Note: Descriptions are shown in the official language in which they were submitted.
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78542-13
HIGH FREQUENCY CONTACT WELDING APPARATUS
WITH SKEWED CONTACTS
Field of the Invention
This invention relates to high frequency contact
welding apparatus and methods in which the shaping and
positioning of the contacts is selected to eliminate or
substantially eliminate overheating and undesirable defacing
of the part or parts being welded.
:Backaround of the Invention
Apparatus and methods for the forge welding
together of a pair of metal parts or the edge portions of a
single part folded to form a tube in which the surface
portions of the metal part or parts to be welded together
are advanced toward a weld point with a gap therebetween are
heated by high frequency electrical currents supplied to the
surfaces by way of. contacts at opposite sides of the gap and
contacting the metal part or parts in advance of the weld
point are well known in the art. See, for example, U.S.
Patents Nos. 2,821.,619; 2,873,353; 2,886,691; 2,898,440;
2,992,319; 3,047,712; 3,056,882; 3,375,344; 3,391,267 and
4,241,284.
In the high frequency prior art heating apparatus,
the downstream edges of the contacts extend at substantially
90° to the paths of advance of the metal to be heated. In
addition, the contacts are tilted so that only a small
portion of the contact faces adjacent to the downstream
edges contacts the metal to be heated. Thus, there is a
very high current density at such portions of the contact
faces. In addition, as will be further explained
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78542-13
hereinafter, with high frequency currents and with the
downstream edges of the contacts extending at 90° with
respect to the path of the metal to be heated, a large
proportion of the current leaves the contacts at the
portions of the contact face nearest the heating current
path in the metal. Therefore, the current density at the
portions of the contact faces nearest the heating current
path in the metal is much higher than elsewhere at the
current faces.
la
The use of high frequency electrical currents, i.e. currents of a frequency of
10 KHz and
higher, and particularly of 400 KHz and higher, for the heating of the
surfaces to be welded
together has certain well-known advantages as compared to the use of direct
currents or currents
of 500 Hz or lower. For example, "skin effect" causes most of the current to
flow at the surface
of a part where it is most useful, and such effect increases with frequency.
Furthermore, the
surfaces contacted by the contacts need not be clean, e.g. they can have scale
or oxides thereon.
When direct currents or low frequency currents, i.e. 500 Hz or less are used,
there is little
skin effect. In addition, the current path, when proximity effect is not
involved, is determined
primarily by the resistance of the path because the inductive reactance of the
current paths is zero
or small. Thus, the current density per unit area of the face of the contact
which contacts the part
is substantially uniform. Accordingly, to decrease the current density, one
merely has to increase
the contact area.
Current density where the contact engages the metal part is important in at
least two
respects. Thus, if the density is too high, undesirable melting of the metal
can occur in the area
of contact or close thereto. Such melting is not desirable for forge welding
because it defaces the
metal and can cause hardening of the metal by self quenching after it cools.
Even if there is no
melting, the metal can become discolored, have burn marks, cause self quenched
hardened areas
or melt metal of the contacts onto the metal part which contact metal must be
removed.
In actual practice with high frequency currents, the current can be several
thousands of
amperes with welding speeds, i.e. advancing speed, of 25-S00 ft/min. It has
been found that with
such large currents, and hence, a high contact current density, the problems
mentioned
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hereinbefore have been encountered but have been tolerated for some purposes
because of the
production speeds available.
Because of various pieces of equipment needed for mechanical reasons, e.g.
forming
rolls, forge welding pressure rolls, etc., the space available for the
contacts and their mountings is
limited. However, while increasing the size of the contacting face of the
contacts causes some
improvement with high frequency currents, there is a need for further
improvement. Even with
an increase of the size of the contacting face and high frequency currents,
most of the current
concentrates at the portion of the face nearest the metal surface to be heated
and particularly, at
the downstream corner nearest the surface being heated.
