Abstract:
A cutting tool that can cut concentric ringed features (e.g. protruding ridges or intruding grooves) onto a weld face of an electrical resistance welding electrode is disclosed. The cutting tool includes a cutter blade that can be rotated about the electrode weld face. The cutter blade has at least one cutting surface configured to cut the concentric ringed features onto the weld face when the cutting surface is rotated relative to the weld face while engaged therewith.

Description:
This application is a division of Ser. No. 11/536,001 filed on Sep. 28, 2006. 
    
    
     TECHNICAL FIELD 
     This invention pertains to welding electrodes for electrical resistance welding. More specifically this invention pertains to the formation of concentric geometric features on the welding face of the electrode for improved electrical contact with the workpiece. 
     BACKGROUND 
     Current automotive vehicle manufacturing operations include, for example, the joining of two sheet metal layers by spot welding. Vehicle body panels such as doors, hoods, deck lids and liftgates are often assembled by joining inner and outer panels stamped from sheet metal of suitable metal alloys. Ferrous or aluminum alloys are often used. The thickness of each sheet metal layer may vary from less than one millimeter to more than four millimeters. Electrical resistance spot welding is often used to join such inner and outer panels or other metal parts. 
     In the case of sheet metal body components, flats or flanges of two or three components are placed together and then a series of spot welds penetrating from the top sheet layer through into the bottom layer are made to securely attach the panels. Welding practices have been developed for such spot welding operations. Good welding practices are particularly critical in joining aluminum sheet alloys because of the high electrical and thermal conductivity of the material and the omnipresent oxide coating on the surface. Similar welding challenges arise in the welding of other light metal workpieces such as parts made of magnesium alloys. The spot welding operation is accomplished by assembling the parts in a suitable fixture and pressing welding electrodes against opposite sides of the overlying or touching parts at the intended weld location. The welding electrodes usually provide both clamping force and current commutation for the weld. 
     Copper or copper alloy welding electrodes are often used in welding aluminum alloy workpieces. U.S. Pat. No. 6,861,609, titled Welding Electrode for Aluminum Sheets and assigned to the assignee of this invention, illustrates some such welding electrodes. 
     As illustrated in the &#39;609 patent, the electrodes are often round cylinders with a welding face at one end shaped to engage the workpieces. The welding electrodes are part of a welding apparatus including a welding head or gun that can be moved and actuated to press two aligned and opposing electrodes against the assembled workpieces. The apparatus then delivers a momentary welding current to the electrodes to affect the weld. Workpiece metal layers between the electrodes are momentarily melted by electrical resistance heating to form a weld nugget joining the layers. The clamping force, the value of the welding current (often single phase alternating current, 60 Hz, or rectified direct current) and current duration (several cycles of the 60 cycle current) are also specified for the electrodes to be used and the welding task. 
     In vehicle manufacturing or other industrial process, each welding gun is typically used to make a rapid succession of welds, for example, around the periphery of two or more overlying panels. The high electrical and thermal conductivity in combination with the insulating nature of the naturally-formed surface oxides of aluminum alloys (or magnesium alloys) makes them difficult to weld using spot welding practices previously developed for steel alloys. In the case of light metal alloys, the spot welding process is sensitive to a large number of variables beyond the normal welding parameters of electrode configuration, electrode force, weld time, and weld current. These other variables include sheet surface oxidation, sheet surface cleanliness, sheet surface topography as well as process variations such as alignment of the electrodes to the sheet, location of electrodes relative to the sheet edge and part radius, metal fit up, gun stiffness, alignment of electrodes on the gun, electrode surface roughness, and wear of the electrode surface. 
     The welding faces of some electrodes are roughened by blasting with small steel or sand particles or abrasion with a coarse abrasive paper as illustrated in the &#39;609 patent. The roughened surface is characterized by randomly distributed craters with peak to valley dimensions, for example, in the range of 5 to 30 micrometers and with substantially the same range of peak to peak spacing. This texture permits the electrode face to penetrate an oxide film on the workpiece surface to reduce electrode resistance (and overheating) at the contact surface of the electrode and part. But, whether textured or not, the tips or welding faces of the electrodes may be altered by erosion or by adhesion and buildup of workpiece material after several welds. Welding operations must then temporarily cease while the electrode faces are cleaned, or re-shaped, or re-dressed. The redressing of grit blasted electrode faces, for example, can require many tens of seconds of off-line processing. 
