By S V Patki & D M Talekar

Going for a high technology solution without studying the economic impact may not be a prudent course of action
In any industrial sector, there is a con- stant quest to improve the product qual- ity and minimise the cost of production. This can be achieved by incorporating suitable automation at the bottleneck points, to compensate for the time lost.

In almost all the major industrial manufacturing units, welding plays a key role. Welding automation of simpler shape jobs such as circumferential welding or linear welding can be done easily by using motorised turntable units or linear travel slides. Welding on profiles are possible by combination of linear and circumferential welding units. Universally, such joints are welded using robotic arms coupled with advanced technology welding machines.

Problem definition

Due to the fantastic flexibility of programming, robots are popularly used abroad for such applications. The robotic welding systems are technically best suited for such work. However, commercially the proposal may not be economically viable in some cases, especially when the production volumes are low. In India, we often face this situation because the production levels are not very high but at the same time the quality requirements are high.

The price constraints prohibit the use of robots due to their high initial investments, and under utilisation of such high productivity equipments. (Due to low volumes). The question arises: "Can we use some low cost automation to avoid the use of un-economical robotic welding system in such situations?" The answer is "Yes! We can definitely introduce special purpose automatic welding system (SPAWS)"

Case study

Recently, we came across a similar situation, where two elliptical shaped parts of an automobile exhaust system were to be welded by the CO2 welding process and due to typical elliptical shape the customer (a multi national company) was considering import of a robotic welding system to weld this component.

On learning their predicament, we proposed to them a comparatively "simple low tech" solution and suggested that this can be an effective substitute of the proposed imported robot and save money in the process.

Salient features of the proposal

• A system with a built-in profile tracer unit (interchangeable tracer template to suit two varieties/dimensions of jobs)

• Automatic speed correction of rotary travel speed of the job to compensate the radius difference when the profile is traced by the tracer unit. This is achieved by electro mechanical feed back system

• Weld cycle sequencing by PLC (programmable logic controller)

• Weld quality and quantity of jobs per shift will be comparable to that of robotic welding system, and

• The cost of the proposal nearly six times less.

The proposal looked very attractive but from the customer's point of view, it was risky to stop the procurement of the robot. However, we got the clearance to "go ahead" from the customer, with a very systematic approach and transparent information on technical issues.

Details of system design

• Rotary cam mechanism is used to trace the elliptical profile. As the job rotates, the cam profile itself creates lateral    movement of the welding torch so that the welding point is always on the elliptical profile.

•  The profile also generates electrical signal proportional to the distance of the welding point from the centre of rotation. As the distance of the weld point from the centre of rotation changes continuously, the linear travel speed varies. For consistent weld bead shape, the linear travel speed has to be constant. The continuous feedback signal generated through electro-mechanical sensor is used to correct the rotation speed to maintain linear travel speed constant.

• The job is placed in a locating fixture and then pneumatically clamped at top. The job clamping signal is used in weld start circuit to avoid job being welded without clamping.

• Total automatic weld cycle includes automatic torch down, auto weld start, auto job profile tracing, auto weld stop after crater filling, torch going up to no weld position and job returning to home position. All these controls are achieved through programmable logic control (PLC) modules. The interlocks to make the system fool proof are also included in the programme. SMPS (switch mode power supply unit) ensures stabilised control voltages to the sensitive control circuits.

• Pneumatic operations are carried out smoothly by using flow control valves. Magnetic reed switch signals taken from pneumatic cylinder operations are used to make the weld cycle sequence absolutely fault free. F-R-L unit fitted on the system ensures moisture free and clean air for the pneumatic operations.

• The unit is integrated with a conventional CO2 welding machine. l The system is protected against tampering by placing the critical parameter setting controls inside the panel (locked) so that the operator cannot disturb the settings inadvertently.

• The unit is designed and manufactured with the above features and jobs welded.

Welding testing & approval

The unit is tested for perfect operations and weld cycle sequencing. Welding parameters include welding current, arc voltage, gas flow, arc length are set after taking few runs.

After welding of few jobs and testing for perfect welding (penetration test, weld strength test, etc), the unit is put for production run. It is observed that the unit has achieved the desired weld quality and desired productivity.

Net result

The proposed robotic welding system is replaced by a totally new innovative and comparatively "simple low technology" system. This resulted effectively saving huge money without compromising on quality.

Lesson learnt

We wish to stress that most of the time, we tend to go for high technology solutions without studying its economic effects, resulting in more spending. The concept of "appropriate technology" should be considered before final decision in this regard.

(S V Patki & D M Talekar, Managing Partners, Technocrat Engineers Pune. Tel: 020-25449064, 25448377. E-mail: techno crat91@vsnl.net)