Summar5r of the Invention
I have discovered, and have confirmed by tests, that by skewing the downstream
edges of
the contacts which supply high frequency electric current to surfaces of metal
parts or opposite
edge surfaces of a part (hereinafter sometimes referred to as "metal
surfaces") to be forge welded
together, the burning, hardening, discoloration, etc. of the part or parts
where the contacts engage
such parts can be substantially eliminated or at least substantially reduced
thereby providing a
better welded product.
In the prior art, the downstream edges of the contacts extend substantially
perpendicularly
to the paths followed by the metal surfaces to be heated as the metal surfaces
are advanced. By
skewing the contact downstream edges is meant that the downstream edges are
oriented so that
they extend at an acute angle of from about 35 ° to about 55 ° ,
preferably about 45 ° to the paths
followed by the metal surfaces to be heated, the angle being intermediate the
downstream edges
and the weld point. To be more precise, the downstream edges lie in planes
perpendicular to the
metal surfaces which are contacted, and such planes intersect the paths of the
metal surfaces to be
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heated at an acute angle so that the angles are defined between the planes and
the portions of
such paths downstream of the contacts. At an angle less than about 35 °
, the current increases at
the downstream end of the contact edge and causes the undesirable effects
mentioned, and at an
angle greater than about 55 ° the desired current distribution is not
obtained and the undesirable
effects again occur.
~ Thus, for example, in a known process for welding of a tube or pipe seam, a
single piece
of metal is folded to bring the edge surfaces adjacent each other but with a
gap therebetween as
the metal is advanced to a weld point where the surfaces at forge welding
temperature are forced
together. The electrical heating current is supplied to the edge surfaces from
a source of the
current by way of a pair of contacts, one at one side of the gap and the other
at the other side of
the gap and both engaging the metal in advance of the weld point. Each contact
has a linear, e.g.
rectilinear, edge nearest the weld point, i.e. a downstream edge, which
engages the metal and
extends at an angle of from about 35 to about 55 with respect to the path of
the edge surface of
the metal as it is advanced. The angle is selected so that the current density
at the edge portion of
the contact is more nearly uniform throughout the width of the edge portion,
i.e. the dimension of
the edge portion in the direction transverse to the metal edge surface.
Similarly, if the metal surfaces to be heated are advanced toward a weld point
in
overlapping relation with a gap therebetween and the heating current is
supplied thereto by
contacts at opposite sides of the gap and engaging the metal in advance of the
weld point, the
downstream edges of the contacts extend at an angle from about 35 ° to
about 55 ° with respect
to the paths of the metal surfaces to be heated as the metal surfaces are
advanced.
Also, if an edge surface of one metal part is to be welded to a surface of
another part,
which is not an edge surface, as in the manufacture of structural members,
e.g. T, H or I beams or
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as on the welding of a fin to a tube, the principles of the invention apply.
Thus, as the parts are
advanced toward a forge weld point with a gap therebetween and the electrical
current is supplied
to the parts by contacts at opposite sides of the gap and engaging the parts
in advance of the weld
point, the downstream edges of the contacts are skewed as described
hereinbefore.
Therefore, in accordance with the invention, the skewing of the contact or
contacts is
selected so that each portion of the edge of the contact nearest the weld
point has, between such
portion and the weld point, a path for the welding current which has
substantially the same
impedance, the resistance component of the impedance being small relative to
the reactive
component. In other words, the orientation of the downstream edge of a contact
is selected so
that the impedance of all current paths from the downstream edge to the weld
point is more
nearly equal whereby the heating current from the contact to the metal surface
adjacent thereto
which is to be heated is substantially uniform. In this way, the heating
current is not
concentrated at one portion of the contact, and hence, the problems described
hereinbefore are
substantially reduced.
If desired, each contact may comprise two or more conductively connected parts
which, if
desired, can be individually pressed toward the metal part, such as by springs
or by an air
cylinder.
