GRINDING WELDING ELECTRODES

Abstract

Grinding a rod-shaped welding electrode includes forming a contact line at a contact surface region of a rotating grinding tool, moving the rotating grinding tool and the welding electrode relative to one another in order to grind a surface of the welding electrode, pressing the welding electrode in an axial direction against an abrasive contact surface region of the rotating grinding tool, and, during relative movement between the rotating grinding tool and the welding electrode, moving the rotating grinding tool along a curved path in a plane of movement which extends transversely to the contact line and which includes a longitudinal axis of the welding electrode. A holder for the rotating grinding tool is arranged on a coupler mechanism which is provided with two actuating drives enabling the holder to be moved to any point within an operating range of a plane of movement.

Description

TECHNICAL FIELD

This application relates to grinding rod-shaped workpieces, such as welding electrodes, by pressing a surface area of the welding electrode against an abrasive contact surface area of a rotating grinding tool.

BACKGROUND

When resistance welding metal sheets, high amperage electric currents are introduced into the sheets using two electrodes pressed against the outer surfaces of the sheets to be welded together. This melts the metal of the sheets and forms a welding lens that firmly joins adjacent sheets together. The same applies to the welding of other metallic components. The welding electrodes are usually made of copper or copper alloys. In aluminum resistance welding in particular, the ideal shape and purity of the surface of the welding electrodes is an essential prerequisite for producing a spot weld of high and reproducible quality. Welding just a few spot welds, for example ten to twenty spot welds, can damage the surfaces of the welding electrodes through deposits and wear, so that the spot welds produced do not have the desired strength. For this reason, welding electrodes are reworked at regular intervals so that their surfaces have an optimum shape and are free of impurities during each welding process. A well-known process for reworking the surfaces of the welding electrodes is grinding the welding electrodes.

Several methods and devices for processing welding electrodes are known from the state of the art. For example, FR 2 738 518 A1 discloses a cutting head whose cutting edges are curved so that they produce the desired contour on the face of the welding electrodes when rotated around the axis of rotation of the cutting head. A surface of a welding electrode produced by cutting has the disadvantage that the surface is too smooth. Surfaces produced by grinding have a certain roughness, which experience has shown leads to better welding results. The roughness is caused by the cutting edges of the abrasive particles of the grinding tool, which create grinding marks on the surface along their path of movement.

For this reason, the applicant has developed a grinding process in which a grinding wheel rotating about an axis of rotation, whose surfaces extending radially to the axis of rotation form the abrasive grinding surfaces, performs a wobbling movement around the center of the end face of the welding electrode. In this way, the end face of the welding electrode is ground in the form of a convexly curved cap and has grooves in the surface produced by the grinding wheel, which give the desired roughness. The applicant's publication U.S. Pat. No. 9,573,237 B2 describes a mechanical rotary and wobble drive of the grinding wheel. The applicant's publication U.S. Pat. No. 9,579,770 B2 describes a rotary drive which is coupled with a digitally controlled actuator in order to impart the desired wobbling motion to the grinding wheel.

Both systems have proven themselves in practice for generating the specified cap-shaped electrode surface for welding electrodes. However, the movement controls for the grinding wheels are somewhat complex to manufacture.

DE 195 18 708 C2 and JP 2003071693 A describe grinding methods in which the workpiece is rotated around its longitudinal axis. WO 2020/106419 A1 describes a grinding process in which the axis of the grinding wheel is moved along a circular path around the end face of the workpiece by a type of plotter drive or a swivel arm.

SUMMARY OF THE INVENTION

It is desirable to create a method and a device for grinding rod-shaped welding electrodes which are easy to implement and can be used flexibly in practice.

In the method described here for grinding rod-shaped welding electrodes, a surface area of the welding electrode to be ground and an abrasive contact surface area of a rotating grinding tool are pressed against each other. A contact line is formed on the contact surface area of the grinding tool and the rotating grinding tool and the welding electrode are moved relative to each other in order to grind the entire surface of the workpiece to be ground.

The method is characterized in that the welding electrode is pressed in the axial direction against the abrasive contact surface area and, during the relative movement between the rotating grinding tool and the welding electrode, the grinding tool is moved along a curved path in a plane of movement which extends transversely to the contact line and includes the longitudinal axis of the welding electrode. In other words, the grinding tool, which contacts the end face of the welding electrode along the curved contact line, is moved along a curved path, in particular a circular path, during grinding. This curved path lies in a plane of movement that extends transversely and in particular at right angles to the contact line and extends parallel to the longitudinal axis of the rod-shaped welding electrode. In the direction parallel to the contact line, the curvature of the contact line is impressed on the end face of the welding electrode. In the direction of the plane of movement, the curvature of the curved path is impressed on the end face. Both curvatures are superimposed and result in a convex shape of the end face after the grinding process if the contact line and the curved path are concave in relation to the end face. If, on the other hand, the contact line and the curved path are convex in relation to the face of the welding electrode, a trough is created in the face of the welding electrode so that an annular contact area is available for welding.

