4.  A power slip roller is often used if the repair patch  needs to be curved to fit the contour of the aircraft.  •  When hand drilling thin or flexible parts, put a support  (such as a block of wood) behind the part.  •  Titanium has a low thermal conductivity. When it  becomes hot, other metals become easily attached to  it. Particles of titanium often become welded to the  sharp edges of the drill bit if the drill speed is too high.  When drilling large plates or extrusions, use a water  soluble coolant or sulphurized oil. •  The largest diameter hole that can be drilled in a single  step is 0.1563-inch because a large force is required.  Larger diameter drill bits do not cut satisfactorily  when much force is used. Drill bits that do not cut  satisfactorily cause damage to the hole. •  Holes with a diameter of 0.1875-inch and larger can  be hand drilled if the operator: -  Starts with a hole with a diameter of 0.1563-inch. -  Increases the diameter of the hole in 0.0313-inch  or 0.0625-inch increments. •  Cobalt vanadium drill bits last much longer than HSS  bits. •  The recommended drill motor rpm settings for hand  drilling titanium are listed in  Figure 4-167 . •  The life of a drill bit is shorter when drilling titanium  than when drilling steel. Do not use a blunt drill bit  or let a drill bit rub the surface of the metal and not  cut it. If one of these conditions occurs, the titanium  surface becomes work hardened, and it is very difficult  to start the drill again. •  When hand drilling two or more titanium parts at the  same time, clamp them together tightly. To clamp  920 to 1830 rpm 460 to 920 rpm 230 to 460 rpm 0.0625 0.125 0.1875 Hole Size (inches) Drill Speed (rpm) Figure 4-167.  Hole size and drill speed for drilling titanium. Titanium can be difficult to drill, but standard high-speed  drill bits may be used if the bits are sharp, if sufficient force  is applied, and if a low-speed drill motor is used. If the drill  bit is dull, or if it is allowed to ride in a partially drilled hole,  an overheated condition is created, making further drilling  extremely difficult. Therefore, keep holes as shallow as  possible; use short, sharp drill bits of approved design; and  flood the area with large amounts of cutting fluid to facilitate  drilling or reaming.  When working titanium, it is recommended that you  use carbide or 8 percent cobalt drill bits, reamers, and  countersinks. Ensure the drill or reamer is rotating to prevent  scoring the side of the hole when removing either of them  from a hole. Use a hand drill only when positive-power-feed  drills are not available.  The following guidelines are used for drilling titanium: them together, use temporary bolts, Cleco clamps, or  tooling clamps. Put the clamps around the area to drill  and as near the area as possible. NOTE: The intimate metal-to-metal contact in the metal  working process creates heat and friction that must be  reduced or the tools and the sheet metal used in the process  are quickly damaged and/or destroyed. Coolants, also called  cutting fluids, are used to reduce the friction at the interface  of the tool and sheet metal by transferring heat away from the  tool and sheet metal. Thus, the use of cutting fluids increases  productivity, extends tool life, and results in a higher quality  of workmanship. Aircraft structural members are designed to perform a  specific function or to serve a definite purpose. The primary  objective of aircraft repair is to restore damaged parts to their  original condition. Very often, replacement is the only way  this can be done effectively. When repair of a damaged part  is possible, first study the part carefully to fully understand  its purpose or function. Strength may be the principal requirement in the repair of  certain structures, while others may need entirely different  qualities. For example, fuel tanks and floats must be protected  against leakage; cowlings, fairings, and similar parts must  have such properties as neat appearance, streamlined  shape, and accessibility. The function of any damaged part  must be carefully determined to ensure the repair meets  the requirements. An inspection of the damage and accurate estimate of the type  of repair required are the most important steps in repairing  structural damage. The inspection includes an estimate of the  best type and shape of repair patch to use; the type, size, and  number of rivets needed; and the strength, thickness, and kind  of material required to make the repaired member no heavier  (or only slightly heavier) and just as strong as the original.  When investigating damage to an aircraft, it is necessary to  make an extensive inspection of the structure. When any  component or group of components has been damaged, it is  essential that both the damaged members and the attaching  Basic Principles of Sheet Metal Repair 4-86