Hydrostatic bearings increase performance in titanium machining

A little push – and the 12-metric-ton column accelerates on the machine bed without any further action. The STC 1250 HD 5-axis machining center, with its horizontal linear axes having hydrostatic bearings for frictionless gliding, gives rise to significantly higher stock removal rates, improved dynamics, and longer tool life.

Starrag machines have been setting standards in the economical machining of structural components, multiblades, and casings, as required in the aerospace industry. The STC series is designed 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 robust gear spindle and a compact swivel milling head, enabling use of shorter, more stable tools for more efficient machining.

To further increase efficiency in titanium machining, heavy machining specialists have performed a 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 tool wear, and therefore on the economic efficiency.”

Hydrostatics increase performance

When it comes to vibration damping, hydrostatic guides can’t be topped. “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 customer demands and the lessons we’ve learned, we wanted to change it back,” explains Rainer Hungerbühler, sales director for aerospace and turbines. So, Starrag developers re-addressed the topic of hydrostatics. “Our new STC 1250 HD is fantastic. The first tests have already shown 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,” Hungerbühler says.

A look behind the scenes

Rolando Senn, lead designer in hydrostatics, 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 there’s no contact between the guide rails and the carriage travels completely without friction.”

Without friction, there’s no wear, and the oil dampens the vibrations occurring during machining. The large contact surfaces also ensure tremendous rigidity. “We’ve increased this even further by choosing one with pre-tensioned handle guides instead of a statically open system,” Senn explains.

To classify these strengths, it’s necessary to look at the essential characteristics of other guide systems. Hydrodynamic guides have 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 must 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 aren’t as prone to friction and wear, but they’re also less rigid and damping. In addition, micro-vibrations occur during the rolling process, which can impair surface quality during finishing operations.

Triple chip volume during roughing

The user derives the greatest advantages from using hydrostatic guides in roughing operations. Due to the significantly higher rigidity and damping compared to roller guides, cutting depths can be increased 3x with roughing times reduced accordingly.

The damping effect makes the chipping barely noticeable. During a test machining operation, even experienced machinists were surprised, 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 the cutting process had already started; the milling cutter was in full engagement and really thick titanium chips were flying.”

The STC 1250 HD also benefits from the hydrostatic bearing during finishing. The increased rigidity reduces pitching of the column, which limits jerking and acceleration. “Instead of using a 2.5" (63mm) diameter ball screw as before, we now use one with a 3.1" (80mm) diameter on the HD. That means a 61% larger cross-sectional area, which is incorporated 1:1 into the rigidity in the axial direction,” Senn says.

The higher dynamics are noticeable in higher starting speeds, accelerations, and faster 5-axis movements, ultimately reducing finishing times.

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 improvements in this respect, i.e. significantly longer service lives, even when the speeds are increased.

The frictionless movement of the two linear axes and absence of micro-vibrations also benefit 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.

True values show themselves

Using the example of a typical structural component, a framed door with dimensions of 3.1" x 11.8" x 48" (80mm x 300mm x 1,220mm), Starrag made a comparison with the previous STC 1250. The 81.8 lb (37.1kg) forged part made of Ti6Al4V had a 0.5" (13mm) allowance on each side and had to be machined to 12.1 lb (5.5kg). 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 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 1/3 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, it consumes 33% less energy.

Minimal maintenance required

Due to the lack of friction, the hydrostatic system’s guide rails don’t wear out, whereas roller guides must be replaced every 5 to 10 years. In addition, the entire hydraulic system is easy to maintain. 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" (0.02mm to 0.03mm) gap filled with oil between the bed and the slide. The oil permanently escaping 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.

Undesired mixing of the hydrostatic oil with other oils is avoided, 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 wouldn’t cause any damage. On the contrary, if, in 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 associated hydrostatic circuit from dirt, water, and chips 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’s significantly softer than the machine bed and can therefore absorb most of the energy. These pockets are easy to replace, limiting damage in the event of minor accidents.

What about the costs?

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