Machining titanium, molybdenum and other hard-to-cut materials

Article from ARNO Werkzeuge

Many materials appear almost impossible to machine. A component with a combination of materials represents a special challenge. High-positive indexable inserts from ARNO Werkzeuge achieve convincing process reliability and finish quality in turning operations. Siemens Healthcare GmbH is delighted with a tenfold increase in tool life.

“If you are X-rayed anywhere in the world on a Siemens machine or undergo a computer tomography or a mammography, you can be sure that the equipment contains components from Rudolstadt,” says Martin Andris. He is factory manager at the Siemens Healthcare GmbH (Siemens) location in Rudolstadt, Germany, a site which has a tradition dating back almost 100 years. The site manufactures equipment for the medical engineering sector, in particular radiating sources such as X-ray tube units for medical applications, radiating sources for industrial applications used in materials testing and high-energy radiating sources for screening containers.

X-rays are generated in vacuum vessels with a number of components made of a variety of materials. Some of these components are manufactured using precision tools made by the Swabian manufacturer Karl-Heinz Arnold GmbH (ARNO Werkzeuge). Tools from ARNO are mainly used to machine difficult-to-cut materials, such as molybdenum. The molybdenum that Siemens uses is in the form of sintered metal, which has high hardness and a high melting point of over 1, 000° C. In addition to these properties, the highly abrasive behavior of the material makes machining even more difficult. These conditions lead to high tool wear and place considerable demands on cost-effectiveness. Cost-effectiveness is what ARNO Werkzeuge has offered for many years.

^{From left to right: Martin Andris, factory manager at Siemens Healthcare GmbH Rudolstadt, and André Scharmer, field representative at ARNO Werkzeuge. Photos courtesy of ARNO Werkzeuge.}

“When the tube factory was relocated from Erlangen to Rudolstadt in the middle of the 1990ies, tools from ARNO moved with the factory. That’s how far back our joint history goes,” reminisces Andris. “Of course, since then the number of parts and turnover have increased many times over. At the same time, the tools used for machining must meet much greater requirements today.”

Besides materials that are difficult to cut, machining also faces a special challenge when it comes to vacuum components with tight tolerances. Although tolerances are maintained by grinding the components in the final step, ARNO tools attempt to approach as close as possible to low tolerances of a few hundredths of a millimeter to shorten total machining time. That is not so simple given the type of materials used.

“Any surface unevenness can disturb the electrical fields in the X-ray tube and cause voltage peaks, which have a negative impact on image results or, in the worst case, make them unusable,” explains Andris. Tools are changed frequently to ensure the constant high quality of results. This makes them a vital economic component which requires regular monitoring.

“We are constantly developing and learning from each other,” says André Scharmer, field representative at ARNO Werkzeuge. “This is how we can always achieve increases in tool life with every new coating, geometry, rake angle and rounding.”

^{The high-positive VCGT indexable inserts with ALU geometry from ARNO Werkzeuge ensure optimal process reliability when machining.}

Highly positive in production

All high-precision parts that are critical for dimensional accuracy (tolerances of ±1 µm) are turned using high-positive indexable inserts from ARNO Werkzeuge. The manufacturer offers an extensive portfolio of high-positive indexable inserts. For ARNO Werkzeuge, “high positive” means indexable inserts with a rake angle of 25°. They are periphery ground with polished chipbreakers and thus have a sharp cutting edge, which means they only require minimal cutting forces. Accordingly the resulting finish quality is good and precision is excellent. When parts are turned at high precision, the subsequent grinding operations are much shorter, thus saving total machining time.

Roughness and finish quality play a vital role in vacuum parts, stresses Andy Jahn, production line manager at the Siemens location: “Any imprecision in surface finish can cause swirling, dirt deposits, flash-overs and even failure. Achieving the demanded machining quality with sintered molybdenum to meet the high requirements is an art in itself. The benchmark, so to speak.”

It took employees almost a year to find the right solution to handle tools, cutting speeds and feed rates. They started with tool lives of three to five parts. Today, the tools machine 50 parts using high-positive indexable inserts from ARNO Werkzeuge. The right tool must be found to match the process – and this is also the prerequisite for automation. When it comes to internal machining, a VCGT indexable insert with ALU geometry is used for undercutting operations in molybdenum.

Scharmer explains: “The insert must be extremely sharp and wear-resistant and in most cases uncoated since the coating would not last very long.” The demands placed on the indexable insert are high. “The grind must be precise, otherwise we cannot achieve the required results without having to rework.”

