GB2415158A - Die failure detection - Google Patents

Abstract

An automated process for detecting a broken die 7 used in riveting operations utilises an array of proximity switches 14 incorporated on a structure 12 for detecting any anomally in the profile of a die 7 at any time between any number of successive riveting operations. The invention obviates the necessity to shut down the riveting process while a die is visually inspected for any damage. An alternative embodiment 19, 21, 23 employs a pneumatic method for detecting a broken die 7.

Description

Die Failure Detection This invention relates to a method and apparatus for

detecting failure of a die and is particularly applicable to dies which are used in a riveting process.

Known riveting machines, such as that described in US 4,615,475, essentially have the form of a C-frame 1 (see figure 1). An upper jaw 2 of the C-frame 1 supports a cylindrical housing 3 for a hydraulicallyoperated punch 4. A lower jaw 5 of the C-frame is provided with a holder 6 for a replaceable die 7. Rivets 8, which are to be driven by the action of the punch 4 into a work piece, are carried on a carrier tape which, in turn, is supported on the cylindrical housing.

Typically, the C-frame is part of a numerically-controlled robot and the riveting operations that it carries out are performed in accordance with a computer program.

A typical riveting machine of the type described above operates as follows. The C- frame is moved into a position so that a work piece (for example two pieces of sheet metal which are to be joined together) is located between the upper and lower jaws 2' 5 and in contact with the die 7. Hydraulic pressure is then applied via a fluid supply line 9 to the machine causing the punch 4 to be forceably extended from its housing 3 and consequently to drive a rivet 8 into the work piece. The die acts to deform the rivet so that a good joint is made. Subsequently, the punch is retracted and the machine moves to a new location for carrying out a further riveting process, either on the same or another work piece.

Other types of riveting machine may be driven by electric rather than hydraulic power.

A typical die 7 is shown in figure 2 and is generally cylindrical and made from hardened steel. Its upper surface incorporates a semi-toroidal cavity 10 around a central projection 11. This particular shape encourages spreading of a self-piercing rivet as it is driven through the work piece and ensures the integrity of the resulting joint.

Die manufacturers usually specify a maximum number of riveting operations that a die may perform after which, it is recommended that a new one be substituted. However, it has been found that some dies fail before the specified maximum number of riveting operations have been completed. Further, failures occur in an unpredictable fashion.

Usually the die "fails" by breaking i.e. a part of the die shears off from the rest of it. This means that any joints subsequently made with a broken die will not have the desired integrity because the rivets will not have formed properly during the riveting process.

Hence any joint made with a broken die has to be repaired.

Currently, methods of detecting a die failure rely solely on visual inspection and coincidently, when a die is to be changed because it has reached the end of its recommended life or when the fitting of a new rivet tape becomes necessary. The riveting process has to be stopped whilst any visual inspection is made. If a die is found to be broken at the stage, then all joints which had been ma-with-the Behave to be traced and checked and repaired where necessary. Such an operation is extremely costly both in monetary terms and in manufacturing time.

Hence it would be advantageous to be able to provide some form of automated method for detecting failure of a die without the need for visual inspection and without the need for temporarily terminating the riveting process.

In a first aspect, the present invention consists of apparatus for detecting die failure, said apparatus including a structure having a formation for receiving a die whilst the die is secured in place in a riveting machine, and at least one sensor assembly for detecting a lack of uniformity in a profile of the die and for generating an output signal for transmission - 3 to a remote riveting machine controller when a lack of uniformity in the die's profiles is detected.

In one embodiment, the structure comprises a disc having a central, circular orifice for receiving the die to be tested, and the sensor assembly comprises a plurality of proximity switches.

The proximity switches may be of any conventional type e.g. those which rely on changes in the capacitive or inductive properties of a tuned circuit or those employing magnetic or Hall effects or incorporating optical or ultra sonic sensors.

A number of proximity sensors, preferably 5, may be symmetrically arranged around the circumference of the die. A broken die, having an uneven profile, will cause differing readings between proximity sensors. The accuracy of the die testing process may be improved by rotating the die through 180 degrees whilst simultaneously monitoring the proximity sensor outputs.

