Cutting tools must be matched to matrix materials, fiber reinforcements
Carbon fiber reinforced polymer (CFRP) composite materials deliver the important performance advantages of high strength-to-weight ratio, durability, and extreme corrosion resistance in lightweight structures, valued especially for demanding aerospace and oil and gas industry applications. Difficulty of machining can vary significantly depending on the combinations of matrix material and fiber reinforcements selected. Due to the wide array of applications, no two CFRP materials are exactly alike. Each composite can take on different characteristics by changing the matrix formulation, fiber type, content, orientation, build-up, and the method of forming, according to Precorp Inc. (Spanish Fork, UT), a company since 2013 in the Sandvik Coromant organization.
Further complicating machining are that stacks of materials of differing strengths and physical properties can be combined in layers for highly specialized and targeted uses. According to Kennametal’s Composite Machining Guide fiber reinforcement materials include carbon fiber/graphite fiber, glass fibers, ceramic fibers, polymer fibers, and tungsten fibers. Polymer matrix materials include epoxy, phenolic, polymide, and polyetheretherketone (PEEK).
Machining characteristics of CFRP/CFRP and CFRP/metal materials are impacted by the abrasiveness of fibers, by fiber size, fiber diameter, fiber length, volume of fibers (percentage), and fiber layout, unidirectional or fabric weave. For example, abrasiveness of fiber increases with strength and diameter. Short fibers tend to delaminate, as do unidirectional layered composites. To counter the tendency to delaminate, Kennametal has developed compression-style routers that generate cutting forces at top and bottom of the materials’ surfaces, as well as other tools including burr-style routers, down cut-style routers, and ball end routers.
Sandwiched Composite Layers Pose Cutting Challenges
As part of continuing efforts to offer lighter, stronger, more cost-efficient products, manufacturers develop and apply high-performance workpiece materials. Sandwiched composites are good examples of that trend, according to Don Graham, manager of education and technical services at Seco Tools LLC (Troy, MI). Here is Graham’s assessment of the current state of composites machining:
Aerospace manufacturers, in particular, rely heavily on sandwiched composite structures in critical components such as aircraft wing skins, fuselage sections, cabin walls and floors. However, machining a stack of materials of differing strengths and physical properties presents several layers of challenges. The main goal is to avoid bending or fraying the core structure or delaminating the face sheets. Sandwiched composite material that is cut unevenly or deformed loses its strength, much as creasing corrugated cardboard destroys its rigidity.
Like corrugated cardboard, a sandwiched composite is comprised of a lightweight core structure, usually resembling the hexagonal cells of a honeycomb, backed by rigid facing sheets. Depending on strength requirements, the honeycomb cells may be formed from high-tech paper, cardboard, carbon-fiber-reinforced plastic or aluminum. The face sheets can be paper, plastic, aluminum or titanium and are bonded to the open ends of the honeycomb cells. A balance of bending, compression and shear forces among the elements of sandwiched composites, results in materials that are lightweight, rigid and remarkably strong.
Sandwiched composite parts typically are flat or mildly curved panels that range in thickness from 0.250 to 0.500″ (6.35–12.7 mm). The panels are fabricated to near-net-shape and finish-machined to trim outer edges, mill out widows and other various shaped openings and holes. For finish machining, shops must use high-speed end mills specifically designed for such sandwiched composites.
Sharp end mill cutting edges and high cutting speeds are key factors in cleanly machining sandwiched composite materials. The situation is much like slicing bread—cut too slowly, and the bread compresses instead of shearing cleanly. While on the other hand, fast-moving, sharp cutting edges generate clean cuts. When machining sandwiched composites, slow cutting speeds can distort the face sheet and the honeycomb structure itself.
High cutting speeds, however, generate heat, and that poses problems because many of the constituents of sandwiched composites are heat-sensitive. Accordingly, light radial engagement—on the order of 5% of cutter diameter-minimizes heat generation. For the same reason, feed rates are kept low. Despite the light engagement and low feed rates, high cutting speeds help maintain productivity.
