Stud welding

A capacitive stud welding having an inductive-capacitive welding current supply circuit is provided with a stud and a tunable inductor. The stud is attachable to a workpiece and has an end portion shaped to retain in place on the workpiece at the weld point the flux from the stud to form an optimum weld. The tunable electrical inductor optimizes resonance of the inductive-capacitive welding current supply circuit and the flow of welding current through the stud and workpiece.

BACKGROUND OF INVENTION 
1. Field of Invention 
The present invention relates to stud welding. 
2. Description of Prior Art 
It has been common practice to attach labels or other information bearing 
tags to steel bars or other metallic product bundles or objects before 
their shipment. So far as is known, these studs have been typically welded 
by using a capacitive stud welding technique. There have been considerable 
problems with the studs being broken or knocked off when contacted by 
other objects. This in turn caused costly reidentification procedures. The 
strength of the weld between the stud and the object or workpiece has, 
within applicants' knowledge, been unsatisfactory. 
SUMMARY OF INVENTION 
Briefly, the present invention provides a new and improved capacitive stud 
welding system having an inductive-capacitive welding current supply 
circuit. The welding system has a new and improved stud for attachment by 
the system to the workpiece and a tunable inductor for optimizing 
resonance of the inductive-capacitive welding current supply circuit and 
current flow through the stud and the workpiece. 
The stud is used to attach a label or other component to the workpiece and 
includes a connector head at one end for fitting into a welding gun of the 
system. The stud also includes a collar formed along an intermediate 
portion of the stud for fitting the stud into the gun and also for 
retaining the label or other component on the workpiece when the stud is 
attached. The stud also includes an outwardly extending skirt formed at an 
opposite end opposite the connector head. The skirt forms an interior 
recess defining an extended, yet sheltered arc length for flow of applied 
welding current to the work piece. The interior recess also retains molten 
material from the heated stud in place against the workpiece during 
welding. Additionally, the skirt functions to provide a larger surface 
area over which an arc can be formed than the prior tip or point on 
conventional studs. The larger surface area thus permits longer weld time 
and a better arc when the workpiece has a rough or irregular surface. 
The tunable inductor of the welding system according to the present 
invention optimizes resonance of the inductive-capacitive welding current 
supply circuit and flow of heating current through the stud and the 
workpiece. The tunable inductor includes a sheet of electrically 
conductive material wound into a cylindrical shape and having a 
longitudinal passage formed through it. A ferromagnetic core is movable 
within the longitudinal passage of the sheet to vary the inductance of the 
coil and thus the resonance of the welding current supply circuit. 
The welding system according to the present invention thus provides a 
tunable electrical inductor/controller to control the current flow in the 
stud welding system. The welding system according to the present invention 
also provides a new and improved welding stud which co-acts with the 
controller for improved welding results on irregular or rough surfaces.

DESCRIPTION OF PREFERRED EMBODIMENT 
In the drawings, the letter W designates a new and improved stud welding 
system according to the present invention. The stud welding system W is 
used to weld a stud S (FIGS. 1 and 4) to a workpiece P. The stud welding 
system W is of the capacitive type having an inductive-capacitive current 
supply circuit 10 connected through a tunable inductor or coil L (FIGS. 
1-3) to a stud welding gun G in which the stud S is mounted. The current 
supply circuit 10 (FIGS. 1 and 5) is connected at a first positive, or +, 
output terminal 12 by a conductor 14 and clamp C to the workpiece P. The 
current supply circuit 10 is connected at a negative opposite, or -, 
output terminal 16 by a conductor 18 to the tunable inductor L. 
The inductive-capacitive current supply or driving circuit 10 (FIG. 5) 
includes a chargeable bank of capacitors which, as will be set forth, is 
controllably discharged through a suitable semiconductor switching device 
(usually a silicon-controlled rectifier or SCR) into an electrical load, 
in this case the welding stud S. 
In prior art stud welder driving circuits, the capacitance of the 
capacitors has been a fixed value, while the electrical properties of the 
load such as welding leads and varying grounds usually varied based on the 
particular welding operation conditions during the welding operation. 
