A dive into squeeze-type resistance spot welding

Aug. 1, 2020
Although not new to our industry, it is a process that virtually every shop should be capable of performing currently.

Q: What did the good spot weld say to the bad spot weld? A: You just couldn't “resist”, could you?  If you don't know much about spot welds I'll let you off the hook. That joke isn't particularly funny. Good welding jokes are hard to come by. Squeeze-type resistance spot welding is the subject we will cover. No more jokes will be attempted. I promise!

Although not new to our industry, it is a process that virtually every shop should be capable of performing currently. I will recap the process if you are familiar with it, and get you up to speed if you are new to it. If you are not currently capable of performing this process and need to justify the purchase of a machine, you will hopefully benefit from this information. I'll give a high-level overview of what squeeze-type resistance spot welding, or STRSW for short, is and why it should be regularly used in today's collision repair shop. I will go over the different types of machines and some of the more common procedures associated with this equipment. In the end, you should be confident about what technicians are doing and why. If you haven't purchased this type of equipment yet you will have some idea of what you should be looking for when purchasing an STRSW machine.

A typical STRSW machine

STRSW is the acronym for squeeze-type resistance spot welding. The process in its simplest form of explanation is that where a weld, or joining of the materials, is made by a resistance weld. To explain it just a bit deeper it is a weld that typically bonds two or more panels/parts together by creating an electrical resistance between two electrodes. The electrodes don't touch, they attempt to pass current through the location, thereby heating the metals to a point of welding them together. Another aspect is the “squeeze-type” part of the process. It's here where a typical resistance spot welder and STRSW separate. In the STRSW process, the panels are squeezed together with a minimum, or specific amount, of pressure while the weld is created. The squeezing of the panels together is typically done first before preheating of the metal and the actual welding process. It may follow this simplified overview: squeeze, preheat, weld, cool down, and release. Tips of resistance spot welders are made of copper which is a better conductor of electricity than steel. That design is what creates resistance as the current attempts to pass through both tips. It is why the machines of today are far better than those of days past or those used in industrial applications. Some resistance spot welders use a manual squeeze action, but it may be limited by the operator's strength. Current equipment applies a much stronger force, more on that later. 

If you are fortunate to have a squeeze-type resistance spot welder in your shop consider this to be a refresher or maybe you know about the machine, but not really what it is or what it does. It may be that it's not being utilized correctly or often enough for various reasons. This can also will help you to better explain to your customer how this part of the repair process works. They may be reassured knowing that you are using a similar method as the manufacturers used when they built the car, truck, or SUV. The use of STRSW is what I would consider a “win win win”. The repairs are done correctly, they resemble the factory appearance and it is much more efficient the MIG/MAG welding. In appearance, it will mimic other similar areas of the vehicle. The ability to have a visually better repair should not exceed the importance of performing the necessary type of repair. Resistance spot welding, in even its most basic form, reduces the amount of heat put into the panels at the time of the weld. That fact also can play a role in the possibility of limiting the amount of corrosion forming at a later time as well. The less heat, which reduces the “heat affect zone” (HAZ), used during a resistance welding process, the less chance there will be unnecessary damage to coatings on the steel. There is an even more important reason we will discuss next.

Shunting pliers and copper pads

Squeeze-type resistance spot welding may be a critical factor in the proper way to join advance steels. It is this notion as to why you should understand the OEMs requirement of STRSW in certain scenarios, whether it be general collision repair guidelines or specific body repair procedures. The use of steels other than mild cold rolled steel is rampant in today's vehicle construction, as you should be well aware. Many of these advance steels do not respond well to methods of days past. Even though advanced steel may appear satisfactory joined after MIG/MAG welding, the amount of heat introduced into the area could have caused that steel to become brittle and not perform well in a subsequent collision. As mentioned above, if corrosion sets in due to the elevated damage of the coatings, that can also affect crash-worthiness at a later time.

As for the shops that do have the necessary equipment, but aren't utilizing it to its full potential there can be several reasons as to why. The two most common factors that have brought the shop to that state are, either the machine is out of date or more likely there is a lack of knowledge/training in play. It may be that the initial investment did not have satisfactory training or the current employees were not part of it, or have not been educated by the ones that may have proper knowledge regarding the equipment. It may be that your machine you have currently is satisfactory, but recurrent or even initial training should be considered to gain its full potential.

The machines are available through many different welding equipment manufacturers. The most important consideration when choosing the right machine for your shop is what, if any, recommendations or requirements are made by the vehicle manufacture. Some manufactures have approved equipment specified while others don't. Vehicle makers that don't specify a particular brand or model of machines may still have minimum requirements that a machine must be capable of. Shops that are engaged in an OEM certification program, or are looking into a possible future relationship, may want to consult that program's requirements. If you find yourself in a market where one particular vehicle make is popular I would suggest you purchase a machine that coincides with that manufacturer's guidelines. If you are considering an upgrade by purchasing a new machine the same considerations should be in play. Nugget size can be called out by some OEMs as well as clamp force or other specifications required. It may be the vehicle maker has amperage or other minimum guidelines only. That may lessen the need to purchase a particular brand or model of equipment but will require the buyer to make sure those specifications can be met with that particular welder when choosing.

An STRSW machine with arms of various lengths

As promised we will cover the “squeeze” aspect of the process. The ratings for the machines as it relates to the squeeze pressure can be represented in Dakanewtons, or daN for short. That value can be converted into a more understandable measure of pound-force. Some automakers specify a particular or minimum amount of force to be applied when squeezing the panels together before performing the weld. The newton is the Standard International (SI) unit of force. I won't go into it any further because it gets a bit scientific. As stated, it is the squeeze force applied but it's important to recognize the machine capabilities as it relates to repair information.

