FIELD OF INVENTION
The present invention relates to a development of an automatic welding fixture
with robotic features for spot welding of steel sheets. Additionally, it is designed
to meet the precision requirement and high productivity of the welded
components. The capability of the fixtures in precisely placing the job for welding
and enable in rapid welding [@8 spots/min] makes it a generically attractive
technology for manufacturing a variety of other automotive components.
BACKGROUND OF THE INVENTION
The life assessment of the copper based spot welding electrodes is essential for
assessing the suitability of all zinc coated steel sheets as well as the welding
electrodes for manufacturing of automotive components. An electrode usually
survives for more than 2500 welds more the number of welds better are the
electrodes/caps for the given steel. To evaluate this property a large number of
spot welds (> 2000) need to be done in the shortest possible time. It takes a
long time to do it manually and hence the best option is to adopt an automatic
(or robotic) process.

A China Patent No. CN 201147870 discloses an invention of "New type device for
binding, measuring and spot welding of purifier comprises binding band,
stationary fixture and movable fixture, stationary fixture, spot welding device and
servo motor."
Another China Patent No. CN 201086179 discloses an invention relating to
"Cover plate spot welding fixture equipment for automatic welding of cover plate
and tag has cover plate and battery cell locating and holding devices that are set
in positions proportional to cover plate and battery cell structure."
A further China Patent No. CN 201061846 discloses a development related to
"Spot welding equipment and optical fiber identifying system for distinguishing
welding parameter and welding number of welding spot, fixture and tire has
identifying fiber which one end connected with fiber amplifier."
A Korean Patent No. KR 440747 discloses an invention related to "welding fixing
type sleeve for bolt assembling structure frame of assembly type sleeve, has
central part formed near punched hole by flanging processing in cylinder pipe,
and extended and processes screw thread."
A Japanese Patent No. JP8057656 discloses a development related to "Fixture for
spot welding which presses electrode chip of welding gun against fixed electrode,
and then extends the cylinder rod of welding gun."

A further Patent No. DD113858 discloses an invention which relates "Spot
welding lacquered or bare copper wire-onto a large part, using a single-sized
fixture containing an auxiliary heater."
The above mentioned cited patents relates to spot welding fixtures but none of
them have mentioned to the claims made in the currently designed and adopted
technology, and the targeted application. Although there are dedicated material
handling robots which possibly can perform similar type of applications, but
having one of them for such specific purpose would be preposterous, considering
the price of the equipment vis-a-vis the critical requirement.
As automatic fixtures are not available to meet this objective, tailor made spot
welding fixtures have been devised by leading steel research laboratories (for
e.g. Corus, Nippon Steel, Thyssen etc.) for assessing the compatibility of welding
electrodes with different coated steels. A number of information gathered reveal
that most of the fixtures are made up of conventional mechanical devices, for
which a large number of parts are required occupying large floor space. The
technology is outdated; the operation is cumbersome with limited applicability,
flexibility and accuracy. Productivity is much less than desired. Moreover there is
no open information on the design of these machines. Therefore it became
necessary to design a fixture and build it locally.

OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose an automatic
welding fixture with robotic feature for spot welding of steel sheets which
eliminates the existing disadvantages.
Another object of the present invention is to propose an automatic welding
fixture with robotic feature for spot welding of steel sheets which increases the
productivity.
A further object of the present invention is to propose an automatic welding
fixture with robotic feature for spot welding of steel sheets which saves
consumption of electrode.
A still further object of the present invention is to propose an automatic welding
fixture with robotic feature for spot welding of steel sheets which saves cost of
welding.
An yet further object of the present invention is to propose an automatic welding
fixture with robotic feature for spot welding of steel sheets which is eco-friendly.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - shows a photo view for automatic spot welding as per invention
Fig.2 - shows a photo view for automatic spot welding fixture and spot welding
machine a per invention
Fig.3 - shows a photo view for delivery stacker (on the left) and dispatch stacker
(on the right) as per invention
Fig.4 - shows a photo view for workstation in welding position showing welding
is in progress. The ball screw that enables sliding of the workstation along the Z-
axis is visible below the workstation tray
Fig.5 - shows a photo view of completed welded array using automatic fixture
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The automatic fixture is shown in Fig.l. The salient parts of the fixture are the
delivery (2) and dispatch (4) stackers, the servomotor driven ball screw (3)
mounted workstation (5). The electrode (7) is part of the spot welding machine
(1).
Location of the fixture (8) with respect to the spot welding machine (1) is given
in Fig.2. On the left of the dotted line is the fixture, whereas on the right is the
spot welding machine. The delivery stacker (3) and the dispatch stacker (4) are

shown in Fig.3. The motorized lead screws (9) used for lifting the stacked sheets
(10), are visible from the rear side of the stacker (Fig.3). The workstation (5)
with the job (11) clamping arrangement is shown in Fig.4.
The details of the workstation are visible in Figs 4-5. In Fig.5, the job is held
between the clamps (12) of the workstation (5) and is placed between the
welding electrodes (7). It also shows that welding is in progress with the sheets
(13) properly locked. The array of spot welds made by the automatic fixture is
exhibited in Fig.5. The spacing between two welds along the x-axis and the z-
axis is 15mm. If necessary the same can be either reduced or increased by
programming.
A new programming is developed using CTIU configuration software to decide on
the array size and the starting weld position. Spacing information in both the Y-
axis and Z-axis are provided as input values. The welding machine is made ready
and the desired welding parameters are set online. The fixture then starts its
function. The PLC of this fixture thereafter controls ail its functions. The
workstation (5) is vertically raised (along Y-axis) from its rest position to home
position by the help of a pneumatic cylinder. At this position the job is placed on
the workstation.

