I. Drilling
Drilling in composite materials differs significantly from drilling in metal aircraft structures. Special drills, higher speeds, and lower feed rates are essential for achieving precision holes. Structures composed of carbon fiber and epoxy are extremely rigid and abrasive, necessitating the use of special flat-flute or similar four-flute drills. Aramid fiber (Kevlar®) and epoxy composites, while not as hard as carbon fiber, are difficult to drill unless special tooling is employed, as the fibers are prone to wear or tearing unless cleanly cut as they are embedded in the epoxy. Special drills with clothespin points and fishtail points have been developed to sever the fibers before they are pulled out of the drilled hole. However, when Kevlar® and epoxy sections are sandwiched between two metal sections, standard twist drills can be utilized.
II. Equipment
Pneumatic tools are utilized for drilling composite materials. The free speed of the drill motor can reach up to 20,000 revolutions per minute. The general rule for drilling composites is to employ high speeds and low feed rates (pressure). Drilling equipment with powered feed control produces superior hole quality compared to drill motors without it. The use of drill guides is recommended, particularly for thicker laminates.
Do not use standard twist drill bits to drill composite structures. Standard high-speed steel is unacceptable as it will become dull immediately, generate excessive heat, and cause delamination, fiber tearing, and unacceptable hole quality.
Drill bits for carbon fiber and glass fiber are made of diamond-coated materials or solid carbide because the fibers are very hard and standard high-speed steel (HSS) drill bits do not last long.
Twist drills are commonly used, but Brad point drills can also be utilized. Kevlar fiber is not as hard as carbon fiber and can accommodate standard HSS drills; however, hole quality may be compromised. The preferred drill type is the sickle-shaped Klenk drill, which first engages the fibers and then shears them, resulting in better hole quality. Larger holes can be cut using diamond-coated hole saws or fly cutters, but fly cutters should only be used on drill presses and not drill motors. (As illustrated in Figures 85, 86, and 87)
(Figure 85) Klenk drill for drilling Kevlar®
(Figure 86) Drilling and cutting tools for composites
(Figure 87) Automatic drilling and cutting
III. Operating Procedures and Precautions
Composite drilling motors operate within a range of 2000 to 20,000 rpm and low feed rates. Drill motors equipped with hydraulic feed chambers or other types of feed control are preferred as they limit the surge of the drill bit out of the composite material, reducing burst damage and delamination. Parts made from tape products are particularly susceptible to burst damage, while parts made from fabric materials are less prone to it. Composite structures require metal sheets or plates as backing to avoid bursting. Holes in composite structures are typically pre-drilled with a small pilot hole, then enlarged with a diamond-coated or carbide drill bit, and finally reamed to the final hole size with a carbide reamer.
Back-drilling is an issue that may arise when carbon fiber-epoxy parts are mated with metal substructure parts. The back edge of the hole in the carbon fiber-epoxy section can be eroded or frayed by metal chips pulled through the composite. This is more prevalent when there are gaps between the parts or when the metal chips are linear rather than fragmented. Back-drilling can be mitigated by adjusting feed rates and speeds, tool geometry, part clamping, adding final reaming, using pecking drills, or a combination of these methods.
When drilling composite parts in conjunction with metal parts, the metal part can dictate the drilling speed. For instance, although titanium is compatible with carbon fiber-epoxy materials from a corrosion standpoint, to prevent metallurgical damage to titanium, the drilling speed needs to be reduced. Titanium alloys are drilled at low speeds with high feed rates. Drills suitable for titanium may not be suitable for carbon fiber or glass fiber. Drills for titanium are typically made of cobalt-vanadium, while drills for carbon fiber are made of carbide or diamond-coated to enhance drill bit life and produce precise holes. Small diameter HSS drills, such as a #40 drill, are commonly used for manually drilling pilot holes due to their relatively low cost, offsetting their limited lifespan. HSS drills are only suitable for a single hole.
The most common issue with using carbide tooling in hand drilling operations is tool damage, (specifically edge chipping). A sharp drill bit with slow and constant feed can achieve holes with a tolerance of 0.1mm (0.004 inches) through carbon fiber-epoxy and thin aluminum, especially when using a drill guide. Hard tooling can maintain tighter tolerances. When the structure beneath the carbon fiber-epoxy is titanium, the drill bit can pull titanium chips through the carbon fiber-epoxy, enlarging the hole. In such cases, a final reaming operation may be necessary to maintain hole diameter tolerance. Holes in carbon fiber-epoxy composite structures require carbide reamers. Additionally, when the reamer removes more than 0.13mm (0.005 inches) in diameter, the exit end of the hole requires adequate support to prevent cracking and delamination. The support can be provided by the substructure or a plate fixed to the back surface. Typical reaming speeds are approximately half of the drilling speeds.
