Valk Welding: 1997

With the images of the recent fair "SCHWEISSEN UND SCHNEIDEN" in the Messehallen in Essen and the numerous robots shown there still fresh in mind, we thought it would be interesting to look at one robot application more in detail.

Let's go to ACV INTERNATIONAL in Ruisbroek. This family business, celebrating its seventy fifth anniversary, is specialised in the construction of heating appliances for the production of hot water for central heating and sanitary purposes. The company is already using Panasonic robots for some time now. The company VALK WELDING  just recently supplied a new Panasonic robot for a very specific application. It has been installed in the new ACV-plant in Seneffe.

The product to be welded
The robot installation has been completely designed for the welding of a typical ACV-heat-exchanger,  consisting of a core in stainless steel (304) surrounded by an envelope in carbon steel. The RVS-core mainly consists of a bundle of tubes with a diameter of 40 mm and a wall thickness of 1.5 mm.

The number of tubes may vary, but in the application we saw, there were 36 (6x6). (Photo 2) The ends of the tubes are caught in collecting plates that are also 1.5 mm in thickness. A thin-walled envelope in carbon steel is fastened under and on top of the RVS-core with fillet welds and in the sides with a few prop welds.

In the heating appliance, the tubes are put in vertical position. The burner is mounted underneath, it sends the flames and hot combustion gasses through the tubes, and is being charged an discharged via tube connections in the carbon steel jacket. The photo clearly proves that manual welding would be a very difficult and labour-intensive job. In particular the 72 welds (in this case) around the tube ends, are hard to weld delicately due to the small plate thickness (1.5 mm) and the positions. Due to the large deformation that is inevitable when welding thin-walled RVS-constructions, the heat-exchanger is completely mounted and attached to a relatively rigid whole, before starting to weld. As a result, the series of tubes along the walls are even more difficult to weld manually. This can only be done with a special extended torch. Furthermore, quality requirements are very high. All welded connections must be absolutely leak-proof, and great importance is attached to the appearance. The density of all heat exchangers is being tested on a test stand with a pressure of 4kg/cm². The above mentioned technical reasons, together with the aim of working as efficiently as possible, have determined the choice of robot welding.

The robot
The Panasonic robot has six axes. The piece is being clamped on a manipulator with two axes. This is done with a simple fork, whereby a mould provides the correct positioning.

The six axes of the robot and the two axes of the manipulator have been integrated via the software. In this way, the workpiece can be put in the optimal positions for welding with a minimum of manipulations and hence a minimum of time. The programme is thus designed that after pushing the start button, the product will be welded completely without interruption and without any intervention of the operator.

The robot has a lifelong repetition accuracy of 0.1 mm. However, the problem is that after clamping we talk of an accuracy in the order of millimetres regarding the position of the heat-exchanger and of the seams to be welded, . During welding, the deviations from the ideal programmed situation, will be even larger due to the deformations. The utmost care paid to the preparation could not prevent this. That is why, the robot will regularly scan the position of the piece and the seams, and will send the result of this scan to the installation's brain after which the robot programme will be adjusted accordingly. "Tactile" searching was chosen. The gas protection sleeve at the end of the welding torch will search the actual position of the welds by making contact with the material. This sleeve is provided with a low tension of 24 V, and with each contact there will be an electrical signal. Therefore, a special (vulnerable) sensor is required.

For a fillet weld for example, the end of the seam is searched by contacting the three surfaces. For round welding of tubes, searching the position of only two tubes will suffice. A central pin fitted in both tube ends will be scanned. This suffices to weld all 36 round seams, since the positions between the tubes are sufficiently accurate.

During welding other additional deformations might occur. The seam must be followed along the complete track with a seam monitoring system, for which use is made of arc controlled weld monitoring. The welding torch makes a constant weaving movement, whereby arc length and hence the arc resistance to both welding sides is being measured. When there is a difference in arc resistance, this will be corrected instantly.

The power source
The final result of robot welding largely depends on the quality of the power source. Welding is done with the MAG-method and use is made of the Panasonic HM 350 Dip Pulse power source, of the inverter type for pulse welding. The inverter disposes of an internal monitoring system, called "artificial intelligence" by Panasonic, that is permanently optimising the welding parameters. This application has the main advantage that even when welding is done with low arc voltage, short circuiting is being avoided. If for example in case of over welding of an attachment point, the arc length is reduced in length and there is a risk of short circuiting, an extra pulse will be given, after which the arc will exude fine drops. Consequently, welding can always be done with a tranquil arc and practically no splashes. This is very important for the subsequent treatment.

Gas protection
A ternary gas, containing helium beside argon and CO2, has been chosen as protection gas. (Inarc 172 of Air Liquide, 63.8% argon, 32% helium, 3.2% CO2, 1% H).

Welding wire
A solid 1 mm wire in stainless steel (309) is being used for welding. A traditional four roll system was chosen as wire feeding cabinet. It goes without saying that the motor is software controlled. The wire feeding cabinet is placed close to the welding torch at about 1.5 m, thus preventing excess friction.

Welding torch
The welding torch has been specifically designed for this application. In order to reach all seams, and in particular those close to the wall, a long torch with a special form was made.

The programme includes regular stops to clean the welding torch,  which is placed on top of a small cutter for internal cleaning and against a brush for external cleaning of the gas sleeve. The latter operation is important because of the scanning function of the torch, and to avoid communication of falsified data.

Efficiency
As mentioned before, the heat-exchanger is being welded in one action without any interruption. The product we saw with 36 tubes, was completed in 57 minutes. The relative arc time is about 70%. This also improves the quality: nice welds, no or little splashing and completely leak-proof.

The decision to chose robot welding in this application was completely justified.

(Source: Lastijdschrift 4 - 1997, by J. Tondeleir, BIL)