Downsizing makes engines more economical. In order to ensure that their performance is not compromised, turbochargers are increasingly being used to compensate in smaller vehicles. The market is growing – but so is the pressure on prices.
Roughing and finishing turbine housingsis particularly challenging
in the case of passenger cars with spark-ignition engines.
Vehicle fuel consumption needs to be reduced – not just in the lab. Legal provisions from all over the world are driving the automotive industry to implement measures in almost every vehicle class. This has given rise to huge challenges for the automotive industry. An important factor: Turbochargers that squeeze high performance out of small yet efficient engines. However, the turbochargers themselves are also under pressure to be smaller, more efficient and, importantly, more cost-efficient.
Reducing Machining Costs
“We expect – and studies by leading turbocharger manufacturers agree – that the number of turbochargers used for petrol engines will experience a 2.5-fold increase over the next five years,” says Rolf Buob, component manager for turbine housings at Walter AG in Tübingen.
Currently, turbochargers for petrol engines place particularly high demands on machining when compared with diesel engines. The exhaust gases in the turbine housing reach temperatures of between 1,000 and 1,050 °C; in diesel engines, however, they reach relatively low temperatures of between 800 and 850 °C.
“Temperatures of 1,000 °C or higher require high temperature-resistant steels, typically chrome-nickel alloy steels that have a material identification number beginning with 1.48 and ending in 49, 48, 37 or 26 – with a tendency towards the material identification number 1.4826. These 1.48 steels are constantly being developed further and it is becoming more and more difficult to machine them,” explains Buob.
They also make the turbine housing the most expensive component in terms of machining. Buob explains further: “We anticipate different machining costs for each component, depending on the presence of an exhaust manifold.” Above all else, a high chrome content reduces service life. “There are applications where tools only last long enough for twenty to thirty components.” For comparison: The materials used for diesel engine turbine housings extend the service life by up to five or ten times, while also being 50 percent faster to machine.
Walter has developed a new milling cutter system to meet the
specific requirements of thin-walled turbine housings.
Walter machining experts have therefore developed a new milling cutter concept especially for roughing, semi-finishing and finishing turbocharger housings. It reduces the all-important cost per finished part, while also significantly improving surface quality. Over the course of the development process, the cartridge system used for finishing, which had previously been the norm, was replaced with an intentionally simple tool design with a fixed insert seat.
This “plug-and-play” solution eliminates the need to carry out presetting operations, which required accuracy to between 3 and 5 µm.
Further saving measures: The use of identical indexable inserts with 16 cutting edges as semi-finishing and finishing inserts. Previously, the market standard was to use 12 for semi-finishing and four for finishing. This also simplifies inventories and eliminates errors when changing the indexable inserts. The indexable inserts are coated with PVD or CVD and are available in various geometries.
Indexable inserts belonging to the Walter WSP45S or WSM45X grades are typically used. Short cutting edges reduce the pressure on the unstable components. This results in reduced vibration, which improves surface quality, increasing it from the usual Rz 7-8 to approximately Rz 5. “Overall, these measures lead to improvements in handling and process reliability,” says Buob.
As a rule, every third to fifth insert is positioned differently on the finishing tool. The position of the semi-finishing inserts can be adjusted by approximately 5° in the same way as when carrying out rough machining; the finishing inserts are inserted so as to cut in a flat plane.
Rolf Buob explains: “This is why we distinguish between semi-finishing and finishing inserts on the same tool, even when the inserts are identical. Only the insert seats are rotated differently when inserted.”
The cutting speed when finishing is approximately 140 m/min at a feed rate per revolution of up to 4 mm. This milling cutter is also available for machining allowances of up to 3 mm for roughing in particular. Here, the indexable inserts are aligned uniformly both axially and radially, in contrast to the finishing face mill. They all have the same function for machining operations.
However, the new milling cutter concept does not mean that development has finished. Walter is expected to make further advances involving new PVD coatings that are currently still in development.
APMEN Cutting Tools, Oct 2016