FORM 2
THE PATENTS ACT 1970
(39 of 1970) COMPLETE SPECIFICATION
(SEE SECTION 10, RULE 13) TITLE
A device for spot welding of sheet metal
APPLICANTS
Tata Motors Limited, Bombay House, 24 Homi Mody Street, Hutatma Chowk, Mumbai 400001, Maharashtra, India, an Indian company
INVENTORS
Under Section 28(2)
Yashwant Anant Joshi and Vithal Pandurang Bandgar both of Tata Motors Limited, Manufacturing Division CVBU, Pimpri, Pune 411018, Maharashtra, India, both Indian nationals
The following specification particularly describes the nature of this invention and the manner in which it is to be performed :
GRANTED


This invention relates to a device for spot welding of sheet metal and is particularly, though not exclusively applicable to spot welding sheet metal part in automobile industry. For instance, this spot welding technique is applicable for welding car body and its sub-parts like bonnets, door assembly and the like.
It is obvious that if the joints are not welded properly, the component becomes weak which leads to customer dissatisfaction, warranty and other quality related issues. Hence, in order to achieve the required strength, adequate care must be taken to ensure that the spots are all welded in each assembly.
In the hitherto known method of spot welding the following steps are undertaken
1) The components to be welded are cleaned and are then mounted on the fixture for welding to which they are clamped firmly. Number of spots to be welded are specified
2) The operator uses the spot welding gun and welds the

components. It is to be understood that only if all the spots specified in the drawings are welded, the completed joint gets full strength. If the number of spots welded are less than the specified or required number, the quality and strength of the component becomes inferior.
3) After completion of spot welding the welded component is declamped and the operator is ready for the next cycle.
Limitation of this process is that the number of spots welded may be less than the specified number. Since the welding operation is operator dependent human error factor exists. The resultant welded joint will become weak if all the spots are not welded.
The object of this invention is to reduce or overcome operator dependency and to ensure that all the spots as specified in the drawings are welded in the manual welding operator. Thus, the quality of the welded joint is assured.

Accordingly this invention relates to a device for spot welding of sheet metal comprising a welding fixture with clamping and declamping means, said fixture being provided with means for mounting parts to be welded thereon, a spot welding means associated with a solonoid, and a make and break relay circuit, said circuit being provided with a comparative counter connected to a relay that becomes operational when only a preset number of spots is welded so as to declamp the welded component from said fixture.
This invention also includes a method of joining metal by spot welding with a device as described hereinabove.
This invention will now be described with reference to the diagrams that accompany this specification.
Fig I is a block diagram showing the improved spot welding process. Fixture will be declamped only when all the spots specified in the drawings are welded. This reduces operator dependency and ensures welded products of the required strength.

Fig II is the circuit diagram of the device according to this invention. Reference 1 and 2 shows the contacts which closes when components are present on the fixture. Contact points 3 and 4 closes when the fixture is clamped and a command to that effect is given. When the push button of the spot gun is operated, contacts 5 and 6 close and the relay RLi, becomes operational. Contacts 7 and 8 close which operates the solonoid provided on the cylinder of the spot gun. The circuit is provided with a counter which counts the number of spots welded during the operation and compares with the predetermined number provided in the comparator. A further relay RL2 provided in the circuit gets activated when a specified number of spots are welded. Declamping is effected when the operator pushes the button which closes the contacts 9-10. The job gets declamped only if the specified number of spots are welded and not other wise. Thus human error is avoided and a job jointed as specified is obtained.
Fig III shows the counter which receives the actual count from the circuit. This counter is set with a predetermined number or count value indicating the specified number of spots to be welded. Relay RL2 becomes

operational only when the actual spots welded are equal or greater than the set count value. Contacts 11 and 12 closes which allows the fixture to be declamped from the system.
Though this invention is described herein above with respect to specific embodiment, alteration and modifications obvious to persons skilled in the art are not excluded from the scope and ambit of the appended claims.

We Claim :
1) A system for controlled spot welding of sheet metal comprising a welding fixture with clamping and declamping means, said fixture being provided with means for mounting parts to be welded thereon, a spot welding means associated with a solonoid and a make and break relay circuit; said circuit being provided with a comparative counter connected to a relay that becomes operational when only a preset number of spots is welded so as to declamp the welded component from said fixture.
2) The system as claimed in claim 1, wherein said spot welding means is a spot welding gun.
3) The system as claimed in claims 1 and 2, wherein said solonoid is electrically actuated.
4) A system for spot welding of sheet metal substantially as herein described and illustrated with reference to the accompanying
drawings