COMPOSITES I TOOLING
SOMETHING C
THAT CERTAIN
WNT’s Adrian Fitts explains why the company has created whole ranges of cutters specifically developed to cope with the unique qualities required to machine composite materials.
omposite materials have their own characteristics and issues when it comes to machining. Thankfully,
cutting tool technology has kept pace with the latest developments in materials technology. However, little, if any of this technology can be applied to machining more conventional materials. According to WNT UK’s business
development manager, Adrian Fitts, one of the big benefits of using composites is also the biggest headache when it comes to machining them: namely the ability to form them into complex freeform shapes. “This creates a requirement for vacuum workholding when machining is required,” he begins. “Consequently, it dictates that the geometry of any cutter has to be designed to assist the vacuum process and push the workpiece into the workholding system, unlike conventional cutters that would tend to have the effect of pulling the component.” Another factor often overlooked when machining composites is the abrasive qualities of composites. While they are relatively easy to cut with the correct geometries and cutting materials, composites can quickly wear out tungsten carbide tools. To combat this, WNT has developed a range of Poly- Crystalline Diamond tools (PCD). The role of PCD tools is reversed from when they are used on ‘conventional materials’ where they are usually used for light finishing applications. For composites the opposite is often the case as the improved wear resistance of PCD can be
composite materials. “In order to achieve best results when machining composites it is important that the following are adhered to: ensure that the workpiece is clamped at the lowest possible clamping pressure to avoid deformation,” states Fitts. “Cutter overhang should be kept to an absolute minimum. The milling procedure should be conventional for aramid fibre, carbon fibre and glass fibre materials whereas climb milling techniques should be applied to all other materials.” For plastic-based composites, high
The push and pull of machining composites: The geometry of any cutter has to be designed to assist the vacuum workholding process and push the workpiece into the workholding system
applied to removing larger amounts of composite material without suffering the same wear issues that carbide would. Finish machining of composites using
PCD creates its own issues as PCD tools are restricted by the fact that their geometries cannot be formed in the same way as carbide and this lack of complex geometry may lead to localised cracking of the component. Tungsten carbide tools, on the other hand, have the advantage of being able to be formed/ground to create the extremely high helix, high top rake, and sharp cutting edge which helps eliminate the cracking problem. In addition, carbide manufacturers are able to ‘fine tune’ the grade of carbide to best meet the cutting conditions for specific
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local temperatures need to be avoided otherwise there is a risk of partial melting or decomposition. Therefore, tools with large chip flutes that enable the widest chip thickness to be achieved should be used to eliminate, where possible, any heating of workpiece and tool. That said, the swarf created during cutting conducts almost no heat. “At WNT we have first-hand experience of the UK subcontract market and we are seeing that many of these companies have moved away from high volume, low cost part production and have invaluable experience machining ‘exotic’ materials for high added value components,” Fitts concludes. “However, the move to machining composites may need the extra support that WNT is able to provide both with its composite materials knowledge and the experience
of our technical sales engineers.” y
www.wnt.com
AEROSPACEMANUFACTURING | SEPTEMBER 2011 29
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