metal. The 50-grit abrasive disc will
sanding, these cut, non-round discs
chance of gouging it.
accomplish this, but 60- and 80-grit
should be used exclusively. The rea-
When metal finishing is com-
discs are better bets.
son is simple: When you use them,
pleted, your work should have a con-
In the final smoothing phase of
you do not have a consistently round
sistent, silvery appearance, with
metal finishing, your disc sander is
sanding disc that is always presenting
curves that remain consistent as you
operated somewhat differently than
the same edge to the metal that you
move your sight line past them.
in the discovery and leveling phases.
are sanding. A round disc can quickly
Enjoy that sight because, after you
For this operation, it is best to run
cut a groove into metal, particularly
perform some filling to remove the
your disc sander at right angles to the
when it is moved into an area like the
few minor surface defects that may
direction that you sanded in the dis-
transition from crown to reverse
remain, your work will move on to
covery and leveling phases of metal
crown. In such an area of changing
finishing or refinishing. After that,
finishing. Your sander should be held
crown, it may be presented to the
you will probably never have the
as flat to the panel as possible, pro-
metal at very sharp angles that cause
opportunity to admire that particu-
ducing as much disc contact as possi-
its edge to dig in. Damage can occur
lar metal surface again.
ble. This creates an averaging effect,
and avoids any likelihood of gouging
the panel, as might happen if only
the outer edge of the disc contacted it.
A good disc cutter is an impor-
tant disc sanding accessory, one that
anyone who uses a disc sander
should own. A disc cutter is used to
trim the edges off discs used for sand-
ing paint or for discovery sanding. It
removes the spent outer inch, or so,
from the disc, allowing the good
abrasive that is left on the disc to be
used. It would be a shame to throw
away a disc with only 30 percent of
its usable area consumed. A disc cut-
ter allows you to avoid doing this.
The second use of a disc cutter is
to allow you to modify a round disc
by cutting a symmetrical and bal-
anced
rounded-side-and-rounded-
point pattern into it. You can do the
same thing with a knife or a pair of
scissors, but a disc cutter lets you do
it faster and more consistently. The
object is to cut a shape into a disc
that has from three to six apexes. I
This disc cutter has two functions. One is to trim off the outer 3/4-inch from
like a pentagon-shaped disc best, but
used discs, so that new abrasive is exposed for general disc sanding. Its other
tastes vary in this matter. The points
function is to cut discs into various shapes, with rounded sides and corners,
between the rounded sides also
for final surface-smoothing sanding.
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
69
C H A P T E R 8
WELDING BODY METAL
Automotive sheetmetal and struc-
tural welding are vast topics, and it is
not possible to give them anything
approaching complete coverage here.
This chapter concentrates on some of
the fundamentals of welding sheet-
metal sections, and on a few ploys
that that may make this type of weld-
ing easier for you to perform, while
producing superior results.
It is important to note that weld-
ing thin metal sections is very differ-
ent from welding bridge girders or
thick plates. For one thing, welding
thick metal pieces rarely involves
having to worry about heat distor-
tion and material warping. Thick
their bulk, and to their tendency to
act as their own heat sinks. The main
In the last 35 years, MIG welding has revolutionized how we join autobody
considerations in that kind of weld-
metal. It is fast and efficient, and it provides good quality joints. Oh, that little
ing are penetration, bead deposit,
screwdriver between the fingers of the welder’s left hand; read on, and find out
bead shape, and strength. Concerns
why he keeps it there.
like welding through base materials
and creating drop-outs are remote.
gas or electric sheetmetal welding
fact that all of these forms of welding
In non-structural panel welding,
mastery. The skill set for this work is
involve carrying a puddle of molten
these issues become paramount,
very different, and must be devel-
metal down a seam, and fusing it to
while strength is usually secondary.
oped separately. If you are familiar
the metal on either side of the seam,
You may have great skills for and
with torch and/or electric welding,
there is no automatic transfer of
success with stick welding thick sec-
that may help you to learn sheet-
skills from heavy section welding to
tions, but these do not translate into
metal welding. But aside from the
sheetmetal welding.
