Graebener® is a medium-sized, family owned machine building company with locations in Netphen (Germany), Houston (USA) and Shanghai (China) as well as a variety of international representatives. Based on 100 years of experience in the metal processing business we develop and realize customized solutions for the core markets of automotive industries, hydroforming, manufacturing of large pipes, shipbuilding, wind tower production, vessel construction and renewable energies. Our product range includes hydraulic forming lines, milling and bending machines, calibration and straightening presses as well panel production lines. Graebener® stands for a holistic approach. As technology and system partner we accompany our customers along the entire process chain – from the initial idea, comprehensive engineering (including hydraulics, automation and control technology as well as intelligent Industry 4.0 applications) up to customized new machines or retrofitted existing lines.
Circular and longitudinal seam milling machines by Graebener® and INGERSOLL
Graebener® developed its first milling machines for the preparation of weld seams on pressure vessels already in the 1980s, initially only for use in its own large-diameter pipe mill. In cooperation with INGERSOLL WERKZEUGE GMBH, the machines became products for the world market, which today are in great demand, among other things, especially for the construction of offshore wind towers and their foundation structures.
Graebener® looks back on a company history of more than 100 years. When the company was founded in 1921, the family business began with pipe and apparatus construction. Until 1955, the focus was on the production of heavy iron and sheet metal structures including steel and vessel construction. Subsequently, a large pipe mill was built and established in Netphen-Werthenbach in the Siegerland region, which was then sold to Indonesia at the end of the 1990s.
Since then, Gräbener Maschinentechnik GmbH & Co. KG has concentrated entirely on mechanical and plant engineering. The company, which currently employs around 80 people, focuses on the development and design of hydraulics, mechanics and automation as well as on the assembly and commissioning of new plants and retrofits. The necessary machine components, such as steel structures as well as all other parts, are purchased from suppliers mainly in the regional area. Therefore, Graebener® sees itself as a "virtual factory", as it does not operate its own facilities for component production.
The past is nevertheless present at the company headquarters. On the one hand, production still takes place in the historic halls. On the other hand, the experience built up here over decades and the constant improvements form the basis for the company's current business success.
Joint development from the beginning
The construction of the company's own circular and longitudinal seam milling machines was started in the mid-1980s out of a serious problem. The pipes manufactured in Werthenbach were becoming thicker and thicker and were increasingly causing transport problems. The pipes, which were up to 70 meters long and weighed up to 70 tons, therefore had to be finally welded together on the construction sites. The idea of a mobile milling machine was born, with the help of which the weld seam preparation on the pipes could be carried out on site.
For the milling machines, Graebener® worked together with INGERSOLL from day one. For machining the tubes, milling heads were jointly developed for circular and longitudinal seam milling, but also for other fields of application, such as weld seam preparation on sheet metal and profiles. At the beginning, they had to contend with considerable difficulties, as Dieter Kapp, Managing Director of Graebener® reports: "The first milling heads only lasted a few days and our gearboxes were damaged just as quickly, as there was no experience for this new machining method. "
But Graebener® and INGERSOLL stuck to the concept and pursued continuous further development of the machines and milling tools over the years. A few years after the first milling machines for internal use, the machines became durable, powerful, process reliable and thus competitive. McDermott in Scotland and SIF from the Netherlands, at that time two market leaders for thick-walled pipes, vessels and apparatus, were the first two customers for Graebener®. Today, SIF is one of the leading manufacturers of, among other things, foundation structures for offshore wind turbines.
Market leader in weld seam preparation
The weld seam preparation machines manufactured by Graebener® were the first of their kind. With over 200 milling machines delivered within the past 25 years, Graebener® is the world market leader. About 150 of these machines are dedicated to the milling of weld seam preparations for circumferential and longitudinal welds on cylindrical and conical components with diameters of up to 15 m and wall thicknesses of up to 200 mm. In the last few years, more than two dozen circumferential seam milling machines and half as many longitudinal seam milling machines have been sold to the wind offshore industry.
In the meantime, all well-known manufacturers of foundation structures for wind turbines in the onshore and offshore sector use milling machines from Graebener®. The offshore market in particular is experiencing a sustained boom and is providing Graebener® with high demand. Wind power is to be used for green energy in more and more parts of the world. The machine manufacturer has already responded to a new market developing in the USA with its own subsidiary in Texas, where a transportable circular seam milling machine is available as a demonstration unit. New wind farms are also being built off the coast of Japan. Here, a manufacturer of monopiles wants to carry out the complete assembly process outdoors rather than in a hall. To meet this requirement, Graebener® is developing machines that are also suitable for outdoor use.
