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Product category: Machinery and Production Equipment
News Release from: Bauromat (UK) | Subject: Robot welding systems
Edited by the Engineeringtalk Editorial Team on 04 August 2000

Robot welding improves over dedicated
line

Resistance spot welding of door frames for the VW Lupo and Seat Arossa has been moved from a dedicated welding line to a twin robot system at the Brownhills plant of Wagon Automotive UK/USA

In June this year (2000), resistance spot welding of door frames for the VW Lupo and sister model, the Seat Arossa, was moved from a dedicated welding line to a twin robot system at the Brownhills plant of Wagon Automotive UK/USA (formerly Edward Rose) Configured and installed by specialist integrator, Bauromat (UK), the new robotic system allows first-tier subcontractor, Wagon Automotive, to produce door assemblies to a high level of reliability and to a target accuracy of +/-0.5 mm over the dimensions of the frames

Now that spot welding has moved over to automated systems equipped with articulated-arm robots, the VW door assembly cell at Brownhills is entirely based on this technology, arc welding operations on the same doors having been performed by two twin-robot systems since 1998.

A total of 48 spot welds and nine MIG (metal inert gas) arc welds are completed in the production cell, handling between the operations being performed manually.

The cell currently operates two shifts, five days a week to meet the required production level, which is an average of 760 door frame pairs per day, called off by VW's Wolfsburg plant in Germany and Spanish subsidiary, Seat, in Barcelona.

Says Richard Edwards, Project Manager in charge of the VW cell at Wagon Automotive, "We were having difficulty holding tolerance on our dedicated spot welding line and a certain amount of rework was necessary.

Also, line reliability was not as good as we would have liked and consumable costs were high.

"By contrast, robotic systems are simplified processes and very reliable.

We have a better chance of achieving the required dimensional accuracy with this type of equipment.

I would say that we have seen a step improvement in product quality.

"Moreover, robots are easier to service as there is ready access to the welding equipment, fixtures and other mechanical elements for cleaning, fault diagnosis and preventative maintenance." Another factor behind the new investment in spot welding plant was a change from single-sided coated material to an FEP03 steel, 0.9 to 2.5 mm thick, which is galvanised on both sides and was very difficult to weld on the previous dedicated line.

Further advantages of Bauromat (UK)'s robotic approach are that automated tip cleaning is possible within the cycle; and one tip carries out a number of welds instead of just one.

These are the first robotic spot welding systems to enter service at the Brownhillsplant.

New also is the use of servo-drive weld guns manufactured by the Japanese company, Obara.

They allow much more precise electrode closing pressure and positioning than air guns, minimising distortion of the component during welding which translates directly into improved accuracy.

The reduced mechanical stress additionally extends the life of the gun.

The distance of the stroke is adjustable as well, resulting in cycle time reductions of up to 15 per cent, according to Mick Widjadw, Senior Welding Engineer at Wagon Automotive.

He points out that the guns use the same servo drives as are built into the all-electric Motoman robots, which are also manufactured in Japan (by Yaskawa Electric Corporation).

Richard Edwards and Mick Widjadw both agree that good tooling is crucial to successful robotic welding, yet sourcing high quality fixtures is invariably one of the biggest problems they encounter.

System integrator Bauromat (UK) was chosen to implement the twin-robot spot welding system (op 25), which contains complex tooling at two turntable positions, one for fixturing, say, a right hand door frame whilst welding is carried out on a left hand frame at the other.

A second system (op 40) incorporates a single robot and two manually attended fixturing stations for the handed doors.

Bauromat (UK) had previously supplied other systems to the Brownhills plant for welding Ford Transit bumpers and towing rings.

Whereas the two older MIG welding systems (op 50, left and right hand doors) are equipped with Motoman SK6 robots and MRC controllers, Bauromat (UK) selected the new Motoman UP130s for this project as they benefit from the latest design features, offering larger working envelopes than their predecessors and incorporating a flexible conduit which routes all services from the base to the upper arm, eliminating many trailing hoses and cables.

In addition, the new UP models have direct drive on all axes, allowing greater speed, accuracy and reliability.

The motions of all moving elements within the systems including the 6-axis robots, servo weld guns (7th axis), turntable and powered fixturing are all orchestrated by Motoman XRC controllers, which are also new.

As many as 27 separate axes can be controlled simultaneously by just one unit, sufficient to control three robots and their peripheral equipment.

Concludes Richard Edwards, "The decision to move away from dedicated welding lines to more flexible robotic systems has been entirely validated.

Not only are they more reliable and productive, but also we can easily reconfigure them to suit a different door frame, or even to weld another assembly entirely should the need arise.".

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