Cutting fluids are not normally used or recommended for  drilling thin (less than 6.3 mm, or 0.25-inch thick) carbon/ epoxy structure. It is good practice to use a vacuum while  drilling in composite materials to avoid that carbon dust freely  floats around the work area.  Cutters that work well for metals would either have a short  life or produce a poorly cut edge if used for composite  materials. The cutters that are used for composites vary with  the composite material that is being cut. The general rule for  cutting composites is high speed and slow feed. Countersinking a composite structure is required when  flush head fasteners are to be installed in the assembly. For  metallic structures, a 100° included angle shear or tension  head fastener has been the typical approach. In composite  structures, two types of fastener are commonly used: a 100°  included angle tension head fastener or a 130° included angle  head fastener. The advantage of the 130° head is that the  fastener head can have about the same diameter as a tension  head 100° fastener with the head depth of a shear-type head  100° fastener. For seating flush fasteners in composite parts,  it is recommended that the countersink cutters be designed  to produce a controlled radius between the hole and the  countersink to accommodate the head-to-shank fillet radius  on the fasteners. In addition, a chamfer operation or a washer  may be required to provide proper clearance for protruding  head fastener head-to-shank radii. Whichever head style is  used, a matching countersink/chamfer must be prepared in  the composite structure.  Carbide cutters are used for producing a countersink in  carbon/epoxy structure. These countersink cutters usually  have straight flutes similar to those used on metals. For  Kevlar$^{®}$ fiber/epoxy composites, S-shaped positive rake  cutting flutes are used. If straight-fluted countersink cutters  are used, a special thick tape can be applied to the surface  to allow for a clean cutting of the Kevlar$^{®}$ fibers, but this  is not as effective as the S-shaped fluted cutters. Use of a  piloted countersink cutter is recommended because it ensures  better concentricity between the hole and the countersink and  decreases the possibility of gaps under the fasteners due to  misalignment or delaminations of the part.  Always use safety glasses and other protective equipment  when using tools and equipment. Use a microstop countersink gauge to produce consistent  countersink wells. Do not countersink through more than 70  percent of the skin depth because a deeper countersink well  reduces material strength. When a piloted countersink cutter  is used, the pilot must be periodically checked for wear, as  wear can cause reduction of concentricity between the hole  and countersink. This is especially true for countersink cutters  with only one cutting edge. For piloted countersink cutters,  position the pilot in the hole and bring the cutter to full rpm  before beginning to feed the cutter into the hole and preparing  the countersink. If the cutter is in contact with the composite  before triggering the drill motor, you may get splintering. The bandsaw is the equipment that is most often used in  a repair shop for cutting composite materials. A toothless  carbide or diamond-coated saw blade is recommended. A  typical saw blade with teeth does not last long if carbon fiber  or fiberglass is cut.  [Figure 7-88]  Air-driven hand tools,  such as routers, saber saws, die grinders, and cut-off wheels  can be used to trim composite parts. Carbide or diamond- coated cutting tools produce a better finish and they last  much longer. Specialized shops have ultrasonic, waterjet,  and laser cutters. These types of equipment are numerical  controlled (NC) and produce superior edge and hole quality.  Cutting Equipment 7-52 Countersinking Cutting Processes and Precautions •  Carbon fiber reinforced plastics: Carbon fiber is very  hard and quickly wears out high speed steel cutters.  For most trimming and cutting tasks, diamond grit  cutters are best. Aluminum-oxide or silicon-carbide  sandpaper or cloth is used for sanding. Silicon-carbide  lasts longer then aluminum-oxide. Router bits can also  be made from solid carbide or diamond coated. •  Glass fiber reinforced plastics: Glass fibers, like  carbon, are very hard and quickly wear out high-speed  steel cutters. Fiberglass is drilled with the same type  and material drill bits as carbon fiber.  •  Aramid (Kevlar$^{®}$) fiber-reinforced plastics: Aramid  fiber is not as hard as carbon and glass fiber, and cutters  made from high-speed steel can be used. To prevent  loose fibers at the edge of aramid composites, hold  the part and then cut with a shearing action. Aramid  composites need to be supported with a plastic backup  plate. The aramid and backup plate are cut through at  the same time. Aramid fibers are best cut by being  held in tension and then sheared. There are specially  shaped cutters that pull on the fibers and then shear  them. When using scissors to cut aramid fabric or  prepreg, they must have a shearing edge on one blade  and a serrated or grooved surface on the other. These  serrations hold the material from slipping. Sharp blades  should always be used as they minimize fiber damage.  Always clean the scissor serrations immediately after  use so the uncured resin does not ruin the scissors.