Nugget size calculation device

Abstract

An example of a calculation device capable of calculating nugget diameter is disclosed. [Solution] In the calculation device 1, that is, the nugget diameter calculation method, the first calculated value Cv1 is calculated and determined using the value obtained by dividing the difference between the input power amount Ip and the heat dissipation amount Hd (=Ip-Hd) by the heat of fusion Hf, and the nugget diameter Nd is determined using the value obtained by multiplying the result of dividing the first calculated value Cv1 by the change amount ΔL by the expansion coefficient α of the workpiece. Therefore, the calculation accuracy of the first calculated value Cv1 is improved, and the accuracy of the calculated nugget diameter Nd is also improved.

Description

Technical Field

[0001] The present disclosure relates to an arithmetic device that calculates the size of a nugget (hereinafter also referred to as the nugget diameter) generated by resistance spot welding.

Background Art

[0002] For example, the method of calculating the nugget diameter described in Patent Document 1 calculates the diameter of the nugget using the measurement results of the stroke, current value, voltage value, and applied pressure between two electrodes. Specifically, in Patent Document 1, until welding is completed, the expansion amount of the nugget is sequentially added (integrated), and the value at the end of the addition is used as the nugget diameter.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present disclosure discloses an example of an arithmetic device capable of calculating the size of a nugget from a different perspective from Patent Document 1.

Means for Solving the Problems

[0005] The nugget size arithmetic device that calculates the size (Nd) of a nugget generated by resistance spot welding in which a plurality of stacked workpieces are sandwiched between two electrodes (3) and energized while being pressurized preferably includes at least one of the following constituent elements.

[0006] In other words, the configuration includes an input power detection unit (6B) that detects the amount of electricity supplied to the workpiece, a heat dissipation detection unit (6C) that detects the amount of heat that has moved from the molten part of the workpiece to a part other than the molten part after the start of energization, a change amount detection unit (6E) that detects the change in the distance between the two electrodes (3), a first calculation unit (6F) that calculates and determines a first calculated value (Cv1) using the difference between the amount of electricity detected by the input power detection unit (6B) and the amount of heat detected by the heat dissipation detection unit (6C), divided by the amount of heat required to melt a unit volume of workpiece, a second calculation unit (6G) that calculates and determines a second calculated value (Cv2) using the result of dividing the first calculated value (Cv1) by the change amount (ΔL) detected by the change amount detection unit (6E) and multiplying it by the expansion coefficient of the workpiece, and an output unit (6H) that outputs the second calculated value (Cv2) as the nugget size (Nd).

[0007] As a result, the nugget size calculation device uses a first calculated value (Cv1) calculated based on the difference between the amount of power detected by the input power detection unit (6B) and the amount of heat detected by the heat dissipation detection unit (6C) to calculate and determine the nugget size (Nd). Therefore, the calculation accuracy of the first calculated value (Cv1) is improved, and the accuracy of the calculated nugget size (Nd) is also improved.

[0008] Incidentally, the symbols in each of the parentheses above are just examples showing the correspondence with the specific configurations etc. described in the embodiments described later, and this disclosure is not limited to the specific configurations etc. indicated by the symbols in the parentheses above. [Brief explanation of the drawing]

[0009] [Figure 1] This is a conceptual diagram showing a nugget size calculation device according to the first embodiment. [Figure 2] A and B are diagrams illustrating the gaps. [Figure 3] This graph shows the relationship between applied pressure and expansion. [Figure 4] This is a flowchart showing the operation of the nugget size calculation device according to the first embodiment. [Figure 5]This graph shows the effect of the nugget size calculation device according to the first embodiment. [Modes for carrying out the invention]

[0010] The following "Embodiments of the Invention" are examples of embodiments that fall within the technical scope of this disclosure. In other words, the features defining the invention as described in the claims are not limited to the specific configurations and structures shown in the embodiments below.

