Drill Point Modifications

I he proper use of controlled drill poini-ing. which consists of adjustments in the included point angle, culling relief angle, and clearance angle, can result in substantially lower drilling costs for a particular operation. The effect of drill point design on drill efficiency is demonstrated in the following example.

Example 3: Chisel Versus Spiral and Self-Centering Drill Points (Fig. 21). Three different types of points on 12 mm ll?A: in.) drills were evaluated for endurance, power consumption, and size reiention. A tape-controlled drill press powered the drills as they cut through 50 mm (2 in.) I hick 1035 steel plate. The results are shown graphically in Fig. 21. As indicated, the spiral and self-centering points were nearly equal in endurance and power consumption, and both produced better results than the chisel points.

Material Considerations

The proper point angle (the angle between the two culling lips of ihe drill) depends on the material being drilled. An included point angle of 118° is usually satisfactory for drilling all carbon and alloy steels in the annealed or normalized condi-lion. For drilling heat-treated sleels as hard as 40 HRC, or for drilling stainless steels, split-point drills are preferred, with ihe poini angle increased to 125 to 135". For drilling steels or other metals al 40 to 52 HRC, drills with split points of 140 to 150° are preferred.

For drilling casl iron lo a depth up lo three diameters. Ihe standard 118° point is satisfactory; for deeper holes in cast iron, a point angle of 90 to 100" is preferred. Drills with point angles of 100 to 1I8'; are generally used lor drilling copper alloys and aluminum alloys. More detailed information on drill points for drilling metals other lhan steel is available in the Section "Machining of Specific Metals and Alloys" in this Volume.

Drill Point Alterations on Stock Tools (Ref 2)

Because drill points form the cutting edges. iheir geometries are critical to tool performance. A variety of point styles are currently being used; some of Ihe more common ones are described in this section. Proper selection, control, and use of drill points can rcsull in substantial savings in drilling costs. Single-point angles on stock drills cannot only be increased or decreased bui also modified lo form double-angle points, reduced-rake poinls. lour- or six-facet points, split points, helical points, rounded-edge poinls. or combined helical/ rounded-edge poinls.

Single-Angle Points. Standard iwist drills having conventional points with a 118" included angle arc the most commonly used because they provide satisfactory results in drilling a wide variety of materials. The culling lips on these drills are essentially straight lines, with the heel side of each land a smooth curve (Fig. 22a).

As the hardness of ihe workpiece material decreases, drill performance can be improved by reducing the included angle of the drill poini from 118" lo between 60 and 90°. Drills having these more acute point angles produce thinner chips for a given feed rate and are commonly used, with low-helix llutes. for producing holes in soil plastics and nonferrous materials. Drills having poinls with a 90' included angle are occasionally used for drilling soil cast irons and certain woods.

On the other hand, as the hardness of the workpiece material or ihe deplh of hole

Twist Drill Web Thickness

/ Widlh ol flat primary clearance equals V? web thickness at point and parallel to culling edge

30'-approximate notch angle 0-5" positive rake

Vj lip length (approximate)

118" included angle point

118 or 130' included v angle point

Drill Geometry
Fig. 23 Geometry of a split-point Twist drill, (o) and (e) Front views, (b), (c), (d), and (f) Side views

increases, the included angle of the drill point is increased from I18s to between 135 and I4(f. These larger point angles produce thicker and narrower chips for a given feed rate. Drills with these Hatter points are generally used to produce holes in harder, tougher materials, and they usually minimize burring. It is especially important to use guide bushings with drill points having higher angles because there is a tendency for the points to skid or walk on the work-piece surfaces when starting holes.

Double-Angle Points. Twist drills with double-angle points (Fig. 22b) are generated by first grinding a larger included angle (IIS or 135°) and then a smaller included angle (typically 90°) on the corners. This provides the effect of chamfers and reduces abrasive wear on the corners.

Double-angle points were originally used in drilling medium and hard cast irons as well as other very abrasive materials to reduce corner wear on the drills. More recent applications include improving hole-sizes and finishes and drilling very hard materials to reduce chipping of the corners of the lips. Twist drills with double-angle points are often used for the same applications as drills with rounded-edge (radiused hp) points (discussed later in this section).

Reduced-Rake Points. A common and easily applied point variation is the Hatted cutting lip. Both cutting edges are flatted on their flute faces (called dubbing) from the culling lip corners to the chisel edge, as illustrated in Fig. 22c. I'his type of point reduces the effective axial rake to I) to 5 positive, causing a pushing or plowing of metal rather than a shearing action. Re duced shearing action is an effective method of preventing the tools from digging in when drilling is performed on low tensile strength materials, such as many types of brass, bronze, and some of the harder acrylic plastics (for example. Plexiglas). Reducing the rake also strengthens the cutting lips, and this type of point is often used in operations in which chipping of the lips has been a problem.

Four- and Six-Facet Points. I'he geometry of a four-facet point (Fig. 22d) is generated by grinding Hat primary relief (10 to 18°) and secondary clearance angles (25 to 35") on the end of each lluic. The width of the primary relief flat is equal to one-half the web thickness, resulting in four facets on the end of the drill that subtend at a point on the drill axis and entirely remove the chisel edge. Six-facet points are produced by adding two cutting edges at the web of four-facet points.

