Resistance welding device

A resistance welding device, comprising a frame for supporting a tool configured like a clamp with two arms arranged facing one another and having respective welding electrodes, of which a first arm is integral to the clamp and the second arm is assembled with the capacity of being moved with respect to the clamp by means of a first actuator, such that respective movements along a straight trajectory can be imparted to the arms of the clamp for moving their electrodes closer to or away from one another. One of the arms has a hollow head housing the corresponding electrode in a guided manner the tip of which is suitable for receiving the support of a component to be welded on the workpiece, the head and the electrode being able to adopt at least one standby position (A) in which the electrode is inside the mentioned head; and a working position (B), in which the tip of the electrode is arranged in or exceeds the inlet of the head, accordingly placing the component to be welded outside the head.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/ES2013/070313 filed May 16, 2013, claiming priority based on European Patent Application No. 12382229.8, filed Jun. 5, 2012, the contents of all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a resistance welding device particularly suitable for welding small components or workpieces, such as welding nuts to a metal sheet arranged in any spatial orientation within the range of the device.

BACKGROUND OF THE INVENTION

Resistance welding devices comprising a tool configured like a clamp in which two arms supporting respective welding electrodes are distinguished are known today. Conventionally, one of the arms has the capacity of being moved with respect to the tool to enable holding between the arms the components or workpieces on which the welding operation must be performed by passing a welding current through the electrodes when the workpiece is held between the arms of the clamp.

When the welding operation requires attaching a small component, such as a nut on the surface of a metal sheet, in ideal situations in which said sheet is horizontally orientated the small component is placed on the sheet to attach them by welding. This small component is placed by means of shuttles or pistons immediately before the upper electrode exerts pressure on the lower electrode arranged under the sheet.

This approach is not applicable when the welding device incorporating the clamp is assembled on the articulated arm of a robot for the purpose of welding workpieces which can have a spatial orientation different from the horizontal, to one another. In these circumstances the small component does not rest naturally and stably on the sheet.

A first objective of the invention is therefore a device assuring the correct attachment by welding in any spatial orientation.

On the other hand, the small components to be welded must be supplied to the mentioned shuttles or pistons which, like the clamp, must have the same capacity for being orientated according to the needs of each welding operation.

Another objective of the invention is a device which in addition to assuring the correct welding, assures the correct supply of the small components to be welded by means of the clamp.

Another objective of the invention is that this supply is performed simply and such that a significant number of welding operations can be carried out consecutively without needing to intervene with the supply means.

Another objective of the invention is to disclose a device capable of offering the performance pointed out above and which is simple in construction and with an optimum number of components or mobile parts.

DISCLOSURE OF THE INVENTION

The device according to the invention is of the types comprising a frame for supporting a tool configured like a clamp with two arms arranged facing one another and having respective welding electrodes, of which a first arm is integral to the clamp and the second arm is assembled with the capacity of being moved with respect to the clamp by means of a first actuator, the clamp being able to be moved with respect to the frame supporting it by means of a second actuator and such that both first and second actuators have the capacity of being driven together for imparting respective movements along a straight trajectory to the arms of the clamp for moving their electrodes closer to or away from one another.

In essence, the device is characterised in that one of the arms has a hollow head defining an outer surface with an inlet and housing the corresponding electrode in a guided manner the tip of which is suitable for receiving the support of a component to be welded on the workpiece, the head and the electrode being able to adopt at least one standby position (A) with respect to one another, in which the electrode is inside the mentioned head, the tip of the electrode being arranged a sufficient distance with respect to the outer surface of the head to enable arranging the component to be welded to the workpiece between them and inside the head; and a working position (B), in which the tip of the electrode is arranged in or exceeds the inlet of the head, accordingly placing the component to be welded outside the head.

In a variant of special interest, the first arm fixed with respect to the clamp supports the hollow head the outer surface of which is intended for receiving the support of the workpiece, said head being subjected to the effect of elastic means which tend to arrange it in a standby position (A) in which the tip of the electrode and the component arranged immediately on it are inside hidden under the outer support surface, when pressure is being exerted on said outer support surface said head can move a sufficient distance in the direction opposite the second mobile arm so that the component resting on the electrode contacts the workpiece to be welded that is applied on the mentioned outer support surface through the inlet of the head.

According to another feature of this variant, the device comprises control means for controlling the second actuator and consequently the linear movement imparted to the clamp with respect to the frame, said control means comprising instructions for moving the clamp towards the workpiece while it is immobilised so that the component resting on the electrode contacts said workpiece through the inlet of the head when the outer support surface of the hollow head is retracted due to the forward movement of the clamp.

The invention contemplates that the mentioned control means also control the first actuator of the second arm of the clamp while the clamp moves forward for retracting the outer support surface of the hollow head, the control means comprising instructions for moving this second arm with respect to the clamp, together with its forward movement, and towards first arm for immobilising the workpiece.

