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How to weld (MIG)

Author: NJR Steel
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Categories: Manufacturing, DIY

This article provides basic instructions on how to weld using a metal inert gas (MIG) welder (also known as a wire feed welder). MIG welding is the amazing process of using electricity to melt and join pieces of metal. MIG welding is sometimes referred to as the “hot glue gun” of the welding world and is generally regarded as one of the easiest types of welding to learn.

Note: this article is not intended to be the definitive guide on MIG welding; it is merely intended to get you started. Welding is a skill that needs to be developed over time, so you must be willing to practise.


MIG welding was developed in the 1940s and, 70 years later, the general principle is still very much the same. MIG welding uses an arc of electricity to create a short circuit between a continuously fed anode (the wire-fed welding gun) and a cathode ( the metal being welded).

The heat produced by the short circuit, along with a non-reactive (hence inert) gas, locally melts the metals and allows them to mix together. Once the heat is removed, the metal begins to cool and solidify, and forms a new piece of fused metal.

MIG welding is useful because you can use it to weld many different types of metals: carbon steel, stainless steel, aluminium, magnesium, copper, nickel, silicon bronze, and other alloys.

The advantages of MIG welding include:

  • The ability to join a wide range of metals and thicknesses
  • All-position welding capability
  • A good weld bead
  • A minimum of weld splatter
  • It is relatively easy to learn

The disadvantages of MIG welding include:

  • MIG welding can only be used on thin to medium thick metals
  • The use of an inert gas makes this type of welding less portable than arc welding which requires no external source of shielding gas
  • It produces a somewhat sloppier and less controlled weld as compared to TIG (Tungsten Inert Gas Welding)

How the machine works

A MIG welder has a couple of different parts. If you open one up, you will be able to see something that looks like what is pictured below.

The inside of a MIG / wire feed welder
The inside of a MIG / wire feed welder
The welder

Inside the welder you will find a spool of wire and a series of rollers that push the wire out to the welding gun. There isn’t much going on inside this part of the welder, so it’s worth it to take just a minute and familiarise yourself with the different parts. If the wire feed jams up for any reason (this does happen from time to time), you will want to check this part of the machine out.

The large spool of wire should be held on with a tension nut. The nut should be tight enough to keep the spool from unraveling, but not so tight that the rollers can’t pull the wire from the spool.

If you follow the wire from the spool, you can see that it goes into a set of rollers that pull the wire off of the big roll. The MIG welding described in this article is for steel, which uses a copper colored wire.

The gas tank

Assuming you are using a shielding gas with your MIG welder, there will be a tank of gas behind the MIG. The tank is either 100% Argon or a mixture of CO2 and Argon. This gas shields the weld as it forms. Without the gas your welds will look brown, splattered and just generally not very nice. Open the main valve of the tank and make sure that there is some gas in the tank. Your gauges should be reading between 0 and 2500 PSI in the tank and the regulator should be set between 15 and 25 PSI depending on how you like to set things up and the type of welding gun you are using.

It’s a good rule of thumb to open all valves to all gas tanks in a shop only a half turn or so. Opening the valve all the way doesn’t improve your flow any more than just cracking the valve open since the tank is under so much pressure. The logic behind this is so that if someone needs to quickly shut off gas in an emergency, they don’t have to spend time cranking down a fully open valve. This might not seem like such a big deal with Argon or CO2, but when you are working with flammable gases like oxygen or acetylene, you can see why it might come in handy in the event of an emergency.

Once the wire passes through the rollers, it is sent down a set of hoses which lead to the welding gun. The hoses carry the charged electrode and the argon gas.

The welding gun

The welding gun is the business end of things. It’s where most of your attention will be directed during the welding process. The gun consists of a trigger that controls the wire feed and the flow of electricity. The wire is guided by a replaceable copper tip that is made for each specific welder. Tips vary in size to fit whatever diameter wire you happen to be welding with. Most likely, this part of the welder will already be set up for you. The outside of the tip of gun is covered by a ceramic or metal cup which protects the electrode and directs the flow of gas out the tip of the gun.

The Ground Clamp

The ground clamp is the cathode (-) in the circuit and completes the circuit between the welder, the welding gun and the project. It should either be clipped directly to the piece of metal being welded or onto a metal welding table. The clip must be making good contact with the piece being welded for it to work, so be sure to grind off any rust or paint that may be preventing it from making a connection with your work.

Step 1: Safety gear

MIG welding can be a pretty safe thing to do so long as you follow a few important safety precautions. Because of MIG welding produces lots of heat and lots of harmful light, you need to take a few steps back to protect yourself.