[0011] At least one of the components or parts described with reference numerals is provided, unless otherwise specified, such as "one". The nugget size calculation device shown in this disclosure comprises at least one of the components described with reference numerals and at least one of the illustrated components.

[0012] (First Embodiment) <1. Nugget Size Calculator> <1.1 Overview of the Nugget Size Calculation Unit> This embodiment applies an example of a nugget size calculation device according to the present disclosure to a calculation method for calculating the size of a nugget (hereinafter also referred to as nugget diameter Nd) produced by resistance spot welding.

[0013] Nugget diameter Nd refers to the size (dimension) of the nugget, which is the largest molten area formed at the joint of a resistance spot weld, in the direction perpendicular to the direction of pressure applied during resistance spot welding.

[0014] Resistance spot welding is a type of electric resistance welding in which multiple stacked workpieces (for example, metal plates) are welded together by sandwiching them between two electrodes, applying pressure, and passing an electric current through them.

[0015] <1.2 Configuration of the Nugget Size Calculation Unit> As shown in FIG. 1, the nugget size calculator 1 according to the present embodiment is incorporated into the resistance spot welder 2 and integrated with the resistance spot welder 2. Hereinafter, including the resistance spot welder 2, it is simply referred to as the "nugget size calculator 1".

[0016] The nugget size calculator 1 includes at least two electrodes 3, an electric motor 4, a strain sensor 5, a control unit 6, etc. The two electrodes 3 conduct current to a workpiece such as a metal plate. The electric motor 4 moves the electrode 3 (in this embodiment, the electrode 3A). Thereby, the two electrodes 3 press the workpiece by sandwiching a plurality of stacked workpieces.

[0017] Note that an encoder 4A is incorporated in the electric motor 4. The encoder 4A detects the rotation angle of the rotor of the electric motor 4. The strain sensor 5 detects the strain generated in the frame 3C that holds the electrode 3.

[0018] That is, when the electrode 3 comes into pressure contact with the workpiece, the frame 3C is slightly deformed by the pressing force. The strain sensor 5 detects the deformation as strain. Then, the detection signals of the encoder 4A and the strain sensor 5 are input to the control unit 6.

[0019] <2. Configuration of the control unit> The control unit 6 executes the method for calculating the nugget diameter according to the present embodiment and controls the operation of the resistance spot welder 2.

[0020] Specifically, the control unit 6 includes a motor drive unit 6A, an input power detection unit 6B, a heat dissipation amount detection unit 6C, a gap detection unit 6D, a change amount detection unit 6E, a first calculation unit 6F, a second calculation unit 6G, an output unit 6H, etc.

[0021] Note that the control unit 6 is composed of dedicated hardware such as the motor drive unit 6A and a computer, etc. The computer executes detection, calculation, etc. in addition to the control commands for the motor drive unit 6A, etc. The computer has a CPU, a ROM, a RAM, etc.

[0022] Incidentally, the input power detection unit 6B to output unit 6H, etc., according to this embodiment are realized by software being executed by the CPU, i.e., the control unit 6. This software is pre-stored in a non-volatile storage unit (not shown), such as ROM.

[0023] <2.1 Motor Drive Unit> The motor drive unit 6A supplies a drive current to the electric motor 4. The drive current supplied by the motor drive unit 6A is controlled by the execution of the software described above.

[0024] <2.2 Input Power Detection Unit> The power input detection unit 6B detects the amount of energy (hereinafter referred to as the power input amount Ip) supplied to the workpiece via the electrode 3. Specifically, the power input detection unit 6B calculates and determines the power input amount Ip using the current value I supplied to the workpiece via the electrode 3, the electrical resistance value R between electrode 3A and electrode 3B, and the energizing time T. In other words, Ip = I^2 × R × T.