Another difficulty is that quality assurance can only inspect parts after they are assembled in the complete plain bearing. For this reason, a precise process sequence takes place between each individual machining and assembly step. Jahn is already thinking ahead: “In the age of Industry 4.0, we would like to reach the stage that tool lives allow machines to continue running in extra shifts.”

^{The variety of materials in the composite place different demands on the tool and make machining twice as challenging. Here is a part comprised of copper, molybdenum and stainless steel.}

Composites of copper, stainless steel and molybdenum

It is especially difficult to machine composites. The challenge here lies in the different properties of the materials which must be machined using only one tool for reasons of time and costs – on top of this, the finish must have the same first-class quality across all material layers. For example, the rotors for X-ray tubes are made of a composite made of oxygen-free copper, stainless steel X10-CR13n and molybdenum. The fit affects all materials in a range of 16 µm. For this task, Siemens uses high-positive indexable inserts from ARNO Werkzeuge for both internal turning and external roughing.

Siemens also uses specially designed indexable insert drills for drilling work. The materials machined include molybdenum, copper and titanium, as well as nickel. The various materials are previously joined together using specific high-temperature brazing technologies. The part could then receive a borehole of approximately 100 mm, for example. It is an enormous challenge to create a reliable process to produce a constant borehole through all material layers, including hard brazing materials, without requiring rework.

“In the beginning, we used drills from the competition but they were incapable of lasting longer than five parts. Then we had to change the insert,” said Karsten Raasch, programmer at Siemens Rudolstadt. “On top of that, we unfortunately sustained a large number of rejects and the machine crashed several times due to drill breakages!”

ARNO Werkzeuge tested and modified its SharkDrill² drill inserts with the result that vibrations were reduced and finish quality was improved. This quadrupled tool life! “It saves us enormous costs, not only tool costs but also changeover times,” says Jahn delightedly.

Scharmer added: “Our tool is reliable and this has drastically minimised reject costs.” If the time and costs caused by a crash are included (machine reset, the costs of servicing, maintenance and repairs), the sum quickly reaches a high five-digit Euro amount per year.

Of course, the increase in productivity is also a factor, in addition to automation. Here too, reliable tools are a basic prerequisite. Stability is vital. The aim is to lengthen tool life as much as possible so that nothing stands in the way of automation. “In that case, we will certainly have to integrate measuring instruments, but ultimately the prime factors are to achieve stable machine throughput and a reliable process,” says Jahn with a view to the future. Besides all these technical arguments in favour of tools from ARNO Werkzeuge, the price-performance ratio must also be right. “And it is,” stresses Jahn.

^{High-precision internal fits with tolerances in the micron range are difficult to manufacture. However, they are achievable through intensive collaboration with tool manufacturer ARNO.}

Honest and sustainable project management

Siemens attaches great importance to continuous monitoring and optimisation of the tools already in use. ARNO Werkzeuge is well known at Siemens for the successful planning and monitoring of new projects over long periods of time and of implementing them sustainably. “We had the unfortunate experience with other suppliers that the improvements they announced were not achieved in high-volume production runs. With ARNO Werkzeuge we are sure that they do not make any empty promises. We have a great deal of confidence here,” says Raasch.

“The response times to our enquires are excellent and tool availability is very good,” emphasises Jahn. "We want to reduce our safety stocks. And to do this, we need stable suppliers.”

The possibility of breaking new ground and integrating logistics processes, for example, is not taken for granted by either company, but it is part and parcel of an effective collaboration. Scharmer says: “Due to the many years of cooperation, we can deal openly with each other and we quickly reach a consensus on what solution would fit the best.”

^{The aim is to continue development in production in future and the next steps will be towards Industry 4.0. From left to right: André Scharmer, ARNO Werkzeuge, and Karsten Raasch and Andy Jahn from Siemens Healthcare.}

Market demands in medical engineering

Factory manager Andris risks a look at future market demands: “X-ray tubes with higher performance to supply images in greater detail, even in deeper lying tissue layers. These images could be used to obtain even better analyses. The demands placed on the components and their tolerances will then also rise.”

He considers that lathed surfaces that are indistinguishable from polished surfaces are absolutely possible. “To achieve this, we need a strong partner like ARNO Werkzeuge,” says Andris and summarises: “I’m very satisfied with the collaboration. Tasks are solved constructively. Above all, the quality, reliability and, ultimately, the price of the tools are right. They meet our high requirements and for us they are currently the best on the market.”