Spot welding device and welding method

Abstract

To provide a spot welding device and a welding method that are able to accurately determine whether a weld zone (nugget) is good or bad even if the plate thickness changes.SOLUTION: The device includes a welding gun 10. The welding gun 10 comprises a pair of electrodes 12A, 12B and a pair of gun arms 11A, 11B for holding them, respectively. The pair of electrodes 12A, 12B is moved in mutually approaching directions by moving the gun arms 11A, 11B by means of electrode contacting / separating means 15, and a member to be welded 30 is pressurized and energized for welding. The device includes a strain detector 40 provided to the welding gun 10, and determination means 28 for determining the quality of welding on the basis of the polarity of a strain displacement amount, which is a temporal change in the amount of strain detected by the strain detector 40.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spot welding apparatus and a welding method for performing spot welding that can determine the quality of a welded portion (nugget) in spot welding and guarantee the quality of the weld. [Background technology]

[0002] In the field of resistance welding, typified by spot welding, a method for determining the quality of welding in real time (during welding) has been known in the past, in which the properties of the welded portion (nugget) formed on the welded material are determined.

[0003] For example, Patent Document 1 proposes a method for detecting the amount of movement between welding electrodes, which derives the amount of movement between welding electrodes by adding the amount of movement of the electrode drive part in the electrode movement direction due to expansion and contraction of the welded part (nugget) during welding and the amount of deflection of the welding gun due to the pressure applied from the electrode to the workpiece. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3593981 Summary of the Invention [Problem to be solved by the invention]

[0005] However, while Patent Document 1 estimates the expansion and contraction of the nugget based on the correlation between the inter-electrode movement amount and time, this expansion and contraction of the nugget deforms the welding equipment and is therefore not detected as the inter-electrode movement amount.In addition, since there are many variable and irregular factors such as the material, plate thickness, and welding conditions of the welded parts, and welding speed is also required in mass production sites, it is thought that this method is not suitable for detecting the quality of welding in-line.

[0006] Specifically, when welding workpieces of different thicknesses, it is necessary to prepare multiple pieces of data correlating the inter-electrode movement amount and time in order to determine whether the nugget is good or bad. Since the current measurement value is compared and determined based on these multiple correlation data, it takes a long time to make the determination, which results in a slow welding speed.

[0007] Therefore, a main object of the present invention is to provide a spot welding apparatus and a welding method that can accurately determine the quality of a welded portion (nugget) even if the plate thickness changes, for example, in an in-line welding process. [Means for solving the problem]

[0008] The embodiment that solves the above problem is as follows. (First aspect) A device for welding a pair of electrodes attached to a gun arm of a welding gun by moving a drive part of an electrode drive means having a drive part connected to at least one of the pair of electrodes to open and close the electrodes relative to each other, and for welding materials to be welded by applying pressure and passing current through the pair of electrodes, a strain detector provided on the gun arm; a determining means for determining whether welding is good or bad based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector; A spot welding device comprising:

[0009] (Second aspect) A method for welding a pair of electrodes attached to a gun arm of a welding gun in a direction toward each other by moving a drive unit of an electrode drive means having a drive unit connected to at least one of the pair of electrodes, and applying pressure to and current through the pair of electrodes to weld members, comprising: a strain detector is provided on the gun arm; determining whether the welding is good or bad based on the polarity of the strain displacement amount, which is a time-varying change in the strain amount detected by the strain detector; Spot welding method. [Effects of the Invention]

[0010] According to the present invention, by focusing on the fact that the polarity of the strain displacement of the gun arm changes from positive to negative over time during the application of pressure and current during the expansion and contraction of the nugget portion of a spot weld, it is possible to determine the quality of the weld, even in an in-line welding process, by determining the time change in the polarity of the strain displacement. This provides a spot welding device and welding method that can accurately determine the quality of the weld (nugget) even if the plate thickness changes. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of a spot welding device. [Figure 2] 10 is a graph showing changes in strain amount and strain displacement amount relating to the explanation of terms. [Figure 3] 10 is a graph showing an example of the change over time in the amount of strain and the amount of strain displacement detected by a strain detector, and the welding current in the case of a good weld. [Figure 4] 10 is a graph showing an example of changes over time in the amount of strain and the amount of strain displacement detected by a strain detector, and in the welding current in the case of a defective weld. [Figure 5] 10 is a graph showing an example of changes over time in the amount of strain and the amount of strain displacement detected by a strain detector, and in the welding current when spatter occurs. [Figure 6] FIG. 10 is an explanatory diagram illustrating the transition of the welding state over time in the case of a good weld. [Figure 7] FIG. 10 is an explanatory diagram illustrating the transition of the welding state over time in the case of a defective weld. [Figure 8] FIG. 10 is an explanatory diagram illustrating a case where tip dressing or tip replacement is required. DETAILED DESCRIPTION OF THE INVENTION