Cutting fluids are generally not used or recommended for drilling thin (less than 6.3mm or 0.25 inches thick) carbon fiber-epoxy structures. Using a vacuum when drilling composites is a good practice to avoid carbon dust freely floating in the work area.
IV. Drilling
When flush fasteners are to be installed in assemblies, counterboring is required for composite structures. For metal structures, 100° shear or tension head fasteners are typical methods. In composite structures, there are two commonly used types of fasteners: 100° tension head fasteners or 130° tension head fasteners. The advantage of the 130° head is that the diameter of the fastener head can be the same as that of a 100° tension head fastener, while the head depth is the same as that of a 100° shear head fastener. For flush fasteners in composite parts, it is recommended to design the counterboring tool with a controlled radius between the hole and the counterbore to accommodate the head-to-shank fillet radius on the fastener. Additionally, chamfering operations or washers may be required to provide adequate clearance for protruding head fasteners. Regardless of the head type used, a matching countersink or chamfer must be prepared in the composite structure.
Carbide tools are used to produce countersinks in carbon fiber-epoxy structures. These countersink cutters usually have straight flutes similar to those used on metals. For Kevlar fiber-epoxy composites, an S-shaped positive rake cutting flute is used. If straight flute or countersink cutters are used, a special thick adhesive tape can be applied to the surface to clean the cut Kevlar fibers, but this is less effective than an S-shaped flute cutter. A pilot countersink tool is recommended as it ensures better concentricity between the hole and the countersink and reduces the potential for gaps beneath the fastener due to asymmetry or delamination of the part.
Use a micro-stop countersink gauge to produce consistent counterbores. Do not countersink deeper than 70% of the surface layer depth, as deeper countersinks can reduce the material's strength. When using a pilot countersink tool, it is essential to regularly check the pilot for wear, as wear can lead to reduced concentricity between the hole and the countersink tool. This is particularly applicable to countersink tools with only one cutting edge. For pilot countersink cutting teeth, position the pilot in the hole and adjust the cutting teeth to maximum RPM before initiating feeding of the cutting teeth into the hole and preparing for countersink cutting. If the cutting teeth come into contact with the composite material before triggering the drill motor, debris may be generated.
V. Cutting Processes and Precautions
Cutting tools designed for metals are either of short lifespan or produce poor cutting edges when used on composites. The tools used for composites vary depending on the composite material being cut. The general rule for cutting composites is high speed with slow feed.
Carbon Fiber Reinforced Plastics (CFRP): Carbon fibers are extremely hard, and high-speed steel tools wear out quickly. For most trimming and cutting tasks, diamond grit blades are the best choice. Grinding can be done with alumina or silicon carbide sandpaper or abrasive cloth. Silicon carbide has a longer lifespan than alumina. Router bits can also be made of solid carbide or diamond-coated.
Glass Fiber Reinforced Plastics (GFRP): Glass fibers are as hard as carbon fibers, and high-speed steel tools wear out quickly when used on them. Drilling holes in glass fibers should be done using the same type and material of drill bits as those used for carbon fibers.
Aramid (Kevlar®) Fiber Reinforced Plastics: Aramid fibers are not as hard as carbon and glass fibers, and tools made of high-speed steel can be used. To prevent fiber loosening at the edges of aramid composites, hold the part firmly before shearing. Aramid composites need to be supported with a plastic backing plate. The aramid and the backing plate should be cut simultaneously. The best cutting method for aramid fibers is to tension them first and then shear them. There is a specially shaped cutter that can grip the fibers and then cut them. When using scissors to cut Kevlar fabric or prepregs, there must be one side with cutting blades and another side with serrated or grooved surfaces. These serrations prevent the material from slipping. Always use sharp blades as they can reduce fiber damage. After use, be sure to clean the serrations of the scissors immediately to prevent damage from uncured resin.
When using tools and equipment, always wear safety glasses and other protective gear.
VI. Cutting Equipment
The bandsaw is the most commonly used equipment in maintenance workshops for cutting composites. It is recommended to use carbide-tipped or diamond-coated blades without teeth. Typical toothed blades will not last long if used to cut carbon fiber or glass fiber. As shown in [Figure 88], pneumatic and manual tools such as routers, jab saws, die grinders, and cutting wheels can be used for trimming composite parts. Carbide-tipped or diamond-coated tools provide better finish and longer lifespan. Professional options include ultrasonic, waterjet, and laser cutting machines. These types of equipment are numerically controlled (NC) and produce superior edge and hole quality. Waterjet cutting machines cannot be used on honeycomb structures as they introduce water into the parts. Never cut anything on equipment intended for composites, as other materials can contaminate the composites.
(Figure 88) Bandsaw
Prepregs can be cut using a (CNC) Gerber cutting table. The use of this equipment accelerates the cutting process and optimizes material usage. Design software can calculate how to cut layers of complex shapes. As shown in Figure 89：
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(Figure 89) Gerber Cutting Table
To be continued
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