In an alternative embodiment, the structure comprises a housing, one face of which is adapted to receive the die and form an air-tight seal around its circumference, whilst allowing compressed gas to flow out from a central orifice in said face into cavities incorporated in the die's upper surface. Gas escaping around the wails of a broken die where it fails to seal properly, may be detected as a lack of back pressure in the gas supply line, thereby indicating die failure.

In a second aspect, the invention comprises a method for detecting die failure including the steps of; securing a die in a riveting machine, connecting die failure detection apparatus to a programmable controller which controls the riveting machine, in the programmable controller instructing the riveting machine to locate the die proximate the die failure apparatus, instructing said apparatus to perform a die failure detection test, and in said apparatus, if die failure is detected, sending a signal to the programmable controller indicating that a die failure has been detected, in which case, the programmable controller instructs the riveting machine to cease riveting operations.

By virtue of the invention, a die failure may be detected soon after failure has occurred so that the number of joints which need to be repaired can be kept to a minimum.

Depending on production line requirements a die may be checked after any chosen number of riveting operations.

Preferably, the checking operation is written into the computer program that controls the riveting process. Hence the operation can be made to be entirely automated. No visual inspection of the die is required and so there is no need for any temporary shutdown of the riveting process in order to check a die.

Some embodiments of the invention will now be described, by way of example only, with reference to the drawings of which; Figure 1 is a partly cut away side view of a conventional riveting machine; - 5 Figure 2 is a cross sectional view of a conventional die; Figure 3 is a part perspective and part schematic view of apparatus for detecting a broken die in accordance with a first embodiment of the invention, and; Figure 4 is a part sectional and part schematic illustration of apparatus for detecting a broken die in accordance with a second embodiment of the invention.

In a first embodiment, a broken die detection apparatus employs a plurality of proximity switches to detect a lack of uniformity in a die's profile.

With reference to figure 3, a circular, metal disc 12 is provided with a central, circular orifice 13 (into which may be inserted a die 7 under test) and five, equi-spaced radial bores, in each of which is located a proximity switch assembly 14. The diameter of the central orifice 13 is chosen so that there is a small (1 mm) clearance between the outer circumferential surface of the die and the disc's inner surface.

The disc 12 is supported by a stand 15 which is secured to the ground close to riveting machinery.

Electrical connecting cables 16 from each proximity switch assembly 14 are fed to a control unit 17. The control unit 17 provides electrical power to the proximity switch assemblies and also communicates with a programmable machine controller 18 which controls a riveting operation.

Proximity switches are well known and generally incorporate a sensor containing a tuned circuit with a capacitor and a coil, whose impedance changes as a metallic object approaches. As a result of the change in impedance of the tuned circuit, an amplitude of the tuned circuit signal changes. The signal is rectified and is converted by a discriminator into a signal indicating the presence or absence of the object. The tuned circuit amplitude depends on the tuned circuit frequency and the position of the object, that is to say its distance from the sensor. Hence in the application of the present invention, the signals from each proximity switch assembly 14 can be compared in the control unit 17 and used to detect if any part of a die under test is missing. The object-sensing part of each proximity switch assembly 14 is placed close to the end of its respective radial bore and the central orifice 13.

In operation, after a riveting machine has performed a chosen number of riveting operations, the machine positions itself so that the die 7 it carries is inserted into the orifice 13 in the metal disc 12.

Figure 1 shows the die 7 in this position, being supported by the lower jaw 5 of a C frame riveting machine. The punch housing 3 and rivets 8 are also shown in this figure.

Once the die 7 is in position in the orifice 13, the computer program which controls the riveting process and runs in the machine controller 18 triggers the control unit 17 to activate the proximity switch assemblies 14. The riveting machine then rotates its jaws so that the die 7 turns through 180 degrees and at the same time, the outputs of each of the five proximity switch assemblies 14 are recorded in the control unit 17. If all proximity switch assemblies 14 record the presence of an object while the die is being rotated (i.e. they all detect an outer surface of the die 7 throughout the rotation process), then the die 7 is deemed to be good and the riveting process continues and the control unit 17 subsequently deactivates the proximity switch assemblies. If however, one or more proximity switch assemblies 14 fail to continuously register the presence of the die's outer surface while the die is being rotated in the orifice, then a fault condition exists as it can be assumed that a piece of the die is missing. In this case, the control unit 17 sends a fail message to the machine controller 18 which deactivates the riveting machine, logs a fault and activates an alarm. The control unit 17 subsequently deactivates the proximity switch assemblies 14.

