Indirect spot welding apparatus

Abstract

In the indirect spot welding apparatus, a lower limit in a stable pressing force region where the pressing force of the spot welding electrode against the metal sheets can be controlled within a tolerance of 10% ranges from 70 N to 200 N and an upper limit in the stable pressing force region ranges from 800 N to 2000 N, and an overshoot OS(%)=(PLAL)/AL100 of the pressing force occurring when the spot welding electrode is pressed against the metal sheets is controlled to be 10% or less.

Claims

1. An indirect spot welding apparatus configured to weld a member composed of at least two overlapping metal sheets by pressing a spot welding electrode against the metal sheets while applying pressure to the spot welding electrode from one side of the member, attaching a feeding point to the metal sheet on the other side of the member at a location separate from the spot welding electrode, and passing a current between the spot welding electrode and the feeding point, wherein the apparatus comprises a drive unit for pressing the spot welding electrode and controlling the pressing force, and a current controller for applying the current to the spot welding electrode, and wherein the drive unit is operable such that a lower limit in a stable pressing force region where the pressing force of the spot welding electrode against the metal sheets can be controlled within a tolerance of 10% ranges from 70 N to 200 N and an upper limit in the stable pressing force region ranges from 800 N to 2000 N, an overshoot of the pressing force occurring when the spot welding electrode is pressed against the metal sheets is represented by OS in Equation (1), and OS is controlled to be 10% or less, and a contact speed of the spot welding electrode being pressed against the metal sheets ranges from 1 mm/s to 30 mm/s:
OS(%)=(PLAL)/AL100(1) where PL represents a peak pressing force (N) in an overshoot range and AL represents an average pressing force (N) applied by the electrode.

2. The indirect spot welding apparatus according to claim 1, wherein the drive unit is configured to linearly move the spot welding electrode to apply pressure to the member to be welded; the drive unit uses a servomotor as a drive source; and when the pressing force is a lower limit in the stable pressing force region, an output of the servomotor is 30% or more of a rated output.

3. A method for indirect spot welding comprising welding a member composed of at least two overlapping metal sheets by pressing a spot welding electrode against the metal sheets while applying pressure to the spot welding electrode from one side of the member, attaching a feeding point to the metal sheet on the other side of the member at a location separate from the spot welding electrode, and passing a current between the spot welding electrode and the feeding point, wherein a mounted drive unit configured to linearly move the spot welding electrode to apply pressure to the member to be welded uses a servomotor as a drive source; and wherein a lower limit in a stable pressing force region where the pressing force of the spot welding electrode against the metal sheets can be controlled within a tolerance of 10% ranges from 70 N to 200 N and an upper limit in the stable pressing force region ranges from 800 N to 2000 N, an overshoot of the pressing force occurring when the spot welding electrode is pressed against the metal sheets is represented by OS in Equation (1 ), and OS is controlled to be 10% or less, and wherein a contact speed of the spot welding electrode being pressed against the metal sheets ranges from 1 mm/s to 30 mm/s:
OS(%)=(PLAL)/AL100(1) where PL represents a peak pressing force (N) in an overshoot range and AL represents an average pressing force (N) applied by the electrode.

4. The method for indirect spot welding according to claim 3, wherein, when the pressing force is a lower limit in the stable pressing force region, an output of the servomotor is 30% or more of a rated output.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(a) illustrates how welding is performed by a direct spot welding method, FIG. 1(b) illustrates how welding is performed by a series spot welding method, and FIG. 1(c) illustrates how welding is performed by an indirect spot welding method.

(2) FIG. 2 illustrates a drive unit of an indirect spot welding apparatus according to an embodiment of the present invention.

(3) FIG. 3 illustrates an indirect spot welding apparatus according to an embodiment of the present invention.

(4) FIG. 4 illustrates an indirect spot welding apparatus used in EXAMPLE 3.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(5) An embodiment of the present invention will be described on the basis of the drawings.

