With advances in material sciences and the ability to design composite parts with new virtual software technology, cutting tool manufacturers are being challenged to continually evolve and develop solutions for these versatile materials. “Common challenges in machining composites are excessive tool wear, delamination of layers, uncut fibers, fiber pullout, and scrapped parts in both milling and drilling,” said Mike MacArthur, vice president-engineering, RobbJack Corp. (Lincoln, CA). “Another challenge when machining composites is that they are made in an autoclave as unmachined parts and don’t always match the CAD file associated with the part. When that happens, multiple operations are needed to bring them into the tight tolerances required.”
For machining composites, there are three common types of drills—solid carbide, diamond-coated carbide, and PCD (polycrystalline diamond), according to MacArthur. “Each has its place. Carbide drills are usually used only for manual operations because of their poor tool life. Both diamond coating and PCD are the drills of choice for most applications in abrasive composites like carbon fiber. Diamond coating is excellent for drilling, and in some applications you can even increase tool life over PCD drills. Diamond-coated drills, however, can’t be resharpened like PCD cutters and are disposable. PCD drills are an excellent choice for drilling in CFRP materials. PCD tools tend to outperform diamond-coated drills in materials with high plastic content because they can maintain a sharper cutting edge.”
RobbJack has developed cutting tool solutions like its solid PCD-tipped W-Point drills for drilling carbon fiber-reinforced polymer (CFRP) parts like wing spars to eliminate poor quality cuts and expensive scrap. “The W-point geometry supports the materials as it pierces through and its sharp outer edge shears the material without delamination, uncut fibers, or fiber pullout. The solid polycrystalline diamond tip outperforms diamond-coated carbide drills for tool life up to 25 times longer than uncoated carbide,” said MacArthur. He recounted how one manufacturer was going through 25 carbide drills to make one ship set of wing spars, scrapping many very expensive parts in a costly and unstable process. “RobbJack’s solid PCD-tipped W-point was the only tool that could meet the manufacturer’s exacting design and performance standards, cutting over 4000 clean holes in tolerance; other tools struggled to get 160 holes. Cutting tool savings totaled $548,290.08 per year,” said MacArthur.
Another tool that RobbJack has developed for composites is a solid diamond-tipped drill with multifaceted geometry. “The compound 135° and 20° tip geometry has tested to be the best drilling solution across the most diverse range of carbon fiber composites,” said MacArthur. “The new multifaceted solid-diamond tipped drill produces the cleanest holes I’ve ever seen in carbon fiber materials. There is no delamination or uncut fibers in almost every material we have tested in our machining lab. “
RobbJack has been able to grow wear-resistant diamond on the cutting edges of carbide tools. “One solution we have developed is a combination tool that trims and bevels composites in one operation. This tool eliminated three other tools and eliminated the operation of trimming the part to match the CAD model. The tool bevels the mating surfaces and trims the part in one operation, reducing cycle time by five days,” MacArthur said.
The sheer volume of composites being used in the aircraft industry seems to dwarf all other applications and is expanding rapidly due to their inherent advantages of durability, resistance to corrosion, and high strength-to-weight ratio. Difficulty of machining can vary greatly depending on the combinations of matrix material and fiber reinforcements chosen. Composites are as varied as the matrix formulation, fiber type, fiber content, fiber orientation, and build-up of the material.
Possible combinations and their machining characteristics, according to Kennametal’s Composite Machining Guide, include fiber reinforcement materials like carbon fiber/graphite fiber, glass fibers, ceramic fibers, polymer fibers, and tungsten fibers. Poly 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, fiber size, fiber diameter, fiber length, volume of fiber percentage, and fiber layout, i.e. unidirectional or fabric weave. For example, fiber abrasiveness increases with strength and diameter. Short fibers tend to delaminate, as do unidirectional layered composites.
Enhanced software is available that increases the ability of manufacturing engineers to analyze, optimize, and document the CNC programming and machining process for advanced composites. For example, the latest release of Vericut CNC from CGTech (Irvine, CA) can simulate all types of CNC machining of composites, including drilling and trimming, water jet cutting, riveting, mill/turn, and parallel kinematics. The release features several enhancements designed to increase workflow, enabling users to quickly access only the menu choices needed at the time. A new ribbon bar helps users find the functionality they need quickly and with minimal mouse clicks.