The current devices comprise control units for controlling the mobile arm of the clamp and also for controlling the latter with respect to a frame for the purpose of substantially maintaining and exerting the same pressure with the electrodes on the workpieces to be welded during the welding operation.

Advantageously, the variant of the invention described above uses the same compensation systems, commonly called equalisers in the scope of welding devices comprising tools configured like clamps for arranging the electrode in its operating position. This allows, for example, eliminating active movement systems for moving the electrode with respect to the corresponding arm of the clamp considerably simplifying the construction of the device.

According to another feature, the electrode housed in the hollow head houses a centring bolt or pin for centring the component to be welded on the workpiece, the distal end of which protrudes beyond the tip of the electrode.

In another variant of the invention, the first actuator is a linear actuator and the second arm comprises a carriage which transports the hollow head, the carriage being linked to the first actuator by elastic means which tend to arrange it in an extended position with respect to the said first actuator, the second arm further comprising an end of travel stop for the carriage intended for preventing the movement of the head which it transports without the forward movement of the actuator being prevented.

Preferably, the electrode guided through the head directly receives the thrust of the same first actuator, such that a relative movement occurs between the hollow head and the guided electrode when, during the movement of the first actuator, the carriage reaches the end of travel stop and the forward movement of the first actuator is not stopped.

According to another aspect of the invention, the device is provided with a reservoir suitable for accumulating a line of components to be welded, integral to the clamp, and with means communicating said reservoir with the interior of the hollow head for transporting the components to be welded from the reservoir to a position juxtaposed to the tip of the electrode housed therein.

In one embodiment, said reservoir comprises an oblong housing and a hold-down element subjected to the action of a thruster tending to arrange it against the last component to be welded of the line of components housed in the reservoir, the hold-down element being assembled with the capacity of being moved with respect to the direction of the line of components inside the reservoir and such that it will always exert a thrust on the last component regardless of its position inside the reservoir.

The means communicating said reservoir with the interior of the hollow head preferably comprise a guide element connecting an outlet of the reservoir with a positioning mechanism for positioning components communicating with a side opening of the head in communication with the cavity of said head.

According to another feature of the invention, the hollow head comprises retaining means for retaining a component to be welded in a position juxtaposed to the tip of the electrode when it adopts its standby position (A), said retaining means being able to be actuated together with the first actuator for releasing the component to be welded when a relative movement occurs between the hollow head and the electrode in the direction arranging the electrode in its working position (B).

DETAILED DESCRIPTION OF THE INVENTION

The drawings which will be referred to below show two variants of a device1according to the invention. In both variants the depicted device1comprises a frame2serving as a support for a tool configured like a clamp3. This frame2is intended to be assembled in a robotic arm provided with several degrees of freedom in order to enable locating and orientating the clamp3in any point in space within the reach of the robotic arm. In the scope of the invention the term welding gun is used for designating clamp-shaped tool of this type.

It is known that the clamp3comprises two arms4and5arranged facing one another and having respective resistance welding electrodes. In the drawings the clamp3has always been illustrated in a vertical position and such that both electrodes are vertically aligned for facilitating the understanding of the drawings. In the two depicted variants of the device1, the first arm4is an arm integral to the clamp3which is bent and has the electrode which is arranged in a lower position; and the second arm5having the electrode which is arranged in an upper position, it is assembled with the capacity of being moved with respect to the clamp3by means of a first actuator8. As will be described in detail below, the clamp3can be moved with respect to the frame2supporting it by means of a second actuator9. It is also known that both first and second actuators8and9have the capacity of being driven together for imparting respective movements along a straight trajectory to the arms4and5of the clamp3for moving their electrodes closer to or away from one another, and at the same time moving them closer and/or away with respect to a fixed point in space, arranged between the electrodes, in which a workpiece on which the welding operation must be performed can be located. In the drawings inFIGS. 5 and 6ato6dthis workpiece is represented by the workpiece21in the form of a C-shaped sheet.

Unlike known clamps, the clamp3of the device1is designed for automatically placing and welding a single or a plurality of components15to the workpiece21, formed in the example by nuts, regardless of the orientation in space of said clamp3. For such purpose, one of the arms4or5has a hollow head10inside which the components15to be welded will be deposited, which components will be subsequently placed in its welding position in contact with the workpiece21by thrusting the electrode13, this sequence being repeated in each welding operation.

In the variant ofFIGS. 1, 2a,2b,3and4ato4c, this hollow head10is assembled on the second arm5which can be moved with respect to the clamp3; while in the variant ofFIGS. 5 and 6ato6dthis hollow head10is assembled on the first arm4fixed with respect to the clamp3.