Safety Steps:

The light that is generated by any form of arc welding is extremely bright. It will burn your eyes and your skin just like the sun will if you don’t protect yourself. The first thing you will need is a welding mask. Auto-darkening welding masks are really helpful if you are going to do a lot of welding, and will make a great investment if you think you will be working with metal often. Manual masks require you to jerk your head, dropping the mask into position, or to use a free hand to pull the mask down. Think of protecting others from the light as well and use a welding screen if it’s available to make a border around yourself. The light has a tendency to draw onlookers who might need to be shielded from being burned too.

Wear gloves and leathers to protect yourself from molten metal splattering off of your work piece. Some people like thin gloves for welding so they can have a lot of control. In TIG welding this is especially true; however, for MIG welding, you can wear whatever gloves you feel comfortable with. The leathers will not only protect your skin from the heat produced by welding, but they will also protect your skin from the UV light produced by welding. If you are going to be doing any amount of welding more than just a minute or two long, you will want to cover up because UV burns happen fast!

If you are not going to wear leathers, at least make sure that you are wearing clothing made from cotton. Plastic fibers like polyester and rayon will melt when they come into contact with molten metal and will burn you. Cotton will get a hole in it, but at least it won’t burn and make hot metal goop.

Do not wear open toed shoes or synthetic shoes that have mesh over the top of your toes. Hot metal often falls straight down. Wear leather shoes or boots if you have them or cover your shoes in something non-flammable to prevent this.

Weld in a well-ventilated area. Welding produces hazardous fumes which you shouldn’t breathe in if you can avoid it. Wear either a mask or a respirator if you are going to be welding for a prolonged amount of time.

Important safety warning

Do not weld galvanised steel. Galvanised steel contains a zinc coating that produces carcinogenic and poisonous gas when it is burned. Exposure to it can result in heavy metal poisoning (welding shivers) - flu like symptoms that can persist for a few days, but that can also cause permanent damage. This is incredibly serious.


Molten metal can spit several feet from a weld. Grinding sparks are even worse. Any sawdust, paper or plastic bags in the area can smolder and catch fire, so keep a tidy area for welding. Your attention will be focused on welding and it can be hard to see what’s going on around you if something catches fire. Reduce the chance of that happening by clearing away all flammable objects from your weld area.

Keep a fire extinguisher beside the exit of your workshop. CO2 is the best type for welding; water extinguishers are not a good idea in a welding shop since you are standing next to a whole lot of electricity.

Step 2: Prep for your weld

Before you start welding, make sure that the welder and the metal pieces are properly set up.

The welder

Check to make sure that the valve to the shielding gas is open and that you have around 20 ft 3 /hr flowing through the regulator. The welder needs to be on, the grounding clamp attached to your welding table or to the piece of metal directly and you need to have proper wire speed and power setting dialed in (more on that later).

The Metal

While you can pretty much just take a MIG welder, squeeze the trigger and and touch it to your work piece to weld, you won’t get a great result. If you want the weld to be strong and clean, taking 5 minutes to clean your metal and grind down any edges that are being joined will really help your weld.

In the picture below, the weldor is using an angle grinder to bevel the edges of some square tube before it gets welded onto another piece of square tubing. By creating two bevels on the joining edges, it makes a little valley for the weld pool to form in. Doing this for butt welds (when two things are pushed together and joined) is a good idea.

Preparing the metal
Preparing the metal

Step 3: Laying a bead

Once your welder is set up and you have prepped your pieces of metal, it’s time to start focusing on the actual welding. If it’s your first time welding, you might want to practice just running a bead before actually welding two pieces of metal together. You can do this by taking a piece of scrap metal and making a weld in a straight line on its surface. Do this a couple of times before you start welding so that you can get a feel for the process and figure out what wire speed and power settings you will want to use.

Every welder is different, so you will have to figure these settings out yourself. Too little power and you will have a splattered weld that won’t penetrate through your work piece; too much power and you might melt right through the metal.

The basic process of laying a bead is not too difficult. You are trying to make a small zig-zag with the tip of the welder, or little concentric circles moving your way from the top of the weld downward. I like to think of it as a “sewing” motion where I use the tip of the welding gun to weave the two pieces of metal together.

First start laying beads about an inch or two long. If you make any one weld too long, your work piece will heat up in that area and could become warped or compromised. It’s best to do a little welding in one spot, move to another, and then come back to finish up what’s left in between.

What are the right settings?

If you are experiencing holes in your work piece, then your power is turned up too high and you are melting through your welds. If your welds are forming in spurts, your wire speed or power settings are too low. The gun is feeding wire out of the tip, then it’s making contact, and then melting and splattering without forming a proper weld. You’ll know when you have the settings right, because your welds will start looking nice and smooth.

You can also tell a fair amount about the quality of the weld by the way it sounds. You want to hear continuous sparking, almost like a bumble bee on steroids.

Step 4: Welding metal together

Once you’ve got your method tested out a bit on some scrap, it’s time to do the actual weld. Grind down the edges of the surfaces that are going to be welded, so that the seam where they meet makes a small “v”.

