Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
This invention relates to a welding apparatus for studs
having a substantially annular cross-section, which comprises
a stud holder that is movable perpendicularly to a workpiece
surface to which the stud is to be welded. A welding arc is
generated between the welding stud and the workpiece surface,
and an electromagnet coil surrounding the stud holder pro-
duces a magnetic field that acts on the arc to move the arc
along the annular face of the stud.
In an apparatus of this type, which is known from DE-B-
1 565 003, the magnet coil consists of a single or multiple
loop through which a current flows which may be the welding
current itself. The magnetic field created by this coil ex-
tends primarily coaxially with the welding stud and is only
little effective to move the arc.
Summary of the Invention
It is an object of the invention to provide a welding
apparatus which permits the welding of hollow studs safely,
strongly and quickly. It is also an object of the invention
to make such apparatus compact and simple in structure so
that it may be readily mounted on existing stud welding appa-
ratus and that it may be used in connection with hand-held
stud welding guns as are common at assembly and construction
sites.
The meet these objects, a welding apparatus for studs of
substantially annular cross-section in accordance with the
present invention comprises a stud holder which is movable
perpendicularly with respect to a workpiece surface to which
the stud is to be welded, means for generating a welding arc
within a welding gap defined between the stud and the work-
piece surface, an electromagnet coil surrounding the stud
holder for generating a magnetic field that acts on the arc,
and a magnetic flux guiding portion coupled to the electro-
magnet coil and including a frusto-conical field former hav-
ing a generatrix which points to the welding gap, an end of
the field former having a substantially annular pole surface
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surrounding the welding gap for producing a magnetic field
component which extends radially of the stud within the weld-
ng gap.
The flux guiding portion with its frusto-conical field
former enables the generation of a strong magnetic field com-
ponent which extends radially within the welding gap and is
effective to move the arc. The field former also reduces the
possibility for the magnetic field of the magnetic coil to be
coupled into the workpiece, which would otherwise weaken this
radial magnetic field component.
The frusto-conical shape of the ferromagnetic field for-
mer further results in efficient magnetic shielding of the
welding zone against external field-distorting influences and
results in a compact structure. The apparatus may thus be
mounted on hand-held stud welding guns so that it can be
flexibly used in mobile assembly applications. Due to the
slender shape of the magnetic field former, the apparatus of
the invention is suited for welding at locations that are
difficult to access.
The coil and the flux guiding portion are preferably
formed as a structural unit which is mechanically secured to
the stud welding apparatus in an axially adjustable manner.
It can be stably placed on the workpiece by adjustable spac-
ers. The stud holder is mechanically connected to a lifting
and lowering mechanism of the stud welding apparatus to pro-
duce the axial movement of the stud necessary to start the
welding arc.
Furthermore, the stud holder is electrically connected
to the terminal of a welding current source for supplying the
welding arc. The coil is electrically excited at least during
the burning phase of the arc.
By means of the apparatus according to the invention,
the magnetic flux lines generated when the coil is excited
pass pre~omin~ntly and substantially along their entire paths
through the magnetically highly conductive material forming
the apparatus and the tubular stud. Only in the vicinity of
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the welding zone, where the flux lines exit from the annular
pole face of the field former and penetrate the air gap, the
flux lines have a radial field component within the welding
gap. After the welding arc has been ignited by lifting the
stud off the workpiece surface, this radial field component
causes the arc to move in a well-defined manner along the an-
nular surface of the tubular stud. The stud welding apparatus
of the invention thus permits high quality welding of a tubu-
lar stud to a workpiece by a closed annular welding seam.
In the apparatus of the invention, the workpiece itself
does not actively take part in the formation of the magnetic
field. The apparatus thus ensures substantially constant
field conditions for moving the arc even with very differ-
ently shaped workpieces.
For easy handling and high reliability of the apparatus
according to the invention, it has turned out useful to ar-
range the field former in an electrically insulated manner
from both the stud and the workpiece. Alternatively, the
field former and/or a magnetic return-path element connected
to the magnetic field former, may be made of ferromagnetic
material exhibiting no or small electrical conductivity, or
part or all of the field former may be coated with electri-
cally insulating material. !~ ,
An electrically non-conductive sleeve, e.g. a ceramic
ring, may be disposed between the annular pole surface of the
field former and the tubular stud and likewise between the
field former and the workpiece for shielding the welding
zone. Such a sleeve or ring assists the formation of a weld-
ing bead and also serves as an insulating spacer between the
field former and the workpiece.
Portions of the apparatus may be designed such that they
serve for supplying a protective gas to form a protective gas
curtain completely surrounding the welding zone. To this end,
the stud holder preferably has at least one through-bore
which is connected to a protective gas supply and communi-
21~7594
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stud and/or that of the magnetic field former.
The chuck of the sleeve holder may be formed either as a
clamping sleeve of preferably non-magnetic material for en-
gaging the outer periphery of the tubular stud or as a peg ofpreferably ferromagnetic material for engaging the bore of
the stud.
Brief Description of the Drawing
The sole figure of the accompanying drawing shows a
schematic sectional view of a stud welding apparatus in ac-
cordance with an embodiment of the invention.
Detailed Description of Pre~erred Embo~im~t~
According to the drawing, the apparatus includes a stud
holder 1 of ferromagnetic material. The end of the stud
holder 1 facing the workpiece 2 carries the tubular welding
stud 4 by means of a chuck 3 that is formed as a holding peg.
Coaxial with the holder 1 is an electrically excitable
annular coil 5 which is embedded in a pot-shaped magnetic re-
turn-path member 6. One end of the return-path member 6 is
magnetically coupled to the stud holder 1 and the other end
is magnetically coupled to a field former 7 which is made of
ferromagnetic material. The field former 7 and the return-
path member 6 together form a magnetic flux guiding portion.
The field former 7 has the shape of a truncated cone and
is so positioned that its generatrix points to the welding
zone 8. The annular pole face 9 of the field former 7, from
which the magnetic flux exits, surrounds the welding zone 8
at a small spacing. An insulating spacer sleeve 11 is dis-
posed between the field former 7 and the workpiece 2 to sup-
port the apparatus with respect to the workpiece 2 in use.
The coil 5, the magnetic return-path member 6 and the
field former 7 are formed as a structural unit which is
mounted on the stud welding apparatus by means of two axially
adjustable columns 10.
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~ In operation, welding current is applied between the
workpiece 2 and the stud 4 which initially contacts the work-
piece. An arc is ignited by lifting the stud off the work-
piece surface. The coil 5 is electrically excited at least as
long as the arc is on. The magnetic field generated by the
coil 5 will pass through the field former 7, the air gap be-
tween the pole face 9 at the lower end of the field former 7
and the welding stud 4, the stud, the stud holder 1 and the
flux return path member 6. Within the air gap, the magnetic
field lines extend in the radial direction so that they cross
the welding current path substantially perpendicularly,
thereby producing a force that will move the arc circumferen-
tially along the annular gap.
The stud holder 1 has an axial through-bore 12 for sup-
plying protective gas to the welding zone 8.
