Tips and Insight to find the Best Spot Welder for your needs
Article by Jeff Coops / Contributing Editor Car-O-Liner
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When my dad purchased his first personal computer, I remember him saying, “It has a one megabyte hard drive, more storage than I will ever need.” Now, we routinely send media text messages from our phones much larger than that. While changes in computer technology are extreme, similarly changes in the automotive industry are not far behind. Th e breakthroughs in building vehicles lighter, more fuel efficient and safer means a whole host of changes for the body shop. Just as that old PC cannot keep up today, expecting your old spot welder to safely repair newer vehicles is also unrealistic. So what has changed and how does that affect the purchase of a new Squeeze Type Resistance Spot Welder (STRSW)?
The basics of resistance spot welding have not changed from Elihu Thompson’s original discovery in 1885. When electric current runs through metal sheets that are tightly clamped together, the inherent resistance to that flow generates heat and creates the weld. The combination of these welding parameters — welding current, weld time and squeeze pressure — creates a molten pool that forms the weld nugget. With regard to welding parameters, one might assume more is always better, right? Actually, that is not the case. First, consider welding current. Too little current results in no fusion, while too much will overheat newer steels, taking the strength out of the weld. Second, consider squeeze pressure.
Limited pressure will have sparks flying everywhere (expulsion); however, too much can limit the size of the weld nugget and excessive pressure reduces resistance, meaning less heat generated. Third, consider weld time. The time and current work together to create the right amount of energy. If you have too little or too much energy, the weld suffers. That total amount of energy varies depending on the thickness and type of material. The three parameters must be combined correctly to create a proper weld. So how do we know the right combination? Welders can learn through practice and experience, and also get some added assistance from a smart welder.
What makes a welder a “smart welder?”
At its simplest, we can think of it as fully automatic versus manually setting the weld parameters.
Traditional welders have just two dials The technician sets the welding current and weld duration themselves. Without exact information from the OEM, the technician must perform sample welds and destroy them until they find the correct settings for the material they are welding
Testing is done and parameters are built into the welder. The technician determines minimal information, usually the material type and thickness, and the welder sets the actual parameters.
As part of the weld process, the spot welder determines the material type and thickness itself, then sets all of the parameters for the technician. Thus the concept of “Pull the trigger and weld.”
In practice, this means when welding a B-Pillar, the smart welder self-adjusts every time the stack up changes. Without this technology, the technician must recognize the change and set the welder manually for the new conditions.
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.
“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 the 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 60Hz 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.
While considering heat, it is worth mentioning the types of cooling systems: air cooled, liquid cooled or a combination. Air-cooled units rely on internal fans and shop air blowing on the cables and electronics to cool, while liquid-cooled welders use a coolant circulation system. Ideally, the welder needs to be cooled everywhere heat is generated. Starting at the weld, electrode caps bring coolant to the back side of the weld. Cables, transformer and power modules all generate heat and therefore require cooling as well. Verify what is being cooled and how. Consider the size of the coolant tank and whether the liquid is actively cooled. It will take much longer to heat up 20 litres of coolant than it does 5 litres. The type of cooling determines the duty cycle you can expect, particularly with the higher current requirements.
Spot welders can be broken into two main categories based upon the location of the transformer. On cable welders, the transformer is larger and located in the base. They have a smaller, lighter gun (welding tong) but require large copper cables to minimize loss of power, typically no longer than 8 feet. Trans-guns house the transformer in the gun itself. Because the transformer is located near the electrodes, it is much smaller and therefore the welding cables are smaller and longer, approximately 20 feet, offering the technician mobility without having to constantly re-position the welder. Trans-gun welders are also more forgiving of poor shop power. There is a tradeoff though — trans-guns are usually heavier than cable guns.
Another shift in the industry comes from increasing OEM program requirements. In an effort to guarantee proper repairs, programs require shops to have correct tools. Some OEMs test welders themselves and publish a list of approved equipment. Other OEMs establish minimum specifications that the welder must meet. Honda recently published a requirement that STRSWs used on 1500MPa repair parts have a minimum welding current of 9000 Amps and 770lbf squeeze pressure. Consider the vehicles you commonly repair and the programs you work with when choosing a welder.
Another major consideration is where you purchase the equipment. What can you expect for training and support? A body shop is a harsh environment for any type of electronic equipment. What resources are available if you have problems? You need a team you can rely upon just like your customers rely upon you.
As the saying goes, "the only thing constant is change" The automotive industry and repair procedures change for the better. Make sure the equipment you rely on is ready to keep up.