(6) FIG. 2 illustrates a drive unit mounted in an indirect spot welding apparatus (indirect spot welding robot apparatus) according to an embodiment of the present invention.

(7) In the embodiment of the present invention, the indirect spot welding robot apparatus needs to have a drive unit mounted therein which uses a servomotor as a drive source to apply pressure to a member to be welded. As illustrated in FIG. 2, a drive unit 30 has a casing 32 containing a motion conversion mechanism. A rotational motion of a servomotor 31 is converted via the casing 32 to a linear motion of a linearly-movable rod 33, which allows a spot welding electrode 34 (23) to apply pressure to the member to be welded. The drive unit 30 is preferably secured to a welding robot by a sufficiently rigid bracket 35 or the like without using an elastic body, such as a coil spring. It is desirable that when the output of the servomotor 31 is converted to a pressing force, the sliding resistance of the drive unit 30 be sufficiently small. The motion conversion mechanism contained in the casing 32 may have a ball screw structure. In FIG. 2, reference numeral 36 denotes a feeding cable and reference numeral 37 denotes a shunt (bypass line).

(8) The drive unit 30 needs to be capable of controlling the pressing force within a tolerance of 10% (+10% to 10%) in a lower limit range of 70 N to 200 N and an upper limit range of 800 N to 2000 N. Additionally, when the pressing force is at the lower limit (ranging from 70 N to 200 N), the output of the servomotor 31 is desirably 30% or more of a rated output. Normally, the servomotor 31 can stably operate in the range of 30% to 300% of the rated output. Accordingly, the pressing force needs to range from 70 N to 200 N when the output of the servomotor 31 is 30% of the rated output, and needs to range from 800 N to 2000 N when the output of the servomotor 31 is 300% of the rated output.

(9) In other words, if the lower limit in a stable pressing force region of the servomotor 31 exceeds the range of 70 N to 200 N, a proper pressing force range for indirect spot welding is not satisfied when the total sheet thickness of the member to be welded is small. Also, if the upper limit in the stable pressing force region of the servomotor 31 is below the range of 800 N to 2000 N, the proper pressing force range for indirect spot welding is not satisfied when the total sheet thickness of the member to be welded is large. Here, the total sheet thickness of the member to be welded is assumed to range from 1.0 mm to 6.0 mm.

(10) When the spot welding electrode 34 is pressed against the metal sheet 21 (upper steel sheet 21a) as illustrated in FIG. 4 (described below), an overshoot of the pressing force occurs if a contact speed of the spot welding electrode 34 is too high. When the overshoot occurs, an excessive pressing force is momentarily applied to the welded portion 25. Therefore, particularly when indirect spot welding is performed while the opposite side of the spot welding electrode 34 is in an unsupported and hollow state, the welded portion 25 is significantly deformed, which may affect the quality of welding.

(11) An effective solution to this is that the overshoot represented by OS in Equation (1) is controlled to be 10% or less:
OS(%)=(PLAL)/AL100(1)
where PL represents a peak pressing force (N) in an overshoot range and AL represents an average pressing force (N) applied by the electrode.

(12) To control OS in Equation (1) to be 10% or less, the contact speed of the spot welding electrode 34 that is being pressed against the metal sheet 21 is desirably 30 mm/s or less. For example, in the case of a drive unit using a pneumatic cylinder as a drive source, the moving speed of the spot welding electrode 34 cannot be variable-controlled. Accordingly, if the moving speed is set in accordance with the contact speed, since it takes time to move the spot welding electrode 34, the overall operation time may be affected and work efficiency may be degraded. On the other hand, in the case of the drive unit 30 using the servomotor 31 as a drive source as in the present embodiment, the moving speed of the spot welding electrode 34 can be variable-controlled. Accordingly, the moving speed (contact speed) needs to be 30 mm/s or less only when the spot welding electrode 34 comes into contact with the member to be welded. Thus, high work efficiency can be maintained, and this is effective in terms of productivity.

