APRIL 2006 / VOLUME 58 / NUMBER 4 ➤ BY LAROUX K. GILLESPIE Counter Intuitive When it comes to composite materials, ‘simple countersinking’ isn’t so simple.

ircraft rivet heads are coun- tersunk because they must be A flush with the outer skin of the plane after assembly. The head/hole assembly must be tight, to ensure that the aircraft’s components stay together for long periods of time. This critical aspect of aircraft assem- bly means special consideration must be given to the countersink tool used. More than 102 standard counter- sink tool styles are available, but they are not adequate to meet the needs of many new composite materials. The countersink tool must cut without leav- ing burrs or tearing out chunks of the composite matrix. It must cut fibers cleanly—not delaminate the layers of the composite—and be able to with- stand the abrasive wear of the fibers and other interior materials. Countersink with slanted hole. In addition, the cutting action must not melt the resin, smear matrix ma- Rick Gazak, a carbide tool designer comprised of glass fibers, polyimide terials on other layers or degrade the at Guhring Inc., Brookfield, Wis., “is glass, Teflon glass, polyimide quartz, properties of the matrix around the finding just the right combination of carbon fibers, graphite, silicon carbide- hole. Melting the resin onto the cut- angles to provide the necessary shear- aluminum, aramid (Kevlar), boron, ting edge, for example, will lead to increased cutting forces. Depth accuracy, which is critical to More than 102 standard countersink tool styles are available, providing the necessary rivet holding but they are not adequate to meet the needs of many new force, must be absolute—often less composite materials. than 0.001" tolerance. Roundness of the countersink is critical, too, as is the smoothness of the countersink and the ing. Today, the aerospace challenge Spectra, and combinations involving hole wall. Also, chips from one layer is to provide countersink tool designs resins of polyester, polyetherether ke- must not damage or contaminate other that provide perfect chamfers for each tone (PEEK), polyphenylene sulfide layers. material and layer in the composite in (PPS), acrylonitrile butadiene styrene “The challenge of newer materials a single pass.” (ABS), bismaleimide (BMI) and other and sandwiches of composites,” said Composite layers, or stacks, can be polyimides. Some are sandwiched with core materials of metal (aluminum, Figure 1: Comparison of a typical straight rake (left) and a hook-shaped rake; steel and titanium), plastic or aramid f = feed rate per tooth per revolution. and joined with adhesives, such as thixotropic bonding paste or epoxies. The challenge is that each layer has significantly different material prop- erties and the quality-of-cut require- ments are high for each layer. The hardness of Al2O3, SiC and B4C fibers, for instance, is higher than that of solid carbide. If that weren’t challenging enough, the top and bottom layers, or face sheets, and the inner layers consist of composite plies oriented in various directions and the machined surface quality is affected by the direction of

each ply. L. Gillespie

Cutting Action hook-shaped rake has a positive axial to evacuate chips (Figure 2). cause they last longer than C2 carbide, Despite being available for 20 years, rake angle of 6° to 15°. Because the Kirk Bennett, engineering manager a standard grade. Kevlar is still one of the most difficult hook angle changes along the cutting of sp3 Cutting Tools, Decatur, Ind., A spindle speed of 1,750 rpm is composites to machine to the required edge from the ID to the OD to produce a manufacturer of diamond-coated recommended as a starting point for quality levels. Some fuzzing, or axial the correct cutting action, a single tool tools, said: “Diamond adds [tool] life typical countersink diameters. splitting of the fibers, occurs where the will probably not be successful for pro- to many applications, but it is a mis- For Kevlar, as well as all composites, tool enters the composite. This mate- ducing several different countersink take to simply take a sharp tool and using a firm backup or sacrificial bottom rial can be removed by wet sanding, diameters. Therefore, different sizes of add a diamond coating to it. It is much sheet prevents a host of problems, such but only if liquids can be tolerated. this design would be needed to accom- more complicated than that. You have as delaminating, tearing and burring. To minimize fuzzing, the Kevlar modate different diameters. to understand the cutting forces as well fibers must be preloaded by tensile In a test performed with a cutting as the material properties or you can Clamp It stresses and then cut with a shearing speed of 45 m/min. and a feed of 0.3 shear the diamond right off.” Successful countersinking requires action. For countersinks, this means mm/rev. (1,771 ipm and 0.012 ipr), more than just a sharp, well-designed fibers must be stretched while being more than 1,500 countersinks were tool. The composites being machined pulled over a sharp cutting edge. made with a 10.3mm (0.406")-dia. tool must be clamped tightly to prevent A sharp cutting edge and a com- of this design without losing quality. burrs, tear-out and delamination. For paratively high cutting speed (158 m/ Only 80 were produced with a standard high-production applications, machines min.) are necessary to apply the correct countersink tool. have a clamping, or pressure, foot built amount of tensile stress and prevent the Carbon-fiber composites are cut around the drill or countersink tool. fibers from pulling back into the matrix with PCD-tipped tools, either single Tight clamping also assures accurate before being cut. Cutting at too high a or twin flutes. The advantage of PCD diameters are produced. Compacting