70
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
W E LDI NG BODY M ETAL
Here are examples (top to bottom) of a butt joint, lap joint, and offset lap joint. The fronts are shown on the left and the
backs to their right. These are realistic examples of shop welding done at average levels of quality.
There
is
one
commonality
Types of Joints
end, against each other, with some
between stick or torch welding thick
amount of gap between them, to
sections and welding sheetmetal. In
Panel welding was discussed
provide for expansion during weld-
both of them, a serviceable weld
briefly in Chapter 3. Let’s now look
ing. Lap joints are made by overlap-
often, but not always, has a good-
at this topic in greater detail.
ping small amounts of metal, and
looking, even, penetrated, and uni-
Welded joints for bodywork fall
welding the exposed edges of each
form
appearance.
Unfortunately,
into three categories: butt, lap, and off-
section to the other, on one, or on
most finished sheetmetal welds are
set lap. While it would sound democ-
both sides. This, of course, creates a
unnoticed because
, very quickly after
ratic to say something like, “…each of
double thickness of metal at the
their creations, they are almost invari-
these joints has its place in panel work,
joint overlap, something that can be
ably ground off and covered over
and each is a good approach,” that
difficult to hide without using exces-
with the likes of filler, primer, and
would not be accurate. Butt joints are
sive amounts of filler.
paint. While the skill needed to per-
the gold standard of welded panel
Offset lap joints require the use
form good sheetmetal welds may
joints. The other two types of jointure,
of an offsetting tool to flange the
equal, or surpass, the skill applied to
particularly lap joints, are sometimes
edge of one of the sections to be
visible welds, like those on motorcy-
desirable. This usually is the case when
joined. The other section edge is
cle frames, you are far less likely to
they are used to duplicate factory lap
then slipped under the flanged area,
attain the fame, accolades, and down-
joints. I suppose that it is true that butt
and welded to it. This creates the
right glory with a crowd of the ador-
joints are more difficult for novice
appearance of continuous metal
ing that the makers of those
welders to master, but once you learn
because the double thickness of the
motorcycle welds receive. Still, you
how to weld them, they are not hard
lap joint exists on only one side of
will see your raw sheetmetal welds,
to achieve with good MIG or TIG weld-
the joined sections, and, naturally, it
before they are ground and painted
ing equipment.
is that side that is chosen as the back
over, so you will have the opportunity
Butt joints are made with the
side, and, therefore, hidden. Again,
to briefly appreciate your great work.
edges of the sections butted end to
in offset lap joints, the weld is
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
71
CHAPTE R 8
sometimes made on both edges of
when panel patching is the purpose
sons, this practice is rare or extinct in
the lapped metal to seal the joint.
of welding thin sections, butt joints
modern volume produced light vehi-
There are multiple problems
are preferred.
cles, it was common as recently as 15
with lap and offset lap joints. One is
years ago. Before that, the side fram-
that you may have to weld the joints
Welding Smaller Pieces into
ing panels of many cars were still
twice, if you want to seal them.
Large Constructions
welded up from as many as 20 sepa-
Another approach is to weld the out-
rate pieces. That practice was
side (or visible side) of a lap or offset
Sometimes large, complex con-
replaced by stamping and roll-form-
lap joint, and then seal the inner side
structions are welded up from
ing techniques that made it possible
of the joint with seam sealer. Sealing
smaller pieces. Many advanced prac-
to form these large, complex struc-
lap joints is critical to preventing
titioners of metal forming and fabri-
tures as single pieces.
corrosion from forming in the laps,
cating tend to frown on this practice,
Early fenders were often fabri-
where capillary action invites mois-
preferring to make their fabrications
cated out of more than one stamp-
ture and electrolytes in for a corro-
from single pieces of stock metal.
ing. For example, the drop skirt edges
sion bash.