A holder for the rotating grinding tool is arranged on a coupler mechanism, which is equipped with two actuating drives that allow the holder to move to any point within an operating range in a movement plane. A coupler mechanism of this type is inexpensive to manufacture using simple means, requires little maintenance and can be controlled very reliably. Stepper motors or servomotors can be used as actuating drives for the two couplers.

In one embodiment, in which the abrasive contact surface area of the grinding tool is formed by a concavely curved annular groove on the side face of a grinding wheel, the welding electrode is pressed against the concavely curved contact surface area of the grinding tool parallel to the axis of rotation of the grinding wheel.

In one embodiment, in which the abrasive surface area of the grinding tool is formed by a circumferential surface of a grinding wheel, the welding electrode is pressed against the circumferential surface of the grinding wheel in a radial direction. The circumferential surface of the circular grinding wheel is convexly curved in the circumferential direction. The circumferential surface can be concave in the axial direction to create a concave contact line. However, the surface can also be convexly curved to create a convexly curved contact line. This allows the production of an annular end face of the welding electrode, which creates an annular contact during welding. An annular contact area can be very advantageous in practical applications. For this purpose, the relative movement between the welding electrode and the grinding tool can take the form of a rotation around the axis of the welding electrode. A depression then forms in the central area of the end face and the contact area effective during welding is ring-shaped. However, it is also possible to move a spherically curved grinding wheel along an arc transverse to the curved contact line along the face of the welding electrode. In this case too, an end face with a trough is created on the welding electrode. The face of the welding electrode is preferably rotationally symmetrical.

The abrasive particles are preferably formed by diamond particles or other high-strength abrasive particles such as cubic boron nitride CBN. The abrasive particles can be applied to a deformable metal foil, in particular galvanically bonded. The metal foil can in turn be applied, in particular bonded, to a rubber-elastically deformable base layer of the grinding wheel. A manufacturing method for such a grinding wheel is described in DE 10 2016 119 746 A1. The flexible but high-strength attachment of the abrasive particles to the metal foil makes it possible to achieve an optimum processing tool for welding electrodes. The flexible abrasive surface prevents hard impacts and knocks during the grinding process. At the same time, the abrasive particles are firmly anchored on the metal foil. Grinding tools of this type have a long service life.

However, it is also possible to attach the drive motor for the grinding wheel to a flexible holder and thus achieve a certain resilient property of the grinding surface. Alternatively or additionally, the drive shaft of the grinding wheel can be flexibly mounted, e.g. by the magnetic forces acting on the rotor in case of an external rotor. The grinding wheel can also be flexibly mounted on the axis of the drive motor, e.g. by using elastic O-rings.

To implement the grinding method described here, a device for grinding rod-shaped welding electrodes is proposed which has a grinding tool, a rotary drive for the grinding tool and a pressing device which presses a surface area of the welding electrode to be ground and an abrasive contact surface area of the rotating grinding tool against each other.

The contact surface area of the grinding tool is a contact line, wherein a movement device moves the rotating grinding tool and the welding electrode relative to each other in order to grind the entire surface of the welding electrode to be ground.

This results in the advantages described above. With a simple design of the grinding device, this device can be used to reliably produce a crowned end face with a uniform roughness due to the grinding marks of the abrasive particles if the contact line on the surface of the grinding wheel is concave and the grinding wheel is guided along a concave path over the end face of the welding electrode. If the contact line is convex, a hollow can be created in the center of the welding electrode so that the end face makes contact along an outer ring area of the welding electrode during welding.

The abrasive contact surface area of the grinding tool can be formed by a concave, ring-shaped groove on the side face of a grinding wheel, for example, or by a concave, curved peripheral surface of a grinding wheel.

The pressure device can be a robot arm or a welding gun that carries the welding electrode. However, it can also be formed by a movable holder for the grinding wheel. This holder can be used to press the welding electrode against the grinding wheel. The pressure device can be set up to press the rod-shaped welding electrode in the axial direction against the abrasive contact surface area, where the movement device can be configured to move the grinding tool relative to the welding electrode on a curved path in a plane of movement that extends transverse to the contact line and parallel to the longitudinal axis of the welding electrode.

