Auto Body Repairs: Spot Welding

Spot Welding: Required in Today’s Auto Body Repairs

Spot welds have been recommended in vehicle repair for years. Now, they’re required.

Squeeze type resistance spot welding (STRSW) has been around the automotive industry for most of our lifetimes. It still amazes me how misunderstood it is in the automotive repair industry.

The use of “spot welds” in vehicle repair has been recommended for many years. Now, we’re starting to see the term “required.” This is due to changes in metal designs and mechanical properties. You hear so much about the issue of heat with many of the new metals. Heat is a key factor in mechanical properties in steels or metals in general. Knowing what is allowed and/or the limits of heat is a factor in repairs of all vehicles today and in the future. That is not going to change anytime soon. Neither is the fact that if your shop does not have a properly working STRSW machine that is capable of the repairs specified by the vehicle manufacturer, you might not have the ability to correctly repair that vehicle. If you choose to go ahead and do the repair, you may be in for some problems. Those problems could be sitting in your shop right now. Even if you have the right equipment, let’s not forget you need a technician who can operate the STRSW machine correctly.


In auto body repair today, we’re looking at so many new terms identifying metals and procedures that it’s hard to keep up. I’m going to list some of them and test your knowledge:
  • MPa
  • Pulse
  • Shunt
  • Weld bonding
  • “HAZ”
  • Three-phase
  • Blowout
  • Twist test
  • Peel test
  • Weld pitch

These are all terms a technician using a STRSW machine must know to effectively make proper repair decisions. And this is a short list; I could list a lot more. The point is, if any of these terms are not in your vocabulary or you do not know their definition, I highly recommend training from vendors or I-CAR now. You don’t have time to waste.

A basic principle we all should know is the effect heat has on steel:

  • Heat on mild-strength steel may cause it to become stronger or more brittle
  • Heat on high-strength steel (HSS) may weaken the steel
  • Heat on ultra-high-strength steel (UHSS) or advanced-high-strength steel (AHSS) will alter the strength by changing the metal’s mechanical properties.

We all must learn the changes in steel terminology as they relate to STRSW. Manufacturer instructions are now identifying repairability of steels in megapascals or MPa. For years, we as an industry have used “psi” ratings to rate strengths of steels. Although this is a simple change, it will have a major effect on reading manufacturers’ instructions.


STRSW equipment is expensive, as many repairers have discovered. The option of wanting a STRSW machine is giving in to the necessity of needing one. If you’re working on mostly older vehicles or vintage ones, you may not need one. However, if you’re working on vehicles built within the past five years, that’s a whole different story.

There are many STRSW machines out there with a wide range of prices. Picking the right one may not be based on price alone. A machine capable of working on most vehicles may cost up to $25,000 or more if a shop is going for a manufacturer certification. Deciding which welder to purchase will depend on many factors:

  • Which manufacturers’ vehicles you want to be able to repair
  • How much repair volume your shop does
  • How much power is available at your shop
  • How much you want to spend
  • Available training


In layman’s terms, an STRSW machine creates spot welds by creating heat very quickly while at the same time applying pressure at the weld. This is done incredibly fast so as not to create heat that will expand and negatively affect the surrounding steel’s mechanical properties. The area the heat affects is called the heat affect zone (HAZ). If the surrounding steel is compromised, then the weld is weakened in terms of function and strength. It could also cause corrosion issues or hot spots. These concerns are normally associated with MIG/MAG plug welds, as STRSW creates welds so quickly that these concerns are generally mitigated.

STRSW machines require power. Since DC current offers more control of the current to achieve this power, many or most machines are inverted from AC to DC. Still, a large amount of AC current is needed. Therefore, a shop may have to be set up for three-phase power to operate the welders. The shop may also need to wire in four-gauge wire to outlets to accommodate the required amount of current. The ability to get this kind of power from the electric company may depend on the area the shop is in; not all shops will be able to do this. An electrician can advise you on what may need to be done to be certain you have the power needed to operate the machine correctly – which could add cost to the purchase.

