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, frequently 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, Germany, which will also be on show at the EMO 2017 trade fair in 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 Magdeburg-based machine tool manufacturer MAP Werkzeugmaschinen 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,” says Christoph Tischmann, Branch Manager of MAP Werkzeugmaschinen. “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.”
In response, MAP decided to develop a machine tool with very powerful drives: for example, 55- and 72-kilowatt spindles (torque 210Nm and 273Nm 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 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% 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, a member of 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 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.”
Matching the design to the 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 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.
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. This is calculated using the Monte Carlo simulation method.”
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 fibre-reinforced-composite and laser-system technologies”, reports 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 fibre-reinforced plastics (FRPs), and provides us with proactive support in the shape of simulation expertise for fibre-reinforced component dimensioning.”
Success through 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 FRPs because in contrast to conventional materials there is no recourse available to existing design and dimensioning standards, ,” explains Tzanetos. “And therefore it’s not that easy to predict an FRP component’s dynamic behaviour in conjunction with the rest of the machine’s structure.
“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. So to solve the problem, the requisite competences are brought together in our company within this project.”
Lazering, not bonding
Another critical consideration is joining CFRPs to metals. Up to now, an adhesive bonding process has been used, which according to Tzanetos has four disadvantages:
- The CFRP surface has to be machined mechanically. This leads to unsteadiness and a weakening of the CFRP’s properties.
- It guarantees only a low level of mechanical strength (per joint: 10 to 40 megapascal).
- It is closely dependent on the ambient conditions (e.g. temperature, soiling, chips, cooling lubricant).
- Bonded joints possess a low resistance to wear.
All these disadvantages are eliminated by a lazering 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 CFRPs as well.”
Despite all these difficulties, the change-over to CFRP has been worth it, opines Tischmann, with a view to EMO Hannover. The machine tool manufacturer is thinking about a shared information stand with the Fraunhofer IPT, in order to showcase the advances and procedures involved with this “new material”.
“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.”
Tzanetos also sees collaborative projects like that with MAP Werkzeugmaschinen as a good option for exploring new paths in a process of mutual feedback with the industrial sector. The project currently ongoing has encouraged the researchers in Aachen to press ahead with industrial partners in the field of CFRPs. Tzanetos and his colleagues from the academic community will be getting further input on comparable material-related questions and on lightweight construction in September at EMO Hannover.