Thus, it has been found that the flow of electrical power through these 
leads and grounds to the welding stud load was not satisfactorily 
balanced. For example, the switching through SCR's into a stud welding 
load produced a high level of harmonic currents, due to the abrupt 
switching and discontinuous waveforms and an abrupt power surge would 
occur. 
With the present invention, it has been found that the supply circuit 10 
and the tunable inductor L operating in conjunction overcome the abrupt 
and unsatisfactory power flow problems, smoothing the power surges. The 
inductor L has bi-directional power flow properties that permit it to 
solve this problem whether the load is a welding stud or an electrical 
power source. The supply circuit 10, as will be set forth, provides an 
initial arc current of high level. 
The inductor L according to the present invention (FIGS. 2-3) has a 
ferromagnetic core 20 in the form of an elongated toroid or plug of 
silicone steel. The core has fitted within it a threaded inner member such 
as spaced nuts 22 or a threaded inner surface or sleeve. The threaded 
members 22 are fitted onto a rotatably movable threaded rod 24. The 
rotatable rod 24 is driven by motor or hand through an adjustment or 
control knob 26 to advance or retract the core 20 along length of the rod 
24. As the rod 24 is rotated, the ferromagnetic core 20 moves along the 
longitudinal axis of threaded members 22 and the rod 24. The direction of 
movement of core 20 is governed by the rotational movement of the rod 24. 
The core 20 is within a tube or sleeve 27 of a suitable resin such as a 
plexiglas. The tube 27 serves as an inner sleeve or roller onto which an 
inductive coil 28 is wound. The coil 28 is in the form of a sheet 30 of 
electrically conductive material wound in a cylindrical spiral roll of 
adjacent layers 32 about the tube 27. For electrical insulation purposes, 
an insulative laminate sheet 34 is mounted between successive ones of the 
spiral layers 32 of the spirally wound reel or sheet 30 of conductive 
material. 
The insulative material 34 is formed of a suitable pliable, relatively thin 
but insulative synthetic resin. A suitable such material is that sold 
under the trade name Mylar, having an example thickness of about 0.010", 
or ten thousandths of an inch or so. 
The conductive material in the sheet 30 may be any suitable electrically 
conductive material such as copper, aluminum, silver or the like. Alloys 
of these and other electrically conductive materials may also be used, if 
desired. 
In the preferred embodiment, the sheet 30 of conductive material in the 
coil 28 is a sheet of copper wound into twelve or so spirally wound layers 
34 spaced from each other by the insulative material layer 32. The 
conductive copper sheet 30 is from about ten to twenty, such as 
approximately fifteen, feet in length. The sheet 30 is from about four to 
eight, typically approximately six, inches in width along the rod 24. The 
sheet 30 is usually between ten- and twenty-thousandths of an inch thick, 
in the preferred embodiment about 0.016" in thickness. 
With the present invention, it has been found that a spirally wound sheet 
of conductive metal, such as copper, foil functions better as an inductor 
coil than wound wire coils. The conductive metal foil sheet exhibits 
practically no loss due to resistance. In contrast, round wire in coils on 
passage of current through them begin to heat up due to what is known as 
the skin effect, increasing the resistance. Such increased wire resistance 
causes a reduction in welding current. 
The coil 28 is electrically connected into conductor 18 of the stud welding 
system W by a first electrical connector 40 mounted along an inner end 42 
of the sheet 30 of conductive material. The coil 28 is also electrically 
connected into conductor 43 (FIG. 1) and the welding gun G of the stud 
welding system W by a second electrical connector 44 (FIGS. 2 & 3) mounted 
along an outer end 46 of the sheet 30 of conductive material. 
As the toroidal core 20 moves along the rod 24 within the coil 28, the 
magnetic permeability and thus the inductance of the inductor L changes. 
Since the inductor L is connected in series with the capacitive welding 
current supply circuit, the resonant frequency or resonance of this 
inductive-capacitive (or L-C) circuit can be changed. This in turn varies 
the amount of welding supply current flowing into the stud S, without any 
variation in the resistive load of the stud S and the workpiece P. Thus, 
with the present invention, the inductance of the inductor can be tuned or 
adjusted to vary the amount of welding supply current flowing to the load 
of the stud S and the workpiece P as required during stud welding 
operations. 
