Known as the established market leader for robotic welding, Yaskawa is no stranger to the nuances of arc welding automation. Robust and reliable, the process of arc welding via the use of industrial robots has evolved over the last few decades, effectively meeting diverse manufacturing needs. While this common method of joining metal pieces together creates incredibly strong parts with many flexible options, it also has its tradeoffs.
There’s no getting around the fact that arc welding automation uses a lot of consumables, such as wire and gas. While effective, this type of adhesive joining brings a new level of potential hazardous material into play, along with the ongoing consumable expense. Moreover, the robotic arc welding process creates a very high level of UV rays that require certain safety precautions.
These reasons, on top of potentially long cycle times and metal thickness requirements, makes one question whether there may be a better welding method for certain tasks.
Napoleon Hill, one of the original motivational hucksters of the mid-1900’s, used to say “Out of resistance comes strength”. That adage may not have made anyone rich quickly as Hill promised, but it holds a lot of truth in welding today.
Resistance welding—specifically “spot welding”—has been a popular metal joining solution for automotive and other manufacturers for many years. Accomplished by passing an electrical current between layers of metal while applying pressure, this method creates a strong metal bond called a “nugget” at the point of execution. It can also be modified by using wheel-like electrodes to run along a seam for a longer weld. While simple in nature, this method provides particular applications with big advantages:
Because there is no filler metal or shielding gas used with spot welding, cost per weld and weight is rapidly reduced by limiting the consumables. Conversely, arc welding can consume torch nozzles and tips. With resistance welding, a spot gun will have tips that can be dressed to refurbish them several times before they need to be replaced.
Typically, the piece parts used for spot welding are stamped, which is typically more affordable than machined parts. The use of stamped parts also adds another level of consistency to the process, contributing to the cost savings. Cost of arc welding and spot welding systems are pretty comparable, but both are much cheaper than the laser alternatives.
Arc welding methods, like MIG or TIG welding, create a great deal of UV light that is damaging to skin and eyes. Users must protect against this with eye protection, such as a welding hood. The arc curtains around welders and robots are considered secondary protection, and they shouldn’t be relied on for full safety when observing a weld in action.
Although spot welding produces some sparks, it is far less than those of MIG welding, which can potentially burn holes in some materials. Overall, safeguarding can be simplified, as it does not need to reduce the arc flash, nor does it have to control the same amount of sparks and spatter.
With spot welding, there is inherently far less weld spatter that can contaminate the finish of a part and floor of the workcell. Similarly, the fumes are drastically reduced, and smoke is minimized as well. On the flip side, arc welding spatter can cause issues with part cosmetics, or dealing with dreaded “weld berries” that can forever rattle around in an enclosed part.
Aesthetics in spot welding are consistent and clean, whereas with arc welding, creating a nice-looking weld bead can be challenging at times. All spot-welded parts are dual-layered, and the weld is essentially hidden within the body of the part, rather than producing a highly visible seam.
Also, the weld timer is going to be comparable in size and hookup requirements to the arc welding power source. However, a barrel of wire and conduit within the workcell will no longer be needed, and the messy reamer can sometimes be replaced by a tip dresser for a spot gun.
Spot welded parts experience less part deformation due to lower heat input, and they have a very high repeatability. The parts are also generally lighter weight, as they can be hollow after combining two stamped parts. There is no filler-metal metallurgy to consider, making the process relatively similar for dissimilar parts.
A typical spot-welding squeeze time only takes a brief partial second, and a robot can move from spot to spot very quickly. There is no surface preparation required, which arc welding can be sensitive to. The travel speed of a MIG torch is limited by the filler metal, heat input and other variables. While some manual arc welding processes can be very difficult to automate depending on accessibility and part size, nearly every spot-welding application can be automated. Overall, the potential fabrication time for a part can be reduced nearly by half using the spot weld method.
The biggest challenge with spot welding is that the part being welded must be designed for spot welding, allowing the spot gun convenient access to both sides of the part to apply the proper current and pressure. Likewise, the metal thickness is typically limited to about 3 mm per sheet.
Some parts will never be suitable for spot welding, depending on their thickness and the shear strength required. Also, parts that must be completely sealed (fluid tanks, pressure parts, etc.) cannot be spot welded. Sometimes, adhesive and sealant can contribute to the part rigidity and strength, but this adds an additional step in the process.
Spot welding requires different expertise than arc welding, though is generally simpler. But, despite the intelligence of today’s robot software, it cannot replace the design and execution that a properly trained weld engineer can offer with either system.
For high-volume parts that can be stamped, spot welding can be an ideal solution. It can decrease cost, weight and cycle time, while increasing safety, consistency, aesthetics and cleanliness with a proper system. It is one more automation tool in the proverbial tool bag to increase competitiveness and win the future!
Josh Leath is a Product Manager, Welding – Yaskawa America, Inc., Motoman Robotics Division