What is a MIG Welder? A Guide for Extraordinary People

By Bill Byers / December 8, 2018
What is A MIG Welder

Extraordinary people like you, me, the curious, the handyman, DIY'er or hobbyist. The people who want to mend and create - we are extraordinary.

I created this using plain simple terms to explain to you what is a MIG welder.

​What is a MIG Welder?

Hobart 140 and Accessories

​Hobart 140 and Accessories

A MIG welder is essentially a semi automatic wire feeding welding machine.

Wow! now that sounds like a mouth full. But stick with me a few moments more ...

Inside the welder is a motor that feeds the welding wire out at a speed you can set and adjust, through dials and switches on the front.

On a MIG welder, shielding gas is also fed to protect the weld.

The welding wire acts an electrode and is electrically positive in MIG welding. The wire becomes electrically 'hot' when the trigger on the MIG gun is pulled.

In a MIG welder the welding wire is a consumable electrode, this means that the weld uses it up as the weld is formed.

The welding wire conducts electricity to the joint to be welded by forming an electrical arc. The arc melts the base metal (the pieces to be welded) and your welding wire. The welding wire is consumed into the joint, also acting as filler metal.

Forming a pool of molten liquid called the weld pool and when that pool solidifies the weld is formed.

A MIG Welded Joint

A MIG Welded Joint

MIG welding is often the most recommended method of welding because it is easy to learn and accessible for those not in industry.

​There are Different Types of Welders Aren't There?

​Yes, there are several types of arc welders - welding machines - that are powered by electricity.  And that electrical energy produces an arc that is used to weld, melt and fuse joints together.

For example there is TIG welding, Stick welding, and flux core welding.

Flux core welding is sometimes also referred to as MIG welding but strictly speaking it isn't and as I explain what a MIG welder is you'll see why.

​First Let Me Define the Term MIG

​The term MIG stands for Metal Inert Gas.

The metal part of the name is pretty self explanatory. It signifies a form of welding that joins two or more metal objects by melting them together. 

When arc welding was first explored weld problems were experienced due to gasses in the air that we breathe. These gasses reacted with the molten pool of metal causing the welds produced to weaken and fail.

This issue was sorted out with the use of Inert gas. Inert gas is call inert because of its non reactive properties.

Inert gas was perfect for covering and protecting the weld pool from the surrounding air.

MIG welders use Inert gas to protect the weld. e.g. Argon and Helium.

And there you have it. That is how the name MIG welding was born.

​What About the Term GMAW?


You will hear the terms MIG and GMAW used interchangeably.

GMAW stands for Gas Metal Arc Welding. This term arrived when a further discovery was made that Carbon Dioxide gas was also effective in protecting the weld pool from the air.

Carbon Dioxide is not an Inert gas at all but many people by then were used to the term MIG and didn't want to change and use another name.

And now both terms are used. Although Gas Metal Arc Welding more closely describes what is actually happening.

Shielding gas and electricity, produces an electrical arc that is stable and controlled enough to produce heat. Lots of heat. Enough heat to melt the metals to be welded together and form a weld.

The molten metal at the point of the weld is known as the weld pool or puddle.

A MIG welder is a welding machine that uses this process.

I have written an article that goes into much more details about MIG welding and GMAW processes. Do take a look.

​Types of Metals and Gauges Welded By a MIG Welder

​A MIG welder is used for joining mild steel mainly. Mild steel is also known as low carbon steel.

 A MIG welder is also used to weld  stainless steel and Aluminum.

This welding process is used where the look of the finished weld is important and the sizes/width of metal is within the range MIG welded.

The typical metal gauges welded with a MIG welder runs from 24 gauge at its thinnest up to 1/2 inch.

Of course the metals that can be welded does depend on the capabilities of the welder. The more you spend on your MIG welder the more raw power you'll have on that welder. And the thicker metals can be welded more easily.

Want to learn more about what a MIG welder is used for I have a document that explains all.

 So there you have it What is a MIG welder at a high level.

The MIG Welding Machine Itself

​Time to look at a typical MIG welder from the outside in.

This YouTube video shows an example of a MIG welder the Hobart Handler 140. This quick 2 minutes seven second video takes you through the main features of a MIG welder - The video touches on the MIG gun, ground clamp, wire feeding, polarity and the welding guide chart. Take look.

Video Credit: Hobart Welding Products

​Providing Electrical Power

​MIG welding is an electrical process and generally the more electrical power you can provide to your welder, the thicker the metals you can weld.

Assuming the welder is capable.  What does that mean?

MIG welders for home use will have at the end of its power lead a 3 pin plug, and you can plug right in.

Kind of; Let me explain.