[0025] <2.3 Heat Dissipation Detection Unit> The heat dissipation detection unit 6C detects the amount of heat (hereinafter referred to as heat dissipation amount Hd) that has moved from the molten part of the workpiece to a part other than the molten part after the start of energization. In this embodiment, the heat dissipation detection unit 6C considers the sum of the electrode-side heat dissipation amount Hde and the workpiece-side heat dissipation amount Hdw as the heat dissipation amount Hd.

[0026] <Electrode side heat dissipation amount> The electrode-side heat dissipation amount Hde is the amount of heat transferred from the molten area to each electrode 3. The heat dissipation amount detection unit 6C considers the value obtained by dividing the temperature difference between the molten area temperature Mt and the electrode cooling water temperature Ct by the thermal resistance Tr1 as the electrode-side heat dissipation amount Hde. Specifically, Hde = (Mt - Ct) / Tr1.

[0027] Furthermore, there is a correlation between the temperature Mt of the molten section and the amount of expansion of the molten section. For this reason, the control unit 6 in this embodiment considers the temperature calculated and determined using the amount of expansion of the molten section to be the temperature Mt of the molten section.

[0028] Thermal resistance Tr1 is a value that indicates the "degree to which heat transfer is hindered" in the heat transfer path from the molten part to the electrode 3, and is a physical property that changes with temperature. For this reason, the control unit 6 in this embodiment considers the value calculated and determined using the temperature of the heat transfer path to be the thermal resistance Tr1.

[0029] Incidentally, the amount of heat dissipated Hd (especially the heat dissipated on the electrode side Hde) differs depending on whether a gap g exists between the workpieces (see Figure 2A) or not (see Figure 2B). In other words, when a gap g exists between the workpieces, the heat transfer path from the molten area to the electrode 3 changes compared to when the gap g does not exist, and the contact area between the electrode 3 and the workpiece changes. Therefore, the thermal resistance Tr1 changes depending on the size of the gap g.

[0030] Therefore, the heat dissipation detection unit 6C according to this embodiment corrects the value calculated and determined using the temperature of the heat transfer path based on the size of the gap g and the temperature of the heat transfer path, and considers the corrected value to be the thermal resistance Tr1.

[0031] In other words, the heat dissipation amount detection unit 6C according to this embodiment has a correction function for correcting the heat dissipation amount Hde. This correction function is a function that corrects the heat dissipation amount Hde calculated and detected using the size of the gap g detected by the gap detection unit 6D.

[0032] <Heat dissipation from the workpiece side> The heat dissipation amount Hdw on the workpiece side is the amount of heat transferred from the molten part to the non-molten part. The non-molten part refers to the part of the workpiece other than the molten part in question. The heat dissipation amount detection unit 6C considers the value obtained by dividing the temperature difference between the temperature Mt of the molten part and the temperature Nmt of the non-molten part by the thermal resistance Tr2 as the heat dissipation amount Hdw on the electrode side. Specifically, Hdw = (Mt - Nmt) / Tr2.

[0033] The thermal resistance Tr2 is a value that indicates the "degree to which heat transfer is hindered" in the heat transfer path from the molten portion to the non-molten portion, and is a physical property that changes with temperature. For this reason, the control unit 6 in this embodiment considers the value calculated and determined using the temperature Nmt of the non-molten portion as the thermal resistance Tr2.

[0034] <Temperature of the non-molten part> The control unit 6 uses the previous heat dissipation amount Hdw from the workpiece side to calculate and determine the current temperature Nmt (hereinafter referred to as "current temperature T(n)") of the non-melting part, which was used in the calculation, and the previous heat dissipation amount Hdw from the workpiece side (hereinafter referred to as "previous heat dissipation amount H(n-1)").

[0035] Specifically, the control unit 6 considers the current temperature T(n) to be the value obtained by adding the previous temperature T(n-1) to the value calculated and determined using the value obtained by dividing the previous heat dissipation amount H(n-1) by the specific heat of the non-molten part (hereinafter referred to as "temperature rise ΔT(n-1)").

