Factors affecting weld quality
The following factors can have a large influence on
the final outcome of the spot weld:
1 The type of metal being welded (for example,
coated metals)
2 Joint configuration
3 Current and timing settings
4 The type of welding equipment being used
5 Electrode maintenance.
Clearance between welding surfaces
Any clearance between the surfaces to be welded
causes poor current flow. Even if welding can be
made without removing such gaps, the welded area
would become smaller, resulting in insufficient
strength. Flatten the two surfaces to remove the gaps,
and clamp them tightly with a clamp before welding.
Metal surface to be welded
Paint film, rust, dust, or any other contamination on
the metal surface to be welded causes insufficient
current flow and poor results. Remove such foreign
matter from the surface before commencing welding.
Corrosion prevention on metal surfaces
Coat the surface to be welded with anti-corrosion
agent that has a high conductivity (e.g. zinc primers).
It is important to apply the zinc primer uniformly
between the panel surfaces to be welded.
Minimum welding pitch
The strength of individual spot welds is determined
by the spot weld pitch (the distance between spot
welds). The bond between the panels becomes
stronger as the pitch is shortened, but if the pitch
becomes too small and the spot welds are too close
together this can allow current shunting, which in
effect drains away the weld current towards the
previous spot weld (Figure 10.15).
Positioning of welding spot from the edge
and end of the panel
The edge distance is also determined by the position
of the welding tip. Even if the spot welds are
normal, the welds will not have sufficient strength
if the edge distance is insufficient.
Table 10.1Recommended tip pressures according
to the gauge of steel
Gauge of panel Pressure Length of arm
(mm) (kg) (mm)
0.55 _ 0.55 50 250
0.80 _ 0.30 60 250
0.95 _ 0.95 70 250
1.00 _ 1.00 80 250
1.25 _ 1.25 100 250
1.25 _ 2.00 120 250
These pressures relate to HS steels: a reduction in pressure may be
necessary for conventional steels
Figure 10.15Current shunting in resistance welds
positioned in close proximity
Inspection of spot welds
Spot welds are inspected either by outward appearance
(visual inspection) or by destructive testing. A
visual inspection is used to judge the quality of the
outward appearance (tip bruises, pin holes, spatter,
number of spots, spot positions), while destructive
testing is used to measure the strength of a weld.
Most destructive tests require the use of sophisticated
Electric resistance welding 287
Figure 10.17Modes of spot weld failure (Motor
Insurance Repair Research Centre)
equipment which most body shops do not possess.
Consequently a simpler method called a peel test has
been developed by general use in the workshop.
Destructive testing: peel test
After setting up the equipment a test weld should
be made, using sheet metal of the same thickness
and condition as the job to be done. When the test
piece is completed, it should be torn apart. If the
settings are correct, the joint will ‘unbutton’, that is
the weld nuggets will be on one sheet and the holes
on the other (Figure 10.16(a)).
Weld tests for shear and peel for HSLA steels
When shear testing spot welds produced when welding
together two HSLA steels, partial spot-weld failure
may occur. If the partial failure does not exceed
20 per cent of the total nugget area, the strength of
the weld should not be impaired (see Figure 10.17).
10.9 ARO Spotrite Pulsa resistance
welding system
For resistance spot welding in vehicle body repair
there is now a British invention called the Spotrite
(made by ARO Machinery Co. Ltd). This is an
adaptive (self-setting timing) in-process control
unit which operates in several stages. Stage 1 is the
monitoring stage for recognizing the material (lowcarbon
or HSLA steel) and for sensing through any
Figure 10.16(a) Peel test (b) destructive test (Motor
Insurance Repair Research Centre)
protective coatings, whilst judging how closely the
clamped materials are fitting between the electrodes
which will form the joint. Stage 2 is automatically
linked and self-setting to provide a burst of current
to produce the weld nugget. Stage 3 (and 4 if
needed) will provide further precise timed pulses
to bring the weld to the desired size, taking into
account the number of sheets of metal and their
thickness and the distance of the weld from the edge
of the workpiece. The adaptive part of the control
decides the number of stages needed and their
duration. A weld cycle which includes all these
stages added together, will take less than 0.75 s.
288Repair of Vehicle Bodies
With this equipment much of the technology is
transferred from the operator to the controls, only
because the operator has no means of knowing which
type of material he is welding. The control monitors
what the welding gun is doing, and uses the information
to take the decision by computing variables such
as electrode tip wear, spot-weld pitch, voltage fluctuation,
changes in metal thickness, and the presence of
scale, dirt and rust. Consequently quality control is
no longer the operator’s responsibility.
Audiovisual signals warn the operator if there is
overheating, if the electrode tip needs reshaping or
cleaning, and if the weld arms are open too early.
For difficult or unusual work, the experienced
body repairer can switch to manual setting to control
the unit’s weld heat and time tolerance controls
(see Figure 10.18).
Figure 10.18CEBORA spot welder (Mig Tig Arc)
Figure 10.19Pincer welding gun used in different
locations in repair work (Motor Insurance Repair
Research Centre)
A pincer-type welding gun with a powerful weld
transformer (to maintain the very short weld times)
is fitted with a strong pressure system to deal with
the majority of work, which can be approached
from two sides in pincer-type fashion. A range of
interchangeable weld arms and electrodes of different
shapes and lengths can be used to convert
the standard gun instantly into the special tool
that a job may require (see Figure 10.19). Special
Electric resistance welding 289
5 Test earth leakage unit by pressing test button.
6 Set spot size to zero.
7 Make splashless welds by selecting the lowest
heat and the longest time setting to make the
desired size of weld nugget.
8 Make two welds approximately 51 mm apart.
Peel test the second weld, i.e. tear the test piece
apart and check whether the nugget is the size
approximately to the weld strength needed. The
nugget size should be 4.5_t.
9 Increase the spot size setting from zero in single
steps, and make five trial welds in each setting.
Figure 10.20Electrode arm sets for different makes
of vehicles (SIP (Industrial Products) Ltd )
sets of arms with electrodes, specifically designed
for use on individual makes of vehicles, have been
designed in collaboration with most car manufacturers
(Figure 10.20).