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The Garage Guy’s Guide To Welding: Mastering MIG

The secret to a sound MIG weld is found by combining several mechanical techniques into one fluid welding motion. It also helps to have the right welder for the job. If you’ve never welded before or looking to improve your MIG welding skills, we’ll take a look at some hot new equipment and techniques to help you master the art on your truck.

Whether you work on your classic truck for a hobby or you’re a hardcore collector aiming for a perfect restoration, welding can be a challenging skill to master and require different equipment for heavier work. Once you’ve honed this talent, though, the world of custom fabrication and design is limited only by your own creativity.

Welding is mastered through practice and technique. Trial and error is the way we all learn, but you have to understand the basics first. At CCT, we’ve decided it was time to take up the MIG gun and show you a few tips to steer you in the right direction beginning with MIG welding. In upcoming issues, we’ll also be covering TIG, Plasma Cutting and even tell you what you’ll need to stay safe as the sparks fly.

MIG Welding – A Brief Introduction

Metal Inert Gas (MIG) welding is a fairly easy process to learn. The welding machine creates an arc between the work piece (what you want to weld) and a continuously consumable electrode (the wire in the MIG gun). The operator need only focus on directing the MIG gun at the joint and proper motion, which is what we’ll be looking at more in depth. Mild steel, stainless and even aluminum can all be welded using the MIG process.

Selecting The Right Equipment

f;">MIG welding is easier with the right machine. Your requirements are based on the level of your welding needs and what you want to do to your truck. To begin our look at MIG welding, we’ll discuss two extremes: the latest entry-level welder for welding in your home shop, and one for the more experienced welder who may want to weld thicker metal and aluminum.

Setting your weld parameters can be tricky when you are starting out. The folks over at Miller recently introduced the Millermatic 140 with Auto-Set™ to take the guess-work out of setup and operation. Auto-Set technology allows the operator to simply set the wire diameter and the material thickness and the machine is ready to weld. It automatically sets wire feed speed and voltage for optimal welding results with relatively no spatter for welding 24-gauge up to 3/16 in. mild steel. Right out of the box, this 30 -140 amp, 115V machine is designed to grow with the operator. When desired, the Auto-Set feature can be turned off and the operator can select voltage and wire feed speed in manual mode. List price for this welder is $789.

For those looking for the flexibility to MIG thicker materials (22-gauge to 1/2 in.) or aluminum, the Millermatic 251 connects to 208 V/ 230V or 230V / 460V / 575 V power and offers an optional direct connect spool gun. The 251 doesn’t have the Auto-Set feature of the 140, but has a higher amperage range (30 - 300) plus, the ability to weld aluminum is really cool. The base list price for the Millermatic 251 is $2,342. The optional spool gun is an additional cost.

The recipe for making a sound MIG weld is the same whether you are just starting out or you have years of fabrication experience: technique. Both machines will give you a great weld provided you use the right technique. MIG welding on steel is different than welding aluminum with a spool gun, not to mention the various angles that you’ll be welding.

Depending on what stage your truck is in, you’ll most likely find yourself welding in a horizontal, vertical or overhead position vs. the more desirable flat position. The angle you hold the gun at will change with the type of joint being welded (e.g., butt joint, T-joint, lap joint, or plug weld). Gun angles help shape the weld bead and determine the degree of penetration into the work piece. We’ll discuss what angles you should use for various positions.

The 115V Millermatic 140 with Auto-Set™ is a welder designed for the pro-hobby garage guy. The Auto-Set feature eliminates guesswork while setting parameters up and can be turned off if so desired. The standard 75 percent argon, 25 percent CO2 gas mix is required for Auto-Set and is great for mild steel.


For those with a preferred weld style, Auto-Set can be switched off, putting the Millermatic 140 in Manual Mode. The chart on the inside of the door is helpful for determining both wire feed speed and voltage parameters for the size of wire. Matching the voltage to the wire feed speed is key for producing a good weld. On machines that give an approximate starting volts and amps, start on the high end and fine tune from there.


Next, take the specified parameters from the chart and dial them in on the front of the welder. Set voltage first. The main objective of welding is to melt the base metal and then add filler wire. Voltage controls the amount of penetration. If you’re unsure, set the voltage high; you can always come down. Beware: you may get a great looking weld with a low voltage, but you could just pile wire on top of the weld piece rather than have good fusion. Next set the wire feed speed.


One of the more common mistakes novice welders make is having too much stickout (3/4 in. or more). Generally maintain a stickout of ¼ - 3/8 in. A short stickout helps ensure a good start. Here’s a tip: Hold your snips at a 45 degree angle from the edge of the nozzle, cut the wire and you’ll automatically get about a 3/8 in. stickout.


To setup the Millermatic 140 using Auto-Set, simply select the wire size on the wire feed knob and the blue Auto-Set light will illuminate.


Next, specify what material thickness you plan to weld. The welder comes with the material thickness gauge seen here. Once you know how thick the material is, select the size on the voltage knob under the setting for material thickness. You’re ready to weld!


This 10-degree gun angle, as seen from the welder’s perspective, is the proper setup for a horizontal lap weld with a push technique. Pushing usually produces lower penetration and a wider, flatter bead because the arc force is directed away from the weld puddle. Not only is this technique optimal for viewing the weld joint, it also ensures good gas coverage and is great for welding on thin materials to avoid burn through.


