METHOD AND APPARATUS FOR DETECTING PRODUCTION CRACKS IN MECHANICAL PARTS MADE BY PRESSING

Abstract: During the pressing operation, the presence of ultra­ sounds in air are detected in correspondence with regions (5) of the part (T) in which production cracks may occur. This presence of ultrasounds, which is indi­ cative of the existence of production cracks, may cor­ respond both to the propagation through the mechanical part (T) of ultrasounds generated by a source (l0) and directed (7) towards the part itself and to the interaction with the cracks of a pressurised air (l4) flow conveyed (ll) towards the surface of the part.

Description: BACKGROUND OF THE INVENTION The present invention relates generally to the problems of detecting and indicating the presence of production cracks in mechanical parts (drawn parts) made by pressing between a punch and a die. As examples of mechanical parts of this type, the various metal parts constituting the body of a motor vehicle may be mentioned. The cracks which occur during the pressing operation are typically localised in the regions of the part subjected to the most intense plastic deformation (drawing). Although we do not wish to be tied to a specific theory, the cracks may in fact arise from the effects of stretching or contraction of the metal constituting the part. Such stretching or contraction usually arises when the interaction of the punch with the die becomes too tight, at least in some regions of the part, due to wear or slight relative displacement during the working cycle. The production cracks most usually take the form of tears or fissures which are detectable by visual examination. In other cases, however, the cracks are not immediately perceptible and their presence may be detected only after assembly of the part or, indeed, only by the user of the final product after a certain period of use. In other cases, as well as not being completely reliable, visual inspection of the parts is an operation which is difficult to carry out in a manner compatible with high production rates on the pressing lines currently used in the metallurgical and mechanical industries and particularly in the motor vehicle industry. It should also be noted that, even though the actual percentage of parts with production cracks is very low (of the order of l%), the final incidence of the phenomenon in absolute terms is considerable due to the high production levels. OBJECT OF THE INVENTION The object of the present invention is thus to enable the presence of production cracks in mechanical parts obtained by pressing to be detected in accordance with criteria which enable the detection operation to be carried out "in process", that is, during the pressing operation. The solution of the invention, however, must enable the checking to be carried out in an easily automated manner with results having a degree of reliability such as to minimise the incidence of such cracks not only in percentage terms but also in absolute terms. SUMMARY OF THE INVENTION According to the present invention the object defined above is achieved by virtue of a method for detecting the presence of production cracks in predetermined regions of mechanical parts made by pressing between a punch and a die, characterised in that it includes the step of detecting the presence of ultrasounds in air in the predetermined regions during pressing, this presence of ultrasounds in air being indicative of the existence of production cracks in the pressed part. According to a first embodiment, the method of the invention includes the step of generating, during pressing, ultrasounds in air in the predetermined regions and on the side of the part being pressed which faces one of the punch and the die, and the step of detecting the presence of ultrasounds in air in the predetermined regions and on the side of the part being pressed which faces the other of the punch and the die. In other words, in this first embodiment, the propagation of ultrasounds in air through the mechanical part being pressed is detected - this being indicative of the presence of production cracks. According to another embodiment, the method of the invention includes the step of supplying pressurised air to the surface of the predetermined regions of the part being pressed during pressing. According to this second embodiment, in order to detect the presence of production cracks, use is made of the phenomenon in which the part itself, by interaction with the pressurised air flow directed at its surface, generates ultrasounds in air which can propagate through the cracks which may be present in the part. The detection of the presence of these ultrasounds thus enables the presence of production cracks to be deduced. The present invention also provides apparatus which enables the presence of production cracks to be detected in predetermined regions of mechanical parts made by pressing, in accordance with the method described above. DETAILED DESCRIPTION OF THE INVENTION The invention will now be described, purely by way of non-limiting example, with reference to the appended drawings, in which: Figures l and 2 illustrate schematically the physical principle on which the invention is based, with reference to two possible embodiments of the invention itself, Figures 3 and 4 are two time graphs illustrating schematically the changes in two signals which can be detected by the use of the method of the invention, in a mechanical part free from production cracks and in a mechanical part including cracks of this type, respectively, and Figure 5 illustrates schematically the internal structure of part of the apparatus of the invention, in block schematic form. In order to facilitate the explanation and understanding of the