[0036] At this time, before the power is turned on, the temperature Nmt of the molten part and the temperature Nmt of the non-molten part are the same. Then, after the power is turned on, the control unit 6, that is, the heat dissipation amount detection unit 6C, calculates and determines the heat dissipation amount Hd at predetermined intervals (for example, 1 millisecond).

[0037] Therefore, the temperature T(n) in this case corresponds to the cumulative value obtained by sequentially adding the "temperature rise ΔT" to the temperature To at the start of energization. In other words, T(n) = To + temperature rise ΔT1 + temperature rise ΔT2 + ... temperature rise ΔT(n-1). Note that this addition is performed each time the heat dissipation amount Hd is calculated and determined.

[0038] <2.4 Gap Detection Unit> The gap detection unit 6D detects the gap g between the workpieces (see Figure 2A). In this embodiment, the gap detection unit 6D calculates and determines the size of the gap g using the rate of change of the pressure applied to the electrode 3 and the rate of change of the distance between the electrodes.

[0039] The gap g between workpieces is the space that exists between multiple workpieces that are stacked on top of each other. In many cases, this gap g is due to the "warping" or "undulation" of the workpieces that already exist before the power is applied. Then, when pressure is applied to the workpiece by electrode 3 while a gap g exists, the rate of change in the applied pressure and the rate of change in the distance between electrodes become smaller, and the change in the distance between electrodes stabilizes (see Figure 3). Note that this pressure is applied before the energization begins.

[0040] Therefore, the gap detection unit 6D according to this embodiment considers the gap g between the workpieces to be the inter-electrode distance when the rate of change of the inter-electrode distance, that is, the rate of change of the distance between the two electrodes 3A and 3B (hereinafter referred to as the inter-electrode distance), falls below a predetermined rate of change.

[0041] Incidentally, if there is no warping or undulation in the workpiece, even if the workpiece is pressurized by electrode 3 before the start of energization, the rate of change in the distance between electrodes will be less than or equal to a predetermined rate of change from the start of pressurization.

[0042] <2.5 Change Amount Detection Unit> The change amount detection unit 6E detects the change amount ΔL in the distance between the two electrodes 3A and 3B. The change amount detection unit 6E detects the change amount ΔL using the detection signal from the encoder 4A and the detection signal from the strain sensor 5.

[0043] In this embodiment, the change ΔL that occurs after the start of energization is considered to be the amount of thermal expansion of the molten portion, and the following nugget diameter calculation method is performed. In other words, in this embodiment, the volumetric thermal expansion of the molten portion is considered to be mainly the change in the distance between electrodes.

[0044] <2.6 1st calculation section> The first calculation unit 6F calculates and determines the first calculation value Cv1 using the value obtained by dividing the difference between the input power Ip and the heat dissipation Hd (=Ip-Hd) by the heat of fusion Hf of the workpiece. Note that the heat of fusion Hf is "the amount of heat required to melt a unit volume of workpiece".

[0045] Specifically, the first calculated value Cv1 = (Ip - Hd) / Hf. The units are in the SI system. In other words, the first calculated value Cv1 represents the value obtained by converting the energy supplied to the molten part from electrode 3 (resistance spot welding machine 2) into a value equivalent to the volume (hereinafter also referred to as the theoretical molten part volume).

[0046] <2.7 Second calculation section> The second calculation unit 6G calculates and determines the second calculation value Cv2 using the result obtained by dividing the first calculation value Cv1 by the change amount ΔL and multiplying it by the expansion coefficient α of the workpiece. Note that the expansion coefficient α is also a physical property that changes in accordance with the temperature change of the workpiece (including the molten part).

[0047] Therefore, in this embodiment, the expansion coefficient α used to calculate the second calculated value Cv2 is determined using "the value obtained by multiplying the volume expansion coefficient of the workpiece (iron in this embodiment) by the temperature rise of the molten part" and "a value based on the amount of expansion due to melting (phase change from solid to liquid)". The temperature rise is the difference between the temperature of the molten part at the time of the previous calculation and the temperature of the molten part at the time of the current calculation.

