Patent Application
20080019787
Further features of the present invention, as well as the advantages derived therefrom, will become clear from the following detailed description made with reference to the drawings in which:
a) and 6(b) compares the hole quality produced by a 5 μm diamond coated drill of the invention and a conventional PCD drill when drilling a type A CFRP composite material, respectively;
Referring to
The drill 10 has a shank 11, a longitudinal axis 12 and includes two flutes, 14 and 16, at a helix angle 18 that is in a range between about 25 and 35 degrees with respect to the longitudinal axis 12. A margin width 24 is maintained between about 5 to 10 percent of the drill diameter 22. A body clearance diameter 26 is maintained at between about 92 to 96 percent of the drill diameter 22. A web thickness 28 (the distance between cutting lips 38 and 40) at the point 30 (before splitting) is about 20 to 30 percent of the drill diameter 22. Point angle 34 is between about 70 and 100 degrees, and preferably about 90 degrees. A clearance angle or lip relief angle 36 is between about 10 and 20 degrees. A chisel edge angle 42 is between about 105 and 120 degrees. A chisel edge length 43 is less than about 0.035 mm. A splitting angle 44 (secondary cutting edge angle) is between about 130 and 150 degrees. A notch angle 46 is between about 30 and 40 degrees with respect to the drill axis 12. A notch rake angle 48 lies between about −5 and 10 degrees.
Drill Geometry
The geometry of the drill 10 of the invention was tested and compared with a number of different geometries as listed in Table 1. The results of the comparison shows that the brad and spur point geometry and the 90-degree split point drill geometry demonstrated the results of the smallest exit hole defect size. However, the brad and spur point drill had chipping issues, and hence had a reduced tool life. Moreover, the brad and spur point drill is more difficult to grind, and its sharp edges make it unsuitable for coating. Thus, the 90-degree split point drill geometry demonstrated the best overall performance and results.
CVD Diamond Coatings
The performance of the drill 10 of the invention on two types of carbon fiber reinforced plastic (CFRP) composite materials (namely, type A and B) that are being used in aircraft skin was evaluated. In particular, the 90-degree split point drill geometry was employed in this evaluation. Diamond coatings of two different thicknesses were deposited on drill substrate made of tungsten carbide (WC) with 6 wt. % cobalt (Co) by using a chemical vapor deposition (CVD) method. It will be appreciated that the invention is not limited by the particular weight percent of cobalt, and that the invention can be practiced with a WC substrate with cemented cobalt in a range of between about 3 to 10 wt. % cobalt. The conventional polycrystalline diamond (PCD) drill is currently the dominant product on the market for CFRP drilling. The radius of the cutting edge and the thrust when drilling the first hole are listed in Table II. As shown in Table II, the 90-degree split point drill geometry produced smaller thrust and sharper cutting edge, which is beneficial to hole quality, as observed by the inventors.
a) and (b) compare the hole quality produced by a 5 μm diamond coated drill of the invention and a conventional polycrystalline diamond (PCD) drill when drilling a type A carbon fiber reinforced plastic (CFRP) composite material, respectively. As shown, the 5 μm diamond coated drill of the invention produced unexpected results of much better hole quality than the conventional PCD drill. Moreover, the 5 μm diamond coated drill of the invention produced unexpected results by outperforming the conventional PCD drill in tool life (97 holes vs. 50 holes). As a comparison, the uncoated WC-6 wt. % drill can only make 10 quality holes due to severe abrasive wear by carbon fibers.
It will be appreciated that the drill of the invention is not limited by the thickness of the diamond coating. For example, the drill of the invention can be made of a WC substrate with a diamond coating thickness in a range between about 3 to 20 μm. Further, it will be appreciated that the drill of the invention can be practiced with a radius of the cutting edge in a range between about 5 to 30 μm.
As described above, the split-point, two-fluted twist drill 10 of the invention with a WC-6 wt. % Co substrate and either a 5 μm of a 12 μm diamond coating produced unexpected results by outperforming a conventional polycrystalline diamond (PCD) drill when machining fiber reinforced composite material, such as a Type A and B carbon fiber reinforced plastic (CFRP) composite material. Further, the invention is not limited to a drill for machining CFRP composite material. For example, the drill of the invention can be used for machining glass fiber reinforced material, and the like. In addition, the drill of the invention can be used in dry or wet environments.
The documents, patents and patent applications referred to herein are hereby incorporated by reference.
While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.