(13) On the other hand, if the contact speed is below 1 mm/s, the work efficiency is greatly degraded because it takes too much time to bring the spot welding electrode into contact with the member to be welded. Accordingly, it is desirable that the contact speed be 1 mm/s or more.

(14) FIG. 3 illustrates an indirect spot welding apparatus (indirect spot welding robot apparatus) according to an embodiment of the present invention.

(15) With the drive unit 30 attached to an articulated robot 46 as illustrated in FIG. 3, an indirect spot welding apparatus (indirect spot welding robot apparatus) 40 can freely change the welding position. Accordingly, the indirect spot welding apparatus 40 can be widely used for welding of automotive components, automotive bodies, and the like. Robots, other than articulated robots, can be appropriately selected in accordance with the intended use.

(16) A welding current flows from an alternating-current power supply 41, passes through a current controller 42 (8), a primary cable 43, and a transformer 44, and is rectified to a set welding current. The welding current further passes through a secondary cable 45 and is supplied to both the spot welding electrode 34 attached at an end of the drive unit 30 and the feeding point 24 attached to the metal sheet 22.

(17) In indirect spot welding, a circuit formed by the secondary cable 45 is long. To minimize impedance, it is desirable that electricity flowing through the secondary-side circuit be a direct current. That is, although examples of a current controller for resistance spot welding include a single-phase alternating current type, an inverter alternating current type, and an inverter direct current type, it is desirable in this case to select a current controller of an inverter direct current type. At the same time, using a resistance spot welding controller capable of controlling a current for each cycle makes it possible to properly control current during welding.

(18) Additionally, as illustrated in FIG. 3, in the indirect spot welding apparatus 40 of the present embodiment, the opposite side of the spot welding electrode 34 with respect to the member to be welded (metal sheets 21 and 22) can be left in an unsupported and hollow state. There is no need to attach a back bar, such as that described in Patent Literature 7, to a support jig 47 that supports the member to be welded (metal sheets 21 and 22) on the opposite side of the spot welding electrode 34.

(19) Thus, in the present embodiment, a pressing force can be stably set in a range of relatively small pressing forces, which has been considered difficult in conventional indirect spot welding. Accordingly, even when the spot welding electrode 34 applies pressure to the overlapping metal sheets 21 and 22 from only one side to perform indirect spot welding of a portion whose opposite side is in an unsupported and hollow state, an accurate pressing force can be stably output in the range of electrode forces (lower limit: 70 N to 200 N, upper limit: 800 N to 2000 N) from the initial stage of application of electrode force. Additionally, even in the case of changing the pressing force during welding as described in Patent Literatures 1 to 4, it is possible to control the pressing force of the spot welding electrode 23 with high precision and to stably obtain a spot welded portion (spot welded joint) having a satisfactory strength.

EXAMPLE 1

(20) EXAMPLE 1 of the present invention will be described.

(21) EXAMPLE 1 examined the performance of each of drive units (Drive Units 1 to 5) based on the drive unit having the configuration of FIG. 2 and mounted in the indirect spot welding apparatus of FIG. 3. Drive Units 1 to 5 are designed to specifications shown in Table 1.

(22) In Table 1, a pressing force lower limit refers to a pressing force applied when the output of the servomotor 31 is 30% of the rated output, and a pressing force upper limit refers to a pressing force applied when the output of the servomotor 31 is 300% of the rated output.

(23) Here, Drive Units 1 to 4 having pressing force lower limits ranging from 70 N to 200 N and pressing force upper limits ranging from 800 N to 2000 N are defined as Inventive Examples A to D, and Drive Unit 5 not having them is defined as Comparative Example A.

