June 17, 2008
When MIG welding, are you experiencing an erratic, sputtering arc; a gradual need to increase voltage at the power source; discoloration of copper cable strands or the liner; increased contact tip burnbacks; or inconsistent weld appearance? If so, your problem could be poor conductivity caused by electrical resistance.
Electrical conductivity is at the core of MIG welding equipment, and yet poor conductivity is one of the most common and frequently overlooked causes of poor welding performance.
Conductivity is a measure of a material's ability to carry an electrical current. A lack of conductivity can be thought of in terms of electrical resistance – the interference of the free flow of electrical current.
A helpful way of thinking about conductivity is to imagine a hose with water flowing through it. If you restrict the hose in one spot, it decreases the amount of water able to travel through the hose. Similarly, an area along a welding circuit that interrupts the flow of electricity produces increased electrical resistance, or decreased conductivity.
Resistance in the circuit causes a voltage drop and a buildup of heat in the area of increased resistance. While typically very low, electrical resistance exists in all welding system guns and cables. Excessive resistance, however, can lead to a number of problems, including premature equipment failure, increased downtime, and poor weld quality.
Among the factors most damaging to welding components is excessive heat, which is both a source and a result of electrical resistance. If the flow of electricity is impeded, the temperature along the circuit increases. When heat builds up as a result of this excessive resistance, it creates more resistance, which in turn generates more heat. This cycle can significantly shorten equipment life in addition to causing poor welding performance. For these reasons, it is critical to use guns and cables that are appropriately sized for your application.
Common signs of poor conductivity/excessive resistance are an erratic, sputtering arc; a gradual need to increase voltage at the power source; discoloration of copper cable strands or the liner; increased contact tip burnbacks; or inconsistent weld appearance.
Thankfully, troubleshooting conductivity problems is often easy and inexpensive.
Electrical resistance can develop and create performance problems in several places along the welding circuit. Mechanical connections are one of the most common sources of decreased conductivity and typically are the easiest places along the circuit to check for failure. Make sure to turn off power before inspecting.
Visually inspecting the connections between the cables, the power source, and the wire feeder usually is sufficient to diagnose conductivity problems. Solutions range from cleaning and tightening connections to completely replacing guns and cables.
Another connection that can be problematic is between the work lead and the workpiece. The work lead should be in good mechanical condition and clamped to clean, unpainted metal, as close to the work area as practical. A conductive grease can help ensure optimum conductivity for rotating work leads, such as turntables and tube positioners.
Conductivity problems also can be caused by damaged copper stranding within the cables. The internal wire strands – particularly in the gun cable – can fray and break with repeated use. The area just below the gun handle is especially prone to frayed and broken wires in guns without a strain relief.
In many cases, the gun can be repaired by shortening and reterminating the cable to remove the damaged area. Depending on the extent of damage in a cable (which often is difficult to determine), replacing the gun or gun cable may be necessary.
The fittings between the cable and the gun also can cause conductivity problems. Different gun manufacturers offer different types of fittings, including compression, set screw, and crimped.
In general, compression fittings offer the best combination of conductivity and repairability. Set screw fittings typically are easy to repair, but they are more susceptible to loosening and increased resistance. Crimped fittings usually have good conductivity, especially when new, but are generally more susceptible to overheating with repeated use.
The neck, diffuser, and contact tip also play important roles in delivering the current to the wire electrode. Loose fittings at the neck, a diffuser that is too small for the application, or a worn contact tip all can cause increased resistance in the circuit. These components should be tightened, cleaned, or replaced as soon as they begin to create problems.
Although many potential locations of poor conductivity and excessive resistance lie along the welding circuit, they usually are easy to identify and the makeup of the equipment often requires replacing the problem areas only. Always be sure to check manufacturer specifications and recommendations for your equipment when troubleshooting conductivity issues.