Abstract: An electrode holder has an optical sensor element for the detection or measurement of the electrode force.

Abstract: The temperature is determined at a spot weld. The spot weld is between parts to be welded to one another. The parts are formed of a first metal and the spot weld is made with an electrode of a second metal placed in contact with the parts. The thermoelectric voltage between parts and the electrode is measured and the temperature is determined from the thermoelectric voltage measured in the measuring step. A device for determining the temperature includes a voltmeter connected between the parts to be welded and the electrode. The temperture determination is utilized in assessing the quality of spot-welded joints in automated manufacture, for example in the automobile industry.

Abstract: A weld monitor system measures and records physical conditions under which welds are made between parts weld fused by an automated welding machine. Analog data of pressure in hydraulic circuits of hydraulically driven operations of an automatic welding machine, and analog data of weld current amperages applied to welding electrodes of the welding machine is acquired and stored by the weld monitor system to provide an archival record of the physical conditions of the formation of all welds made by the automatic welding machine. The weld monitor system compares the data to a predetermined range of values and sends a control signal to a welding machine controller indicative of the results of the data comparisons. An automatic part identification unit of the weld monitor system sequentially applies permanent unique indicia to parts weld assembled so that weld assembly data for uniquely identified parts can be readily retrieved from the stored data.

Abstract: A welded tube monitoring and control system includes a programmable logic controller (PLC) which receives data from a plurality of input apparatus which monitor the tube manufacturing process, and which generates output data to control various components of the tube manufacturing system. An apparatus for measuring the width of the flat metal strip prior to forming the tube transmits measurements to the PLC. The width of the strip is compared to a set of threshold parameters programmed in the PLC, and an alarm is triggered if the measured width exceeds these threshold parameters. A temperature measuring apparatus is located at the welding site to measure the temperature of the weld. The weld temperature is transmitted to the PLC and compared with a set of threshold parameters, and an alarm is triggered is the measured temperature exceeds the threshold parameters. An eddy-current testing apparatus is located downstream of the welding apparatus and transmits data to the PLC.

Abstract: Techniques for assuring the accuracy of the welding of a part including a part calibration cycle, a tap check cycle and a gap detection cycle. The part calibration cycle determines the electrical load presented by the part to the welder transformer. The tap check cycle verifies that the operator has set the correct tap on the primary of the welder transformer, The gap detection cycle finds the actual spacing between the platen when the ends of the part are substantially touching so that the ends of the part can be accurately spaced at the beginning of the weld cycle.

Abstract: A controller for a welding machine which requires no change of control programs, even if there is any change in material, thickness or others of workpieces is provided. After start of welding, welding current and/or pinching force increase, until there occurs a phenomenon that interelectrode resistance shifts from an upward tendency to a downward tendency, and thereafter the values at this time are maintained. The values are proper, and proper welding is performed. This is achieved, irrespective of material, thickness or others of the workpieces, so that there is no need of changing control programs one by one.

Abstract: A welding voltage output from a power source of a resistance welder is derived on the basis of a welding current and an interelectrode voltage, and diameter of a nugget which is formed in a sheetlike work to be welded is estimated on the basis of the welding current, the welding voltage, data of material constant and a thickness of the sheetlike work, subsequently, the estimated diameter of nugget is composed with a target diameter of nugget, and thereby quality of the resistance welding is determined.

Abstract: The measurement of the voltage over the welding point in order to supervise the welding quality takes place by means of voltage taps on the wire electrodes. As a result, the additional parasitic resistances during measurement by means of conventional voltage taps on the roller head or the roller head mounting are completely eliminated, which improves the measuring accuracy.

Abstract: The present invention relates to a process for controlling the parameters of a welding operation performed by a spot-welding machine. The machine has two movable electrodes which are connected to a source of current that supplies a welding current. The process controls the welding parameters on the basis of at least one electric signal that is a function of the temperature in the welding zone. The electrical signal or signals are taken between the terminal of one of the electrodes (10, or 20) and one of the parts to be welded (30, 40). The electrode and part together constitute a thermocouple.

