The Shed – September-October 2019

Polarity

A common fault is not checking the

polarity. This leads to complaints

about the running characteristics

of the machine. Gas-shielded solid

wires, including steel, aluminium,

stainless steel, and silicon bronze,

run when the machine is set to

direct-current electrode positive

(DCEP) — or negative earth and

positive electrode.

Gasless flux-cored wires run the

opposite way: positive earth and

negative electrode.

bad earth, not to mention old wire or
wrong gas. Most of these problems are
HDVLO\À[HG

Wire
The MIG wire used with shielding
gas supplied from a gas cylinder
is hard wire, a simple, continuous
solid wire consumable. It is copper-
coated to reduce surface rust, increase
electrical conductivity, and aid the
feeding process. The smooth copper
coating also helps increase the life
of the welding contact tip. Solid wire
provides deep penetration in the root of
the weld and usually has little spatter.
It is recognized as one of the cleaner,
simpler processes for manufacturing
with and welding thinner-type steels,
such as in panel beating.

Rollers
The most standard set-up in small MIGs
for hard wire is a smooth V-groove
roller in the wire feeder.
Gasless wire uses a different roller.
When and if you do switch back to
hard-wire MIG after gasless, be sure to
change your wire feeder to the smooth

V-groove drive rolls. The knurled drive

UROOVXVHGIRUKROORZÁX[FRUHGJDVOHVV

wire) can dig into the copper coating on

VROLGZLUHVDQGFDXVHÀOLQJVWREXLOGXS

inside the rollers and your liner.

Gas shielding

When choosing a shielding gas you

basically have the choice of two: argon-

based gases and straight CO 2. The main

function of a shielding gas is to protect

the molten weld pool from the effects

of the surrounding atmosphere. The

R[\JHQDQGQLWURJHQLQWKHDLUZLOO

cause porosity, or brittleness, in the

weld metal and moisture can also be

a problem. Argoshield-type gases for

general-purpose MIG work will have

DSSUR[LPDWHO\  SHU FHQW &2 2 and

WZR SHU FHQW R[\JHQ 7KLV PL[WXUH

ZLOO KDYH DQ LQÁXHQFH RQ WKH VL]H

and force of the molten wire droplets

while providing a stable arc. The small

DPRXQWRIR[\JHQKHOSVEUHDNXSWKH

surface tension of the molten weld pool,

giving the weld a smoother, cleaner

surface appearance. A nice-shaped weld

bead with very little spatter is an added

ERQXVZKHQXVLQJWKLVJDVPL[WXUHf

Contact tip

Tip holder.

Make sure

you have a

few spare

MIG tips

Check the polarity:

with gas shield, the

earth is negative;

with gasless, it

is positive

Gas nozzle

Contact tube

Arc

Weld metal

Gas shield

Weld pool