speed causes thermal degradation of vs. carbide is tool life. Where a carbide sp3 forces for drilling and countersinking the matrix and too slow a speed allows tool produces 100 countersinks, a PCD Figure 2: PCD grains have a negative Airbus A340-600 wing panels can ex- the fibers to pull back into the matrix tool produces 1,000 to 2,000. PCD rake, which increases the heat ceed 2,000 lbs. The pressure foot must generated while cutting. before being cut. Slow feed rates also tools should be used in fixed machines also be “normal to,” or perpendicular allow the fibers to recede before being rather than portable drills because the Table 1 shows speeds and feeds suc- to, the surface. A lack of perpendicu- cut. Feed rates of 0.1 to 0.15 mm/rev. brittle tools are easily broken. When a cessfully used for drilling composites. larity of 20 minutes can lead to a 0.073 per cutting edge are best. pure diamond coating is applied with Countersinking should work at these mm (0.003") error in the countersink Kevlar is highly abrasive, so tool life chemical vapor deposition, the coating conditions also, because drilling is a on the A340-600 holes. Dimensional may be as short as 10 countersinks per should be 20 to 24µm thick. Coatings much less-forgiving operation than tolerances are 0.001" or less. standard tool; a special may produce half this thick can reduce tool life by a countersinking. Accurate countersink size repeat- thousands of countersinks before dull- factor of 500. For less-abrasive thermoset com- ability demands control, or compensa- ing. Because a HSS tool only produces While PCD tools are great for car- posites, speeds of 180 to 300 m/min. tion for, spindle growth caused by ther- a handful of holes before it dulls, al- bon fibers, they are typically not suit- (600 to 1,000 sfm) are used with feeds mal expansion (the spindle can grow most all applications involve the use of able for aramid fibers (like Kevlar) of 0.05 to 0.13 mm/rev. (0.002 to 0.005 by 0.0015" or more during machining). tungsten-carbide tools. because individual diamond grains ipr). Carbon-epoxy composites fall in This is a major source of inaccuracy for Guhring produced a special sickle- have a negative rake, which raises heat the low- to mid-section of these ranges. countersink dimensions. shaped, double-cutting-edge carbide levels in the elastic aramid fibers when If tungsten carbide is the only choice, Because the location of the top sur- tool for cutting Kevlar (Figure 1). Its cut, and there is insufficient clearance submicron carbides should be used be- face of stacked composites can vary significantly, some compensation is Hole Material Cutting Feed Workpiece Tool required for this variance. A spring- diameter thickness speed rate material material loaded override holder attached to a (mm) (mm) (m/min.) (mm/rev.) microstop countersink cage allows the cage to rise when compressed against 4.85-7.92 0-12.7 42.7 0.0254-0.0508 Carbide a thick workpiece. The microstop en- 4.25-7.92 12.7-19.1 33.5 0.0254 sures that the tool depth is accurate, Unidirectional graphite-epoxy and the override holder ensures that 4.25-7.92 0-12.7 61.0 0.0508-0.0889 PCD the microstop pushes tightly against 4.85-7.92 12.7-19.1 51.8 0.0508-0.0889 the top surface. 4.85-7.92 0-12.7 61.0 0.0254-0.0508 Trade-Offs Carbide 4.85-7.92 12.7-19.1 42.7 0.0254 There are no easy answers to the Multidirectional problems surrounding the counter- graphite-epoxy 4.85-7.92 0-12.7 68.6 0.0508-0.0889 sinking of composites. Aerospace and PCD cutting tool manufacturers have been 4.85-7.92 12.7-19.1 61.0 0.0508-0.0889 working for years, researching tool designs and better approaches to ma- Graphite-epoxy Carbide 4.85 6.35 60.9 0.0254 chining these materials. They still do not have all the answers. For most applications, it is a matter of finding the conditions that minimize problems Glass-epoxy HSS __ 12.5 15.0 0.028 rather than trying to figure out how to prevent them. Such trade-offs are critical in many Glass-epoxy HSS 3 10 33.0 0.05 applications. It typically requires col- laboration between the manufacturer, the next-assembly user and the cutting Carbon-epoxy Carbide 3 10 33.0 0.05 tool manufacturer to assure the indi- vidual countersink characteristics meet the requirements. High-power micro- Glass-epoxy HSS 8 1.2 1.5-40.2 0.04-0.333 scopes, dyes, nondestructive testing and temperature sensing may be required to Aluminum ensure that the “best” tool is used. PCD metal-matrix 6 19.2 15-75 0.05 “The first step for any machining Carbide composite of composites is to research what is already known,” Bennett stated. “The Web site, www.matweb.com, is the Carbon-epoxy PCD 6.0 10.4 28-79 0.10 first place I go to gather data on the material properties. It is a free resource with a good library of properties. It Kevlar-epoxy Carbide 5.6 ___ 158 0.05 may not have the machining sugges- tions you want, but you will need all Table 1: Feeds and speeds for drilling composites, which also can be used for countersinking. (S. Abrate and D.A. Walton, “Machining of composite materials. Part The following companies I: Traditional methods,” Composites Manufacturing, Vol. 3, No. 2, 1992, pp. 75-83.) contributed to this report: those properties to understand the cut- About the Author Guhring Inc. ting trade-offs.” LaRoux K. Gillespie is a retired man- (262) 784-6730 Most major carbide and diamond ufacturing engineer and quality con- www.guhring.com tool manufacturers provide some guid- trol manager with a 40-year history ance. Bennett noted that he maintains of precision deburring and edge fin- sp3 Cutting Tools a database of experiments he runs for ishing. He is the author of 10 books (888) 547-4156 customers. In return, they share infor- on deburring and almost 200 techni- www.sp3inc.com mation about how the tools performed cal reports and articles on precision under shop conditions. q machining.

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