Figure 7-85.  Klenk-type drill for drilling Kevlar®. Figure 7-86.  Drilling and cutting tools for composite materials. Figure 7-87.  Autofeed drill. 7-51 Processes and Precautions are often predrilled with a small pilot hole, enlarged with a  diamond-coated or carbide drill bit and reamed with a carbide  reamer to final hole size.  Back counterboring is a condition that can occur when  carbon/epoxy parts mate metal substructure parts. The back  edge of the hole in the carbon/epoxy part can be eroded or  radiused by metal chips being pulled through the composite.  The condition is more prevalent when there are gaps between  the parts or when the metal debris is stringy rather than small  chips. Back counterboring can be minimized or eliminated  by changing feeds and speeds, cutter geometry, better part  clamp-up adding a final ream pass, using a peck drill, or  combination of these.  When drilling combinations of composite parts with metal  parts, the metal parts may govern the drilling speed. For  example, even though titanium is compatible with carbon/ epoxy material from a corrosion perspective, lower drilling  speeds are required in order to ensure no metallurgical  damage occurs to the titanium. Titanium is drilled with low  speed and high feed. Drill bits suitable for titanium might not  be suitable for carbon or fiberglass. Drill bits that are used  for drilling titanium are often made from cobalt-vanadium;  drill bits used for carbon fiber are made from carbide or  are diamond coated to increase drill life and to produce an  accurate hole. Small-diameter high-speed steel drill bits, such  as No. 40 drill, which are used to manually drill pilot holes,  are typically used because carbide drills are relatively brittle  and are easily broken. The relatively low cost of these small  HSS drill bits offsets the limited life expectancy. High-speed  steel drill bits may last for only one hole.  The most common problem with carbide cutters used in hand- drill operations is handling damage (chipped edges) to the  cutters. A sharp drill with a slow constant feed can produce  a 0.1 mm (0.004-inch) tolerance hole through carbon/epoxy  plus thin aluminum, especially if a drill guide is used. With  hard tooling, tighter tolerances can be maintained. When the  structure under the carbon/epoxy is titanium, drills can pull  titanium chips through the carbon/epoxy and enlarge the hole.  In this case, a final ream operation may be required to hold  tight hole tolerances. Carbide reamers are needed for holes  through carbon/epoxy composite structure. In addition, the  exit end of the hole needs good support to prevent splintering  and delaminations when the reamer removes more than  about 0.13 mm (0.005-inch) on the diameter. The support  can be the substructure or a board held firmly against the  back surface. Typical reaming speeds are about one-half of  the drilling speed. Composite materials are drilled with drill motors operating  between 2,000 and 20,000 rpm and a low feed rate. Drill  motors with a hydraulic dash pod or other type of feed control  are preferred because they restrict the surging of the drill  as it exits the composite materials. This reduces breakout  damage and delaminations. Parts made from tape products  are especially susceptible to breakout damage; parts made  from fabric material have experienced less damage. The  composite structure needs to be backed with a metal plate  or sheet to avoid breakout. Holes in composite structures 