STRSW Welding: Get spot on with spot welding

Before you can perform proper welds with an STRSW machine, you need to fully understand how it works. Current flows through the arms (aluminum first with the case of C gun style), which are made of a copper alloy. Copper and aluminum are excellent conductors of electricity since they provide low levels of resistance.

To better understand how electricity is used here, let's adopt the analogy of water rushing through a hose, with the welding arms as a hose and the electricity as water. Volts are the pressure of electricity, and amp (current) is the volume. When you turn on the water and put your thumb over the end of the hose, the water stops. If you increase the volume by opening the hose bib, the pressure increases. When you put your thumb over the end of the hose again, the water squirts out the sides to relieve the pressure.

With an STRSW machine, the electricity flows nicely until the current comes in contact with steel — the thumb on top of the water hose. The pressure is relieved by the formation of heat — water squirting out of the hose.

Paul Wilcox, president of Tite-Spot Welders, provided this description of spot welding in a company bulletin. A spot weld is formed when a large amount of current (amperes A) passes through copper. As the electricity flows through copper and comes in contact with the metal being welded, resistance forms in the electrical path and heat is generated. As the heat builds up, the metal (in contact with tips of the electrodes) melts. The pressure exerted on the weld site causes the metal to deform, producing a "weld nugget" or a small circular dent as it cools.

Technology

Most of the newest generation of STRSW welders use inverter technology and require three-phase power. Three-phase power uses three sine waves instead of one during one cycle. When one wave is at zero, the other two are still delivering power. With all lines producing electricity a constant voltage is achieved.

Constant voltage is required to use inverter technology, which is why shops need to have a 220V three-phase power source. Besides having three-phase power available, shops also need a way of delivering the correct electricity to the equipment. Let's look at the necessary wiring requirement.

Remember that we need the proper current (amps) supply for the welder. Most inverter welders need between 50-65 amps of service. What does that mean?

Think again of water flowing through the pipes as current. If you can get by with the amount of water that the .5-inch pipe delivers in a set amount of time, you are in good shape. If you need more water, you need a larger pipe. With electricity, electrical wire is that pipe. The gauge of the wire is the diameter of the pipe.

Most machines need a minimum of service (wire and size). And that equates to a No. 6 wire. A 65 amp service needs No. 4 wires. If the electrical run is longer (more than 80 feet), you should use the next size wire. So, before investing in an inverter spot welder, you should consult with an electrical engineer to determine if you have the three-phase power and proper electrical setup needed to run additional conduit to handle the circuitry. Let's move onto the components of the weld cycle.

Components of the spot weld

Three components create a resistance spot weld: squeeze pressure, current flow and hold time. These components determine the weld strength. Here's how they work together.

The welder's electrodes are placed into position. The trigger forces the electrodes together. Holding them in place is the pressure exerted by the STRSW machine, which is hooked up to the shop air supply. When the current is turned on, it flows through the arms, heats the metal and forms a weld nugget. The pressure forces the molten metal together.

The hold time is the final stage. During this stage the current is shut off while pressure remains applied to the weld site. The metal cools until it solidifies (the cycle takes approximately one or two seconds).

You need to note a couple of things on squeeze time. First, if you are using three panels, you may need more pressure. Second, as you increase the length of the arms (not in the case of "C" type design), you may need to increase the pressure. Lastly, you need to know that the pressure is not for fit up. This point really needs to be addressed.

Often, techs think the pressure will draw the two pieces of metal together. If the fit is not good, they add more pressure. Too high pressure will result in variable weld strength, higher current, mushrooming of the tips and/or excessive indentation.

Using too little pressure also can create problems. Not enough squeeze pressure can result in expulsion of metal, the electrode sticking to the weld, lower electrode life, possible internal cracking and a larger heat affect zone (more resistance is needed, thus more heat).

We already know about current and how it forms the weld. With advanced metals, we can now apply two stages of current in one weld cycle. The first stage pre-heats the metal, and the second one produces the weld nugget. This process produces a smaller heat affect zone (a larger heat affect zone on advanced metals weakens the metal).

If your weld time is too long, you will shorten the electrode life or produce excessive indentation at the surface or internal cracking in the weld. A weld time that is too short will result in low weld strength. It is extremely important to make test welds and perform destructive tests prior to welding on a vehicle (to be discussed later in the article).

Hold time is used to keep pressure on the weld until it solidifies. This process takes about a second. Too short of a hold time can result in surface expulsion, electrode sticking and internal cracking.

Available guns and electrodes

There are three types of guns — a single-sided gun, a C gun and an X gun.