     There is a need to provide a resistance welding electrode with a contoured welding face that improves electrical contact with a workpiece surface and the reliability of resistance welding, and decreases the time required for re-dressing of the welding face during welding operations. Such an electrode would be useful in many welding applications and would be particularly useful in welding light metal alloy workpieces with their oxide surface films. 
     SUMMARY OF THE DISCLOSURE 
     This invention is applicable to electrodes or electrode caps or welding face surfaces especially for electrical resistance welding. Such electrode members often have round cylindrical bodies for easy securing in the electrode holder of a welding gun. An end of the electrode may be tapered from the diameter of the body to form a welding face. The welding face of the electrode may be machined so that the welding surface comprises concentric circular geometric features (ridges or grooves) instead of the random roughened surface achieved by grit blasting. Suitably, a pattern of concentric circular ridges or grooves starts at the center of the welding face and extends radially outwardly over the face of the electrode. The circular contours may extend from the welding face onto a tapered portion of the electrode because a tapered portion of the electrode may be brought into engagement with a part to be welded. 
     The concentric ridges or grooves are formed, respectively, to have circular surfaces projecting outwardly from the face of the electrode or inwardly into the face of the electrode so that these formed surfaces engage and possibly penetrate the surface layer or film of a workpiece. Each (or some) of the concentric ridges or grooves may be separated from their radially-spaced neighbors by a flat ring-shaped surface in the face of the electrode. By way of example, the circular ridges or grooves are suitably formed to have a peak-to-base height in the range of about 20 to 200 μm with a peak-to-peak or base-to-base spacing (depending on the profile of the ridges or grooves) of 80 μm to 1500 μm. This circularly contoured welding face pattern is easily formed on new electrodes. 
     Thus, a single-step process is provided that simultaneously provides the benefits of both electrode dressing, i.e., electrode reshaping, good alignment, electrode surface cleanliness, etc., with the benefits of a texturing process, i.e., improved mechanical stability, low contact resistance, and reduced external expulsion. This may be achieved by preparing (for example, machining) electrode dressing blades to cut or otherwise produce a circular contoured surface on the welding face (at least) of the electrode tip. This circular pattern is achieved by putting a series of fine grooves or ridges into the face of a dressing blade. For example, the circular grooves may have cross-sections that are semi-circular or saw-tooth (triangular) or sinusoidal in configuration. During electrode dressing, these grooves/ridges cut and produce corresponding ridges or grooves on the dressed surfaces of the electrode. As an example, grooves or channels can be machined into a tool steel blade using EDM machining. 
     The concentric circular ridge or groove design for the electrode cap provides improved welding performance of the electrode. It is also a textured pattern that can be restored very rapidly to the electrode face as it is re-dressed for continued welding operations. 
     Other objects and advantages of the electrodes will become apparent from a detailed description of some exemplary preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of aligned and opposing welding electrodes engaging assembled sheet metal panels for a resistance welding operation. 
         FIG. 2  is a schematic representation of the electrodes of  FIG. 1  positioned to have their welding faces re-dressed by a cutter blade shaped for cutting concentric round ridges in the faces of the tools. 
         FIG. 3  is an enlarged view of a cutter blade positioned under a welding electrode for the formation of concentric round ridges in the face of the tool. 
         FIG. 4  is an enlarged view of an electrode with a tapered conical tip and a machined crowned welding face with a ridge-containing surface. 
         FIG. 5  is an enlarged view of an electrode with a truncated hemispherical tip portion and a crowned welding face with a machined ridge-containing surface. 
         FIG. 6  is an enlarged view of a portion of the face of the welding electrode of  FIG. 3  showing two of the ridge rings and an intervening “flat” area in the welding face of the electrode. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A welding electrode cap (or welding face) design is provided that is useful for forming spot welds in metal workpieces. The welding electrode cap is useful in spot welding operations generally, and it offers advantages for welding light metal workpieces such as aluminum alloy and magnesium alloy sheet materials. These materials often have an oxide film on surfaces contacted by the aligned and opposing electrodes and it is preferred that the electrode faces be shaped to engage and pierce the oxide film during welding. 
