Most resistance welding controllers lack readings for welding current and force.Therefore, it is a good idea to purchase a dedicated portable resistance welding ammeter and dynamometer.
Resistance spot welding looks simple and easy until the weld cracks, at which point the process suddenly takes on a whole new level of importance.
Unlike arc welding, which produces a pass that is easy to inspect visually, spot welds look normal, but can still come apart due to lack of proper fusion.However, it is not the fault of the process.This may indicate that your spot welder is too small or set up incorrectly for the application.
While a small, lightweight machine may be suitable for some applications, you should be well informed so that you know what you’re getting before making the investment.
Resistance spot welding is unique because it is a high-speed method of joining metals without adding filler metal.When a resistance welder is properly sized and set up, the localized application of precisely controlled heat created by the resistance of the metal to the welding current creates a strong forged joint – called a nugget.Correct clamping force is also a key variable as it helps determine resistance.
When applied properly, resistance spot welding is the fastest, strongest and cheapest method of joining metal sheets.However, although spot welding has been used in manufacturing for over 100 years, it is still not well understood outside the automotive industry.
While the process may seem simple, you must understand many variables and how to adjust each to achieve the desired result—a forged joint that is stronger than the base metal.
Resistance spot welding has three main variables that must be set correctly.These variables can be denoted as FCT:
Resistance spot welding looks simple and easy until the weld cracks, at which point the process suddenly takes on a whole new level of importance.
Failure to fully understand the importance of these variables and the relationship between them can result in weak, unsightly welds.Unfortunately, these problems are often blamed on the process itself, which has led shops to replace them with slower and more expensive metal joining methods such as arc welding, riveting, riveting and adhesives.
Choosing the right resistance spot welder and controller can be confusing for shop owners because there are so many brands and price ranges to choose from.In addition to the commonly used AC resistance welders, intermediate frequency DC and capacitor discharge models are now available.
Electronic controls installed on resistance welders are usually of different brands and individual choices.In addition to controlling weld time and amperage, most modern control models now include digitally programmable features that were previously expensive options, such as upslope and pulsation.Some even offer feedback and welding process monitoring as built-in features.
Today, many imported spot welders are sold in the United States, but only a few meet the Heavy Duty Resistance Welding Manufacturing Alliance (RWMA) amperage and force capability specifications.
Some machines are sized and compared based on their kilovolt-ampere (KVA) ratings, and welder manufacturers can manipulate thermal ratings to exaggerate the capabilities of their machines, which can confuse buyers.
The RWMA industry standard requires spot welders to be equipped with a transformer with a 50% duty cycle rating.The duty cycle measures the percentage of time a transformer can conduct current without overheating during one minute of integration.This value is used to ensure that electrical components do not operate above their thermal capacity.However, to confuse buyers, some machine builders rate their transformers at just 10%, which is more than double their nameplate KVA rating.
Also, KVA ratings are generally not related to the actual welding capability of a spot welder.The available secondary welding current output varies widely with the arm length (throat depth) of the machine, the vertical gap between the arms, and the secondary voltage of the transformer.
As with water pressure, the transformer’s secondary voltage must be high enough to push the secondary welding current out of the transformer and through the welder’s copper arm and spot welding electrode (tip).
The secondary output of a spot welding transformer is usually only 6 to 8 V, if your welding application requires a deep throat machine with a long arm, you may need a transformer with a higher secondary voltage rating to overcome the inductance of the large secondary loop.
When a resistance welder is properly sized and set up, the localized application of precisely controlled heat created by the resistance of the metal to the welding current creates a strong forged joint – called a nugget.
This is especially true if the welding location requires the part to be loaded deep into the throat of the machine.Steel in the throat disrupts the magnetic field between the arms and robs the machine of a usable welding amplifier.
The welding forging force is usually generated by the cylinder.For example, on a swing arm machine, the available welding force varies according to the ratio of the arm length to the distance of the cylinder or foot rod mechanism from the fulcrum.In other words, if the short arm is replaced by a long arm, the available welding force will be greatly reduced.
Foot-operated machines require the operator to push down on a mechanical foot pedal to turn off the electrodes.Due to limited operator strength, these machines rarely generate the forging force required to meet the most ideal Class A spot weld specifications.
Class A spot welds have the highest strength and the most attractive appearance.These optimized results were obtained by setting the machine to produce relatively high secondary amperage, short welding times, and appropriate force.
