GB2103136A

GB2103136A - Arc stud welding

Info

Publication number: GB2103136A
Application number: GB08221923A
Authority: GB
Inventors: Hermann Netasch; Willi Rodig
Original assignee: Tucker Fasteners Ltd
Current assignee: Tucker Fasteners Ltd
Priority date: 1981-07-31
Filing date: 1982-07-29
Publication date: 1983-02-16
Also published as: BR8204514A; JPS5825877A; JPH02157B2; FR2510447A1; FR2510447B1; DE3130389A1; GB2103136B; DE3130389C2

Abstract

In drawn arc stud welding, a testing system is provided to check the voltage across the gap before the welding current is imposed upon the pilot arc. If the voltage exceeds a preset maximum or falls below a preset minimum, the welding cycle is interrupted, or the next cycle is prevented from being initiated; an alarm may be given. Too high a voltage across the gap may be indicative of an oily or otherwise contaminated workpiece which might lead to a faulty weld, and too low a voltage may be due to a short circuit.

Description

SPECIFICATION Welding This invention is concerned with welding, and more particularly with a method of testing that conditions are satisfactory for the welding parts, for example welds studs, on to metal workpieces by the drawn arc process.

It has been learned from experience that in stud welding by the drawn arc process unsatisfactory welding results are obtained occasionally, which are to be attributed to the faces to be welded to one another being contaminated. This problem is caused particularly by oil or grease residues being present on the workpiece, which in most cases is made of sheet metal that has previously undergone a drawing process, which for this purpose has been provided with a drawing oil. Drawn arc stud welding is used mostly in production lines which to a large extent operate on an automatic cycle. It is therefore unavoidable that parts contaminated in such a way come to the production line. An insufficiently penetrated weld may then result and in most cases only become apparent at the end of the production line, where the imperfect weld can be rectified only at unacceptably high cost.

It is accordingly an object of this invention to detect inadequately or improperly prepared surfaces in the zone to be welded at an early enough stage on a production line that further welding operations are prevented or the current welding operation interrupted. This problem is solved in accordance with the invention by comparing the mean voltage of the pilot arc with a pre-set maximum voltage value and causing a signal to be generated if the maximum permissible voltage is exceeded.

It has appeared surprisingly that a surface severely contaminated for example with oil or grease has practically no influence on the voltage across the welding arc, although it is reckoned to be the welding arc that is responsible for the welding. On the other hand the pilot arc is relatively sensitive to contaminations or insufficiently prepared surfaces. It has at any rate been established, that insufficiently prepared surfaces lead to an increased voltage across the pilot arc, presumably due to evaporation phenomena or suchlike. These phenomena affect the pilot arc to such an extent that they can be determined by carrying out a check without involving any great expense. Under normal clean operating conditions the voltage present over the pilot arc is approximately 20 to 15 volts, while in the presence of contamination voltages of approximately 30 to 40 volts are produced.The normal operating voltage of the pilot arc stands out therefore quite clearly from the voltage prevailing in the case of contamination. The voltage of the pilot arc expecially in the case of contamination is a direct current voltage subject to voltage peaks closely following each other, from which for example through an RC circuit a mean voltage can be filtered which can be measured and give rise to a signal. Such signal, given only when exceeding a maximum voltage, is indicated before the start of the welding arc as a warning that an insufficient welding is about to take place, so that preventing measures can be taken.

One such measure that can be taken in response to a signal as aforementioned is to convert the signal into an alarm, so that the operator is made aware instantly of the imminence of an inferior weld. In addition to that it is possible to use the signal for stopping the work cycle control unit for continuing operation of the welding unit. In this case the work cycle control unit can be stopped either before initiation of the welding arc, so that no weld takes place or, if there is not time for that, the initiation of a subsequent working cycle can be prevented.

It may also happen that the voltage across the pilot arc falls below a minimum value, and in this event also a signal may be caused to be emitted.

A falling below a minimum voltage measured over the pilot arc can for example arise if a short circuit over the arc gap is caused due to any malfunctioning. Such a case for example is likely to occur if the pilot arc leads to such an intense melting of the metal of the stud or other part to be welded to the workpiece that a molten drop of metal forms a bridge. The proper initiation of the welding arc would then be doubtful, and an interior weld would probably result. Such a case can thus be detected also by the measuring of the mean voltage of the pilot arc and comparing it with a pre-set value.

A method of testing conditions for stud welding in accordance with the invention and illustrative thereof will now be described with reference to the accompanying drawings. It will be realised that this illustrative method of testing has been selected by way of example and not of limitation of the invention.

In the accompanying drawings:~ Figure 1 is a block diagram of a circuit for the illustrative testing method including in the basic circuit a representation of a stud welding gun; Figures 2 and 3 are diagrams for different operating sequences within the scope of the illustrative testing method.