Brief Description of the Drawings
The invention will be better understood by reference to the following detailed
description
of preferred embodiments of the invention which description should be
considered in connection
with the accompanying drawings in which:
Fig. 1 is a diagrammatic, fragmentary, perspective view illustrating
prior art high frequency forge welding of abutting edge surfaces of a
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metal strip or metal strips using contacts to supply high frequency
electrical current from a source thereof to the metal strip or metal
stnps;
Fig. 2 is similar to Fig. 1 with the contact modifications
of the invention;
Fig. 3 is a graph related to the footprint of a contact on a
metal surface of Fig. 1 and illustrates the current distribution at
the downstream edge portion of the contact;
Fig. 4 is similar to Fig. 3 but illustrates the current
distribution at the downstream edge portion of the contact when
the contact is disposed in accordance with the invention;
Figs. 5-8 are end views of contacts illustrating various
cross-sectional shapes which the contacts can have;
Fig. 9 is a diagrammatic, fragmentary, perspective view
of one embodiment of holders for the contacts;
Fig. 10 is a diagrammatic, fragmentary, end elevation view
of an alternate embodiment of the contact holders;
Fig. 11 is a diagrammatic, fragmentary, bottom view
illustrating contacts of the invention with two parts;
Fig. 12 is a diagrammatic, fragmentary, perspective
view of the forge welding together of overlapping portions of a
metal sheet or of metal sheets;
Fig. 13 is a diagrammatic, fragmentary, perspective
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view of the manufacture of tubing by spirally winding a ribbon and
forge welding edge portions of the ribbon together;
Fig. 14 is a diagrammatic, fragmentary, perspective
view of the forge welding of a fin on a tube;
Fig. 15 is a diagrammatic, fragmentary, perspective
view of the manufacture of a T-shaped element by forge welding
an edge portion of a metal strip to an intermediate portion of
another metal strip;
Fig. 16 is a diagrammatic, fragmentary, perspective
view of the forge welding together of the lips on a metal sheet
or on metal sheets; and
Fig. 17 is a diagrammatic, fragmentary, perspective
view of the forge welding of two longitudinal metal strips to a tube.
Detailed Description of Preferred Embodiments
The principles of the invention are applicable to the various types of high
frequency
contact welding systems, e.g. butt and lap welding of a longitudinal or spiral
tube seam,
structural members, such as T, I and H members, spiral fins on a tube, tubular
cable sheathing,
lip tubes with either a longitudinal or spiral lip and the welding of one
metal strip to another
metal strip as the strips are moved longitudinally with either a butt or lap
weld.
Fig. 1 illustrates diagramatically the prior art forge welding together at a
weld point 1 of a
pair of edge surfaces 2 and 3 which can be the edge surfaces of a pair of
metal strips or the
opposite edge surfaces of a single metal strip which has been folded to form a
tube. The edge
surfaces 2 and 3 are advanced in the direction of the arrow 4 and are
separated by a gap 5 in
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advance of the weld point 1. To take advantage of the "proximity effect", the
gap is relatively
small, and the angle 6 between the edge surfaces can be about 4 ° to
about 7 ° . A weld seam 7 is
present following the weld point 1.
High frequency electric current, e.g. current of a frequency of at least 10
KHz, is supplied
to the edge surfaces 2 and 3 by way of a pair of contacts 8 and 9 in sliding
engagement with the
top surfaces 10 and 11 of the metal part or parts with one contact 8 at one
side of the gap 5 and
the other contact 9 at the other side of the gap 5. The contact 8 is adjacent
the edge surface 2,
and the contact 9 is adjacent the edge surface 3. Normally, there is a small
spacing between the
edge surfaces and the respective contact as shown.
The contacts 8 and 9 have contact faces which contact the surfaces 10 and 11
and which
have downstream rectilinear edges 12 and 13 which lie substantially on lines
14 and 15,
respectively. Such lines 14 and 1 S are respectively substantially
perpendicular to the planes of
the edge surfaces 2 and 3, and hence, the paths of the heating current in the
metal.
