Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
1999-12-28
2002-05-14
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S098000, C219S127000
Reexamination Certificate
active
06388224
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to welding systems. More particularly, the present invention relates to systems for robotic stud arc welding of large studs without the use of a ferrule.
BACKGROUND OF THE INVENTION
Stud welding is a process in which the contact surfaces of a stud, or similar fastener, and a workpiece, are heated and melted by an arc drawn between them. The stud is then plunged rapidly onto the receiving surface of the workpiece to form a weld. Arc initiation, arc time, and plunging are controlled automatically.
Two basic methods of stud welding are known as stud arc welding and capacitor discharge stud welding. Both methods usually involve direct current and arcing. A conventional stud arc welding system
10
is shown in FIG.
1
. As shown, system
10
can include an electrical input
12
for incoming, three-phase power. Input
12
is connected through a fused disconnect switch
14
to a power/control unit
16
. For stud arc welding, a motor-generator, a transformer-rectifier, or a storage battery can provide the power supply. The power supply for capacitor discharge stud welding is typically a low-voltage electrostatic storage system, and the arc is produced by a rapid discharge of stored electrical energy.
Control unit
16
includes a welding current controller and is coupled to a welding tool
18
, which is usually a stud gun. Stud gun
18
typically includes a chuck or collet
20
into which a stud
26
can be received. In the arc method, a ceramic arc shield
22
(which is commonly known as a “ferrule”) is generally used to shield the arc and retain the molten weld metal. Where a ferrule is used, stud gun
18
includes a ferrule holder
24
to hold ferrule
22
in place during the welding process. Control unit
16
can also be coupled to a work clamp
28
that holds workpiece
30
in place during the welding process.
In both methods, the stud serves as the electrode, while the gun is the electrode holder. Flux is generally used for stud arc welding of ferrous alloys and is an integral part of the stud. Flux provides cleaning action, arc stability, and a protective atmosphere. The arc time for capacitor discharge welding is so short that flux is not needed. Typically, in arc welding applications, a shielding or “assist” gas is introduced. Assist gas is typically not required with the capacitor discharge method.
Two types of stud arc welding guns that are commonly used are known as “portable” and “fixed” (i.e., production type). The principle of operation is the same for both. A portable or hand-held stud gun resembles a pistol, and is usually designed to be lightweight and durable. For example, a small gun used for welding ⅛ to ½ in. diameter studs can weigh approximately 4½ lb. A larger gun, weighing approximately 11 lb, can be used for welding ⅝ to 1¼-in. diameter studs. Typical gun bodies are made of high-impact strength plastic.
FIG. 2
is a schematic drawing illustrating a hand-held stud arc welding gun. Stud arc welding guns can also incorporate a means for causing the stud to plunge or move slowly as it enters the molten pool of metal at the completion of the weld. This cushioning effect reduces weld splatter considerably and also improves weld integrity.
As shown in
FIG. 2
, stud gun
18
has a fixed core
44
and a movable core
46
. Fixed core
44
is fixed to the rear end of stud gun
18
, while movable core
46
is aligned with fixed core
44
along gun axis
43
, and is situated toward the distal end of stud gun
18
. An air gap
52
exists between fixed core
44
and movable core
46
to enable movable core
46
to move along gun axis
43
. Stud gun
18
also includes a solenoid
42
, and clutch assembly
38
, and a lifting rod
36
that cooperate to move movable core
46
during the welding process.
Stud gun
18
can also include a foot
34
disposed on a distal end of a leg
32
that extends from the main body of stud gun
18
. Foot
34
surrounds stud
26
between ferrule
22
and chuck
20
. Stud gun
18
can be coupled to the power/control unit trigger via a control cable
50
and a weld cable
48
. A trigger switch
40
is used to initiate the arc welding process.
Stud arc welding systems with automatic feed are available with both portable and fixed welding guns. A hand-held gun
18
′ with an automatic stud feed attachment
52
is depicted in FIG.
3
. Typically, studs are automatically oriented in a parts feeder and transferred through a flexible feed tube into the welding gun chuck. A ferrule or arc shield is hand-loaded for each weld. For special applications, inert gas shielding or a semi-permanent ferrule is used to eliminate the loading of a ferrule for each weld. Using automatic feed systems such as this, welding rates in the range of 20 to 45 studs per minute can be obtained.
Capacitor discharge stud welding, because no ceramic ferrule is required, is suited for high-speed automatic stud feed applications. Portable capacitor discharge equipment with automatic stud feed is available for studs ranging from No. 6 through ¼ in. diam. Studs are automatically oriented in a parts feeder and transferred through a flexible feed tube into the welding gun chuck. The automatic feed attachments add very little weight to the gun and do not encumber its use. Welding rates with portable equipment range up to 60 studs per minute on applications where stud location tolerances are such that no templating or only a loose-fitting templating is required.
Typically, in the arc welding method, larger studs (i.e., studs having a diameter greater than about ⅜″) are welded using a ceramic ferrule. The main reason for the ferrule is to control the shape of the weld puddle. In these systems, the studs and ferrules are loaded manually as it is very difficult to feed the brittle ceramic ferrules. Similarly, the remains of the ferrule need to be removed manually after the weld process. This results in a slow process, and tedious, labor intensive work.
Ferrules are required for the stud arc welding process except under highly specialized conditions. The ferrule surrounds the weld area and performs several important functions during the weld cycle, such as concentrating the heat of the arc in the weld area during the weld, reducing oxidation of the molten metal during welding by restricting passage of air into the weld area, and confining the molten metal to the weld area. The ferrule also protects the eyes of the operator from the arc; however, safety glasses with side shields and shade No. 3 filter lenses are recommended.
Two types of ferrules are used: expendable and semipermanent. The expendable ferrule has the broadest commercial use. It is composed of a ceramic material and breaks easily for removal. Because the expendable ferrule is designed for only one weld, it is much smaller, and its design, relative to venting and fillet cavity dimensions, can be optimized. Better fillet control and weld quality can be achieved with the expendable ferrule than with the semipermanent ferrule. Stud shape is not limited, because the ferrule does not have to slip over the stud shank of the welded stud for removal.
The semipermanent ferrule is seldom used and is suitable for special applications involving automatic stud feed systems in which fillet control is not important. The number of welds that can be obtained with a semipermanent ferrule varies considerably, depending on the stud diameter, weld setup, and weld rate, but is generally between 2500 and 7500. The ferrule fails because of the gradual erosion of the ferrule material by the molten metal, causing welds to become unacceptable.
FIGS. 4A-4E
depict a conventional stud arc welding sequence in which a ferrule is used. The welding sequence begins by loading stud
26
into chuck
20
and a ferrule
22
into the ferrule holder. The relationship between stud
26
and ferrule
22
prior to positioning stud
26
on workpiece
30
is shown in FIG.
4
A. Stud
26
protrudes beyond ferrule
22
(by a distance d known as a “plung
ABB T&D Technology Ltd.
Pittman Zidia
Woodcock & Washburn LLP
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