     In the manufacture of car doors, deck lids, liftgates, and the like, for example, it is often the practice to form these parts of complementary inner and outer sheet metal panels. The panels are of complex curvature for overall design effect and to contain any necessary electrical wiring and/or hardware between them. The formed panels usually have flanges at their peripheral edges for joining. An inner panel is placed against an outer panel and the assembled workpieces positioned for the formation of a series of progressively formed electrical resistance spot welds in a bonding pattern along their flanges. In one type of welding operation, the assembled panels might be moved and positioned between the welding arms of a stationary pedestal welding machine In another type of welding operation, the assembled panels might be held in a fixed position and a robot progressively move a welding gun around the periphery of the workpieces to sequentially form the welds. 
       FIG. 1  illustrates a welding operation in which a spot weld is to be formed at a welding site  14  (shown as a dashed line weld nugget to be formed) in two juxtaposed aluminum alloy panels  10 ,  12  (only the overlapping edges of panels  10  and  12  are shown for simplification of the illustration). An upper welding gun arm  16  has an electrode holder  18  that holds welding electrode  20  in shank  22 . The welding electrode is often water-cooled by means not illustrated. Welding gun arm  16  is part of a fixed welding apparatus or robot-carried welding apparatus, not shown. A lower welding arm  24  is also carried on the welding apparatus. Lower welding arm  24  has an electrode holder  26  that holds welding electrode  28  in shank  30 . 
     Welding electrode  20  carried by the upper welding arm  16  is shown in spot weld forming engagement with the outer (upper in  FIG. 1 ) surface of panel  10  and welding electrode  28  carried by lower welding arm  24  is shown engaging the outer (lower) surface of panel  12 . In a spot welding operation electrical current of suitable current value and duration is passed between the tips of opposing and aligned electrodes  20 ,  28  through the overlying panels  10 ,  12  at weld site  14 . The electrodes  20 ,  28  are pressed together, suitably in a predetermined pressure schedule, to press the panels  10 ,  12  together at the weld site  14  and to obtain a suitable preprogrammed momentary current flow for resistance heating of the metal at the weld site  14 . Metal in the current path is momentarily melted. The welding current is stopped; the molten metal rapidly loses heat to the water-cooled electrodes and the surrounding panel material and solidifies as a weld nugget joining panels  10 ,  12  at weld site  14 . The opposing electrodes  20 ,  28  are then withdrawn. 
     This welding sequence is usually completed in a matter of a second or so. The panels or electrodes are moved to a nearby weld site and the process is repeated until a suitable predetermined number of spot welds are formed to secure panels  10  and  12 . Then another workpiece assembly is brought into proximity of the welding apparatus and a new sequence of welds formed. As will be described, the faces of the electrodes play a role in forming of each weld and in the efficiency of the ongoing welding process. 
     The welding faces of the electrodes gradually become eroded and/or accumulate unwanted deposits. The welding apparatus is then usually temporarily withdrawn from “on-line” operation so that the faces of the electrodes can be repaired or an electrode replaced. 
     In this example, electrode  28  is identical to electrode  20 , but the electrodes are not necessarily the same shape. Electrode  20  is further illustrated in  FIGS. 2 ,  3 ,  4 , and  6 . Referring to  FIG. 3 , electrode  20  has a round body  40  with a hollow receptacle  42  adapted to receive a shank  22  for insertion into holder  18  of a welding arm  16 . And electrode  20  has a tapered transition section  44  with a spherically crowned welding face  46 . A series of concentric circular ridges  48  are formed in and constitute the welding face surface  46  of welding electrode  20 . The pattern of circular ridges  48  (or in another embodiment, grooves) may extend onto the tapered surface  44  of electrode  20 , but this is not illustrated in  FIG. 3 . 
       FIG. 4  illustrates a portion of the body  40  and tapered end portion  44  of electrode  20  in cross-section. Referring to  FIG. 4 , and by way of example, the diameter (dimension A in  FIG. 4 ) of electrode body  40  is often about 12.5 mm to 22.2 mm The diameter of the electrode body is usually not critical but it must be strong enough to withstand the 700 to 1500 pound (or so) weld force applied for welding a variety of aluminum gauges, and it must be at least the diameter of the welding face  46 . 