At AeroDef Manufacturing 2017, CGTech will showcase the steps needed to get from a CAD-designed composite part to CNC programs. Highlighted will be the implementation and use of machine-independent, off-line NC programming software for automated fiber placement (AFP) and automated tape laying (ATL) machines, such as the work being done at NASA’s Langley Research Center using a 16-tow Electroimpact automated fiber placement machine. Current customer projects to be highlighted include the use of robots, lasers, probing, and ultrasonic knives. AeroDef is the leading annual aerospace manufacturing conference and exposition, where aerospace companies can find new ways to apply, machine, finish, and automate composite processes. Technical session topics will focus on advanced manufacturing technology, including modeling/simulation, autoclave, thermoplastic composites, additive, automation, NDT, and bonding and assembly.
A new generation of cutting tools from Iscar Metals Inc. (Arlington, TX) is tailored specifically for machining all types of advanced composites. The new breed of carbon fiber-reinforced plastics/laminates are lighter and stronger than ever and elevate performance levels in applications ranging from downhill skis and tennis rackets to military aircraft and automobiles. Although composites technology is helping automakers reach their lightweighting goals faster, as a workpiece material composites have raised fabrication challenges, especially machining.
Iscar offers a wide selection of PCD drilling and milling cutters for a wide range of composite-specific industry applications. “There are drills suited for thicker materials with aluminum on the bottom, as well as for thinner materials with the CFRP on the bottom,” said Pat Cline, national product manager-drilling. “There are combination drill-countersinks and drill-reamers, slotting cutters and combination mill/drill cutters. The bottom line is that possibly for the first time since reinforced composites came on the scene, you can find a no-compromise tool for any conceivable composite machining task,” said Cline.
According to Iscar, the current cutting tool material of choice for composites is a solid-carbide shank with thin PCD coatings, brazed-in PCD inserts or PCD veins at the cutting edge. The solid carbide shank provides the rigidity and dimensional accuracy necessary to maintain close tolerances on size and location as well as smooth surfaces. Solid carbide also makes it possible to start with an optimal cutting geometry that minimizes cutting forces, heat, uncut fibers, cups, fuzz and burrs while controlling chips. The thin PCD coating (or insert) provides the wear resistance at the cutting edge to maintain that optimal geometry over long service intervals. Thin PCD coatings deliver the wear resistance of diamond while preserving the ideal cutting geometry machined into the carbide shank. By contrast, CVD (chemical vapor deposition) inevitably creates a thicker diamond layer that may detract from optimal geometry.
One recent innovation for composites and stack drilling is Iscar’s new head for the Multi-Master line, which provides the ability to mount a variety of changeable heads on one shank quickly and easily. “The novel new head comprises PCD cutting edges and composite-optimized geometries for performing roughing, semi-finishing, and finishing operations in sequence. This geometry enables opening of cavities and slots and performing shoulder applications. Since the shank is thinner than the cutting tip, contact and friction with walls is reduced, so the operation runs cooler,” said Cline. Another promising newcomer is the Iscar SOLIDRILL-REAM, a combination drill and reamer for producing more accurate holes in a single step. Separate brazed-in PCD inserts on the same shank handle the drilling and reaming.
Iscar has developed new ICF (interchangeable) drilling heads geometry for drilling CFRP and CFRP laminates, combining small point angles and positive rakes. “The geometry provides low axial forces for smooth penetration during the cutting process without splintering phenomenon which is critical on thin-wall workpieces. The new heads are produced in the new grade IC107 which combines IC07 hard submicron substrate and CVD diamond coating for prolonged and predictable tool life. SUMOCHAM for composites is suitable to be used on CNC or PKM (parallel kinematic machines) machines, and for robots and powered feed machines, a special connector is available,” said Cline.