From the contacts 8 and 9, the high frequency current flows in the metal part
or parts
along a plurality of contiguous paths to the edge surfaces 2 and 3, only three
of the paths for each
contact, paths 16-18 and 19-21, being indicated in dotted lines in Fig. 1. It
will be observed that
the paths of current flow have different lengths. With direct current or low
frequency current, the
amounts of current in each path is determined only by the resistance of each
path, and therefore,
the current in each path does not vary significantly. However, with high
frequency current, the
amounts of current in each path is determined not only by the resistance of
each path which, due
to skin effect, is higher than the direct current resistance, but also by the
reactance of each path.
In addition, due to the proximity of the contact 8 to the contact 9, the
current density is greatest at
~~~ the downstream corners of the contacts 8 and 9 which are nearest each
other due to the proximity
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effect. It has been found that currents in the paths 16, 17, 19 and 20 are
much larger than
currents 18 and 21 and larger than currents along paths intermediate paths 17
and 18 and paths
20 and 21. Therefore, the current density at the edges 12 and 13 of the
contacts 8 and 9 varies
from a large value at the end of the edge 12 nearest the edge surface 2 to a
significantly smaller
value at the end of the edge 12 farthest from the edge surface particularly at
current frequencies
of 400 KHz and higher. The current density along the edge 13 similarly varies.
It has been
discovered that such variation in current distribution is the cause of the
contact welding problems
described hereinbefore.
After discovering the cause of the prior art contact welding problems, I have
performed
experiments and have found that the problems can be substantially eliminated
or reduced by
skewing the downstream edge of one or both, preferably both contacts, so that
the downstream
edge extends at an angle from about 35 ° to about 55 °,
preferably about 45 °, to the path of the
metal to be heated as the metal is advanced . Such angle is intermediate the
contacts 8 and 9 and
the weld point 1.
Fig. 2 is a diagrammatic illustration of contacts skewed in accordance with
the invention.
Reference numerals in Fig. 2 which are the same as reference numerals in Fig.
1 designate the
same elements. Although the contacts 8 and 9 can have the same size,
respectively, as the
contacts 8 and 9 in Fig. 1, the downstream edges 12 and 13 can be longer as
shown in Fig. 2.
In Fig. 2, the downstream edges of the contacts 8 and 9 extend at acute angles
22 and 23
with respect to the paths of the edge surfaces 2 and 3 as they are advanced.
Preferably, both
angles are about 45 ° , but the angles can be in the range of about 35
° to about SS ° and need not
be the same. With angles less than about 35° and more than about
55° , the current distribution
alters so that the problems described hereinbefore again arise.
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It will be observed from a comparison of Figs. 1 and 2 that the lengths of the
current
paths which are spaced from the ends of the contacts 8 and 9 nearest the
surfaces 2 and 3 are
shorter in Fig. 2 than in Fig. 1. Accordingly, the impedances of such paths
are smaller, and the
magnitude of the current flow therein is greater. Therefore, for the same
amount of heating
current at the surfaces 2 and 3, the current is better distributed across the
contact faces of the
edges 12 and 13, and the currents in the paths 16, 17, 19 and 20 can be less
and hence,
overheating, burning, marnng, etc. of the metal surfaces 10 and 11 by the
currents in the paths
16, 17, 19 and 20 can be substantially eliminated.
Figs. 3 and 4 further illustrate diagrammatically the differences between the
contact
current density with prior art contacts and high frequency current and the
contact current density
with the contacts of the invention. Although, for simplicity, the effects with
the contact 8 are
described in connection with Figs. 3 and 4, similar effects result with the
contact 9.
As mentioned previously, the faces of the contacts are tilted with respect to
the surface of
the metal to be heated so that only a small portion of the contact face
adjacent the downstream
edge actually engages the metal surface at any given time. Also, for the most
part, the heating
electrical current leaves the contact face close to the downstream edge of the
contact.