[0012] Next, embodiments of the present invention will be described in detail below with reference to the drawings.

[0013] (Welding equipment overview) FIG. 1 shows the main part of the welding device, in which a welding gun 10 has a pair of a first gun arm 11A and a second gun arm 11B. First gun arm 11A and second gun arm 11B hold first electrode 12A and second electrode 12B at their distal ends, respectively.

[0014] The intermediate portions of the first gun arm 11A and the second gun arm 11B are connected by a support shaft 13. The first gun arm 11A and the second gun arm 11B are connected to a holding arm 14, which is connected to the tip of a robot arm (not shown), as a means for moving them. The tip of the holding arm 14 holds both the first gun arm 11A and the second gun arm 11B so that they can swing around the support shaft 13.

[0015] An extension mechanism 15 of the electrode approaching and separating means is provided at the base end of the first gun arm 11A, and the tip of the rod thereof is connected to the base end of the second gun arm 11B. As a result, as the extension / retraction mechanism 15 extends and retracts, the first gun arm 11A and the second gun arm 11B rotate around the support shaft 13, and the first electrode 12A of the first gun arm 11A and the second electrode 12B of the second gun arm 11B move closer to and away from each other (open and close). When the first electrode 12A and the second electrode 12B are moved in a direction approaching each other, a predetermined pressure is applied to the target welded members (e.g., overlapping automotive steel plates) 30 via the tips at the ends of the first electrode 12A and the second electrode 12B.

[0016] The welding apparatus in the embodiment is equipped with a control device 20, which can include a central processing unit (CPU) 22 that performs various signal processing related to the welding apparatus including the robot and the above-mentioned welding gun 10, an auxiliary processing unit 24, and a memory device 26.

[0017] The central processing unit 22 controls the welding pressure required for welding by outputting commands to the extension / retraction mechanism 15 to apply or release pressure to the first electrode 12A and the second electrode 12B in accordance with a welding program previously stored in the storage device 26. Furthermore, the central processing unit 22 also controls the current applied to the electrodes. Although the welding device in the embodiment maintains constant values ​​of the welding pressure and welding current, they may be changed.

[0018] The storage device 26 stores a welding program and welding conditions (welding pressure, welding current, current application time, current application interval), etc.

[0019] When the welding gun 10 is mounted on an articulated robot, the control device 20 can be built into the robot control device (not shown) or can be independently arranged in parallel.

[0020] (Transition of welding condition) The transition of the welding condition over time will be outlined below. FIG. 6 is a schematic diagram illustrating the process of producing a nugget in the period from the point at which the first electrode 12A and the second electrode 12B in the welding gun 10 are moved in a direction toward each other (closing) and a predetermined pressure force is applied to the two overlapping welded members 30 to the point at which the welding gun 10 begins to open.

[0021] When current begins to flow, resistance heating begins around the interface where the welded parts 30 overlap, the base material is heated and expands toward the electrodes (stage S1), and with this expansion, a reaction force (expansion force) in the opening direction from the base material side to the first electrode 12A and the second electrode 12B moving away from each other increases (stage S2).

[0022] After that, the base material continues to expand, but on the other hand, the temperature of the base material rises due to resistance heating and it begins to melt and soften. As a result, the force (push-in force) that moves the first electrode 12A and the second electrode 12B closer to each other due to the applied pressure takes precedence over the expansion force (stage S3). When this force reaches its maximum, it is determined that the welded portion (nugget) Na has been sufficiently formed, and the current flow is terminated (stage S4). When the cooling is finally completed, the first electrode 12A and the second electrode 12B are separated from each other, and the welding gun 10 is opened (Step S5). Once the above steps have been completed, sufficient welded portion (nugget) Na will be produced, and it can be determined that the welding is performed well.

