GB2074731A

GB2074731A - Machining monitor

Info

Publication number: GB2074731A
Application number: GB8012485A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1980-04-16
Filing date: 1980-04-16
Publication date: 1981-11-04
Also published as: GB2074731B

Abstract

A method and apparatus for detecting faults in the material of a workpiece during machining. The method involves the use of a transducer (13) which detects the acoustic emission from the machining process, and signal processing and display equipment (14, 15, 16, 17) which indicates when the acoustic emission is greater than a predetermined value and the position of the tool at that time. These peak values of acoustic emission have been found to correlate with faults in the material, hence the positions of faults may be indicated. <IMAGE>

Description

SPECIFICATION Method and apparatus for detecting faults This invention relates to a method and appa ratus for detecting faults in the material of a workpiece during machining.

A wide variety of methods have been used for testing workpieces to ensure that their material is free of faults and defects. These have had different degrees of success, but almost without exception have necessitated the workpiece being subjected to a separate examination or test step before or after it has been machined.

The present invention provides a method and apparatus by which faults in the material of a workpiec may be detected actually while the workpiece is being machined.

In the present invention, use is made of the vibrational phenomenon known as acoustic emission. Acoustic emission consists of an acoustic wave or waves emitted in the defor mation and fracture processes of solid materi als in association with the relief of strain energies. It is generally detected in frequency ranges beyond the normal audible spectrum, and is hence less prone to interference by extraneous noise or conventional vibration ef fects.

We have discovered that the acoustic emis sion of a tool machining a workpiece peaks when the tool passes over a fault in the material of the workpiece, and that using this discovery faults in the material may be de tected and located.

According to the present invention, a method of detecting faults in the material of a workpiece during machining comprises mea suring the acoustic emission from the machin ing operation, detecting peaks of this emission due to the machining of said faults and dis miaying each said peak and the position of the cutting tool on the workpiece surface at the time when the peak occurs so that the posi tion of said faults may be determined.

Preferably the motion of the machining tool over the surface of the workpiece is used to perform a scanning of the surface of the workpiece from which a plot of the workpiece surface may be presented with the level of acoustic emission displayed thereon.

The invention also comprises apparatus for detecting faults in the material of a workpiece during machining comprising a transducer acoustically coupled to the workpiece or ma chining tool and adapted to provide an electri cal output dependent upon the degree of acoustic emission from the machining opera tion, detecting means adapted to detect when the signal from said transducer indicates acoustic emission from the workpiece greater than a predetermined value, and indicating means adapted to provide an indication each time the detecting means detects that the acoustic emission is greater than said predetermined value and to indicate the position of the cutting tool on the workpiece surface each time such an event is detected.

In many instances the machining operation will involve the machining tool effecting a scan of at least a portion of the workpiece surface; in this case the 'scan' of the tool may be duplicated on a plot of acoustic emission to provide a representation of the portion of the workpiece surface on which those areas where the acoustic emission is greater than the predetermined level are indicated.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of apparatus in accordance with the invention, Figure 2 is a developed view of the surface of a test workpiece having holes and a sawcut therein, Figure 3 is a scan of the surface of the workpiece of Fig. 2 using the apparatus of Fig. 1, Figure 4 is a view similar to Fig. 2 but of a second workpiece in which faults occur, and Figure 5 is a scan similar to that of Fig. 3 but taken of the workpiece of Fig. 4.

In Fig. 1 a machining process is diagrammatically indicated by the chuck 10 of a lathe (not shown) in which is held a cylindrical workpiece 11 which is being turned by a cutting tool 12. In this case therefore the machining process comprises turning, but it will be appreciated that the workpieces of other machining operations could be inspected using the method of the invention.

In order to monitor the acoustic emission from the machining operation, a piezoceramic transducer 13 is attached to the shank of the cutter 12. In this case the transducer is clamped to the cutter by a simple G clamp (not shown) and silicone grease is used to couple the transducer acoustically to the cutter. It will be appreciated that while this approach is suitable for machining where static tools are used, for operations such as milling and grinding it is difficult to couple a transducer to the cutter and hence it is preferable to mount the transducer on the workpiece itself.

The small electrical signal from the piezoceramic transducer 13 is amplified in an amplifier 14 and is displayed on a meter 15, by an audio monitor loudspeaker 16 and on the screen of a storage oscilloscope 17. The acoustic emission input is to the Z input terminal of the oscilloscope 17, so that variations in this emission cause changes of the brightness of the point image on its screen.

