Cutting Tools for Composites
Users can select from a variety of cutting-tool strategies to cut composites efficiently
By Bruce Morey
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-µm 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 µm 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.
Lach Diamond (Grand Rapids, MI) offers a variety of drills, end mills, and router bits for cutting composites. The company offers only tools with PCD-brazed inserts and CVD-coated diamond, according to Jason Lindsey, an application engineer for Lach Diamond. In their tests, CVD-coated tools reportedly extend tool life 20x over common carbide tooling. "The catch is that you have to have a tool with sufficient surface area to braze the PCD to. Therefore, for drills less than 4 mm in diam, we recommend CVD diamond coating. For drills greater than 4 mm in diam, we recommend PCD-tipped drills." Besides longer life, PCD tools can be reconditioned and used again, after resharpening and resetting in the tool.
An example of Lach's composite tool engineering is its Type Z end mill. Designed for perimeter roughing, it features PCD tips positioned at varying shear angles. The company offers tools for plunging or nonplunging applications. "During perimeter milling around the material, this design keeps thinner, unstable carbonfiber elements from vibrating rapidly up and down," explains Lindsey. He recommends running this tool at about 2200–3000 fpm (670–914 m/min) for best results. They also offer four other tools useful for composite cutting: The Type M end mill, designed for finishing ID and OD contours; the Type P two-flute end mill with positive hook angle, designed for perimeter-milling Kevlar; the Type C two-flute medium-finishing end mill for perimeter milling; and the Type WA two-flute end mill for skinned panels. The Type WA's up and down shear lessens delamination via counteracting cutting forces that reduce cutting pressures.
Suppliers often engineer tools for composite cutting as custom products, rather than catalog items. Currently, Sandvik Coromant (Fair Lawn, NJ) develops all its composite cutting tool offerings as custom jobs. "What steers cutting data on the market today are component quality, demands on surfaces, hole cost, and material combinations. The material's broad usage, and the various types of machines in the field, makes cutting it very demanding and customer-specific," says Francis Richt, manager, application management. He agrees that drilling stacks in a single operation is a particular challenge.
Sandvik Coromant recently introduced the CoroDrill R854 for drilling composites. This drill is manufactured from fine-grain carbide with a diamond coating. Its geometry reduces axial forces, according to Richt. Coromant also offers carbide and PCD drills, depending on need and materials. "For surfacing, edging, and trimming of CFRP, we offer not just a tool, but a process solution, including milling cutters of various kinds, brazed or indexable. The milling process and tool solution depend very much on machine type, whether it's a PKM machine, CNC machine, or robot," says Richt.
AMAMCO (Duncan, SC) is another supplier of custom cutting tools that believes in the future of composites. Its tool design is the company's response to this expanding market. "Our composite drill has a very small point that tapers back to the main diameter. As the small point penetrates, you actually get a splintering effect within the composite. As the taper continues to exit through the hole, it cleans up the splintering. This double action produces a clean hole with no delamination or splintering," explains Andrew Gilpin, a manager with AMAMCO.
The company also provides tooling for drilling stacks of CFRP with aluminum or titanium. "You are never going to find a tool design that satisfies all types of material in a stack. Tool life is going to be sacrificed to maintain hole quality." Gilpin observes that uncoated tooling is usually best for laminate stacks.
At present, the only coatings that AMAMCO uses are the DiaTiger coating provided by Diamond Tool Cutters (North Tonawanda, NY.) "Diamond coating is excellent for composite applications," says Roger Bollier, president of Diamond Tool Cutters. "It conducts heat well. The diamond is chemically inert, so it will not react with the resins used in composites at high cutting temperatures. It also has a very low coefficient of friction." Rather than a single layer of diamond, the process alternates layers of larger polycrystalline structure with nanocrystalline diamond structure in 1-µm slices. The alternating layers prevent crack propagation from the surface of the coating to the substrate, a leading cause of coating failure, according to Bollier. Cracks in the large-crystal polycrystalline tend to grow perpendicular to the surface, while cracks tend to propagate at 45° in the nanocrystalline. This behavior inhibits crack propagation by constantly shifting the propagation angle from one layer to the next, according to the company. The top nanocrystalline layer provides a smooth surface with crystals only 0.01 µm in diam.
