Smooth gliding, productive machining

Hydrostatic bearings bring about a leap in performance in titanium machining

A little push – and the twelve-tonne column accelerates on the machine bed without any further action. EMO visitors can experience this live at the Starrag exhibition booth. The machine manufacturer will be showing its newly developed STC 1250 HD 5-axis machining center there, with its horizontal linear axes that have hydrostatic bearings. That is where the frictionless gliding comes from, which gives rise to significantly higher stock removal rates, improved dynamics and longer tool life.

“The essential feature of hydrostatic guides is the permanent oil film between the slide and the bed or the standing structural part.”

The STC series is predestined for simultaneous 5-axis heavy machining of titanium and Inconel parts due to its excellent static and dynamic properties.

For many years now Starrag machines have been setting standards in the economical machining of structural components, multiblades and casings, as required in the aerospace industry and for power generation. The STC series is predestined for simultaneous 5-axis heavy machining of titanium and Inconel parts due to its excellent static and dynamic properties. The machines are equipped with Starrag’s own robust gear spindle and an extremely compact swivel milling head, which enables the use of shorter and therefore more stable tools, thus contributing to efficient machining. To further increase this efficiency in titanium machining, the heavy machining specialists at Starrag have carried out a wide range of studies. Bernhard Güntert, head of the test field at Starrag AG, explains: “The main sticking points are the machine’s rigidity and damping properties. These factors have far-reaching effects on the possible cutting depth and the tool wear, and therefore on the economic efficiency.”

Hydrostatics increase performance

The most important test result: when it comes to vibration damping, hydrostatic guides cannot be topped. This is an area in which Starrag already has experience. Rainer Hungerbühler, Sales Director for Aerospace and Turbines, takes a look back: “More than 15 years ago we already had hydrostatically guided machining centers. However, the dynamics of these machines left a lot to be desired, which is why we equipped the subsequent STC series with roller guides. Given today’s customer demands and the lessons we’ve learned, we wanted to change it back.” So the Starrag developers re-addressed the topic of hydrostatics and achieved an impressive result, as Rainer Hungerbühler enthusiastically confirms: “Our new STC 1250 HD is fantastic. The first tests have already shown that it sets a completely new benchmark in terms of stock removal rates during roughing. It even has considerable advantages to offer in terms of dynamics.”

A look behind the technical scenes

Rolando Senn is the lead designer in hydrostatics. He explains the strengths of this technique: “The essential feature of hydrostatic guides is the permanent oil film between the slide and the bed or the standing structural part. This means that there is no contact between the guide rails and the carriage travels completely without friction.” Owing to the lack of friction, there is no wear and the oil dampens the vibrations that occur during machining. The large contact surfaces also ensure tremendous rigidity.

“We have increased this even further by choosing one with pre-tensioned handle guides instead of a statically open system,” explains Rolando Senn. To be able to classify these strengths, it is necessary to look at the essential characteristics of other guide systems. Hydrodynamic guides, for example, have many similar properties to hydrostatic guides, but one major disadvantage: the guide elements come into contact when stationary. The separating oil film only forms when they are moving, so static friction has to be overcome when starting up, which causes the undesirable stick-slip effect. Pure sliding guides also exhibit a stick-slip effect and are additionally subject to high friction and corresponding wear. Roller guides are not as prone to friction and wear, but they are also less rigid and damping. In addition, micro-vibrations occur during the rolling process, which can impair the surface quality during finishing operations.

Triple chip volume during roughing

The user derives the greatest advantages from the use of hydrostatic guides in roughing operations. Due to the significantly higher rigidity and damping compared to roller guides, the cutting depths can be increased threefold. The roughing times are reduced accordingly.

The damping effect can be heard and felt – or in other words: the chipping is barely noticeable. During a test machining operation, even experienced machinists were surprised by this, as Rainer Hungerbühler reports: “We were standing right next to the machine and hardly noticed any noise or vibrations. Only a glance through the window showed us that the cutting process had already started; the milling cutter was in full engagement and really thick titanium chips were flying.” The new STC 1250 HD also benefits from the hydrostatic bearing during finishing. The increased rigidity reduces pitching of the column, which limits jerking and acceleration. “To support the dynamics, we have also made the work strands on the STC 1250 HD more robust,” adds Rolando Senn. “Instead of using a 2.5 in. (63 mm) diameter ball screw as before, we now use one with a 3.1 in. (80 mm) diameter on the HD. That means a 61% larger cross-sectional area, which is incorporated 1:1 into the rigidity in the axial direction.” The higher dynamics are noticeable in higher starting speeds, accelerations and faster five-axis movements. This ultimately also reduces the finishing times.