This permits more equal distribution of heat between the workpiece P and 
the stud S. It also allows for the workpiece P to be more effectively 
heated up, so that the workpiece P is more receptive to welding. 
The stud S (FIGS. 1 and 4) of the welding system W of the present invention 
is generally cylindrical and formed of any conventional welding stud 
material. The dimensions of the stud S also may vary based on intended end 
use. In one embodiment, the stud S is one inch or so in length. 
The stud S is adapted to be fitted at a connector head 50, such as a chuck 
or collet, in a connector end 52 into the stud welding gun G. The stud S 
extends from the connector end 52 to a laterally extending collar or ring 
54 formed along an intermediate portion 56. The collar 54 is adapted to 
function as a stop or stay when the stud S is inserted into the gun G. The 
collar 54 also retains a component to be attached, such as a label or tag 
T, in place on the stud S adjacent the workpiece P once stud welding is 
completed. Usually, the diameter of the collar 54 is forty or fifty 
percent of the length of the stud S. 
It should be understood that the label or tag T is only an example 
component which is attachable by the stud S. Other types of components 
such as attachment structure for insulation and fireproof materials for 
furnaces and high temperature equipment may also be attached. 
The stud S extends from the intermediate portion 56 to a work or lower end 
58 opposite the gun end 52. A skirt 60 is formed extending longitudinally 
outwardly from the lower end 58 of the stud S, forming an interior recess 
62 adjacent an interior end wall or surface 64. For a one inch stud, a 
skirt of twenty five mils or so in length and wall thickness is suitable. 
The interior end surface 64 may be a substantially flat surface, as shown, 
or may be an inwardly curved or hemispherical surface. 
The skirt 60 is integrally formed with the stud S, having an outer diameter 
equal to that of the intermediate portion 56 and gun head 52. For a stud S 
which is about one inch long, an outer diameter of approximately one-sixth 
inch or so is usually suitable. 
The skirt 60 may take any of several forms. In a simplest form, the skirt 
60 has a substantially flat end surface 66 for initial contact with the 
workpiece P. However, with the present invention, it has been found that 
other forms of end surfaces for the skirt 60 may offer added efficiencies 
in stud welding. For example, an undulating or rippled end surface 65 
(FIG. 4A) of alternating crests 65 and troughs 66 for an alternate end 
portion 67 on the skirt 60 of a generally sinusoidal configuration has 
been found to,be suitable. Alternatively, a notched or indented end 
surface 70 (FIG. 4B) for the skirt 60 of alternating toothlike members 72 
and gaps 74 may also be used. Other toothed surfaces may also be used. 
The skirt 60 of the welding stud S according to the present invention 
performs several functions. First, the skirt 60 provides an initial 
contact or pressure point for the stud S on the workpiece P. Depending on 
the particular stud welding operations, the stud S may be selected 
according to the most suitable type of end surface needed. The skirt 60 
also provides a larger surface area over which an arc can be formed than 
the prior conventional tip or point. This permits longer weld time and 
better arc formation, particularly on rough or irregular workpiece 
surfaces. Further, the reduced wall thickness of the skirt 60 affords a 
relatively high resistivity path for the welding current which causes 
vaporization of the skirt material, and thus initiating the arc. This more 
rapidly produces a melting point in the stud S at the workpiece P. The 
recess 62 also defines an extended, yet sheltered arc length within the 
skirt 60 for flow of applied welding current to the workpiece P. The stud 
S and its recess 62 also form a pocket for accumulation of molten flux on 
the workpiece P, providing adequate molten stud material for a strong and 
reliable weld. The skirt also standarizes arc lengths on rough or ground 
workpiece surfaces. 
Considering the welding supply circuit 10 (FIG. 5) more in detail, a bank 
of parallel connected capacitors 80 is charged with direct current 
provided from a rectifier bridge circuit 82. The bridge circuit 82 
receives operating alternating current electrical power from terminals 84 
and 86 which are connected to any suitable source of alternating current 
electrical power. A protective fuse 88 is connected between the terminal 
84 and the bridge 82. 
