How to select the right welding tool
Spot welding looks simple and easy until your products fall apart while being delivered to the job site.
When that happened to one contractor several years ago, spot welding suddenly took on a whole new level of importance because this sheet metal fabricator had to rivet his entire production run at great expense.
Although the surface appearance of his spot welds looked somewhat normal, the weld joints fell apart because they did not have proper fusion. In this case, it turned out that the customer’s tired old 20-kilovolt-amp foot-operated spot welder was too small for the metal thicknesses being used for their new product, and they soon purchased a heavy-duty 50-KVA air-operated machine capable of higher amperage and forging force.
Resistance spot welding is unique because it’s a high-speed method of joining metals without the addition of any filler metal. When the welder is capable and set up correctly, a strong forged joint is formed through the localized application of accurately controlled electric heat and clamping force.
Properly applied, resistance spot welding is the fastest, strongest and least expensive method of sheet metal joining. However, even though spot welding has been used in manufacturing for decades, the process is still not well understood outside of the automotive industry.
Not so simple
Spot welding looks simple, but there are numerous variables at work that must be understood and properly adjusted to achieve the desired result: a weld that is stronger than the parent metal. And since many resistance spot welds must have a smooth show surface as well as being strong, that requirement adds another challenge for welder setup personnel. Three main variables must be considered to achieve success with spot welding, and they can be remembered as “PCT”: pressure (forging force), current (the amount of welding amps used), and time (the duration of current flow).
Without a full understanding of the importance of these variables and their relation to one another, weak and/or ugly weld joints can result. Unfortunately, the spot-welding process often gets the blame for these problems, causing sheet metal shops to substitute expensive and slow metal-joining methods, such as arc welding, riveting and adhesives.
Choosing the right spot welder can be confusing for an HVAC construction or sheet metal shop owner because there are numerous brands and price ranges available. And now, with so many imported spot welders being sold in the U.S., only a few machines meet the heavy-duty specifications for amperage and force capability from the Resistance Welding Manufacturing Alliance.
An additional source of confusion comes from spot welders being sized and compared according to kilovolt amps, which is a thermal rating that can be manipulated by welder manufacturers to inflate their machine’s capabilities. As an example, RWMA specs call for a spot welder to have a transformer with a 50 percent duty-cycle rating, but some machines have a transformer rated at only 10 percent, which more than doubles the published KVA rating.
It may be surprising to learn that the KVA rating can sometimes have little to do with a spot welder’s actual welding amperage output, which varies greatly with the arm length (aka “throat depth”), the vertical gap between the arms and the turns ratio of the transformer.
And another important variable is the secondary voltage of the welder’s transformer, which must be adequate to “force” the secondary welding amperage coming out of the transformer through the welder’s copper arms to the spot-welding electrodes (tips).
Typical spot welder transformers have a secondary voltage rating of 6-8 volts, but if you need a deep-throat machine with long arms, you should specify a welder with a higher secondary voltage, something in the range of 10-12 volts.
This is especially true if reaching all the spot locations requires the part being welded to be loaded in the machine’s throat. Steel in the throat disrupts the magnetic field between the arms and robs the machine of available welding amps.
Another critical spot-welding variable to consider is the available weld forging force (clamping pressure), which is typically produced with a foot lever or an air cylinder.
Since most sheet metal shops use a spot welder with a rocker arm force-delivery system, it is important to understand that the available weld force varies according to the ratio of arm length to the distance of the air cylinder or foot-lever mechanism from the fulcrum point. If short arms are replaced with long ones, the available weld force will be reduced substantially.
It is important to note that foot-operated machines, although widely used in sheet metal works shops, are rarely capable of producing the forging force required to meet the most desirable class A spot-welding specs, which are settings that result in the highest weld strength and the most attractive appearance due to a short weld time and proper force.
Charts that list class A, B and C weld settings for various metal thicknesses are readily available from reference books, with class C welds being relatively strong, but ugly because of a large heat-affected zone resulting from an extended weld time. Unfortunately, most spot welders do not have readouts for welding amps and force, and having a portable amp meter and weld force gauge available is recommended in order to properly adjust these important variables.
Class A spot-welding specs for two pieces of clean, 18-gauge, low-carbon steel are 10,300 welding amps, 650 pounds of weld force and only eight cycles of weld time. One cycle is only one-sixtieth of a second, so eight cycles is very quick. Spot welding 18-gauge galvanized steel requires higher settings: 14,000 amps, 800 pounds and 12 cycles.
A Class C welding schedule for our same 18-gauge uncoated mild steel example is 6,100 amps, 205 pounds of force and a whopping 42 cycles of weld current flow. This extended weld time of well over half a second may overheat the electrodes and eventually burn out the transformer.
Proper water cooling of spot welders is important and some shops use small radiator-type water recirculators that may not be able to keep up with the heat generated. A better choice is a self-contained refrigeration-type water chiller and recirculators properly sized for the machine.
Although the tensile shear strength of a single class C spot weld compared with a class A weld is only reduced from 1,820 pounds to 1,600 pounds, products look much better with an attractive, low-marking class A weld made with a properly sized spot welder. Plus, you’ll never have to worry about your products falling apart.
The same principles apply to handheld, portable spot-welding guns with integral transformers. A light-duty spot welder may be entirely suitable for some applications, but you should be an informed buyer and fully understand what you’re getting.
Tom Snow is CEO of T.J. Snow Co. in Chattanooga, Tennessee, a family-owned company that has been selling new and used spot welders and supplies for over 50 years. In addition to being an American Welding Society-certified resistance welding technician, Snow is currently chairman of the Resistance Welding Manufacturing Alliance.
For reprints of this article, contact Jill DeVries at (248) 244-1726 or email firstname.lastname@example.org.