Resistance-welding-tipsuseful Hints and References.SOLUTIONS with Effective, Practical Advice
(Sponsored Links) We dealt with this process in two previous pages, Resistance Welding Processes and Resistance Welding Equipment. Now we want to present additional details and Resistance-welding-tips not addressed adequately till now, that influence setup and conduct of welding. Classic application of resistance spot welding refers to overlapping joints of the same material and of the same thickness. In such case the electrode tips are identical in material, shape and dimensions resulting in heat balanced distribution and in a central spot welded nugget located symmetrically at the interface. If the thickness of the elements and/or the thermal conductivity of the materials are different, the distribution of the current and of heat build up may be non symmetric, causing the nugget to be lop-sided and possibly unacceptable. One or more of the following Resistance-welding-tips can be applied to redress the situation. An electrode tip with a smaller face area can be used from the side of the higher conductivity alloy or of the thinner sheet. Otherwise an electrode of higher electrical resistance (e.g. tungsten) may face the higher conductivity material. Such provisions will shift the current density and the heat distribution. When using sets of different electrodes, proper controls must be established by design to avoid their erroneous location. Electrode tip faces are progressively deteriorating with use. To assure continuing high quality production it is imperative to follow Resistance-welding-tips that require to check periodically and to dress again their surface after the prescribed number of spot welds has been performed. In automated systems rigid schedules of electrode maintenance should be established. Depending on the configuration of the parts to be welded, electrode Resistance-welding-tips of various shape and dimensions are available, mostly available as shelf items from suppliers of equipment and accessories as advertised in their catalogs. Special bent tips may be needed to reach difficult spots but should never be used when the work can be done with standard cylindrical electrodes. Of the various Resistance-welding-tips one should remember that cooling water direction should be checked to make sure that the central cool water jet impinges on the internal back side of the electrode tip closest to the weld face to ensure maximum effective cooling. Portable spot welding machines may be required to perform spot welding on large and cumbersome objects that cannot be readily brought to a standing welding machine. They are usually held by a structure that bears the weight and eases manual manipulation. While the machine carries the electrodes and the force application mechanisms to deploy the necessary compressive force, the required power source can be either separate or integral with the unit. In the first case electrical cables are connected to fixed transformers. Because of large power losses due to the cables, a higher secondary voltage is needed than in a stationary machine of the same current output. In the second case as recommended by Resistance-welding-tips the transformer is integral with the portable unit which is called a "transgun", with the advantage that there is no voltage drop on the secondary cable. Although this configuration is used in manual units it can be easily mounted on robotic arms, especially after the adoption of Inverter technology since such transformers became much smaller and lighter. This application of Medium Frequency Direct Current (MFDC) Power Sources was briefly reviewed in an article published (2) in Issue 42 of Practical Welding Letter for February 2007. (see PWL#042.) The introduction of Insulated Gate Bipolar Transistors (IGBT) for MFDC, and the increased use of Silicon Controlled Rectifiers (SCR) permit fine adjustment of the amount of energy flowing to the transformer. Resistance-welding-tips suggest that modern control devices based on feedback should be employed to permit supply of constant direct current to each weld, even in cases of primary alternating current (AC) line voltage fluctuations. The other necessary functions are easily regulated with standard equipment. Resistance-welding-tips recommend to use the latest technological developments available for feedback based process control. Force gauges are currently used for monitoring and controlling the applied force. Time control functions are now based on microprocessor digital technology. Obtaining consistently high quality welds is nowadays a much easier task than it was only a few decades ago. Plated and coated steel sheets can be joined by resistance welding. Welding procedures and schedules should preserve the coating presence and function, while providing welds of acceptable strength. Machine settings remain quite similar to those used on bare steel of similar thickness, but adjustments are required. Compensation of coating type and thickness on contact resistance, effects on indentation, alloying of coating with electrode material, and coating pick-up should be cared for. Coating thickness and uniformity is most important, for good weldability the thinnest the better. Resistance-welding-tips of coated steel call for higher welding current, greater electrode force and shorter welding time that that used for uncoated steel of same thickness. Good tip maintenance and effective electrode cooling are the best preventive actions. An Article on Consumable Process Tape for Spot Welding was published (4) in Issue 77 of Practical Welding Letter for January 2010. A clarifying contribution on Spot Welding Duplex Stainless was reported (11) in issue 81 of Practical Welding Letter for May 2010. An Article on Proper Water Cooling of Spot Welding Electrodes was published (3) in Issue 87 of Practical Welding Letter for November 2010. An Article on Automated tip dressing for robotic Resistance Spot Welding was published (7) in issue 94 of Practical Welding Letter for June 2011. Click on PWL#094 to see it. Get at no cost all the issues of Practical Welding Letter as they are published. Please subscribe. You will also get the book "PRACTICAL HARDNESS TESTING MADE SIMPLE", see further down this page. A Q&A note published at page 16 in the July 2011 of the Welding Journal, provides useful Resistance-welding-tips on the extension of time between dressing electrodes used for welding galvanized steel sheets. Besides stressing the role of efficient electrodes water cooling, the note introduces two new concepts. The first is a special Heat/Cool/Weld Schedule that, although somewhat explained, will leave readers baffled when trying to realize it in practice for themselves. The second is a commercial device, called Heat Stepper, designed to increase gradually the welding current with the number of spot welds, to compensate for area increase and contamination, providing up to 3000 acceptable spot welds between electrode dressing. Although this is a commercial product that we do not endorse nor recommend, interested readers might want to check it for themselves and see if it is likely to improve their productivity. Readers are urged to read that note, and possibly to contact the author, if they think that it may help them. Resources Overview of Resistance Welding (10 pages) Handbook for Resistance Spot Welding (20 pages) Resistance Welding - Trouble Shooting Guide Understanding Control Modes in Resistance Welding Testing Resistance Welding Transformers A few important AWS Resources are available for purchase: AWS A10.1M-2007 ANSI/AWS C1.1M/C1.1:2000 (R2006) ANSI/AWS C1.4M/C1.4:2009 ANSI/AWS D8.1M:2007 AWS D8.6-77 ANSI/AWS D8.7M:2005 ANSI/AWS D8.9M:2002 ANSI/AWS D17.2-D17.2M-2007 Any questions or comments or feedback? Write them down and send them to us by e-mail.Click on the Contact Us button in the NavBar at top left of every page.
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