For instance, a 0.500″ (12.7-mm) diameter Seco Jabro 860 solid-carbide end mill at a 10% radial engagement would run at a speed of 400 sfm (121 m/min) with a feed rate of 0.005 ipt (0.13 mm/t). Such parameters would apply to a sandwiched composite material with an internal Ti-Al honeycomb structure.
Tool geometry can also enhance productive machining of sandwiched composites. For instance, Seco’s Jabro 860 end mills are engineered specifically to machine these types of composite materials. The tool’s double-helix tool flute configuration eliminates fiber breakout, prevents delamination, and improves part edge finishes. As the Jabro 860 rotates, the flute helix on the lower part of the tool forces material upward while the upper helix forces material down. The opposed and balanced cutting forces make for a clean-cutting action.
The components of sandwiched composites can be abrasive, so the tools used to machine them usually are manufactured from micrograin carbide to maximize edge integrity and wear resistance. To further resist abrasive wear, some cutting edges receive diamond coatings. Seco’s DURA thin diamond coating, for example, is applied via CVD and combines low surface roughness for lubricity with high adhesion characteristics that reinforce its wear resistance. The coating requires a balance in tool engineering in that it is thin enough to minimally affect sharpness but thick enough to provide resistance to abrasion.
Machining the open construction and lightweight materials characteristic of sandwiched composites involves very low cutting forces, so high-torque machine tool spindles are often unnecessary. However, most sandwiched composite parts are big, such as expansive aerospace wing skins, and the machines that cut them are very large in size as well as powerful.
Industry is examining different methods of machining sandwiched composites, including waterjet and other abrasive cutting methods. All the alternatives have advantages and disadvantages. The bottom line is that flawless machining of sandwiched composites, especially those intended for aerospace applications, is crucial. Any imperfection on the skin of the wing can be a crack initiation site, and vibration and other forces in the aircraft will cause a crack to grow. In the interest of reliability and safety, manufacturers will continue to employ the cutting tools and techniques that have been developed and proven over time when machining sandwiched composite materials, concludes Seco’s Graham.
Composites for Engines, Structural Components
Composite materials for aerospace applications may include polymer matrix composites for structural components for frames and ceramic matrix composites for engine applications. “Ceramic matrix composites developed by GE, Pratt & Whitney, and Rolls Royce for the newest aircraft engines pose special machining challenges,” said Linn Win, industry specialist composites, Sandvik Coromant (Fair Lawn, NJ). “The ceramic matrix composite material is extremely abrasive and very brittle. Adding to the difficulty in machining is the required lightweighting. When you factor in the practical machining for light-weighting applications, it’s very difficult to get the cost savings benefits from the light-weighting components because of the more expensive tooling that is required for complex components like blades and blisks,” said Win.
Polycrystalline diamond-veined tooling is a technology that has proven to be very effective for machining composites. “PCD is great for its wear resistance properties in composite machining applications, which may lead to longer tool life, but in order to gain the full potential of PCD, the tool will need to have positive cutting geometries,” said Win. “Conventional brazed tooling, which involves brazing a PCD wafer into a pocket, at best allows for minute positive formations, not the optimum for machining composites. PCD-veined tooling produced by Precorp, a subsidiary of Sandvik Coromant, enables PCD cutting tools to be manufactured with the high rake angles and helix angles required to effectively machine composites.
Here’s how Precorp’s PCD-veined tooling is produced.
A carbide blank is slotted and filled with diamond powder. The carbide blank is inserted into one of Precorp’s high-pressure, high-temperature presses and subjected to 270°F (132.2°C) and 876,000 psi (60,398 bar). In this process the diamond powder is compressed and the diamond crystals are bonded to each other and to the carbide blank. The PCD nib is then brazed to a solid-carbide shank. The braze is located sufficiently far away from the tip of the tool to avoid any potential thermal damage. This allows the use of a high-temperature high-strength braze joint between the nib and the carbide shank. The drill geometry is ground to produce the finished PCD tool. This patented process allows for many tool geometries that are impractical and/or impossible using conventional PCD insert processes.
