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Drilling compositematerials can be
problematic, but solutions are available for making high-quality
By Alan Richter, Editor
composite materials are not metallic, they shouldn’t be treated
like metals when holemaking. Drilling a composite with a tool for
cutting metal results in poor hole quality and excessive tool wear.
End users need to apply drills specifically designed for tackling
the abrasiveness of the material, which has a layered structure
that tends to delaminate during drill entry and exit.
Composites provide a number of benefits for a variety of parts,
such as those for the aerospace, automotive, medical and defense
industries. Composites have high strength-to-weight ratios, reducing
fuel consumption. They also resist the effects of fatigue and high
temperatures, don’t corrode and are electrically insulating.
Not all composites are created equal, though. “There’s
quite a variety and all of them machine differently,” said
Bob Maxey, vice president of business development for Onsrud Cutter
LP. The Libertyville, Ill.-based company specializes in making cutting
tools for machining composites. “Even within what the aerospace
industry utilizes, there are a variety of different composite configurations.
Even on some portions of an airplane, there are different types
According to Kennametal Inc., usage of one type of
composite is growing: carbon fiber-reinforced polymer (CFRP) materials.
“The big players are using this material for aircraft,”
said Marcelo Campos, the Latrobe, Pa.- based toolmaker’s product
manager for holemaking. “We are also seeing a movement to
use CFRP for medical parts and by some automakers.”
CFRP, or carbon fiber as it is commonly called, consists
of strong, abrasive, reinforcing carbon fibers within a relatively
soft polymer matrix. For other composites, the matrices include
metal and ceramic, and the reinforcements include glass fibers and
polymers. Kennametal stated that in CFRP, the polymer can be either
thermoplastic or thermosetting, and epoxy, a thermosetting polymer,
is the most common.
fiber/epoxy composite has a tensile strength of 1,500 to 3,000 MPa
and a density of 1.5 to 2.0 g/cm3, compared to steel’s tensile
strength of 1,200 to 1,500 MPa and density of 8.01 g/cm3 and aluminum’s
tensile strength of 600 MPa and density of 2.76 g/cm3. These favorable
properties have increased composites’ structural weight percentage
from 6 to 7 percent in existing aircraft to 43 percent in next-generation
aircraft, according to Kennametal, which defines structural weight
as the total weight of wing, fuselage, empennage, propulsion and
landing structure. “Aluminum will drop from about 65 percent
to 20 percent,” Campos said.
Focus on the Fiber
Depending on how the carbon-fiber composite is fabricated, machinability
varies. For example, Campos noted that when the fiber structure
is like a woven fabric with a crisscrossing pattern, the material
is easier to machine than when the fibers are in a unidirectional,
tape-laid pattern, which is more abrasive and more prone to delaminate
and have uncut fibers when drilled.
Crisscrossing fibers limit possible delamination
and reduce the material’s sensitivity to thrust forces, but
the tolerances for allowable delamination are tighter. (See table
on page 51.) A material with unidirectional fibers requires applying
a drill with sharp edges at a feed as low as 0.0015 ipr, Campos
said. Kennametal recommends a 400-sfm cutting speed for producing
a clean exit hole in difficult-to-machine composites and 450 sfm
and a feed up to 0.004 ipr for improved productivity in easier-to-cut
materials. When drilling composites that require a thrust force
greater than 25 lbs., the speed and feed should be reduced.
Bulk resin-infused composite, another type of carbon-fiber
material, exhibits higher thrust forces because of the material’s
high fiber density. Kennametal recommends reducing the speed and
feed if a drill chips prematurely.
As previously noted, effectively drilling composites requires a
tool designed for the specific workpiece material and hole quality
criteria. Numerous options exist, but finding the one that provides
the best solution requires research. Maxey noted that companies
manufacture a variety of drill styles and configurations for machining
different materials in the metalworking industry, and any one of
them can drill a hole, but one will drill that hole much more efficiently
and effectively than another. “The same holds true for composites,”
Because of the material’s abrasiveness, toolmakers
discount using HSS tools. For carbide tools, Maxey recommends a
grade with lower cobalt content for abrasion resistance and increased
tool hardness to withstand high temperatures in the cut. A tool
with less cobalt has reduced toughness, but a high level of toughness
isn’t that critical when drilling composites because the tool
experiences lower cutting forces than when cutting metal.