In particular, a holder for the rotating grinding tool is arranged on a coupler mechanism which is provided with two actuating drives which enable the holder to be moved to any point within an operating range of a movement plane. This embodiment is explained in more detail in connection with the drawings.

In particular, this embodiment enables a particularly simple and compact design of the grinding device. In practice, the grinding device with the drive motor, which rotates the grinding wheel and the movement device, which generates the relative movement between the grinding wheel and the welding electrode, can be designed as an integral part of the welding head or the welding gun, which includes the welding electrodes. In this way, the device for regrinding the welding electrodes can be integrated into the welding head, particularly in the case of precision welding heads for fastening small parts, so that the electrodes can be reground without significant interruption of the welding process or intervention by a fitter. Systems for welding small parts usually have stationary welding heads to which the workpieces are transported. However, there are also welding systems with movable welding heads. Small parts are usually welded with welding electrodes with a diameter of 6 mm to 8 mm and flat end faces. A linear drive usually presses the end faces of the welding electrodes against the workpieces. Such welding electrodes can be excellently shaped into a preferably spherical form using the device and method described here, whereby the surface of the electrode is given the desired structure by grinding. A good welding result can be achieved very reliably with a convex end face of the welding electrodes of welding heads of a small parts welding system. Slight tilting of the welding electrodes in relation to the workpiece surface is compensated by the convex end face.

Finally, the device described here may also provide that the abrasive contact surface area of the rotating grinding tool is held resiliently relative to the welding electrode. As described above, the abrasive surface with the abrasive particles can have a resilient base. The grinding tool can be resiliently attached to the drive motor or the holder for the drive motor of the grinding tool can be resiliently designed. The resilience of the grinding surface in relation to the end face to be machined prevents hard knocks against the end face and possible damage to the end face during the grinding process.

BRIEF DESCRIPTION OF THE DRAWINGS

Further practical embodiments and advantages of the system described herein are described below in connection with the drawings.

FIG. 1 shows a sectional view of a first embodiment of a grinding wheel with hub.

FIG. 2 shows a second embodiment of a grinding wheel with drive motor.

FIG. 3 shows a holder for the drive motor of the grinding wheel from FIG. 2 with schematically depicted welding electrodes.

FIG. 4 shows a front view of the holder from FIG. 3.

FIG. 5 shows a side view of the holder shown in FIGS. 3 and 4.

FIG. 6 shows the coupler mechanism from FIGS. 3-5 in a first position.

FIG. 7 shows the coupler mechanism from FIG. 6 in a second position.

FIG. 8 shows a side view corresponding to FIG. 5 with the position of the coupler mechanism from FIG. 7.

FIG. 9 shows a schematic representation of the coupler mechanism in a position that corresponds to the position shown in FIG. 7.

FIG. 10 shows three representations of the coupler mechanism corresponding to FIG. 9 in three different positions for machining the upper welding electrode.

FIG. 11 shows corresponding representations of the coupler mechanism in positions for machining the lower welding electrode as in FIG. 10.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

FIG. 1 shows a first embodiment of a grinding wheel 1, which forms the grinding tool for carrying out the method described here. The grinding wheel 1 is clamped in a rotationally fixed manner on a hub 2, which is rotated about its axis of rotation 3 using a drive motor (not shown). Grinding wheels are usually rotated at speeds of over 1,000 revolutions per minute. The grinding wheel 1 is intended for wheel-face grinding and has on an upper side face of the grinding wheel 1, which extends perpendicular to the axis of rotation 3, a flat annular groove 4 with a slightly concave curve. The grinding wheel 1 is circular in plan view. The groove 4, which is concave in the radial direction of the grinding wheel 1, extends in an annular area on the circular grinding wheel 1. A rod-shaped welding electrode 5 is shown schematically in the right-hand half of FIG. 1. The welding electrode 5 has a longitudinal axis 6 and a cap-shaped end face, which is pressed in an axial direction into the annular groove 4 of the grinding wheel 1 with a concave surface. Grinding removes dirt from the end face and produces a uniform surface with a fine grit marks caused by the abrasive particles.

FIG. 2 shows an alternative embodiment of a grinding wheel 7. The grinding wheel 7 is intended for peripheral grinding. This means that the grinding wheel 7 has abrasive particles, preferably diamond particles or CBN particles, on a circumferential surface 9 of the grinding wheel 7, which is concave in the axial direction. A shaft 10 is integrally formed on the grinding wheel 7, which is rotated at high speed around the axis of rotation 8 by a drive motor 11. FIG. 2 also shows the welding electrode 5, which is pressed against the concave circumferential surface 9 of the grinding wheel 7 in a radial direction. This also establishes a linear contact along a concave contact line between the grinding wheel 7 and the welding electrode 5.

