Info

Web Thickness

H ^ Margin

Shank

Fig. 11.20. Twist Drill Geometry steel, such as M33 or M42, yield longer lives. Carbide drills, such as grade C-2, give even longer life in titanium, but are more prone to chipping on the cutting edges.

While standard twist drills are used for drilling metallic structure, a number of unique drill geometries have been developed for composites, several of which are shown in Fig. 11.21. The design of the drill and the drilling procedures are very dependent on the materials being drilled. For example, carbon and aramid fibers exhibit different machining behavior, and therefore require different drill geometries and procedures. In addition, composite-to-metal stack-ups will also require different cutters and procedures. The flat two-flute and four-flute dagger drills were developed specifically for drilling stack-ups of carbon/epoxy. The two-flute variety is normally run at 2000-3000 rpm, while the four-flute is run at 18 000-20 000 rpm.3 When drilling through composite-to-metal stack-ups, the drill geometry is usually controlled more by the metal and standard twist drill geometries are often used. Due to their low compressive strengths, aramid fibers have a tendency to recede back into the matrix rather than being cleanly cut, resulting in fuzzing and fraying during drilling. Therefore, the aramid drill contains a "C" type cutting edge that grabs the fibers on the outside of the hole and

Two Flute Dagger

Two Flute Dagger

Four Flute Dagger

Four Flute Dagger

Note "C" Type Cutting Edge

Aramid Drill

Fig. 11.21. Composite Drill ConfigurationsL

Aramid Drill

Fig. 11.21. Composite Drill ConfigurationsL

keeps the fibers in tension during the cutting process. Typical drilling parameters for aramid fiber composites would be 5000 rpm and a feed rate of 0.001 ipr.7

While standard high speed steel (HSS) drills work well in glass and aramid composites, the extremely abrasive nature of carbon fibers requires carbide drills to obtain an adequate drill life. For example, a HSS drill may only be capable of drilling one or two acceptable holes in carbon/epoxy, while a carbide drill of the same geometry can easily generate 50 or more acceptable holes. For drilling carbon/epoxy in rigid automated drilling equipment, polycrystalline diamond (PCD) drills (Fig. 11.22) have exhibited outstanding productivity improvements. While

Carbide Slotted Blank Blank Filled with

Diamond Powder

Carbide Slotted Blank Blank Filled with

Diamond Powder

Finished Blank

Finished Blank

Press Consolidated 2700° F 900000 psi

Blank Brazed to Carbide Shar

Press Consolidated 2700° F 900000 psi

Blank Brazed to Carbide Shar

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