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Resistance spot welding and what to know about the technology

Everything you need to know to make an informed purchasing decision

Demystifying resistance spot welding technology

Most resistance welding controls lack readouts for welding amps and force. Therefore, it’s a good idea to invest in a specialized portable resistance welding amp meter and force gauge.

Resistance spot welding looks simple and easy until the welds fall apart, at which point the process suddenly takes on a whole new level of importance.

However, success with spot welding does not need to be elusive and mysterious.

Unlike arc welding, which produces a bead that can easily be visually inspected, spot welds can look normal but still fall apart because they did not have proper fusion. However, that’s not the fault of the process. It may be a sign that your spot welding machine is too small for the application or set incorrectly.

Although a small, light-duty machine may be suitable for some applications, you should be fully informed so that you know what you’re getting before you make an investment.

Resistance Spot Welding Variables

Resistance spot welding is unique because it’s a high-speed method of joining metals without adding filler metal. When a resistance welding machine is sized properly and set up correctly, a strong forged joint—called a nugget—is formed through the localized application of accurately controlled heat generated by the metal’s resistance to the flow of welding current. The correct clamping force also is a critical variable because it helps determine the resistance.

Properly applied, resistance spot welding is the fastest, strongest, and least expensive method of joining sheet metal. However, even though spot welding has been used in manufacturing for more than 100 years, it still is not well understood outside of the automotive industry.

While the process looks simple, you must understand numerous variables and how to make adjustments to each one to achieve the desired result—a forged joint that is stronger than the parent metal.

Resistance spot welding has three main variables that must be properly set. These variables can be remembered as FCT:

1. Force - forging force

2. Current - the amount of welding amps used

Demystifying resistance spot welding technology

Resistance spot welding looks simple and easy until the welds fall apart, at which point the process suddenly takes on a whole new level of importance.

3. Time - the duration of current flow

Failure to fully understand how important these variables are and how they relate to one another can result in weak, ugly welds. Unfortunately, the process itself often gets the blame for these problems, which causes shops to substitute slower and more expensive metal joining methods, such as arc welding, clinching, riveting, and adhesives.

Choosing the Right Resistance Spot Welding Machines

Choosing the right resistance spot welding machine and control can be confusing for a shop owner because so many brands and price ranges are available. In addition to the commonly used alternating current resistance welders, there are now mid-frequency direct current and capacitor discharge models available.

The electronic controls installed on a resistance welding machine often are a different brand and a separate choice. In addition to controlling welding time and amperage, most modern control models now include digital programmable functions that previously were expensive options, such as upslope and pulsation. Some even offer feedback and weld process monitoring as built-in features.

Spot Welding Machine KVA Ratings

Today many imported spot welding machines are sold in the U.S., but only a few meet the heavy-duty Resistance Welding Manufacturing Alliance (RWMA) specifications for amperage and force capability.

Some machines are sized and compared according to their kilovolt-amps (KVA) rating, a thermal rating that can be manipulated by welding machine manufacturers to inflate their machine’s capabilities, and this causes confusion for buyers.

RWMA industry standards call for a spot welding machine to have a transformer with a 50% duty cycle rating. Duty cycle measures the percentage of the time the transformer can conduct current during a one-minute integrating period without overheating. This value is used to ensure that the electrical components are not operating above their thermal capability. However, to confuse the buyer, some machine manufacturers rate their transformers at only 10%, which more than doubles their nameplate KVA rating.

Furthermore, the KVA rating often has little to do with a spot welding machine’s actual welding capability. The available secondary welding amperage output varies greatly with the machine’s arm length (throat depth), the vertical gap between the arms, and the secondary voltage of the transformer.

Like water pressure, the transformer’s secondary voltage must be high enough to push the secondary welding amperage out of the transformer and through the welder’s copper arms and spot welding electrodes (tips).

Spot welding transformers typically have a secondary output of only 6 to 8 V, and if your welding application requires a deep-throat machine with long arms, you probably need a transformer with a higher secondary voltage rating to overcome the inductance of the large secondary loop.

Demystifying resistance spot welding technology

When a resistance welding machine is sized properly and set up correctly, a strong forged joint—called a nugget—is formed through the localized application of accurately controlled heat generated by the metal’s resistance to the flow of welding current.

This is especially true if the weld locations require the part to be loaded deep into the machine’s throat. Steel in the throat disrupts the magnetic field between the arms and robs the machine of its available welding amps.