FIG. 3 shows an embodiment of a holder for the drive motor 11 of a grinding wheel 7. In the embodiment of FIG. 3, the grinding wheel 7 is guided along a curved path that lies in a plane that is essentially perpendicular to the contact line. The curvature of the contact line between the grinding wheel 7 and the welding electrode 5 and the curvature of the curved machining path overlap and give the face of the welding electrode 5 a convex shape. The curved machining path is generated using a coupler mechanism.

Components of the holder of the drive motor 11 for the grinding wheel 7 are shown in FIG. 6 in an isolated manner. In the embodiment of FIG. 3, the holder for the drive motor 11 is formed by a free end 14 of a first coupler 15, the other end of which is connected to a first crank 16 of a first actuating drive 18 via a first joint 17. In the embodiment shown, the first coupler 15 has the shape of a cranked lever. A second coupler 19 is connected at one end via a second joint 21 to a second crank 20 of a second actuating drive 22 and via a third joint 23 to the first coupler 15. The third joint 23 is located approximately in the area of the crank of the first coupler 15. In FIG. 3 it can be seen that the stepper motors 18 and 22 are attached to a support plate 24. By turning the cranks 16 and 20 using the stepper motors 18, 22, the free end 14 of the first coupler 15, which forms the holder for the drive motor 11 of the grinding wheel 7, can be moved within an operating range in a plane parallel to the support plate 24.

The support plate 24 is located on the side of an electrode holder 25 of a welding device, which is shown as a transparent plate in FIG. 3 for reasons of clarity and carries two welding electrodes 5, 5′. As the grinding wheel 7 can be moved essentially freely in a plane perpendicular to the axis of rotation of the grinding wheel 7 by the coupler mechanism, the grinding wheel 7 can be moved both to the upper welding electrode 5 and to the lower welding electrode 5′ in order to machine the end face with the grinding wheel 7

After machining, in which the coupler mechanism has approximately the position shown in FIG. 6, the coupler mechanism can be moved to the position shown in FIG. 7, in which the grinding wheel 7 and the drive motor 11 of the grinding wheel 7 are at the maximum distance from the welding electrodes 5, 5′. In a rest position shown in FIG. 7, the grinding wheel 7 does not interfere with the welding process executed by the welding electrodes, as can be seen in FIG. 8. If reworking of the end faces of the welding electrodes 5, 5′ is required, the grinding wheel is moved back to the position shown in FIGS. 3-5.

FIG. 9 shows the rotary position of actuating drives, which move the grinding wheel to the position shown in FIG. 8. FIG. 9 is only a schematic representation of the coupler mechanism. An axis of rotation 26 of the a actuating drive and an axis of rotation 27 of a second actuating drives are shown here. The couplers 15, 19 and the cranks 16, 20 are merely shown as lines. Furthermore, a machining path 29 for the end face of the upper welding electrode 5 and a machining path 30 for the end face of the lower welding electrode 5′ are shown schematically. When the axis of rotation 8 of the grinding wheel moves along the machining path 29 or the machining path 30, the concavely curved circumferential surface 9 of the grinding wheel 7 has a linear contact with the respective one of the welding electrodes 5, 5′ to be machined. For both cranks 16, 20, the crank angle is shown in relation to a zero position extending horizontally to the right. In FIG. 9, the crank angle α between the first crank 16 and the zero position is 328.91°. The crank angle β of the second crank 20 in the position of the coupler mechanism in FIG. 9 has a value of 148.97°.

Starting from the position shown in FIG. 9, the cranks 16, 20 can be rotated in order to move the axis of rotation 8 of the grinding wheel, for example, into the vicinity of the machining path 29 of the upper welding electrode 5.

FIGS. 10A, B and C show three positions of the coupler mechanism during machining of the upper welding electrode. It can be seen that the machining path 29 is concavely curved so that the axis of rotation 8 of the grinding wheel moves along a concave machining path 29 to cause the circumferential surface to move along the concave machining path 29 past the end face of the welding electrode 5. The concave contour of the peripheral surface of the grinding wheel is superimposed on the concave path of movement of the axis of rotation 8 of the grinding wheel and in this way produces a cap-shaped contour on the end face of the upper welding electrodes 5.

FIG. 11 shows in representations A, B and C three positions of the axis of rotation 8 of the grinding wheel on the machining path 30 for the lower welding electrode 5′.