invention, it is specified that, in the present specification and in the following claims: - "production cracks" is intended to indicate all those anomalies such as tears, fissures, cracks, etc., which may occur in a part manufactured by pressing as a consequence of the deformation (drawing) to which the part, usually of metal, is subject; - the term "punch and die" generally indicates the two bodies with complementary shapes (also termed die and matrix, etc.) which plastically deform the part in order to give the part itself the final conformation by means of an operation generally termed "pressing"; - the term "during the pressing operation (or cycle)" or, more briefly, "during pressing" indicates those operations carried out during the pressing steps in which there is relative movement between the punch and the die (as defined above) and in the immediately subsequent steps in which the pressed part is still between the punch and the die; - the term "ultrasounds in air" is intended to indicate generally all those vibrations or ranges of wave forces which are propagated in a gaseous medium and generally have frequencies ranging from frequencies immediately above the range of audible frequencies to l00kHz and beyond; more particularly, the term "in air" - which should be understood as extending in general to all gaseous media - is intended to distinguish ultrasounds used in the present invention from ultrasounds which are propagated in liquids or in solids (for example, in a metal such as that constituting the part subjected to pressing). In Figures l and 2 a region of a mechanical part of metal (for example, sheet metal of the FePO₄ type) subjected to pressing between a punch or die P and a die or matrix C is generally indicated S. By way of example, the part T, only partially shown in the drawings, may constitute the so-called framework of the door of the motor vehicle, while the region S constitutes one of the parts of this framework. In each case the region S under examination is one of the regions in which the plastic deformation or drawing characteristic of the pressing operation is most marked. The general criteria for carrying out this operation are known in the art and will not therefore be described in the present specification in that they are not essential for the understanding of the invention. For this purpose, it should be noted, however, that during the prolonged performance of successive pressing operations (which are carried out in the motor vehicle industry, for example, with rates of the order of about five pressing operations per minute) deterioration or wear or slight relative movements between the punch and the die may occur, which can bring about production defects such as tears, fissures or cracks. These defects occur in the region S and are such as to prejudice the mechanical strength and working life of the part T. The occurrence of cracks may also arise from non-uniformities in the material (sheet metal) constituting the part which is pressed and/or from the presence of solid, hard particles thereon. For an orderly description, reference is made initially to the right-hand part of Figures l and 2 where a duct or guide which can allow the propagation within it of ultrasounds in air (as previously defined) is indicated l. The ducts or guides l are of known type and are readily available on the market. For this purpose, reference may be made to the plastics guides or ducts tested positively for ultrasonic transmissions and known under the trade name "RINSAL". The duct l has a first end 2 which extends through the punch P so as to face the surface of the part T being produced. At its opposite end 3, however, the guide 2 terminates at an ultrasound detector 4, the detector 4 being sensitive to the intensity of the train of ultrasonic pressure waves propagated within the guide l and generating at its output 5 an electrical signal which changes correspondingly. Preferably, the ultrasound detector 4 operates in a range of frequencies of around 40kHz. The output 5 of the detector 4 is connected to a signal processing circuit 6 the structure of which will be better explained below with reference to Figure 5. A further duct or guide, structurally identical to the duct l, is indicated 7 in Figure l and can enable and guide the propagation of ultrasounds in air within it. The duct or guide 7 also has an end 8 which extends through the die C in such a position that, during the pressing operation, the end 8 is in fact applied to the surface of the part T. However, the opposite end of the duct 7, indicated 9, terminates at an ultrasound generator l0 which can release into the guide or duct 7 an ultrasonic signal within a frequency range of around 40kHz, corresponding to the maximum sensitivity range of the detector 4. Naturally, several pairs of "transmitting" and "receiving" guides or ducts 7, l may be provided in the punch P and the die C, these facing each other in correspondence with zones of the part T in which the formation of production cracks is probable. In general, the ducts or guides l and 7 may be very short depending upon the manner in which the connection is made adjacent the part subjected to "in process" checking. The criteria which enable the coupling of several receiving ducts l to a single detector 4 and several transmitting ducts 7 to a single ultrasound generator l0 are generally known in the art and will not therefore be discussed further since the invention can be effectively described and understood with reference to the operating criteria of only one of these pairs of ducts or guides. Figure 2 shows a tubular duct, generally indicated ll, having respective ends l2 and l3. The end l2 of the duct ll extends through the die C in a configuration