[0023] In contrast, if the stage shown in FIG. 7 is passed, the welded portion (nugget) Na is not sufficiently formed, and it can be determined that the welding is defective. That is, even though current is turned on and resistance heating of the workpieces 30 begins (stage S1), sufficient heat is not generated and the expansion of the base metal portions to be welded is slight (stage S2). Therefore, the reaction force in the opening direction that separates the first electrode 12A and the second electrode 12B from each other from the base metal side also increases slightly, but to a small extent (stage S3).

[0024] Thereafter, although the welded portion (nugget) Na continues to expand, the current flow is completed without sufficient melting or softening of the base material (stage S4). When cooling is finally completed, first electrode 12A and second electrode 12B are separated from each other, and welding gun 10 is opened (Step S5). In this type of case, sufficient formation of a welded portion (nugget) is not observed, and the welded portion (nugget) Na is small, or peeling of the two-ply welded members 30 occurs.

[0025] (Weld quality judgement) In this embodiment, a strain detector 40 is provided in welding gun 10 to determine whether the welding is good when the process shown in FIG. 6 is observed and whether the welding is poor when the process shown in FIG. The strain detector 40 may be installed at either the first gun arm 11A or the second gun arm 11B. Although it is possible to provide a distortion detector 40 on both the first gun arm 11A and the second gun arm 11B, this would complicate the processing of distortion signals, so it is sufficient to provide one of the gun arms. In this embodiment, it is provided on the first gun arm 11A. Note that providing it on the fixed gun arm allows for a more stable waveform to be acquired.

[0026] As shown in Figure 2, the strain detector 40 defines the difference between two consecutively sampled strain amounts Lt1 and Lt2 as strain displacement ΔLt2 = Lt2 - Lt1. The polarity of the strain displacement is positive (+) due to the expansion process. On the other hand, during the pressing process, the difference between the two strain amounts Lt3 and Lt4 is defined as strain displacement ΔLt4 = Lt4 - Lt3, and the polarity of the strain displacement is negative (-). In this way, the quality of the weld is judged based on the change over time in the polarity of the strain displacement under current and pressure. The change over time in the amount of strain detected by the strain detector 40 is shown in both FIG. 6 and FIG. In the case of "good welding" shown in Figure 6, when current begins to flow, the gun arm opens, so the strain amount shows a positive change in displacement, and after reaching a maximum in a relatively short period of time, the base material melts and the gun arm is pushed in, gradually showing a negative change in displacement. When the energization is completed and cooling is started, the degree of change in the negative displacement of the strain increases until the cooling is completed.

[0027] In the case of the "poor welding" shown in Figure 7, the strain amount shows a positive change in displacement from the start of energization until the end of energization, but the rate of change in the strain amount is slow even after a long time has passed. When the energization is completed and cooling is started, the degree of change in the negative displacement of the strain increases until cooling is completed.

[0028] From the comparative explanation of Figures 6 and 7, it can be inferred that capturing the change over time in the amount of strain displacement detected by the strain detector 40 during pressure and current application is effective for determining the properties of the spot weld, i.e., the formation of a nugget. The control device 20 shown in FIG. 1 includes a determining means 28 for determining whether the nugget has been formed, that is, for determining whether the spot welding is good or bad.

[0029] An example of the output of the strain detector 40 in the case of a "good weld" is shown in Fig. 3. This is an example of the output of the strain detector 40 for each sampling unit time (for example, 20 milliseconds). When current is applied, resistance heating causes expansion, increasing the amount of strain. The overlapping workpieces 30 are heated, and although the change in strain slows down after that, at a certain point, the base material (workpieces 30) begins to melt, mainly at the interface between the overlapping workpieces. As the current continues to flow and heating continues, the base material continues to melt and soften, and as a result, the nugget in which the base material melts grows and softens, and the pressure applied by the first electrode 12A and the second electrode 12B becomes dominant over the expansion, resulting in a period in which a force is generated in the direction in which the first electrode 12A and the second electrode 12B approach each other (this is the period marked "pressing" in Figure 3). The term "pushing in" means that as a result of the base material melting and softening, the pressure force of the first electrode 12A and the second electrode 12B becomes dominant over the expansion force, and the workpiece 30 is pushed in. When the pressure applied by the first electrode 12A and the second electrode 12B becomes dominant over the expansion and this period continues for a predetermined time, it can be determined that a sufficient amount of welded portion (nugget) Na has been formed. Thereafter, the welding current is reduced to zero and the current supply is terminated. Once cooling is complete, first electrode 12A and second electrode 12B are separated from each other, and welding gun 10 is opened.

