Thread: Gmaw Course Lesson 3

Hi all

Mig welding skills.

Lesson 3
(Apologies for the delay, but hard drive failures, illness and a uni graduation have all conspired to delay the third lesson.)

The real skill, my friends, is skill in understanding what your GMAW machine can do ,what it can’t do and how to tell the bloody difference.

Learners, of course, immediately want to learn the mechanics and techniques of laying a downhand (flat) weld bead.
However if you do not understand the very basics of what your machine is really capable of, this can not happen. Once this understanding is reached, one is able to assess any particular job and the suitability of the process to it..

Firstly, GMAW – Gas Metal Arc Welding (old name MIG Metal Inert Gas) means precisely what is says.

It is a gas shielded process .Yes some machines can run gasless, but when they run with out gas they are in fact not a GMAW machine but a FCAW – Flux Core Arc Welding machine. Sure its jargon & terminology but this is important. The processes are as different as chalk and cheese and while both are capable of the same weld in some positions and materials this is not universal for all welds .In general they are different and FCAW is not a substitute for MIG.

The two major differences between the two processes are
1. The wires- GMAW is solid –FCAW is tubular with a core of flux -in industry some FCAW wires are gas assisted-which is something which will be explained latter
2. The polarity that each process uses

I mention this as many purchasers buy a FCAW machine with the expectation that it will substitute for GMAW model. Gmaw machines are often purchased to weld up vehicle sheet metal .Fcaw or gasless is incompatible with vehicle sheet work of this gauge because

• Fcaw runs on the opposite polarity making the 2/3 rds of the heat concentrated build up in the parent metal, making burn through very likely.
• Fcaw flux is acidic and if a bit of it is caught under paintwork it will corrode through in time.

Gmaw machines are often purchased with price as a major consideration often ignoring the function that it is intended for and with little forethought the real ability (as against perceived ability ) of the said machine to fulfill the intended function.

The rub is that the high level function of any GMAW machine is majorly affected by the number of settings of the voltage switch can be set at. Other features on the more expensive rigs are nice to have but not critical

Take the sip type machines sporting two double rocker switches marked high and low and 1 &2.The reality there is when the rubber hits the road, the are only four places on the volt/amp graph where the machine can be expected to run correctly. A good welding bead can only expected of course if all the other parameters are correct.

LET’S CHECK THOSE FACTORS OUT
Amperage /feed speed set to match burn off rate provided by the volt switch setting
Correct rollers size for wire
Correct roller profile to suit the type of wire
Roller correctly tensioned
Wire feed spool tensioned correctly
Correct set of in/out feeds to the roller drive
The correct diameter and type of the wire feed liner
The correct fitting of the wire feed liner.
Correctly positioning the liner (gun cable – keeping it as near straight as possible during operation)


While above features on a machine are usually present the quality is still dependent on who manufactured the machine and its initial price. Any of the above if made cheaply or not able to be adjusted properly can cause headaches.

Wire feed drives are a case in point. Wire drive feed frames, used to be made from cast alloy, but it seems plastics, even in the better makes of machines, have taken over.
The set of feeds in and out of those drives seems to be getting crappier too- hence poor wire feed and birds nesting of wire.
Some wire spool tensioning devices do not do the job very well at all.

And liners, well! There’s the test in itself. With some of the cheapies you can’t buy a liner. Get a bad kink in a liner and see how the bloody things mis feed. A kink in the liner is easily achieved by dropping a heavy lump of metal across the gun cable or by wheeling a hard tired oxy set. across it.




Having a greater number of voltage adjustments is essential for higher quality work.

Further to this variation in arc voltage is also affected by the degree of stick out which affects
• Better control on thin metals
• Out-of-position welding
• Deep penetration for welding thick section


Anyways lets open this up for discussion


Grahame