Understanding MIG Gun Liners

How to Optimize Performance and Troubleshoot Problems

The liner is often overlooked as the cause of MIG welding problems, partially because it is difficult to check for problems without disconnecting the gun from the wire feeder.

The liner is both one of the simplest and most important components of a MIG gun. Its sole purpose is to guide the welding wire from the wire feeder, through the gun cable and up to the contact tip. If it is not performing this task properly, the gun is virtually worthless.

A number of problems can interfere with the liner’s ability to properly guide the wire through the welding cable. When the wire does not feed correctly, weld quality problems may arise that lead to increased operator downtime and costly rework.

The following is a discussion to help you get the best performance from your MIG gun liner and troubleshoot problems when they occur.

Optimizing Performance
There’s an old saying that prevention is the best medicine, and this holds true with MIG gun liners as well. Proactive maintenance can eliminate problems before they arise and reduce operator downtime, more serious equipment failures and costly rework.

Buy quality: The quality of a liner can also impact your welding performance, productivity and operator downtime. A premium quality liner maintains a consistent inside diameter throughout its length. This is critical because variances as small as a few thousandths of an inch can result in wire feeding problems requiring time-consuming liner replacement.

Choose the right size: To maximize the performance of your MIG equipment, choose the correct liner size for the wire being used.

Small diameter welding wires, .023-in. through .045-in., have relatively low columnar strength, which, when paired with an oversized liner, can cause the wire to wander or drift within the liner. This in turn leads to poor wire feeding and premature liner failure due to excessive wear.

By contrast, larger diameter welding wires, 1/16-in. through 1/8-in., have much higher columnar strength. Just make sure the liner you choose is large enough to feed the welding wire you are using.

Don’t overdo it: Most MIG gun liners are made from coiled steel wire, known as music or piano wire, which gives the liner a good balance of rigidity and flexibility and allows it to guide the welding wire through a tightly bent cable without kinking. Nevertheless, bending the cable too much can cause poor wire feeding, premature liner wear and birdnesting (explained below).

Proper replacement: Improperly installing the liner can lead to wire feeding problems such as birdnesting, which results in downtime and reduced productivity. Properly trimming the liner to the right length sounds like an easy enough task, but frequently overlooked is the fact that MIG gun cables are wound in a helix pattern, which causes them to shrink when twisted. A liner trimmed with a twisted cable could be too short when the cable is untwisted. Gaps between the end of the liner and the gas diffuser/contact tip can lead to erratic wire feed or birdnesting.

Some manufacturers print markings on the outside of the weld cable to show when it is twisted, allowing for more accurate trimming. If your gun does not have these markings, make sure the cable is fully extended when inserting the new liner.

Regular maintenance: Tight bends in the cable increase friction between the liner and the welding wire. This friction makes it more difficult to push the wire through the liner, causing wear and metal fragments to accumulate inside the liner. Eventually these tiny particles can build up and cause serious wire feed blockages. This occurs to a lesser degree even with cables that are not bent tightly, so it’s a good idea to periodically clear out the liner with compressed air.

In addition to guiding the wire, the back end of the liner contains o-rings and seals to ensure the shielding gas doesn't escape the gun cable.

Troubleshooting
Poor or erratic wire feeding, a loss of amperage or frequent contact tip burnback are all signs of liner problems. Unfortunately, because of the time it takes to replace the liner, this is often one of the last components checked during a troubleshooting effort.

Poor wire feeding: Erratic or poor wire feeding can result when the liner becomes worn out in certain spots or excessive debris builds up in the liner. Excessive debris can usually be cleared out by removing the wire and forcing compressed air through the liner (without removing it from the cable).

In most cases, a worn out liner will need to be replaced. Changing the liner is a time consuming process and requires disconnecting the gun from the wire feeder, disassembling the power pin, replacing the old liner and trimming the new one to fit the gun.

The portion of the liner that passes through the neck tube is usually the most curved part of the liner and the first area to wear out. For this reason, some MIG gun manufacturers make replacement neck liners that allow operators to replace only the part of the liner that travels the length of the neck tube. These neck liners can reduce operator downtime by roughly 15 minutes every time this part of the liner needs replacing. Just as importantly, these neck liners cost roughly one-third the price of the full liner, saving companies significant equipment costs.

Birdnesting: If the welding wire’s path of travel through a MIG gun is blocked while the wire feeder is pushing it, a tangled mess of wire called a birdnest could be the result. A birdnest can be caused by a liner that is trimmed too short, clogged, or the wrong size (too small or large for the wire diameter). The above section on proper liner replacement describes how birdnesting can result from trimming a liner too short.

Electrical short: The liner is not designed to carry welding current, so a dramatic loss of current at the arc will result if loose cable connections or a degraded cable make it the path of least electrical resistance. One common sign that this has occurred is a discolored liner and excessive heat build up along the power cable. The discoloration is caused by heat and indicates weld current is being carried through the liner instead of the gun’s power cable. A degraded cable will require repair or replacement.

The liner is not visible during normal welding operations, but that doesn’t mean it can be overlooked as a significant factor in weld quality and productivity. Thankfully, performing some very basic maintenance and being able to troubleshoot a malfunctioning liner will help you maintain consistent weld quality and optimum productivity.