Fig. 3 illustrates the footprint 8a of the contact 8 shown in Fig. 1. Almost
all the current
will leave the contact 8 in the downstream portion 24 of the area defined by
the footprint 8a. The
dimension d in Fig. 3 can be of the order of 0.030 inches at 300-400 KHz.
The current distribution in the portion 24 is illustrated by the graph in the
upper part of
Fig. 3. Thus, the current in portion 26 is relatively large at the end of the
footprint 8a nearest the
surface 2 to be heated and is relatively small at the opposite end.
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In contrast, when the downstream end of the contact 8 is skewed as shown in
Fig. 4, the
current density in the portion 24 can be nearly constant as illustrated by the
graph in the upper
part of Fig. 4 or at least, the difference between the current magnitudes at
opposite ends of the
portion 24 can be substantially less whereby the problems described
hereinbefore can be
substantially reduced or eliminated.
From the foregoing, it will be observed that only a relatively small portion
of the contact
face is necessary for supplying heating current to the metal surfaces 10 and
11. Therefore, it is
necessary that the cross-section of a contact be only of a size and shape
which will provide the
necessary physical strength and wear life.
As shown in Figs. 5-8 which show end views of contacts 8a-8e, the contacts 8a-
8e being
viewed facing the end of the contact which engages the metal surface, the
contacts can have
various cross-section shapes. Contact 9 can also have such shapes in reverse.
Because the contacts are connected to the high frequency power source by
transmission
lines, it is not convenient to skew the contact holder. Accordingly, it is
preferred to use contacts
with downstream edges, e.g. edge 12, skewed at the desired angle as shown in
Figs. 5-8 without
changing the position of the contact holder.
Preferably, the downstream edge is rectilinear, but if it is desired to modify
the current
density at the contact face, the downstream edge, e.g. edge 12, can be
curvilinear as shown in
Fig. 8.
The contacts used to supply heating current from the source to the metal to be
heated can
be mounted on supports or holders in any conventional manner. For example, the
contacts 15
and 16 illustrated in U.S. Patent No. 3,056,882 can be replaced by the
contacts 8 and 9 of Fig. 2
herein so that the downstream edges of the contacts extend at an angle of
about 35 ° to about 55 °
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with respect to the paths of advance of the metal surfaces to be heated. Thus,
the contacts 8 and
9 are pressed against the metal at opposite sides of the gap between the metal
surfaces to be
heated by springs 60 described in the 3,056,882 patent.
Alternatively, the contacts 15 and 16 of Patent No. 3,056,882 can be replaced
by copper
alloy bars 25 and 26 which extend through the holders 27 and 28 and are held
in place by
clamping screws, only the screw 29 being visible in Fig. 9, but a similar
screw being present on
holder 27. Thus, as the bars 25 and 26 wear, the clamping screws can be
loosened, the bars 25
and 26 moved down and the screws tightened. The holders are urged toward the
metal surfaces
and 11 by springs as described in said Patent No. 3,056,882.
As a further alternative, the supports or holders can be held in fixed
positions and slidably
receive the bar contacts which air urged toward the metal by the piston rods
of air actuated piston
and cylinder assemblies as illustrated diagrammatically in Fig. 10. Thus,
after the holders 30 and
31 are placed in the positions shown in Fig. 10, the holders 30 and 31 are
held in fixed positions.
The contact bars 25 and 26 extend through openings in the holders 30 and 31
and are slidably
received in such openings. The contact bars 25 and 26 are urged toward the
metal to be heated
by the piston rods 32 and 33 actuable by pistons in the air cylinders 34 and
35.
Although contact bars 25 and 26 of square cross-section have been shown in
Figs. 9 and
10, it will be apparent to those skilled in the art that bars of other cross-
sections, such as those
shown in Figs. 5-8, can be used in place of the contact bars 25 and 26.