(24) TABLE-US-00001 TABLE 1 Pressing Force Pressing Force Servomotor Ball Screw Lower Limit (N) Upper Limit (N) Size (W) Lead (mm) (Motor Output 30%) (Motor Output 300%) Remarks Drive Unit 1 200 10 100 960 Inventive Example A Drive Unit 2 400 25 80 800 Inventive Example B Drive Unit 3 750 25 150 1440 Inventive Example C Drive Unit 4 750 20 180 1800 Inventive Example D Drive Unit 5 1000 10 500 5000 Comparative Example A

(25) For each of Drive Units 1 to 5 shown in Table 1, without passing a welding current, a pressing force was measured 10 times when the electrode contact speed was 20 mm/s and the pressing force was set at 200 N and 800 N. Then, a smallest value, a deviation of the smallest value from the set value, a largest value, and a deviation of the largest value from the set value were determined. The results are shown in Table 2.

(26) TABLE-US-00002 TABLE 2 Set Pressing Force: 200 N Set Pressing Force: 800 N Deviation (%) Deviation (%) Deviation (%) Deviation (%) Smallest of Smallest Largest of Largest Smallest of Smallest Largest of Largest Value Value from Value Value from Value Value from Value Value from (N) Set Value (N) Set Value (N) Set Value (N) Set Value Remarks Drive Unit 1 199 0.5 203 1.5 800 0.0 808 1.0 Inventive Example A Drive Unit 2 189 5.5 200 0.0 798 0.3 808 1.0 Inventive Example B Drive Unit 3 191 4.5 211 5.5 796 0.5 810 1.3 Inventive Example C Drive Unit 4 193 3.5 210 5.0 795 0.6 808 1.0 Inventive Example D Drive Unit 5 190 5.0 235 17.5 794 0.8 811 1.4 Comparative Example A

(27) Table 2 shows that when the set pressing force was 200 N, a deviation of the smallest value from the set value and a deviation of the largest value from the set value were both within the 10% tolerance and no significant deviations were observed in any of Drive Units 1 to 4 (Inventive Examples A to D). On the other hand, in Drive Unit 5 (Comparative Example A), a deviation was outside the 10% tolerance and significant variation was observed. When the pressing force was 800 N, the deviations described above were both within the 10% tolerance in each of the drive units (Drive Units 1 to 5).

EXAMPLE 2

(28) EXAMPLE 2 of the present invention will be described.

(29) Drive Unit 2 shown in Tables 1 and 2 was used in EXAMPLE 2. The pressing force was set at 200 N and 800 N, and the electrode contact speed was varied from 10 mm/s to 80 mm/s. Then, an overshoot of the pressing force occurring when the spot welding electrode was pressed against the metal sheet was evaluated using OS in Equation (1) described above. The result is shown in Table 3.

(30) Here, the cases where the electrode contact speeds were 10 mm/s, 20 mm/s, and 30 mm/s are defined as Inventive Example a, Inventive Example b, and Inventive Example c, respectively, and the cases where the electrode contact speeds were 40 mm/s, 60 mm/s, and 80 mm/s are defined as Comparative Example a, Comparative Example b, and Comparative Example c, respectively.

(31) TABLE-US-00003 TABLE 3 200 N 800 N Average Peak Average Peak Contact Value Value Value Value Speed AL PL OS AL PL OS (mm/s) (N) (N) (%) (N) (N) (%) Remarks 10 203 215 6 794 791 0 Inventive Example a 20 203 217 7 796 787 1 Inventive Example b 30 204 220 8 799 788 1 Inventive Example c 40 208 239 15 801 795 1 Comparative Example a 60 208 405 95 808 840 4 Comparative Example b 80 209 566 171 802 843 5 Comparative Example c

(32) As shown in Table 3, when the pressing force was 200 N, OS was less than 10% in Inventive Examples a to c where the contact speed was 30 mm/s or less. On the other hand, in Comparative Examples a to c where the contact speed was higher than 30 mm/s, OS exceeded 10% and reached 171% particularly when the contact speed was 80 mm/s. When the pressing force was 800 N, OS was less than 10% at a contact speed of 10 mm/s to 80 mm/s (Inventive Examples a to c, Comparative Examples a to c).

EXAMPLE 3

(33) EXAMPLE 3 of the present invention will be described.

