Sharp attack

The use of composites has been steadily growing as the demand for strength and lightness increases, but what are the problems faced and what is the most efficient way to cut them?

Most engineers will be aware of the problems of machining composites. Yet in the not-too-distant future composites will increasingly take the place of traditional materials, and even some of the more exotic alloys.

Composites provide distinctive manufacturing advantages thanks to properties such as high specific strengthto- weight ratios and/or stiffness, low electrical conductivity, transparency to radio emissions, corrosion resistance and good thermal stability. But they are easily damaged and can cause unbelievable degrees of tool wear without the appropriate coating technologies and geometries.

One example of this is de-lamination caused when a drilling force exceeds a threshold value at the entry and exit of a drill bit.

So tool sharpness is a must. An equally important quality is tool hardness, as the abrasive nature of most composites wears sharp tools quickly.

From humble beginnings — for instance pre-history mud bricks reinforced with straw — composites have become the major topic in materials technology today. Applications include aerospace, automotive, and prosthetics. Recently a number of man-made fibres have been developed for polymericmatrix composites.

Continuous fibre-reinforced polymeric composites are materials comprising continuous reinforcing fibres held together by a surrounding polymer binder. The fibres bear the structural loads, and are known as the ‘reinforcement’, while the polymer transfers the load from fibre to fibre, and is normally called the ‘matrix’.

Plies of this composite are stacked layer upon layer, and at specific angles relative to one another, to form a useable laminate. The most common fibre materials used in polymeric composites are glass, carbon, and aramid.

Aramid fibre is organically produced by axially-aligned aromatic polyamide polymer molecules that are hydrogen bonded together into radial plates. The most widely used aramid fibres are Kevlar, developed by Dupont.

Naturally, the same properties that make these materials so good in high-end technologies make them so difficult to machine. Because the high-strength fibres in composites don’t break easily, they tend to be pulled by the cutting tool, leading to microcracking and de-lamination along the cut.

In the case of aramid, the tough fibres are hard to shear, resulting in fuzzy surface finishes. Overheating when drilling or cutting can heat the resin above its transition temperature, locally damaging the laminate. If a coolant is used to prevent this, fibres can absorb moisture which further degrades the laminate. In addition, the heterogeneous combination of fibre and matrix resin means that a cutting tool encounters varying resistance due to the inter-layered hard abrasive fibres and softer resin, which puts stress on the machine.

Some of the key solutions for successfully machining composites include high spindle speeds, light or shallow cuts, and aggressive feed rates, where part geometry and fixture rigidity largely dictate speeds and feeds. However, tool geometry plays an equally important role.

Each manufacturer’s exact rake angles and cutting edge geometries are proprietary and often patented, but as a rule of thumb positive angles are preferred because they promote a good shearing action, which reduces heat when cutting. An analogy is shovelling snow. If you hold your shovel perpendicular to the snow — a neutral rake — it takes considerably more strength to push through the snow than it would if you lowered the shovel to a more acute angle — positive rake — and got under the snow.

Tooling and inserts supplied by Sandvik Coromant (SC) are playing an integral role in precision machining in BAR Honda’s newly-installed composites facility at its UK F1 Team Operations Centre at Brackley, Northants.

As the official insert supplier to BAR, SC has worked closely with the F1 constructor’s engineering teams to establish the best tooling set-ups for a range of machining operations on the facility’s newly-installed Jomach Linx Compact 5-Axis machining centre. Mindful of the need to machine this range of materials, BAR worked with SC to create an inventory of optimised insert grades that would achieve the necessary cutting criteria coupled with minimal insert changing.

To combat the highly-abrasive properties of the carbon composite, SC recommended its PCD CD10 insert grade to maintain the tip sharpness necessary to achieve high productivity, while avoiding de-laminating.

Composed of fine-to-medium grain crystals, CD10 is SC’s preferred choice for finishing and semi-finishing of nonferrous and non-metallic materials. In its composite machining at BAR, it is estimated to be achieving a usable cutting life up to 10 times greater than some comparable composites inserts.

General Tool Company (GTC) found that an eight-faceted, thinned web, diamond tipped carbide drill was most suited for drilling small bore holes in composites.

Tecvac, with partners including the Fiat Research Centre and Airbus Spain have recently completed a three-year EC funded project, ‘Coming-Dry’, aimed at developing a cost-effective method of dry or near-dry machining of exotic materials such as Inconel and organic matrix composites.

Its resulting Nitron- CA coating reduced the cost per hole of an Airbus drilling operation by six times and reduced production time for this operation by 43 per cent — for just 15 per cent on the tooling costs.

The process required a number of 12mm dia through-holes to be drilled and reamed in situ on Airbus tailplanes using pneumatically-driven hand drills with an inbuilt pecking action.

The organic matrix composite was sandwiched between two 15mm thick titanium alloy (Ti6-4V) plates, further adding to Airbus’s tool wear problems. For reasons of convenience and health and safety the drills and reamers were used dry — with no coolant. Trials with commercially-available PVD coatings such as TiN, CrN, TiAlN, TiCN from various major PVD coating companies provided little or no improvement.

Then, in the course of the ‘Coming-Dry’ project, Tecvac developed the TECVAC Nitron-CA coating with outstanding results: drill life was extended from 2.7 to 22 holes on average — over eight times life, well exceeding the Airbus aspirations.

Technologies such as Waterjet cutting are also proving useful regarding composite materials. In contrast to tooling, Waterjets don’t introduce any heat, and many machines can cut through cured composites like Kevlar fibre-reinforced plastic and graphite up to 60mm thick in a single pass. When abrasives such as garnet are entrained in the water stream the abrasive waterjet is powerful enough to cut through 100mm of carbon fibre without creating heat-affected zones.