Forging Force in Resistance Spot Welding

Weld forging force typically is produced with an air cylinder. On a rocker-arm machine, for example, available weld force varies based on the ratio of arm length to the distance of the air cylinder or foot lever mechanism from the fulcrum point. In other words, if short arms are replaced with long ones, the available weld force will be reduced substantially.

Foot-operated machines require the operator to push a mechanical foot treadle down to close the electrodes. Because of limited force from the operator, these machines rarely produce the forging force required to meet the most desirable Class A spot welding specifications.

Class A spot welds have the highest strength and the most attractive appearance. These optimized results are obtained by setting the machine to produce relatively high secondary amperage, a short weld time, and proper force.

It is important to note that the weld force needs to be in the proper range. A force setting that’s too low likely will result in metal expulsion and deeply indented, ragged-looking spot welds. And a setting that’s too high reduces resistance at the joint, which lowers weld strength and ductility.<_p>

Selecting the Right Welding Schedule

Charts that list Class A, B, and C machine settings for various metal thicknesses are included in reference books such as RWMA’s Resistance Welding Manual, Revised 4th Edition.

Although still relatively strong, Class C welds often are considered unacceptable because of the large heat-affected zone (HAZ) resulting from an extended weld time.

As an example, Class A spot welding specifications for two pieces of clean 18-ga., low-carbon steel are 10,300 welding amps, 650 lbs. of weld force, and 8 cycles of weld time. (One cycle is only 1/60 second, so eight cycles is very quick.)

A Class C welding schedule for the same steel combination is 6,100 amps, 205 lbs. of force, and a whopping 42 cycles of weld current flow. This extended weld time of well over half a second may overheat the electrodes, create an extremely large HAZ, and eventually burn out the welding transformer.

Compared to a Class A weld, the tensile shear strength of a single Class C spot weld is only reduced from 1,820 lbs. to 1,600 lbs., but products look much better with an attractive, low-marking Class A weld made with a properly sized spot welding machine. Plus, in a production line environment, Class A weld nuggets will be consistently strong and electrode life will be much longer.

Invest in Setup Tools

Adding to the mystery of the process, most resistance welding controls lack readouts for welding amps and force. Therefore, to adjust these important variables properly, it’s a good idea to invest in a specialized portable resistance welding amp meter and force gauge.

The Welding Control Is The Heart of the System

Every time you make a spot weld, its quality and consistency depend on the resistance welding control. Older control technology may not produce the exact same value of time and heat for every weld. As a result, you’ll need to perform continuous destructive testing of the weld strength to ensure that your welding department is not producing su-par welds.

Updating your resistance welding control is the most cost-effective way to bring your resistance welding operation up to consistent quality standards, weld after weld.

For the ultimate spot welding operation, consider installing a new welding control with built-in current and electrode force monitoring of every weld in real time. Some of these controls even allow you to set the welding schedule directly in amps, while the control’s programmable air feature sets the desired welding force. In addition, some of these modern controls operate in a closed-loop manner to ensure uniform welds even if the material and shop line voltage change.

Water Cooling in Resistance Spot Welding

Spot welding machine components must be properly water-cooled to ensure quality welds and long electrode life during production runs. Some shops use small, nonrefrigerated, radiator-type water recirculators that, at best, can deliver water near room temperature. These recirculators can negatively affect production rates when spot welding tips mushroom from the heat and require dressing or replacement several times a shift.

Since the ideal water temperature for resistance welding machines is 55 to 65 degrees F (or above the prevailing dew point to prevent condensation), it’s best to connect the machine to a self-contained refrigerated water chiller/recirculator. When properly sized, chillers keep the electrodes and other welding machine components cool, which will greatly increase the number of welds made between electrode dressing or replacement.

Research has shown that you can achieve 8,000 welds in low-carbon steel or 3,000 welds in galvanized steel without dressing or changing the electrodes.

Still Need Additional Information?

It pays to work with a qualified dealer to help you select and maintain your resistance welding machines.

Want to learn more? The American Welding Society (AWS) has several resistance welding publications available for purchase. Also, the AWS and other organizations offer training classes that teach the basics of the resistance welding process.

In addition, the AWS offers Certified Resistance Welding Technician certification, which is awarded upon passing a 100-question multiple-choice exam on resistance welding process knowledge.

About the Author

Tom Snow

Chairman

120 Nowlin Lane

Chattanooga, TN 37421

423-308-3165