Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2002-11-27
2004-09-14
Silverman, Stanley S. (Department: 1754)
Electric heating
Metal heating
For bonding with pressure
C219S086250
Reexamination Certificate
active
06791056
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to projection welding of metal sheet to another metal body and, more specifically, to an improved projection and projection forming process for thin aluminum sheet for projection welding thereof, and an improved welding gun for use therewith.
BACKGROUND OF THE INVENTION
Projection welding is a known technique for joining two overlapping metal sheets. In conventional projection welding, a small projection is provided on one of the sheets and extends transversely to a side of the sheet so that the tip thereof contacts the other sheet. An electrode of a welding gun is pressed into contact with one of the sheets in alignment with the projection, and a combination of force and welding current is applied to the electrode to cause the projection to collapse and form a weld nugget which joins the two sheets together at the contact area defined by the projection.
FIG. 1
illustrates a known arrangement
10
for projection hem welding. This welding arrangement
10
is particularly desirable for projection welding of thin sheet-like metal components disposed in overlapping relation, typically light-gauge steel sheets which commonly have a thickness in the range of about 0.020 to about 0.050 inch. The thin sheets
11
and
12
have portions which directly overlap, and in a hem welding process one of the sheets
11
has a flange or hem part
13
which is bent to overlap another side of the other sheet
12
. The sheet
12
is sandwiched between the sheet
11
and its hem part
13
. Intermediate sheet
12
has a bead or projection
14
projecting transversely therefrom so that the tip of the projection contacts the adjacent surface of the hem part
13
. A projection weld is created Directly between the sheet
12
and the hem part
13
at the region of the projection
14
.
An adhesive
16
can be placed between the two sheets
11
,
12
(FIGS.
2
and
3
). The adhesive
16
is a heat curable adhesive that will further secure the two metal sheets together after it is activated by applying heat thereto, usually in an oven.
In the projection welding arrangement
10
as shown in
FIG. 1
, the overlapping sheets
11
,
12
are typically positioned on a support or die
15
, and a movable welding head assembly
17
is positioned adjacent the die
15
to effect the projection weld between the overlapped sheets. The welding head assembly
17
includes a hollow housing
18
having an electrode
19
movably supported therein and projecting outwardly for contact with the overlapped sheets in the region of the projection, and a spring
21
is confined within the housing
18
and acts against an inner face of the electrode
19
so as to urge the electrode outwardly into an extended position, in which position the electrode abuts an interior stop surface formed on the housing. The welding head assembly
17
is electrically connected to a stationary transformer
22
. The power supply to the transformer
22
, and hence the welding current supplied to the welding head assembly, is controlled by a suitable control unit
23
. The transformer
22
has the primary coils
24
thereof connected to suitable electrical conductors
25
and
26
that supply electrical energy to the transformer
22
. The secondary coils
27
of the transformer are in turn connected to electrical conductors
28
and
29
, one of which is connected to the welding head assembly
17
and the other is connected to the workpieces
11
,
12
, such as being connected either to the die
15
or to an electrical contact gun
31
. The contact gun
31
includes a support
32
such as a conventional double-acting pressure cylinder, normally an air cylinder, having a conventional electrode
33
movably supported thereon. The electrode
33
engages a part
34
of the intermediate sheet
12
, which part
34
is shown as spaced from the overlapping portions of the sheets. The conductors
28
and
29
are typically constructed of a conventional flexible lamination so as to permit respective movement of the welding head assembly
17
and contact assembly
31
relative to transformer
22
.
The welding head arrangement also includes a drive device
36
for effecting movement of the welding head assembly
17
. The drive device conventionally comprises a pneumatic cylinder
37
having a housing
38
, which is typically stationarily mounted spaced and separate from the welding head assembly
17
. An extendible and contractible piston rod
39
extends from the housing
38
and couples the housing
18
of the welding head assembly
17
to the drive device
36
. The piston rod
39
controls the movement of the assembly
17
into engagement with the sheets
11
,
12
when a projection welding operation is carried out.
When projection welding as summarized above, the projection is typically stamped or embossed on the thin steel sheet by opposed dies which deform the sheet by forcing a portion thereof sidewardly so as to define a projection which projects sidewardly of the sheet generally in the direction of the force applied by the forming die. The projection typically comprises a geometric shape such as a truncated conical or partial spherical shape as it projects transversely from the sheet. This method of forming the projection and the resulting shape thereof necessarily results in the wall thickness of the projection being thinner than the thickness of the base sheet, and also typically results in the base wall of the projection (i.e. the portion of the wall where the projection joins to the base sheet) being disposed in a sloped or angled relationship relative to the plane of the sheet. These latter configurational features, however, have not detrimentally effected the ability of the projection to create proper quality welds between thin steel sheets since steel possess a high tensile strength and thus is able to withstand the significant compressive force applied thereto prior to reaching the actual melting or welding temperature. Premature collapse of the projection during projection welding of sheet steel has thus not presented a significant problem.
When projection welding an aluminum sheet, however, totally different melting temperature and tensile strength properties are exhibited by aluminum sheet in comparison to steel sheet, and accordingly repeatably and successfully effecting projection welding of aluminum sheets can not normally be achieved. More specifically, not only does aluminum possess a significantly lower strength than steel, but more significantly it has been observed that the yield strength of aluminum undergoes a significant decrease when aluminum is heated to a temperature between two and four hundred degrees F. (FIG.
4
), and in fact this significant decrease in strength occurs over a very small temperature range which is still significantly below the melting or welding temperature for aluminum. The many prior attempts to projection weld thin aluminum sheets have hence mostly met with failure since the projections have exhibited premature collapse thereof at a temperature which is significantly below welding temperature. Thus, it has not been repeatably possible to properly maintain the requisite electrode pressure on the projection, nor has it been repeatably possible for the electrode to properly follow up the collapsing of the projection so as to maintain proper current-transmitting contact therewith. The proper contact and hence transfer of current to the projection, and the proper concentration of the current through the small electric contact area defined by the projection, have thus not been dependably and repeatably achievable, and accordingly proper weld nuggets have not typically been achievable when attempting to projection weld aluminum sheets.
Accordingly, it is an object of this invention to provide improvements for projection welding of thin metal sheets and particularly improvements applicable for permitting successful projection welding in situations where the thin metal sheet having the projection formed thereon is constructed of aluminum.
More specifically, it is an objec
Patrick Edward P.
Spinella Donald J.
VanOtteren Robert G.
Cooke Colleen P.
Flynn ,Thiel, Boutell & Tanis, P.C.
Newcor, Inc.
Silverman Stanley S.
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