WELDERS, SPOT
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What is the welding Process?
Welding is the technique
used in joining metallic parts usually
through the use of heat. Resistance
welding was invented in 1877 by Elihu
Thomson, an electrical engineer and
inventor whose discoveries in the field
of alternating-current led to the development
of successful alternating -current motors.
The earlier type of resistance welding
was accepted long before arc welding
for the spot and seam joining of sheets.
Spot, seam, and projection welding are
resistance welding processes where the
required heat for joining of metals
is created at the interface by the electrical
resistance of the joint. Welds are typically
made in a short time of approximately
0.2 seconds using a low-voltage, high
current power source with force applied
to the joint through two electrodes,
one on each side. Spot welds are
made at systematic intervals on sheet
metal that has an overlap. The strength
of the joint depends on the number and
size of the welds. Seam welding is a
continuous process where the electric
current is successively pulsed into
the joint to form a series of overlapping
spots or a continuous seam welder .
This process is used to weld containers
or structures where spot welding is
insufficient. A projection weld is formed
when one of the parts to be welded in
the resistance machine has been dimpled
or pressed to form a bulge that is melted
down during the welding cycle. This
process permits a number of determined
spots to be welded at one time. All
of these processes are capable of very
high production rates with continuous
quality control. Modern equipment in
resistance welding includes complete
feedback control systems to correct
any weld that does not meet the desired
specifications.
Selecting The Right Welding Machine
For Your Application?
Selecting the right spot welding
machine can be a confusing
experience for the average metal
fabricator or welding
engineer because the price differences
for the various machines are difficult
to understand and justify. Selecting
the wrong spot welding machine
can be an expensive mistake. Many
recalls have been traced to improperly
made spot welds. A properly made
spot weld will not shear or break
in the fused zone. Instead, the
base metal will tear around the
weld. In addition to strength,
spot welding offers speed and cost
effectiveness that no other metal
-joining method can match. You
can determine the right welding
machine for your job by following
the recommendations prescribed
by the Resistance Welder Manufacturers'
Association (RWMA). The RWMA has
been dedicated to setting and improving
industry standards, as well as
education for over 50 years. It
currently represents about 40 manufacturers
of resistance welding machinery,
electrodes and component supplies,
including transformers and controls.
Resistance spot welding machines
have always been rated in KVA (a
thermal rating), where the RWMA's
rating of standard machines is
based on a 50% duty cycle. However,
not all resistance welding machine
manufacturers hold to these standards.
Another performance measure is
the available short-circuit current
of the machine. RWMA standards
specify the short-circuit current
based on the machine size, KVA
rating and throat dimensions, along
with the allowable duty cycle when
operated under short-circuited
conditions. In order to comply
with this standard, a resistance
welding machine must have adequate
copper sections in the secondary
loop, well-designed water cooling,
and the KVA rating of the welding
transformer based on a 50% duty
cycle.
Weld force is the other part of the resistance welding equation. The spot welding machine is a sophisticated device used to heat and forge metal together, the force applied to the weld area is critical to the weld's strength. Today, most resistance welding machines are operated with air cylinders of various diameters that apply the weld force either directly (press-type) or through a lever mechanism (rocker arm types). The force produced by a press-type machine is easily computed from the diameter of its air cylinder and the air pressure applied. Rocker arm machines are entirely different. Extending the arm length (throat deep) of a rocker arm has a dramatic effect on the maximum welding force available since the welding force decreases in the same ratio as the throat depth increases. Therefore, a deep-throat rocker arm not specifically designed to have arms of that length can reduce the force and adversely affect weld strength. RWMA specifications specify the maximum force expected from each spot welding machine frame size for both rocker arm and press types. In today's market, automation is increasingly important to manufacturers using the resistance welding process, it is important to find a company that also builds special design machines to weld customers' parts at high production rates.
