CHAPTE R 8
One Way to Reduce Heat Problems
welds ground flat, your weld line should
look like a perforated tear line.
Now, the entire seam must be closed.
This can be done by filling in the gaps
between the ground tack welds, and
grinding the resulting welds flat to the
panel, or by welding a continuous bead
over the entire seam. In either case, panel
edge position should be perfect, and heat
distortion from the whole operation will be
reduced from what it would have been
with any approach other than alternate
tack welding.
New fender-section metal has been fitted into this old fender with almost
The same approach works equally
no fit-up gap, and TIG tack welded into place. MIG welding would have
well with MIG welding, although our
required a considerable fit-up gap to handle heat expansion, and would
example lacks the apparent neatness of
have produced much more distortion than did TIG welding.
the same process performed with TIG
welding. That is because MIG welding is
Keeping welding heat at the minimum flat to the panel. If you think of each tack not as accurate, controllable, or tidy as is necessary for making good joints is
weld as a line of continuous and adjacent
TIG welding.
critical to joining sheetmetal parts and
buttons, you want to make tack welds at
The main point is that this welding
sections. This limits distortion in the areas
every second or third button position in
technique keeps the buildup of welding
of the welds. One way to accomplish this,
that line. The actual spacing depends on
heat way below what it would otherwise
while ensuring the accurate positioning of
the situation, and on your personal prefer-
be, if you attempted to weld continuous
pieces,
is
to
make
tack
welds
ence. At this point, with the alternate
beads. This is because it allows more
incrementally along a seam. This works
with MIG, TIG, and gas welding
techniques. Our example uses TIG, and
then MIG, welding processes.
You start by making enough tack
welds to position the pieces that you are
welding. As you make the first few tack
welds, at roughly equal distances from
each other, you can make minor adjust-
ments in piece positions by prying them
This is the finished weld bead.
with a small screwdriver. For example, you
Note that it is uniform and well
can keep the edges of the pieces level
Tack welds were added between the
penetrated. Also, note how little
with each other this way, as you fill in more
existing tack welds, until between
metal on either side of the bead
tack welds along the seam.
1/2 and 1/3 of the seam had been
has been discolored by heat. There
When you have filled in most of the
welded. The tack welds were then
is no visible distortion in the
welds, that is, enough to absolutely
ground flat to the panel, leaving the
original fender, or in the new metal
secure the seam, you grind those welds
seam with a perforated appearance.
that was welded into it.
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W E LDI NG BODY M ETAL
After grinding and finishing, here is
the completed fender. It required
little work to bring it to this point,
mostly filing and sanding. The
other photos are of the right
The underside of this fender repair required even less finishing work than
fender, while this one shows the
its topside. A little, light surface grinding and sanding did the job. This is
left. Its repair was almost identical
as close to the perfect restoration of metal as you are ever likely to see.
to that of the right fender.
The repaired metal is almost indistinguishable from the original.
cooling and heat-dispersion time between
riority of the welding in the TIG example.
equipment and time and skill. Maybe so,
adjacent short welds. When it is used with
but it has the potential to restore and
MIG welding, I prefer to just fill between
in Cameron, Wisconsin. It represents
fabricate metal that makes it more like
the ground welds, and not to weld a con-
the achievement of perfection in a very
original metal than any other welding
tinuous bead over them, because this
challenging
job,
from
fabrication
approach. It finishes more easily and
involves less heat and distortion.
through welding, and finishing the
better than any other technique, and
When you compare these examples
welds. Some may argue that TIG weld-
creates far less distortion around welds.
of MIG and TIG welding, note the supe-
ing is overkill, in terms of the cost of
I don’t think that is overkill.
You can use the same type of
The welds shown in the previous
When the seam had been completely
alternate tack welding approach
photograph are shown here, after
welded and ground flat, it was
with MIG welding as with TIG
partial grinding. After they were
finished with a small orbital disc
welding. This is illustrated here
ground flat to the panel, it was tack
sander. The result was a strong and
with a sample that has been
welded in the areas between the
good-looking bond. Note that this
welded with spaced MIG welds.
ground welds.
was a butt-welded joint.