The different types of power supply are typically either 3 phase electrical supply requirements or the machine may be battery operated. Some machines may operate on 230V but those with 3 phase requirements are the most common. The battery-operated machines may be used where there is no 3 phase power supply to the shop or other limiting circumstances. There are several factors when selecting a machine if you are not using the OEM recommendations. Most are based on current and applied force. Welding time is also a component to consider in the ability of a machine. The specifications that a vehicle manufacturer may require could have all or more parameters listed to perform the correct weld. The body repair manual of the specific make, model, and year should be referenced before any work begins. Machines may have the ability to sense the type of metal and create welds based on what the machine determines to be the appropriate settings. It may be that the machine can sense more than the type of metal alone. It may be able to detect the type, thickness, and if there is anything in between the metals such as adhesives. If not the automated type, the user may have to manually input parameters such as the type of metal, thickness, and whether or not weld bonding is being performed. 

Some equipment makers have programs installed into the machine that are based on a particular vehicle make and/or model settings. Feedback from the equipment may include a pass/fail on each weld and they could have the ability to document the process for record-keeping by the shop. Software updates are typically done as called out by the welding equipment manufacturer. Some machines will have the ability to connect directly with their respective manufacturer for updates and may even provide training videos for the user within the machine.

We will now cover some of the typical repair procedures that may be required when it comes to STRSW. Shunting of the panels is a common requirement. Shunting is typically done with specific pliers similar to locking pliers but allows the multiple work-pieces to be “connected” before the first weld is performed. Pliers that are made specifically for shunting will most likely have copper “feet” with a cable connecting those feet so the current has an easy path to follow at that specific location. Shunting may be done with typical locking pliers but for the best possible results, specific shunting pliers should be used. For further information refer to I-Car Collision Repair News article: Shunting The Current When Spot Welding, Posted on 10 June 2016

An STRSW machine with a single-sided optio

Test welds may also be listed as part of the repair procedure and are very important just as in MIG/MAG welding. The welds may look OK and the machine may verify a good weld but ultimately a destructive procedure on the test weld is the only true way to know if the performance is going to be there based on your settings and other preparations. Another note about clamping is that all clamps or other means of fitting a panel together, besides the shunting clamp used for the initial weld, should be insulated. There are various common methods to do this such as using tape or other non-conductive materials used on the areas where the clamp comes in contact with the metal. 

Air pressure is typically used for the clamping force during the welding process. The amount of air pressure set during the weld will affect the clamp pressure and therefore can affect the weld quality. Vehicle manufacturers will sometimes specify minimums in those parameters that will be used with settings and machine capabilities. It is also worth noting here, it is a common practice to remove all coatings where the electrode tips come in contact with the steel. Typically OEM applied coatings, such as e-coat, are left intact on the inside where the panels meet. It may be that the vehicle manufacturer specifies a weld-through coating to be applied between the panels or the process may include weld bonding. Weld bonding is a process where an adhesive, typically two-part, is used between the panels to aid in the joining process. If weld bonding is required the technician should follow the vehicle manufacture guidelines on the proper way to prep the panels before applying the adhesive. If no manufactures guidelines exist it is common practice to follow the product manufactures guidelines.

STRSW machines may have several functions that can be performed by one machine outside of their primary function. Examples may be a dent removal capability with a weld-on puller such as a slide hammer or manual pulling attachment. Other functions may include the ability to weld studs, nuts, washers, or pins on a panel used for attaching such things as a heat shield, closeout panels, and such. Metal shrinking abilities may be built into the machine for straightening/shrinking processes as well. Some machines may have the ability to perform a “stitch” or “seam” type of weld with the single-sided function. Machines may also have the ability to perform single-sided spot welds. These welds will require a ground attachment to the material being welded and will not have the ability to squeeze the panels together. It is reliant upon the operator to make that happen. It is why these types of welds are less efficient when compared to STRSW, but it may be necessary as limits can exist where the arms of the squeeze-type mode will fit. Vehicle manufacturers will, at times, specify what type of welds are to be made if there are limits to where the arms can reach.

As with any other tool or piece of equipment, training is paramount. Welding equipment manufacturers may have training available at purchase or provide it in another form. It will be important to determine how that will take place before purchase. There is nothing worse than investing in a process or equipment and not utilizing it to its full potential. Liability concerns should be at play here as well. Maintenance should also be planned before purchase. There are some wear/consumable items on most machines. The electrode tips are very important in many ways when it comes to making a proper weld. Alignment, condition, and size can all have effects in this area. Training is important, equipment makers and OEM facilitated programs that provide training may have Alliance Courses that could qualify for I-Car credits. General types of education, that are not machine specific, are available as well.  For further online and in-shop training refer to I-CAR:

Online- Introduction to Squeeze-Type Resistance Spot Welding

Hands-On- Squeeze-Type Resistance Spot Welding Hands-On Skills Development

The most important guideline is to follow the vehicle makers' repair manual or criteria for the machines and processes before beginning the repairs. As stated in the beginning, choosing a machine for initial purchase or when updating current equipment, the vehicles you are repairing should weigh into the decision making process. Training and support are important as well. We all know the only thing constant in our field is change. As with everything else, it should come as no surprise there have been changes in this arena too. Mostly for the better!

Sponsored Recommendations

Best Body Shop and the 360-Degree-Concept

Spanesi ‘360-Degree-Concept’ Enables Kansas Body Shop to Complete High-Quality Repairs

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Banking on Bigger Profits with a Heavy-Duty Truck Paint Booth

The addition of a heavy-duty paint booth for oversized trucks & vehicles can open the door to new or expanded service opportunities.

Boosting Your Shop's Bottom Line with an Extended Height Paint Booths

Discover how the investment in an extended-height paint booth is a game-changer for most collision shops with this Free Guide.