In Figure 1 is shown a welding gun 1 in diagrammatic representation. Out of the front end of the welding gun 1 protrudes a support sleeve 2, which is placed to rest on a base metal workpiece 3. Inside the sleeve 2 extends a stud holder 4, which carries at its front end a weld stud 5. The stud holder 4 is retractable in a known manner by means of a lifting solenoid 6 into the stud welding gun 1 against the resilience of a compression spring (not shown). With the energizing of the lifting solenoid 6 the stud holder 4 together with the weld stud 5 is pulled into the stud welding gun 1 by about 1.5 mm (backward stroke); when de-energizing the solenoid 6, the stud holder 4 together with the weld stud 5 is driven forward again by the spring (forward stroke). These operations and the structural parts required for them are known.Into the handle 7 of the stud welding gun 1 run a number of electrical leads which are necessary for the normal operational use of the gun 1 . Through leads 10 and 1 1 runs the energizing circuit for the solenoid 6. From the handle also runs a lead 13 which carries the current for both the pilot and welding arcs. Furthermore, the workpiece 3 is connected to an electric lead 14, so that a circuit is set up comprising the lead 13, the stud holder 4, the stud 5, the workpiece 3 and the lead 14.

The leads 10,11, 13 and 14 are connected to a work cycle control unit 8, which is of a known type for use in connection with stud welding guns. The unit 8 supplied to its terminals 16 and 17 the voltage for energizing the lifting solenoid 6. In addition to that, the unit 8 delivers to its terminals 18 and 19 the voltage required for generating the pilot arc and the welding arc. In the work cycle control unit 8 is a timing control contact 9, which, when closed, actuates the lifting solenoid 6 to withdraw the stud holder 4 together with the stud 5 from the workpiece 3 thus to draw the pilot arc, which lasts for a period of time determined in a known manner by the unit 8.The circuit for the solenoid 6 is also broken again by the work cycle control unit 8 opening the contact 9 at practically the same time as the welding arc is initiated so that with the fading of the welding arc to the weld stud is plunged into the fused mass created on the workpiece 3, through which the welding process is brought to an end.

The operations just referred to will now be described in more detail with reference to Figures 2 and 3. Line a represents the contact 9 which at 52 is shown to be closed as controlled by the work cycle control unit 8 for a period of time from t1 to t5. Closing of the control contact 9 at the point of time t, energizes the lifting solenoid 6, represented on line b, whose backward stroke is completed at the point of time t3. The deenergizing of the solenoid 6 due to the opening of the control contact 9, determined once again by the work cycle contol unit 8, takes place at the point of time t5, which, after a time delay which is characteristic of the electrical component, is followed by the de-energizing time of the solenoid 6. This is completed with the plunging of the stud 5 into the fused mass on the workpiece 3 at the point of time t6.With the withdrawal of the stud 5 from the workpiece 3 immediately after the point of time t1 is initiated the pilot arc between stud 5 and workpiece 3, namely at the point of time t2.

The voltage measured across the pilot arc is illustrated on line c. It has in the normal course of operation the mean level U1 which lasts roughly up to the point of time t5, at which point the voltage for the welding arc is imposed upon the pilot arc. Across the still existing arc gap then forms under the effect of this voltage the voltage increase 50, which is short-circuited with the plunging of the stud 5 into the fused mass on the workpiece 3 at the point of time t6. The welding arc current itself is illustrated at 51 on line d, which shows clearly that in the descending curve of the welding arc current 51 at the point of time tf, there is a sharp rise of current on account of the short-circuit, which is then followed by the final fading of the arc.

Referring again to Figure 1, in order to measure the mean voltage of the pilot arc the terminals 18 and 19 are connected to an impedance transformer 24, the purpose of which is to ensure that the voltage supplied to it across the terminals 18 and 19 is passed on at a steady value representing the voltage across the arc gap to output terminals 22 and 23, and thence to a capacitor 27 via a gate 25 (shown in Figure 1 in simplified form as a control contact) and resistor 26. The gate 25 is provided with pilot lines 28, 29 connected to the terminals 16 and 17 thus to respond to energization of the lifting solenoid 6.

The voltage supplied by the pilot lines 28 and 29 corresponds therefore with line a in Figure 2; that is to say during the period between the points of time t, and t5 the gate 25 is closed-see line e in Figure 2, allowing the voltage supplied by the impedance transformer 24 to pass through.

The voltage present at the terminals 16 and 17 (line a in Figure 2) is also supplied over the aforementioned pilot lines 28, 29 to a switch 30 acting as a monostable multivibrator, which from the closing of the contact 9 thus assumes a closed condition indicated at line fin Figure 2 from the point of time t1 to a point of time t4 shortly afterwards.