(34) In EXAMPLE 3, indirect spot welding of steel sheets (the upper steel sheet 21a and the lower steel sheet 22a) was performed as illustrated in FIG. 4 using the indirect spot welding apparatus 40 illustrated in FIG. 3.

(35) Drive Units 1, 3, and 5 shown in Tables 1 and 2 were used as drive units, and a current controller of a direct-current inverter type was used as the current controller 8. The spot welding electrode 34 made of a chromium copper alloy and having a uniform curvature radius of 40 mm at an end thereof was used.

(36) The upper steel sheet 21a and the lower steel sheet 22a were placed on a U-shaped metal support jig 47a illustrated in FIG. 4. The upper steel sheet 21a is an SPC270 steel sheet having a tensile strength of 270 MPa or more and a thickness of 0.7 mm, containing chemical components shown in Table 4, and having the balance of Fe and inevitable impurities. The lower steel sheet 22a is an SPC270 steel sheet having a tensile strength of 270 MPa or more and a thickness of 1.2 mm, containing chemical components shown in Table 4, and having the balance of Fe and inevitable impurities. A support distance L was 30 mm, and a ground electrode 24a serving as the feeding point 24 was attached to the bottom of the metal support jig 47a. Then, the spot welding electrode 34 applies pressure to the upper steel sheet 21a and the lower steel sheet 22a from above for indirect spot welding. The contact speed of the spot welding electrode 34 was 30 mm/s or 50 mm/s.

(37) Welding conditions (pressing force F, current C) were set as shown in Table 5. In Welding Condition 1 in Table 5, the pressing force F and the current C were constant during the period from turning on to turning off electricity. In Welding Condition 2 in Table 5, the time period was divided into first and second stages. The pressing force F was smaller in the second stage than in the first stage and the current C was larger in the second stage than in the first stage, and the pressing force F and the current C were controlled at the same time.

(38) TABLE-US-00004 TABLE 4 Chemical Component C Si Mn P S (mass %) 0.003 tr 0.09 0.016 0.004

(39) TABLE-US-00005 TABLE 5 Upper Row: Time Period (s)/Pressing Force F (N) Lower Row: Time Period (s)/Current C (kA) First Stage Second Stage Welding Condition 1 0.30/400 0.30/7.0 Welding Condition 2 0.06/400 0.24/200 0.06/2.0 0.24/7.0

(40) As shown in implementation conditions in Table 6, the cases where Drive Units 1 and 3 (which are Inventive Examples A and C shown in Tables 1 and 2) were used as drive units and the contact speed of the electrode 23 was 30 mm/s are defined as Inventive Examples 1 to 4. Also, the cases where Drive Unit 5 (which is Comparative Example A shown in Tables 1 and 2) was used as a drive unit are defined as Comparative Examples 1, 2, and 4. The cases where the contact speed of the spot welding electrode 34 was 50 mm/s are defined as Comparative Examples 3 and 4.

(41) For each of the implementation conditions (Inventive Examples 1 to 4 and Comparative Examples 1 to 4) shown in Table 6, spot welding was performed at 10 points and an average value of nugget diameters of the resulting welded joints, a smallest value, a deviation of the smallest value from the average value, a largest value, a deviation of the largest value from the average value, and an appearance defect occurrence ratio were examined. The results are shown in Table 6.

(42) In Table 6, the nugget diameter is, in a cross section taken at the center of the welded portion, a length of a molten portion on the overlapping line, formed between the upper steel sheet 21a and the lower steel sheet 22a. The appearance defects refer to surface cavities caused by melting and scattering of the welded portion between the spot welding electrode 34 and the upper steel sheet 21a.

(43) If the nugget diameter is 3.0 mm or more, the nugget can be determined to be a biconvex shaped desirable nugget formed in a molten state, and can be regarded as a welded portion having a satisfactory strength. Also, if deviations of smallest and largest nugget diameters from an average value are both within the 15% range, it is possible to assume that stable welding can be achieved.

