Valk Welding: 1997

The company ACV from Ruisbroek, manufacturer of heating installations and boilers both for domestic and industrial use, recently invested in an automatic welding robot system to weld a "family" of heat exchangers. The installation was carried out by the "royal supplier" of welding robots, Valk Welding, who previously already had supplied various welding robots.

Too critical for manual work
The welding of heat exchangers is a critical job,  because it involves the welding of RVS products, and that is mainly done by hand. For these types of heat exchangers obtuse welds between flat plate and thirty tubes (more or less, depending on the required type) have to be welded. The wall thickness of both plate and tubes is 1.5 mm. After welding, the whole must be 100% leak-proof. In addition, the form of the heat exchangers causes other problems. The welder must work with an extended welding torch, and has a limited  view during welding. He cannot put his head in between the tubes to see the other side. Last but not least, austenite material will deform during welding, despite the fact that the whole is clamped and has been pre-mounted via several short welds. Furthermore, in order to obtain a leak-proof weld, the weld must be made as continuously as possible. That is why manual welding is extremely labour-intensive.

The only way to solve such a welding problem while guaranteeing quality work, is automation with a welding robot. Easy operation of the installation was one of the requirements. As ACV already disposed of several Panasonic welding robot installations, it was obvious to further collaborate with Valk Welding. The result is a combined robot manipulator system. The pre-mounted heat exchangers are being clamped in a fork-like mould using four clamps. The mould is mounted on a two-axe manipulator that will offer the product in an optimal way to the robot torch during welding. These two "external" axes are completely integrated in the control of the six axe welding robot via the so-called "Harmoniser" software. With this software, the eight axes for the complete movement, welding robot and manipulator, are being controlled to obtain an optimal welding position with a minimum of manipulation time. This integration strongly reduces programming and programming time.

Clamp and push the button
A robot works with a repetition accuracy of ± 0.1 mm, thus allowing to obtain high qualities, providing the positioning accuracy of the part to be welded is high. And here pinches the shoe, because hanging a pre-mounted heat-exchanger in the mall cannot be done very accurately. The repetition accuracy of the suspension in the mould is expressed in millimetres. In addition, the installation had to be suitable for the welding of different types of one family (similar heat exchangers with more of less tubes, of with different size). Furthermore, the company wished an installation whereby the operator clamps the heat-exchanger, pushes the start button, after which the robot faultless does the required welding.

These objectives could be realised by equipping the robot with so-called  "tactile searching". First, the robot will use its welding torch to "touch" the product. The gas protection cap of the welding torch works as a sensor (an "open" contact with 24 V), with which the localisation programme can be executed. The robot runs at maximum speed until f.e. 10 mm of the required positioning of a reference point and then "senses" at a speed of 1 m/sec. When the sensor touches metal, the contact is closed and the 24 V-signal is detected. These data are stored in buffers and compared with the programmed types,  resulting on the one hand in information about which heat-exchanger is involved, and hence the latter's programme is loaded into the controller. On the other hand information is received about the position of the heat-exchanger and the shifting thereof compared with the position provided in the programme. Based on localisation of reference points, the  robot movements are adjusted in the welding programme by shifting, rotating and rescaling of the computer model to the actual position of the heat-exchanger.

Optimising the weld
As mentioned before, the product will strongly deform during welding. Since a high welding quality is required, a follow-up system will be indispensable. The most elementary system was chosen: arc controlled weld monitoring. The robot will continuously measure the arc resistance and adjust its track so that arc resistance is kept to a minimum. Solving the deformation problem with a special mould would be too expensive and welding the product in one clamping would not be possible.
The continuous monitoring of the pulse arc is also used to adjust the welding process. When the arc pressure between two pulses falls under the critical limit, the control will generate an extra "dip-pulse" after which the welding wire will exude fine metal drops,  thus preventing that the welding arc comes in the (adverse) short circuit area (more splashes and a greater chance of mistakes). This process, developed in Japan by Panasonic, is known as "Dip-Pulse" MIG-welding. It is only used on Panasonic welding power sources, thus making these welding machines better suitable for the welding of thin-walled stainless steel and aluminium products: hence MIG-welding can be applied where otherwise the TIG-process would be used for welding.

With automatic robot welding, wire feeding and power source are equally important to maintain a stable welding arc. Various manufacturers recommend push-pull feeding of the welding wire (beside the normal drive, there is also a "pull" drive in the welding head itself), because the RVS-wire is quite ductile and can only support little buckling load. Nevertheless, Valk Welding chose a normal (technically uncomplicated and thus more robust) wire feeding in this installation because the same effect can be reached as with push-pull with a correct concept of wire feeding A four roll drive computer controlled by the power source was used and the wire distance between spool and welding torch was kept as short as possible (less than 1.5 m, thus limiting friction). Depending on the programmed welding parameter, the feeding speed and feeding acceleration of the welding wire can be adjusted to guarantee an optimal welding start.

Performance
The performance of this robot installation is acquired with the speed and uniform quality delivered by automation. One type that is welded onto the welding robot in 55 minutes, carried out by an experienced by hand will require 3 to 4 times more time. Any operator, even without training, can remove a welded heat-exchanger, clamp a pre-mounted piece and start the robot. The end result is a perfectly welded part. It is obvious, that pay-back time for this installation is extremely short. Anyway, with manual welding the quality cannot be guaranteed and faultless production is essential to maintain our high reputation on the market.

(Source: Professional supplement Technisch Management Sept. 1997)