For the period of time t, to t4, contact 32 of the switch 30 short circuits the capacitor 27 and ensures its complete discharge. At the point of time t4 the switch contact 32 opens, the capacitor 27 is available for recharging by the voltage present at the output terminals 22 and 23 of the impedance transformer 24 via the resistor 26.

Resistor 26 and capacitor 27 act thereby as an RC circuit, which ensure the levelling out of possible voltage peaks across the arc gap so that the charge on the capacitor 27 represents the mean voltage of the pilot arc, namely the voltage U2 illustrated in line g of Figure 2 (or, that is to say, the voltage U1 on line c).

To ensure that the capacitor is fully charged before relying on the voltage U2 as a measure to verify that conditions are satisfactory for welding, the gate 25 is closed by the work cycle control unit 8 at the point of time t5 and measuring of the voltage on the capacitor 27 is thus initiated by means of the time switch 33 just as the solenoid 9 is de-energized.

The time switch 22 is made operational by inserting a bridge A. Its mode of operation lies in that it is started at the end of the operational period of the control contact 9, i.e. at the point of time t5, thus to release the outflow of a voltage discriminator 34 for the duration of a pulse 42 produced by a time switch 33 (see line h in Figure 2). For this purpose the time switch 33 is connected to the pilot line 28 29 from terminals 16,17.

In order to establish if the voltage U2 on the capacitor 27 exceeds a maximum voltage U3 (line i in Figure 2) or falls below a minimum voltage a comparator 35 is provided in the voltage discriminator 34 to which in a knowm manner the maximum voltage value U3 and the minimum voltage value are supplied at the same time to terminals 47 and 48. In the event of the voltage either exceeding or falling below these voltage values respectively, then the voltage delivered at the output point 49 of the voltage disciminator 34 will operate a relay 36, provided that the time switch 33 is closed at the same time. The closing period of the time switch 33 is defined by the above-mentioned pulse 42. For reasons of time, in the circuit as described with reference to Figure 2, it is possible only to prevent further welding cycles; the welding arc pulse already started cannot be stopped.The contact 36a, however, connected to the work cycle control unit 8 through leads 38, breaks a circuit without which there is no possibility of starting a further welding cycle. The relay 36 is interlocked with a holding contact 36b, which has to be re-set by hand by means of a manual switch 60. The relay 36 can also be arranged to operate an audible or visible alarm.

If it is desired to interrupt a welding cycle before the welding current is superimposed upon the pilot arc then the measuring of the voltage across capacitor 27 has to occur before the point of time tS, i.e. before the de-energizing of the lifting solenoid 6 in the gun. Contrary to the above-described mode of action of the time switch 33 with the pulse 42 being released upon de-energizing of the solenoid 6 at the point of time t5, it may be arranged, by inserting a bridge B, that a time switch 61 is introduced which closes at a predetermined time after the energization of the solenoid 6 at the point of time tt; that is to say a test pulse 43 is initiated immediately in front of the point of time t5 (see line e in Figure 3).At this stage the pilot arc is nearing its end, consequently the capacitor 27 is fully closed (see line fin Figure 3), so that now at the time of the pulse 43 the measuring of the voltage by means of the voltage discriminator 34 can be reliably effected.

The voltage delivered at the output point 49 of the voltage discriminator 34 is supplied via the time switch 61 that is closed by the pulse 43 to a relay 37 at such an early stage that if the maximum voltage value U3 is exceeded the initiation of the welding arc in that cycle can still be prevented by the closing of the contact 37a of the relay 37.

The relay 37 is arranged to stop the welding only for that one cycle of the work cycle control unit 8. With the insert placed over the bridge A, the relay 36 would also be energized as hereinbefore described, which would prevent further initiation of the work cycle control unit 8 until the circuitry has been re-set by opening the switch contact 60.

Claims

1. A method of testing that conditions are satisfactory for the welding of studs or other parts to a metal workpiece in drawn arc welding wherein the mean voltage across the gap towards the end of the duration of the pilot arc is compared with a pre-set maximum value, and if that value is exceeded a signal is generated.

2. A method according to claim 1 wherein the signal is caused to prevent initiation of a further welding cycle is untilua manual switch has been operated.

3. A method according to claim 1 wherein the signal causes interruption of the welding cycle by preventing initiation of the welding arc.

4. A method according to any one of the preceding claims wherein the signal initiates an alarm.

5. A method according to any one of the preceding claims wherein the signal is alternatively generated if the said mean voltage falls below a pre-set minimum value.

6. Apparatus for testing that conditions are satisfactory for the welding of stud or other parts to a metal workpiece comprising circuitry arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.