Composite drilling is different from metal aircraft structure drilling. Different types of bits, higher speeds and lower feeds are necessary to drill precision holes. Structures made of carbon fiber and epoxy resins are very hard and abrasive, requiring special flat-grooved bits or similar four-grooved bits. Aramid fiber (Kevlar®), epoxy composites are not as stiff as carbon fiber, but are difficult to drill unless using a special cutter, as the fibers are prone to wear or tear unless they are cut clean when embedded in epoxy. Special drills with clothespin tips and fishtail tips have been developed to cut through the fibers before removing them from the borehole. If the Kevlar® or epoxy parts are sandwiched between two metal parts, you can use a standard twist drill.

equipment

Pneumatic tools are used to drill holes in composite materials. The drill motor has a free speed of up to 20,000 revolutions per minute. The general rule for borehole composites is to use high velocity and low feed (pressure). Drilling equipment with power feed control can produce better aperture quality than drilling motors without power feed control. Drilling guides are recommended, especially for thick laminates.

Do not drill composite structures with standard twist drill bits. Standard HSS is unacceptable because it dulls immediately, generates excessive heat, and results in layer separation, fiber tearing, and unacceptable hole quality.

Bits used for carbon fiber and glass fiber are made of diamond-coated material or solid carbide, because the fibers are very stiff and standard high speed steel (HSS) bits do not last long.

A twist drill is usually used, but a Brad bit can also be used. Kevlar fiber is not as stiff as carbon fiber and can be used with standard high speed steel drill bits. If a standard bit is used, the aperture quality may be poor, and the bit type is a sickle Kroenke bit. The drill first pulls on the fibers and then shears them, allowing better quality holes to be drilled. Larger holes can be cut with a diamond-coated hole saw or a fly cutter, but only with the fly cutter on the drill press, not on the drill motor.

Operation process and precautions

The composite boring motor operates between 2000-20,000 rpm and low feed rates. Bit motors with hydraulic flushing or other types of feed control are good choices because they limit bit surge out of the composite. This reduces burst damage and delamination. Parts made of belt products are particularly prone to burst damage; Parts made of fabric materials are less damaged. Composite structures need to be backed with sheet metal or sheet metal to avoid cracking. Holes in composite structures are usually pre-drilled with a small pilot hole, then enlarged with a diamond-coated bit or carbide bit, and then enlarged to the final bore size with a carbide reamer.

Back reaming is one of the conditions that may occur when carbon fiber epoxy resin parts are fitted with metal substructural parts. The back edge of the hole in the epoxy portion of the carbon fiber can be eroded or irradiated by metal shavings pulled through the composite. This is more common when there are gaps between parts, or when the metal pieces are linear rather than small pieces. Backdrilling can be reduced or eliminated by changing feed and speed, tool geometry, better part clamping, increasing final reaming, using pecking drills, or a combination of these methods.

When composite parts are drilled in combination with metal parts, the metal parts can control the drilling rate. For example, even if titanium is compatible with carbon fiber epoxies from an corrosion point of view, the drilling rate needs to be reduced to ensure that the titanium does not suffer from metallurgical damage. Titanium alloy is drilled with low speed and high feed rate. Suitable for titanium bits may not be suitable for carbon fiber or fiberglass bits. The bits used to drill titanium are usually made of cobalt vanadium; Bits used for carbon fiber are made of carbide or diamond coatings to increase bit life and produce precise holes. Small diameter high speed steel (HSS) bits, such as the No. 40 bit, are commonly used to drill pilot holes manually because carbide bits are relatively brittle and prone to breakage. The relatively low cost of these small HSS bits offsets the limited life expectancy. A high speed steel drill can only drill one hole.

The most common problem with carbide cutters in hand drilling operations is dealing with tool damage (edge off). A sharp, slow and constant feed bit can produce 0.1 mm (0.004 in) tolerance holes through carbon fiber epoxy and thin aluminum, especially if bit rails are used. With hard tools, tighter tolerances can be maintained. When the structure underneath the carbon fiber epoxy is titanium, the drill can pull the titanium chips through the carbon fiber epoxy, widening the hole. In this case, a final reaming operation may be required to maintain the aperture tolerance. The hole of the carbon fiber epoxy composite structure requires a carbide reamer. In addition, when the reamer is removed with a diameter greater than 0.13 mm(0.005 in), the outlet end of the hole needs to be well supported to prevent cracking and delamination. The support may be a substructure or a plate attached to the back surface. The typical reaming speed is about half of the drilling speed.