The motion of the point image in the X direction across the screen is controlled by the oscilloscope's built-in timing circuit to take 50 seconds to traverse the entire screen width, while in the Y axis a sawtooth input of once per revolution of the workpiece 11 is provided. This is derived as described below, and its effect is to provide a vertical motion of the point image which parallels the helical motion of the cutter relative to the workpiece but in a 'developed' manner. The picture built up on the screen of the oscilloscope 17 is therefore equivalent to a developed view of the cylindrical surface of the workpiece 11 formed as the cutter traverses it and having a brightness contour which corresponds with the acoustic emission produced by the machining action over the surface.

In order to produce the sawtooth signal referred to above, the chuck 10 is provided with a card 18 attached to it and positioned so that once per revolution of the chuck it interrupts an optical switch 19. The switch 19 is energised by a power supply 20, and the pulse resulting from the interruption of the switch is fed to the trigger input of a second oscilloscope 21. This oscilloscope has an inbuilt sawtooth generator which is triggered by the pulse from the switch 19 to produce a once-per-revolution ramp signal in phase with the rotation of the chuck. A potentiometer 22 provides the correct voltage of sawtooth to feed in to the Y terminal of the storage oscilloscope 1 7.

In order to test the operation of this apparatus, a test bar was used as the workpiece 11. This bar comprised a cylindrical mild steel bar having a plurality of holes of 3.5 and 2 mm diameter and a single angled saw cut.

The positions of these features, which are intended to represent faults, are shown in Fig.

2 which is a developed view of the surface of the bar.

A cut was taken from the surface of the bar with the tool following the usual helical pattern relative to the surface so as to remove a surface layer from the bar. The plot of the acoustic emission produced during this process is illustrated in Fig. 3. Although Figs. 2 and 3 differ in scale, it will be seen that each of the simulated faults shown in the plan of Fig. 2 is visible as a contrasting area on the plot of Fig. 2. Clearly the display illustrated in Fig. 2 could be used to locate at least the simulated faults of this test bar.

In order to try a more realistic situation, the test bar was next replaced by a substantially cylindrical bar of forged metal which had axial cracks in its surface produced by the forging process. Fig. 4 is a developed view of the surface of this bar showing the location of these cracks.

A machining operation similar to that of Figs. 2 and 3 was carried out on this bar, and the resultant display of the oscilloscope 17 is illustrated in Fig. 5. Again it will be seen that the cracks are quite visible. This is a good result because these cracks are truly representative of the faults sometimes present in materials, and the experimental apparatus used only the 'raw' acoustic emissions from the cutting process to give this result. There is obviously considerable scope for modification of the signals to give even better results which may enable the detection of faults not visible to the naked eye such as 'peened-over' cracks and faults which exist but have been closed up by residual stress in the material.

It will be appreciated that the use of a storage oscilloscope as referred to in the examples above is a convenient and clear way of presenting the information required to detect the faults in the workpiece surface. However it should also be noted that other methods, such as a quasi-three dimensional plot of the surface could well be used, or even a series of linear scans.

Also the acoustic emission information used to detect the flows could well be subject to more complex signal processing than that described in connection with the above embodiment.

Claims

1. A method of detecting faults in the material of a workpiece during machining comprising measuring the acoustic emission from the machining operation, detecting peaks of the emission due to the machining of said faults and displaying each said peak and the position of the cutting tool on the workpiece when the peak occurs so that the positiion of said faults may be determined.

2. A method as claimed in claim 1 and comprising measuring said acoustic emission using a transducer acoustically coupled to the cutting tool.

3. A method as claimed in claim 1 and comprising measuring said acoustic emission using a transducer acoustically coupled to the workpiece.

4. A method as claimed in any one of the preceding claims and comprising using the motion of the machining tool over the surface of the workpiece to perform a scanning of the surface of the workpiece from which a plot of the surface is presented with the processed signal level of acoustic emission displayed thereon.

5. Apparatus for detecting faults in the material of a workpiece during machining comprising a transducer acoustically coupled to the workpiece or machining tool and adapted to provide an electrical output dependent upon the degree of acoustic emission from the machining operation, detecting means adapted to detect when the signal from the transducer indicates acoustic emission greater than a predetermined value, and indicating means adapted to provide an indication each time the detection means detects that the acoustic emission is greater than said predetermined value, and to indicate the position of the cutting tool on the workpiece surface each time such an event is detected.

6. Apparatus as claimed in claim 5 and in which said indicating means duplicates the scan of the tool over the workpiece surface on a plot of acoustic emission to provide a representation of at least a portion of the workpiece surface on which those areas where the acoustic emission is greater than the predetermined level are indicated.

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7. Apparatus as claimed in claim 6 and in which said plot comprises the tube face of a storage oscilloscope.

8. Apparatus as claimed in claim 7 and in which said tool moves in a helical path with respect to said surface, and said oscilloscope is swept in the X or Y direction by a once-perrevolution waveform to provide a developed plan view of the workpiece surface.

9. Apparatus substantially as hereinbefore particularly described with reference to the accompanying drawings.