MIG's are sold as 110v, 115v, 230v machine and more.

A 110 volt welder being at the cheaper less able end of the market and the 230 volt welder is more expensive and more capable.

A MIG welder will need a socket provided to it on a circuit that can power that welder. For example

Plugging a 230 volt welder into a 120 volt power supply isn't going to work.

As part of your decision on buying a MIG welder look at the power that is needed and the fuse for that circuit.

Some MIG welders will run on 120 volts but need a 30 Amp fuse to be able to run at its maximum power output - welding output - or you risk forever tripping your fuses.

A good ground connection in your cabling is needed to complete the electrical circuit. While you are welding dedicate the circuit to your MIG welder.

Anything else plugged into the circuit takes away power your MIG welder needs to use.

​Providing The Shielding Gas to your Weld

​Still looking at the MIG welder from the outside in, your MIG welder's cabinet will also have a place where a gas hose can be connected. You connect your shielding gas up here.

You know - the gas that shield your weld pool.

The gas hose may be 8 or more feet in length.

Once connected that gas hose has at the other end connections to a regulator.

A device that controls the rate at which the gas flows to the MIG welder.

The regulator may or may not have dials as a visual cue to the flow rate. A high pressure gas tank or cylinder is connected to the other end of the regulator.

Depending on what you are welding the shielding gas can be an Argon, Carbon Dioxide mix - C25 for example which is 75% Argon gas and 25% Carbon Dioxide gas.

Pure Argon gas.

Argon/CO2 Shieding Gas Cylinder

Argon/CO2 Sheilding Gas Cylinder and Regulator

Pure Carbon Dioxide or a gas mix called Tri-Mix (90% Helium + 7.5% Argon + 2.5% CO2) for stainless steel.

The shielding gas mix is chosen based on the metals welded and the kind of weld penetration you want.

For example Carbon Dioxide is used because it is a cheaper alternative than some of the gases I've mentioned. But more clean up will be required of your weld.

On the flip side it creates deeper welding penetration on some metals.

Typically a shielding gas flow rate of 20-25 cubic feet per hour is set on your regulator.

​The MIG Gun



​A MIG welder will have a cord (also known as a cable or hose) that feed up to the MIG gun.

The MIG gun cord can be 8 or 10 feet in length. Look for a cord length to suit you and the flexibility you want for your welding comfort.

Inside the cord is a liner through which the welding wire is fed.

The MIG gun (also known as a welding torch) is controlled by a trigger switch which when depressed triggers the welding wire to be fed at a speed set on the MIG welder.

MIG Gun Trigger

MIG Gun Trigger

​The trigger also enables the welding wire to be electrically 'live' when you are welding. This is the positive side of the electrical welding circuit.

Watch out for those cheaper MIG welders where the welding wire is always live. This can lead to unexpected arcs when resting your MIG welder against or near a metal surface.

At the end of the MIG gun is the contact tip and over that is the nozzle.

The contact tip helps direct the welding wire and aids in electrical conductivity.

When you are first feeding your welding wire up to the MIG Gun the contact tip is removed. This aids in feeding the welding wire. The contact tip is put on then the nozzle is put on and over the top.

The nozzle has an important function on a MIG welder as it directs your shielding gas over the wire to properly shield your weld pool from the surrounding air.

MIG Gun Nozzle

MIG Gun Nozzle

​Completing Your MIG Welding Electrical Circuit

I've covered the power input, now to look at the other side of the electrical circuit.

​Your MIG Welder's Work Clamp

The work clamp, usually made of electricity conducting metal, is clipped directly onto your metal welding project. Or some folks like to clip their clamp onto the metal surface they are welding on. For example, their welding table.

A cord connects your ground clamp to your MIG welder and through your MIG welder takes the electrical current back to earth.

A good solid contact is needed or your electrical circuit will fail or an unstable arc produced.

Lotos MIG 140 Ground Clamp and Regulator

Lotos MIG 140 Ground Clamp and Regulator

Look for a MIG welder with a quality ground clamp, made of good electrical conducting material. If your MIG welder has a poor ground clamp change it out. Put one on your MIG welder that is better.

Some folks like to change out their metal ground clamp for a magnetic one.

​If you are not sure what a magnetic clamp is then take a look at my document all about it here.

​Whatever the type of clamp you use, make sure your ground clamp has a good connection.  And that it isn't affected by paint or other surface treatments on your project piece that could cause a poor electrical connection.

If you're unable to connect it to the project piece itself, connect your ground clamp as close as you can to the project your welding. The further away your ground clamp is the more likely something will reduce electrical conductivity.

And lastly look for a cord length on your ground clamp that fits in with the distance away from your MIG welder you need to comfortably weld.

