Following Tesla's success, companies such as Volvo are following suit: With giga- or mega-casting, structural components such as the underbody of a car are no longer welded, glued or screwed together from many individual parts, but cast in one piece. This leads to a maximum reduction in the number of components produced and to the elimination of most joining operations in the production process. This results in savings throughout the entire production chain. In addition, gigacomponents are designed to reduce vehicle weight, which is particularly important in the field of electromobility: less weight means greater energy efficiency and therefore a longer vehicle range.

However, there are complex requirements in production - from the huge casting molds and their temperature control to the casting process itself and the subsequent cooling and the associated component distortion. Even after successful die casting, giga parts remain challenging components, as Product Manager Michael Kreuzberger from Schwäbische Werkzeugmaschinen (SW) emphasizes: "Up to now, the discussion about giga casting has mainly focused on the die casting process itself. However, this is by no means the end of giga-casting. The machining of such large cast components also poses a number of challenges that need to be considered."

Large machines, high space requirements

While some companies have already developed special large machines for die casting, such as the Gigapress from Tesla, the reality of CNC post-processing is different: It is often still carried out on portal milling machines from large parts manufacturing. "The components from the giga-casting sector are simply far too large for conventional CNC machining centers and there is not enough space in the machine," explains Kreuzberger. "On the other hand, the systems used in large part production are actually too large and, above all, too slow: Efficient machining is hardly possible on these single-spindle portal milling machines, machining times are too long and loading and unloading is too time-consuming."

On the one hand, companies are faced with enormous space problems: The die casting machines alone, which are required for giga-casting, are the size of a house. When huge portal milling machines are added to the mix, many production halls quickly reach their limits. Few companies have the opportunity to build a new production hall of the same size on a greenfield site as Tesla.

Cycle times do not meet automotive requirements

Even if there is enough space for large portal milling machines, the machining times of such machines become a problem in the fast-paced automotive industry, as the machines are usually driven by ball screw drives. Kreuzberger explains: "Generally speaking, the non-productive times for aluminum machining are significantly longer than the actual machining times. The larger the workpieces, the longer the feed axes have to travel and the more non-productive time is required, for example when changing tools." Ball screw drives work with lower accelerations and speeds, which leads to even longer non-productive times. Machines used in the production of large parts are therefore usually unable to deliver the cycle rates required in the demanding automotive sector. However, the task is to achieve the same speeds when machining gigantic components as are common in the automotive industry today.

Another challenge is the sensitivity of the components: As stresses arise in the workpiece during die casting, large-volume cast parts are susceptible to warping. In addition, the wall thicknesses of the components in electric cars in particular need to be as thin as possible to save weight. This also makes the workpieces warp more easily. "The precision of the machine is not the problem, as both large portal milling machines and smaller machining centers can easily achieve the required accuracies," explains Kreuzberger. "It is much more important that the clamping device in the machining center and the gripper technology of the automation are optimally matched to the workpiece in order to avoid distortion during machining."

Higher speed thanks to linear motor

One solution for the sensitivity of gigantic components during machining: "The clamping device is developed specifically for the respective component. Good advice and cooperation with the respective manufacturer is important," says Kreuzberger. "We have a dedicated department at SW that works closely with our customers to ensure an optimum match between the clamping fixture and the workpiece."

It becomes more difficult with the required cycle rates. This calls for suitable machines with high dynamics that have enough space for the gigabyte components. In any case, users should opt for systems with linear motors: The direct drive of a linear motor generates the desired movements without mechanical transmission elements. As a result, it achieves maximum acceleration and maximum travel speeds. It also works wear-free. "The fastest CNC machines on the market all work with linear motors and torque motors, including most of our own machines," says Kreuzberger. "We also rely on a weight-optimized design. This allows us to reduce non-productive times and minimize cycle times: Our machining centers achieve acceleration values of over 2 g and rapid traverse speeds of 120 meters per minute." SW has already launched the BA space3 in 2021 to make this speed available for larger workpieces. This machine combines high cycle rates with space for large castings. Users can only achieve even higher speeds with multi-spindle machines.

New machine relies on multi-spindle capability

SW is already working on a machine that will introduce the twin-spindle machine to the Space series. The two spindles will work completely independently of each other. SW is using two autonomous three-axis units to enable maximum flexibility with different components. For medium-sized components, both spindles can each machine a component in parallel as usual. For large components, however, where only one workpiece fits into the machine, both spindles can work simultaneously on one workpiece and change tools independently of each other. Loading parallel to machining time via a double swivel carrier ensures a further reduction in cycle time.

This article appeared in issue 12/23