PROJECTION
WELDING
It
is a modified form of spot welding. Projection welding is an electric
resistance welding process that uses small projections, embossments, or
intersections on one or both components of the weld to localize the heat and
pressure.
INTRODUCTION
In
this process localization of heat between the ends are affected by projection.
The metal surfaces to be joined, have projection in which the current is
passed. The welding takes place only at these points.
PROCESS
1.
The machines are basically flat platens with T slots for the attachment of
special tools as shown in fig.
2.
In this method protrusions (protrusion means something that bulges out) are
pressed on one of the sheets to be welded and exact location of the weld is
determined.
3.
Sheet metal is first put through a punch press that makes small projections or
buttons in the metal.
4.
The projections are made with a diameter on the face equal to the thickness of
the stock and extend from the stock about 60% of its thickness. Such projection spots or ridges are made at
all points where a weld is desired.
5.
When current is passed and the electrode pressure is applied the projection
collapses and the sheets are welded together. The projections are flattened
during the welding.
ADVANTAGES
1.
Filler metals are not used.
2.
It is in effect, a form of multi-spot welding in which a number of welds are
made simultaneously.
2.
Welding current and pressure required is less and is .
3.
This process is suitable for automation.
DISADVANTAGES
1.
Equipment cost is very high.
2.
It is not suitable for thin work pieces.
3.
Thin work pieces cannot withstand the electrode pressure.
APPLICATIONS
1.
A very common use of projection welding is the use of special nuts that have
projections on the portion of the part to be welded to the assembly.
2.
This welding process is used for steel radiator, coupling elements, brake
shoes, tin-plate tank handle etc.
3.
This process is also used for crosswire welding and for parts where the ridges
are produced by machining.
SEAM
WELDING
Spot
welding are spaced so closely that they overlap each other, giving rise to seam
welding. Seam means joint consisting of a line formed by joining two pieces.
INTRODUCTION
Seam
welding is similar to the spot welding with the difference that the electrodes
are in the form of rollers. This used for joining two metal sheets and to end
in a continuous line.
PROCESS
1.
To obtain sound weld the work pieces to be joined are cleaned to remove dirt,
grease and other oxides either chemically or mechanically.
2.
The welding electrodes are motor driven wheels as opposed to stationary rods.
3.
The work pieces are overlapped and placed firmly between two wheels shaped
copper alloy electrodes, which in turn are connected to a secondary circuit of
a step-down transformer.
4.
The electrode wheels are driven mechanically in opposite directions with the
work pieces passing between them, while at the same time the pressure on the
joint is maintained.
5.
Welding current is passed in series of pulses at proper intervals through the bearing
of the roller electrode wheels.
6.
As the current passes through the electrodes, to the work piece, heat is
generated in the air gap at the point of contact (spot) of the two work pieces.
7.
It is desirable to have intermittent flow of current to prevent overheating
which may occur resulting in burning of the sheets.
8.
Under the pressure of continuously rotating electrodes and the current flowing
through them, a series of overlapping spot welds are made progressively along
the joint.
9.
The number of spots per cm varies between 2 and 4. Such a weld is normally gas
and liquid tight.
10.
The weld area is flooded with water to keep the electrode wheels cool during
welding.
ADVANTAGES
1.
Filler metals are not required. Hence, there are no associated fumes or gases.
This results in clean welds.
2.
The production of single seam weld and parallel seams can be obtained
simultaneously.
3.
Gas tight as well as liquid tight joints can be made and the overlap is less
than spot or projection welding.
4.
It can be automated.
DISADVANTAGES
1.
It requires complex control system to regulate the travel speed of electrodes
as well as the sequence of current to provide satisfactory overlapping welds.
2.
The welding process is restricted to a straight line or uniformly curved line.
3.
The metal sheets having thickness more than 3mm can cause problems while
welding.
APPLICATIONS
1.
Seam welding is extremely useful for fabricating all types of exhaust systems,
barrels etc.
2.
This process can produce continuous pressure tight seams at high speeds.
3.
It is used for oil transformers, refrigerators, evaporators and condensers,
aircraft tanks, paint and varnish containers etc.
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