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MIG Welding Process -  Definition

Brand new Expert MIG Welding Page

MIG welding gets its name from Metal Inert Gas, and is also known as Gas Metal Arc Welding (GMAW) and can sometimes be called Metal Active Gas (MAG).

 For the purpose of simplicity I will just call it MIG welding.

If you are visiting my site & looking for specific information on 'how to MIG weld car bodywork', then please read all of this page first, then look for the links to the other pages that contain all the 'set up' data as regards your MIG welder set up. This will help you to set up your MIG welder from scratch, using 0.6mm or 0.8mm solid MIG wire specifically for the repair of thin gauge car body work panels.

For full information on MIG Welding Settings, know as Welding Parameters; look for the link near the bottom of this page.  If you are having any problems in selecting your correct welding volts or amps, then you will not want to miss this.

  • This is the biggest mistake that learner mig welders make.
  • The control knobs on the front of your MIG welding machine are for adjusting the welding voltage, not the amperage.
  • are your welds big & lumpy?
  • or do you have a 'line of holes'?
  • If you don't know how to set the welding volts - you will just love this! 

Further down this page you will also find a photograph album example of car body panel replacement, & I used a Landrover for clarity of the panel removal & replacement procedure. 

The detailed written captions that accompany the photos will be of a major benefit to anyone thinking of starting a simple job like this one detailed.

Please forget about using 1.0mm diameter solid MIG wire for repairing your car bodywork, as this is best suited to experienced welders, for reasons that you will shortly understand!

The basic mig welding process is easy to understand, and the weld is achieved by heating steel to its melting point using an 'eletric arc', then feeding 'filler wire' into the molten 'weld pool'.

The steel is heated by an electric arc produced between the steel workpiece and the end of the filler wire, hence the reason that MIG welding is an electric arc welding process. 

This is pretty much the same principle as stick welding with rods (MMA), but the filler material (electrode/welding rod) is delivered in a different way.

The Basic Principle...

  • The filler wire that feeds out of your MIG torch contact tip is more commonly known as mig wire, and is available in various types and sizes for different mig welding processes.
    This mig wire feeds automatically from your mig torch when you press the torch trigger.

  • The molten weld pool, and weld metal being deposited from your mig torch contact tip, is protected from the atmosphere by using what’s called a shielding gas.
    This shielding gas is normally
    Argon, which is an inert gas, meaning that its not flammable and does not support combustion, and is in fact, an
    The shielding gas flows automatically from your mig torch bezel/shroud when you press the torch trigger.

Now you know where the ‘i’ and ‘g’ in ‘mig’ comes from, it simply stands for inert gas. (& the 'm' simply stands for metal.)

Shielding Gas Specifics...


  • CO2 gas is normally mixed with small amounts of Argon (& sometimes oxygen) oxygen to give the best weld metal shielding properties and can be a complicated gas mix for some mig welding processes.

  • This shielding gas is stored under pressure, in a steel bottle of some kind, & sits on the back of your mig welding machine.

  • A pressure regulator fitted to the bottle, controls the gas flow & maintains your pre-selected delivery pressure at around 10lbs per square inch upwards, to as high as 25lbs per square inch.

  • Cheaper straight CO2 can also be used as a shielding gas, but this gives a much hotter weld and lots of weld spatter, hence the reason for an argon mix.
    Full details on this later...

  • I am regularly asked: What do I set my Shielding Gas 'Flow Rate' At?
      Read The Simple Answer Here!


How A MIG Welding Machine Works... 

  • When you press the trigger on the mig torch, the mig wire/ filler wire is mechanically propelled into the molten weld pool.

  • an electric solenoid valve opens and allows the shielding gas; from its pressurized storage bottle, through to the MIG torch bezel, & over your work piece weld pool.

  • A large electrical contactor (big switch) also closes to allow the welding current to flow from your mig welding machine transformer to your mig wire electrode contact tip sticking out from the end of your mig welding torch .

These are the three things that the small trigger switch on your mig welding torch controls, & they all happen at the same time!

This is why mig welding is considered to be a semi-automatic welding process.


With torch trigger pressed & everything connected properly, the mig wire melts off & forms your weld bead/weld metal, according to your wire feed speed settings & welding amperage setting.
You only really have two basic settings to control on a mig welding machine: welding volts (wire feed speed) & welding amps, & both are pre-selected before you start to weld, (presuming you know your weld setting parameters of course), some machines have rotary knobs or switches for pre selection only, while others have accurate digital displays to enable very precise weld parameter adjustments.
Full details later...

Diagrams and photos will also be added soon…

A MIG Welding Electrical Circuit...

The mig wire is gripped by feed rollers and pulled from its storage roll & at the same time pushed through the mig torch liner to the torch contact tip.

The consumable copper torch contact tip (easily replaced) should be suited to the diameter of mig wire being used.

The torch contact tip is also the positive electrical connection which transfers the welding current (rated in amps) from the mig welding machine to the mig wire.

This arc welding circuit is commonly known in the welding trade as DC+ positive polarity to the electrode.

