How to Maintain Your GMA Gun

To achieve long-lasting performance out of your gas metal arc gun, inspect its liners, tips and other essential components.

AWS Publications | February 9, 2021 | Processes
Welding Digest ►  How to Maintain Your GMA Gun

To achieve long-lasting performance out of your gas metal arc gun, inspect its liners, tips and other essential components.

When the gas metal arc welding (GMAW) arc becomes erratic, should the operator adjust the power source? Not necessarily.

If the voltage and wire feed speed values currently used previously worked well, and if the correct shielding gas and flow rates have been selected, worn components are likely at the root of the problem.

Liners, tips, and other gun components need routine inspection to ensure consistent GMAW performance. Good maintenance also prevents rework, eliminates unplanned downtime and maximizes gun life.

The following maintenance practices apply to most brands of guns. However, before working on a gun, review the operator’s manual and follow appropriate safety precautions to prevent burns or electric shock.

 

Cleaning Liners

 

Erratic GMA performance occurs when excessive friction causes the electrode to stick or slip. When wire feed speed slows, yet voltage remains constant, the electrode will melt back from the weld pool more quickly and produce an erratic arc. One of the most common culprits is a gun liner that has become clogged with wire shavings and other particulates

Operators everywhere should get in the habit of blowing out the liner with compressed air when installing a new spool of wire (Figure 1). For GMAW systems with a high arc-on time, operators might consider blowing out the liner more frequently (experience will dictate the need).

How rapidly a liner clogs varies greatly. Overtensioning the drive rolls on flux cored electrodes can clog a liner quickly. Electrodes exposed to the elements, especially humidity and salt air, can quickly clog a liner, as can a poor quality electrode. Different electrode manufacturers use different drawing compounds and substances to coat and finish the electrode. As a result, some clog liners faster than others — sometimes in half a spool or less.

To keep liners cleaner longer, consider installing cleaning pads during the next scheduled maintenance. Use good cleaning pads and secure them properly, otherwise bits of the pad may clog the liner.

For a clearer indication of particulate volume inside the gun liner, place a sheet of paper or cardboard under the gun to provide better contrast. If the arc is erratic and blowing out the liner reveals only small amounts of particulate, the liner is worn and should be replaced. For example, in the Tweco test lab, using a brand of 1⁄16-in.-diameter ER70 S-6 electrode, applying these guidelines, we most frequently replace the liner after two 60-lb spools.

Fig 2 - Use as LEAD alsoFigure 1: Blow out liners when installing a new spool of wire.

 

Replacing Liners

 

To replace a liner, remove the old liner, lay the new liner next to it on the floor, and cut the new liner to the exact same length (Figure 2). A liner that is too short can interfere with feeding performance and lead to an erratic arc. Further, if a void exists between the liner and diffuser, it could create a spot that traps the wire and leads to a bird’s nest.

When trimming the new liner, use a sharp, premium quality side cutting tool (Figure 3). To produce a clean cut, place the cup side of the cutting tool against the liner and orient the device so that it cuts against the curl of the liner. Cutting with the curl tends to produce a burr on the inside edge of the liner, where it could drag against the wire.

Never use dull cutting tools to trim a liner. Rather than cleanly cut through the tough piano wire used for liners, they will most likely deform the liner and/or spread out the coils. Never use a cutting disc as it can leave a sharp edge that drags against the electrode and creates shavings. Should the cut liner have a burr, use a hand file to remove it. Otherwise, discard it and try again.

As with any mechanical system, don’t use excessive force when installing a new liner. If the liner hangs up, twist it in a counterclockwise direction so as not to uncoil the liner. If the liner doesn’t fit, the liner could be too long (in which case, trim it) or it could be bent. If the liner is bent, discard it, as a bent liner can promote erratic feeding.

Some gun models use small screws to keep the liner from twisting. Do not remove or discard those screws. First, if the liner twists independently from the gun cable, it can increase or decrease the length of liner relative to the length of the gun cable. Second, an absent screw may permit shielding gas to escape through the hole instead of the diffuser, which may lead to insufficient gas coverage or excessive gas flow to compensate for lost gas.

Fig 3-1Figure 2: To simplify liner stocking, companies running several models of guns can use a “universal conduit liner.” These two-piece liners feature a liner crimped with a threaded wire guide and adapters to make it compatible with most guns.

 

Fig 4-1Figure 3: Trim liners using a sharp side cutting tool. To avoid creating a burr, cut against the curl of the liner using the cup side of the device.

 

Spatter and Tips, Nozzles and Diffusers

 

The GMA process, especially short circuiting transfer, inherently produces spatter. A percentage of operators feel compelled to knock spatter off by banging the gun on the welding table or workpiece. Banging the gun doesn’t do much to remove spatter, but it’s a great way to loosen component connections at the front of the gun. Don’t bang the gun.