Looking Inside a MIG Welder

​1. Is A Wire Feed Welder

Everlast 140 Wire Drive Assembly

Everlast 140 Wire ​Feed

​A MIG welder is also called a wire feed welder because inside the welder is a spool of wire.

In a MIG welder the spool of wire is solid wire because it is formed of solid metal. The wire is very important as it acts as the electrode forming the positive end of your electrical circuit.

​2. Choosing Your Spool of Wire For your MIG Welding Machine

​Look for quality wire, that's wound well to save yourself a lot of frustration with wire that doesn't feed through your welder properly or produces poor welds.

Spools of welding wire are chosen to match the metal you are welding, the welding positions you plan to use and how many passes you plan to make.


Let me explain.

​3. Match the Metal You Are Welding With Your MIG Welder

Mild steel wire is used for welding mild steel.

Lincoln Electric Solid MIG Wire for Mild Steel

Lincoln Electric Solid MIG Wire for Mild Steel

​Stainless Steel is chosen for stainless steel.

Aluminum wire for welding Aluminum. There are two types of Aluminum welding wire so be sure to choose the right wire to match your project.

​4. Size of Wire to Use In Your MIG Welder

​Solid welding wire comes in various diameters 0.023, 0.030, 0.035 0.045 are some examples.

Check what wire diameters your MIG welder uses. Your MIG welder should have inside the cabinet or in its manual a welding parameters chart.

It will suggest the size of wire to use for the gauge of metal you're welding. Generally thinner wire is used for slimmer gauges of metal, thicker wires for wider gauges.

​5. MIG Wire Welding Positions

​Welding wires are certified for certain welding positions - flat, horizontal, vertical and overhead. Flat being the easiest position to weld in and all wires will support that.

Check out this YouTube video made by the Kansas City Community College it is 3 minutes 39 seconds long. The welding instructor at the college takes you through the different welding positions.

Learn Welding Positions and Joints

Video Credit: Crypto Freak

Should you be welding overhead or uphill you will need welding wire with certain properties, so look for welding wires that support this.

​6. Multiple Pass Wire

​Welding thicker gauges with a lower powered welder may need you to prepare the weld joint and take multiple weld passes to 'fill up' the joint. Look for welding wire that supports multiple passes if you're doing this or risk a failed weld joint.

​7. MIG Welder Wire Feeding

​Install your chosen spool of wire is in your MIG welder. Take care as spools are wound under pressure and keep hold of the end to stop it springing off the spool.

In a MIG welder the wire feeds out of the spool and into the drive assembly.

There is knob that is used to adjust the pressure on the wire.

Lincoln 140 Drive Roll System

Lincoln 140 Drive Roll System

​The wire feeds through the drive assembly onto a drive roll with grooves on it. The wire needs to sit in the right sized groove or you will have feed problems. On pressing the welding gun trigger the wire is fed up through a liner in the MIG gun cable up to the MIG gun where it appears.

​8. MIG Welder Polarity

​Making sure your MIG welder is set to the correct polarity is important when MIG welding. Especially as many MIG welders can also weld flux core.

Flux core wire welding demands opposite settings.

The electrical characteristics of successfully welding with shielding gas and solid welding wire mean that your welder has to be set DCEP (Direct Current Electrode Positive).

Example of MIG Polarity Terminals

Example of MIG Polarity Terminals

There will be cable connections inside your welder (check the manual to find out exactly where) where you can ensure your welding wire is set to be electrically positive.

​8. Your MIG Welding Machine's Fan

​Welders for the home, or garage workshop are normally fan cooled. There are MIG welders that are water cooled but these welders are toward the high end cost wise.

A MIG welder will have ventilation holes both front and back and you'll be advised in its manual on the minimum distance to keep away from other objects so that your welder stays well cooled.

Leave your MIG welder connected to external power to allow the fan to turn and cool your welder during its resting period.

9. Duty Cycle of Your MIG Welder

Each MIG welder will have stated its Duty Cycle. The duty cycle tells you how long the MIG welder can weld for before the welder overheats and needs to be rested to allow the internal fan to cool down the MIG welder.

The duty cycle is measured over a ten minute period and is expressed in a percentage at a certain amperage.

e.g. 20% at 88 Amps means that the welder can weld for 2 minutes at an 88 Amp setting and then will need to be rested to cool down.

Now we have explored the insides of a typical MIG welder and the features that make it a MIG welder let's look at the controls.

​MIG Welding Controls

​MIG welding can be automatic. As in industrial welding robots where the welding is controlled from a central station but the actual welding takes place without human hands.

