Serial production of composite components – efficient machining with compression tools
From fenders to supporting structural elements to interior linings, more and more automotive parts are being made of fibre-reinforced composite (FRC) materials. Precision tools are a key technology in this field – especially when it comes to high-volume serial production, where tools are a decisive factor in realizing competitive processes. Two questions are crucial here: How can you avoid damage to components resulting from delamination? How can you reliably machine clean edges without chipping or fibre protrusions? LMT can provide a “compressed answer” – in the shape of compression tools produced by LMT Onsrud and LMT Belin.
The special problems that arise when machining fibre-reinforced composites can be summarized under the heading “delamination”. Different kinds of damage can result depending on the material and machining technique involved. Currently, duroplastic resins are still mainly used, which become very hard and brittle after the irreversible hardening process. Glass or carbon fibres can literally “break out” of the brittle matrix material during machining, especially along the edges of the workpiece, where they are then left protruding from the material. In the case of sandwich materials with honeycomb structures or other filling layers, however, forces occur at an angle to the main cutting direction and create a “peeling” effect that leads to the delamination of the core and the covering layer. In addition, so-called layer delamination can also occur inside the laminate when the bonding between individual layers of material fails.
Main problem: layer delamination
How can you prevent these different forms of delamination, protrusions and flashing? “Overheating of the workpiece can only be addressed by efficient chip removal, minimal friction along the cutting edge and, ideally, cooling with air or appropriate coolants,” explains Martin Danielczick, head of the Composites and Plastics Machining Sales Segment at LMT. “LMT believes that compression technology represents the best way of avoiding all forms of mechanically caused delamination.”
Balancing tension and compression
The principle that compression tools use is a combination of upcut and downcut. Ideally, during machining these tools exert pressure on the workpiece from both directions, thereby literally “compressing” it. “Compression tools offer various advantages,” explains Danielczick. “These start with smooth running. The opposing spirals mean they behave in a very neutral way. The chopping cut of straight grooved tools causes chatter marks and chipping, especially in hard, brittle plastic materials. Tools with only upcut or downcut spirals can also cause vibration. That is not the case with compression tools.”
High-end tool solutions from LMT
A current example from the aircraft industry demonstrates the enormous potential of this tool technology. One of LMT Onsrud’s customers in America has been using the DFC Compression Mill to produce a wing component since the middle of last year. “Previously a standard finishing cutter had been used for finishing – with all the usual problems,” says Leslie Banduch, Senior Vice President Sales & Marketing at LMT Onsrud. “There was chipping and delamination caused by melting of the resin matrix. Both are prevented by using the DFC Compress Mill.” At the same time the aircraft manufacturer also benefits from the tool’s significantly higher feed rate: this machining step on the wing component is now completed three times faster.