Joint preparation. New ways to increase precision.

Precise joint preparation is the key requirement for the production of reliable and load-bearing weld seams. Depending on the sheet thickness, material and joint geometry, various processes are used, each with their own advantages and disadvantages.

Many modern designs are based on steel materials. From machine building with low to medium sheet thicknesses between 1 and 40 mm through conventional steel construction to the relatively new industry of wind power plant production, in which sheet thicknesses of more than 100 mm are often used, all users are concerned with the professional connection of the individual components. Welded joints are the most economical way to achieve that. To produce a reliable and load-bearing weld seam, precise preparation of the joint between the components is the key requirement.

Saving material and weight

Welding bonds components together. The welding joint between the components must be executed properly and is based on the welding process and the desired joint geometry.

A fundamental distinction is made between butt and fillet welds.

Fillet welds require no joint preparation and are often used for components perpendicular to each other. They interrupt and divert the flux of force, which means they are only suitable for load conditions with small to medium forces.

Butt welds make it possible to connect the components over their entire cross-section. Their notching effect is comparatively low, so they are also suitable for dynamic loading. Users report substantial material and weight savings in their designs after introducing professional machines for weld edge and joint preparation, which enables them to produce more rigid joints between components.

Metalworking generally involves the use of arc welding processes, which require a V, K or X-shaped welding joint to produce a butt weld. This joint is filled with weld metal to produce the fused bond between the components.

A fundamental quality criterion of a butt weld is full penetration welding, i.e. the fusion of the joint edges on the underside of the sheet. This can be assisted by a defined root gap. Another important parameter is the joint opening angle. Like the root gap, this should be as small as possible, to save weld metal, time and cost while also minimising distortion. On the other hand, both must be large enough to ensure the fusion of the joint edges and a reliable welded joint. Typical values for the root gap range from 1 to 3 mm, with those of the joint opening angle between 30 and 60°.

Joint preparation on just one mating part reduces costs and produces single-bevel or double-bevel groove welds. However, this increases the risk of incomplete fusion – particularly with metal active gas welding – due to poor accessibility.

Comparison of oxy-fuel and plasma cutting

Manual joint preparation can still be found at companies without series production components. In this case, the individual components are manually prepared with the required weld edge, either by means of machining or with a thermal cutting torch. For series components or large production volumes, mechanised thermal cutting processes such as oxy-fuel and plasma cutting are increasingly used for joint preparation for economic reasons.

In geometric terms, the tool for oxy-fuel flame cutting is a beam. This makes it possible to produce any ruled surface. It creates a parallel kerf without groove trailing. The kerf tolerance can be kept within a range of a few tenths of a millimetre. Cutting length is virtually unlimited and bevel angles of up to 60° are possible.

Oxy-fuel flame cutting is preferred in the case of thicker sheets, where joint preparation requires a butt weld as a double-bevel groove weld with an upper and lower bevel as well as a root face. The advantage of the process lies in the small size of the single torch, which allows three torches to be combined in a three-torch VBA (VBA = variable bevel angle). The three-torch VBA features variable angle and lateral adjustment (adjustment of lateral inclination) of the torches and can be rotated in the centre. With the aid of this aggregate, joint preparation can be completed in one work step with the separating cut and is particularly economical. The range of application extends to sheets up to 120 mm thick. The disadvantages of oxy-fuel flame cutting lie in its limitation to non-alloy and low-alloy steels as well as the high heat input into the component.

The advantage of plasma cutting lies in the variety of materials that can be cut. This cutting process makes it possible to cut high-alloy steels, aluminium and other conductive metals. The bevel angle is limited to 45°. The kerf is typically wedge-shaped, dependent on the condition of the wear parts and always has a groove trail. The kerf tolerance and therefore the component tolerance is two-to-three times greater than with oxy-fuel flame cutting.

The plasma cutting process is used for weld edge and joint preparation in conjunction with the plasma VBA, which can tilt the plasma torch at the required cutting angle and rotate it around the central point. This makes it possible to produce any bevel angle in all directions of the component.

This process is particularly suitable for producing V weld joints, because it allows the separating cut and joint preparation to be performed in one work step. At the same time, it should be noted that tolerances of the distance between the component and nozzle tip will affect the cutting quality and the dimensions of the cut component as a result of the process, so highly effective distance control is required.

Robots: cheap, universal and robust

A small jointed-arm industrial robot can be used to guide the cutting torch. The robot program is written offline at the office and the robot is used like an NC machine. In the workshop, the robot is simply moved manually to the starting point and the program is selected and started. The operator needs no knowledge of programming and little understanding of industrial robots, but must be an expert in thermal cutting.

The robot's working area is comparatively small, because it is split into subspaces that cannot be left during the robot's path movement. On a conventional robot, the working area to allow +/-45° orientation of the tool in all directions is smaller than 500 mm × 500 mm × 500 mm.

Robots are cheap, universal and robust, but also comparatively inaccurate, problematic in terms of safety and more difficult to operate than a CNC machine.

The working area is significantly larger and production accuracy is far higher if a gantry-type flame cutting machine is fitted with a bevel aggregate. The VBA EXPERT PRO aggregate, for example, is tailored to medium plasma currents. The distance is not controlled contactlessly but with a drag foot, which allows particularly high accuracy. The plasma VBA works according to the same principle but, unlike the VBA EXPERT PRO, it is endlessly rotating and designed for very high plasma currents of up to 750 A. These aggregates are used in steel construction and shipbuilding for sheets mostly between 5 and 30 mm thick, which are finished with a V joint preparation for further processing.

In contrast, the VBA Wrist is designed for cutting three-dimensional components with a sheet thickness of up to 30 mm in the plasma process and is particularly slim. Using the TubeCut and DomeCut programming systems makes it possible to finish tubes and tank bottoms with all kinds of intersecting shapes, cut-outs and markings, each combinable with a wide variety of joint shapes. The software is designed so that no special programming or CNC knowledge is required and any skilled worker can work productively with it after one day at the most.

The COLUMBUS™ software is used for programming two-dimensional, plate-shaped components with or without bevels and with simple or composite joint shapes. Building on a basic version, it offers a wide variety of separate modules that customers can combine according to their individual needs.

With its casing, cooling and special kinematic design, the VBA Excavator bevel aggregate for oxy-fuel flame cutting is specifically adapted to the needs of high-power flame cutting with a cutting length of up to 500 mm and very large bevel angles. For example, this aggregate can simplify the production of abrasion-prone bucket wheel excavator teeth for open-cast mining in a single work step, thus eliminating the need for laborious and costly reworking with machining processes.