Abstract: What is described is an arrangement for monitoring the quality of electric welds on bodies (10) for the production of cans. Welding parameter values (F.sub.i), which have been measured on a number of bodies (10) found to be good are statistically evaluated in an evaluation device (22). For this purpose, an average welding parameter profile is determined. Then a reject limiting-value band (F.sub.g.+-.) enveloping the average welding parameter profile is determined from the mean values and the product of the standard deviation of the welding parameter values (F.sub.i) assumed to be normally distributed and at least one sensitivity factor. A permissible number of erroneous rejects per one million of welding spots is selected as a criterion for the sensitivity factor so that, for example with 100 welding spots on a body (10) of a medium-sized can, every ten thousandth body will be accidentally rejected on the average.

Abstract: A self-diagnosing resistance welding cable and method for monitoring the relative increase in cable resistance while a cable is in use, without disconnecting the cable. The cable incorporates a reference resistor in one of the terminals in thermal communication with the cable, such that a comparison in resistance variation between the cable and reference resistor cancels the effects of temperature variation on resistance change. The reference resistor is connected in series with the cable in an electrical test circuit. The test data is sampled each time the welding machine is cycled, and the data is updated and processed by a computer on a continuous basis. The system also monitors the actual temperature of the cable and the flow of coolant through the cable. Multiplexer circuits are used to monitor multiple welding cables with the same computer.

Abstract: The motorized weld head employs a control system which allows user control of critical welding parameters such as speed, force, position, and duration. The control system includes a position servo loop attached to a motor-driven output shaft on which a welding electrode is mounted via a spring. During a weld cycle, the control system moves the welding electrode toward a workpiece at a rapid user-defined rate. Just prior to when the electrode contacts the workpiece, the speed of the electrode is reduced to prevent damage to the parts. When contact is made, a pressure transducer measures the force exerted by the electrode on the workpiece. A comparator continuously monitors the force measured by the transducer and changes state when the measured force equals a user-defined optimum welding force. A signal is then passed to the servo loop commanding it to hold the position of the electrode effectively maintaining the applied force on the workpiece.

Abstract: A decrease amount in plate thickness at a spot weld is detected at a predetermined time period when application of pressure is still maintained after termination of power supply to electrodes for spot welding. The decrease amount is correlated with the magnitude of diameter of nugget formed by spot welding, e.g., a larger diameter of nugget signifies a stronger the joint, and therefore, the quality of joint at the spot weld can be examined on the basis of the detected decrease amount. Since a value obtained by integrating plate thickness detection data of the spot weld during the predetermined time period is correlated, as the decrease amount is, with the magnitude of the diameter of a nugget, the quality of the joint may also be examined on the basis of this integrated value.

Abstract: A seam weld machine includes a voltage monitoring circuit (9) for determining weld quality. The voltage is sensed by two pick-ups. A first pick-up is in the form of a conductive rider (10), mounted to directly sense the voltage from a weld wire (4). The wire (8) from the second pick-up is short and is fed through a gap in the z-bar (6) of the machine to emerge close to the wire from the first pick-up, thus reducing electromagnetic interference and noise.

Abstract: A parallel-gap welder applies a precisely controlled force to the weld head (40) by means of a solenoid (60) having a plunger (72) aligned with the axis of the welding electrodes (50,52). To precisely control the amount of force applied by the solenoid a load cell (80) is interposed between the weld head (40) and the solenoid plunger (72) to provide a feedback force signal (158) indicative of the force applied by the solenoid. The feedback force signal is used to control current through the solenoid coil (68) and thereby servo the applied force to a force command input. The solenoid plunger (72) and load cell (80) are axially aligned with the axis of the electrodes (50,52) and solenoid force is applied through a universal joint (76,78) connection to assure an in-line driving force on the electrodes.

Abstract: For improving the quality of welding operations, in a process for alternating current welding, in which during a period the actual r.m.s., welding current is measured and the phase displacement of the use of current flow during the next period of the a.c. current is regulated. Variations of the actual r.m.s. welding current during all the welding periods already performed are taken into account and compared with a desired r.m.s. current during the welding period to be performed. A welding apparatus for alternating current welding using two welding electrodes, a power supply for the same and a device for regulating the r.m.s. welding current during each a.c. voltage period has a computer for taking into account variations of the actual r.m.s. welding current during preceding welding periods of a welding operation compared with a desired r.m.s. welding current flow during the further welding periods to be performed of the particular welding operation.