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
79
CHAPTE R 8
Actually, you can’t weld with a
torch by melting and carrying a puddle
down the seam. The two pieces can’t fill
the seam without thinning them unac-
ceptably. For that reason, a steel filler
rod is applied to, and melted into, the
puddle as needed to form a bead, as the
puddle is carried down the seam by
torch movement in that direction.
All of this is possible and has
worked reasonably well for genera-
tions. On the other hand, MIG and
TIG welding also have worked much
better for fewer generations. The first
disadvantage of torch welding thin
sections is that it requires consider-
able skill, more than MIG welding,
and about the same as TIG welding.
Note that the considerations and
manipulations of the TIG electric
torch and filler rod are somewhat sim-
Gas welding is difficult to master, due to t
he many operator variables involved
ilar to those employed in gas welding.
in its performance. Note the extreme torch angle and the use of filler rod to
Torch welding also imparts much
add metal to the puddle and, sometimes, to shield it from excessive heat. Also
more general heat to a weld area
note the distortion in this example.
than either of the common electric
welding formats. This means more
great welds in a variety of situations.
6,300 degrees F at the cone tip of a neu-
distortion and more fun and games
As with your personal computer, you
tral flame. A neutral flame has a perfect,
chasing collateral damage off the
can operate a TIG welder without a
combustible mixture of oxygen and
welding scene, after welding is com-
deep understanding of how it works.
acetylene for complete combustion of
pleted. For those reasons, torch weld-
A good welding course at a vocational
each gas, with no excess of either in the
ing sheetmetal seams is rarely
school will have you started down the
mix. This means that it is possible for
employed these days. Put simply,
path of mastering TIG techniques.
that flame, in sufficient size, to melt the
you can do better work with much
surface of steel, which begins to melt at
less skill, knowledge, and effort with
Oxy-Acetylene Gas Welding, Brazing
around 2,700 degrees F.
electric welding techniques.
and Braze Welding
In practice, oxy-acetylene welds
Brazing and braze welding also
These are older methods of join-
are made by moving the flame, with
have limited use in good autobody
ing thin metal sections that still have
its inner cone near the metal to be
jointure practice. Like torch welding,
some application in today’s world of
joined, angled at roughly 45 degrees
these techniques are performed with
(mostly) electric welding. In most
to the surface, and oriented in a fore-
an oxy-acetylene flame. In this case, a
cases they will prove inferior, in one or
handed direction (the direction that
slightly carburizing flame is preferred.
more of several ways, to electric weld-
the weld is being made). The torch is
That is, a flame with a slight feather
ing approaches. But there are times
moved along with a slightly oval or
around its inner cone, caused by a
when you may find uses for torch
circular tip motion to make welds
richer-than-neutral amount of acety-
welding and brazing techniques.
that have characteristics indicated by
lene in the oxygen/acetylene mixture.
The oxy-acetylene flame, generated
the approved ripple appearance in
Brazing is somewhat like solder-
by mixing oxygen and acetylene gases
their weld beads. All of that takes
ing with tin/lead-based and silver-
to fuel a torch, is infernally hot—about
some coordination and practice.
based solders, but is done at higher
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AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
W E LDI NG BODY M ETAL
temperatures (around 1,000 degrees
F) and with filler rod that is a brass
alloy or, less commonly, a bronze-
based alloy. Note: Brass alloys are
based on copper and zinc, while
bronze alloys are based on copper
and tin. Like soldering, brazing does
not produce a fusion joint; that is,
the molecules at the surfaces of the
metals do not intermingle as they do
in true fusion processes, like MIG,
TIG, and torch welding. Instead, the
brazing material is attracted by capil-
lary action between the sections
being brazed together. There is some
surface mingling of braze and base
metal molecules, but nothing like
the alloying action that occurs in
true fusion welding.
You might think that brazing
is the low-temperature and low-
distortion solution to the challenges
of joining thin-metal materials. It
might seem that all you have to do is
bring joints up to a relatively low
temperature, and flow brazing rod
into the space between the pieces.
The weld on the left is a pretty good gas weld. The one on the right is a MIG
And because brazing rod is very cor-
weld. Note how much smaller the heat-affected zone is in the electric weld.
rosion resistant, it should automati-
This translates into less warping and distortion.
cally seal areas like lap joints.
Alas, it isn’t that simple. Brazing
be done in production, but usually is
(or bead) of brazing material in braze
materials typically don’t have the
very difficult in repair and custom
weld areas. This type of joint is
strength required for making sound
fabrication situations. Another prob-
stronger than simple brazing, but has
butt welds, so their use is reserved for
lem with brazing is the flux that is
all of brazing’s other drawbacks,
lap and offset lap joints. In fact, in
used. It is most often borax based,
mentioned above. There are some
years past, some automobile manu-
and it can be persistently difficult to
places where brazing and braze weld-
facturers used some brazed lap joints
remove from finished joints. If some
ing have application in sheetmetal
in their original constructions of cars
is left behind after cleaning, it does
work, particularly when they are used
and trucks. However, for most pur-
not take on primer and paint well.
to repair or to replace factory joints
poses, butt joints are more desirable.
Finally, brazing fluxes have a ten-
that were originally brazed or braze
That eliminates brazing them.
dency to cause hydrogen embrittle-
welded. In the main, however, brazed
Even with lap joints, the fit-up
ment in the metal adjacent to brazed
and braze welded joints are perilously
for brazing is critical to getting the
joints, and this can cause cracking in
close to being substandard tech-
right capillary flow of the brazing
that metal, as panels vibration cycle
niques for most panel jointure pur-
material into the joint. Without this
over miles and time.
poses today. While these techniques
factor, brazed joints tend to be too
Braze welding, unlike brazing,
once may have seemed attractive,
weak for automotive panel jointure.
goes beyond capillary action and
modern electric welding approaches
Maintaining
proper fit-up gaps can
deposits a strengthening thickness
have supplanted them.
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
81
CHAPTE R 8
Six Welding Shop Tricks and Tips
There are hundreds of little tricks,
habits, and minor ploys that will help
you make great welds. Some of them are
in books devoted to welding, and others
in books, DVDs, and seminars by
advanced professionals in the sheetmetal
and welding crafts. Still others can be
garnered by watching seasoned body
shop welders at their work. Here are six of
my favorites:
Tip #1
The edges of the sheetmetal that you
weld are often somewhat jagged, from
contact with the likes of saws and
aviation-type snips. If you take a minute
to run an inexpensive hand-deburring tool
over these edges, before you weld them,
you will improve your welding results by
making them more uniform. It is a small
Behold the screwdriver trick for keeping edges in alignment as you tack
weld them. The heat from tack welding can cause edges to lose lateral
alignment as you add tacks. Some judicious prying with a small-blade
screwdriver can reestablish alignment, before more tack welds are added.
point, but it is so easy to deburr edges
this way that it is a good idea to add this
trick to your routine preparations for
welding autobody metal.
Tip #2
Sometimes, despite your best efforts
to keep surfaces properly laterally posi-
tioned to each other, the heat of tack
welding or seam welding causes them to
An inexpensive edge-deburring
move out of position. When this happens,
tool can improve your welding
it can be corrected as you go along by
when it is applied to the sheet-
prying edges back into alignment with a
Cooling welds with compressed air,
metal edges that you weld. The tool
small screwdriver or other prying tool. I
right after you make them and as
is drawn along an edge to cut off
tend to keep a cheap, small screwdriver in
you go along, reduces heat buildup
any minor defects that may have
my left hand for this purpose as I weld
and hardens the metal in the welds
been created when the edge was
sheetmetal tacks and seams, probably
and weld area. Both of these
originally cut.
because I am right-handed.
results are highly desirable.
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