     The planar diameter (dimension B in  FIG. 4 ) of the spherically crowned or domed welding face  46  of electrode  20  is, for example, between about  6  and  12  mm Welding face  46  may preferably be rounded or crowned with an exemplary radius (dimension C in  FIG. 4 ) of about 25.4 mm 
     A plurality of round concentric ridges  48  ( FIGS. 3 ,  4 , and  6 ) are formed in the spherically crowned welding face  46  of electrode  20 . These ringed-ridges  48  extend radially outward from the center of weld face  46 , the longitudinal axis (axis  47  in  FIG. 6 ) of round cylindrical electrode  20 . In a preferred embodiment, each contoured ridge  48  is nearly semi-circular in cross-section (see  FIG. 6 ), with its cross-sectional circumference arising upwardly from the surface of the crowned profile of face  46  with a sloped “flat” ring  49  (or base) on the spherically crowned surface between each contoured ridge ring  48 . Of course, each contoured ridge  48  and each intervening flat ring  49  is of increasing radius as it is formed radially outwardly from the center of the electrode face  46 . By way of example, the diameter of the base of each contoured ringed ridge may be about 125 micrometers and the width of each intervening flat ring may also be about 125 micrometers. 
     In a preferred embodiment, the contoured rings are machined in the crowned face of the electrode. As illustrated in  FIG. 2 , a single piece cutter blade  50  is prepared with two cutting surfaces  52 ,  54  for cutting upraised concentric rings in the welding face surfaces of two electrodes  20 ,  28 . The welding arms have positioned the welding faces of welding electrodes  20 ,  28  against the cutting surfaces  52 ,  54 , respectively. This operation could be for the purpose of forming the concentric rings on new welding electrodes or for re-dressing the welding faces of used electrodes. The end  56  of the cutting surfaces  52 ,  54  of cutter blade  50  extends to the aligned longitudinal axes of electrodes  20 ,  28 . 
     Cutter blade  50  is carried in a rotating cutting tool (not shown) that rotates the cutter blade  50  around the aligned center axes of the opposing electrodes.  FIG. 3  presents an enlarged view of a portion of  FIG. 2  with the lower electrode  28  removed to better illustrate the lower cutting surface  54  of cutting blade  50 . The end  56  of the blade is at the center axis of the aligned electrodes. The electrodes are pressed against the cutting surfaces  52 ,  54  of cutting blade  50  which rotates around its end  56  to cut concentric circular ridges  48  and intervening flat rings  49  and in the faces of the electrodes  20 ,  28 . 
     Cutting surfaces of blade  50  are curved in complementary conformance with the domed face surfaces of electrodes  20 ,  28  and provided with cutting surfaces for forming or re-forming the concentric contours in the electrode faces. The cutter surfaces  52 ,  54  may be shaped by electrical discharge machining or other suitable process to have curved circular cutter teeth  59  spaced by intervening “flat” (actually sloped) recessed cutter surfaces  58 . Cutter teeth  59  and recessed cutter surfaces  58  are sized and located along cutter surfaces  52 ,  54  for forming the contoured faces (e.g., face  46 ) in electrodes  20 ,  28 . Cutter teeth  59  are illustrated in  FIG. 3  as extending straight across cutter surface  54  of cutter  50 , but they may be curved for more accurate cutting of ridges  48  in electrode  20 . The welding face of each electrode then has formed upstanding concentric rings of ridges of semicircular cross-section separated by concentric flat ring spaces. Two of these ridges  48  with an intervening flat ring  49 , starting from the center of the crowned face of electrode  20 , are illustrated in  FIG. 6 . 
     The rings of ridges  48  start at the center of the round welding face  46  and become progressively radially larger across the face. Ridges  48  are used to improve engagement of welding face  46  with the surface of a work piece to be welded. They assist in gripping the workpiece and penetrating a surface oxide layer. They improve electrical conductivity and reduce overheating and oxidation of the workpiece surface. 
     Electrode face ridges  48  may be formed in different continuous concentric or spaced concentric shapes such as, for example, saw tooth (triangular) or sinusoidal shapes. While the formation of the contoured surface has been illustrated by the use of a rotating cutter blade other surface shaping methods may be used. 
