Zimmer Group’s high-performance spindles optimized for demanding milling requirements

5-axis system for edge finishing for CFRP components
Photo by ZIMMER GmbH

The immense load-carrying capacity and low weight of carbon fiber-reinforced plastics will continue to increase the importance of the material in the future. The high-performance spindles from Zimmer Group make it possible to meet the highest quality requirements for milling, making them especially ideal for final edge machining work on workpieces.

The importance of carbon fiber-reinforced plastics (CFRP) continues to increase. They offer outstanding mechanical characteristics while still being lightweight, feature 16x more tensile strength than steel in the direction of tension and provide the same load carrying capacity as their steel counterparts at a fraction of the weight.

CFRP components are very easy to integrate for meeting load requirements. The material’s inherent ability to dampen vibrations is much higher than steel’s and can also be controlled easily through the use of ribbing in specific places. The more free-form areas a workpiece has, the more beneficial the use of CFRP components. In addition, they allow wall thickness and component weight ranges that are impossible to achieve with steel.

Last but not least, the lightweight construction segments in the automotive, aerospace and wind energy market are expected to see widespread use of CFRP materials due to these factors with regard to environmental and climate control policies. Due to their lightweight characteristics, extensive freedom of design and many options for functional integration, carbon fiber-reinforced plastics can make a significant contribution to resource efficiency when building and operating automobiles, airplanes and many other products.

The use of carbon fibers in industry is expected to increase from 20,000 tons in 2011 to 500,000 tons by 2030. This immense increase will also lead to a significant drop in the cost of CFRP materials by 2030. By 2030, the average cost of CFRP components is expected to drop to 90% of the cost of steel, compared to being 570% the cost that it is today.

This revolution is leading to comprehensive changes in many products and their manufacturing processes, and many companies from every industrial sector will be forced to adapt to the specific nuances of manufacturing components from CFRP.

The manufacturing process

Manufacturing components made of CFRP typically entails inserting several layers of CFRP mats in a mold and soaking them in a special liquid resin. The mats are usually cut using lasers and inserted layer by layer into the resin-coated molds by hand. Each new mat layer is also soaked with resin so that the component grows layer by layer, gradually taking on its later shape. Not all of the layers have the same outer contour, however, because components often become thinner working inward from the edge. In addition to the actual CFRP mats, other components can also be inserted into the mold, such as sensors, inserts for screw connectors or even fabric meshes made of other materials like Kevlar. These materials can adapt the component specifically to its intended use.

Because no significant load has to be applied to the molds—unlike for forged molds—they can be economically made from materials that can be shaped easily, such as aluminum sheets. This makes it possible to create components made of CFRP materials shaped to withstand loads with minimal time and effort. Using traditional steel machining, these same designs would only be possible through extensive effort and by using three-dimensional forging techniques.

Once the mold has been completely filled, it is placed in a vacuum furnace. This uses a vacuum to extract any remaining air bubbles from the resin while heating the workpiece to temperatures around 200 °C. This fully cures the resin, allowing the component to be removed from the mold in a nearly finished state once it has cooled. All that is left is final shaping work on the edges, because this is where the ends of the CFRP mats are located and where cured resin may have formed burrs at the seams in the mold.

Trimming with high-performance spindles

Trimming the edge and ultimately giving the component its defined shape requires removing the protruding mats and resin residue—a perfect task for high-speed milling.

One of the options for edge processing involves placing the components onto a reference matrix and vacuum-clamping them in place to obtain a precisely defined position. Depending on the number of processing axes that are needed, the milling work can be performed by a robot, by a processing portal or even on a wood machining center.

High cutting speeds are required to obtain clean edges. This means only high-performance assemblies like the processing spindles from Zimmer Group are used for this task. These motor spindles are the core element of modern machine tools because directly powered spindles carry the tool and provide a critical factor for determining the performance and precision of the entire system with their output, speed and the quality of the bearing.

Zimmer Group provides its customers with a wide range of motor spindles that combine compact, high-performance motor packages with automatic tool exchange systems and precision bearings that are lubricated for life.  The delivery program includes both air-cooled and liquid-cooled.

HF spindles which are powered by high-quality asynchronous motors in most cases. Maximum speeds between 18,000 rpm and 30,000 rpm, power levels between 3 and 24 kW and torque values between 3 Nm and 38 Nm make the spindles ideal, especially for dry machining of non-ferrous metals, wood, plastic and even CFRP materials. Despite having smaller dimensions, the water-cooled variants feature exceptionally high outputs, allowing high-performance machining work even when space is limited.

The motor spindles from Zimmer Group use high-quality hybrid bearings to achieve maximum concentricity and optimal production quality as a result. These are steel bearings with ceramic balls that are lubricated for life. The bearings are dynamically pre-tensioned to compensate for thermal expansion and achieve the best possible service life time. Each individual spindle also goes through an extensive test that is used to check and log all of its functions. This also includes accurately defined endurance programs that guarantee a perfect distribution of lubrication in the bearings and make it possible to create an operating data log. During the endurance programs, the temperature values and vibration data are continuously monitored to detect deviations from the specified target values immediately and take corresponding actions. This guarantees consistent quality.

The air-cooled spindles are sealed off to the outside by contact-free labyrinth seals and can also be pressurized with sealing air. The liquid-cooled spindles are equipped throughout their entire length with a space-saving cooling sleeve. The installation space gained within the spindle housing allows larger motors with better performance to be installed without increasing the construction volume. This is an important requirement for high power density and optimal usage of the available installation space.

Depending on the version, Zimmer Group spindles are available with manual or automatic tool exchange where the customer can currently choose between the HSK F63, HSK A63, HSK E63 clamping systems and, as of recently, the Solid Fix S3. The spindles do more than just hold tool mounts with their integrated clamping systems. They can also act as equipment holders and even allow for 5-axis processing when combined with the right accessories. Zimmer Group offers a wide range of suitable exchangeable assemblies for numerous task areas, such as bracket assemblies, assemblies for doubling rotational speed, sensor assemblies or even customer-specific exchangeable assemblies for level-free edge trimming needed in CFRP production.

For more information, please visit http://www.zimmer-group.de.

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