Cutting fluid is not usually used or recommended for drilling thin (less than 6.3 mm or 0.25 in thick) carbon fiber epoxy structures. It is a good practice to use a vacuum when drilling the composite to avoid carbon dust floating freely in the work area.

borehole

When flat head fasteners are to be installed in assemblies, a composite structure is required to be sunk. For metal structures, 100° Angle shear or tension head fasteners are typical methods. In composite structures, two types of fasteners are commonly used :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 the diameter of the tensioner 100° fastener, while the shear head 100° fastener has the same head depth. For rinsing fasteners in composite components, it is recommended that the countersunk tool be designed with a controlled radius between the hole and countersunk hole to accommodate the head-to-shank fillet radius on the fastener. In addition, chamfering operations or washers may be required to provide appropriate clearance for protruding head fasteners. Regardless of the type of head used, a matching sink or chamfer must be provided in the composite construction.

Carbide cutters are used to produce sinks of carbon fiber epoxy structure. These sink cutters usually have straight grooves similar to those used on metal. For Kevlar epoxy composites, S-shaped rake cutting grooves are used. If using a straight slot or sink slot cutter, a special thick tape can be applied to the surface to clean cut Kevlar, but this is not as effective as an S-slot cutter. Pilot countersunk knives are recommended because they ensure better concentricity between the hole and countersunk hole and reduce the possibility of clearance under the fastener due to asymmetry or layering of the part.

Use a micro countersunk bore gauge to produce consistent burials. Do not go through deep holes that are more than 70% of the surface depth, as deeper deep holes will reduce the strength of the material. When using pilot burials, the pilot must be checked regularly for wear, because wear will cause the concentricity between the hole and the burials knife to decrease. This is especially true for countersunk knives with only one cutting edge. For pilot sink cutters, the pilot is placed in the hole and the cutters are set to maximum speed before starting to run the cutters into the well and prepare the sink cutters. If the cutters come into contact with the composite material before triggering the drill motor, fragmentation may occur.

Cutting process and precautions

Metal cutting tools used in composite materials either have a short life or a poor cutting edge. The cutting tools used for composites vary depending on the composite being cut. The general rule of cutting composite materials is high speed and slow progress.

• Carbon fiber reinforced plastics: Carbon fiber is so hard that HSS tools wear out quickly. For most trimming and cutting tasks, a diamond scrub knife is best. Sanding with alumina or silicon carbide sandpaper or emery cloth. Silicon carbide has a longer life than alumina. Groove planer bits can also be made of solid carbide or diamond coating.

• Fiberglass reinforced plastics: Fiberglass, like carbon fiber, is very strong and wears out fast steel knives. The glass fiber is drilled using the same type and material drill bit as the carbon fiber.

• Kevlar® fiber reinforced plastics: Aramid fibers are not as strong as carbon fiber and glass fibers and can be used with tools made of high speed steel. In order to prevent the edge fibers of aramid composites from loosening, the parts are cut after being held. The aramid composite needs to be supported by a plastic back plate. Aramid and backup plates are cut at the same time. Aramid fibers are cut by tightening and then cutting. There's a special shape cutter that pulls on the fibers and cuts them. When using scissors to cut arnylon fabric or prepreg, there must be one side of the cutting edge and the other side of the serrated or slotted surface. These zigzags prevent the material from slipping. Sharp blades should always be used, as they reduce fiber damage. Be sure to clean the serration of the scissors immediately after use, so as not to damage the scissors with uncured resin.

Always use safety glasses and other protective equipment when working with tools and equipment.

Cutting equipment

Band saws are the most commonly used equipment for cutting composite materials in repair shops. Toothless carbide or diamond coated saw blades are recommended. A typical toothed saw blade will not last long if carbon fiber or glass fiber is cut. As shown in [Figure 88], pneumatic hand tools such as slot planers, saber saws, die grinders, and cutting wheels can be used to trim composite parts. Carbide or diamond coated knives have a better finish and longer service life. Specialized operations include ultrasonic, water jet and laser cutting machines. These types of equipment are numerically controlled (NC) and produce superior edge and hole quality. Water jet cutters cannot be used in honeycomb construction because they introduce water into parts. Do not cut anything on equipment used for composites, as other materials can contaminate the composites.