As previously mentioned, each of the contacts can be made in a plurality of
conductively
connected parts, each part being separately movable toward and away from the
metal to be
heated and each part being separately biassed or urged toward the metal. It
has been found that
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with such construction of the contacts, less arcing occurs between the
contacts and the metal
contacted.
Fig. 11 illustrates diagrammatically two part bar contacts which can be used,
for example,
in the embodiment shown in Fig. 10. In Fig. 11, the holder 30 slidably
receives two bars 25a and
25b which are longitudinally movable with respect to each other and are
separately urged toward
the metal to be heated, such as by two air cylinder and piston assemblies 34.
Similarly, the
holder 31 slidably receives two bars 26a and 26b movable longitudinally with
respect to each
other and separately urged toward the metal to be heated, such as by two air
cylinder and piston
assemblies 35. The bars 25a and 25b and the bars 26a and 26b preferably are
spaced from each
other by high temperature insulation spacers 36 and 37. The parts or bars 25a
and 25b have
downstream edges 12a and 12b, respectively, which extend at an acute angle of
about 35 ° to
about 55 ° to the path of the metal to be heated and which are aligned
with each other. The parts
or bars 26a and 26b similarly have downstream edges 13a and 13b which are
similarly oriented
and aligned. The holders 30 and 31 being made of conductive metal,e.g. copper,
conductively
interconnect the contact parts held thereby.
Although bar parts 25a and 25b and bar parts 26a and 26b have been shown as
square in cross-
section, it will be apparent to those skilled in the art that the parts can
have other cross-sectional
shapes.
As previously mentioned, the principles of the invention are applicable to
contact welding
methods and apparatus which are different from the methods and apparatus shown
in Figs. 1-11
and described in connection therewith. Figs. 12-17 illustrate schematically
other applications of
the invention.
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For example, if it is desired to form tubing with a weld seam or to weld two
metal parats
together with the metal overlapping at the weld, such as is disclosed in U.S.
Patent No.
2,886,691, the contacts 8 and 9 are disposed as shown in Fig. 12, i.e. with
their downstream
edges 12 and 13 extending at an angle of about 35 ° to about 55
°, preferably about 45 °, to the
paths of the overlapping portions of the metal surfaces to be heated as they
are advanced to the
weld point 1 a.
Fig. 13 illustrates the positioning of the contacts 8 and 9 in the welding of
helically
formed tubing as described in U.S. Patent No. 2,873,353, Fig. 13 specifically
illustrating butt
welding but the positioning of the contacts 8 and 9 being the same for lap
welding. Thus, as the
tube 38 with a helical weld seam is rotated in the direction of the arrow 39,
a metal ribbon 40 is
advanced in the direction of the arrow 41 toward a weld point lb. The
downstream edges 12 and
13 of the contacts 8 and 9 extend at an angle of about 35 ° to about 55
°, preferably about 45 ° to
the paths of the edge surfaces 42 and 43 as they are advanced to the weld
point lb.
Fig. 14 illustrates the positioning of the contacts 8b and 9 in the welding of
a fin 44 on a
tube as described in U.S. Patent No. 3,047,712 except that the contact 8b
engages the side of the
fin 44 rather than the top edge of the fin 44. Depending upon the width of the
fin 44, the contact
8 or 8a, or any of the contacts previously described, can be used to contact
the fin 44. In the fin
44 is relatively narrow, it may be preferable to use the contact 8a as shown
in Fig. 14.
The downstream edges 12 and 13 of the contacts 8a and 9 are oriented as
previously
described with respect to the paths of the metal to be heated as such metal of
the tube 45 and the
fm 44 are advanced to the weld point I c.