Abstract: An apparatus and method for monitoring weld quality and for establishing a proper amount of weld energy and the upper and lower limits of the proper weld energy to be supplied by a precision resistance welding apparatus in order to produce acceptable welds. In the present apparatus and method the difference between the change in voltage and current supplied to a test weld is compared to a reference weld and this difference is used to obtain the upper and lower limits of the proper amount of energy to be supplied by the welder in a production context.

Abstract: A preweld resistance check for resistance welding machines uses an open circuited measurement to determine the welding machine resistance, a closed circuit measurement to determine the parallel resistance of a workpiece set and the machine, and a calculation to determine the resistance of the workpiece set. Any variation in workpiece set or machine resistance is an indication that the weld may be different from a control weld.

Abstract: Apparatus for welding longitudinal seams in can bodies includes upper and lower electrode wheels mounted on arms, a transducer for detecting vertical displacement of the upper wheel, a pair of pick-ups for detecting the rotational positions of the wheels, and a single processing circuit. The transducer output is divided into a set of traces, each corresponding to a single wheel rotation. The traces are averaged to produce a representation of the wheel profile. Each wheel profile is subtracted from the transducer output to produce a good indication of weld thickness. Alternatively, the transducer output is divided into a series of traces, each of which represents the vertical displacement of the upper wheel while welding a single can body. These are averaged to provide a good indication of weld thickness. The wheels have surface irregularities and each is inevitably mounted with some degree of eccentricity. Unwanted transducer output components due to these irregularities and eccentricities are removed.

Abstract: There is disclosed a resistance measuring mechanism in a resistance welding machine for calculating a resistance value in an arbitrary portion of the resistance welding machine. The resistance measuring mechanism comprises means for detecting temperature of an exit through which cold water for cooling a cable flows and the temperature of parts of cables, means for detecting voltage at a secondary or a primary winding of a welding transformer and means for detecting welding current at the secondary or the primary winding of the welding transformer characterized in that resistance value at a measuring temperature is calculated by correcting the resultant detected temperature, voltage, welding current for voltage, welding current at a reference temperature.

Abstract: An indicating system has a current pick-up inductively coupled to one of the cables of a welding system. A predetermined fraction of the voltage from the pick-up is applied to a programmed circuit arrangement which includes a timer and which controls a warning device to indicate when the predetermined fraction of the pick-up voltage has been less than a predetermined value for a predetermined time after current has begun to flow in the cable.

Abstract: A portable meter for measuring the impedance of a resistance welding circuit between first and second points on the apparatus. The meter is powered by conventional alternating current and conducts a test alternating current through the welding apparatus at such an amperage so that the voltage measured between the first and second points is equal to the impedance therebetween.

Abstract: What is described is a circuit for measuring the effective voltage drop at the welding position (30) between the welding electrodes (14,16) of a resistance seam welding machine. A first measuring loop (40) for measuring a first loop voltage is secured to the lower welding arm (28) close to the lower welding electrode (16) and is in communication with the upper welding electrode (14) through a sliding contact (44). The second measuring loop (60) is taken close to the welding electrodes (14,16) through the head portion of the Z-rail (12). The conductor turns (42,62) of both measuring loops, which lead to an evaluation circuit (48), are twisted together. Thus the stray effects picked up in the two measuring loops are the same. The difference between the two loop voltages formed in the evaluation circuit (48) is the effective voltage drop which is further processed in the usual manner to monitor the quality of the welding seam produced.

Abstract: A control system for parallel gap welders typically employed to weld contacts to solar cells or wires to integrated circuit chips is disclosed. The system monitors the temperature generated by the welder at the work pieces. The system automatically terminates the weld operation should the weld temperature reach a maximum predetermined temperature, thereby preventing overheating the weld joint and adjacent parts. Alternatively, if the weld temperature does not reach a minimum predetermined temperature the system automatically signals the weld operator that the joint must be rewelded to avoid an incomplete or cold weld. A quality weld can therefore be repeated.

Abstract: Method and apparatus for monitoring the quality of a weld performed with an electrode mounted on an automatically controlled electrode support arm which moves between a rest position and a series of spaced operative positions in each of which a weld operation is performed. The apparatus includes a radiation detector for connection to a monitoring system and a radiation collector attached to the support arm adjacent the electrode and coupled to the radiation detector by optical fibers to receive in use a first level of radiation from electrode and a second level of radiation from the weld zone when the arm is in its operative position. A reference source of radiation is also provided that is located such that when the support arm is in its rest position, the collector receives a third level of radiation from the reference source, whereby the radiation detector generates three signals corresponding to the three levels of radiation impinging on the detector for supply to the monitoring system.