entirely similar to that of the end 8 of the duct 7 of Figure l. The other end l3, however, terminates at a pressurised air supply l4. More particularly, the duct ll may be constituted by an ultrasound guide entirely similar to the guide 7 of Figure l. The function of the duct ll, however, is different in that this duct is intended solely to convey the pressurised air generated by the source l4 towards the surface of the part T gripped between the punch P and the die C. The air may be at a pressure of between l and 6 atmospheres, the value considered preferable at present being about 2 to 3 atmospheres. In the embodiment shown schematically in Figure l, the method of the invention is based on the observation that the propagation of the ultrasounds in air conveyed by the guide 7 to the guide l takes place solely in the production cracks in the region S, that is to say, only when there are tears, fissures or cracks in the region 6. In fact, the latter contain air which constitutes a propagation medium for the ultrasounds. Consequently, the detection of the presence of production cracks in the part T may be carried out "in process", that is, during the pressing operation, the ultrasound generator l0 being activated and it being observed by means of the detector 4 whether the ultrasounds in air produced by the generator l0 are propagated not only through the guide 7 but also through the region S of the part and the guide l. As indicated above, the propagation through the region S and the guide l takes place only if there are production cracks; the picking up of ultrasounds in air by the detector 4 is thus indicative of the presence of production cracks. The embodiment illustrated schematically in Figure 2, however, is based on a different principle. In this case, during the pressing operation, the source l4 is activated so as to direct pressurised air through the guide or duct ll towards the part T. When production cracks are not present in the region S, the part T so to speak "stops" the guide ll and acts as a shield against the ultrasounds produced by the smaller flow of air from the end l2, which are not able to propagate through the part T since it is flawless. When there are production cracks in the region S, however, the pressurised air penetrates these cracks and enables the ultrasounds generated to be propagated both in correspondence with the end l2 and due to the interaction with the walls of these cracks. These ultrasounds in air are mainly in the frequency range around 40kHz and propagate in the guide l until they reach the detector 4. Again in this case, the detection of ultrasounds in air in correspondence with the region S is thus indicative of the presence of production cracks in this region. Of the two possible embodiments of the invention, the embodiment illustrated in Figure l is more advantageous in terms of sensitivity of detection. The experiments carried out by the inventors have in fact shown that with this solution it is possible reliably to detect production cracks with an overall length of less than a centimetre and a width of less than 0.5 mm. The solution schematically illustrated in Figure 2 has limits with regard to the sensitivity of detection, mainly due to the distribution of frequencies of the ultrasounds around the frequency of 40kHz. When this embodiment is chosen, it is more difficult to identify production cracks with overall lengths of less than l centimetre. It can be installed more easily, however, particularly with regard to the possibility of the use of simple tubes or ducts on the transmission side (that is, on the side of the die C in the embodiment illustrated in Figure 2) for conveying pressurised air and no guides for the ultrasounds, which, as is known, attenuate the ultrasounds themselves. Moreover, the practicality of the ducts for the pressurised air is not seriously affected by the possible infiltration into the ducts themselves of oil or powder which may be present in the industrial environment in which the pressing is carried out. However, such infiltration can seriously compromise the operating characteristics of ultrasound propagation guides. Figures 3 and 4 are two graphs in which the scale on the abscissa relates to the time coordinate and the scale on the ordinate relates to the changes in the signal (5';5") at the output 5 of the detector 4. More particularly, the changes in the signal are illustrated in Figure 3 with reference to changes detectable during a pressing operation in which no production cracks are made. Figure 4 relates to changes in the signal detectable when the pressing operation causes such cracks to form. In both Figures, a square-wave synchronising signal S C is shown in chain line, its course being indicative of the prosecution of the pressing operation. More particularly, the change in the synchronising signal S C to a high logic level is indicative of the moment at which the drawing process is started, that is, the moment at which the punch P starts to act on the part T. The ascending front and the descending front of the signal S C are thus respectively indicative of the start and finish of the pressing operation proper. With reference to Figure 3, it can be noted that, even in the case in which no production cracks are formed, the detector 4 always picks up ultrasounds during the pressing operation, these being due essentially to the general noise of the pressing apparatus in which mechanical parts knock and slide against one another. In each case the generation of these ultrasounds is caused in strictly predetermined time intervals. More particularly, as illustrated in Figure 3, in the overall interval τ in which the pressing operation is carried out, there can be seen at least two time "windows" W₁ and W₂ in which - when the part T does not have cracks - the signal generated by the detector 4, indicated S', remains at around a nil value. On the other hand, when the pressing operation results in the formation of production cracks (Figure 4), the signal generated by the detector 4, here indicated S", has an alternating or undulatory course even in the time windows W₁ and W₂, which is clearly distinguishable from the practically zero level observable in the absence of production cracks. If the course of the signal generated by the detector 4 is observed during at least one selectively predetermined fraction (window W₁ or W₂ or both) of the time interval τ in which the pressing is carried out, it is possible to note the presence of production cracks without ambiguity. A comparison of Figure 3 and Figure 4 also makes it possible to see that the signals S' (part without cracks) and S" (part with cracks) behave differently even at the end of the interval τ. Thus, the detection may usefully be extended even for a certain period of time after the completion of the pressing operation. If we leave out of consideration their different characteristics of sensitivity and the different degrees of difficulty of putting them into effect, the results obtainable from the embodiments of Figures l and 2 are essentially alike, particularly with regard to the course of the ultrasonic signal picked up by the detector 4. Figure 5 illustrates in block schematic form the internal structure of the processing circuit 6 to which the signal at the output 5 of the detector 4 is supplied. In this block diagram, an electrical supply unit, indicated l5, provides the operating power to all the other elements constituting the device 6 through lines, not illustrated. When the embodiment of Figure l is used, the unit l5 may be used to advantage to supply the ultrasound generator l0. The input circuit which amplifies and rectifies (after any conversion to low frequency) the signal present at the output of the detector 4 is indicated l6. The output of the input circuit l6 is connected to a first threshold circuit for blocking and interrupting the propagation of signals whose amplitude is less than the predetermined threshold towards the other parts of the device 6. The circuit l7 is intended essentially to annul any disturbances or background noises which reach and/or are generated in the detector 4 and in the circuits associated therewith. A counter circuit, indicated l8, counts the number of half waves or the alternations in the (rectified) signals which pass through the first threshold circuit l7. The counter circuit l8 has an enabling input l8a at which a signal generated by a sensor l9 mounted on the pressing device (press) is present. The sensor l9 is able to detect the relative movement of the punch P and the die C. In other words, the sensor l9 generates a signal approximately corresponding to the synchronising signal S C illustrated in Figure 3, enabling the counter l8 to operate only during the pressing operation. The sensor l9 therefore has the task of making the counter l8 function only when necessary. More precisely, the sensor l9 may be formed so as to enable the counter l8 only for a selectively predetermined fraction (such as, for example, one of the time windows W₁ or W₂) of the interval τ in which the pressing is carried out. This avoids the counting carried out by the counter l8 being disturbed by spurious noise or signals which are not meaningful for the purposes of the detection of the presence of production cracks. In the embodiment illustrated, the output signal of the sensor l9 is also directed to the counting input of a further counter 20 which totals the number of pressing operations carried out successfully by the press on which the detection device of the invention is mounted. The counting signal emitted by the counter l8 is fed to the input of a further threshold circuit 2l. In the circuit 2l, the count value of the counter l8 is compared with a reference count defining the level of discrimination between the parts to be considered good, that is, without production cracks, and the parts wasted, that is, parts with cracks. The threshold level of the circuit 2l may be adjusted experimentally in dependence on the characteristics of the parts T and the pressing apparatus. In general, parts for which the count of the counter l8 is less than the threshold of the comparator circuit 2l are considered good. The parts for which the count of the counter l8 surpasses the threshold are waste due to the presence of production cracks. The need to throw away the part during pressing is indicated by a warning device such as an acoustic or optical warning device 22 activated by the comparator circuit 2l when the threshold is surpassed. The operators may thus remove the defective part from the pressing line or operate the conveying apparatus associated therewith to discharge the part. When an automatic conveyor line is used for the parts, the alarm signal emitted by the circuit 2l may be used to control a selector device which can convey the waste parts from the line. The surpassing of the threshold level of the circuit 2l, indicative of the formation of a defective part due to the presence of production cracks, is also indicated to a further counter circuit 23 which totals the number of waste parts. By comparing the count of the counter 20 (overall number of parts pressed) with the count of the counter 23 (number of parts wasted) and observing the changes of these values with time, the operators are able quickly to identify anomalous situations requiring stoppage of the production, adjustment of the pressing apparatus, or possibly replacement of the punch P and/or the die C worn by use quickly.