A sample of 250 mm X 100 mm size coated sheets of 0.8 mm to 2 mm thickness
is stacked in the delivery stacker (2). A motorized lead screw (9) arrangement
lifts the stack (10) as soon as sheets are removed from the top, which is sensed
by a limit switch. A single sheet is picked up by the vacuum grippers and placed
on the workstation by a pneumatically controlled slide. The same operation is
carried out twice. Pneumatic clamps are firmly applied to fix two sheets which
are placed on the workstation, one above another.
The workstation then slides along the Z-direction and setting the job precisely
between the electrodes of the spot welding machine. This operation is performed
by a servo-motor driven ball screw. Once set in the weld position, the spot
welding process is triggered by a signal from the fixture to the welding machine
panel. The end of each spot welding cycle is detected by a limit switch, which
directs movement of the job to the next welding by sliding along either the X-
axis or the Z-axis. The welding signal is then again triggered and the entire array
is completed by this process. Once the array is completed, the workstation
returns to the station position. At this point of time the vacuum grippers' pick-up
the completed job and places them on the despatch stacker. With increasing
number of sheets the stacker position gradually lowers aided by the lead screw
mechanism. Once the stack gets completely filled they need to be unloaded and
made empty.

Manual intervention is limited to loading of the stacker, starting the welding
machine, setting the welding parameters and starting the fixture. The rest of the
process is automatic. It is possible to weld more than 2000 welds uninterruptedly
without any human interference. An entire life cycle study of an electrode can be
completed in a day, which usually takes a month if done manually.
The home position of the workstation has been designed in such a manner that it
provides access to the spot welding machine for manual operation, without the
physical intervention of any parts of the fixture. A pneumatic cylinder retracts the
workstation to a lower position and clears the space in front of the machine and
provides enough space to the welding operator. This way the same machine can
be used under automatic mode as well as manual mode.

WE CLAIM
1. An automatic welding fixture with robotic feature for spot welding of steel
sheets comprises:
- a movable delivery stacker (2) and a movable despatch stacker (4)
mountable with lead screws (9);
- a pneumatic gripper for lifting steel sheets from delivery stacker (2) and
positioning steel sheets (12) on a workstation (5) and again after welding,
transferring the sheets from workstation (5) to despatch stacker (4);
- a two-axis movable slide assembly workstation (5) operated by a
servomotor driven ball screw;
- a PLC controller for stacker movement, material handling, weld trigger
control and workstation movement;
- a weld position array for automatically locating weld positions; and
- a pneumatically retractable workstation to provide access for manual
operation of spot welding machine
characterised in that the said fixture automatically provides steel sheets
from a delivery stacker (2) to a movable slide assembly workstation (5)
that pushes the sheets to welding machine (1) for automatically spot
welding according to array and automatically removes the welded sheets
to a despatch stacker (4).

2. The fixture as claimed in claim 1, wherein steel sheets are loaded
manually on the delivery stacker and after welding, welded steel sheets
are unloaded manually from the despatch stacker.
3. The fixture as claimed in claim 1, wherein the movable delivery stacker
and the despatch stacker are operated by lead screws.
4. The fixture as claimed in claim 1, wherein the slide assembly for two axis
workstation movement are operated by a servo motor driven ball screw.
5. The fixture as claimed in claim 1, wherein automatic movements are
controlled by a PLC controller.
6. The fixture as claimed in claim 1, wherein the fixture can be by-passed for
manual mode also.
7. The process of operating the automatic welding fixture as claimed in
claims 1-6 for spot welding of steel sheet comprises the steps of:

- making the welding machine (1) ready;
- setting the desired welding parameters on line;
- starting the fixture for operation;

- raising the workstation (5) vertically from its rest position to home
position by the help of a pneumatic cylinder;
- picking up a single sheet by a vacuum gripper from delivery stacker (4);
- placing the sheet on the workstation in home position;
- lifting the stack as soon as sheets are removed from top which is sensed
by a limit switch;
- pneumatically clamping firmly to fix two sheets which are placed on the
workstation one above another;
- sliding the workstation along Z-direction towards the welding machine;
- setting the job precisely between the electrodes of the spot welding
machine by a servo motor driven ball screw;
- triggering the spot welding process by a signal from the fixture to the
welding machine panel;
- end of each spot welding cycle is detected by a limit switch which directs
movement of the job to next welding by sliding along either the X-axis or
the Z-axis;
- triggering the welding signal until entire array is completed;
- completing and finishing the welding of entire array and returning the
workstation to station position;
- gripping the welded sheets and placing them on a despatch stacker; and
- thereafter starting the next cycle of operation.

8. The process of operation of automatic welding fixture as claimed in claim
7, wherein with the increasing number of sheets the delivery stacker
position gradually lowers aided by the lead mechanism to maintain same
top height.
9. The process of operation of automatic welding fixture as claimed in claim
7, wherein once the despatch stacker gets completely filled they need to
be unloaded and make empty manually.

The present invention relates to an automatic welding fixture with robotic feature for spot welding of steel sheets comprises a movable delivery stacker (2) and a movable despatch stacker (4) mountable with lead screws (9) and a pneumatic gripper for lifting steel sheets from delivery stacker (2) and positioning steel
sheets (12) on a workstation (5) and again after welding, transferring the sheets from workstation (5) to despatch stacker (4) and a two-axis movable slide assembly workstation (5) operated by a servomotor driven ball screw and a PLC controller for stacker movement, material handling, weld trigger control and workstation movement and a weld position array for automatically locating weld positions and a pneumatically retractable workstation to provide access for manual operation of spot welding machine characterised in that the said fixture
automatically provides steel sheets from a delivery stacker (2) to a movable slide
assembly workstation (5) that pushes the sheets to welding machine (1) for automatically spot welding according to array and automatically removes the welded sheets to a despatch stacker (4).