The shape of things to come

Although advanced composite materials - particularly carbon fibre reinforced plastics (CFRP) - have desirable attributes due to their inherent light weight and strength, we’re still in the dark when it comes to fully understanding their true characteristics. No two materials are exactly alike and each possesses many different properties due to changes in the formulation of its matrix, fibre type, orientation, build-up and the methods used to form them.

Another factor often overlooked is their abrasive qualities. Whilst relatively easy to cut, without the correct geometry and cutting material, composites can quickly wear out tungsten carbide tools. To combat this, many tooling companies have developed a range of polycrystalline diamond tools (PCD).

However, finish machining of composites using PCD tools creates its own issues as they are restricted by the fact that their geometries cannot be formed in the same way as carbide. This lack of complex geometry may lead to localised cracking.

So for new entrants to this niche technology sector, it’s important to first understand why specialised cutting tools are needed as opposed to traditional carbide metalcutting tools and what exacting requirements particular industries and niche applications demand.

“Due to the many different configurations and types of fibre and resin matrices, no one material is the same as another,” begins Dormer Tools’ key account manager, aerospace and composites, Ricky Payling. “This makes predicting a machining strategy very difficult and very specific tools with unique geometries are often required.”

The same but different

According to Sandvik Coromant’s product specialist for composites, Francis Richt, the properties of composites differ greatly from those of conventional metallic materials, driving the need for specially developed tools.

“Factors such as specific material and component/assembly size dictate cutting tool selection,” he explains. “For instance, with large aerospace parts it’s not always possible to use conventional floor standing machine tools, especially within assembly. In these situations, portable machines or even handheld units are necessary, so cutting tools must be capable of accommodating greater resonant frequencies and/or non-linear forces.”

Adrian Fitts, business development manager at WNT (UK) suggests that little, if any composite tooling technology can be applied to the machining of more conventional materials.

“Composite materials have their own characteristics and issues and thankfully, cutting tool technology has kept pace with material technology,” he states. “Many companies have created ranges of cutters - including carbide and PCD tooling - specifically for machining composites. For example, when machining honeycomb-style composites, the tool needs to push and pull at the same time to avoid damaging the upper and lower layers of the composite material, so cutters with left and right hand helix flutes have been designed.”

For OSG UK’s sales director, Tony Cooper, the key lies in getting the ingredients of the base material, the coating and the geometry all working in harmony. “For most applications OSG uses PCD coatings with various unique tool geometries,” he maintains. “However, it’s vital that the PCD coated tool has sharp edges for machining carbon fibre. Diamond coatings can be quite thick and the more that’s put on the tool, the rounder the cutting edges will become.”

Although Seco Tools is involved in all aspects of composite tooling, its main focus is in aerospace. The company’s aerospace business development manager, David Pearson says that one complex issue for airframe designers is in using carbon fibre composites for applications around the engine pylons, landing gear and wingbox.

“They need to strengthen these areas so they’re looking to combine carbon fibre with aluminium and titanium,” he reveals. “It’s difficult when there’s a desire for one tooling solution to primarily drill and edge rout carbon fibre, aluminium and titanium as a stack. Traditionally it tends to be carbon fibre on top with combinations of material underneath. For major structural areas, titanium tends to be the ‘exit’ side of the stack and this can mean drilling large diameter holes which can be a problem if the requirements aren’t fully understood.”

So, we have the right tool - what other elements of composite material machining, such as speeds and feeds, swarf removal, heat build-up, delamination, cracking and workholding need to be considered?

“The main issues are delamination and splintering caused by the thrust forces involved when drilling,” notes Payling. “Heat build-up can be an issue if the machining process has to be conducted dry and the temperature approaches the melting point of the resin. For these reasons, the speed and feed need to be tailored to suit tool geometry and composite composition.”
Ultimately, an aerospace assembly like a CFRP wingbox features thousands of holes, so deriving an efficient production process is vital. For Richt, the key to successful composite drilling lies in the geometrical shape of the drill tips.