The substrate is limited to carbide with 6% cobalt, and Bollier reports coating both round-tools and inserts. DiaTiger CVD-coated tools cannot be resharpened after they begin to show wear, though Bollier points to instances where drills have been re-tipped. "Diamond coating is becoming more accepted as the aerospace manufacturers move to industrial-scale machining of composites," asserts Bollier. "Our focus now is to reduce deposition time. It currently takes 20–60 hr to deposit a 12-µm layer. Reducing that time will make diamond coating more attractive."
Another supplier of custom designed cutting tools for composites is OSG Tap and Die (Glendale Heights, IL). The company manufactures drills, end mills, router bits, and reamers designed for composite machining. Tod Petrik, senior applications engineer for OSG, says that each composite application can be so different that tuning the tool design delivers maximum performance. They supply uncoated carbide, PCD-brazed, and diamond-coated cutting tools. "Tailoring the tool gives us the freedom to tweak it for maximum performance. In one instance, we changed the drill design three times in the space of a couple of months to keep increasing the customer's productivity when production requirements had changed. With a standard catalog offering, it's hard to change it three times in two months." By controlling point design, helix angles, radial rake, and drill web thickness, they target specific constraints, according to Petrik. "For example, we often alter helix angle to control delamination on the offside only. Holes are commonly chamfered for fastener heads, and so the customer does not care about entrance-hole quality. By combining the right physical design features, we can keep tool thrust force below 30N, where delamination generally occurs."
"We prefer CVD-diamond-coated carbide tools, because with CVD we can produce complex and varied geometries. It gives us much more freedom of design compared to PCD-tipped tools," Petrik says, though he notes that they supply PCD when the application warrants it. One reason why the company may prefer CVD coatings is that OSG has its own proprietary process. This process permits economical replacement of worn coatings. Instead of throwing the tool out, it's resharpened and reused.
Can PCD be used to create complex geometries? The drawback to traditional use of PCD in cutting tools is the flat wafers cut and brazed to the tool. Not only does this design limit cutting edge geometries, but the braze joint is close to the hot cutting surface, and prone to failure, according to Scott Horman, product engineering manager for MegaDiamond (Provo, UT). One solution is producing carbide tool blanks for routers and drills with complex geometries formed or pocketed into them. Diamond powder is filled into the pockets and sintered into PCD.
Once fluted and sharpened, these hybrid tool blanks provide complex-geometry cutting tools. "A key point with these tools is that the braze joint is farther away from the cutting surface than with traditional PCD-edged tools," says Horman, reducing failures due to joint failure. MegaDiamond provides semifinished tool blanks to partner companies that then flute and sharpen the tools to their finished geometry.
From the perspective of the machine, there are advantages in cutting composites. "You can cut at much higher surface speeds than when you cut metals, especially titanium, and to some extent steel or cast iron. With composites, we run at maximum spindle speed and achieve several thousand fpm compared to, say, 200–500 fpm [61–152 m/min] with titanium, which is at the opposite end of the spectrum," says Mike Sess, applications manager for MAG Cincinnati (New Hebron, KY).
He agrees that hole exits are a problem when drilling composites. "Sometimes we use a sacrificial piece behind the back layer of the composite. It supports the material as the drill breaks through." To protect the tool from both abrasion and heat, Sess recommends brazed-PCD tools. He is not a fan of CVD diamond coatings.
There are issues to watch for. "What we found is that sometimes the fibers in the composites will pick away at the brazed joint, or if you're running a dry process, sometimes the heat will cause the braze to melt." In this latter situation, coolant might help. He notes, however, that some composites have issues with coolants, especially oil-based coolants that might interact with the composite material and leave residue behind, or require the user to perform another operation to clean the part after machining.
Because excessive tool thrust and friction is what leads to delamination, can a different cutting machine concept solve some of the problems? Enter orbital drilling with its helical-interpolation motion. Developed and patented by Novator AB (Spanga, Sweden), the orbital drill unit is a self-contained spindle that spins the drill on its own axis while rotating (orbiting) around its central axis. Kennametal developed a family of tools with proprietary geometries, substrates, and coatings to complement Novator orbital drills. The multiflute design of the Kennametal cutting tool reduces thrust pressure compared to a twist drill, according to Kennametal's Sampath.
MAG Cincinnati's Sess agrees that helical interpolation is a good cutting strategy for composites. "On our smaller machines, we can actually create that helical motion for efficient composite cutting. On our larger machines, however, we would prefer to attach the Novator Orbital drill directly to the spindle, which we can do."
This article was first published in the April 2009 edition of Manufacturing Engineering magazine.