Ultra-high-strength materials are highly popular in aircraft and automobile manufacturing, as well as the mechanical engineering sector, because they are often comparatively light and at the same time very sturdy. Machine tools, however, frequently come up against their physical limits when constructed using these materials. Until now. Researchers in Germany believe they may have found a plastic remedy.
The researchers at the Fraunhofer Institute for Production Technology (IPT) in Aachen, Germany usually adopt a holistic approach to optimizing designs. In other words: they consider the machine's design as a coherent whole, thus 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-fiber-reinforced plastic (CFRP) behaves in a machine tool and how the Z-slide can be optimized.
The Fraunhofer IPT examines how a machine component for vertical movements (Z-axis) made of carbon-fiber composites (CFRP) behaves in a new machine tool and how the Z-slide can be optimized.
"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 high-strength materials like the titanium alloy Inconel they do not possess the requisite drive power."
MAP decided to develop a machine tool with very powerful drives: for example, 55- and 72-kilowatt spindles (torque 210 and 273 Newtonmeters 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 percent 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 of how the changeover 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 analyzed the thermal and dynamic reactions of the entire machine on the Z-guide slides. "The machine was subjected to an exhaustive scrutiny," reports Tischmann. "We used these measurements to develop several solutional approaches, in order to improve the design."
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 behavior at certain critical points by means of reinforcements or ribs."
Replacing steel: using CFRP's reduces the mass of the Z-axis by 60 percent.
Working with CFRP is a 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 fiber-reinforced materials the mechanical strength or rigidity will depend on the direction of the fibers. A CFRP component, however, behaves differently in a simulation to its behavior in reality.
In projects of this kind, the Fraunhofer Institute is often assisted by other institutes or spin-offs, but in this case the scientists found the support they needed in-house. "In our institute, we have a department for fiber-reinforced-composite and laser-system technologies," says Tzanetos. "This department has over the course of many years accumulated a lot of can-do competences in the field of dimensioning machine tool components made of fiber-reinforced plastics (FRPs), and provides us with proactive support in the shape of simulation expertise for fiber-reinforced component dimensioning."
Success Assured by Synergised Competences
Support of this kind is indispensable for solving questions encountered when it comes to using FRP components in plant and machinery construction, since these materials, by virtue of their anisotropic properties, are not often used here.
"Up to now, there has been a notable reluctance to use FRP's because in contrast to conventional materials there is no recourse available to existing design and dimensioning standards and therefore it's not that easy to predict an FRP component's dynamic behavior in conjunction with the rest of the machine's structure," explains the Aachen-based scientist.
"Mistakes are made, for example, when a component is dimensioned in terms of its mechanical strength in just one axis direction, while ignoring the mechanical strength in the other axis directions. But if we use simulation tools to fine-tune the interrelationship between the FRP component and the machine tool's own dynamics, nothing can go wrong."
Lasering, Not Bonding
Another critical consideration is joining CFRP's to metals. Up to now, an adhesive bonding process has been used, which according to Tzanetos has four disadvantages:
All these disadvantages are eliminated by a lasering process. But it's not only the joining technology that MAP's Branch Manager sees as problematic. "In order to assure precise positioning and reproducibility accuracies in the machine even in the case of high dynamic response, we scrape off the layers on the linear guides by hand," says Tischmann. "It's now an enormous challenge for us to accomplish this with CFRP's as well."
Despite all these difficulties, the changeover to CFRP has been worth it, opines the expert. "Basically, at the end of this project we aim to be putting a dynamic, high-precision, and above all powerful machine on the market," explains Tischmann. "We would like to see it becoming widely accepted in the aerospace sector, particularly."
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