Abstract: In an electron-fushion welding process a fault, e.g. due to incorrect assembly of the pipe joint components, is detected by sensing the welding current and emitting a signal to actuate an audible or visible alarm and/or to interrupt the welding process, if the welding current increases unexpectedly. A reference signal dependent upon the current dependent signal produced by the sensor and RMS to DC converter at a time t.sub.1 is held by a converter and is compared by a comparator with the current dependent signal transmitted through converter at subsequent intervals. So that the device does not react to transient current increase a delay arrangement delays the output signal for a given time.

Abstract: Apparatus, for measuring the thickness of a welding seam produced by resistance welding between a resiliently mounted upper roll and a lower roll, comprises a source of light mounted laterally of the resiliently mounted roll and a light collector mounted to receive such light as is not eclipsed by the periphery of the roll so that variation in the amount of light received as the roll periphery interrupts the light is indicative of weld thickness. The light received is preferably converted by means to an electrical signal which is analyzed to discern the quality of the weld.

Abstract: Apparatus and accompanying methods for use therein for measuring and controlling weld indentation during resistance welding operations are disclosed. During a welding operation, the apparatus measures changes in a distance separating a reference point located on an electrode assembly and a region situated on the surface of the workpiece being welded wherein the region is not substantially indented by the electrode. Specifically, a distance measuring system first establishes the location of the surface relative to the electrode before any welding current is passed through the weld area. The indentation is repeatedly measured by detection of ultrasonic echoes from the surface of the material being welded. Such measurements are taken at frequent intervals during the current flow period during which the weld is being formed in order to determine on a continuous basis the indentation of the electrode into the surface. The welding power may be terminated when the measured indentation reaches the desired value.

Abstract: A welding current measuring apparatus for a resistance welding machine comprises a current detector for use in measuring and detecting welding current flowing through the resistance welding machine, a memory for successively storing and holding the current values detected over the whole energizing time, and a device for specifying a desired energizing cycle interval to compute and display current values after completion of welding, wherein any energizing cycle interval is specified at will after completion of welding so that the current values or the current waveform in the specified interval can be displayed.

Abstract: The quality of a percussion weld is sensed by measuring and analyzing certain features of the weld current signature. Instantaneous current is measured by a Hall sensor and permits isolation of the welder from the monitoring system. Weld quality is strongly correlated to the time to vaporize the nib separating the parts, and the dwell time to forge the parts together after nib explosion. A weld is assumed defective if nib time is too short or dwell time is too long. These quality parameters may be supplemented by weld and forge timing, line voltage, and acoustic information; a weld is rejected if any of these are outside of acceptance limits. Experiments on the percussion welding of silver-cadmium oxide to brass show that cadmium-oxide is decomposed and expelled by the forge pressure and the joint is a silver to brass weld.

Abstract: The Hall effect current sensor is dipped in a coating of magnetically saturable and electromagnetically saturable material, such as a mixture of powdered nickel in an epoxy binder. The coating has little effect on the sensor's ability to sense the magnetic field attributable to current flow in the welding transformers primary or secondary. The coating shields the Hall effect device from potentially damaging electromagnetic radiation which is produced by the welding apparatus as a by-product of the welding operation.

Abstract: The invention relates to the art field of resistance welding utilizing rollers in conjunction with a continuous copper wire electrode and effected with welding current that remains constant on average, at least within each half-period. According to the invention, the weld between members (generally tinned metal strip) can be monitored by measuring the current that flows through the circuit created by the electrode wire, this being a function of the welding resistance that registers between the welding rollers; the value of the current thus measured is compensated in respect of the inductive effects produced by the magnetic field originating from the welding current, and relayed in digital form to a microprocessor that compares it with given maximum and minimum limits within which the reading must fall for quality of the weld to be assured.

Abstract: Ultrasonic transducers spring biased into seating engagement in a pair of opposed spot welding tips transmit and receive ultrasonic waves and associated circuitry measures the time of flight between the transducers which is a function of tip separation, workpiece temperature and nugget size which are indicators of the progress of a weld. The rate of change of flight time is measured and the weld current is adjusted early in the weld cycle to maintain a preset rate of weld growth. The weld current is terminated later in the cycle if the transit time rate falls below another preset value. Otherwise the weld current and termination time follow a standard schedule.