The negative electrical connection in the actual welding circuit comes from the welding machine transformer & makes a direct contact with the work piece, & is normally called the earth clamp, but should really be known as the work piece return, & I will call it the 'return clamp' for the purpose of 'correctness'!


Circuit Summary...
Therefore the mig wire, technically known as the electrode, is the positive side of the welding circuit & the work piece return clamp is the negative side.

But this polarity can be different, especially in the case of some ‘self shielding mig wire’ welding processes.

You have just read the basic
MIG welding process...

& Just to really confuse you...
...there are also two very different kinds of mig welding torch...

Torch Positive...

  • as it suggests, the mig welding torch bezel/shroud (the end tube that directs the gas shield), is permanently live.
  • ...not too good if you are into auto body repair work & your welding location is amongst awkward steel panels etc!
  • & definitely not good if your gloves or hands are damp, for obvious reasons of electric shock.
  • ...200amps through your body is not funny, & fortunately, these machines are becoming rare.

Torch Negative...

  • Your modern mig welding machine should have this feature...
  • much safer to use, but the bezel can still become live when weld spatter builds up on the copper contact tip & makes an electrical circuit to the inside of the bezel...

...don't ask how I know this!


& then you have these...

Industry standard mig welding torches have lots of 'continuous welding on-time' features, such as...

  • water cooling of the torch components, right up to the contact tip.

  • automatic welding fume extraction directly from the external bezel area: brilliant systems.

  • smart switches: for rapidly changing weld parameter settings from the torch head.

Full details later... 

Diagrams and photos will be added soon…


But, this is further complicated by the methods in which the mig wire is deposited as weld metal.


  • For example: the mig welding process can be split into two distinct different types...

The MIG Welding Transfer Modes...

  • You have dip transfer( more commonly known as short circuit transfer)

  • and spray  transfer, with some slight variations in between.

If you imagine a home decorator painting your house walls, he may apply the paint using a paintbrush, roller or spray equipment.

The same type of idea applies for mig welding; your mig torch can apply the weld metal in short circuit transfer mode (which you think of as the paint brush method),  and is most suited to thin sheet steelwork like vehicle auto body panel repair.

Or full spray transfer, which is obviously the fastest mode of weld metal deposition, and best suited to steelwork which is 5mm thick and upwards.

There are some ‘in-between’ transfer modes,  which you can imagine as being the roller method, and they are commonly known as semi-dip transfer, and are faster than bottom amperage range short circuit transfer, but slower than full spray transfer.

You may also have heard the term 'globular transfer' & lots of welders weld within this 'upper short circuit amperage range', & I will devote a whole page to this later.

  • There are major technical differences here between dip & spray transfer regarding the 'weld penetration' characteristics.  

  • This is heavy reading even for experienced welders, so I will cover it in great detail later.


A quick guide to the general applications of these different modes of weld transfer are as follows...


  • Small 'home user' or DIY mig welding machines can only really use the short circuit transfer mode with 0.6mm or 0.8mm mig wire, & reasonable results can be obtained from 0.8mm at 180amps on a single phase machine.

  • however, if you are looking to MIG weld car bodywork, which is mainly made up from 0.8mm thick mild steel sheet, then you will want to read this Extremely Popular email Question & Answer.


  • Trade or small professional mig welding machines of up to about 200 amps welding output; normally run using short circuit transfer, but semi-dip transfer is easily possible with 0.8mm mig wire and spray transfer can be achieved using  0.6mm wire, but this is difficult to control. These machines can also run 1.0mm mig wire, but they are fully stretched to do this continuously.


  • Large professional and industrial machines that are rated at minimum 300 amps welding output normally have a three phase 415v input supply, and can easily handle 1.0mm mig wire using spray transfer mode. These machines also use 1.2mm and 1.6mm mig wire.

This is the size and type of machine that I use for all of my general steel fabrication work, and I run 1mm mig wire at full spray transfer most of the time or 1.2mm mig wire using semi-dip transfer.(short circuit).


 Mig Wire Specifics...

Mig wire is readily available in 4 popular diameters.

  • 0.6mm is the smallest and is suitable for vehicle autobody repairs.
  • 0.8mm is for light sheet steelwork, but too heavy for vehicle work.
  • 1mm mig wire is the most popular for general steelwork fabrication.
  • 1.2mm mig wire is best for steelwork 5mm thick and over.

I will give the full rundown on all mig wire diameters and their best uses later.


& The Different Types...

