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WELD SENTRY 2
Page 1
SECTION 1 - WELD SENTRY 2 DESCRIPTION
WELD MONITORING
Two conditions must be met to produce a good weld. First, the oxide layer covering the metal surfaces must
be eliminated. Second, the irregular surface of the metals must be brought into intimate contact with each
other.
All present day welding processes are just different ways of achieving these two goals by using various
combinations of heat, pressure and time. In the resistance welding process, the pressure is applied by the
electrodes which are mounted on a welding head. These electrodes carry a very high electrical current for a
fraction of a second. Heat, generated by the resistance of the workpieces to the flow of electricity, either
melts the workpieces at their interface or at least reduces their strength to a level where their surfaces
become plastic. When the flow of current stops, the electrode force is maintained, for another fraction of a
second, while the weld rapidly cools and solidifies. The size and shape of the weld is generally limited by
the size and contour of the faces of the electrodes.
General practice in resistance spot welding is to base quality control on weld properties as well as on the
uniformity and consistency of results. The quality of spot welds is usually checked by visual inspection and
by destructive testing. Nondestructive testing methods tend to be inconsistent and are not ordinarily used as
an inspection criteria except in the semiconductor industry. Quality standards should be related to the
requirements of the specific application. For example, it is rarely possible to obtain maximum joint strength
and minimum indentation using the same machine settings.
Weld monitors are devices which measure one or more specific electrical and/or mechanical parameters
which dynamically change during the welding process.
Somewhat in order of popularity, these
measurements include weld current, the voltage drop across the electrodes, workpiece expansion and
deformation, the electrode force, the size of the electrode face, the acoustic energy emitted while the weld is
being formed, and the temperature of the workpieces. Variations in the thickness, tensile strength, hardness,
surface finish and cleanliness of the workpieces have a significant effect on weld quality. The shape of the
electrode face also affects weld quality. Modern measurement techniques make it possible to accurately
measure the energy and pressure used to make a resistance weld. Weld monitoring is effective to the extent
that the electrical and mechanical measurements made during the welding process reflect the variations in
the physical properties of the workpieces and the welding equipment.
The successful user of a weld monitor usually conducts extensive experiments in order to determine which
combination of measurement parameters correlates with the quality of their specific parts. Once correlation
is verified in a production environment over a reasonable time, the weld monitor becomes a vital
manufacturing tool. To date no weld monitor has been commercially marketed which simultaneously
monitors all of the possible parameters; nor has any monitor been successful in predicting the quality of all
parts which might be welded in a given manufacturing area.
The Unitek Peco Weld Sentry accurately and consistently measures the voltage and current used to make a
direct energy (AC), stored energy (DC), constant current (CI), or constant voltage (CV) resistance weld.
This makes it superior to any other precision instrument of its type. Testing, on the production line, has
shown that the precision of the measurements made by the Sentry allows users to logically evaluate the
quality of their welding processes. If the user carefully controls the quality of the workpieces and uses good
manufacturing process control, the Sentry can provide the necessary electrical data for statistical process
control which in turn should increase quality and reduce manufacturing costs.
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