Welding fasteners: Spot or projection?
Both spotand projection weld fasteners have their place. Choosing which process touse—spot or projection welding—depends, like anything else, on the application.
Both spot and projection weld fasteners have their place. Spot welding fastenersresistance-weld directly to a sheet with either a rocker-arm or a press-typeresistance welding system . Projection fasteners, which have small protrusionson the surface that fuse to the base metal, require a press-type machine.Fasteners that provide a hermetic seal have ring projections that circle aroundthe flange of a weld screw, pin, or nut
Projection weld fasteners generally produce stronger, more precise welds than spot weldfasteners. The process uses two flat electrodes; the top one descends straightdownward to produce complete compression, and the fastener projections turn toliquid nuggets and initiate fusion. Spot welding fuses fasteners withoutprojections. The process shows its advantages when working in corners or otherareas where a projection fastener wouldn't fuse properly, or in areas difficultfor the electrodes on a press-type welder to reach.
Choosing which process to use—spot or projection welding—depends, like anything else, onthe application.
Allresistance welding requires a balance of heat (current), time, and pressure.Current can be adjusted by a regulator, which changes the ratio of primary tosecondary voltage. The amount of heat generated increases with the square ofthe current, expressed as secondary amperes. The maximum rating shown on mostmachine nameplates, secondary amperes represent the current required to makethe weld. They range from 5,000 to 15,000 amps for small fasteners, 15,000 to25,000 for larger fasteners, and as high as 75,000 amps for large ringprojections.
Onlow-carbon material, pressure settings are from 300 to 1,000 PSI for smallfasteners, 1,000 to 2,000 PSI on larger sizes, and up to 4,000 PSI for largerring projections. Stainless steel requires from 1,500 to 5,000 PSI.
Time inresistance welding—divided into squeeze time, weld time, and hold time—ismeasured in cycles, each of which represents 1/60 second. Weld time should bethree to 10 cycles for small fasteners and 10 to 20 cycles for larger fasteners.Welding force should continue long enough to include 20 to 30 cycles of holdtime to allow the weld to cool slightly.
When settingup a new job, start with pressure on the high side, use short weld cycle times,as well as low heat. As weld trials are made, increase the heat and weld cyclesprogressively until the machine produces good welds. This way, pressure can belowered or increased as required without burning up the work and damaging theelectrodes or equipment.
Withoutproper cooling, electrodes wear quickly. Flowmeters show how well fluid ismoving throughout the welding machine. Insufficient cooling can be a problemespecially with spot welding, because fasteners have no projections todissipate the heat during the weld. But you should monitor water flow even whenprojection welding. Certain projection electrodes have internal water coolingcoming to within ½ inch of the welding face. Hard-to-cool applications can useexternal water-cooling chambers, available in 5/8- and 1¼-in. diameters, to supplementinternal cooling. Ideally, any resistance welding electrode should feel cool tothe touch after welding.
Additionally,oil and dirt on the part can significantly reduce electrode life. Machinecontrols may have preweld options that burn off oil before performing the weld,but it's always better to start with the cleanest surface possible.
Dressingelectrodes regularly ensures proper contact and better welds. For this, youshould first remove electrodes per the machine manufacturer's instructions—neverwith a pipe wrench, which can damage the electrode surface. Most machines haveelectrodes with pin and rod assemblies inside the electrode holder; once youtap the tip of the rod in the holder, the electrode should kick out.
For spotwelding, dress electrodes at the end of the shift using an electrode sharpener(never a coarse file); sharpeners approved by the Resistance WeldingManufacturing Alliance (RWMA) give the best results. Otherwise, the electrodescan erode or mushroom. Although projection electrodes do not require suchfrequent dressing, they should be faced in a lathe frequently enough tomaintain good weld quality and appearance.
Be sure tokeep several sets of electrodes on hand to reduce downtime for electrodedressing. Also, the top and bottom electrode material should be the same toensure consistent quality.
Projection electrodes carry more current and weld thicker material than spot weldingelectrodes. For this reason, the projection process begins to show itsadvantages at about 0.035 in. and thicker (though it certainly can weld thinnermetal). During the weld, the extra metal within each projection heats up into aliquid nugget, which in turn helps control and focus heat to initiate fusion.Proper fusion happens when both the weld fastener and the base metal reach thewelding temperature simultaneously.
Projectionwelds use two flat electrodes large enough to cover the entire face of thefastener, and the projections themselves help absorb heat, thereby producing aclean weld without discoloration. The result: The process produces strong weldsthat are close to being cosmetically perfect.
The largetooling has another benefit: The more bearing surface, the longer the electrodelife. Projection welding electrodes also can have a harder copper facing thatextends their life even more. And unlike spot welding electrodes, projectionelectrodes can weld a wide range of sheet thicknesses, reducing changeoutsbetween different runs.