The die 7 can then be removed and replaced and the riveting process can be reset and restarted.

In a second embodiment, to be described with reference to figure 4, a broken die detection apparatus employs a pneumatic method for detecting an anomally in the shape of a die under test.

A compressed air supply 19 supplies compressed air via a conduit 20 to a valve and pressure switch unit 21. This unit 21 is connected by a further air-line 22 to a rectangular box shaped plastics housing 23.

The housing 23 has an orifice 24 of around 5mm diameter in one of its faces 25 and the air-line 22 enters an adjacent face and terminates at the orifice 24. The face 25 having the orifice 24 has a rubber seal 26 afixed thereto on its outer surface. The seal 26 has a hole in its centre which aligns with the orifice 24 in the housing 23.

The housing 23 and unit 21 are supported on a stand (not shown) which is secured to the ground close to the riveting machinery. The unit 21 is electrically connected to a programmable machine controller 27.

In operation, after a riveting machine has performed a chosen number of riveting is operations, the machine positions itself so that the die 7 is pushed upwards against the rubber seal 26 of the housing 23.

Figure 4 shows a good die in this position and also (ghosted) a broken die with an upper part missing.

Once the die is in position, the program which controls the riveting process and runs the machine controller 27 triggers the unit 21 to allow compressed air to flow from the air supply to the housing 23. Air is allowed to flow for several seconds so that in the case of a good die, a back pressure builds up in the air line 22 by virtue of air being trapped in the die cavity 10 by the action of the seal 26. The back pressure then triggers the pressure switch in the unit 21 which causes a signal to be sent to the machine controller 27. This signal - 8 indicates that the die 7 under test is a good one and so the valve in the unit 21 closes off the air supply. The riveting machine can then continue with further riveting operations.

If the die 7 is broken, however, as shown in the ghosted part of figure 4, then when the air supply is turned on, no back pressure will build up in the air line 22 because the air will escape around the broken upper surface of the die 7 (see arrow). When this situation occurs, and no back pressure is detected, then the unit 21 sends a fail message to the machine controller 27 which deactivates the riveting machine, logs a fault and activates an alarm. The unit 21 subsequently shuts off the air supply to the supply line. The die can be removed and replaced and the process reset. - 9 -

Claims (8)

1. Apparatus for detecting die failure, said apparatus including a structure having a formation for receiving a die whilst the die is secured in place in a riveting machine, and at least one sensor assembly for detecting a lack of uniformity in a profile of the die and for generating an output signal for transmission to a remote riveting machine controller when a lack of uniformity in the die's profile is detected.

2. Apparatus as claimed in claim 1 in which the structure comprises a disc having a central circular orifice for receiving a die.

3. Apparatus as claimed in claim 2 in which the sensor assembly includes a plurality of proximity switches each mounted in one of a plurality of equi-spaced radial bores within the disc.

4. Apparatus as claimed in claim 1 in which the structure comprises a housing, one face of which is adapted to receive a die and form a gas tight seal around the circumference of the die and which incorporates an orifice for allowing a supply of a gas to flow onto an upper surface of the die.

5. Apparatus as claimed in claim 4 in which the sensor assembly includes a gas supply line for supplying gas under pressure to said orifice and a gas pressure sensitive switch for reacting to the presence of a back- pressure in said gas supply line.

6. A method for detecting die failure including the steps of; securing a die in a riveting machine, connecting die failure detection apparatus to a programmable controller which controls the riveting machine, - 10 in the programmable controller, instructing the riveting machine to locate the die proximate the die failure apparatus, instructing said apparatus to perform a die failure detection test, and in said apparatus, if die failure is detected, sending a signal to the programmable controller indicating that a die failure has been detected, in which case, the programmable controller instructs the riveting machine to cease riveting operations.

7. Apparatus for detecting die failure substantially as hereinbefore described with reference to figures 3 and 4.

8. A method of detecting die failure substantially as hereinbefore described with reference to figures 3 and 4.