There are economical STRSW machines, but you may need to see if they’re capable of achieving welds that meet OEM requirements. One of these requirements is that the STRSW machine is capable of weld bonding. Welding through the adhesive requires that the machine have pulse mode, which is used to preheat metal and adhesive to allow the weld to be made. The pulse mode may also be used with multi layers of steel and UHSS to prevent a blowout of the weld. Blowout is when the nugget is literally blown out of the weld, leaving holes or gaps. When you see sparks fly while using a STRSW machine, you may have to inspect the welds for blowout. The sparks flying looks cool, but may indicate weld failure. Failed welds are usually not the fault of the STRSW machine, as long as it was designed for that weld process. Most often, they are the result of operator error.

Operator Error

We all know the old adage about “assuming.” So many shops assume technicians know how to weld or use the equipment correctly that it concerns me greatly. I see shops spend great amounts of money on equipment and never properly train technicians how to use it and why. I can’t tell you how many STRSW machines sit in shops not being used because technicians were never taught how to use them. At some point, they may have tried and found it to be difficult or could not get the machine to weld correctly. I continuously hear, “That machine sucks,” or “That welder sucks.” They may be having problems due to improper setup of the machine, or the prep of the weld zone could be wrong. This leads to a machine getting parked in the corner and nobody using it.

A shop spends all that money but does not invest in or seek training. The process runs counterproductive to what the desired outcome for the owner is. It is training that can teach technicians to use the welder correctly, but training also gives the why and where STRSW must be used. The evolution of steels today dictates the attachment process for the vehicle. Knowing why a steel plug weld may no longer be an alternative to STRSW is a critical issue. Education is a major key to what is happening in the repair world and is the difference between surviving and prospering. To be blunt, I do not know how a shop will succeed without a STRSW machine and training.


The advantages to using STRSW are amazing. Not having to dress welds or weld and seal the weld area at the same time is priceless to a shop where time is money and quality of repair is life. The cost to buy equipment is high but is a worthwhile investment. The big advantage in my world is being able to duplicate the OE build process. STRSW is almost always the default attachment process due to advantages of low HAZ, lightweight, corrosion protection as well as desired effects. Some of the procedures and advantages include:

  • Weld bonding. STRSW with adhesives. Used extensively by many vehicle makers. It is recommended that we follow the OE procedures during repairs. This means when applicable, shops should duplicate this procedure. Vehicle manufacturers can add strength to the vehicle and seal against corrosion at the same time. STRSW machines are capable of duplicating the OE build process. This allows the shop to do a quality, long-lasting repair.
  • Seam sealer welding. This is a relatively new process of welding through urethane seam sealers. This process does not replace adhesives during welding in any way, but gives a shop the ability to use seam sealers to provide the best corrosion protection during welding. This offers incredible corrosion protection while not changing the vehicle structure.
  • Duplicating welds. STRSW allows shops to duplicate the weld process and maintain factory-looking weld zones. Shops should verify the number and location of the welds before attaching panels.
  • Cycle time. When done correctly, STRSW can speed up the time it takes to weld panels. Since many repair times are based on STRSW, this will benefit the shop greatly. As the welds are done correctly, this also gives a great quality repair for the long-term.

When Problems Arise

If or when you do pull the trigger and purchase an STRSW machine, I sincerely encourage you to set a culture of quality in your shop. What I am implying is that you made a huge investment in equipment to repair vehicles correctly. Using the STRSW machine and training your technicians to use it correctly is critical today. Encourage its use and encourage training and learning of how and when to use it. Look up OE procedures and follow them.

Everyone is accountable to a culture of quality. Make sure the technicians use and maintain the welder. Failing to maintain it can cost you dearly in the pocketbook. Training on how to use but also how to care for the tips and the machine itself reduces repair costs and any downtime due to the machine not operating correctly. Set up scheduled visits from the vendor to make sure everything is operational and not being neglected. A preventive maintenance program may seem like a waste, but wait until something breaks and you have a shop at a standstill. This is a headache no one wants to deal with. A maintenance program can give you a little relief knowing that somebody is looking out for your investment.

The investment in a STRSW machine is a sound step to improving quality and maintaining the ability to repair vehicles correctly. If you already own one, you already know that value. I encourage those who already own one or are looking to buy to set up a maintenance program and learn to operate and maintain the equipment. Making sure all equipment is cared for is every person’s job.

We’re talking about everyone’s livelihood here. Mechanical breakdowns or lack of training will cost you more than you know.

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