Or semi automatic MIG welding, which is the welding more commonly seen around the home, yard, garage or done by and in fabrication workshops.

Semi automatic because the welding settings are set on the MIG welder. The human welder controls the MIG gun itself, the speed of welding, the direction of weld and the distance the electrode is held from the weld.

All are in the hands of the person welding.

​Output Voltage or Amperage

​The amount of output voltage or Amp (essentially heat) available to you is defined by the type of welder you have. A 110 volt, 115 volt or 120 volt welder is only capable of a certain output voltage - perhaps a maximum of 140 Amp.

Maximum output is also a function of the power supply you have available to connect your welder to.

Once you step up in capability and cost to a 230 volt welder then you have a greater range of output Amps available to use.

The output voltage is set on the welder itself, using dials - which can be tapped or a continuous dial.

Hobart Handler 140 Front Controls

Hobart Handler 140 Front Controls - A five position tapped voltage dial

​A tapped dial can only be set in certain positions for particular levels of welding outputs. A continuous dial can be set anywhere within the minimum and maximum setting of the MIG welder.

The level of welding Amps used needs to be selected for the metal gauge to be welded.

As you go up the price range you get to MIG welders that have digital LED dials for setting the amperage.

​Wire Feed Speed

​Is also set on the MIG welder, normally with a continuous dial. The minimum and maximum wire feed speeds available depends on the capabilities of the welder.

The wire feed speed may also be shown as an LED display on some MIG welders.

​Safety Features on MIG Welders

​I have touched lightly on some of them already, but it is well worth doing a round up.

1. Some MIG welders have an overload cut off to prevent burning out the motor on your drive roll should there be a wire feeding problem.

2. Most MIG welder have Overheating / Over Temperature protection should the duty cycle be exceeded and your MIG welder starts over heating.

The internal fan then runs to cool down the MIG welder

3. The MIG gun trigger staying electrically 'cold' until switched to prevent unexpected arc. Reducing the risk of Arc Eye for the person welding.

​MIG Welder Options

​For welding Aluminum on a MIG welder an extra tool is needed called a spool gun. Aluminum is a soft metal, bends easily and feeding Aluminum wire from a spool inside your MIG welder up 10 foot of MIG gun cord is going to cause you a world of frustration.

Hence something called a spool gun.

Spool Gun For Aluminum Welding

Spool Gun For Aluminum Welding

​The spool gun replaces your normal MIG welder gun.

The Aluminum wire is held inside the spool gun, hence its name. And the wire is fed a short distance 6 or so inches to the contact tip and nozzle on the spool gun.

​Preparing for Welding

​1. Welding Wire Stickout

It is worth checking what the welding wire you are using recommends on the length of welding wire to be left at the end of the contact tip on the MIG gun.

This is called the wire Stickout.

Welding Wire Stickout

Welding Wire Stickout

​The welding wire needs to "stick out" just enough distance from the end of your contact tip.  About 3/8th of an inch.

2. MIG Welding Guide Chart

It's important for a good weld and is called the stick out for a good reason.

Be guided by the welding guide chart - usually on the inside cabinet of your MIG welder or in the operating manual.

Hobart Cabinet Welding Guide Chart

Hobart Cabinet Welding Guide Chart

​The guide chart will advise on the welding wire size, output voltage setting, for the gauge of metal your welding and the shielding gas to be used for the type of metal you are welding.

​3. Preparing the Metal for your MIG Welding Machine

​A MIG welder is particularly sensitive to 'dirty metal'. The process needs good clean metal surfaces for an effective weld to be made.

​Your MIG welder will leave you with a good looking weld with the right settings, however preparation is the key.

Clean off any Mill Scale, rust, paint, oil, galvanization with a grinder fitted with the right kind of disc for the gauge and type of metal you're using.

A flap disc on your grinder is kinder on thin metal. Use for getting rid of mild mill scale, rust, dirt or other impurities on your metal.

​Or use a wire brush if you are dealing with light dirt on your metal. Remove any contamination till you get down to bright clean metal before you attempt to use your MIG welder.

​MIG Welder Manufacturers

​When choosing a MIG welder you'll find it is best to go to one of the top manufacturers such as Hobart, Lincoln Electric, Miller, Eastwood as these companies have been in the welding industry for tens of years.

Their welders have stood the test of time and are well proven.

They can be there to service your welder for you and you know there are spares available should you need them.

And you easily get consumables such as contact tips, liners and nozzles.

Should you be looking for more detailed help on making a choice on the right MIG welder do check out my article "How to Choose a MIG Welder - the Secret Sauce"

​Last Words

​Thank you for reading my document 'What is a MIG welder? A Guide for Extraordinary People' I hoped you enjoyed my explanation.