Machining composites presents unique challenges compared to metals.
Reinforcement fibers are abrasive, shortening tool life. The
plastic matrix carries away little heat, unlike metal chips, and
overheating can melt the matrix. Composites can delaminate, and
burrs and fibers show on poorly drilled holes or poorly trimmed
edges.
Other challenges include machining or drilling laminate stacks
composed of composites combined with titanium or aluminum. Creating
tools that drill such stacked holes in a single operation is
particularly difficult. The common strategies for tool designs for
cutting composites include uncoated carbide, tools with a diamond
coating applied by chemical vapor deposition (CVD), and
polycrystalline-diamond (PCD) edged or tipped tools. Traditional
PCD is produced by sintering diamond crystals embedded in a metal
matrix. The tool’s cutting section is cut to shape and brazed or
sintered onto a carbide shank. Traditional PCD is limited in tool
geometry, but some companies now offer PCD-veined tools that
produce more-complex geometries.
Tool designs need to minimize cutting force pressures, especially
in drilling. Avoiding delamination is paramount. Delamination
typically occurs on the tool breakout, as the axial-thrust force
puts pressure on the lower surface laminations. It also happens at
the surface during entry,” explains Karthik Sampath, a senior
engineer with Kennametal (Latrobe, PA). Although thrust correlates
with breakout delamination, Kennametal also believes variations in
fiber position and the presence of voids contribute. Unlike
machining metals, where shearing and formation of consistent chips
is desirable, machining composites means fracturing fibers, coupled
with shearing the matrix material, according to Sampath.
To cut cleanly with the least amount of tool wear, Kennametal seeks
to optimize tool geometry. Our tests prove that composite drills
need a high helix angle, a severe clearance angle, and a high-rake
gash angle for easier entry into the material,” states Sampath. He
notes paying particular attention to the clearance angle behind the
cutting edge. In one test that compared 10, 20, and 36° clearance
angles, as clearance increased hole quality improved dramatically.
Tool sharpness is also critical. Our findings conclude that having
a sharp edge ≤10-m radius) before applying a coating material works
best.”
As for materials, Kennametal recommends PCD-veined drills and
diamond coating of low-cobalt steel drills. Diamond-coated drills
deliver a 10:1 improvement in tool life over uncoated carbide and,
in some cases, a 50% longevity increase over PCD-drill technology,
according to Sampath. We recommend a diamond-coating thickness of
12 m for maximum wear resistance and good cutting properties. A
thinner coating can lead to edge chipping, and a thicker coating
does not improve the performance in proportion to its extra cost,”
explains Sampath. Kennametal offers its B531 and B532-series drills
for composite cutting.
A tool geometry designed to minimize cutting pressures is critical
for cleanly cutting composites, agrees Stephen Jean, milling
products manager of Emuge Corp. (West Boylston, MA). Our end mills
are unconventional in appearance, and resemble a thread mill more
than a typical carbide end mill. The tool incorporates two serrated
cutting edges. One edge cuts in an upward direction, and the other
cuts downward. In rotation, the effect is a scissors-like cutting
action that efficiently mills the base material, while shearing the
fibers and eliminating the fraying effect.” Emuge also offers PCD
end mills and inserts, as well as a variety of CVD diamond-coated
carbide inserts and end mills for machining composites.