Fig. 15 illustrates the positioning of the contacts 8 and 9 in the manufacture
of welded
structural elements as described in U.S. Patents Nos. 2,821,619 and 3,391,267,
the welding of a
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T-shaped element being specifically shown in Fig. 15 but the positioning of
the contacts with
respect to H-shaped elements being apparent from Fig. 15 and Patent No.
3,391,267. Thus, two
pairs of contacts of the invention replace the two pairs of contacts used in
the welding of H-
shaped structural elements.
In Fig. 15, a pair of metal strips 46 and 47 are advanced toward a weld point
1 d in the
direction of the arrow 4 with a gap therebetween in advance of the weld point
1 d. The
downstream edges of the contacts 8 and 9, such as the edge 12 of contact 8,
extend at an angle of
from about 35 ° to about 55 °, and preferably about 45 °,
with respect to the paths of the metal of
the strips 46 and 47 to be heated. Since the path adjacent to contact 9 in
which the heating
current flows is not at the edges of the strip 47, it can be desirable to
increase the length of the
path of the heating current in the strip 47 relative to the path of the
current in the strip 46.
Accordingly, the contact 9 can be further upstream from the weld point 1 d
than the contact 8 as
shown in Fig. 15.
Fig. 16 illustrates the positioning of the contacts 8 and 9 for the welding
together of lips
48 and 49 which can be formed at the opposite edges of a metal sheet formed
into a tube as
shown in U.S. Patent No. 2,992,319 or at the edges of a pair of sheets to be
secured together.
Thus, there are a pair of metal surfaces 50 and 51 terminating in lips 48 and
49 to be
forge welded together at a weld point 1 e. The lips 48 and 49 are advanced
toward the weld point
1 a with a gap therebetween and the heating current flows on the facing
surfaces of the lips 48 and
49 and is supplied thereto by way of the contacts 9 and 8 respectively
engaging the surfaces 50
and 52. The downstream edge of the contact 9 extends at an angle from about 35
° to about 55 °,
preferably about 45 °, with respect to the path of the surface of the
lips 48 being heated and the
CA 02349628 2001-05-04
downstream edge of the contact 8 extends at a similar angle with respect to
the path of the
surface of the lip 49 being heated.
Fig. 17 illustrates the positioning of the contacts 8 and 9 when a pair of
metal strips are
being welded to a metal tube as described in U.S. Patent No. 3,375,344. Thus,
as the tube 52 and
the strips 53 and 54 are advanced in the direction of the arrow 4, the strips
53 and 54 are heated
at their faces 53a and 54a and the longitudinal surface portions of the tube
52 adjacent thereto are
heated by the electric current supplied by way of contacts 8 and 9 connected
to a high frequency
electric power source by way of leads 55 and 56 overlying the tube 52. In
advance of the weld
points l f and l g, there are gaps between the strips 53 and 54 and the tube
52. At the weld points
if and lg, the strips 53 and 54 are pressed adjacent the tube 52 to form forge
welds
therebetween.
The downstream edges of the contacts 8 and 9 extend, with respect to the paths
of the
metal to be heated, at an angle between about 35 ° and about SS
°, preferably about 45 °.
In the embodiments described hereinbefore, the downstream edges of the
contacts can
extend at the same angle with respect to the paths of the metal being heated
and advanced.
However, the downstream edges of the contacts can extend at different acute
angles with respect
to such paths. Also, in some cases, such as when marking of one of the metal
pieces is not
objectionable, the downstream edge of the contact engaging such one metal
piece can extend at
90° with respect to the path of the metal being heated, as in the prior
art.
In addition, in the embodiments described hereinbefore, both contacts can have
the same
cross-sectional shape, or the cross-sectional shape of one contact can be
different from the cross-
sectional of the other contact.
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It will be understood that the various methods and devices for cooling the
contacts and
their supports which are known in the art can, and normally would, be used in
the apparatus of
the invention.
Although preferred embodiments of the present invention have been described
and
illustrated, it will be apparent to those skilled in the art that various
modifications may be made
without departing from the principles of the invention.
17
