(go straight to how to set up a MIG welding machine to weld a particular joint)
Here we are under way, starting late Summer going into Autumn 2005.
I'm now learning MIG welding at two different courses, at two different colleges - and they are very different!
My impression of MIG is in the context that I have qualified in all other main manual welding processes first (oxy-acetylene, TIG, SMA) and MIG, the most commercially used process, is the last one I am coming to.
Well... Here's my impression:
With MIG as in other welding processes you can do all the different welding scenarios with one welding machine.
Comparing MIG mainly to SMA, the other widely used manual high-deposition welding process...
With MIG the welding machine can have exactly the same consumables - the "system" stays with exactly the same components - and all the range is covered by the different settings on the machine - and the machine has a plurality of independent controls amounting to huge "contour map" of a lot of process knowledge the user must have.
With SMA the welding machine has just one control setting, the current, with the user selecting from a range of fundamentally different types of welding rods, each type available in range of different sizes, to cover the range of welding requirements.
How is my impression of the process' applications so far? Well, it seems that MIG is the predominant factory and workshop process for reasons which become absolutely obvious as you start to "get" the process. Fast, efficient, capable, easily giving welds which absolutely storm the criterion for a good weld.
On the other hand, SMA is still there because it can master welding-on-site conditions which bar use of MIG. The vision of a site welder is someone carrying a welding machine by its carrying-handle in one hand, while the other hand holds the straps of a bag of cartons of welding rods, a chipping hammer, wire brush, ... And this person can go up a ladder, down a hatchway into a boat, under a bridge buttress...
Thing is, MIG can't stand wind and/or draughts. The gas shield is blown away, destroying the weld. So not much good outside... Also, MIG machines have to have a bottle of shielding gas, and overall it is unavoidable that a MIG machine will be physically a large size. For site use, a SMA welding machine is small (even very powerful machines in these days of computer-controlled power-electronics "inverter" welding sets), tough and SMA welding is quite resistant to wind, damp and dirt. However, back in the workshop, MIG machines easily make good welds at a very high productivity rate...
There are exceptions of course - but this is the majority view.
There are now MIG welding machines from which the welding scenario is selected at a console on the machine and the machine chooses the fundamental electrical and wire-feed conditions which will give a good weld.
Furthermore, with computer control, it is now possible for a MIG machine to have a third "voltage sensor" cable which clips to the workpiece, sensing the voltage drop between the torch and the workpiece. With overall computer control, this enables the operator to select their welding current (Amps) and electrical potential (Volts), with the machine doing whatever it has to do to to make this so given the complex interaction of the many variables which affect these two fundamental of MIG welding.
Most MIG machines are not of these recent types! While current and voltage remain the fundamental measures of MIG welding, their values float in complex ways depending on all variables. This list is huge - the wire-diameter, shielding gas composition, length of welding leads, resistance in the electrical contacts, ... (keep going!). All of which means that for the majority of MIG machines the controls are marked in arbitrary numbers. For instance the voltage fine control might be marked from "1" to "6" - which in no way gives any hint of what voltage you are actually ending up with between torch and workpiece when welding! Even for the wire-feed-speed, typically marked from "0" to "10", while it would be possible to make a chart of wire-feed-speed to dial-setting, if you changed the drive rollers they might give a slightly different diameter at point of grip, giving a different wire-feed-speed! So the manufacturer can't put an absolute scale on even this control!
So for most users of MIG machines, you have to be able to recognise where you are in the "contour" of conditions when you try the machine - point the torch at a piece of scrap metal and press the trigger - and know therefore what changes to make upon the current voltage-tap and wire-feed-speed settings.
Summarising the independent variables you have on "traditional" MIG machines:
On most of these welding machines, the voltage selection has at least two knobs, one for selecting parts of the overall range of the machine and the other to finely subdivide that range.
Then just to prevent complacency! No two welding machines are exactly identical, even if they are the same make and model. So you cannot absolutely guarantee that if you have a "settings book" of the arbitrary scale settings at which you got a good weld for a particular joint, if you are doing exactly the same joint on a different supposedly identical machine you will get good welding conditions by setting the same selections. You will be close, but not necessarily exactly on the conditions!
MIG offers three "domains" of transfer mode of metal from the welding wire to the joint, with effect on all other characteristics including the electrical characteristics experienced by the MIG machine.
This is in general order of heat input to the weld. You would generally look to "dip" for thin sheet metal where you need to restrict heat input, whereas "spray" would be selected to get sound fusion on thick plate sections.
Using a MIG welding machine demands a lot of "process knowledge". In particular that means having a very clear map in your mind of the very different operating modes which are achievable - the "transfer modes". Generally you need to know which of three transfer regimes - dip-transfer, globular-transfer (rarely!) and spray-transfer - you want to be in and recognise where you are now.
Two methods to set up a MIG welding machine to weld a particular joint.