Contact tip welding electrode
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a contact tip for a welding electrode. Such contact tips are generally used in welding devices as for example in arc welding torches. Contact tips of this kind constitute contact devices for the transfer of the welding current to the welding electrode, for example a solid wire electrode or a tubular electrode with a powder filling, as it is fed through the contact tip. 
     2. Description of the Prior Art 
     Prior art contact tips have the form of a tube, the internal diameter of which is slightly larger than the diameter of the electrode. The electrode slides during its passage through the contact tip tube against the inner wall of the tube thereby establishing the desired contact for the transfer of current from the contact tip tube to the welding electrode. Known are certain types of contact tip tubes which are slightly curved in one plane thereby improving the contact between the electrode and the tube. As a general rule, the position of the contact area of the electrode and the contact tip is not fixed. The length of the electrode between the tip of the electrode and this contact area which is passed by the welding current, varies. As a consequence, the energy supply to the welding pool fluctuates, which adversely affects the welding quality. Prior art contact tip tubes pose another problem because of surface impurities and coatings on electrodes. 
     There is described in the U.S. Pat. No. 3,112,393 a tubular contact nozzle with a helix-shaped passage for the electrode. Continuous electrical contact between the electrode and the inner wall of the nozzle is established throughout the length of the nozzle. The manufacture of this nozzle, which starts from a tube with small bore, is quite complicated and therefore costly. 
     Electrodes are often treated with a protective coating to prevent oxidation and deterioration. Morevoer, the surface of the electrode may become contaminated with dust and other deterious substances as for example abrased material from the electrode guides. These materials may constrict the narrow bore of a tubular contact nozzle and prohibit a uniform advancement of the electrode. 
     The contact nozzle should be an inexpensive item as it is worn out very quickly. Its service life is generally reckoned in hours. 
     OBJECTS OF THE INVENTION 
     It is the intention and general object of this invention to provide a simple and inexpensive non-tubular contact tip having an accessible contact surface. 
     A further object of the subject invention is to provide a contact tip which ensures a very good contact even for imperfectly straightened wire electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects of the subject invention will become more fully apparent as the following description is read in conjunction with the attached drawings wherein: 
     FIG. 1 shows the main features of a contact tip viewed from its front end in the direction of the axis of the tip; 
     FIG. 2 shows an embodiment of a contact tip; using the invention 
     FIGS. 3, 4 and 5 are sections along III--III, IV--IV and V--V in FIG. 2; 
     FIG. 6 shows the contact tip in accordance with FIG. 2 together with a welding torch nozzle; 
     FIG. 7 shows a modified embodiment of a contact tip; 
     FIGS. 8, 9 and 10 are sections along VIII--VIII, IX--IX and X--X in FIG. 7; 
     FIG. 11 shows a modified embodiment of a contact tip in accordance with FIG. 2. 
     FIG. 12 shows the contact tip of FIG. 11 viewed from its front end. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The contact tip as shown in FIG. 1 consists of a bar 1 of rectangular cross-section helically wound for at least one turn about an axis C. The surface 2 closest to the axis C and the other lateral surfaces of the bar constitute helical surfaces. The axis C is situated outside the bar 1 at a distance A from the bar surface. An electrode 3 is in contact with the surface 2. Without this electrode the tip exhibits a straight cylindrical passage 4, which is indicated by a broken circle 4a with the diameter 2A. The diameter D of the wire electrode may be seen here to be greater than the diameter of the passage 4. The electrode is therefore prevailed upon as it advances through the contact tip to contact theoretically the surface 2 along the helix which is closest to the axis C. Because of the geometrical imperfections of the electrode and those of the contact tip, the electrode makes contact with the bar within a narrow, stripe-shaped contact surface on the bar which encompasses the said helix and extends axially along the bar. 
     The bending of the wire electrode as it is advanced along the contact tip depends partly on the situation of the axis C with respect to the bar, partly on the diameter D of the electrode, partly on the height of the pitch of the helix and last but not least on the geometrical imperfections of the contact tip and the electrode, which normally is not perfectly straight when it enters the contact tip. 
     It has been found that for obtaining a good contact between the electrode and the contact tip the distance A of the helix from the axis of the tip, which is also the axis of the helix, is defined by 
     
         A≦0.6D 
    
     A positive value for A denotes that the axis C is situated outside the bar, wherein a negative value for A denotes that the axis C passes through the bar. 
     If A=0.5D the diameter of the passage 4 is the same as the diameter D of the wire electrode. If a perfectly straight electrode enters the tip, where 0.5D≦A≦0.6D, the electrode will only incidentally slide against the contact surface of the tip. However, because of the above mentioned imperfections the distance A may be as great as 0.6D and nevertheless a good contact will normally be established. Especially thicker electrodes having a diameter of a few millimeters will properly slide against the helical contact surface. A suitable value for A can be found by tests. 
     A preferred range for A is 0≦A≦0.5D. If the value of A is negative, bending of the electrode on the contact surface of the tip is quite considerable and ensures a very strong contact. It leads then to a rapid wear of the tip, especially when using thick wires. Such wear may be acceptable in certain cases, when very high demands are imposed in respect of a very low contact resistance between the welding electrode and the contact tip. If the value of A is smaller than 0.5D a good contact between the electrode and the tip is also achieved for perfectly straight electrodes. 
     A smoother bending of the electrode on the contact surface of the tip produces a lower contact pressure and less wear. It has been found that a contact tip which possesses optimum properties in these respects should have a distance A to which the following is applicable: 
     
         0.2D&lt;A&lt;0.5D. 
    