In order to feed and arrange the components to be welded15automatically in the suitable position so that they are moved by the electrode13towards the workpiece12, it can be seen inFIG. 1that the device1according to the first variant of the invention is provided with supply means comprising a reservoir6integral to the clamp3suitable for accumulating a line28of components15to be welded.

FIG. 7shows the cross-section of this part of the device1and specifically the reservoir6and the means connecting said reservoir6with a positioning mechanism31(shown in greater detail inFIGS. 2aand 2b) which will resend the components15to be welded into the head10one by one each time a welding operation must be performed.

The reservoir6comprises an oblong housing7and a hold-down element16which is subjected to the action of a thruster tending to arrange it against the last component15bto be welded of the line28of components15housed in the reservoir6. This hold-down element16which may be driven by elastic or pneumatic means is further assembled in the reservoir6with the capacity of being moved with respect to the direction of the line28of components inside the reservoir6, such that it will always exert a thrust on the last component15bregardless of its position inside the reservoir6, a thrust which will be transmitted to the first component15aof the line28for arranging it in a chamber29, illustrated in detail inFIGS. 2aand 2b, of the positioning mechanism31.

In the example ofFIGS. 1 and 7, the means connecting the reservoir6with the interior of this positioning mechanism31comprise a guide element17extending between an outlet18of the reservoir6and a side opening not shown in the drawings of the aforementioned chamber29. The invention contemplates using a conduit, hose or the like having inner section same as or complementary to the component15as guide element or, alternatively, a rail or an open guide structure, which does not completely envelop the components15but is suitable so that these can slide towards positioning mechanism31in a guided manner.

A piston30describing an alternating back and forth movement together with the electrode13has been arranged in a moveable manner in the chamber29.FIG. 2athus shows how the piston30adopts an extended position in which it arranges, by thrust, a component15to be welded in a position juxtaposed to the tip14of the electrode13, whileFIG. 2bshows the same piston30when it adopts a retracted position, allowing the entry of the first component15bof the line28into the chamber29, ready to be placed inside the head10, juxtaposed to the tip14of the electrode13in the following movement manoeuvre of the piston30and when said electrode allows it, as will be explained below.

FIGS. 2a, 2b,3and4a, but especiallyFIG. 3, show that the head10houses the electrode13in a guided manner the tip14of which is suitable for thrusting the component15to be welded on the workpiece21(seeFIG. 4). The electrode13can be moved with respect to the head10such that it can adopt at least one standby position (A), depicted inFIGS. 2a, 2band3, in which the electrode13is inside the mentioned head10, the tip14of the electrode13being arranged a sufficient distance with respect to the outer surface11of the head10to enable arranging the component15to be welded to the workpiece21between them; and a working position (B), depicted inFIG. 4c, in which the tip14of the electrode13is arranged in or exceeds the inlet12of the head10, accordingly placing the component15to be welded outside the head10.

The sequence of a welding operation is exemplified inFIGS. 4ato4c.

Based on the situation depicted inFIG. 4a, having duly placed the clamp3such that the portion of the workpiece21on which the component15must be welded is arranged between the arms4and5of said clamp3, resting on the electrode13′ of the lower arm4and preferably traversed by a centring bolt or pin24provided in the mentioned electrode13′ for such purpose, in a first step the first actuator8works to move the second arm5towards the workpiece21, in this case downwards, the head10and the electrode13being moved together in unison until reaching the position depicted inFIG. 4b.

Subsequently, and due to the effect of the same first actuator8, the movement of the head10is stopped but not that of the electrode13, which starts a relative movement with respect to the head10, being moved downwards driving the component15along until adopting the aforementioned working position (B), all this is as depicted inFIG. 4c.

It must be noted that to keep the component15inside the hollow head10without the risk of accidentally dropping it, the head10is provided with retaining means20for retaining the component15to be welded in the position juxtaposed to the tip14of the electrode13when it adopts its standby position (A), said retaining means being able to be actuated together with the first actuator8for releasing the component15to be welded when a relative movement occurs between the hollow head10and the electrode13in the direction arranging the electrode13in its working position (B). In the example these retaining means20are formed by a pair of clips which can be actuated in transverse direction with respect to the direction of the electrode13inside the head10and are illustrated inFIG. 3.

It should be noted that a single actuator, in this case the first actuator8, is used for performing the movement in steps described above. To that end, the second arm5comprises a carriage25which transports the hollow head10, the carriage being linked with the actuator8by elastic means26which work by traction and which tend to arrange the carriage in an extended position with respect to the mentioned actuator8, as illustrated inFIG. 4b. In contrast, the electrode13guided through the head10is moved integral to the actuator8.