Abstract: The body blanks (10) are moved through, with overlapping longitudinal edges, between an inner and an outer electrode roller (14, 24), the axes (16, 26) of which lie substantially in a common plane (II--II) normal to the direction (A) of the axes of the body blanks (10). Also lying in this plane (II--II) are the axes of a plurality of sizing rollers (34, 36, 38) which are disposed in a ring and one of which (38) is disposed diametrically opposite to the outer electrode roller (24). This sizing roller (38) and a guide roller (42) which is disposed in front of it in the direction (A) of the axes of the body blanks (10) are encircled jointly by an endless conveyor element (46). The guide roller (42) and hence also the conveyor element (46) can be driven by a servomotor (50) which is controlled according to the signals of an angle-of-rotation transmitter (30) rotating with the outer electrode roller (24).

Abstract: A method and apparatus are disclosed for analyzing the dynamic resistance of a resistance spot welding process to determine the time of an expulsion event and use that time as the weld completion time for controlling weld termination via a weld feedback signal, controlling automatic current stepping, and identifying maintenance needs. A preconditioning pulse is used to stabilize the weld process and aid in obtaining interpretable resistance information. After an initial blanking period the resistance curve is analyzed by calculating first and second derivative values and applying several algorithms that make possible the computer recognition of characteristic resistance curve shapes resulting from weld expulsion under good geometry, bad geometry and edge weld conditions.

Abstract: The invention is directed to a control circuit for controlling the current of a welding transformer integrated in an industrial robot. For the purpose of achieving a simpler, more accurate and more reliable detection of contact between the welding tongs (4, 5) and the parts (11, 12) to be joined by welding, there are provided a high-frequency generator (1) for supplying measuring current to the secondary circuit (2) of the welding transformer, an impedance measuring circuit (3), an additional impedance (7) for increasing the impedance of the secondary current (2) to high frequency, an interrupter circuit (8) for interrupting the welding current, and a comparator for enabling the welding current merely in the presence of a differential voltage signal which corresponds to complete contacting of the two parts to be joined by welding.

Abstract: A secondary conductor supervising device of a resistance welder control system for supervising short-circuit, disconnection, and/or deterioration of a secondary conductor by means of a current flowing through a resistance welder, which comprises a voltage detecting circuit for detecting a voltage from the secondary conductor, inclusive of a secondary cable, of the resistance welder, a current detecting circuit for detecting a current flowing through the resistance welder, a divider for dividing the output of the voltage detecting circuit by the output of the current detecting circuit, an average computer for averaging the output of the divider over a part or the whole of a welding period, a supervisory level setting unit for setting a resistance value such as of the secondary conductor, and a comparator for comparing the output of the setting unit with the output of the average computer based on the welding period.

Abstract: Apparatus provides for measuring weld resistance of each of a group of weld sites that are subject to variation with respect to weld resistance. The apparatus includes circuitry for inducing a flow of alternating current in a weld site. The magnitude of the induced alternating current is substantially constant; in particular, the percentage variation in its magnitude is substantially less than the percentage variation in resistance to which the weld sites are subject. The apparatus further includes voltage responsive circuitry for producing a signal representative of the magnitude of the resistance of the weld site.

Abstract: An electrode for resistance pressure welding equipment having current control has frictionally connected thereto an optical accelerometer. The parts of the optical accelerometer as well as its optical signal leads are fabricated from electrically non-conductive and non-magnetic material.

Abstract: The quality of welds produced during a percussion welding process are determined in real time by sensing the vibrational impulse signals produced during the percussion welding process, filtering the impulse signal to select a frequency band containing frequency components associated with the vibrations imparted to the forge rod of the welding machine, measuring the mechanical damping characteristics of the selected spectral portion of the signal by determining the log-decrement of the envelope of the signal, and comparing the measured log-decrement value obtained with a preselected threshold value.

Abstract: A multivariable process monitor and method of monitoring a resistance spot welding process to yield instantaneous information on weld quality during the weld cycle. Sensors on the machine measure welding input variables and workpiece response variables and provide signals to a microcomputer system. A simplified analytical model of the spot welding process is incorporated in the microcomputer and is the reference for the weld quality logic that accepts or rejects the weld. Faulty welds are diagnosed and the reason displayed.