Milling composites, like drilling, is equally challenging. If the tool encounters several different layers at the same time, you need to engineer the process, taking all limitations into account. “If you are progressing through the layers pass by pass, you may have to change parameters to match the layer,” said Tom Raun, national product manager- milling. “That’s the reason why Iscar has designed many of the most popular styles of milling cutters and routers available in versions optimized for CFRPs and complex laminates. The SOLIDMILL solid carbide end mills for rough and finish end- and side-milling now come with optimal diamond coatings, and TANGSLIT slitter and TANGSLOT slotters now are available with PCD tips, as are Heli2000 indexable inserts for 90° milling applications. All have proven effective in wing strut and airframe applications,” said Raun.
OSG USA Inc. (Glendale Heights, IL) offers a full range of tools engineered to machine abrasive composite materials for aerospace, automotive, and medical applications, among others. OSG continues to develop custom roughing and finishing routers and drills for CNC applications. The company’s composite tools include drills and routers that feature OSG’s patented ultrafine diamond coating for applications ranging from trimming carbon fiber and glass cloth to machining tough laminates.
Two recent product introductions, the EXOPRO AERO-LHX and EXOPRO AERO-D-REAM, combine special geometries and ultrafine diamond coatings to achieve extended tool life and solutions for common difficult-to-machine material combinations. Typical applications include carbon fiber, glass fiber, CFRP/Nomex honeycomb, glass-fiber reinforced plastic (GFRP)/Nomex honeycomb, CFRP/Al honeycomb, Al/Al honeycomb, and carbon/carbon combinations.
The EXOPRO ARTO-LHX is a diamond-coated, low-helix drill designed to eliminate uncut fibers and delamination when cutting tough laminates. The drill features triple angle geometry and OSG’s diamond coating, ensuring smoother and sharper cutting edges that provide up to 40 times longer tool life, according to the company. Triple-angle geometry reduces push-out exit delamination and the low-helix style geometry provides sharper cutting edges to help shear tough fibers.
OSG’s EXOPRO ARTO-D-RRSM diamond-coated drill/reamer is for cutting carbon and glass fiber composites. It features a tapered four-flute design and OSG’s diamond coating. The drill/reamer features a straight-flute, elongated double-angle geometry. According to OSG, benefits include limited peel-up at hole entrance, limited exit delamination, and smoother and sharper cutting edges, providing up to 50 times longer tool life.
Iscar offers these tips for successful machining of composite materials. The harder the matrix and higher the reinforcing fiber content, the more PCD coatings/inserts will be needed at the cutting edge. When hole size allows, orbital drilling with a solid-carbide end mill or helical interpolation using the Multi-Master with PCD-coated tips is preferable to straight twist drilling.
For shallower holes, use stubby, straight-shank drills. For deeper holes, design the process for absolutely reliable ejection of all types of chip. Consider “peck drilling” and even coolant flushing, if possible.
Match speeds and feed to the layers in the laminate. Be ready to change parameters for each layer as the drill progresses.
Select the tool geometry based on the last material in the stack. If the last material is a plastic, use a tapered drill with a long point angle. If the last layer is aluminum or titanium, a high-shear drill with a sharp point angle will exit more cleanly and leave less burr. The tapered drill would just smear aluminum.
In thicker composite structures, beware of heat buildup as well as chip jamming. Select drills with narrow flutes, wide gullets and tighter spirals that complete the hole before things get too hot and in addition consider coolant.
When titanium is in the stack, the preferred practice is everything that composites hate. To avert work hardening and overheating and to keep chips controllable, select tools with low reliefs and rake angles and a low spindle speed. Although coolant or mist is not generally used, it may be unavoidable for titanium because of heat and/or chip flushing.
In short, take everything into account in tool selection, including the relative thicknesses and location of the metal and plastic layers. It is a balancing act. A stack heavy towards the metal favors solid-carbide tools with internal coolant. If CFRP is the main part, PCD carbide tools would be preferable.
Be sure the process reliably breaks up the titanium chips into small, easily ejected pieces. You especially don’t want to risk a titanium chip jamming the hole in a composite. Again, the most effective remedies are slower speeds and pecking cycles.
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