[0030] In the progress of the above welding, the welded portion (nugget) continues to expand, while at the same time the base material begins to melt and soften, so that the pressure of the first electrode 12A and the second electrode 12B becomes dominant over the expansion, and a period in which a force is generated in the direction in which the first electrode 12A and the second electrode 12B approach each other (the "push-in" period) occurs during the period of current flow due to steady current flow, which means that the welded portion (nugget) Na has been sufficiently formed. As shown in the example output of the strain detector 40 in FIG. 3, during the "pressure" period, the "polarity" indicating whether the "displacement amount" of the strain amount at a certain sampling point relative to the strain amount at the previous sampling point is positive or negative is negative. Therefore, if multiple consecutive samples with negative "polarity" are taken partway through the period of energization with steady current, it can be determined that the welding is good.

[0031] 4 shows an example of the output of the strain detector 40 in the case of a "poor weld." This occurs when the electrode abuts on the workpiece at an angle, the electrode tip is worn, the current density is low, or dirt or other debris adheres to the overlapping interface of the workpieces, making it impossible to obtain an appropriate current density to produce a nugget. In this case, the resistance heat is small, so the pressure applied to the first electrode 12A and the second electrode 12B does not prevail over the expansion. Therefore, it is shown that the "no pressing" state continues throughout the entire energization period with a steady current. Moreover, throughout the entire energization period of the steady energization current, the "polarity" is positive and never becomes negative. This expansion and push-in phenomenon is unrelated to the thickness of the welded parts, and welding judgment based on this amount of strain displacement is faster than conventional welding judgment based on the amount of strain for each plate thickness, making it possible to inspect welding during automatic welding on a mass production line.

[0032] As described above, when the change in polarity of the strain displacement over time is negative, it can be determined that the weld is good. However, although a good weld can be determined if the polarity of the strain displacement changes negatively only once, the determination may be unstable. For example, even if the polarity of the strain displacement changes negatively only once, it may later change to positive due to the vibration of the welding gun or the reaction force of the plunger. Therefore, it is desirable to judge the welding to be good when the polarity is negative or zero repeatedly. For example, as shown in Figure 3, there are three consecutive negative readings, namely the first sampling, which marks the beginning of the "good welding" period, the second sampling, and the third sampling. In such a case, it can be determined with stability that the welding is good. Furthermore, if the polarity alternates between negative and zero, the welding can be determined to be good.

[0033] FIG. 5 shows the change in the amount of strain and the change in the amount of strain displacement when sputtering occurs. When sputtering occurs, there may be multiple times when the polarity is negative. When sputtering occurs, the molten metal splashes, causing a sudden push-in, which can result in a sudden change in polarity to the negative side, followed by multiple periods of negative polarity. Therefore, if the polarity suddenly changes to the negative side and then there are multiple instances of the polarity being negative, it can be determined that spatter has occurred. The record of spatter occurrence can also be used as data for adjusting the welding conditions for the next welding. When compared with the case where spatter occurs when the polarity is negative, the meaning of determining that the weld is good based on the criteria explained in the previous paragraph 0032 becomes even clearer.

[0034] On the other hand, in order to determine whether the welding is good or bad, it is desirable to incorporate a monitoring device (not shown) for the primary current value during welding, in consideration of the possibility of unexpected factors occurring, and to determine that the welding is good when both of the following conditions are met: when the polarity is negative, or when negative or zero is repeated multiple times, and when the welding current from the monitoring device for the primary current value is constant.

[0035] When determining the quality of spot welding, it is desirable that the device shown in FIG. 1, for example, can be used in common even when the thickness and number of welded members change in a plurality of combinations. Fortunately, according to the above embodiment, a good weld can be determined when the polarity is negative, or when the polarity is negative or zero multiple times. Even if the plate thickness of the welded member is changed, or even if the degree of strain displacement differs, the polarity remains the same, and the same determination method can be used to make the determination.