     Lower values of A are applicable for thin electrodes and higher values for thick ones. 
     To extend the service life of the tip it is proposed to provide the contact surface with a layer of wear resistant, conductive material as for example nickel and nickel alloys. Such a layer can easily be deposited on the surface by electro-plating or other methods of deposition as the surface is accessible. 
     Another dimension of the helix which considerably affects the wear of the tip is the dimension or height of the pitch of the helix. The pitch should preferably be gripped and guided not less than 10D. The smaller the height of the pitch of the helix and the smaller the distance A, the more tightly the wire electrode will be gripped and guided curved as it passes the tip. A tighter grip on the electrode causes increased friction between the electrode and the tip. It slows down the rate of feed of the electrode and increases the rate of wear. 
     Further it has been found that the length of the tip should preferably correspond to at least to one complete turn of the helix about the axis. The electrode will be straightened uniformly in all directions as it passes the tip, if the length of the helix is at least one turn. A desirable straightening effect is applied to the electrode in this way such that it will leave the tip in a certain direction which for example facilitates the correct positioning of the welding torch in relation to the workpiece. 
     The contact tip illustrated in FIG. 2 consists of a bar 5, which is helically wound about an axis C&#39;. The bar 5 has at least one helical opening on a circumferential side thereof. The bar has an essentially semi-cylindrical cross-section with an outer cylindrical surface 6 and a somewhat concave surface 7. A welding electrode 8 is advanced along the concave surface 7 and makes contact with it. The contact tip shown in FIG. 2 is dimensioned in such a way that the distance between the helix closest to the axis C&#39; is greater than zero. The axis C&#39; is therefore situated outside the bar. Moreover, this distance is also smaller than half the diameter of the electrode which is forced to adopt a helical motion as it passes the tip, as may be appreciated from FIGS. 2 to 5. 
     Any contaminating material which is fed to the contact tip together with the electrode falls radially out from the contact tip and will not disturb the advancement of the electrode. 
     The contact tip is provided at its rear end with a thread 9, which is secured in a corresponding holder 11 of a welding torch 10 with a gas nozzle 12 (FIG. 6). 
     The somewhat modified contact tip shown in of FIG. 7 has a bar 13 provided with a V-shaped helical slot 14 for an electrode 16. The slot has a rounded bottom 15 which has a radius of curvature greater than the radius of the electrode. The electrode slides against the bottom of this slot, as shown in FIGS. 9 and 10. 
     As a result of the very good contact between the electrode and the helical contact surface, the contact resistance is small and the heating of the contact tip is so low that spatter from the welding pool normally does not adhere on the moderately heated contact tip. However, in such cases where great spatter losses occur it is desirable to provide the contact tip at its front end with a nozzle 17 which protects the contact tip from collecting and retaining a great part of the spatter and also ensures that the electrode leaves the nozzle 17 in a certain direction. 
     The contact shown in FIGS. 11 and 12 which corresponds to the tip in FIG. 2, is surrounded by a bushing 18. In some cases the flow of a shielding gas which surrounds the contact tip--as for example shown by the arrows 19 in FIG. 6--may be disturbed by the helical surface of the contact tip and the quality of the weld might be adversely affected. As the speed of the gas flow through the nozzle of the welding torch is comparatively low, such disturbances can be normally neglected as they occur only to a small extent. The bushing should only be used if the gas flow needs to be significant and very high demands must be met. Advantageously a wear resistant layer is deposited on the contact surface before inserting the tip into the bushing. 
     The contact tip can be manufactured in many differet ways. What immediately comes to mind is to produce such a tip by forming by winding an originally straight bar about an axis which is situated at the distance A from the surface of the bar. Certain problems may arise, however, if the material, which is usually a hard copper alloy, can be deformed only with difficulty in this manner. It is also conceivable to manufacture these contact tips by extrusion and continuous casting processes. Further, it is also envisaged to machine a suitably bar-shaped starting material in such a way that it will be given the desired helical contact surface. For example contact tips 13 according to FIG. 7 without the nozzle can be manufactured in a similar to twist drills. 
     The invention as described hereinabove in the context of the preferred embodiments is not to be taken as limited to all of the provided details thereof, since modifications and variations thereof may be made without departing from the spirit and scope of the invention.