Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2000-10-31
2003-02-04
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S091100, C219S091200, C219S110000, C219S119000, C428S594000
Reexamination Certificate
active
06515251
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a welding system. The present invention more particularly relates to a welding system for producing a spot weld.
BACKGROUND
It is generally known to resistance weld metallic workpieces together using an electric current. Specific applications of resistance welding include spot welding, seam welding, projection welding, and butt welding. In general, resistance welding involves squeezing the workpieces between a pair of electrodes for a set period of time (i.e., the “squeeze” time), and locally heating the workpieces by supplying an electric current until a molten pool of material from the workpieces forms at an interface between the workpieces. The current may be turned off and the workpieces are allowed to cool so that the pool solidifies to form a weld nugget at the interface to attach the two workpieces together.
Electrodes are typically used to conduct the electric current through the workpieces. The heat generated in the welding system depends, at least in part, on the electrical resistance and thermal conductivity of the workpieces, the amount of force applied to the workpieces, the amount of current supplied, and the period of time that the force is applied and the current is supplied.
During the welding operation, formation of the molten pool tends to form a surface defect (e.g., bump, depression, indentation, depression, dimple, etc.) in the surface of the workpieces. The surface defect is caused by pressure and current being applied to the workpieces by the electrodes. As the surface defect is formed, the contact between the electrodes and the workpieces may decrease. One problem with known welding systems can be that molten “expulsion” or “weld splash” may occur as the depression is formed if the force applied to the workpieces is too low, with a given current. Another problem with known welding systems can be that the defect is visible in the finished product even if later painted.
In order to overcome such problems, during the welding operation the force initially applied to the workpieces may be increased so that even after the depression forms, there is a sufficient amount of force being applied to the workpieces. However, such methods of compensating for the anticipated decreased applied force and contact between the electrode(s) and the workpieces does not necessarily render the defect perceptibly not visible. Also, such a defect may result in additional steps in the manufacturing process (e.g., filling, sanding, application of a coating, etc.) to obscure the defect, or may result in special design to hide the defect from view. In order to further overcome the problem of a surface defect, some have attempted to obscure the depression by filling it with multiple layers of paint or by designing the article so that the welded workpieces are hidden from view, which may add additional manufacturing steps and material costs.
Accordingly, it would be advantageous to provide a resistance welding system that produces a workpiece having a depression of a generally reduced depth. It would also be advantageous to reduce the depth and size of the depression to eliminate additional processing steps such as sanding, grinding, refurbishing, or applying multiple layers of a coating. It would also be advantageous to provide a resistance spot welding process for forming a spot weld that does not substantially “show” on the visible side of a welded article. It would also be advantageous to provide a resistance welding system that produces a workpiece that does not necessarily require obfuscation such positioning the weld marks out of view by the customer.
It would be desirable to provide a welding system with or providing any one or more of these or other advantageous features.
SUMMARY OF THE INVENTION
The present invention relates to a method of resistance spot welding. The method includes providing a first metallic workpiece having a primary surface and a second metallic workpiece having a secondary surface, the first workpiece and the second workpiece each substantially free of a welding projection; positioning the primary surface into physical contact with a first electrode and a second electrode into physical contact with the secondary surface, wherein the second electrode includes a secondary actuator; measuring contact force between the first electrode and the second electrode; initiating a current through the first electrode when the contact force reaches a preselected force value; forming a weld nugget in at least the second workpiece and a depression in the primary surface of the first workpiece; and controlling the location of the weld nugget by using two different classes of electrodes. The depression in the primary surface is substantially undetectable by a visual examination.
The present invention also relates to a method of resistance spot welding. The method includes providing a first metallic workpiece having a primary surface in a facing relationship to a second metallic workpiece having a secondary surface, the first workpiece and the second workpiece each substantially free of a welding projection; positioning the primary surface into physical contact with a first electrode; positioning a second electrode into physical contact with the primary surface, the second electrode including a fast follow-up cylinder; measuring contact force between the first electrode and the second electrode with a sensor; initiating a current through the first electrode on reaching a preselected contact force; forming a weld nugget in at least the second workpiece; controlling the growth of the heat affected zone by using at least two different classes of electrode material; measuring a depression formed in the primary surface by visually determining whether the depression is substantially visible to the unaided eye from a distance; and conducting a quality control test on the depression formed in the primary surface.
The present invention further relates to a method of resistance spot welding. The method includes providing a first metallic workpiece having a primary surface in an abutting relationship to a second metallic workpiece having a secondary surface, the first workpiece and the second workpiece each substantially free of a welding projection; positioning the primary surface into physical contact with a first electrode; positioning a second electrode into physical contact with the secondary surface, the second electrode coupled to a fast follow-up cylinder; directly measuring with a pressure sensor a signal representative of a contact force between the second electrode and the first electrode; initiating a direct current through the electrode on reaching a preselected contact force; forming a weld nugget in the first and second workpieces; controlling the growth of the heat affected zone; measuring a depression formed in the primary surface having a lesser depth than a depression formed in the secondary surface wherein the depression in the primary surface is less than about −0.0008 inches in depth; and conducting a quality control test on the depression formed in the primary surface.
The present invention further relates to an article of furniture including a metallic primary surface having a depression that is not substantially visible to the unaided eye and a metallic secondary surface having a depression, wherein a weld nugget provides a structural attachment of the primary surface to the secondary surface capable of bearing a relatively significant load. The article of furniture is produced by a process of resistance spot welding, which includes positioning the primary surface into physical contact with a stationary electrode of a second class; positioning a fast follow-up electrode of a first class into physical contact with the secondary surface; directly measuring with a sensor a signal representative of a contact force between the fast follow-up electrode and the secondary surface; initiating a direct current through the fast follow-up electrode on reaching a preselected contact force; forming
Elve M. Alexandra
Steelcase Development Corporation
Stoner Kiley
LandOfFree
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