If the circumferential surface of the welding electrode is not concave but convex in the axial direction, the movement sequence of the coupler mechanism can be reversed, i.e. the grinding wheel is moved along a convex path towards the welding electrode during grinding and pressed into an end face of the welding electrode until the grinding wheel reaches the center, and the grinding wheel is then moved away from the welding electrode in the axial direction again. The result is not a crowned end face but a depression in the center of the end face of the welding electrode. In this way, the end face of the welding electrode is provided with an annular surface projecting in the axial direction on the outer periphery, which contacts the workpiece during welding.

The features of the invention disclosed in the present description, in the drawings and in the claims may be essential, both individually and in any combination, for the realization of the invention in its various embodiments. The invention is not limited to the described embodiments. It can be varied within the scope of the claims and taking into account the knowledge of the person skilled in the art.

Claims

1. A method for grinding a rod-shaped welding electrode, comprising: pressing a contact surface region of the welding electrode in an axial direction of the welding electrode against an abrasive contact surface region of the rotating grinding tool, so that a contact line is formed at the contact surface region of the rotating grinding tool; andmoving the rotating grinding tool and the welding electrode relative to one another in order to grind a surface of the welding electrode along a curved path in a plane of movement which extends transversely to the contact line and which includes the longitudinal axis of the welding electrode, wherein a holder for the rotating grinding tool is arranged on a coupler mechanism which is provided with two actuating drives enabling the holder to be moved to any point within an operating range of a plane of movement.

2. The method according to claim 1, wherein the abrasive contact surface region of the rotating grinding tool is concavely curved and the contact line is concavely curved.

3. The method according to claim 1, wherein the abrasive contact surface region of the grinding tool is formed by a concavely curved, annular groove on a side face of a grinding wheel.

4. The method according to claim 1, wherein the abrasive contact surface region of the grinding tool is formed by a peripheral surface of a grinding wheel.

5. The method according to claim 4, wherein a circumferential surface of the grinding wheel is concavely curved in an axial direction of the grinding wheel.

6. The method according to claim 1, wherein a surface region of the welding electrode and the abrasive contact surface region of the rotating grinding tool are pressed resiliently against one another.

7. A device for grinding a rod-shaped welding electrode, comprising: a rotating grinding tool;a rotary drive coupled to the rotating grinding tool;a pressing device that presses a surface region of the welding electrode to an abrasive contact surface region of the rotating grinding tool, wherein a contact surface area of the grinding tool forms a contact line;a moving device that moves the rotating grinding tool and the welding electrode relative to each other in order to grind a surface of the welding electrode, wherein the pressing device presses the welding electrode in an axial direction against the abrasive contact surface region and wherein the moving device moves the rotating grinding tool relative to the welding electrode on a curved path in a plane of movement, which extends transversely to the contact line and is parallel to a longitudinal axis of the welding electrode; anda holder for the rotating grinding tool that is arranged on a coupler mechanism which is provided with two actuating drives that enable the holder to be moved to any point within an operating range of a movement plane.

8. The device according to claim 7, wherein the contact line is concavely curved.

9. The device according to claim 8, wherein the abrasive contact surface area of the grinding tool is formed by a concave, ring-shaped groove on the side face of a grinding wheel.

10. The device according to claim 7, wherein the abrasive contact surface region of the rotating grinding tool is held resiliently relative to the welding electrode.

11. The device according to claim 7, wherein the device is an integral part of a welding head or a welding gun.

12. The method according to claim 3, wherein a surface region of the welding electrode and the abrasive contact surface region of the rotating grinding tool are pressed resiliently against one another.

13. The method according to claim 4, wherein a surface region of the welding electrode and the abrasive contact surface region of the rotating grinding tool are pressed resiliently against one another.

14. The method according to claim 5, wherein a surface region of the welding electrode and the abrasive contact surface region of the rotating grinding tool are pressed resiliently against one another.

15. The method of claim 2, wherein a circumferential surface of the grinding tool moves past an end face of the welding electrode.

16. The device according to claim 8, wherein the abrasive contact surface area of the grinding tool is formed by a concavely curved peripheral surface of a grinding wheel.

17. The device according to claim 8, wherein the abrasive contact surface region of the rotating grinding tool is held resiliently relative to the welding electrode.

18. The device according to claim 9, wherein the abrasive contact surface region of the rotating grinding tool is held resiliently relative to the welding electrode.

19. The device according to claim 16, wherein the abrasive contact surface region of the rotating grinding tool is held resiliently relative to the welding electrode.

20. The device of claim 8, wherein a circumferential surface of the grinding tool moves past an end face of the welding electrode.