With the hydrostatic STC 1250 HD, it was possible to halve the roughing time and reduce the total machining time by a total of 30%.

Greatly reduced tool wear

One of the biggest cost factors in titanium machining are the tools, which are subject to heavy wear. The excellent damping provides enormous improvements in this respect, i.e. significantly longer service lives, even when the speeds are increased. According to Rolando Senn, this offers a huge savings potential: “With intensive machine use, the savings put the additional costs of hydrostatics into perspective.” The frictionless movement of the two linear axes and the absence of micro-vibrations also benefits accuracy and surface quality, particularly in 5-axis simultaneous machining with its frequent changes of direction. Starrag ensures further process optimization by equipping the hydrostatic guides with pressure and temperature sensors. Users can thus track the loads on the guides during the process and use the recorded data for further adjustments.

The true values show themselves in practice

Using the example of a typical structural component, a framed door with dimensions of 3.1 × 11.8 × 48 in. (80 × 300 × 1,220 mm), Starrag made a comparison with the previous STC 1250. The 81.8 lb (37.1 kg) forged part made of Ti6Al4V had a 0.5 in. (13 mm) allowance on each side and had to be machined to 12.1 lb (5.5 kg). The specification: a perfect surface with thin walls and floors. With the hydrostatic STC 1250 HD, it was possible to halve the roughing time and reduce the total machining time by a total of 30%. These values can be used to calculate another, initially unexpected advantage: the reduced energy consumption during roughing. This is because the halved roughing time saves the user – despite the one-third higher power consumption – half of the drive power previously required plus that of the basic load, which consists of the power consumption of the cooling lubricant system, hydraulics, pneumatics, cooling, etc. Overall, the exemplary component consumes 33% less energy.

Another plus point: the minimal maintenance required

Due to the lack of friction, the guide rails of the hydrostatic system do not wear out, whereas with roller guides they have to be replaced at great expense every five to ten years. In addition, the entire hydraulic system is extremely easy to maintain. Here is a brief look at the technology of hydrostatic bearing.

On the Starrag STC 1250 HD, the hydrostatic unit actively delivers the oil into the pressure pockets of the guide plates at a pressure of 80 bar. A dynamic pressure builds up there, which creates a 0.0008 to 0.0012 in. (0.02 to 0.03 mm) gap filled with oil between the bed and the slide. The oil that permanently escapes at the guide edge flows back along the bed into a collection container. Another pump delivers it back to the hydrostatic unit via a changeover filter. The undesired mixing of the hydrostatic oil with other oils is simply avoided by Starrag. Rolando Senn explains: “We also use the hydrostatic oil to lubricate the turntable and other bearings. Even if small amounts of oil were lost from these components, they would end up in the hydrostatic tank, where they would not cause any damage. On the contrary. If, over time, the amount increases slightly, you can drain some oil and use it to dispose of impurities collected at the drain plug.”

To prevent service and maintenance measures and ensure safe processes, Starrag protects the guide rails and the associated hydrostatic circuit from dirt, water and chips in two ways: with telescopic steel covers and bellows underneath.

Starrag has also thought ahead for the worst case scenario, a crash, and defined an initial wear point. The pressure pockets installed in the guides are made of a material that is significantly softer than the machine bed and can therefore absorb most of the energy. As these pockets are easy to replace, the damage is limited in the event of minor accidents.

“We were standing right next to the machine and hardly noticed any noise or vibrations.” Rainer Hungerbühler, Sales Director Aerospace and Turbine

And what about the costs?

The hydrostatic guide is more expensive than a roller guide. Of course, this is reflected in the costs when buying a machine. But in the end, it is the unit costs that count. Due to shorter processing times, the higher purchase price is usually amortised in a relatively short time.