While not universal, coated drills,
such as PCD-coated and PCD-tipped drills, can be effective with
composites. To successfully coat a drill with diamond, selecting
the appropriate carbide grade is vital, said Doug Mueller, general
manager of Diamond Tool Coating, a North Tonawanda, N.Y., toll coating
service. “Diamond is mechanically bonded to the substrate,
so we have to get carbide that we can coat,” he said.
indicated that the best carbide for PCD coating has 6 percent cobalt,
has an average grain size of 1 to 3 microns and lacks grain-growth
inhibitors, such as vanadium or chrome carbide. To enable the PCD
to mechanically bond to the carbide, the company removes some cobalt
at the surface layer, creating a rough surface where diamond crystals
form and grow between the carbide matrix. “The crystals wedge
themselves within that matrix and continue to grow until they form
a continuous layer on the surface,” he said.
The thickness of the resulting coating is 8 to 12
microns. The appropriate thickness depends on the application because
drilling composites often requires sharp cutting edges and the thicker
the coating, the more the edge rounding.
“The concern is always edge rounding
because when we remove some cobalt in our pretreatment process,
carbide also falls away and then we’re putting a rather thick
coating on that cutting edge,” Mueller said. “The end
result is about 20 microns of edge rounding.”
Because the diamond coating creates a radius on the
cutting edge, Onsrud’s Maxey noted that a tool’s edge
prep shouldn’t be more than 0.0005". “The key is
to have as keen a cutting edge as possible, so the carbon fibers
are cleanly sheared and not torn,” he said.
Many end users may want to avoid the cost of a PCD-coated
drill, but Mueller maintains that PCD coating is a wise tooling
investment. “With an uncoated tool, they can’t drill
an acceptable hole that has the quality they’re looking for,”
he said. “With a diamond coating, they’re able to do
that and do it 10 times as long.”
Not just any PCD-coated drill is effective for cutting composites,
according to Kennametal’s Campos. Like in a machining system,
various elements of a drill function in concert to achieve success.
With that in mind, Kennametal introduced its Split-Point Fiber (SPF)
drill that has a combination of substrate, geometry and coating
designed to machine CFRP composites with, reportedly, a substantially
lower cutting force than other PCD drills while reducing the cost
per hole by 50 percent.
“Because of the high cutting force, there’s
a lot of delamination when you use, let’s say, normal PCD
drills,” Campos said. “It took more than a year trying
different kinds of geometries and coatings to find the best combination.”
The drill’s solid-carbide substrate is coated
with a smooth CVD multilayer diamond coating. The tool’s low
point angle reduces axial forces to minimize or eliminate delamination,
and special point thinning increases the drill’s centering
capability and reduces thrust to improve hole quality, according
In one Kennametal test pitting an SPF drill against
a PCD-veined drill, the SPF tool drilled 300 1⁄4"-dia.
through-holes in 0.3"-thick difficultto- machine CFRP plate
before burrs formed and delamination occurred, compared to 150 holes
for the other drill. The tools were run at a 400-sfm cutting speed
and a 0.0015-ipr feed without coolant on a Makino A55 horizontal
The tool also has a shank with an H6 tolerance to
minimize TIR. “It needs to have low runout,” Campos
said. “We suggest using this drill in a hydraulic chuck or
shrink-fit holder, or if you use a collet, it needs to be a high-precision
collet.” In addition, the SPF drill is for CNC machine applications.
“It should not be used in hand tools or other manual devices
because they’re not rigid enough to support the drill,”
Typically, the so-called chips generated when drilling composites
are a dry dust or sand. But, depending on the material’s density,
the chip can be small and granular, noted Onsrud’s Maxey.
“The variety of different types of materials, like anything
else, generates different types of swarf,” he said.
Maxey added that the key to successful drilling is
to form the chip, accommodate the chip and evacuate the chip. To
assist in evacuating the chips and allowing the tool to operate
at a lower temperature, shop air is often directed around the tool.
coolant is an option but generally not required or preferred, in
part, because composites have a tendency to absorb coolant. However,
that occurs only at the surface, said Earl Benton, director of sales
and marketing for composites supplier Toray Composites (America)
Inc., Tacoma, Wash. “It’s not like a sponge,”
he added. “Coolant isn’t carried deep into the laminate.
Unless the laminate is exposed to cutting fluids over long periods,
pickup is [inconsequential].”
When coolant is applied, though, it’s generally
done using minimumquantity lubrication techniques, such as through-tool
misting. “Typically, people thought through-coolant tools
would not be commonplace for these types of applications,”
said Maxey, “but more and more in the production mode, manufacturers
are doing MQL or air through a coolant-fed drill.”
As composites become more prevalent in the aerospace
industry and for other applications, millions more holes will be
drilled annually. “We are really moving forward in using this
lightweight material,” said Campos. “That should result
in a big, big change in the metalworking industry.”