“Here, the design needs to improve hole entrance and exit quality on high fibre content materials,” he affirms. “Where splintering or fraying is the problem, or if composite delamination is proving to be difficult in resin-rich materials, then our drills come with geometric features such as small point angles and high rake angles that improve quality as well as reduce axial forces. This also helps with the elimination of burrs and enhancement of bore finish.”

For 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 and 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 tend to have the effect of pulling the component,” he states.

Seco’s Pearson is concerned about the confusion surrounding the exact classification of delamination: “Everyone is trying to understand what’s acceptable. We talk in terms of the number of plies or percentages of delamination relating to the diameter of the drilled hole and if it’s within a certain value then it’s okay. There’s a desire to re-write the machining application guides like those of conventional materials to define the quality parameters.”

Composite compositions

Broadly speaking, what are the main challenges posed in machining different materials, such as carbon/glass fibre, plastics and bio-fibres? To achieve the best results, Fitts says it’s important to ensure the workpiece is clamped at the lowest possible pressure to avoid deformation and that cutter overhang is kept to an absolute minimum. “Milling cutter procedures should be conventional for aramid (Kevlar) fibre, carbon fibre and glass fibre materials, whereas climb-milling techniques should be applied to all other materials,” he notes.

Cooper adds that the one thing we’ll learn with carbon fibre is that it tends to be fairly consistent under machining conditions: “You generally end up with the same results if the geometry is incorrect, i.e. delamination and unwanted heat. Even with the most problematic metallic materials, carbide tools available on the market mean that machining can be completed to a degree, but with carbon fibre, if the base tool geometry is wrong you won’t achieve anything.”

With so much focus on composite tooling research and development, just how much is tool life and consistency of performance being improved over time?

Payling says Dormer continually researches and develops tool substrates, coatings and geometries to offer customers the best solution for their applications: “We also investigate component materials to identify and understand them, which then enables us to apply a good solution first time round.”

And while mainly the OEMs and tier ones undertake tooling development work, Fitts believes that mainstream production will be the domain of thousands of subcontractors across the world. “We have first-hand experience of the UK subcontracting market and see many companies with invaluable experience machining exotic materials for high added-value components moving away from high volume, low cost part production. However, this trend towards composites may need the extra support we provide with our knowledge and experience.”

For Cooper, the biggest change is that OSG needs to develop the required levels of tool life that will help customers migrate from being unable to machine any holes with one tool up to desirable quantities of between 100-150.

“We’ve optimised our tool geometries,” he affirms. “The biggest step is now to increase tool life further. The quickest way to spot tool wear is when delamination appears, or if the board starts splitting.”

And as the requirement for volumes of machined parts ramp up, the attributes of what PCD tools can really offer become more prominent.

“With six thousand 6mm holes to drill in an aircraft wing spar, the customer will be most concerned with promoting tool life,” states Pearson. “We’re developing our PCD tool knowledge and how to obtain the geometry of cutting tool helices as well as working on specific grades. There’s a trend to try and widen the PCD grade selection to suit different applications, so we’re trying to offer more choice of tool geometries whilst keeping costs down.”

In conclusion, what kinds of innovations can we expect to see in the field of composite tooling in the future? Further PCD tool activity seems likely, along with developments for difficult stack machining applications. PCD alternatives for carbide substrates with diamond coatings will also be an area ripe for research.

“Other areas will be the likes of orbital drilling, waterjet machining and even lasers, but as yet I get the impression that these are less common strategies at the moment,” notes Payling.
For Sandvik’s Richt, as production volumes increase in aerospace, automotive and other areas, there will be a demand to reduce production costs to maintain profit margins.

“Sandvik will be working to develop tools that can reduce the number of passes and increase security in production,” he confirms. “New machine tools with higher cutting speeds will also demand more advanced tools.”

Cooper closes by pointing to a growing interest in composite materials from other industry sectors: “During the last Composites Engineering Show some companies said they were looking for weight reduction, but hadn’t considered using composites,” he concludes. “It’s certainly a material that more companies will be working with over the coming years.”

www.secotools.com
www.dormertools.com
www.wnt.com
www.osg-uk.com
www.coromant.sandvik.com