[0048] By the way, the coefficient of volume expansion αv is (ΔV / V) / ΔT. However, V is the volume of the molten region during the previous calculation. ΔV is the difference between the volume of the molten region during the previous calculation and the volume of the molten region during the current calculation. ΔT is the temperature rise mentioned above.

[0049] In this embodiment, the volume expansion accompanying the temperature rise of the molten portion can be considered to proceed mainly in a direction that increases the distance between electrodes. Therefore, the expansion coefficient α in this embodiment can be considered in the same way as the linear expansion coefficient.

[0050] Specifically, the expansion coefficient α according to this embodiment can be considered as (ΔL / L) / ΔT. L corresponds to the distance of the portion of the molten material parallel to the distance between electrodes before expansion (before current is applied) (hereinafter referred to as the nugget height h).

[0051] In other words, "multiplying the result of dividing the first calculated value Cv1 by the change amount ΔL by the expansion coefficient α of the workpiece" means Cv1 / ΔL × α = Cv1 / (ΔL / α) = Cv1 / h. Note that in the above formula, ΔT = 1 is used to simplify the equation.

[0052] The first calculated value Cv1 corresponds to the theoretical molten volume, so "Cv1 / h" is the size corresponding to the cross-sectional area of ​​the theoretical molten volume. Since the cross-sectional area of ​​the molten region is empirically generally circular, in this embodiment, the second calculated value Cv2 is the value corresponding to the radius of that circle.

[0053] Specifically, the second calculated value Cv2 = 2 × [(Cv1 / ΔL × α) / π]^0.5. In this embodiment, the user can choose to set the second calculated value Cv2 to {A × 2 × [(Cv1 / ΔL × α) / π]^0.5}, where A is a predetermined correction coefficient.

[0054] <2.8 Output Section> The output unit 6H outputs the second calculated value Cv2 as the nugget diameter Nd. Specifically, the output unit 6H displays a numerical value representing the second calculated value Cv2 on a display (not shown). The user can select to display either one or both of the two types of second calculated values ​​Cv2.

[0055] <3. How to calculate the nugget diameter> <3.1 Overview of Calculation Method> The nugget diameter calculation method according to this embodiment involves calculating and determining a first calculated value Cv1 using the difference between the input power Ip and the heat dissipation Hd (=Ip-Hd) divided by the heat of fusion Hf, and then determining a second calculated value Cv2, i.e., the nugget diameter Nd, using the result of dividing the first calculated value Cv1 by the change amount ΔL and multiplying it by the expansion coefficient α of the workpiece.

[0056] <3.2 Details of the calculation method> Figure 4 is a flowchart detailing the calculation method. The calculation method shown in this flowchart is executed by the control unit 6. The software for executing this calculation method is stored in advance in the non-volatile memory unit mentioned above.

[0057] The control unit 6 first moves the electrode 3 to pressurize the workpiece (S1). This pressurization is carried out until the rate of change of the distance between the electrodes falls below a predetermined rate of change. After that, the control unit 6 determines a correction coefficient to correct the heat dissipation amount Hde on the electrode side based on the detected gap g (S3).

[0058] Next, the control unit 6 starts energizing for welding (S5). Subsequently, the control unit 6 performs calculations by the first calculation unit 6F and the second calculation unit 6G at predetermined intervals (for example, 1 millisecond) (S7).

[0059] Then, after a predetermined time has elapsed since the start of energization, the control unit 6 stops the energization (S9) and then makes it possible to output the nugget diameter Nd (S11). <4. Features of the nugget size calculation device and nugget diameter calculation method according to this embodiment> In the nugget size calculation device 1 according to this embodiment, that is, the nugget diameter calculation method, a first calculated value Cv1 is calculated and determined using the value obtained by dividing the difference between the input power amount Ip and the heat dissipation amount Hd (=Ip-Hd) by the heat of fusion Hf, and the nugget diameter Nd is determined using the value obtained by multiplying the result of dividing the first calculated value Cv1 by the change amount ΔL by the expansion coefficient α of the workpiece.