It should be noted that the welding force needs to be in the proper range.Too low a force setting can result in metal flaking and deep dented, jagged-looking spot welds.Setting too high will reduce the electrical resistance at the joint, thereby reducing weld strength and ductility.Choosing the Right Welding Schedule Charts listing Class A, B and C machine settings for various metal thicknesses are included in reference books such as RWMA’s Resistance Welding Handbook, Revised 4th Edition.Although Class C welds are still relatively strong, they are generally considered unacceptable due to the larger heat-affected zone (HAZ) due to prolonged welding time.For example, two pieces of clean 18-ga. mild steel have a grade A spot weld specification of 10,300 weld amps, 650 lbs.Welding force and 8 welding time cycles.(One cycle is only 1/60th of a second, so eight cycles is very fast.) Class C weld schedule for the same steel combination is 6,100 amps, 205 lbs.force, and up to 42 welding current cycles.This extended welding time of more than half a second can overheat the electrodes, create an extremely large heat-affected zone, and eventually burn out the welding transformer.The tensile shear strength of a single Type C spot weld is only reduced from 1,820 lbs compared to a Type A weld.up to 1,600 lbs, but with an attractive, low mark, Class A weld made with an appropriately sized spot welder looks much better.In addition, in a production line environment, the Class A weld nugget will always remain strong and the electrode life will be longer.Adding to the mystery of investing in a setup tool is that most resistance welding controls lack readouts for welding current and force.Therefore, to properly adjust these important variables, it is best to purchase a dedicated portable resistance welding ammeter and dynamometer.Weld Control is the Heart of the System Every time a spot weld is made, its quality and consistency depend on resistance weld control.Older control techniques may not produce the exact same time and heat values for each weld.Therefore, you need to perform continuous destructive testing of weld strength to ensure that your welding department does not produce out-of-spec welds.Updating your resistance welding controls is the most cost-effective way to bring your resistance welding operations to a consistent quality standard, one after the other.For final spot welding operations, consider installing a new welding controller with built-in current and electrode force to monitor each weld in real time.Some of these controls even allow you to set a welding schedule directly in amps, while the control’s programmable air function sets the desired welding force.In addition, some of these modern controls operate in a closed-loop fashion, ensuring uniform welds even with changes in material and shop voltage.Importance of Water Cooling Spot welder components must be properly water cooled to ensure weld quality and long electrode life during production.Some stores use small, unrefrigerated, radiator-style water circulators that, at best, deliver water near room temperature.These recirculators can have a negative impact on productivity, as spot welding tips can rapidly increase due to high temperatures and require multiple trims or replacements per shift.Since the ideal water temperature for a resistance welder is 55 to 65 degrees Fahrenheit (or above the primary dew point to prevent condensation), it is best to connect the machine to a separate chilled water cooler/recirculator.When properly sized, coolers can keep electrodes and other welder components cool, which will greatly increase the number of welds between electrode trims or replacements.Studies have shown that you can achieve 8,000 welds on mild steel or 3,000 welds on galvanized steel without trimming or replacing electrodes.Need additional information?It pays to work with a qualified dealer to help you select and maintain your resistance welder.Want to learn more?The American Welding Society (AWS) has several publications on resistance welding available for purchase.Additionally, AWS and other organizations offer training courses that teach the basics of the resistance welding process.Additionally, AWS offers the Certified Resistance Welding Technician certification, which is awarded after passing a 100-question multiple-choice exam on knowledge of the resistance welding process.
Charts listing Class A, B, and C machine settings for various metal thicknesses are included in reference books, such as RWMA’s Resistance Welding Handbook, Rev. 4th Edition.
Although Class C welds are still relatively strong, they are generally considered unacceptable due to the larger heat-affected zone (HAZ) due to prolonged welding time.
For example, two pieces of clean 18-ga. mild steel have a grade A spot weld specification of 10,300 weld amps, 650 lbs.Welding force and 8 welding time cycles.(One cycle is only 1/60 of a second, so eight cycles is very fast.)
Class C welding schedule for the same steel combination is 6,100 amps, 205 lbs.force, and up to 42 welding current cycles.This extended welding time of more than half a second can overheat the electrodes, create an extremely large heat-affected zone, and eventually burn out the welding transformer.
The tensile shear strength of a single Type C spot weld is only reduced from 1,820 lbs compared to a Type A weld.up to 1,600 lbs, but with an attractive, low mark, Class A weld made with an appropriately sized spot welder looks much better.In addition, in a production line environment, the Class A weld nugget will always remain strong and the electrode life will be longer.
To add to the mystery, most resistance welding controls lack readings for welding current and force.Therefore, to properly adjust these important variables, it is best to purchase a dedicated portable resistance welding ammeter and dynamometer.
Every time a spot weld is made, its quality and consistency depend on resistance welding controls.Older control techniques may not produce the exact same time and heat values for each weld.Therefore, you need to perform continuous destructive testing of weld strength to ensure that your welding department does not produce out-of-spec welds.
Updating your resistance welding controls is the most cost-effective way to bring your resistance welding operations to a consistent quality standard, one after the other.
For final spot welding operations, consider installing a new welding controller with built-in current and electrode force to monitor each weld in real time.Some of these controls even allow you to set a welding schedule directly in amps, while the control’s programmable air function sets the desired welding force.In addition, some of these modern controls operate in a closed-loop fashion, ensuring uniform welds even with changes in material and shop voltage.
Spot welder components must be properly water cooled to ensure quality welds and long electrode life during production.Some stores use small, unrefrigerated, radiator-style water circulators that, at best, deliver water near room temperature.These recirculators can have a negative impact on productivity, as spot welding tips can rapidly increase due to high temperatures and require multiple trims or replacements per shift.
Since the ideal water temperature for a resistance welder is 55 to 65 degrees Fahrenheit (or above the primary dew point to prevent condensation), it is best to connect the machine to a separate chilled water cooler/recirculator.When properly sized, coolers can keep electrodes and other welder components cool, which will greatly increase the number of welds between electrode trims or replacements.
Studies have shown that you can achieve 8,000 welds on mild steel or 3,000 welds on galvanized steel without trimming or replacing electrodes.
It pays to work with a qualified dealer to help you select and maintain your resistance welder.
Want to learn more?The American Welding Society (AWS) has several publications on resistance welding available for purchase.Additionally, AWS and other organizations offer training courses that teach the basics of the resistance welding process.
Additionally, AWS offers the Certified Resistance Welding Technician certification, which is awarded after passing a 100-question multiple-choice exam on knowledge of the resistance welding process.
WELDER, formerly Practical Welding Today, showcases the real people who make the products we use and work with every day.This magazine has served the welding community in North America for over 20 years.
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Post time: Jul-05-2022