Still, it can be a useful approach,
of very early automobile fenders were
If you double weld these joints,
when limitations of your equipment
riveted to the bodies of those fenders.
that is, weld them at both seams, you
and/or skills make single-piece fabri-
Later some large panels, like fenders
must apply more than twice as much
cation impossible.
and hoods, were gas welded, brazed,
heat to them as you would with a
It is interesting to note that in
or electrically welded out of two or
butt joint. In lap welding, one or
the past, some OEM large panels
three smaller stamped pieces.
both welds involve welding an edge
were made from smaller pieces,
When you have to fabricate a
to a flat, and this requires more heat
welded together. While, for cost rea-
panel or structure that is too large or
than is used to make a butt weld,
end-to-end. That extra heat is an
invitation to local distortion and
panel warping. Another problem
with lap joints is that they may later
show themselves through filler and
paint, as a panel experiences vibra-
tion cycles. It takes many miles for
this to occur, if it does occur, but it is
a haunting possibility. Finally, there
is no advantage to using lap and off-
set lap joints, where situations do
not mandate them, other than the
misguided idea that they are easier to
make than butt joints. The two
exceptions are when you are dupli-
cating a factory weld that was origi-
nally a lap joint, and when space and
access considerations make butt
welding undesirable, or impossible.
Butt joints usually return panels
more closely to their original format
Some common fixturing tools are shown holding small metal pieces. Left:
than do the other two types of joints,
locking pliers. Center top: magnets. Center bottom: standard Clecos (left)
and are simply the cleanest solution
with both sides shown, and Cleco edge type clamp (right). Install tool is
to the issues of laterally joining thin
below them. Right top: thumb clamps. Right bottom: pressure-type edge
sections of sheetmetal. Certainly,
clamps and install tool.
72
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
W E LDI NG BODY M ETAL
complex, or both, for you to form it
from a single piece, always remem-
ber that, as a last resort, you can
form parts of it and then join them.
Fixturing
Of the things that I really hate in
life, there are three that stand out: 1)
the sound of a horse pulling its hoof
out of deep mud, 2) the smell of the
inside of a Russian horse doctor’s
valise, and 3) trying to weld a mov-
ing target. It is that last one that I am
usually able to avoid.
The way I avoid it is to properly
fixture the pieces that are to be
These specialized locking pliers are incredibly handy for holding metal pieces
welded together. This means two
in place, prior to tack welding them. The right-angle configuration of the two
things: providing and maintaining
sets (top) works in many tight spots. The interchangable jaw set (front) has
adequate fit-up gaps between them,
five sets of interchangeable jaws.
and holding the pieces firmly in
place for tack welding. Once you
company that first manufactured it,
Wing nut clips can be positioned
have done those things, you can tack
Vise-Grips. Locking pliers are terrific
anywhere in a butt joint, regardless of
weld attachments that maintain
for holding pieces in alignment while
their depth from its edges. They work
proper positioning and fit-up gaps
tack welds are made. They come in
well on straight-line joints, and inher-
for final welding.
angled, long-reach, and pivoting-end
ently maintain a consistent fit-up
There are many methods of fixtur-
designs, and represent the first line of
gap. However, they do not work along
ing pieces for welding. Which one you
holding parts in place. Specialized
curved joints because they hold edges
choose for your work depends partly
locking pliers devices are also available
unacceptably far apart in that applica-
on the situation and partly on your
for many specific purposes.
tion. Still, if you are butt welding
personal preference. For example,
The main limitation on the use of
along a straight line, these inexpen-
welding magnets might be adequate
locking pliers to hold metal in posi-
sive clips work impressively well.
for holding a patch panel in place for
tion for welding is the reach of their
tack welding, but are probably not the
arms. They may work well for you,
Electric Welding
best approach to holding a whole rear-
when your positioning needs require
quarter section in position. Of the
holding at 12 inches, or even 16
Many welding techniques apply
many fixturing methods, devices, and
inches, from the nearest accessible
to autobody work. Most of this weld-
gadgets out there, the main ones are:
edge of what you are welding. But
ing is now electric, though gas weld-
locking pliers, welding magnets, edge
even at that reach, locking pliers tend