparing said signal with a preselected signal and means for controlling drive to the tool in dependence upon said comparison.

The present invention also provides a method of monitoring the operating condition of a power driven tool comprising sensing a frequency of vibration of the tool during operation, comparing said frequency of vibration with a preselected value and controlling drive to the tool in dependence upon said comparison.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a schematic circuit diagram of a system according to the present invention; and Fig. 2 is a graph showing the variation in amplitude of vibration against frequency for both a worn tool and a new tool.

The circuit 10 shown in Fig. 1 comprises a vibration sensor in the form of an accelerometer 12 which is mounted normally as close as possible to a tool which is to be monitored so that as the tool operates on a workpiece the vibrations set up are detected by the accelerometer 12. The accelerometer generates an electrical signal which is representative of the vibration generated at the tool-workpiece interface and this signal is amplified through amplifiers 14 to a gain control 16. This comprises a potentiometer which can be adjusted to set the sensitivity of the circuit. A buffer amplifier 18 connects the gain control to a filter 20. The filter 20 is a tunable bandpass filter with a frequency range of from approximately DC to 20 KHz. Tuninig is effected via control network 22 and a manually adjustable potentiometer 24.

The filter 20 is conveniently voltage tuned and enables a specific frequency of vibration to be selected as the monitored frequency.

From the tunable bandpass filter 20 the signal is fed through a buffer amplifier 22 to a rectifier 24, conveniently a full wave rectifier. The DC signal produced by the rectifier is then fed through a normally closed electronic switch 26 to a signal conditioning network 28. The latter has a main output to a further buffer amplifier 30 and a number of subsidiary outputs (here shown four in number) which may be selectively connected through a six position switch 32 and an amplifier 34 to a meter 36. Each of the subsidiary outputs of the signal conditioning network 28 is associated with a different time constant of the network, each providing different rise and fall times, and can be selected to vary the response of the system to rapid changed in the DC signal generated by the rectifier 24.The amplifier 34 has a peak hold facility, here shown in the form of a switch 38, which enables the peak levels of the varying DC signal and thus the peak levels of vibration to be held and displayed on the meter 36. A potentiometer 40 is provided for calibration of the meter 36.

The output from the buffer amplifier 30 is fed to a respective input of each of two comparators 46 and 48. The other input of each of these comparators is fed with a preset voltage from a potentiometer 42, 44 which may also be coupled through switch 32 to the meter 36 to enable the voltage levels to be set to the required comparator trip level during an initial setting up procedure.

The comparator 46 is normally set to trip at a lower input signal from the buffer amplifier 30 than is comparator 48 and applies its output signal to a self-latching circuit 50 which, on tripping of the comparator 46 turns on a transistor 52 to energise an indicator means, here shown as a light-smitting diode 54, thus drawing attention to the fact that the tool is approaching the end of its useful life. The indicator means, may of course, be audible or visual or indeed both audible and visual.

The output of the comparator 48 is also connected to a self-latching circuit 56 which also operates, when the comparator 48 is tripped, turn on a transistor 58 and energise a suitable indicating means, again here shown as a lightemitting diode 60. The output of the transistor 58 is, however, also coupled to a control circuit of the machine such that when the transistor 58 is energised the machine is deactivated. In this instance, the comparator 48 is set to trip when the vibration of the tool indicates that the tool has or is about to fracture or has been damaged in some way or other.

Resetting of the two self-latching circuits 50 and 56 is effected by momentarily closing switch 62.

Referring now to Fig. 2, this graph shows the considerable difference in the amplitude of the vibrations generated by a worn tool (curve a) compared with those of a new tool (curve b). The plotting of such a graph enables a suitable monitoring frequency to be chosen to which the filter 20 is tuned. Such a frequency is 1 1 KHz for a typical cutting tool although of course the frequency chosen may vary from tool to tool.

Finally, the output of the buffer amplifier 30 may also be coupled through a further amplifier 64 to an ouput terminal for connection to, for example, be connected to a pen recorder or the like.

As will be appreciated the present invention allows tool use to be optimised thus providing efficient utilisation of such tools.

Claims

1. A method of monitoring the operating condition of a power driven tool comprising the steps of sensing a frequency of vibration of the tool during operation, comparing the amplitude of said frequency of vibration with a preselected value and providing a control signal in dependence upon said comparison.

2. A method as claimed in claim 1 wherein the step of sensing a frequency of vibration of the tool comprises monitoring the frequency of vibration of the tool over a preselected frequency spectrum and selecting a frequency within said spectrum for comparison with said preselected value.

3. A method as claimed in claim 1 or 2 wherein the step of comparing the amplitude of said frequency of vibration with a preselected value comprises converting said frequency to a d.c.

voltage and comparing the level of said d.c.

voltage with a preselected d.c. voltage level.

4. A method as claimed in claim 3 wherein said control signal activates a visual or audible alarm when said d.c. voltage level exceeds said preselected level.

5. A method as claimed in any one of claims 3 or 4 wherein said control signal inhibits drive to said tool when said d.c. voltage level exceeds a further d.c; voltage level greater than said preselected level.

6. A system for monitoring the operating condition of a power driven tool comprising means for generating a signal representative of a frequency of vibration of the tool, means for comparing the amplitude of said signal with a preselected value and deriving a control signal in dependence upon said comparison.

7. A system as claimed in claim 6 wherein said generating means comprises a vibration sensor for monitoring the frequency of vibration of the tool over a preselected frequency spectrum and a filter for selecting a frequency within said spectrum for comparison with said preselected value.

8. A system as claimed in claim 7 wherein said filter is tunable over the range d.c. to 20 KHz.

9. A system as claimed in any of claims 6 to 8 further comprisng means for converting said selected frequency to a d.c. voltage and comparing the level of said d.c. voltage with a preselected d.c. voltage level.

10. A system as claimed in claim 9 further comprising a signal conditioning network coupling said detector to a visual display means for controlling said visual display means as function of said d.c. voltage.

1 A system as claimed in claim 10 wherein the time constant of said function is variable.

12. A system as claimed in any of claims 9 to 11 further comprising a visual or audible alarm actuable by said control signal responsively to said d.c. voltage level exceeding said preselected d.c.

voltage level.

13. A system as claimed in any of claims 9 to 12 further comprising means actuable for inhibiting drive to said tool responsively to said d.c. voltage level exceeding a further preselected d.c. voltage level greater than said first-mentioned preselected d.c. voltage level.

14. A method of monitoring the operating condition of a power driven tool substantially as hereinbefore described with reference to the accompanying drawings.

15. A system for monitoring the operating condition of a power driven tool substantially as hereinbefore described with reference to the accompanying drawings.