Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2000-07-19
2001-09-25
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S107000, C219S091210, C219S091220
Reexamination Certificate
active
06294751
ABSTRACT:
The present invention relates to a method of electrically welding a member or part such as a screw or a bolt around a bore in a composite metal sheet, in situations where this metal sheet has an intermediate layer of substantially insulating material located between two metal layers. Said screw or bolt will preferably be provided with an annular or spherical rib facing the sheet, so as to make the electrical welding easier.
This intermediate layer of substantially insulating material may be a synthetic material, such as a polymer, and the metal layers may be made of low-carbon steel, for example used to make housings for engines. The thicknesses of said layers may be in the order of {fraction (5/100)} of a mm in the case of the polymer, which in this case is therefore a very fine film, and {fraction (7/10)} of a mm in the case of the metal. Such sheets are commonly known as MPM sheets.
A problem arises when the heads of screws or bolts have to be welded onto such sheets using a conventional resistance spot welding method because the sheet is clamped between two electrodes with the part to be welded placed in between: current is not able to pass between the layers of metal if the polymer film affords total insulation between them, whereas if this is not the case, the abrupt increase in the temperature of the sheet around the bore will cause an explosion due to sudden evaporation of the polymer, which can pierce the sheet under certain circumstances.
Furthermore, if low intensities are applied by flat electrodes clamped onto a single MPM sheet for long welding times, the current passes through at different points in a random manner, giving rise to random visible heat spots moving around the surface, which do not follow the direction of the electrodes and do not systematically start at the edges of the sheets, bringing to light the internal defects of the MPM sheet, which is not totally satisfactory.
The objective of this invention is to remedy the disadvantages inherent in the prior art and, to this end, the invention proposes a method of the type outlined above which is characterised in that said sheet is pre-heated in the vicinity of said bore in order to remove said substantially insulating material therefrom, after which a conventional resistance spot welding operation is performed on said part on said sheet.
In the case of certain parts, in particular heads of screws with a relatively small diameter, said pre-heating operation can be applied by passing an electric current between a counter-electrode applied to the sheet from the same side as said part, in the vicinity of said bore, and an electrode applied either to said part to be welded, or on a particular contact member taking the place thereof, if it is wanted to avoid any risk of deterioration of the part to be welded during the pre-heating step.
This counter-electrode may be applied to the sheet non-symmetrically relative to the bore.
In the case of other parts, for example bolts with a relatively large diameter, on the contrary, it is of advantage if said counter-electrode is annular and completely surrounds said part to be welded or said particular contact member in rotational symmetry so that heat is applied uniformly to the sheet around the bore.
REFERENCES:
patent: 3657509 (1972-04-01), Beneteau
patent: 3666910 (1972-05-01), Anderson
patent: 4020316 (1977-04-01), Schaft et al.
patent: 4766283 (1988-08-01), McCrovey et al.
patent: 5248869 (1993-09-01), DeBell et al.
patent: 388 486 (1965-06-01), None
patent: 53124142-A (1978-10-01), None
patent: 360137580-A (1985-07-01), None
Aro
Cooke Colleen P.
Heinrich Samuel M.
Riper Marbury Rudnick & Wolfe
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