- November 6, 2013
by Tim Wilson
Composites hole drilling comes clean
Composites are finding their way into more and more industry applications, with aerospace leading the way. From a holemaking perspective, challenges emerge as materials change, with stacked composites including metals, such as aluminum and titanium. As well, carbon fiber composites can be very thin, which increases the risk of delamination or splintering.
"The titanium barrier in a stacked composite can create a challenge, as can the breakout," says David Vetrecin, holemaking product manager for Iscar Tools, Oakville, ON. "Fortunately, we have geometries that can work with stacked as well as pure carbon fiber composites."
Composites are unique in that they don't tend to conduct heat very well. Problems then arise when the resins warm and soften. Some composites have literally dozens of layers of different types of material glued together. And though many industries use composites, it is the aerospace sector that is on the forefront of the materials challenge.
"I would say 90 per cent of composite use is in aerospace," says Vetrecin. "Advances in aluminum and titanium originate there, and will trickle into other sectors such as automotive, as they become more affordable. The problem is that though composites are not hard they are abrasive, which makes it difficult to predict tool life. It really all depends on the machine and the set up."
Issues include whether or not a hole needs to be pre-drilled and if reaming is required. With the right tool materials and edge geometry, reliability can be enhanced, thus avoiding costly correctives. In this regard, it is crucial not to skimp out on tooling, because the cost-return benefit on secure machining for expensive components is huge.
Diamonds are forever
"When it comes to composites, diamonds are the machinist's best friend," says Randy McEachern, product and application specialist, holemaking and tooling systems, Sandvik Coromant, Mississauga, ON. "Solutions include diamond coated and diamond veined for cemented carbide tools. These come in the form of standard, semi-standard, and engineered."
These options can deliver results throughout the range of composite and stacked materials. The important thing is to take into consideration a material's properties as well as the hole size, depth, and quality. From there it is a matter of ensuring a stable set up to support the desired volume of holes. If volumes are higher, it can be necessary to optimize with a more cost-effective tool.
"Hole volume, as well as consideration for the type of composite and set-up, will partly determine tool choice," says McEachern. "We have drills to make holes optimally in composites varying from fiber-rich to resin-rich. They can offer all-round alternatives and suitability for stacked materials, including titanium."
For example, Sandvik Coromant's CoroDrill line has options that can address fiber-rich materials and minimize fraying. Spurs at the periphery help to better cut fibers and avoid splintering.
"We also have an alternative standard drill, designed for resin-rich composites, that has a double-angled cutting geometry," says McEachern. "This can minimize delamination by allowing for softer entries and exits. It also works for glass-layered composites."
Typically, a PCD-vein drill will work in an automated set up to ensure quality consistency and optimal performance, with a sharp geometry on the cut to handle a composite's abrasiveness. Higher cutting speeds, as well as tight limits on the entry and exit, are abetted by a strong tool corner.
"A PCD-vein drill can also have micro-grinds for areas of high stress," says McEachern. "And with low thrust force where the edge cuts the fibers, there is minimal delamination, fiber breakout, or exit burrs where there is metal in the stack."
Taking flight with composites
Innovation in composites has the world's big tool companies racing to offer the best solutions. With the aerospace sector setting the gold standard, advanced composites in high-value components are making advances in tool technology for drilling of utmost importance.
"If you think of a wing skin, or the barrel section of a 787, hole drilling and trimming are at the end of the process," says Michel Hudek, manager, business development and operations at the Composites Innovation Centre, an industry and government funded non-profit in Winnipeg, MB. "At that stage, hundreds of hours of labour have already been put in. These are large pieces. If you mess up a hole, the rework is costly."
To address these issues, Seco Tools has its new CX1 and CX2 solid polycrystalline diamond-tipped drills, which are specially designed for composites. The CX1, which has three flute geometry, uses a solid PCD dome, and the CX2 uses a solid PCD cap, both of which help prevent delamination and uncut fibers on a solid carbide drill body.
"This is our newest technology," says Joel Radner, market segment specialist, aerospace, Seco Tools, Hartford, CT. "It took three or four years to develop, and involved us working closely with our PCD supplier. It addresses uncut fibers on the exit and delamination, as well as hole quality and surface finish."
Seco's solid PCD tips can be sharper than diamond coated drills, with the third flute on the CX1 providing stability in the hole while reducing vibration. The dome-shaped tip of the CX1 also offers a double-angle geometry which reduces uncut fibers and delamination, while also allowing for reconditioning and resharpening. By comparison, the CX2 is a flat geometry better suited for stacked composite materials, with the drill point at a 180° point angle for better break and evacuation.
"The real difference with our new technology is when it is compared to the standard vein or the brazed PCD," says Radner. "Composite fibers are difficult to cut on the exit, which makes edge sharpness crucial. A diamond can cut to 25 microns, and we can then grind to 5 microns, with the reduced cutting forces minimizing delamination." SMT