[0060] Therefore, the calculation accuracy of the first calculated value Cv1 is improved, and consequently, the accuracy of the calculated nugget diameter Nd is also improved. In other words, in this embodiment, the size corresponding to the cross-sectional area of ​​the molten region is calculated by dividing the theoretical molten region volume by the nugget height h, and the nugget diameter Nd is determined. Therefore, in order to accurately calculate and determine the nugget diameter Nd, it is necessary to accurately calculate and determine the theoretical molten region volume.

[0061] As described above, the theoretical molten zone volume is the value obtained by converting the energy supplied to the molten zone from electrode 3 (resistance spot welding machine 2) into a value equivalent to its volume. Therefore, in order to accurately calculate and determine the theoretical molten zone volume, it is necessary to accurately calculate and determine the energy supplied to the molten zone.

[0062] A portion of the energy supplied to the workpiece from electrode 3, i.e., the input power, is transferred as heat from the molten area to the non-molten area and back to electrode 3. In other words, the heat remaining in the molten area from the input power (energy) is the input power (energy) minus the heat dissipation amount Hd.

[0063] Therefore, in this embodiment, the energy injected into the molten portion can be calculated and determined with high accuracy, and thus the accuracy of the calculated nugget diameter Nd is also improved. As shown in Figure 5, in this embodiment, the calculated and determined nugget diameter Nd and the measured nugget diameter Nd match with high accuracy.

[0064] Incidentally, Figure 5 is a graph showing the relationship between the calculated and determined nugget diameter Nd and the measured nugget diameter Nd. In Figure 5, the horizontal axis represents the calculated and determined value, and the vertical axis represents the measured value.

[0065] The black dots on the graph represent the coordinates of the calculated / determined nugget diameter Nd and the measured nugget diameter Nd. The dashed line is a line connecting the coordinates where "calculated / determined nugget diameter Nd" = "measured nugget diameter Nd".

[0066] The heat dissipation amount detection unit 6C according to this embodiment calculates and detects the heat dissipation amount Hd according to a calculation formula that is stored in advance and utilizes the temperature difference between the molten part and the non-molten part. As a result, the nugget size calculation device 1 can significantly reduce the calculation time compared to methods that calculate heat dissipation using analysis methods such as the finite element method. Consequently, the nugget size calculation device 1 can output the nugget diameter Nd in a very short time, either during or after welding.

[0067] The heat dissipation detection unit 6C has a correction function that corrects the calculated and detected heat dissipation amount using the size of the gap g. This can improve the accuracy of the calculated nugget diameter Nd. (Other embodiments) In the embodiments described above, the size of the nugget was shown as being equivalent to its diameter. However, this disclosure is not limited thereto. That is, the disclosure may also show the size of the nugget as being equivalent to its area or radius, for example.

[0068] In the embodiments described above, the nugget diameter could be output as either a corrected or uncorrected value. However, this disclosure is not limited to this. That is, the disclosure may, for example, be configured to output only one of the values.

[0069] In the above-described embodiment, the system was configured to perform corrections when calculating and detecting the first calculated value Cv1, the second calculated value Cv2, the expansion coefficient α, the thermal resistance Tr, and the heat dissipation amount Hd. However, this disclosure is not limited thereto.

[0070] In other words, the presence or absence of correction, and the correction method, are not limited to the methods shown in the embodiments described above. For this reason, for example, a configuration may be available in which the "function for correcting the thermal resistance Tr1 for calculating the electrode-side heat dissipation amount Hde based on the gap g" is abolished.

