Thermal Effects

The most significant and often underestimated cause of machining deviations remains thermal effects, particularly temperature fluctuations at the machine’s installation site. If a customer ensures ideal conditions with high temperature stability, this directly improves machining accuracy. However, achieving such stability requires complex climate control systems, which are energy-intensive and increasingly out of step with today’s sustainability goals. Therefore, Röders prioritized designing machines that remain geometrically stable and precise even under non-ideal temperature conditions.

For this development, Röders created specialized climate-controlled rooms, each housing a single machine. The temperature control system for these rooms was quickly developed in-house based on existing expertise. A simplified Röders control system directly regulates a standard climate unit, allowing for highly consistent temperatures. This setup enables engineers to evaluate what is achievable under optimal conditions while also simulating various temperature profiles to analyze machine behavior, particularly the shifting of reference points.

Any expert understands that a machine with perfectly temperature-controlled components is inherently resistant to thermal fluctuations. However, Röders’ engineers knew that this was not a feasible solution due to the excessive costs and effort involved. Instead, they developed a tiered system with three configuration levels, rigorously testing machines in climate-controlled environments over extended periods under different temperature variations. Depending on customer requirements, the machines can be adapted to different precision levels, known at Röders as PRECITEMP I, II, and III. Each level integrates a varying number of circuits for precisely temperature-controlled water and air regulation in different areas of the machine. Any remaining minimal deviations are compensated for by software. The results at the highest configuration level are impressive: even with temperature fluctuations of ±3K, the machine maintains geometric stability within ±1 µm.

This approach offers significant advantages, particularly for customers operating machines from different manufacturers in the same production hall, where not all machines are used for ultra-precision work. Instead of climate-controlling an entire hall to ensure high precision on one machine, Röders' technology allows precision machines to function optimally in varying temperature conditions. This saves both costs and energy.

Of course, for this strategy to work, the machine’s internal heat sources—such as the spindle and axis motors—must not impact its geometry. Röders has long mastered this challenge with its RXP machines. For instance, temperature fluctuations in the milling spindle are isolated from the Z-axis using a two-stage system with a high-flow temperature control sleeve. Additionally, a contactless external sensor measures spindle elongation at the sub-micrometer level to ensure high Z-axis precision. When needed, such as for high-speed reciprocating grinding, the guide rails can also be temperature-controlled with water.

However, an additional improvement was required for the 5-axis machines to enhance the thermal behavior of the rotary-swivel unit’s bearings. The direct-drive torque motors are already water-cooled as standard, but in the new machine series, the bearings and machine tables are now also temperature-controlled. This significantly reduces deviations at higher rotational speeds. Any remaining minimal errors can be corrected via software, ensuring that higher speeds no longer noticeably affect machining precision.

Mechanical Precision and Surface Quality

In 5-axis machines, the rotary-swivel unit often plays a crucial role in determining overall precision. Röders re-evaluated this component, optimizing tolerances and key elements, which has significantly enhanced accuracy. As a result, not only do the workpieces milled on the new RPT machines achieve outstanding precision, but the machines themselves also set new benchmarks. Measurements taken during development, as well as initial customer trials at Röders' technical center, confirm that machining tolerances have been halved compared to RXP machines. "Of course, the achievable accuracy also heavily depends on the machining process itself," explains Sebastian Knaack, Deputy Head of the Röders Technical Center. "For example, long tool lengths inevitably cause deflections and deviations, which even a significantly more precise machine cannot completely eliminate. But when the process is well-optimized, the new RPT machines represent a major leap forward."

Since the factors influencing high precision and excellent surface quality are somewhat different, Röders also re-evaluated its RXP machines in this regard. If the company was already investing in a new machine series, it wanted to improve across all aspects. At the same time, Röders aimed to retain the highly rigid and cost-effective roller guides in the linear axes, which have proven particularly effective for high-dynamic axis movements and roughing operations. A renowned guideway manufacturer’s offer to collaborate on further improvements came at just the right time.

For surface quality, cross-movements—orthogonal to the feed direction—are crucial. Even in the sub-micrometer range, these movements can impact surface finish when machining with a ball-end mill. Minimizing these movements was a key focus. By optimizing the raceways for the rollers in the guide carriages, Röders significantly reduced cross-movements, as demonstrated in the accompanying graph. This innovation, called NANOTOL, is a defining feature of the new RPT machines, setting them apart from the RXP series with substantially improved motion quality.

Design and Ergonomics

"As a highly pragmatic company, we have traditionally prioritized technical performance over machine aesthetics," says Managing Director Jürgen Röders. "This even cost us a contract once because of our machine’s appearance. But a better machine design doesn’t necessarily have to increase costs significantly if considered early in the design phase. That’s exactly what we aimed for this time." The result is a true eye-catcher with a modern, dynamic look. However, design alone is not enough. Röders also analyzed customer feedback from RXP machines, particularly regarding service and usability. The goal was to retain the benefits of existing machines—such as clear visibility and access to the work area from two sides, as well as easy automation—while making ergonomic improvements.

The result is a series of user-friendly enhancements, such as push-button door operation instead of manual handles and a new lighting concept that directs light onto the workpiece in the operator's line of sight. Additionally, the electrical cabinet is now detached from the machine, standing independently. This prevents vibrations from high-dynamic machining or roughing operations from affecting the electronics, enhancing system longevity. At the AMB trade show in Stuttgart, Röders showcased the second machine size of the RPT series, the RPT800DSH, a 5-axis machine that can be equipped with various spindles and options depending on the application. The rotary-swivel unit accommodates an Erowa UPC chuck, making the machine ideal for medium-sized workpieces. A 3-axis version is also available.

Previously, at EMO in Hannover, Röders presented the smaller RPT600DSH for the first time. "More machines in the RPT series will follow, but development will also continue for the existing RPT machines," hints Jürgen Röders.

Source | Röders GmbH