[0036] These judgments are made by judgment means 28, but in order to not only judge the quality of the weld but also to monitor the welding state or ensure long-term welding stability, an additional welding quality assurance judgment means can be provided. This welding quality assurance judgment means records and outputs the temporal change in the polarity of the strain displacement amount for each production lot in storage device 26, thereby ensuring production quality. In other words, traceability can be established because the quality of each and every item in a lot is guaranteed before delivery.

[0037] The first function of the welding quality assurance judgment means is to monitor that, when the polarity of the strain displacement is positive during the upslope process in which the welding current is gradually increased and the period thereafter, the expansion process is in progress, and subsequently the melting of the workpiece is initiated, as shown in Figures 8 and 6, which are reproductions of Figure 3.

[0038] On the other hand, since the electrode tip wears due to multiple welding operations, tip dressing or tip replacement has generally been performed periodically depending on the number of welding points. Tip dressers and the use of tip dressers for this purpose are disclosed in Japanese Patent Application Laid-Open No. 2021-79399 and Japanese Patent Application Laid-Open No. 2018-103200, etc. The above-described embodiment can be used as an indicator of tip dressing or tip replacement timing.

[0039] For this purpose, as a second function of the welding quality assurance judgment means, during the upslope process, when the polarity is positive as shown in Figures 3 and 8, an upper limit value for the distortion displacement in the positive polarity region in the case of a good chip at the limit of use can be determined in advance, and for example, an upper limit threshold value S for the distortion displacement can be set to "plus 7".

[0040] At a certain point when the wires are actually stacked at the welding point, if the time-dependent change in polarity of the strain displacement detected by the strain detector 40 changes from positive to negative, and the positive displacement falls below the threshold value S (i.e., does not exceed the threshold value S), it is determined that the tip has deteriorated or that irregular welding has caused the target part of the welded material to expand but not melt sufficiently, and the tip can be dressed or replaced using a tip dresser, or the irregularity can be resolved.

[0041] Furthermore, when the polarity of the strain displacement detected by the strain detector 40 is positive and the duration of the positive polarity is longer than a predetermined duration (as shown by Ca1 in Figure 8), that is, when it is the time when a good tip would have started to be pushed in, the welding quality assurance judgment means can determine that no push-in is occurring and that tip dressing or tip replacement is required.

[0042] Furthermore, even if the polarity of the strain displacement detected by the strain detector 40 changes over time to negative, but the degree of negative polarity is extremely large (as shown by Ca2 in FIG. 8), it can be determined that tip dressing or tip replacement is necessary. In the case of Ca2, welding is insufficient, and there is a risk of peeling.

[0043] Furthermore, instead of periodically dressing or replacing the tip, the wear state of the electrode can be detected while it is in use and the timing for dressing or replacing the tip can be determined, thereby improving environmental friendliness by extending the life of the electrode.

[0044] The distortion detector can output a distortion detection signal at intervals of 20 milliseconds or less. If the interval is too long, it is difficult to determine whether the polarity is correct. If the interval is too short, noise may be mixed in, making the determination unstable.

[0045] It is preferable to provide a device for monitoring the resistance and voltage between the tips of the first electrode 12A and the second electrode 12B and combine this with the welding quality assurance and determination means. [Explanation of symbols]

[0046] 10...Welding gun 11A, 11B...Gun arm 12A,12B…electrode 15…Expansion mechanism (electrode contact / separation means) 20...Control device 28…Judgment means 30...Welded parts 40...Distortion detector

Claims

1. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; The gun arm is opened and closed by an electrode contact / separation means, thereby moving the pair of electrodes in a direction in which they approach each other, and welding is performed by applying pressure to and passing current through the workpieces to be welded, a strain detector provided on the gun arm; a determining means for determining whether welding is good or bad based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector during pressure application and current application, The determining means determines that the welding is good when the polarity changes from positive to negative during pressure application and current flow. Spot welding equipment.

2. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; The gun arm is opened and closed by an electrode contact / separation means, thereby moving the pair of electrodes in a direction in which they approach each other, and welding is performed by applying pressure to and passing current through the workpieces to be welded, a strain detector provided on the gun arm; a determining means for determining whether welding is good or bad based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector during pressure application and current application, The determining means determines that the welding is good when the polarity changes from positive to negative during pressure application and current application, and when the negative polarity, positive polarity, or zero polarity is repeated multiple times. Spot welding equipment.

3. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; The gun arm is opened and closed by an electrode contact / separation means, thereby moving the pair of electrodes in a direction in which they approach each other, and welding is performed by applying pressure to and passing current through the workpieces to be welded, a strain detector provided on the gun arm; a determining means for determining whether welding is good or bad based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector during pressure application and current application, The determining means determines that the welding is good when both of the following conditions are met: the polarity changes from positive to negative during pressure application and current flow, and the negative polarity is repeated multiple times with a positive polarity or zero polarity; and the welding current is constant. Spot welding equipment.

4. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; A method for welding by opening and closing the gun arm using an electrode contacting / separating means, thereby moving the pair of electrodes in a direction toward each other and applying pressure and current to the workpieces to be welded, a strain detector is provided on the gun arm; The quality of the welding is determined based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector, and When the polarity changes from positive to negative during pressure application and current application, or when the negative polarity or zero polarity is repeated multiple times, the welding is judged to be good. Spot welding method.

5. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; A method for welding by opening and closing the gun arm using an electrode contacting / separating means, thereby moving the pair of electrodes in a direction toward each other and applying pressure and current to the workpieces to be welded, a strain detector is provided on the gun arm; The quality of the welding is determined based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector, and Even when the plate thickness of the workpieces to be welded is of multiple types, the welding is judged to be good when the value changes from positive to negative during pressure application and current flow, and when the negative or zero state is repeated multiple times. Spot welding method.

6. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; A method for welding by opening and closing the gun arm using an electrode contacting / separating means, thereby moving the pair of electrodes in a direction toward each other and applying pressure and current to the workpieces to be welded, a strain detector is provided on the gun arm; The quality of the welding is determined based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector, and The welding quality assurance determination means further comprises a welding quality assurance determination means that determines that the melting of the workpiece is insufficient and that the workpiece is in an expansion process when the polarity of the strain displacement amount, which is the change in the strain amount over time detected by the strain detector, is only positive. Spot welding method.

7. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; A method for welding by opening and closing the gun arm using an electrode contacting / separating means, thereby moving the pair of electrodes in a direction toward each other and applying pressure and current to the workpieces to be welded, a strain detector is provided on the gun arm; The quality of the welding is determined based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector, and The welding quality assurance determination means further comprises a welding quality assurance determination means that determines that tip dressing or tip replacement is required when the polarity of the strain displacement amount, which is a time-varying change in the strain amount detected by the strain detector, is negative and the negative amount of the strain displacement amount exceeds a predetermined threshold value. Spot welding method.

8. a welding gun including a pair of electrodes and a pair of gun arms each holding the pair of electrodes; A method for welding by opening and closing the gun arm using an electrode contacting / separating means, thereby moving the pair of electrodes in a direction toward each other and applying pressure and current to the workpieces to be welded, a strain detector is provided on the gun arm; The quality of the welding is determined based on the polarity of the strain displacement amount, which is a time change in the strain amount detected by the strain detector, and The welding quality assurance determination means further includes a welding quality assurance determination means for determining any one of the following (1) to (4): (1) When the polarity of the strain displacement amount, which is the change in the strain amount over time detected by the strain detector, is only positive, it is determined that the melting of the workpiece is insufficient and that the workpiece is in the process of expanding. (2) When the polarity of the strain displacement amount, which is the change in the strain amount over time detected by the strain detector, is positive and the positive amount of the strain displacement amount is below a predetermined threshold, it is determined that tip dressing or tip replacement is required or that irregular welding is occurring. (3) When the polarity of the strain displacement amount, which is the change in the strain amount over time detected by the strain detector, is positive and the duration of the positive polarity is longer than a predetermined duration, it is determined that tip dressing or tip replacement is required. (4) When the polarity of the strain displacement amount, which is a change in the strain amount over time detected by the strain detector, is negative and the negative amount of the strain displacement amount exceeds a predetermined threshold, it is determined that tip dressing or tip replacement is required. Furthermore, the welding quality assurance determination means records the change in polarity of the strain displacement amount over time in each production lot in a storage device and outputs the record. Spot welding method.

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