I have previously only talked about solid mig welding wire, and now it gets really interesting, as there are also these following types of mig wire:

  • Gasless mig wire, just as it says on the tin: no shielding gas needed! There are two basic types: the cheapest version looks like a very thin stick welding rod on a roll, which is really just solid mig wire with a flux coating on the outside. Truly awful to use and I cannot recommend it.
    then you've got this one...
  • Self Shielding, & again, no gas needed & similar to previous, which is quite expensive, looks like a solid mig wire but is really tubular in section, or ‘hollow’ mig wire, and contains powdered flux, and you can compare this to being similar to a regular stick welding rod inside out! but, obviously, very small in diameter. The diameter of this self shielding mig wire is 0.8mm & upwards.
    I have used this extensively over the years, and I have a dedicated mobile mig welding machine specifically for mobile on-site mig welding in windy conditions.
    I use a 1.1mm dia wire that is fully certified & approved to the highest standards & it comes on 12lb reels.
    it is classed as a cored wire electrode.
    It is driven from the wire feed unit by special drive rolls that have teeth in their grooves, and pull/push the mig wire without crushing it.
  •  However...there is also a 'cheap' version of gasless fully self shielding mig wire, and it is normally supplied as standard issue for small home user or DIY welding machines. Avoid this like the plague unless youre really desperate, as you have to be an expert at mig welding to use this in the first place!
    The precise setting up of fully self-
    shielding mig wire is critical to a good weld.
    More expert info on this later…
  • Flux Cored mig wire: basically, this is a tubular mig wire, or ‘hollow’ mig wire, with flux throughout its center, to improve the weld pool shielding characteristics. This is always used with a shielding gas, and is used throughout the welding industry. Again, this type of flux cored mig welding wire needs special wire feed unit drive rollers.


Vertical Up MIG Welding 

I receive a lot of questions on this subject.

here you will find my latest replies to
Vertical Up MIG Welding.
click here

More info on this and its flux cored mig welding process later.

You will find full technical descriptions, explanations & specific uses for all of these different situations on their own pages.

Diagrams and photos will also be added soon…

You will by now, appreciate that there are many different wire feed unit types, for different mig wires, and different welding machines for different mig welding applications.

More of this content is being added soon…

Diagrams and photos are on the way…

No Matter What You NEED to know about
Expert MIG Welding,
this is the place to find it!
Every single detail that you can ever imagine is going on this page.
Mobile or Workshop, all the processes & all the Knowledge, filler materials, shielding gases, torches, machines, expert MIG Welding Parameters, & NOT the crap that you read on the Manufacturer's websites, they just mention the basic machine operation & miss out the things that you really must know before you start!
When I read a 'user manual' that comes with a new MIG Welding machine, I often wonder how anyone can possibly weld properly when they are given a pile of complete crap to read.

for example:

  • why do these user manuals never tell you that Spray Transfer for 1.00mm solid wire starts at about 280amps?
This is why you MUST know about Dip/Short Circuit Transfer Processes.
This is not theory - this is fact. 
contents are being typed up daily....
....please call back regularly.
unfortunately, I have to go to work everyday!   please be patient!
For those of us that are about to start a car body work repair or restoration project, I have chosen a Landrover 110 A post replacement, as a good example of what's involved in general panel replacement. 
I have chosen this particular job example, purely because it was easier to photograph each stage for your clarity of understanding just exactly what is happening.
For a full visual photo display, please see my my Landrover repair photo album below :
This is my public album display for welding topics that you may be interested in.
The photo captions contain detailed explanations of what is displayed, along with some very helpful tips. 
I use Google Picasa secure web albums to display photos for your benefit.
RIGHT CLICK on the link below, & SELECT OPEN IN NEW WINDOW, this will keep this web page open so that you may browse Picasa & return here when finished.
Once inside Picasa, select any album by clicking on the cover photo, then when the album opens, you can read the Detailed Descriptions by 'hovering' your mouse over the 'thumbnail' photo views, or just select 'slideshow' from the top of the page.
When 'slideshow' starts,click on 'hide captions' then 'show captions' twice, to reveal the captions
as you look at the photos in full view. You can also change the speed at which the photos are rotated to read the longer captions.
  1. & what about the BEST gas shielding to use for the various jobs?
  2. & the correct 'stick out' for flux cored?
  3. & how's about something on why the shroud is not adjustable?
  4. & something on correctly setting the wire feeder would be handy!
  5. Troubleshooting: why is there no mention of the following:
  • what to check first when your welds are bad?
  • why is your weld penetration crap?
  • what causes weld porosity when my settings are spot on?
  • what to do when your cables start to smoke! 
  • why are there eighty different mild steel filler wires?
  • what's the best way to weld car bodywork?
  • how do you weld upside down?
  • how's about vertical up?
  • or down?

Get the answers here

& a special section for home users of
DIY MIG TIG & Stick Welding Machines will be added

You need to know about the electrical supply that you plug your welder into at home.

does 16amps ring a bell? - I didn't think so!

If you don't know this,
 your welder will never work properly.

The same goes for cheap extension cable reels,
you MUST know about voltage drops, resistance,
& supply amps.

This is VITAL

Essential Advice with Simple Solutions.

prepare to be amazed!

Do you know how to set your MIG welder amperage?
& what about the voltage?

You Must Know How To Do This
or you will never make a good weld
  • Do you struggle to set your MIG welder properly?
  • Do you have problems getting a good weld?
  • Do you really know how to set those switches?
This will shock anyone that thinks the front panel contol knobs are for the amps!
click this link for Expert MIG Welding Parameters

In The Meantime...

...No matter What Your Question
Get Your Them Answered Here!....
Just Ask!

please use your own email facility


Coming Very Soon....
....The Worlds Latest & Best Practical Guide To MIG Welding!

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