The monopiles for wind towers are always assembled from several individual cylindrical and conical steel rings, the so-called pipe shells. Typical dimensions for such a pipe shell are a diameter of 13 meters and maximum lengths of 4 meters. With wall thicknesses of 120 mm, this results in an individual weight of up to 150 tons per pipe shell. After bending, the pipe shells are first welded longitudinally from the inside and then from the outside. This is followed by the assembly of up to 6 sections into one section. 3 to 4 of these assembled sections form the so-called monopile. A monopile has a total of approx. 192 m longitudinal and 940 m circumferential welds and a total weight of up to 2,500 tons.
The milling machines from Graebener® enable very efficient, economical preparation of the external weld seams. The process is fast, material-saving and precise. For the external weld seam preparation it is necessary to cut out the root area of the internal weld. This was traditionally done with a so-called air gouge by spark erosion and manual grinding. With the Graebener® longitudinal and circular seam milling machine and the INGERSOLL side milling cutter, a tulip-shaped groove is milled right into the root of the inner weld seam. The angle of this external weld seam preparation, which was conventionally approx. 60°, is reduced to 16° by the milling process using a side milling cutter as standard. This reduces the welding consumables and the relating welding time. The total cost saving for this weld seam preparation with Graebener® machines is 38% compared to conventional processing. In several cycles, the cutter penetrates up to 150 mm deep to the root layer of the inner seam. The clean, uniform groove, which requires no further finishing, is the prerequisite for an automated submerged-arc welding process. The milling process is also 10 times faster than conventional gouging. The process is also well suited for repair work on the weld seams in order to mill out local defects.
For circular and longitudinal seam milling INGERSOLL supplies V-shaped side milling cutters with radius R7/R8 in the radius section and opening angles from 14° to 16 °. In addition, customized side milling cutters are also available. The diameters of these large tools are mainly between 850 mm and 1060 mm. The side milling cutters are equipped with different milling tips for the radius and sides of the tool. Generally, these are milling tips with negative cutting geometry. The radius milling tips are mounted in a hardened and replaceable L-insert due to the extremely high load. This is not needed for the side milling tips, since the side angle results in a lower load.
Vibrations are a challenge
Depending on the component, vibrations occur on the pipe body during machining. This is a major problem, because these can cause chipping on the milling tips and subsequently lead to wear. When designing the machine, Graebener® therefore takes care to match the milling head and gearbox to the expected vibration behavior. The bigger problem, however, are the huge workpieces. Graebener® draws on all its experience to offer the optimum solution for the customer's particular machining situation.
On the one hand, the pipe turning device has a major influence on the vibration behavior of the components, but on the other hand, the diameter and wall thickness of the tubes also play a significant role. Thin-walled sections in particular tend to vibrate during machining. In order to avoid milling tips chipping in this process, INGERSOLL uses milling tips with a particularly durable base substrate, which is given the necessary wear resistance by an appropriate coating.
Additionally, the pipe shells have a certain ovality. In order to compensate for this during circular seam milling and still achieve a constant milling depth, the milling depth is automatically corrected with the help of sensors. Lateral drift of the workpiece on the pipe turning device is compensated for the circular seam milling machines by means of a sideways adjustment device for the milling head.
Customized manufactured plants
The machines built by Graebener® are standardized and can be specifically adapted according to customer requirements. The main focus is to bring the machine into the ideal working position depending on the respective production environment. Christian Landau, Sales Manager at Graebener®, outlines the possibilities: "For example, our machines can be moved on rails or brought to the appropriate milling height with various substructures. A typical example is monopiles with different pipe diameters and cone transitions."
A selection of basic machines in the power ranges between 30 kW and 65 kW, which are suitable for different milling depths, serves as the basis for these customized solutions. "We have designed the power of our machines in a way that we can run optimal cutting speeds for the respective application," explains Dieter Kapp. "In doing so, we pay a lot of attention to the optimum torque on the milling spindle."
Meanwhile, there are new development goals for Graebener® and INGERSOLL: machines and tools optimized for machining demanding materials. They are not used for monopiles, but in other industrial sectors in which Graebener® is active. In pressure vessel, tank and special pipe construction, Graebener® milling machines also machine materials such as stainless steels, high-strength aluminium alloys and special steel grades.
One challenge is the range of different material compositions with different properties. "There is no standardized solution for all materials," says Andreas Bulla, Product Manager Tangential Tools/Thread Milling at INGERSOLL, who explains the strategy. "In order to reduce the heat at the edge of the milling tip as much as possible, we optimize the cutting geometry, for example. Additional grooves in the chip area minimize the material-related sticking of chips to the cutting edge and reduce frictional heat by minimizing the contact surfaces."
Depending on the material, INGERSOLL uses different geometries and qualities for the milling tips. In contrast to the machining of steel pipes, positive cutting geometries are preferably selected for the machining of stainless steel, both for the radius milling tips and the side milling tips.
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