(44) TABLE-US-00006 TABLE 6 Results Nugget Diameter Smallest Value Largest Value Implementation Conditions (mm) (mm) Appearance Contact Average [Deviation(%) [Deviation(%) Defect Drive Speed Welding Value from Average from Average Occurrence Unit (mm/s) Condition (mm) Value] Value] Ratio(%) Inventive Example 1 1 30 1 3.2 3.0 3.4 0 [6] [6] Inventive Example 2 1 30 2 3.6 3.4 3.8 0 [6] [6] Inventive Example 3 3 30 1 3.3 3.0 3.4 0 [9] [3] Inventive Example 4 3 30 2 3.6 3.3 3.8 0 [8] [6] Comparative Example 1 5 30 1 3.1 2.5 3.3 20 [19] [6] Comparative Example 2 5 30 2 3.3 2.7 3.5 10 [18] [6] Comparative Example 3 1 50 1 2.5 2.2 2.7 0 [12] [8] Comparative Example 4 5 50 1 2.3 1.5 2.8 0 [35] [21]

(45) As shown in Table 6, in Inventive Examples 1 to 4, average nugget diameters were all more than 3.0 mm and welded portions with satisfactory strengths were obtained. Also, deviations of the smallest and largest nugget diameters from the corresponding average values were within the 15% range, and stable welding could be achieved. No appearance defects were observed.

(46) In Comparative Examples 1 and 2, average nugget diameters were both more than 3.0 mm. However, in each of Comparative Examples 1 and 2, a deviation of one of the smallest and largest nugget diameters from the average value was outside the 15% range, and stable welding was not achieved. Also, appearance defects were observed. In Comparative Example 3, deviations of the smallest and largest nugget diameters from the average value were within the 15% range. However, since the average nugget diameter was less than 3.0 mm, a welded portion with a satisfactory strength was not obtained. In Comparative Example 4, the average nugget diameter was less than 3.0 mm, and deviations of the smallest and largest nugget diameters from the average value were outside the 15% range. That is, a welded portion with a satisfactory strength was not obtained, and stable welding was not achieved.

INDUSTRIAL APPLICABILITY

(47) For indirect spot welding in which a member composed of at least two overlapping metal sheets is welded by pressing a spot welding electrode against the metal sheets while applying pressure to the spot welding electrode from one side of the member, attaching a feeding point to the metal sheet on the other side of the member at a location separate from the spot welding electrode, and passing a current between the spot welding electrode and the feeding point, aspects of the present invention can provide an indirect spot welding apparatus in which a set pressing force is stably achieved within a proper pressing force range which may be lower than that in direct spot welding, an overshoot of a pressing force that occurs when the spot welding electrode is pressed against the metal sheets is reduced, and thus a welded portion with a satisfactory strength can be stably obtained.

REFERENCE SIGNS LIST

(48) 1: metal sheet

(49) 2: metal sheet

(50) 3: spot welding electrode

(51) 4: spot welding electrode

(52) 5: welded portion (nugget)

(53) 6: force controlling system

(54) 7: force controlling system

(55) 8: current controller

(56) 11: metal sheet

(57) 12: metal sheet

(58) 13: spot welding electrode

(59) 14: spot welding electrode

(60) 15-1: welded portion (nugget)

(61) 15-2: welded portion (nugget)

(62) 21: metal sheet

(63) 21a: upper steel sheet

(64) 22: metal sheet

(65) 22a: lower steel sheet

(66) 23: spot welding electrode

(67) 24: feeding point

(68) 24a: ground electrode

(69) 25: welded portion (nugget)

(70) 30: drive unit

(71) 31: servomotor

(72) 32: casing

(73) 33: linearly-movable rod

(74) 34: spot welding electrode

(75) 35: bracket

(76) 36: feeding cable

(77) 37: shunt

(78) 40: indirect spot welding apparatus

(79) 41: alternating-current power supply

(80) 42: current controller

(81) 43: primary cable

(82) 44: transformer

(83) 45: secondary cable

(84) 46: articulated robot

(85) 47: support jig

(86) 47a: metal support jig