FIELD OF INVENTION
The present invention relates to an apparatus for off-line measurement of
contact resistance of metal sheets under varying pressure conditions required to
ensure the steels suitability for spot welding purpose.
BACKGROUND OF INVENTION
Contact resistance is an essential parameter that governs the process of spot
welding of metal sheets. During spot welding a high current is passed through
copper electrodes into two overlapped clamped metal sheets. The heat
generated at the interface depends on the contact resistance that exists between
the two sheets and therefore the process is known as resistance spot welding.
Surface roughness and asperity strength determine the extent of surface
resistance at ambient temperature. Contact resistance is created when two
surfaces are brought in close contact with each other. The contact resistance can
vary from one steel grade to another, and provides significant input towards
development of suitable welding parameters. It is a quality parameter and is
therefore essential to determine accurately.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose an apparatus for off-line
measurement of contact resistance of sheet steels under varying pressure
conditions in order to assess their suitability for spot welding.

Another object of the invention is to propose an apparatus for off-line
measurement and control of contact resistance of sheet steels, subjected to spot
welding, which can readily display the measurement data.
A further object of the invention is to propose an apparatus for measurement of
surface resistance value of a single sheet.
SUMMARY OF INVENTION
Contact resistance is an essential parameter that provides in-depth
understanding of the welding process for various bare and coated automotive
steel grades. In order to cater to this requirement, a contact resistance
measurement and control apparatus has been developed. The apparatus can
determine both surface and contact resistance values of plurality grades of steel
with tensile strengths up to lOOOMpa.
Accordingly, there is provided an apparatus for off-line measurement of contact
resistance of steel-to-steel under varying pressure conditions required to check
suitability of these steels for spot welding, comprising; a servo-valve controlled
pneumatic pressure cylinder including apiston for generating pressure on
clamped steel - to-steel sheets, joinable by the spot -welding process; a pair of
copper electrodes each disposed within one each

insulated holder; a micro ohm-meter connectable to the pair of copper electrodes
to measure the low resistance being inversely related to the generated contact
pressure of the steel sheets; a load sensor cell sensing the pressure and
displaying through a load indicator; a low voltage high current source injector to
inject high current to the contact surface for causing voltage drop that generates
detectable resistance and thereby measuring the contract resistance in the micro
ohm meter; and one each air-inlet and air - outlet for monitoring and control of
the contact pressure through operation of the survo-valve controlled pneumatic
system.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - illustrates the positioning of overlapped sheet steels during spot welding
according to the invention.
Fig. 2 - illustrates the schematic diagram of the apparatus of the invention.
PETAIIEP DESCRIPTION OF THE PREFERRED EMBQPIMENT
Fig (2) illustrates the salient features of the proposed apparatus which
comprises, a servo-valve controlled pneumatic pressure cylinder (1), a pneumatic
piston (2), an insulated electrode holder (3), a pair of electrode (4), two joinable
steel sheets (5), a load cell (6), which are fitted within a steel structure. The
apparatus further comprising a load indicator (7), a source current injector (8)
and a micro-ohm meter (9).

The apparatus has been developed to measure contact resistance for soft and
high tensile steels of tensile up to 1000 MPa. The diameter of the pneumatic
cylinder (1) and the load cell (6) are selected based on the tensile strength of
the steel. The pressure of contact is controlled by the servo valve of the
pneumatic cylinder (1) through operating an air inlet (10) and an air outlet (11).
An external low voltage current source has been provided to inlet current up to
80A at 5V with terminals connected to the copper electrodes (4), the copper
electrodes (4) are insulated by insulated electrode holder (3) to prevent any
grounding effect.
During the process of measurement of the contact resistance, the micro ohm-
meter (9) is operably connected to the copper electrodes (4) to measure the low
resistance which increases with increasing contact pressure generated through
the pneumatically driven piston (2) on the job (4), which is controlled by the
servo valve. The most influencing parameter for the spot welding process being
the contact resistance which is related to the contact pressure of the sheet steels
(5), the apparatus of the invention assesses suitability of sheets and help set
parameters for spot welding. The pressure generated is sensed through the load
cell (6) which is indicated by the load indicator (7).

The low voltage high current source injector (8) injects high current through the
electrodes (4) to the contact surface of the job (5) for an appreciable voltage
drop by which the contact resistance is measured in the micro-ohm meter (9)
adapting a process of simulation.
The apparatus has been found to provide reliable results according to the desired
values. Moreover, variation in resistance as observed for different grades of
steel, are shown in Table 1.
Table 1 : Contact resistance values for two overlapping sheets obtained from the
apparatus.

Note : IF and DP stand for interstitial free and dual - phase steels respectively.
The affixed numbers 300 and 590 are the specified tensile strength in Mpa of
each of the grades.

WE CLAIM
1. An apparatus for off-line measurement of contact resistance of steel
sheets by controlling pressure and their assessment for spot welding
purpose, comprising;
- a servo-valve controlled pneumatic pressure cylinder (1) including a
piston (2) for generating pressure on clamped steel - sheets (5),
simulating the spot-welding process;
- a pair of copper electrodes (4) each disposed within one each
insulated holder (3);
- a micro ohm-meter (9) connectable to the pair of copper electrodes
(4) to measure the low resistance being inversely related to the
generated contact pressure of the steel sheets (5),
- a load sensor cell (6) sensing the pressure and displaying through a
load indicator (7);
- a low voltage high current source injector (8) to inject high current
to the contact surface for determining resistance accurately with
voltage drop and thereby measuring the contract resistance in the
micro ohm meter (9); and

- one each air-inlet (10) and air - outlet (11) for monitoring and
control of the contact pressure through operation of the survo-
valve controlled pneumatic system.
2. The apparatus as claimed in claim 1, wherein the voltage drop is used to
derive the contact resistance value for measurement by the micro ohm
meter (9).
3. The apparatus as claimed in claim l, wherein the low-voltage current
source is capable of injecting up to 80A current at 5V to the copper
electrodes.
4. An apparatus for off-line measurement of contact resistance of joinable
steel sheets for spot welding process, as substantially described herein
and illustrated with reference to the accompanying drawing.

This invention relates to an apparatus for off-line measurement of contact
resistance of steel sheets by controlling pressure and their assessment for spot
welding purpose, comprising a servo-valve controlled pneumatic pressure
cylinder (1) including a piston (2) for generating pressure on clamped steel -
sheets (5), simulating the spot-welding process; a pair of copper electrodes (4)
each disposed within one each insulated holder (3); a micro ohm-meter (9)
connectable to the pair of copper electrodes (4) to measure the low resistance
being inversely related to the generated contact pressure of the steel sheets (5),
a load sensor cell (6) sensing the pressure and displaying through a load
indicator (7); a low voltage high current source injector (8) to inject high current
to the contact surface for determining resistance accurately with voltage drop
and thereby measuring the contract resistance in the micro ohm meter (9); and
one each air-inlet (10) and air - outlet (11) for monitoring and control of the
contact pressure through operation of the survo-valve controlled pneumatic
system.