Stiffer than steel but much lighter—carbon is an amazing material. In Formula 1 it enhances crash safety and acceleration. But it’s not just ideal for car racing. The characteristics of carbon’s black fibers also play an important role in metrology. Zeiss manufactures its measuring styli for coordinate measuring systems with a carbon-fiber technology specifically developed for these systems.
“The ideal stylus system consists of one piece, weighs zero grams and is perfectly stiff,” jokes Marius Sauter, an industrial engineer with Carl Zeiss 3D Automation GmbH.
In reality a stylus has not one, but three main parts: first, the adapter, which is the interface to the probe head of the machine; second, the sphere, which touches the work piece and can be, for example, a ruby or diamond; and third, the shaft, which connects adapter and sphere.
Most of the time, not only one stylus is used, but rather a system of various styli which are all connected to each other through cubes, angles, rotary joints and stylus extensions. This is the only way that complex geometries can be quickly captured from different angles.
Light and Stable
The lighter and stiffer the stylus system is, the faster it can be used to measure something without the bending force and torque affecting the precision of the system. Even if the ideal weight of zero grams remains a dream, Sauter and his colleagues at the Zeiss factory are trying their best to get as close to that as possible. According to Sauter, this works best with carbon fiber, which Zeiss uses for styli and stylus extensions between the elements of the stylus system.
Light weight and flexural rigidity are not the only things that distinguish the fiber. Thermal stability also makes carbon ideal to use in metrology. Aluminum, for example, loses its shape by 23.4 micrometers per meter of material if the temperature changes by just 1 degree Celsius. “For manufacturing tolerances in the hundredths to thousandths millimeter range, this can lead to an enormous loss in precision,” emphasizes Sauter.
As a result, using aluminum for coordinate measuring machines located outside a measuring room with a constant temperature is out of the question. By contrast, carbon fiber changes by only 1.4 micrometers per meter if the temperature changes by 1 degree Celsius—much less than with stainless steel, steel, titanium or ceramic. Furthermore, unlike most other materials, carbon fiber does not expand when heated, but shrinks. This characteristic has to be taken into account when manufacturing styli and stylus extensions, notes Sauter.
However, not all carbon fiber is the same. And the way it is manufactured into bars and tubes for specific applications allows for optional performance. Which is why for more than ten years Zeiss has been developing and producing its own carbon fiber material designed specifically for measuring technology—together with specially-developed compression and wire-wrap technology, as well as ultra high-modular fibers. In the past few years, Zeiss has continuously fine-tuned this procedure.
How the Special Fiber is Created
Carbon fibers are made of organic basic raw materials that are turned into carbon while other elements are separated out. Even more complex than this process is the processing that happens afterwards. The five-to-nine micrometer-thin fibers are first combined in bundles made of many thousands of strands.
Next, they are woven and glued together with synthetic resin. Manufacturers use different technologies to do this. The structure of the mesh is the most crucial part and must be completely customized for each intended purpose. For bicycles, for example, where force is exercised from above by the cyclist, structures in which the fibers are oriented in one direction will provide the best result. For golf clubs and fishing rods, where force is applied from the side, manufacturers often use meshes with a helix structure.
For its measuring technology, Zeiss weaves the bundles of fiber into a type of fabric. “That way, the material has a high degree of rigidity in all directions, without bending or twisting – the basis for exact measuring probes,” explains Sauter.
Tricks in the Workmanship
But a highly specialized raw material is only half the story. How precisely measuring devices with styli and styli extensions actually measure in the end depends largely on how the fibers are further assembled.
Even if the material shrinks by only 1.4 micrometers per meter with a 1-degree Celsius rise in temperature, this can still negatively affect the precision. This change in the stylus system is especially apparent when highly precise measuring devices are used, or when measuring machines are subject to big changes in temperature in the factory. Therefore, Zeiss measuring technology experts apply a technical trick: They combine a carbon fiber tube with adapters made from titanium. While the carbon fiber tube contracts under the influence of heat, the titanium adapters expand. Zeiss has synchronized the tube and adapter to each other to such an extent that the thermal expansion is, in total, practically nil.
However, since the measuring technology experts knew the glue gap between the carbon fiber tube and the titanium adapter is soft and can change its shape, they were still not satisfied with their development. In order to guarantee the consistently high precision of the measuring device, they screw the titanium adapters with a self-cutting thread into a carbon fiber tube to fit perfectly. Via the thread, the titanium and carbon fiber always remain connected, even if the gap from the thread is also filled with glue.
Thanks to the highly specialized carbon fiber and processing techniques, Zeiss has created the light-weight stylus system ThermoFit, which is both thermally stable and has a high degree of rigidity. The latter is crucial when doing measurements with a high throughput because a higher scan speed also increases the flexural forces that deform that system.
The stylus systems have an even higher rigidity because various groups of components are solidly connected with each other and the individual styli are mounted in a way to prevent twisting. Sauter believes that carbon fiber stylus systems will continue to gain popularity in the coming years. “Metrology is becoming a more dynamic endeavor. And because this can often only be achieved with a light weight and rigid material, carbon fiber is becoming more critical in the future.”