[0071] In the above-described embodiment, the change amount ΔL was detected using the detection signal of the encoder 4A and the detection signal of the strain sensor 5. However, the disclosure is not limited thereto. That is, the disclosure may, for example, use laser light to detect the change amount.

[0072] In the embodiments described above, the temperature of the molten portion was detected by utilizing the relationship between the amount of expansion and the temperature. However, the disclosure is not limited thereto. That is, the disclosure may, for example, detect the temperature of the molten portion by utilizing a heat ray emitted from the molten portion. Similarly, the temperature of the non-molten portion may be detected by utilizing a heat ray emitted from the non-molten portion.

[0073] The control unit 6 according to the above embodiment had a first arithmetic unit 6F and a second arithmetic unit 6G, respectively. However, this disclosure is not limited thereto. That is, the disclosure may, for example, be a single arithmetic unit in which the functions of the first arithmetic unit 6F and the functions of the second arithmetic unit 6G are integrated. Alternatively, the functions of each arithmetic unit 6F and 6G may be separated.

[0074] Furthermore, this disclosure is not limited to the embodiments described above, but is sufficient to be consistent with the intent of the disclosures described in the embodiments described above. Therefore, it may be a configuration in which at least two of the embodiments described above are combined, or a configuration in which any of the illustrated components or components described with reference numerals in the embodiments described above are omitted. [Explanation of symbols]

[0075] 1… Nugget size calculation unit 2… Resistance spot welding machine 3… Electrode 4… Electric motor 5… Strain sensor 6… Control Unit 6A… Motor drive unit 6B... Input power detection unit 6C… Heat dissipation detection unit 6D… Gap detection unit 6E… Change amount detection unit 6F... 1st calculation section 6G... 2nd calculation section 6H… Output section

Claims

1. In a nugget size calculation device that calculates the size of a nugget produced by resistance spot welding, which involves welding multiple stacked workpieces by clamping them between two electrodes, applying pressure, and then applying an electric current, An input power detection unit that detects the amount of power supplied to the workpiece, A heat dissipation amount detection unit detects the amount of heat transferred from the molten part of the workpiece to a part other than the molten part after the start of power application, A change amount detection unit for detecting the amount of change in the distance between the two electrodes, A first calculation unit calculates and determines a first calculation value using the value obtained by dividing the difference between "the amount of power detected by the input power detection unit" and "the amount of heat detected by the heat dissipation detection unit" by "the amount of heat required to melt a unit volume of workpiece", A second calculation unit calculates and determines a second calculation value using the result obtained by dividing the first calculation value by the change amount detected by the change amount detection unit and multiplying it by the expansion coefficient of the workpiece, An output unit that outputs the above second calculation value as the size of the nugget, A nugget size calculation device equipped with the following features.

2. The nugget size calculation device according to claim 1, wherein the heat dissipation amount detection unit calculates and detects the amount of heat according to "a calculation formula that is stored in advance and utilizes the temperature difference between the molten part and the part other than the molten part."

3. It is equipped with a gap detection unit that detects the gap between workpieces, The nugget size calculation device according to claim 2, wherein the heat dissipation amount detection unit has a correction function that corrects the amount of heat to be calculated and detected using the size of the gap detected by the gap detection unit.

4. In a method for calculating the size of a nugget produced by resistance spot welding, which involves welding multiple stacked workpieces by clamping them between two electrodes and applying pressure while energizing them, When the amount of electricity supplied to the workpiece is defined as the input power, the amount of heat transferred from the molten part of the workpiece to the part other than the molten part after the start of energization is defined as the heat dissipation amount, and the change in the distance between the two electrodes is defined as the change amount, A method for calculating nugget size, which involves calculating and determining a calculated value using the difference between the input power and the heat dissipation amount divided by the amount of heat required to melt a unit volume of workpiece, and then determining the size of the nugget using the value obtained by multiplying the result of dividing the calculated value by the amount of change by the expansion coefficient of the workpiece.

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