The downward movement of the actuator8in the direction indicated by the arrow ofFIG. 4bwill cause the simultaneous movement of the carriage25and consequently of the head10and of the electrode13. When the carriage25reaches an end of travel stop27, preferably an adjustable stop, provided in the clamp3, the forward movement of the carriage25and of the head10will be prevented although the movement of the actuator8continues, the elastic means26being compressed as illustrated inFIG. 4c, the actuator driving the electrode13along in its downward movement and causing the relative movement between the electrode13and the head10from that instant.

FIG. 5schematically shows another variant of particular interest for a device1according to the invention.

Unlike the first variant, the first arm4which supports the hollow head10housing the electrode13is fixed with respect to the clamp3.

In this case, the head10is subjected to the effect of elastic means22which tend to arrange it in a standby position (A), illustrated inFIG. 5, in which the tip14of the electrode13and the component15arranged immediately above it are inside hidden under the outer support surface11for the workpiece21. As will be shown below, when pressure is being exerted on said outer support surface11the head10can move a sufficient distance in the direction opposite the second arm5so that the component15resting on the electrode13projects outwards through the inlet12of the head10.

The sequence of movements during a welding operation is shown inFIGS. 6ato 6d. Based on the situation depicted inFIG. 5, in a first step the robotic arm moves the device1arranging the first and second arms4and5of the clamp3on either side of the workpiece21, as illustrated inFIG. 6a; next, the first actuator8moves the second arm5in the direction indicated by the arrow until placing the electrode13′ on a first surface of the workpiece21, as illustrated inFIG. 6b; then the second actuator9is driven for moving the assembly of the clamp3with respect to the frame2in the direction indicated by the upward arrow ofFIG. 6cuntil the outer surface11of the head10rests on the opposite surface of the workpiece21. Since the second arm5is moved together with the clamp3, this last manoeuvre does require driving the first actuator8for moving the second arm5downwards, as indicated by the downward arrow of the sameFIG. 6c, for compensating the upward movement of the clamp3.

The following step is carried out in a manner similar to the previous step for moving the clamp3and thereby the head10upwards but, the movement of the workpiece21being prevented due to the intervention of the first actuator8on the second arm5, the retraction of the outer support surface11of the head10now occurs until the component15contacts the facing surface of the workpiece21, an instant in which the electrode13adopts the working position (B) with respect to the head10and said welding can be started since both electrodes13and13′ are in contact with the component15and the workpiece21, respectively, as illustrated inFIG. 6d.

In order to coordinate the movement of the clamp3with respect to the frame2and at the same time that of the second arm5with respect to the clamp3, the device1comprises control means23for controlling the second actuator9and consequently the linear movement imparted to the clamp3with respect to the frame2, said control means23comprising instructions for moving the clamp3towards the workpiece21while it is immobilised. The mentioned control means23also control the first actuator8of the second arm5of the clamp3while the clamp moves forward 3 for retracting the outer support surface11of the head10, therefore these control means23also comprise instructions for moving this second arm5with respect to the clamp3, together with its forward movement, and towards the first arm4for immobilising the workpiece21.

This variant of the invention offer certain advantages with respect to the first variant, since it constructively simplifies the second arm5and uses the resources which some welding devices with clamps already have so that they perform the function of the second actuator9. More specifically, this function can be performed by the conventional equaliser systems which are conventionally used for moving the clamp during the maneuvers to move the workpiece to be welded closer.

On the other hand, it can be seen that the reservoir6for the line of components15is assembled integral to the first arm4of the clamp3which means that it travels together with the electrode13, which in this second variant is fixed with respect to the first arm4. There is therefore no relative movement between the reservoir6and the electrode13.

According to another advantage of this variant of the device, there is no active mechanism operating directly on the electrode13housed in the hollow head10, but the means moving the clamp3are utilised for arranging the electrode13in its working position (B).

Regarding the features of the reservoir6, it is understood that it can be similar to that of the device ofFIG. 1.

Although it is not discussed in greater detail, in this second variant the head10is also provided with retaining means20for retaining the component15to be welded in the position juxtaposed to the tip14of the electrode13when it adopts its standby position (A), said retaining means being able to be actuated together with the position the outer surface11of the head10adopts at all times, the component15which they hold being released when the said welding operation starts, i.e., when the component15is applied by pressure on a surface of the workpiece21.

Similarly to the device ofFIG. 1, these retaining means20can be formed by a pair of clips which can be actuated in transverse direction with respect to the axial direction of the electrode13inside the head10.

Although not depicted, it is also contemplated that the electrode13housed in the hollow head10houses the centring bolt or pin24for centring the component15to be welded on the workpiece21instead of said bolt being arranged in the electrode13′ such as the case of the example ofFIGS. 5 and 6ato6d. Naturally, the length of the bolt24must be such that its distal end protrudes beyond the tip14of the electrode13.