Plunging to new depths

Take the raw power of 1,200 Porsche 911 superfast cars and you have the capacity of a single SGT5-8000H gas turbine from Siemens, which weighs the equivalent of a fully fuelled Airbus A380.

Today the SGT5-8000H is one of the largest and most powerful gas turbines in the world, built to profit from the underlying trend of the global gas market, which according to some forecasts is growing by more than 6 percent annually.

These huge components are manufactured in Berlin, Germany, in the Siemens gas turbine plant, built in 1909 in a neoclassical style with glass-and-steel walls some 120 metres long and 25 metres tall. Since 1956 the plant, part of the Siemens Power Generation Division, has been classified as a historical building. The machinery and the technologies it houses are top-notch and feature state-of-the-art machining centres.

In 2012, when Siemens introduced a new maxi housing for its huge gas turbine, the plant had to adapt its technology. The established production process, based on rough milling and finish boring using angle heads and helical milling, was not stable enough for the 16 borings with a 600-millimetre diameter that were required for the burners.

“We had huge problems,” says Markus Zapke, head of technology, tooling processes and machine supply in the turbine factory. “The angle head was the weak part in the process. We had too much vibration and tools breaking down. We had reached the limit of our installed technology.”
Sandvik Coromant was contacted, along with other suppliers, and in August 2013 the factory chose the Sandvik Coromant concept, which was based on plunge milling and a new spindle strategy. “Sandvik Coromant has been a technology partner for many years,” says Zapke. “They were able to draw up a potential solution to our problems almost immediately.”

Olaf Zahn, head of regional sales and technical consulting for Sandvik Coromant, is the man behind the new concept. “We knew that the angle head was very unstable,” he recalls. “So we opted for plunge milling, which means that the tools plunge straight into the precast borings in the housing.”
Plunge milling itself is nothing new. “I have seen this technology for 20 years in the automotive industry,” says Herbert Imrich, team leader for production in the Siemens plant. “It’s a wonder nobody thought of it before, or at least when the problems arose.”

In fact, Sandvik Coromant did: Plunge milling is the first step in the new production process. As plunge milling automatically leads to rough surfaces, it requires finish boring in a second step to meet high surface demands and maintain the required tolerances. In this final step the diameter of the borings is increased from 598 to 600 millimetres. The new concept was tested and further developed in close cooperation between Siemens and Sandvik Coromant.

The testing was done in a real-life situation. Thomas Reich, Siemens expert for technology, cutting tools, explains: “Since our old technology could not cope with the new requirements, we wanted to go ahead with the new process without any delays. So we made all the necessary adaptations and new programming in real production.”
The results so far are extremely positive. Plunge milling reduced the machining time from 100 hours per component to 42. The time for finish boring was cut from 480 minutes to 160 minutes. In total there is a time reduction of more than 63 hours for each housing – or 11 weeks of machining time saved in three years of production, according to Siemens calculations.

Further positive effects are obvious but difficult to put figures on. “We are very pleased with the fact that we are able to use standard tools all along,” says Zapke. “That means lower prices, high reliability and tools always in stock.”
Most important, though, the Sandvik Coromant solution lends itself to further adaptation. “Plunge milling can easily be transferred to other processes,” says Christian Lendowski, Sandvik Coromant senior key account manager for Siemens. “It has already been carried over to another Siemens housing. And more projects are envisaged. In that sense it is really a model technology.”

Techical insight

Plunge milling is key to the new Sandvik Coromant process at the Siemens gas turbine factory in Berlin, Germany. The housings for the world’s biggest gas turbines have 16 borings for the burners, which are precast with a tolerance of 12 to 15 millimetres. When finished the borings have a
diameter of 600 millimetres and a length of 450 millimetres.

Sandvik Coromant uses CoroMill 210 for plunge milling. Some 430 millimetres long, the tool is plunged 300 millimetres straight into the precast boring, avoiding the high leverage on the angle head in the former process. The tool plunges up and down in a circle, 126 times for each boring. For this rough milling the machine time for each component has been reduced to 42 hours from 100 hours using angle head and helical milling. In the next step the diameter of the borings has been increased from 598 millimetres to 600 millimetres by finishing the surface. This is done with the Sandvik Coromant CoroBore 820 XL, which allows increased feeds compared with fine boring tools. At the same time it maintains the required tolerance and surface demands.

In the implementation phase a new tool-clamping concept was introduced. Reducing the setups from three to one was another very positive side effect. Compared with the old process, the new concept saves 320 minutes per housing. Moreover, using standard tools off the shelf helps keep tool prices low and guarantees a stable supply of reliable tools. Altogether the new concept, developed in close cooperation between Siemens and Sandvik Coromant at the gas turbine plant in Berlin, produces a stable process with significant time savings. Siemens calculates that total machine time is reduced by more than 63 hours per component.