More about MIG welding

WWW.THEFABRICATOR.COM JANUARY 2005

January 11, 2005

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Editor's Note: This article is a companion piece to Marty Rice's article MIG welding—The basics and then some.

My first time using MIG (also called gas metal arc welding, GMAW) in the field was working on four stainless steel hoppers (tanks) at an Owens Corning plant. X-rays of the hoppers' welds revealed that all the seams were defective. We had to cut out each weld and then reweld, both on the outside and inside of the tanks.

The tanks were about five stories tall and located about 10 stories high. As an apprentice ironworker, I was about to get some serious on-the-job experience.

After welding the outside seams, we had to climb into a porthole and then weld bars below us to make a ladder down into the tank. The bottom of the hoppers had been removed, so there was roughly a 10-foot-diameter hole leading straight to the ground below.

Was I scared going down inside the hoppers? No terrified would be a better word! Looking at my face, you would've seen a macho iron head doing his job, cool, calm, and collected. Inside my mind, you would've heard a whole lot of begging and promising. "Please, God, I won't go to the bar after work if you'll keep me from falling!" Then the next day: "Please God, I really won't go to the bar today; last night was just to settle my nerves!"

Choosing a More Efficient Process

Our subcontractor was going bankrupt paying big bucks for stainless steel electrodes the process required. The decision was made to switch to MIG welding, which reduces waste, such as incompletely burned electrodes, and requires little to no cleanup. Using electrodes requires chipping and brushing each bead, which can take quite some time when you're welding thousands of feet. In the field, time is money.

MIG welding also eliminates slag inclusion (slag in the weld) and reduces restarts that can cause weak points. Where the weld stops, the restart should begin at the front of the crater left in the steel, then worked back to the beginning of the bead. You should then continue in the direction of travel previously welded. If the restart doesn't tie in properly with the previous bead, weak points could occur.

Some Drawbacks

Shift changes created major problems on this job. On each of the three shifts, workers would adjust the machine settings, because no two welders use the same settings. I swear, even if the machine was perfectly set, the new shift would make changes just to say they were necessary!

Another major drawback was how big and cumbersome the MIG machines were. They were a real pain to move, heavy as a load of lead. Moving them required constant rigging. The machines also were finicky. I'd use one setting successfully one day, then perform the exact same weld the next day and have to fiddle with the machine 20 minutes before I got it set right.

Things Have Changed

That was 25 years ago, and MIG machines have come a long way since then. Now affordable, compact, reliable units plug into 110 volts and weld up a storm. I won't mention specific brands I don't really care for, because I don't want to offend anyone, especially company lawyers. I've said in previous articles that most big names in welding offer good machines. Even some inexpensive models are OK, if you don't plan on doing a lot of heavy-duty welding.

Just remember that you do get what you pay for. One of my students told me about a cheap machine he purchased that lasted about four months. (The warranty ran out after 90 days.) I believe that you will pay anywhere from $425 to $550 for a good 110-V machine. I arrived at those prices after researching different welding supply stores and supplier Internet sites. Prices vary big time, so be sure to shop around.

You will also need gas when using MIG. Again, prices vary, so shop around. I've noticed more and more stores carry bottles now, both oxyacetylene and shielding gas. The increased accessibility is excellent; it should keep prices reasonable and save you from having to travel all over the place looking for a bottle.

Bottles vary on the cubic feet they hold, and the bottle size you need depends on how much welding you'll be doing. Talk with your supplier before buying or renting a cylinder. I'd start with a midsize bottle, and then go for more or less as necessary. More than one person has regretted getting the smallest bottle, because it ran out so quickly.

Need a Cart?

You may choose to use a cart. You can purchase one or fabricate it yourself according to your exact specifications. When making your own, make sure to check out all the steel suppliers in your area. Buying steel from some of the hardware stores and building suppliers can be very expensive. Steel prices have doubled, and in some cases tripled, over the last couple of years. I haven't had to order any for my shop lately as we've had some very generous donations. I got a price list from my supplier the other day and about fell out of my chair. Again, you must shop around, grasshopper. Steel also is available on the Net, although I imagine shipping charges could be fairly high.

Pay Attention to Safety

I've seen MIG used and abused. Although it is easy to learn and use, as with any process, you should learn and follow all safety procedures. I tell my students to be sure not to mimic some of the cable TV shows, on which most safety practices are nowhere to be found. Over and over I see people risking burns and electric shock by not wearing gloves, and instead of wearing a hood, just closing their eyes before making a tack weld. That thin little onionskin of an eyelid is going to let in some ultraviolet light, which over time can damage your eyes. Why take a chance on permanent, irreversible eye damage?

Short Circuit

Among the MIG processes are axial spray transfer, globular, pulsed arc, buried arc, and short-circuit. As a structural ironworker, and head garage jack of all trades / master of none, I always used short-circuit. This process is great for structural steel and most hobby projects. You can weld in all positions— flat, horizontal, vertical, uphill, and downhill— and you can vary the process to achieve good penetration for thick steel or surface penetration for thin steel.

Besides steel, you also can use short-circuit to weld chromoly, stainless steel, and aluminum for projects such as racecar and motorcycle frames and parts. For welding aluminum, you need an adapter, a gun with an attached drive roller that pulls the wire through your lead. The adapter is necessary because aluminum is too soft to be driven by the drive rollers in the machine without bunching up.

Wire and Stick-Out Recommendations

I recommend 0.030- or 0.035-in. wire for the hobbyist. The thinner the wire diameter, the easier it is to control the weld pool. The 0.035-in. wire is fine for structural applications also, and usually is what I used in the field. You can purchase wire in 2-, 5-, 10-, and on up to 44-lb. or larger spools. The less you buy, the more expensive it is. Go figure. I guess it's more of a pain to produce a small amount.

I advocate using a wire stick-out as small as possible; however, you have to be careful not to fry the tip. The tip is the contact tip that is electrically hot and transfers heat into the wire. Stick-out is how much wire is protruding from the gun nozzle tip to the surface of the base metal being welded. Tips have increased in cost from $0.20 to $1.20 over the last few years. At $1.20 a tip, you'll see me with a hacksaw, sawing off the end to reuse it, as opposed to throwing the tip away, as I did when they were 20 cents a pop.

Frying the tip occurs when you get too close to the weld pool and the wire melts into the tip, thus preventing it from feeding. Keep pressing the trigger and you will have a birdnest as the wire bunches up around the drive rollers in the machine. That's good for a cuss word or two as you try to detangle the mess!

Push or Drag?

You can push the weld pool (forehand) or drag the weld pool (backhand) when MIG welding. I teach my students to push it if at all possible. Although dragging backhand gives better penetration, pushing forehand makes it easier to see the puddle and to achieve better feathering in on the edges of the weld.

MIG also is excellent for welding downhill. Downhill welding is when you start at the top of a vertical weld joint and weld downward to the bottom. Although not a good process for structural welding, it is great for projects. Downhill welding doesn't achieve enough penetration, or leave enough filler metal deposit for most structural applications. Pipe fitters often use it for the first pass on pipe applications, but they then run additional passes over it, usually vertically uphill. However, for projects that don't require critical welds, downhill welding is excellent. It produces a really pretty, flat bead that typically needs no machining. It also puts very little heat in and around the joint— the heat affected zone, where heat can damage the base metal.

For downhill MIG, point the gun up into the weld puddle. Then advance downhill as fast as the puddle will allow while moving left and right slightly in a weave pattern. By weaving, you feather in the bead and ensure that it ties in well to both sides of the base metal. You should travel as fast as possible, and turn the machine up a good five to 10 amps, depending on your machine.

The longer the wire stick-out, the colder the weld. You actually can control your temperature when welding downhill by allowing more or less wire stick-out. Most books recommend anywhere from 1/8 in. to 3/8 in.

Gas Flow

Get in the habit of always turning on your bottles first. That way you won't start out with a garbage-looking weld because of no shielding gas. If your welding sounds like bacon frying, then you are doing well. If you don't hear that sound and the pool is bubbling, either the gas isn't on or the flowmeter isn't set right. The flowmeter is the gauge that reduces the shielding gas in the cylinder to a working pressure. It is measured in cubic feet per hour. Generally, I set the flowmeter at 20 to 25 lbs. If you set the flow too low, it won't shield the weld pool properly. If you set too high, it actually interferes with the puddle, just like wind blowing into it.

Remember to learn all the theory you can about whatever welding process you are using, and follow all of the safety guidelines. This advice goes for beginners and old hands. As one of my favorite college professors, Mickey Wircenski, used to say, "We all are TAB, or temporarily able-bodied. It takes just a split second to become disabled." I know. A couple of split seconds resulted in two surgeries and several years of healing for me. Take the time to do it right and do it safely.



High School Career Center in Texas

Marty Rice

Contributing Writer
High School Career Center in Texas
Marty Rice is a welding instructor at a high school career center in Texas. He is an honorary member of the Ironworkers Local 263.

 

Questions for the author can be e-mailed to vickib@thefabricator.com

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