Take the welder and make a sewing motion across the top of the seam. It’s ideal to weld from the bottom of the stock up to the top, pushing the weld forward with the tip of the gun. However, that isn’t always comfortable or a good way to start learning. In the beginning it’s perfectly fine to weld in whatever direction/position that is comfortable and that works for you.

Weld the pieces together
Weld the pieces together.

Once you have finished welding, you will be left with a big bump where the filler came in. You can leave that if you like, or you can grind it flat, depending on what you are using the metal for. Once ground down, you may find a spot where the weld didn’t penetrate properly. That means you need to go back and redo the weld with more power and more wire.

If your weld isn’t on a piece of metal that will show, or if you don’t care about how the weld looks, then you are done with your weld. However, if the weld is showing or you are welding something that you want to look nice, then you will most likely want to grind down your weld and smooth it out. Slap a grinding wheel onto an angle grinder and get started grinding on the weld. The neater your weld was, the less grinding you will have to do, and after you have spent a whole day grinding, you will see why it’s worth it to keep your welds neat in the first place. If you use a ton of wire and made a mess of things, that’s okay, it just means that you might be grinding for a while. If you had a neat, simple weld, then it shouldn’t take too long to clean things up.

Grind the weld for a smooth finish.
Grind the weld for a smooth finish.

Be careful as you approach the surface of the original stock. You don’t want to grind through your nice new weld or gouge out a piece of the metal. Move the angle grinder around like you would a sander so as not to heat up, or grind away any one spot of the metal too much. If you see the metal get a blue tinge to it, you are either pushing too hard with the grinder or not moving the grinding wheel around enough. This can happen especially easily while grinding thin sheets of metal.

Grinding welds can take a while to do depending on how much you have welded. It can be a tedious process - take breaks while grinding and stay hydrated. (Grinding rooms in shops or studios tend to heat up, especially if you are wearing leathers). Wear a full face mask when grinding, a mask or respirator, and ear protection. Make sure that all your clothing is neatly tucked in and that you don’t have anything hanging down from your body that could get caught in the grinder - it spins fast and can suck you in!

Step 5: Common problems

It can take a good amount of practice to start welding reliably every time, so don’t worry if you have some problems when you first start. Some common problems are:

  • No or not enough shielding gas from the gun is surrounding the weld. You can tell when this happens because the weld will start splattering little balls of metal, and will turn nasty colors of brown and green. Turn up the pressure on the gas and see if that helps.
  • The weld is not penetrating. This is easy to tell, as your weld will be weak and won’t be fully joining your two pieces of metal.
  • The weld burns a hole right through your material. This is caused by welding with too much power. Simply turn down your voltage and it shouldn’t happen again.
  • There is too much metal in your weld pool, or the weld is globby like oatmeal. This is caused by too much wire coming out of the gun and can be fixed by slowing down your wire speed.
  • The welding gun spits and does not maintain a constant weld. This could be because the gun is too far from the weld. You want to hold the tip of the gun about 1/4” to 1/2” away from the weld.

Step 6: If wire fuses to the tip, change the tip

Sometimes if you are welding too close to your material, or if you are building up too much heat, the tip of the wire can actually weld itself onto the tip of your welding gun. This looks like a little blob of metal at the tip of your gun and you’ll know when you have this problem because the wire won’t come out of the gun anymore. Fixing this is pretty simple if you just pull on the blob with a set of pliers.

If you really scorch the tip of your gun, and fuse the hole closed with metal, then you need to turn the welder off and replace the tip. (Follow the steps below to see how it’s done).

1. The tip is fused closed.

2. Unscrew the welding shield cup.

3. Unscrew the bad welding tip.

4. Slide a new tip into place.

5. Screw the new tip on.

6. Replace the welding cup.

7. It’s now good as new.

Step 7: Replace wire feed to gun

Sometimes the wire gets kinked and won’t advance through the hose or the gun, even when the tip is clear and open. Take a look inside of your welder. Check out the spool and the rollers, as sometimes the wire can become kinked in there and needs to be re-fed through the hose and the gun before it will work again. If this is the case, follow these steps:

1. Unplug the unit.

2. Find the kink or jam in the spool.

3. Cut the wire with a set of pliers or wire cutters.

4. Take the pliers and pull out all of the wire from the hose through the tip of the gun.

5. Keep pulling, it’s long.

6. Unkink the wire and feed it back into the rollers. To do this on some machines, you have to release the tension spring holding the rollers down tight on the wires.

7. Check to make sure the wire is properly seated between the rollers.

8. Re-seat the tension bolt.

9. Turn on the machine and depress the trigger. Hold it down for a while until the wire comes out of the tip of the gun. This can take 30 seconds or so if your hoses are long.

Source: Instructables

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