Claims: 1. Method for detecting the presence of production cracks in predetermined regions (S) of mechanical parts (T) made by pressing between a punch (P) and a die (C), characterised in that it includes the step of detecting the presence of ultrasounds in air in the predetermined regions (S) during pressing, this presence of ultrasounds in air being indicative of the existence of production cracks in the pressed part (T). 2. Method according to Claim l, characterised in that it includes the step of generating, during pressing, ultrasounds in air in the predetermined regions (S) and on the side of the part (T) being pressed which faces one (C) of the punch (P) and the die (C), and the step of detecting the presence of ultrasounds in air in the predetermined regions (S) and on the side of the part (T) being pressed facing the other (P) of the punch (P) and the die (C). 3. Method according to Claim l, characterised in that it includes the step of supplying pressurised air to the surface of the predetermined regions (S) of the part (T) being pressed during pressing. 4. Method according to any one of Claims l to 3, in which the pressing action involves relative movement between the punch (P) and the die (C) for a predetermined time interval (τ), characterised in that the step of detecting the presence of ultrasounds in air is carried out in at least a selectively predetermined fraction (W₁; W₂) of the interval (τ). 5. Apparatus for detecting the presence of production cracks in predetermined regions (S) of mechanical parts (T) made by pressing between a punch (P) and a die (C), characterised in that it includes: - at least one guide (l) for propagating ultrasounds in air associated with one (P) of the punch (P) and the die (C) and having two opposite ends (2,3), of which a first end (2) faces the surface of the mechanical part (T) in correspondence with one of the predetermined regions (S) during pressing, and - a detector (4) of ultrasounds in air connected to the other end (3) of the at least one guide (l) for propagating ultrasounds in air. 6. Apparatus according to Claim 5, characterised in that the detector (4) is sensitive mainly to ultrasounds in air having a frequency of the order of 40kHz. 7. Apparatus according to Claim 5 or Claim 6, characterised in that it includes: an ultrasound detector (4) which can generate a generally alternating signal (S'; S") indicative of the changes in the pressure wave associated with the detected ultrasounds in air with time, and - a comparator circuit (l8,2l) sensitive to the number of alternations in the signal generated by the ultrasound detector (4), the comparator detector (l8,2l) having a respective threshold of alternation and being able to generate an alarm signal (22) when the number of alternations of the signal generated by the ultrasound detector (4) surpasses the alternation threshold level. 8. Apparatus according to Claim 7, characterised in that it includes a sensor element (l9) sensitive to the relative movement between the punch (P) and the die (C) and capable of generating an enabling signal (l8a) for the comparator circuit (l8, 2l) in order to activate the comparator circuit (l8,2l) only during at least one selectively predetermined time interval (W₁, W₂) in the pressing operation. 9. Apparatus according to Claim 7 or Claim 8, characterised in that there is interposed between the ultrasound detector (4) and the comparator circuit (l8,2l) an auxiliary comparator circuit (l7) which has a respective intensity threshold and is able to allow only those signals generated by the ultrasound detector (4) whose intensity surpasses the respective intensity threshold to be propagated to the comparator circuit (l8,2l). l0. Apparatus according to any one of Claims 5 to 9, characterised in that it includes: - at least one further guide (7) for the propagation of ultrasounds in air associated with the other (C) of the punch (P) and the die (C) and having two opposite ends (8,9), of which a first end (8) faces the surface of the part (T) in correspondence with one of the predetermined regions (S) during pressing, and - a generator (l0) of ultrasounds in air connected to the other end (9) of the at least one further guide (7) for propagating ultrasounds in air. 11. Apparatus according to Claim l0, characterised in that the generator (l0) produces ultrasounds with a frequency of the order of 40kHz. 12. Apparatus according to any one of Claims 5 to 9, characterised in that it includes: - a pressurised air supply (l4), and - at least one duct (ll) for conveying the pressurised air and having a first end (l2) which faces the mechanical part (T) in correspondence with one of the predetermined regions (S) during pressing, and a second end (l3) connected to the pressurised air supply (l4). 13. Apparatus according to Claim l2, characterised in that the supply (l4) generates air at a pressure of substantially within the range of one to six atmospheres.

Inventors: ALPINI, ALESSANDRO, ANSALDI, ERMANNO
Applicants: FIAT AUTO S.P.A.
Application year: 1986
Main classification: G01N29
IPC codes: G01N29, G01N19