The processrequires even contact and pressure between the base metal and projections onthe fastener. Excessive electrode pressure actually reduces the resistancebetween the fastener and material being welded, causing weak, incomplete weldsor no weld at all. Insufficient pressure can cause flashing, burning, and discoloration.
Thepositions of fastener projections can vary, and each has its pros and cons.Some fasteners have projections located at the center of the head or, for nutsor screws, the midpoint of the flange. This helps concentrate heat in thecenter of the fastener itself or its flange, preventing excessive heat at theedges and eliminating spatter. Other designs place projections near the edge ofthe fastener. For fasteners with external threads, projections near the edgeconcentrate heat farther away from those threads, reducing distortion. However,because those projections are near the flange edge, spatter can be an issue.
Forthrough-hole applications, insulated locating electrodes help protect threads.Locating electrodes have a hole in the center where the screw's threads areinserted. These electrodes come with an insulated sleeve designed to protectthreads from the weld current.
Weldingprojections require the fastener to be aligned perpendicular to the flatelectrodes, and to the centerline of the piston cylinder that sits directlyabove the top electrode holder. Even pressure and contact are paramount. If theelectrodes have more wear on one side, they probably are misaligned ornonparallel.
You canmeasure projection electrode alignment with a carbon-paper test. Insert carbonpaper between the electrodes, cycle the machine without current, and theelectrodes will produce an impression. A half-moon impression showsmisalignment, which means the electrode holders need to be adjusted.
Problemsarise when the fastener does not align parallel to the part surface, or if theweld is close to a corner or edge, where the welds can have excess spatter fromoverheating. For these and similar circumstances, spot welding may be a betteralternative.
Spot weldingfasteners work well with material less than 0.035 in. thick. In fastenerapplications, a spot welding system applies heat and pressure directly on thesheet and fastener. The electrode tip diameters control the size of the weldarea.
Picture asheet and fastener inserted between two spot welding electrodes; the fastenercan be thinner or thicker than the base metal. Typically, the electrode with athicker diameter makes contact with the thinner material, while thesmaller-diameter electrode contacts the thicker material. The smaller electrodeemits a greater concentration of current, which better fuses the thicker metal.
Some spotweld nuts have electrode target areas, consisting of an indentation on theoffset tab of the fastener. These not only help locate the electrodes, theyalso create a more favorable balance of material thicknesses between thefastener and sheet metal. This is especially helpful if the fastener is beingwelded onto lighter stock.
When weldingnuts with through-hole alignment pilots, if the electrodes do not hit thefastener at a consistent position, pressure misalignment can cause an offsetnut to rock out of the hole. The more centered the electrode contacts are,using consistent pressure, the easier it will be to hold the nut in position.
Longerrocker-arm systems have a tendency to apply uneven pressure on the top andbottom electrodes, because of the long distance between the tips and the air pistoncylinders applying the pressure. In these cases, offset electrode tips, bentslightly so the two electrode faces are perpendicular at the contact point, canimprove alignment.
Spot weldinghas advantages for hard-to-reach areas. For instance, a spade screw, which hasa flat section jutting perpendicularly at the top, can be spot-welded in tightcorners and confined spaces (see Figure 4 and 11).
Spot weldinghas a few tendencies to consider. The process can discolor the material, mainlyfrom excessive heat. Backing off on amperage, voltage, or pressure can helpcontrol this. The process also tends to mar the material because the electrodesput pressure directly against the surface of the fastener and the materialitself. This is difficult to avoid. However, if a slightly marred surfacedoesn't affect overall part integrity and meets the application's qualitystandards, spot welding can be a cost-effective option.
Choosingwhether to projection- or spot-weld fasteners often hinges on what equipment ashop has. Most have a rocker-arm spot welding system (usually less expensivethan a press-type welder), and if the rocker arm attaches fasteners with spotwelds of sufficient quality, then it may suffice.
If a shophas both a spot and press-type welding system, other factors enter into theequation. If the fastener can be located perpendicular to the top and bottomelectrodes, projection welding usually will produce better results. But if youcannot locate the weld projections because of difficult geometries (such as aweld against an edge or corner), spot welding would be the better choice.
Also, ifyour operation involves welding with and without fasteners, spot welding may bethe way to go, if the application meets several criteria: First, spot weldingmust provide sufficient weld quality for the product; second, all materialsmust be of similar thickness to avoid electrode changeouts. If these criteriaare met, you can use the same electrode set for both fastener and nonfastenerjobs. If you were to projection-weld the fasteners, you would need to switchout electrodes between welds, extra setups that can add some serious time atthe end of the day.
For burning, discoloration of screw head, try one of the following:
For flashing of projections, or "spitting" of molten metal onthreads, try one of the following:
If the operation produces poor, weak, or no welds at all, try one of thefollowing:
For electrode indentations, holes, or expulsion, try one of the following: