QUALITY MANAGEMENT IN ROBOT WELDING

The quality level of robot welding varies in many welding shops. The quality of the outcome depends often on the worker, even though quality should be repeatable independent of the welding robot operator. In several welding shops, the best quality is achieved by one worker. Often the problem comes evident only when the best worker leaves the company.

The newest offline programming systems have made quality planning and management an integrated part of robot welding. The most advanced offline programming systems include welding quality management and quality level control as well as an opportunity to affect the weldability of the product. High quality in robot welding can be achieved by recording weld type-specific welding methods into the software’s own WPS (Welding Procedure Specification) -file or database, which has been created by the best welding expert in the welding shop.

PROGRAMMING BY TEACHING AND WELD QUALITY

The quality of welding programmes is very much affected at the stage when creating the programme. Programming a robot affects basically two quality-related groups:
1. welding parameters and
2. geometric variables of the torch and the workpiece, like
– torch angle and
– length of the cord

Welding parameters are defined in method tests every time there is a need for new parameters. In programming by teaching, the way the weld looks like depends mainly on the creator of the programme. The experience and know-how of a welding robot operator shows in better quality of welding joints compared to beginners or users who have less experience of programming. Although good quality can finally be achieved, it can take several weeks. Controlling geometric variables can be challenging also for an experienced programmer. The challenge is emphasised especially with curved or double-curved welds and in welds in which the so called coordinated movement of the robot and the positioner is utilised: how to always hold the torch in desired pushing or pulling angle and how to maintain the standard length of the cord?

AN OFFLINE PROGRAMMING SYSTEM IN WELDING QUALITY MANAGEMENT

Changing the torch angle and the free cord length has an effect on the quality in robot welding. In programming by teaching and especially in welding curved welds, torch angles and the cord length are determined by the fact how precisely the robot operator can estimate the torch’s positions and distances in relation to the weld groove.

Picture (courtesy of Rolan Robotics). A body frame of a loader contains a lot of curved welds. The gantry robot installation may include more than 11 synchronously controlled axes.

Especially in welding in any other than standing position, changes in the tilt and movement angles of the torch during welding naturally affect the position of the weld in relation to the groove, the form and penetration of the weld and the smooth connection to the element. Variations in the distance of the nozzle have a known effect on the welding flow. An incorrect distance can result either torch through or a short penetration and an increased amount of spatter. The standardising of the above-mentioned parameters is of significant importance especially in welding demanding joints, in which the fitting precision of the groove and penetration management are critical.

Picture. The torch angle remains desired through the whole length of the welding groove (Delfoi ARC).

Instead, in parametric offline programming quality management is supported by the software. Quality has been recorded in a WPS (Welding Procedure Specification) file/database as weld type-specific objects, in other words as an arc welding method library. The weld type-specific torch angle and the length of the cord are set as desired in the software. Whether the torch moves along a straight or an curved groove, the length of the cord and the torch angle remain the same during the whole length of the weld. Welding values are also recorded in the same WPS file or database. Combined with groove search and follow-up, if possible, depending on the structure of the workpiece, the welding quality achieved in robot welding can be standardised in a very reliable manner. In the software welding points are not created by teaching and entering welding values, like in manual programming, but teach points and their search functions are created as weld type-specific groups. Therefore offline programming is also fast. The welding robot operator chooses a method from the menu and shows on a screen a weld to which the method is to be applied. This is repeated until all the welds have been gone through in desired order. The software itself does not decide the correct welding order or the most suitable welding direction, so here the expertise of the operator comes into the picture.

Picture. The desired length of the cord remains standard through the whole length of the weld (Delfoi ARC, OTC Daihen).

MAINTAINING WELDING EXPERTISE AND QUALITY IN A COMPANY

Offline programming utilising a WPS file is one factor in enabling constant and high quality in robot welding. Weld type-specific values recorded on a file are determined by a welding expert or an experienced welding robot operator. When welding quality is recoded in the system, same methods can be used also by less experienced operators. Maintaining welding quality as standard and good is thus less dependent on the operator than in programming by teaching. In many companies an experienced welder is the guarantee of quality, and this is the way it should be. However, the best quality is often lost if the experienced worker leaves the company. Instead, if quality is recorded in a system, a less experienced worker can also take the place of the more experienced one and take advantage of the quality that the expert has recorded in a WPS file.

23.05.2015

Heikki Aalto, Delfoi Ltd.
Olli-Pekka Holamo, Kemppi Ltd.