Subject: MIG Welding thin Mild Steel

From: [email-masked] (wayne weedon)
Date: Mon, 16 Sep 96 21:19:50 GMT

I have owned a cheap MIG welder for some years.  A Seely 120 XT, which
is the same as many other machines available on the DIY market under
various other names.  It's fan cooled and nominally rated at 120 Amps.

Anyway to my problem I have enormous difficulty welding thin sheet
steel, like car bodywork.  Blowing holes though on butt welds very
easily!  I use an Argon/Co2 mix formalated for this application but
cannot seem to get the thing to weld properly (Or should i say myself!)

I can run a weld though the middle of the same gauge sheet without
blowing though, with a small bead appearing on the opposite side of the
material.

My question is are there any secrets to doing this on these small MIG
welders?  Or should I invest in a more expensive machine?

-- 
wayne weedon

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From: [email-masked] (Jens)
Date: Mon, 16 Sep 1996 19:58:05 -0700

>welders?  Or should I invest in a more expensive machine?

Are you currently on your lowest heat setting ? 
When welding in the middle, there is plenty of heat sinking around
that keeps the metal from melting through (and even if the metal
melts, there is material on all sides to support the molten section).
You need to play a bit with you heat and possibly voltage. A lot of
the smaller Mig welders don't have fine controls (some have detented
steps etc). If this is so, you need to play with your timing and sort
of spot weld. DO a small section and let it cool. Do another section
somewhere else to let the first section cool some more and then come
back to it. This is also a good thing to do when you have a fully
adjustable mig.
I presume you have played with the wire that you are running? A fat
wire will always do you in on thin stuff.
I am able to weld 22 ga but even with that I choose to braze a lot of
time 'cause it's easier. It's very difficult not to blow through - you
need lot's of patience and the ability to hold your cool when you do
go through (otherwise the patchup attempt will make things worse).
I myself can not generate a nice bead on 22 ga with 0.023 wire -
things just get too hot.
The other trick I know of (although I haven't really tried it) is to
back the weld area up with something that helps to keep things cool
and also helps to support the molten section. Off hand I would say a
heavy copper bar but that is a guess. I think it also helps if there
is a small depression in the copper where the bead will be run (gives
more substance to the joint.
Even with all that, watch the distortion ... again, doing smaller
sections helps a lot.

Hope this helps - no guarantees though
Jens

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From: Robb Simmons <[email-masked]>
Date: Sun, 22 Sep 1996 23:00:24 GMT

>>welders?  Or should I invest in a more expensive machine?

As a structural welder working on heavy steel using FCAW (flux core arc
welding)(which is similar to MIG (GMAW) I experience the same blow through.

I will agree with Jens to use a lower amperage setting. If you are welding
vertical the heat will travel through the metal and cause the metal to blow
out. A method to prevent this is called back-step welding (ie. start 1 in.
>from  the top, weld your way up, go back to the start of the weld, go down
one in. below that and weld up. continuing this from the the top to bottom,
welding upward. 

As for a backing, copper is good, we use a ceramic backing strip that has a
depression in it for welding up gaps of .25" to 3.5 in. Check with your
local welding supply for availability of ceramic backing. 

The best method gor welding sheet metal is TIG (GTAW) welding, it has more
control than MIG, because you add the filler metal as you go, and has better
amperage control.

Good luck!

Robb Simmons
[email-masked]