Finish Drilling

Next to turning, drilling is the most widely used machining process, accounting for about 25% of all machining operations, and as much as 60% in some small and medium industries. Despite this wide use, conventional drilling remains a rough process that generally needs to be finished by boring or reaming. The tool materials used for drills are high-speed steels, carbides, CBN, and diamonds. High-speed steel in the form of twist drills comes in all sizes and is used for rough machining. Carbide drills come in three forms: solid carbide drills of diameters of 8 mm (0.3 in.) and less, drills with brazed tips, and drills with indexable inserts for diameters of 14 mm (0.6 in.) and above. Carbide drills need to be used at much higher speeds than high-speed steel drills but result in higher productivity and better surfaces, often not requiring a secondary reaming or boring operation. Getting high speeds with small drills necessitates very high spindle speeds, and such spindles require air bearings.

Drilling of holes in printed circuit boards is done with carbide drills. As in milling, machining centers are popular for drilling. Hole fabrication by ultraprecision drilling has not gained ground, primarily because the width of cut, which is equal to the hole diameter, is very high. Using PCDs does not help because they are not good enough for ultraprecision turning and milling, let alone drilling. PCDs are used, however, for finish drilling of aluminum and copper. Precision hole fabrication can be done by techniques such as grinding (for small diameters) and boring (for large diameters where single-

crystal diamonds can be used at low depths of cut and low feeds). Nontraditional machining techniques such as electrical discharge machining, electrochemical machining, and extrusion honing are other finishing techniques that are good for putting quality holes in difficult-to-machine materials.

The strategy for finish drilling is to modify twist drills to improve the hole quality, but not at the expense of productivity. A highly successful drill is the indexable drill, or endrill, where two indexable carbide inserts are used. The two inserts are positioned so as to get a negative (Fig. 6a) or a hybrid negative-positive (Fig. 6b) point angle. The negative point angle in drilling is equivalent to a negative side-cutting-edge angle in turning, and speeds of 70 to 120 m/min (230 to 395 ft/min) give excellent surface finish, cylindricity, and roundness. At speeds of 70 m/min (230 ft/min) a builtup edge is formed. Figure 7 shows a chip root obtained by using an explosive quick-stop device. Chip roots obtained in high-speed machining should show a secondary shear zone, and this has not been reported for indexable drilling (Ref 15).

Fig. 6 Indexable drills using (a) square and rhomboid-shape inserts and (b) two trigon inserts to give (a) negative and (b) negative-positive point angles. Source: Ref 15

Fig. 7 A chip root obtained when machining 1018 steel with an indexable drill shows the presence of a builtup edge at a speed of 70 m/min (230 ft/min). Source: Ref 15

In a twist drill the two lips do the cutting while the chisel edge removes material by extrusion, which is less efficient than cutting and produces higher temperatures. This results in a lower removal rate and, hence, lower productivity. The chisel edge has a high negative rake angle equal to half the point angle (i.e., 59° in standard drills). By grinding a groove on both sides of the chisel edge (Fig. 8), the rake angle can be reduced from -59° to +5°, the thrust force can be reduced considerably, and extrusion action can be avoided. Surface roughness, roundness, and cylindricity can be improved by reducing vibration, which is often due to inefficient chip disposal. This can be achieved by providing nicks on the cutting edge of twist drills, as well as indexable drill inserts, to improve chip splitting and breaking (see Fig. 9).

Fig. 8 Thrust force reduction achieved by eliminating the chisel edge by grinding grooves on either side. (a) Line diagram. (b) Three photographic views
Fig. 9 Nicks or grooves on indexable inserts, used to reduce vibration by improving chip splitting and chip breaking. Source: Ref 15

Burr formation is undesirable, especially in fine finishing, and deburring can be a tedious and time-consuming operation. Burr height can be decreased by reducing the point angle from 118° to 90°, or by having a double cone with 118° and 90° for the two stages. A substantial decrease in burr height can be obtained by vibrating the drill in the feed direction at ultrasonic (Ref 17) or low (Ref 18) frequencies. In both cases, there is an optimum amplitude for each cutting condition.

References cited in this section

15. V.C. Venkatesh, D.Q. Zhou, W. Xue, and D.T. Quinto, A Study of Chip Surface Characterisitics during the Machining of Steel, Ann. CIRP, Vol 42 (No. 1), 1933, p 631-636

17. H. Takeyama and S. Kato, Burrless Drilling by Means of Ultrasonic Vibration, Ann. CIRP, Vol 40 (No. 1), 1991, p 83-86

18. H. Takeyama, S. Kato, S. Ishiwata, and H. Takeji, Study on Oscillating Drilling Aiming at Prevention of Burrs, J. Jpn. Soc. Precis. Eng., Vol 59 (No. 10), 1993, p 137-142

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