November 7, 2011
When designing and operating a robotic welding cell, don't overlook the consumables. Choosing the wrong consumables can over time lead to serious costs.
A welding automation investment should provide the productivity gains and quality improvements that set your welding operation apart (see Figure 1). To achieve success, however, you must ensure that the parts you are welding are consistent and repeatable, and confirm that your welding operation has good work flow and properly trained welding operators to oversee the system. You also need the right equipment for the job. Take care to select the appropriate robotic gas metal arc welding (GMAW) gun and consumables—contact tips, nozzles, liners, and retaining heads—for the application.
The consumables, in particular, are an easily overlooked part of an automated welding system, but they can have a measurable impact on downtime and day-to-day costs.
The contact tip-to-nozzle relationship for an automated welding system varies with each application. Complex joints or tooling often requires an extended contact-tip-to-nozzle relationship. This provides greater access into more complex joints and can help you accommodate for complex tooling better.
However, this also makes your contact tip more prone to spatter accumulation and, because it is exposed to more heat from the arc, a reduced tip life. Applying an antispatter compound can offer some protection against such situations, but you still need to monitor the contact tips regularly for signs of wear. Remember, preventive maintenance is better than downtime for resolving problems. Change over your contact tips before problems occur.
Using heavy- or extended-duty contact tips composed of chrome zirconium is also a good option for gaining longer performance. Chrome zirconium contact tips are harder and more durable than copper ones, and while they offer slightly less conductivity, the difference is negligible. You usually can identify these types of contact tips by the machined groove at the base of the thread.
Checking contact tips, retaining heads (or diffusers), and nozzles for good connections also can affect welding performance (see Figure 2). Solid connections help ensure reliable electrical conductivity and minimize heat, which in turn provides more consistent weld quality and helps consumables last longer. Also look for consumables designed to thread together and mate securely, as these can further increase their longevity.
The welding wires can affect the performance of contact tips and what size you should use. Larger drums of wires—500 to 1,000 pounds—commonly are used for automated welding systems to minimize changeover; however, the wire in these drums tends to have less of a cast, or helix, than wire that feeds off of a smaller spool. As a result, the wire often feeds through the contact tip relatively straight, making little or no contact with it. The effect is twofold: One, it minimizes the electrical conductivity necessary to create a good arc and a sound weld; two, it can cause the welding wire to contact the part being welded and arc back into the contact tip, creating a burnback. This condition causes downtime to change over the contact tip.
As a solution, consider undersizing your contact tips, particularly if you are using a solid wire. For example, a 0.040-inch-diameter contact tip could work for a 0.045-in. wire. Check with a robotic integrator or welding distributor if you are using metal-cored wires, because their tubular construction doesn’t always make undersizing them feasible.
Also consider how the wire affects contact tip longevity. For example, non-copper-coated solid wires tend to wear contact tips and liners more quickly than copper-coated ones. The copper on a copper-coated wire acts like a lubricant to improve feedability and often can extend consumable life. Copper-coated wires do cost more upfront but, depending on the application, may be worth it, considering the increased cost of purchasing more contact tips for use with a non-copper-coated wire, as well as the downtime for changeover.
The GMAW transfer mode affects the type of consumables you require. For example, pulsed welding programs, in which the power source pulses between low background currents and high peaks, are especially harsh on consumables because of the higher levels of heat the process generates. They tend to cause the contact tip to erode more quickly and therefore require more frequent changeovers.
With such a welding program, carefully monitor your contact tip usage so that you can determine how often the contact tips need to be replaced. Changing over these consumables before they have problems can help prevent issues like loss of electrical conductivity, burnbacks, or excessive spatter accumulation. Such spatter accumulation tends to occur when the contact tip becomes too hot and the consumable material softens.
Use the time during routine pauses in production to change over the contact tip; this prevents interrupting arc-on time. Also consider using heavy-duty contact tips for higher-heat applications; again, chrome zirconium contact tips are a good choice.
The tooling usually dictates the type of nozzle to use. Bottleneck, straight, and tapered nozzles are common choices because they are narrower than standard nozzles and can provide better access around tooling or into complex joints.
Still, always consider the duty cycle and amperage when deciding which nozzle to use. In most cases, the more tapered a nozzle, the thinner it is and the less it can withstand high-amperage or high-duty-cycle applications. If your automated system welds at 300 amps or greater and has high levels of arc-on time, it may be a good idea to select heavy-duty-style nozzles, which have thicker walls and insulators and are better able to resist heat. Nozzles composed of copper also are a good option, as are those featuring high-temperature fiberglass insulators. Remember that you need to be sure to select a nozzle that can access the joint but is not so narrow (especially in relation to the contact tip) that you compromise shielding gas coverage or unnecessarily shorten the consumables’ life.
For all nozzle styles and types, you should employ a nozzle cleaning station or reamer to help maintain them (see Figure 3). A nozzle cleaning station cleans the robotic gun and nozzle of spatter and clears away debris in the retaining head that accumulates during the welding process. These stations can be outfitted with a sprayer that applies a water- or oil-based antispatter compound to protect the nozzle, retaining head, and workpiece from spatter after it has been cleaned.
The nozzle cleaning station should be placed close to your robot so it is easily accessible. Also, you should program your robot to use it between cycles—during part loading or tool transfer—so it won’t interrupt the welding operation. The nozzle cleaning station should take only a few seconds to complete its job.
Select consumables that are well-machined and have smooth, round surfaces, as these are less prone to collecting spatter and tend to last longer. Also, try to use the heaviest-duty consumables for your application that will still give you access to tooling. Doing so can help extend their life.
Pay attention to your retaining head selection and the liners in the robotic GMAW gun. The retaining head should work with your nozzle and contact tip, and offer a secure connection so that you obtain the best conductivity. Also, always trim and install liners according to the manufacturer’s recommendation, using a liner gauge to determine the appropriate length. A liner that is too short or too long can cause wire-feeding problems that require downtime to rectify.
As with any part of an automated welding system, the strategy is to keep your consumables in working order so that you spend more time reaping the benefits of the process—and less time troubleshooting problems.
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