“We have found that in drilling applications in ceramic matrix composites, we do not encounter too many issues when entering the material; the problems occur when we exit the material. On the exit surface, we find that the breakout can be quite poor, due to the high axial pressures that are being applied to the workpiece upon the tools exit. The back side of the material in an unsupported environment tends to blow out, which may lead to quality issues. In an unsupported application, we believe there is not an effective way to machine ceramic matrix composites, due to the materials inherent brittleness,” said Win. “But what we have found is that when we apply positive tool geometry with the PCD-veined diamond tooling, you can actually reduce the amount of stress forces that being applied because now you have freer cutting edge and are able to produce the features that are needed with lower thrust force.”
“PCD diamond-veined tools can be applied to almost every type of machining operation,” said Win. “With structural products for the frame, I think it’s a little easier. What I found in drilling carbon fiber and titanium stacks is that once you clear the carbon material and encounter the titanium material interface, the titanium chips will sometimes score the composite material, again potentially causing a quality issue. To alleviate the problem, we’ve adopted a micro-peck technique in the drilling cycle that allows the formation of smaller titanium chips to clear. In regards to the amplitude of the micro-peck, we are talking on the scale of millimeters. The peck distance is going to be around 0.1–0.2 mm at a frequency of 1.5 to 2.5 micro-pecks in a 360° rotation. What the micro-peck technique does is break up the titanium chips during a drilling cycle and allows for proper evacuation of the smaller more manageable chips. The result is that we’re cutting both materials—the composite and the titanium—with one diamond-veined tool.” Sandvik Coromant is developing a new 88 series geometry PCD-veined tool for CNC applications and a new 86 series for power-feed tools.
Due to the wide array of applications, no two CFRP materials are exactly alike. Each composite can take on different characteristics by changing the matrix formulation, fiber type, content, orientation, build-up, and the method of forming, according to Precorp.
Innovative Tools for Machining Composite Materials
Iscar Metals Inc. (Arlington, TX) has developed a new range of tools, both indexable and solid-carbide, for machining composite materials. Because intensive abrasion of a cutting tool can lead to dramatic deterioration of cutting tool geometry and, as a result, to performance problems, Iscar R&D has focused on wear that can cause delamination during drilling and milling operations. In order to significantly improve the cutting tools’ performance during drilling, Iscar has developed a solution based on interchangeable heads of its SUMOCHAM product line. The new ICF drilling head geometry, which has been especially designed for drilling composite materials, provides low axial forces for smooth penetration during the cutting process without splintering phenomenon. The new heads are based on a new carbide submicron substrate and diamond coating for prolonged and predictable tool life.
SUMOCHAM for composites is suitable for use on any type of machine-tools such as CNC machines, robots, and even powered feed machines (ADU) for which special thread connectors are available. The fast head replacement and high positioning repeatability provide minimum machine downtime. Relatively small indexable drilling heads with diamond coating provide an economic advantage, compared to long full solid-carbide drills, as well as easy stock management. The SUMOCHAM range for composite materials covers today a diameter range from 0.250 to 0.500″ (6.35–12.7 mm).
Iscar also offers a range of solid-carbide drills, starting from 0.118″ (3 mm). The tool geometry of the CFD family has been designed with a stepped point and with two working sections, considerably improving surface finish and allowing a smooth cut on very difficult-to-machine composites, like RTM or thermoplastic materials.
For milling applications, the versatile Multi-Master tool system, with interchangeable heads, features a carbide head with brazed PCD tips. Due to this innovative design, a machined composite workpiece experiences less loading and swarf evacuation issues and surface finish is improved. The main applications for these tools are orbital milling, edging, and ramping down.
Another milling family, EPX, is intended mostly for machining carbon fiber reinforced polymers (CFRP). This family of compression end mills features opposite cutting edge directions—a combination of right and left helix along one flute. This progressive cutting edge geometry reduces delamination and improves tool performance when milling CFRP and the technology is especially recommended for increased feed rates.
Iscar’s EPN-F family of solid-carbide end mills feature cutting edges that are divided into sections. This design results in better distribution of load on the end mill and machined workpiece, and thus provides increased tool life and improved surface finish, especially when machining carbon fiber and honeycomb composites.
This article was first published in the September 2016 edition of Manufacturing Engineering magazine.