As the term suggests, spot welding is a process in
which two sheet metal parts are fused together at localized points by passing a
large electric current through the parts where the weld is to be made. The
fusion is accomplished at relatively low voltage levels by using two copper or
copper alloy electrodes to squeeze the parts together at the contact points and
apply the current to the weld area. The electric current results in sufficient
heat in the contact area to fuse the two metal parts, hence producing the weld.
The two electrodes have the general shape of a
pincer. With the two halves of the pincer open, the electrodes are positioned
at the point where the parts are to be fused. Prior clamping or fixturing of
the parts is usually required to hold the
pieces together for the process. The two electrodes are squeezed together
against the mating parts, and the current is applied to cause heating and
welding of the contacting surfaces. Then the electrodes are opened and allowed
to cool for the next weld. A water circulation system is often used to accelerate the cooling of
the electrodes. The actual welding portion of the sequence typically requires
less than a second. Therefore, the rates of production in spot welding are
largely dependent on the time required for positioning of the welding electrodes
and the parts relative to each other. Another factor that affects production
rate is the wear of the electrodes.
Spot welding has traditionally been performed
manually by either of two methods. The first method uses a spot-welding machine
in which the parts are inserted between the pair of electrodes that are
maintained in a fixed position. This method is normally the pair of electrodes
that are maintained in a fixed position. This method is normally used for
relatively small parts that can be easily handled.
To create heat, copper electrodes pass an electric
current through the work pieces. The heat generated depends on the electrical
resistance and thermal conductivity of the metal, and the time that the current
is applied. The heat generated is
Expressed by the
equation: E=I2*R*t
Robots in spot welding:
As a result of these difficulties, robots have been
employed with great success on this type of production line to perform some or
all of the spot-welding operations. A welding gun is attached as the end effectors
to each robot's wrist, and the robot is programmed to perform a sequence of
welds on the product as it arrives at the workstation. Some robot spot-welding
lines operate with several dozen robots all programmed to perform different
welding cycles on the product. Today, the automobile manufacturers make
extensive use of robots for spot welding. In 1980, it was reported that there
were 1200 robots used in this application.
The robots used in spot welding must possess certain
capabilities and features to perform the process. First, the robot must be relatively large. It
must have sufficient payload capacity to readily manipulate the welding gun for
the application. The work volume must be adequate for the size of the product.
The robot must be able to position and orient the welding gun in places on the
product that might be difficult to access. This might result in the need for an
increased number of degrees of freedom. The controller memory must have enough
capacity to accomplish the many positioning steps required for the spot-welding
cycle. In some applications, the welding line is designed to produce several
different models of the product. Accordingly, the robot, must be able to switch
from one programmed welding sequence to another as the models change. For
welding lines in which there are multiple robots, programmable controllers are
used to keep track of the different models at the various welding stations and
to download the programs to the robots at individual workstations as needed.
Benefits for using robots in spot welding:
The benefits that result from automation of the
spot-welding process by means of robots are improved product quality, operator
safety, and better control over the production operation. Improved quality is
in the form of more consistent welds and better repeatability in the location
of the welds. Even robots with relatively unimpressive repeatability
specifications are able to locate the spot welds more accurately than human
operators. Improved safety results simply because the human is removed from a
work environment where there are hazards from electrical shocks and burns.
Some spot welding series:
F-200iB, M-710iC/50, M-710iC/70,
M-900iA/200P, M-900iA/260L, M-900iA/350, M-900iA/400L,
M-900iA/600, M-900iB/700, R-1000iA/100F, R-1000iA/80F, R-2000iB/100P,
R-2000iB/125L, R-2000iB/165F, R-2000iB/165R, R-2000iB/210F, R-2000iB/220U