Abstract: A system is provided to measure, store and display data relating to a series of steps in a plurality of welding operations. The data stored may be printed out to assist in locating steps in an operation which do not conform to predetermined time standards to permit corrective actions to be taken, if necessary.

Abstract: A method and apparatus of monitoring the resistance characteristics of a circuit carrying an alternating current or a current having an alternating component is described. The apparatus includes first integration or averaging means for integrating or averaging the potential drop for the period between instants when the instantaneous current has the same value in a current loop; and display means for displaying a value representative of the output from the integration or averaging means.

Abstract: Adjusted voltage measurements of monitored AC electrical energy supplied during a welding operation are evaluated by integration with respect to time during half cycles to develop a variable criterion each half cycle that is compared with adjusted limits to electronically make weld quality judgements. The generation of a fail signal during two consecutive half cycles as a result of such judgements, is operative through logic gating to produce a reject signal to control the marking of defective welds, interrupt the supply of energy to the welding electrodes and effect timely operation of an ejector removing articles with defective welds from a production line.

Abstract: A method and apparatus are disclosed for detecting weld nugget growth on a workpiece during a welding operation using a resistance spot welding gun. In the preferred embodiment, a piston rod is slidably mounted in a cylinder and is connected at one end to an electrode of the gun. The opposite end of the piston rod extends rearwardly through the cylinder and is coupled to a sensor assembly for detecting the amount of movement of the piston rod created by weld nugget growth on the workpiece during the welding cycle.

Abstract: A method and system for determining weld quality in resistance welding in such a manner that acceptance or unacceptance is determined by comparing with their respective reference values the electrode voltage, the electrode current time, the number of electrode voltage pulse signals in excess of the reference value, and the integral of difference between the electrode voltage in excess of a base voltage and the base voltage, independently or simultaneously, in addition to the comparison of the electrode resistance with its reference value. The relatively low-priced system according to the present invention can determining weld quality accurately even when the electrode tips are deformed or crushed so as to form other current path areas.

Abstract: A system is provided for use in a microprocessor controlled controller for a resistance welding machine for calculating the RMS value of the welding current so as to provide a closed loop control of the RMS current in order to eliminate the effect of variations in load and line voltage on the welding current and to provide a constant welding tip temperature in the presence of such variations.

Abstract: During a weld period the weld parameters are monitored and data representing the weld resistance curve and the power curve are acquired and stored for analysis by a suitably programmed computer. The derivative of R, R is calculated and stored and the function is divided by power P to obtain R/P. The resistance curve is searched to obtain the maximum value R.sub.m which occurs during the heating phase and the function R/P is searched prior to the time of R.sub.m to find the maximum of that curve representing the highest rate of resistance increase. Then the R/P curve is searched subsequent to its maximum to determine when the function reaches a specified percentage of the maximum. That value occurs at the knee of the resistance curve and approximates the onset of melting in the weld.

Abstract: Means for making electrical contact with a pair of spot welding electrodes and workpieces engaged by the electrodes. The means includes two rigid insulating blocks for mounting on each electrode of an electrode pair. Each block has a first electrical conductor for physically engaging the electrode upon which it is mounted and a second electrical conductor for physically engaging an exposed surface of the workpiece the electrode will engage in a welding process. The electrical conductors extend in a direction substantially perpendicular to the electrodes when the blocks are mounted thereon. In addition, means integral with the blocks and conductors for respectively electrically connecting the conductors to a device for measuring the voltage drop between electrodes and between the exposed surfaces of the workpieces when welding current is applied to the workpieces by the electrodes.

Abstract: During a weld period the weld parameters are monitored and data representing the weld resistance curve and the power curve are acquired and stored for analysis by a suitably programmed computer. The derivative of R, R is calculated and stored and the function is divided by power P to obtain R/P. The resistance curve is searched to obtain the maximum value R.sub.m which occurs during the heating phase and the function R/P is searched prior to the time of R.sub.m to find the maximum of that curve representing the highest rate of resistance increase. Then the R/P curve is searched subsequent to its maximum to determine when the function reaches a specified percentage of the maximum. That value occurs at the knee of the resistance curve and approximates the onset of melting in the weld.