Ron Covell
Owner, Covell Creative Metaworking
Let’s start with a definition: MIG stands for Metal Inert Gas welding. Typically called Gas Metal Arc Welding (GMAW), the term MIG welding is much more common, so we’ll use that here.
MIG has been the most popular welding process for decades, for both professional welders and hobbyists. It’s fast, strong, versatile and fairly easy for most people to learn. Because it’s such a great process, there’s a steady stream of newcomers determined to learn. Whether you're new to MIG welding or simply brushing up, this guide offers a solid foundation to build your skills.
MIG welding relies on a power supply that delivers constant voltage, most commonly Direct Current Electrode Positive (DCEP). This power supply uses transformers and rectifiers to modulate line voltage, which stabilizes the arc and provides good arc starts. It also includes circuitry to protect against overloading.
To complete the circuit, a work clamp connects the base material to the power supply. There is a spool of wire, usually housed inside the power supply case, along with a drive mechanism to feed the wire through the cable, toward the gun.
The hand-held gun is the “business end” of a MIG machine. The gun has a trigger that controls several functions simultaneously:
MIG welding requires shielding the weld from the atmosphere. To do this, direct shielding gas over the weld area, and control the flow of gas by the trigger on the gun. In other instances, welders might use a flux-cored wire, either alone or with a gas shield.
Setting up the machine and preparing the material is the easy part. The way you hold and move the gun is the key that governs the quality and appearance of your welds. Whenever possible, it’s best to use a two-handed grip on the gun and to support your hands, wrists, forearms or elbows in a way that allows you to move the gun smoothly, while maintaining precise control. The position of the gun and the distance from the gun to the work are crucial.
The farther you hold the gun from the work, the farther the wire has to extend to meet the metal. The length of wire between the gun and the metal—called the stickout—has a major impact on weld quality. A typical stickout is around 3/8 inch. Extending it too far reduces heat input and can compromise gas coverage.
Most MIG welding applications require shielding gas. The most common choices are carbon dioxide and argon/CO2 mixes. The gas bottle has a regulator or flowmeter to set the gas flow.
For light-duty welding, a good starting point is 20 cubic feet per hour.
As you gain experience, you can fine-tune the flow rate to suit your needs.
Before making a weld, there are two essential settings to adjust on the welder: the voltage and the wire feed speed. Pro tip: nearly every MIG welder has a chart — including my Millermatic® 211 PRO — often just inside the hinged access cover, which gives you the suggested settings. These are based on the material type and thickness, and the diameter of the filler you’re using. Use these values to adjust the settings on the face of the machine.
If you don’t want to review the chart, Miller pioneered advanced Auto-Set™ technology, which allows you to simply set the process, the material thickness and wire diameter, and the machine adjusts the settings automatically.
This has worked so well for me that I haven’t read a chart in years!
The material needs to be clean to get a good weld; remove any grease or oil before using abrasives. MIG welding is more tolerant of minor surface contaminants than TIG welding, but the cleaner the metal, the fewer problems you’ll have. I often use sanding disks or a non-woven abrasive for cleaning rust, paint or scale off the metal.
Welding technique plays a big role in weld quality. I normally push the puddle when I weld. Since I’m right-handed, that means the motion of the gun is toward my left. In most cases, you should angle the gun slightly in the direction of motion—this is called the travel angle, and 15 degrees is a good place to start.
When looking at the gun from the end of a seam, the angle of the gun to the work is called the work angle. Here are some common setups:
Butt joint: Hold the gun at 90 degrees to the workpiece.
Fillet welds: Use a 45-degree angle.
Inverted T joints: Angle the gun slightly away from the vertical element to prevent burn-through, since the vertical part can’t dissipate heat as effectively.
The speed you move the gun is very important too. Going too slowly builds up an oversized bead and going too quickly may diminish penetration. Some welders hold the gun steady as they progress, but there are a variety of techniques for weaving or oscillating the gun that may be beneficial. I encourage you to experiment with subtle changes in the way you move the gun, paying close attention to how each change affects the weld. You can learn a lot by talking to and observing other experienced welders.
Ideally, the weld bead should be slightly crowned, with the toes or edges of the bead flowing nicely into the base metal. There should be full penetration, but not so much that there is excessive bleed-through on the back of the joint. The width and height of the bead should be fairly consistent, and there should not be any craters or voids. Many people test their practice welds by holding a welded part in a vise and bending the joint until it breaks. Ideally, the metal NEXT to the weld should fracture before the weld bead does.
It takes a lot of practice to get your welds to meet all these criteria. But by following these MIG welding tips and spending time with your helmet down, your welds will only get better!
Ron Covell is a talented welder and metalworker in the automotive industry. Covell is a contributor for Hot Rod Network, where he has a popular column titled, "Professor Hammer's Metalworking Tips." He also owns Covell Creative Metalworking, offers instructional metalworking DVDs and hosts workshops around the country.