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What to look for in a spot welder

Tips and insights to find the best spot welder for your shop’s needs
Thursday, May 19, 2016 - 07:00
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Setting the parameters up front can be limiting. How are you certain the welder did what it was supposed to do? Just like heat can be a problem at the weld, similarly heat builds up in the shop electrical system and the machine itself. That heat then steals energy that is supposed to go into the work pieces. Advanced machines monitor and adjust throughout the weld cycle to ensure the amount of energy needed at the tips is actually delivered. The system then provides feedback on the results of the weld. That feedback can be as simple as red and green LEDs or a full display of the actual measurements. Many newer welders capture this information, logging details about the weld, settings used, results, weld location, etc., then generating a report to accompany the repair paperwork.

Spot welder gun comparison
A CTR12000 welder

“Smart” controls offer advanced features in addition to initially setting the parameters. Features vary by equipment manufacturer, but some of the potential tasks include:

  • Checking the welder status prior to welding. Are the electrode tips too dirty to create a good weld? Do you have the proper gap?
  • Recognizing material between the layers and adjusting accordingly. Simply put, resistance spot welders are creating an electrical circuit. If there is no connection, there can be no weld. Connection barriers, such as heavy E-Coatings, waste energy meant to create the weld to establish the connection. Smart welders, however recognize this situation and add a pre-pulse to the weld. Typically this is a fixed amount of current and time. More advanced models actually determine when the contamination has been burned through before starting the weld, ensuring all of the energy from the weld goes into forming the nugget. This will be critical as structural adhesives and repair procedures calling for Weld Bonding continue to increase.
  • Recognizing a shunt. Like water, electricity takes the path of least resistance. In spot welding that means some of the current will flow through the previous spot weld rather than directly between the electrodes. While this helps establish the connection, it also means energy is stolen from forming the nugget. Some systems recognize when a shunt is drawing power away from the weld, adding extra energy to compensate, ensuring the quality of the second weld.

It’s also important to consider how heat affects new metals. To create high strength (HSS) and ultra-high strength steels (UHSS), special processes trap extra carbon in the molecules. When repairing the vehicle, if the heating and cooling are not controlled properly, carbon escapes converting even the UHSS back to mild steel. Changes in the characteristics of the metal mean it will not react as designed in a collision.

When talking about any type of welder, one of the key questions is “how many amps?” How much welding current does it generate at the tips? Most spot welders these days use inverters and require three phase power. They are converting the incoming 60 Hz AC Main to a DC wave at higher frequencies up to 10,000 Hz. This means they apply the energy much quicker and more efficiently; instead of getting peak current 120 times a second, they are hitting it 10,000 times a second, for virtually constant power. To apply the same amount of energy on single phase, you would have to dramatically lengthen the weld time. That is more time for heat to dissipate out into the surrounding steel and a greater risk of destroying its strength.

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