Electric heating – Metal heating – By arc
Reexamination Certificate
1999-10-15
2001-10-23
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S137610
Reexamination Certificate
active
06307179
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to welding guns and, more particularly, to a connection between a welding gun tip and a retaining member.
2. Related Art
In metal inert gas (mig) welding, the welding gun contact tip must satisfactorily perform two important functions. First, it must accurately guide a weld wire to the point of contact with the workpiece. Second, it must conduct adequate current, which can be as high as 400 amps or more, to the weld wire. To achieve those results, it is known to provide a relatively long contact tip made of copper and having an axial hole therethrough. The hole is sized slightly larger than the diameter of the weld wire. The contact tip extends from a gas diffuser through which an inert gas is directed around the welding arc. Current to the contact tip is supplied to it through the gas diffuser.
Contact tips are particularly subject to at least two modes of failure. The first is burnbank, which occurs if the feeding of weld wire through the contact tip is stopped while an arc is maintained. The second mode of failure is wearing of the through hole due to the abrasive nature of the weld wire material. In addition, because the contact tip is so close to the welding arc, and because the tip must carry high electrical currents, it becomes very hot. The high temperatures also have a deleterious effect on the contact tip, due in part because the hotter the tip becomes the softer it becomes, which makes it less resistant to wear from the weld wire. A properly worn contact tip has a groove on one side of the inside hole.
Due to the severe operating conditions, contact tips have relatively short service lives. Consequently, they are treated as consumable items in the industry. Typically, contact tips are made as separate pieces that are threaded into a gas diffuser. The thread is a single thread and, therefore, each contact tip has only one seating location with respect to the gas diffuser. When a contact tip has worn, it is threaded out of the gas diffuser, is discarded, and a new one is screwed into its seating location.
As noted above, in addition to heating caused by the welding arc, another potential source of heat to the contact tip is the threads between it and the gas diffuser. That is, the threads can cause resistance heating. To minimize electrical resistance, the contact tip and gas diffuser threads have a relatively long engagement length and a fine pitch. However, the long engagement, fine pitch, thread presents a problem. Removing a worn contact tip from the gas diffuser requires several turns of the contact tip. If the weld wire has burned back and fused to the contact tip, the weld wire must twist through the same number of turns as the contact tip. The potential springback of the fully removed contact tip due to the twisted weld wire is annoying if not potentially dangerous. Moreover, the time required to disassemble the multiple-turn contact tip also contributes to down-time of the welding production area.
Although the prior art has individually addressed the problems of short service life of a contact tip, resistance heating, and quick replacement of a contact tip, there has not been one convenient configuration which solves all of these problems.
U.S. Pat. No. 5,440,100 to Stuart et al. and U.S. Pat. No. 3,469,070 to Bernard et al. each discloses a contact tip which can be seated in two different positions within a gas diffuser. Stuart and Bernard use a projection on the gas diffuser, wherein the projection fits into a groove on the contact tip. But such a connection is weak both mechanically, and electrically. Mechanically, there is only a point contact between the gas diffuser and contact tip which holds them together. In Stuart, a dimple or protrusion extends from the gas diffuser, however, the dimple is subject to wear each time a contact tip is inserted within the gas diffuser thereby making the mechanical connection loose over time. In Bernard, a set screw in the gas diffuser extends into a bore which receives the contact tip. The tip of the set screw fits into recessed guideways on the contact tip. But the set screw may loosen due to vibration during use of the welding gun and, therefore, weaken the mechanical connection between the contact tip and gas diffuser. Further, the point contact tends to loosen easily thereby allowing the contact tip to rattle within, and quite possibly come out of, the gas diffuser which would be a big problem during welding operations. Electrically, each Stuart and Bernard suffers the drawback of resistance heating near the limited point contact between the projection and the groove which hold the parts together.
U.S. Pat. No. 5,760,373 to Colling address the problem of resistance heating by providing a welding device having a tapered connection between a contact tip and a gas diffuser. However, Colling still suffers the drawback of requiring a full turn of the contact tip to remove or insert it within the gas diffuser. Moreover, Colling's contact tip seats in only one position relative to the gas diffuser and, therefore, suffers the additional drawback of a short service life.
Similarly to Colling, U.S. Pat. No. 5,726,420 to Lajoie provides a welding device having a tapered connection between a contact tip and a gas diffuser. Lajoie claims to enable quick replacement of the contact tip, however, Lajoie's device requires 2½ turns of the contact tip relative to the gas diffuser in order to remove or insert it. Lajoie's required 2½ turns is still quite time consuming and, therefore, contributes to a long down-time during replacement of the contact tip. Further, Lajoie's contact tip seats in only one position relative to the gas diffuser and, therefore, suffers the additional drawback of a short service life.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the disadvantages of the prior art by providing one welding apparatus configuration which solves the problems of short service life, resistance heating, and quick replacement of the contact tip, with one simple design. It is another object of the invention to provide a welding apparatus with a contact tip that has a longer service life than conventional contact tips, thereby reducing the cost of operating the welding apparatus. Further, it is an object of the present invention to provide a welding apparatus in which a contact tip and gas diffuser quickly may be connected and disconnected, thereby reducing the time necessary for replacement of the contact tip and thus decreasing down-time of a welding production area. Further, although the contact tip may be quickly removed and inserted within the gas diffuser, the present invention provides a strong connection therebetween which also minimizes resistance heating.
In accordance with the present invention, a welding apparatus is provided with a contact tip having a longer service life—about twice as long as a conventional contact tip. In order to provide longer service life, the contact tip has an interwoven-double-start thread, wherein the thread-cut starting points are 180 degrees from one another. Additionally, the gas diffuser has contact-tip-mating thread which is also an interwoven-double-start thread, wherein the thread-cut starting points are 180 degrees from one another. By providing an interwoven-double-start thread on both the contact tip and the gas diffuser, the contact tip may be seated in two positions, 180 degrees apart from one another, relative to the gas diffuser. Thus, once the inside hole of the contact tip is worn on one side—as it is in a properly worn contact tip—the contact tip can be removed, rotated 180 degrees, and reinserted so that the unworn side of the inside hole may then be used. By rotating the tip, the wire can access the remaining good side of the hole, and the welding operation can continue with the same tip.
Additionally, in accordance with the present invention, a quick release contact tip is provided that minimizes resistance heating bet
American Torch Tip Company
Shaw Clifford C.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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