       FIG. 5  illustrates (in cross-section) a different electrode shape with a different face contour. Electrode  60  has a round cylindrical body  62  with a hemispherical tip  64  having the radius of the body portion  62 . The hemispherical tip  64  is truncated and spherically crowned using a larger radius than the tip radius to form domed face  66  on the rounded hemispherical peripheral tip  64 . By way of example, the diameter (dimension A in  FIG. 5 ) of electrode body  60  may be about 12.5 to 22.2 mm The planar diameter (i.e., in plan view, dimension B in  FIG. 5 ) of the welding face  66  of electrode  60  is, for example, about 6 to 12 mm Welding face  66  may preferably be rounded or crowned with an exemplary radius (dimension C in  FIG. 4 ) of about 20 to 150 mm or greater. The concentric rings of ridges  68  formed in the welding face  66  are of triangular cross-sectional shape. In this example no relatively flat ring surfaces (like surfaces  49  in  FIG. 6  on hemispherical face  46  of electrode  20 ) have been formed between the concentric, circular, radially spaced, triangular-cross section ridges  68 . 
     The forming or dressing of the concentric rings of ridges or grooves on the welding faces (and, optionally, the tapered side surfaces of the faces) of the welding electrodes can be done following different strategies. Obviously, provision must be made in the original length of the electrode body and tip portions to accommodate repeated removal of material if the welding face of the electrode is to be repeatedly redressed. For example, in one strategy, if the grooves/ridges on the electrode can be brought into registry with the ridges/grooves on the cutting blade during dressing, then a small amount of metal can be removed to refinish the electrode without completely re-cutting the ridges/grooves. Experience in spot welding aluminum in production runs has shown that as little as 50 μm of metal can be removed to refinish the weld face. Where the size of the electrode permits a total depth of cut of 8 mm into the electrode face, which is also possible, this would result in up to 160 dresses. Where obtaining registry between the electrode and dressing blade is not possible and new ridges/grooves need to be cut for each dressing, then the size of the ridge/groove features should be such that they can be cut without removing an excessive amount of the electrode face. In this case, to achieve a reasonable number of dresses on an electrode (&gt;40), less than ˜200 μm of metal would be removed per dress and still maintain an adequate amount of copper (˜2 mm) before penetrating the water passage. This would suggest that the ridges/groove features to be machined into the electrode should have a peak-to-peak height of at most 200 μm. In general, to be effective the weld face should incorporate a minimal number of ridges/grooves, i.e., three or more. To accommodate three concentric ridges/grooves on an electrode face, for example, 8 mm in diameter the maximum spacing between each feature would be about 1500 μm. For complete re-cutting of the electrode face, the grooves/ridge features would most likely have a peak-to-peak height of 20 μm to 200 μm with a spacing of 80 μm to 1500 μm, respectively. 
     Besides machining of grooves or ridges into the cutter face, the cutter could be designed from the outset with a textured face such as a saw tooth wave or sine wave. This would be able to produce even rougher surfaces for a given peak-to-peak height of the texture, but may be much more difficult to produce than the previous designs. 
     A simpler alternative to machining grooves or ridges into the cutting face of the blade would be to grind the cutting face with a rough grinding tool that puts a random set of grooves and ridges into the dressing blade. During dressing, a mirror image pattern of these features will be produced on the electrode surfaces. Since registry of the features of the blade and electrode might be more difficult to obtain in this case, the peak-to-peak height of the machined blade should be less than ˜200 μm. 
     For blades that contain multiple cutting flutes (2 or more on a single electrode face), it may become apparent that the texturing pattern on the cutting flutes does not produce the desired pattern on the electrode face because it is not possible to perfectly align the flutes with each other and the electrode face. In this case, only one of the cutting flutes could be designed to produce the texture while the other flutes are machined so they do not contact the electrode face. Alternatively, the multiple flutes could be designed to each texture a different radial area of the electrode face leaving the remainder of the face undisturbed. 
     Use of welding electrodes with concentric contoured welding faces can significantly improve process robustness and weld quality for resistance spot welding of light metals. This is achieved by producing geometrically consistent, clean electrode surfaces that will be perfectly aligned on the weld gun. In addition, the surface texture produced on the welding electrodes will mechanically stabilize the welding process and significantly reduce surface expulsion, which not only harms weld quality, but detrimentally impacts paint surface quality. 
     In general it is preferred to form welding electrodes of copper or copper alloys because of the strength and electrical conductivity properties which are very useful in making spot welds using electrical resistance heating. 
     The welding electrodes have been described in terms of certain preferred embodiments but other welding face shapes may be used.