EXHIBITIONS Making light work of it:
Ultra-high-strength materials are highly popular not only in aircraft and automobile manufacturing, but also in the mechanical engineering sector, because they are often comparatively light and at the same time very sturdy. Machine tools, however, not infrequently come up against their physical limits when processing these materials.
This can be remedied by using structural parts for machinery that are made of lightweight fibre-reinforced materials. This entails mastering some serious obstacles, as evidenced by an as- yet-uncompleted research project at the Fraunhofer Institute for Production Technology (IPT) in Aachen which will also be on show at the EMO Hannover.
CFRP replaces steel for enhanced dynamics
The researchers in Aachen usually adopt a holistic approach to optimising designs. In other words: they consider the machine’s design as a coherent whole, thus also including the development of important drive elements in the machine tool. They have currently joined forces with a machine tool manufacturer from Magdeburg to examine how an innovative machine component for vertical movements (Z-axis) made of carbon-fibre-reinforced plastic (CFRP) behaves in a machine tool and how the Z-slide can be optimised. “We began development work on the CFRP slide in 2013,” relates Christoph Tischmann, Branch Manager of MAP Werkzeugmaschinen GmbH from Magdeburg. “We already possess plenty of experience with linear and rotary axes, for machining aluminium, for instance. But for
32 IMT June 2017
enhanced dynamics with CFRP EMO Hannover 2017 provides inspiring ideas for lightweight construction
reactions of the entire machine on the Z-guide slides. “The machine was subjected to an exhaustive scrutiny,” reports Christoph Tischmann. “We used these measurements to develop several solutional approaches, in order to improve the design.”
high-strength materials like the titanium alloy Inconel they do not possess the requisite drive power.” So MAP decided to develop a machine tool with very powerful drives: for example, 55- and 72-kilowatt spindles (torque 210 and 273 Newtonmetres respectively in S1 or S6 mode) are now used, which are significantly heavier and larger. “So as not to have to compromise on the dynamics, we were looking for a way to compensate for the greater weight,” explains Christoph Tischmann. “That’s why we opted for the CFRP variant.” By way of comparison: the machine tool used to operate in the Z-axis with spindles rated at 28 to 36 kilowatts.
So what’s involved here is roughly doubling the drive power. At the same time, using CFRP reduces the mass by around 60 per cent compared to an axle made of steel. “However, we’re not aiming for any particular weight, we’re targeting an optimum ratio between weight and mechanical strength,” explains Filippos Tzanetos from the scientific staff of the Fraunhofer IPT.
The question arises here of how the change-over from a steel guide slide to a CFRP design with a drive weighing around twice as much will affect the design as a whole. The Fraunhofer IPT has for this purpose analysed the thermal and dynamic
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The entire design is modified to suit the new material Because materials cannot be simply replaced on a one-for-one basis, the design needs to be modified to suit the new material concerned. Finite-element simulation has proved its practical worth in this context. “At the computer, we take a detailed look at the specific points in the design that are the most yielding, in order to determine the causes involved,” explains Filippos Tzanetos. “We then attempt to replace some of the existing components by their equivalents in aluminium or CFRP, or to improve the dynamic behaviour at certain critical points by means of reinforcements or ribs.” Working with CFRP is a particular challenge for design engineers, since the material behaves anisotropically: “anisotropy” is a term describing the direction- dependence of a property or an operation. This means that in the case of fibre-reinforced materials the mechanical strength or rigidity will depend on the direction of the fibres. A CFRP component, however, behaves differently in a simulation to its behaviour in reality. Filippos Tzanetos lays out the details for specialists: “The meaningfulness of the simulation is estimated using the uncertainty propagation defined in DIN ISO 21748:2014-05. The uncertainty of the model’s parameters exerts a certain influence on the uncertainty of the model’s output variables.
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