The C gun works using pressure that pushes the electrode out of a cylinder to make contact with the fixed electrode on the C arm (you will need larger C arms to reach further into the vehicle's structure). The C gun style has some distinct advantages. The first is changing the C arm. One locking handle allows the arm to be changed (figure with open position, closed position). No adjustment is necessary. As the arm gets bigger (to reach deeper into the vehicle), there is virtually no change in the clamping pressure needed for the weld. This style of gun does have some access limitations in certain applications.

The X gun uses a center pivot (like an X) and forces the arm to pivot into the other electrode. There are many different types of electrodes for nearly all welding applications. The X gun electrodes come in matching pairs and are designed to reach virtually every spot weld on a vehicle. Exchanging electrodes takes more time compared to the C gun.

One manufacturer uses a pin on the electrode to aid in the alignment of the electrode. A couple of other welders use set screws to attach the electrodes to the gun. Of these two, one has a cam action that locks in electrodes to a standard position (it required virtually no alignment of the tips).

There are three other details on electrodes. First, as the electrodes increase in length, the squeeze pressure at the tips decreases, increasing the possibility for a weld to fail. Second is the cost factor. Additional electrode sets are not cheap, especially the water-cooled variety. Third, the tips. They can be either solid or press on. Sold tips will last longer, but require dressing after a number of welds. Press-ons are usually not dressed due to their shape, so they require changing on a shorter interval. They require no sharpening tool and are very easy to replace.

Techs need to keep in mind that as they repeatedly weld, heat builds up in the electrodes. With an increase in heat comes an increase in resistance. The increase in resistance reduces the current available for welding. Electrodes are cooled either with air or water — water providing a better method of cooling.

Test welds

The following steps demonstrate how to conduct test welds. (Note: An inverter welder is used in this demonstration.)

Test 1

1. Start by turning the machine on and following the prompts on the computer screen. The computer lists many of the steels found in today's cars including advanced steel with boron. Let's weld two pieces of 22 gauge steel together. The actual measured thickness of the metal turns out to be .77 mm.

2. Set the type of steel followed by the metal thickness. You're now ready to make a test weld.

4. Note that I-CAR teaches in the WCS 04 class that the weld nugget should pull a 6 mm hole to be a good weld. This test involves two pieces of galvanized steel that are 1.2 mm thick.

5. The program was set, but we deliberately set the parameter to the wrong thickness to see what would happen. Result: The weld showed very little penetration, which was evident by the non-existent heat affect zone.

6. We reset the machine to 1.2 mm (type also was changed).

7. This time, with the new setting, the second weld had a much larger heat affect zone. The peel test also produced a 6 mm tear out hole.

Test 2

This test was performed with a single piece of galvanized steel (1.4mm) and 22 gauge steel.

2. We reset the computer to the proper thickness. This time the weld passed the peel test.

Test 3

The next test was with three pieces of 22-gauge steel.

We set the machine's computer to the proper setting and performed a peel test and again the test passed.

Weld bonding: The art of combining adhesives and STRSW welds

Why weld bond? Two reasons: First, the joint it produces is stronger, and second, it reduces NVH (noise, vibration and harshness). Let's look at the process.

Start with adhesive, metal, locking pliers and, of course, an STRSW welder. In this example we're starting with clean metal. In the shop, it's a little different because the mating surfaces are either painted or E-coated. Ford recommends removing the paint, galvanization and E-coat from the mating surfaces.

Next step is to apply the adhesive. You might be asking, "If adhesive is applied, how will current flow through it?" It won't. You need to create what is called a "shunt." A shunt is an alternative path for the current to flow.

We'll apply adhesive to the entire panel and create a shunt with a pair of locking pliers. The adhesive acts as an insulator between the two pieces of metal being welded.

The locking pliers shall have electrical tape wrapped around the ends. We do not want the electrical path to go through them, just the clamp. Once the first weld is completed, an electrical path is created (by the weld) for the next weld and so on.

Three last notes:

• Welding can be done with the glue wet or uncured and cured.
• A single weld should be made with the glue and a peel test should be done. (Note that a twist test is not recommend with adhesive).
• A shunt is needed when weld through primer is applied to the mating surfaces.

Conclusion: Making the right purchase

What do you need to look at when purchasing an inverter spot welder? First, make sure you have the power. Next, consider service and training. If a problem occurs, who will fix it and where? Who will teach your techs how to use the welder? Also, you need to consider what comes with the machine and what the cost will be for accessories, along with upgrades for the computer. Perform due diligence when deciding what machine is good for you. These machines are the next generation of welders, and they are no longer luxury items, but are a necessity and a requirement for shops.