Use both hands to steady the gun in a comfortable position. Maintain the 10-degree gun angle and start welding with a small, back and forth motion. The rate of your travel speed influences the shape and quality of a weld bead to a significant degree. Keep the arc on the leading edge of the puddle; don’t let the molten metal get ahead of you. Also, watch the toes (edges) of the weld to ensure you’re getting good penetration.


This is what the completed weld should look like. Note the discoloration of the base material, this is known as the Heat-Affected Zone (HAZ). Excessive heat or too slow of a travel speed can cause this area to enlarge and on thin materials, heat is a bad thing that leads to warping. To help maintain the amount of heat, weld in small segments and allow for cooling between welds.


When describing the fundamentals of a good weld, there are three different gun angles to keep in mind: the direction of the travel (travel angle); the angle of the direction of travel and the angle of the gun to the joint (work angle). Think of welding as drawing a line down the center of the joint, as demonstrated here.


Here is a look at the work angle between the gun and the joint on a lap weld, which is roughly 45 degrees.


The travel angle viewed here shows the 10- to 15-degree gun angle required for the push technique.


These two welds show the difference between a good weld (below) and a poor weld (above). A common mistake is having too slow of a travel speed as shown in the poor weld. Note the increased amount of heat and excessively large weld bead. Hint: Make your weld bead no wider than the thinnest part of your base metal.
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The same welds, seen from a side angle shows warping of the thin material caused by excessive heat. If this were a larger piece of sheet like that found in body panels, the warping would not allow for a correct fit and throw off any other body measurements.

Work angles change with the type of joint being welded. Here is a traditional T-Joint fit up. Tack-weld the ends of the base material into the position and try to maintain a 10-degree travel angle while using the push technique.

Keep an eye on your travel speed and gun angles. Travel angles beyond 20 to 25 degrees can lead to more spatter, less penetration and general arc instability. This is due to poor metal transfer, poor direction of the weld metal and poor shielding gas coverage.

Another way to determine you are welding correctly is by sound. A smooth sizzling bacon sound will tell you you're doing everything correctly. Loud popping and flying batons can signal that the gun is too far away from the joint or the wire feed speed needs to be turned up.

Practice. To make a sound weld, you must learn to combine all the mechanical techniques into one fluid motion. A good weld should look like a stack of dimes, as seen here. Practice so that welding on all joints in all positions becomes second nature. Visually and structurally test your welds on pieces of scrap before putting the gun to your truck for the first time.

Here’s another joint that you’ll likely encounter on your truck. A plug weld is a technique used to fill in holes or plug-weld two pieces of thin material together. To fill a hole, start with the gun at the 10-degree travel angel and weld around the edge of the hole, making complete circles and spiraling inward until it is filled.

This is what the plug weld looks like from the back of the material. A quick hit with the grinder and it’ll look as if there was never any hole at all.

If you intend to do any chassis welding, (thicker metal) or have the desire to weld aluminum components, you’re going to need a welder that can handle the increased demands for power and process. The Millermatic 251 seen here is connected to 230V power and offers an optional direct-connect spool gun for MIG welding aluminum.

A common weld found while fabricating on trucks is the vertical down position seen here on mild steel. Vertical welding, both up and down, can be difficult. Since you are fighting gravity, consider reducing the voltage and amperage 10 to 15 percent from the settings for the same weld in the flat position.

Begin at the top and work your way down. This technique helps when welding thin metals because the arc penetrates less due to a faster travel speed. Let gravity be your friend, maintain a 45-degree work angle between the nozzle and the joint. When welding, angle the gun slightly back into the puddle at a 5 to 15 degree angle.

For thin metal where burn-through is a concern, angle the gun slightly up and pull it downward, keeping the wire on the leading edge of the weld puddle.

The Spoolmatic 30A spool gun has a built-in drive system for hassle-free aluminum welding. Because aluminum wire is softer than mild steel wire, it can not easily be pushed through a drive system and gun liner without bird nesting. A spool gun eases the delivery of aluminum wire by shortening the distance it needs to travel.

The spool gun has three connections seen here. The gas hose, control cable and the weld power connection. Remember to set the welder for reverse polarity (DCEP), which is the same for mild steel. For welding aluminum, use 100 percent argon shielding gas with a 20 to 40 cfh flow rate. Proper gas coverage is key around the weld puddle so that the arc and argon cleans off the oxide coating on the aluminum creating good fusion.

When welding aluminum, the push technique is the only technique that ensures good gas coverage of the weld puddle and provides better cleaning action.

MIG welding of aluminum progresses at a faster travel rate than for steel. Due to the high thermal conductivity and low melting point of aluminum, the travel speed will increase as the welding progresses. If travel speed is not increased, there is a chance of excessive melt-through on thin aluminum parts.

When exposed to air, aluminum forms a hard, oxide layer that completely covers the aluminum surface. The white area seen around the weld is the result of the arc and argon gas cleaning the oxide coating off the aluminum.

Always use the push technique when welding aluminum to ensure proper gas coverage as seen here on the left. The weld on the right is an example of the drag technique where there was not enough gas coverage to clean the aluminum.

This chart shows two types of weld bead patterns, the Stringer Bead and Weave Bead. The slight motion used in a weave pattern is helpful in covering an area with one pass of the electrode and when done right, the bead will resemble a stack of dimes.

Whether you weld occasionally or you pound wire like crazy, practice your welds until the technique becomes second nature. This diagram shows examples of a good weld and several bad MIG welds.

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