Use whelpers, a nozzle reamer, or even an old pocketknife to remove spatter from the nozzle and contact tip. If spatter has adhered to the contact tip to the point where it can’t be easily removed, discard the tip rather than deform the orifice trying to remove spatter.

Note that the spatter from a well-tuned arc is finer and easier to remove (and antispatter dip or spray will also help) (Figure 4). Large spatter balls may indicate the need to further fine tune parameters, or perhaps your base metal has excessive mill scale or oil.

Spatter also finds its way into gas diffuser ports. Inspect the diffuser for spatter, remove the spatter, or replace the diffuser if necessary. Caution: Do not remove the nozzle and lay a live gun on the welding table. The gas diffuser is electrically hot. If the trigger is accidentally activated, electricity will take the path of least resistance through the diffuser, causing damage. In addition, note that the diffuser threads are prone to damage if a gun is left sitting around with the diffuser threads exposed.

Fig 5-1Figure 4: Antispatter compound can minimize the effects of spatter and extend consumable life.

 

Contact Tips

 

Contact tips should be replaced when the operator can observe wear inside them. Drop-in style contact tips can be rotated 90 deg to extend service life. Threaded contact tips should be wrench-tight to ensure good conductivity.

When welding at higher amperages and holding a tight arc, the electrode becomes preheated almost to the contact tip. There, the tip acts like a kiln; the electrode drawing compound, shavings, and particulates fuse into a sludge. As sludge builds, the tip narrows inside, eventually causing the electrode to bind, creating an erratic arc or even a meltback. In such situations, change the tip more frequently.

Note that the elemental composition and grain structure of a metal directly affects its electrical and thermal conductivity, as will a parts’ design and machining tolerances. As such, use premium quality contact tips to help ensure consistent GMA performance — (Figure 5).Fig 6-1Figure 5: When storing contact tips, keep them in a plastic bag or plastic bin. Storing them in anything metal can quickly lead to galvanic corrosion and ruin them.

 

Insulator, Diffuser and Conductor Tube

 

As insulators repeatedly heat and cool over time, they can lose their volume, and the connection between them and the gas diffuser and/or conductor tube becomes wobbly (and using the gun as a chipping hammer greatly exacerbates the problem). Worn insulators should be replaced, as loose connections may lead to gas leaks, poor conductivity, or excessive electrical resistance, all of which will manifest themselves as arc instability or porosity. Some diffusers and conductor tubes have O-rings, which should be inspected and replaced if worn. Inspect all threaded connections for wear or damage and replace if worn.

 

Gun Handle and Trigger

 

These components can take an incredible amount of use and abuse and rarely need maintenance, other than checking to ensure the screws in the gun handle are secure (Figure 6). Triggers do sometimes become magnetized, attract metallic dust, and cause contactors to close prematurely or the trigger to stick. In this case, replace the trigger.

There is a contingent of old-school welders who bend the contactors inside the trigger to reduce the distance the trigger travels before it engages the welding machine. They prefer this “hair trigger” to reduce hand fatigue. Gun manufacturers cannot endorse such unauthorized modifications. A better solution, and one which doesn’t involve altering the gun, would be to use a wire feeder with trigger hold control or a power source with 4T capabilities for GMAW.

Fig 7Figure 6: Periodically check the screws on gun handles to ensure that they remain tight.

 

Cables

 

Inspect the gun and work clamp cables daily to ensure the cable jacket is free from cuts that could expose copper wiring and lead to electrical accidents. As displayed in (Figure 7), replace — never tape or otherwise repair — any cut cable. Otherwise, the risk from shock is too high.

When done welding, don’t drape the gun over the edge of the welding table, power source, or in a position where it places excessive stress on the area just past the gun handle. In shops that get cold overnight, the once warm gun cable may have cooled in a kinked position that, at a minimum, can be a pain to use until it warms up again.

Also, while guns can weld with the cable tied into a knot, why place excessive stress on the system or risk binding the wire and/or a bird’s nest? Keep the gun cable relatively straight when welding, and coil and hang it properly at the end of the day.

Fig 8Figure 7: Never tape a cut cable, as that is insufficient protection against shock. Always replace damaged cables with new ones.

 

Feeder Connection

 

Periodically inspect the O-rings on the feeder connection; replace worn O-rings to prevent shielding gas leaks. Avoid dropping the back end of the gun, as the “power pin” on some models is especially susceptible to deformations that hinder wire feed performance.

When reconnecting the gun to the wire feed system, push it all the way in and lock it in place with the set screw. Check this connection every week or so, as loose connections lead to a voltage drop that especially impacts advanced processes, such as modified/enhanced short circuiting transfer and pulsed GMAW.

Gun maintenance procedures follow common sense guidelines to ensure consistent GMAW performance and maximum uptime. Maintaining components that cost just a few dollars and a couple of minutes to replace is a much better choice than grinding out a bad arc start or unplanned downtime in the middle of a shift.

 

This article was written by Ross Fleischmann for the American Welding Society.