A low-voltage, transformerless power supply circuit 90 is also connected in 
supply circuit 10, usually between the input terminal 86 and the bridge 
82. The voltage supply 90 includes a pair of capacitors 92 and 94 
connected in series with each other between the terminal 86 and the bridge 
82. A rectifier or diode 96 is connected in parallel with the capacitor 
92, while a diode 98 of reverse polarity to the diode 96 is connected in 
parallel with a capacitor 94 and in series with the diode 96. The series 
connected capacitors 92 and 94 in effect perform the function of and 
replace a conventional iron core transformer. The voltage supply 90 
provides a much more efficient transfer of energy from the alternating 
current source at terminals 84 and 86 to the bridge 82 in place of a ferro 
magnetic, step-down transformer. 
When a first of the capacitors, such as capacitor 92, is charged through 
the diode 98 during a first half-cycle of alternating current the 
alternate capacitor 94 is discharging current into the bridge 82. The 
capacitance of the capacitors 92 and 94 is selected based on the output 
voltage to be provided to the bridge 82. For example, for a 120 volt 
alternating current power level being present at terminals 84 and 86, 
capacitors 92 and 94 with a capacitance of 252 microfarads provide an 
output voltage of 12 volts alternating current. Of course, other 
capacitance value for the capacitors 92 and 94 may be chosen based upon 
particular operating conditions for the supply circuit 10. 
A semiconductor switch 100, preferably a silicone-controlled rectifier (or 
SCR) in the supply circuit 10 is electrically connected at an anode 
terminal 101 to the inductor coil L at terminal 16 by the conductor 18. A 
gate terminal 102 of the semiconductor switch 100 is connected to a 
terminal or connection 104 between a fixed resistor 106 and a variable 
resistor or potentiometer 108 of an oscillating gate circuit O. The 
resistor 106 is connected between the gate 102 and a cathode terminal 110 
of the semiconductor switch 100. 
The potentiometer or variable resistor 108 is connected at an opposite end 
from the juncture 104 through a parallel circuit arrangement of a resistor 
112 and capacitor 114 of the oscillating gate circuit O to a welding 
control switch 116. The switch 116 controls the activation of the welding 
supply circuit 10 to furnish current from the charged capacitors 80 to the 
stud S through the inductor L. 
The resistor 112 and capacitor 114 together with potentiometer 108 form an 
R-C timing network in the oscillating gate circuit O which controls the 
time duration of firing of the semiconductor switch 100. The resistance of 
variable resistor 108 is adjusted according to desired welding operations. 
The timing of the oscillating gate circuit O is adjusted in connection 
with controlling the inductance of turnable inductor L, so that only an 
initial surge of high intensity transient current is furnished to the stud 
S. This insures a high current flow for formation of a hot contact in the 
initial arc present when switch 116 is activated. As the resistance of 
resistor 108 is decreased, the voltage present at the junction 104 and 
consequently at the gate 102 of the semiconductor switch increases much 
more rapidly, thereby causing the semiconductor switch 100 to turn on more 
rapidly when the switch 116 is depressed. 
Conversely, when the resistance of resistor 108 is increased, the ability 
of the voltage presented when the switch 116 to the gate 102 at terminal 
104 through the R-C timing network of gate circuit O is depressed 
increases more slowly, therefore decreasing the time intensity of 
application of current through the discharge of the capacitor bank 80 
through the switch 100 to the inductor L. 
Thus, in addition to the tunable inductor L overcoming power flow problems, 
the oscillating gate circuit O with its R-C timing adjustment permits 
tuning of the intensity of the current pulses formed during discharge of 
current to the stud S from the capacitor bank 80 through the semiconductor 
switch 100 and the tunable inductor L. 
Having described the invention above, various modifications of the 
techniques, procedures, material and equipment will be apparent to those 
in the art. It is intended that all such variations within the scope and 
spirit of the appended claims be embraced thereby.