Because these points are exactly in the middle of the drills, the tools are self-centering, and accuratc. straight holes can be produced. They also require less power and thrust and permit increased feed rates. Drills with these points, however, are subject to more wear on their margins, and they cannot be modified to suit the drilling of various materials. Another disadvantage is the cost of resharpening with a special machine.

Four- and six-facet points have found their greatest application in solid-carbide drills used to produce holes in printed circuit board materials such as llberglass-ep-oxy. The points can also be used on small-diameter high-speed tool steel drills that do not lend themselves to normal point-splitting techniques.

One major automotive manufacturer has doubled the feed rate over conventionally pointed carbide drills in drilling high siltcon-aluminum engine heads on a transfer line by using six-facet point drills. The six-facet drill points hold 0.05 mm (0.002 in.) or belter on location without a bushing plate. According to one major gear manufacturer, ihe average drill life of the six-facet point is three times that of drills with a standard chisel point in a variety of materials. This longer service life is accompanied by reduced thrust, better hole-size accuracy, and increased production.

Split Points. This type of point, also called crankshaft point, was originally developed for use on drills designed for producing small-diameter, deep holes in automotive crankshafts. Since then it has gained widespread acceptance for drilling a wide variety of hard and soft materials. Heavy-duty types with thicker wehs arc used for drilling stainless steels, titanium, tough alloys. and high-temperature resistant alloys. Drills with this type of point are also extensively used for applications in which guide bushings cannot be used, as well as for portable drilling applications.

In generating split points on drills, ihe clearance face of each cutting edge is given a sharp (55° typical) secondary relief to the ccnier of Ihe chisel edge (Fig. 23), thus creating a secondary cutting lip on Ihe opposite cutting edge. I'he angle between these lip segments acts as a chip breaker when drilling is done on many materials: this produces smaller chips, which arc readily ejected through the (lutes. More important, however, the additional culling edges produced and the reduction in width of the original chisel edge reduce thrust requirements (typically 25 to 30% compared to conventional Il8r' points) and improve the centering capability. A disadvantage is the need for a point-splitting grinding machine.

Helical (Spiral) Points. This type of point is generated by reducing the drill point from a chisel edge to a helical (spiral) point, as illustrated in Fig. 24. This produces an S-shaped chisel with a radiused crown effect thai has its highest point at Ihe cenierof the drill axis. This S-shaped chisel creates a continuous edge extending from margin to margin across the web.

The advantages of drills with a helical point include a self-centering capability and some reduction of thrust. Their use also results in better hole geometry and improved hole size.

A possible disadvantage of this type of point is that burrs are sometimes produced at hole breakthrough. In addition, Ihe S-shaped chisel is weaker than straight chisel points, resulting in faster dulling when hard materials are drilled. Special machines are required for grinding these points.

Fin 0/L helical drill point that consists of

9* an S-shaped point rother than a straight-

line chisel edge, (a) Side view, (b) Front view

Rounded-edge (radiused-iip) points arc generated by grinding a blended, rounded edge (radiused corner or lip) on conventional points (Fig. 25). Points such as these provide a continuously varying point angle, with the lips and margins blended by a smooth curve. Because the drill (also known as Racon or radiused conventional point) cuts on long, curved lips, there is less load per unit area and less heat generated. Elimination of the corner reduces margin wear. Breakthrough burrs are eliminated, and tool life can be lengthened by a factor of eight to ten times compared to conventionally pointed drills when cast iron is drilled. Feed rates can also be increased because of the improved heat dissipation.

Twist drills with rounded-edge points are used when drill life is most important. Drills with these points are not self-centering and are best applied where guide bushings are used. When ihey are used on NIC machines, prior center drilling is required. The time required for center drilling, however, may be more than offset by longer tool life. A possible limitation is that special grinding machines are required for producing these points. In addition, when steel is drilled, these points cut closer lo size, which may reduce drill life compared lo that possible with conventional points because of greater corner and margin wear.

Combined Helical/Rounded-Edge Points. Drill grinding machines are available that combine Ihe features of both the helical and rounded-edge points. The point produced (Fig. 26) provides the self-centering capability of helical points and Ihe long life, burr-free breakthrough, and higher feed capacity of rounded-edge points. These features make the drills capable of producing accurate holes on NC machines without the need for prior center drilling.

Radical Drill Point Modification

Figure 27 shows the extensive reworking done on a 17.V mm (jVm in.) drill to convert it into an end-mill type tool containing a pilot. ;ts discussed in ihe following example.

Example 4: Forming Sharp Corners in Counterbores. The two sharp-cornered counterbores in the turbine-governor cast-shown at the left in Fig. 27 were produced with a specially ground piloted drill. This tool lai right. Fig. 27l resembled a two-lip

Machinists Handbook Surface Finish

1i% C Rounded-edge drill point that contains lips " ond margins blended by smooth curves,

Hat-bottom end mill and had a primary relief of 8 to 10" and a secondary relief of 30 to 40".

I he complex hole in this part was produced by the following procedure:

• The pan was drilled through with a 7.9 mm (Vir, in.) diam standard iwisi drill

• T he through hole was enlarged, for about 41.3 mm IIV« in.I from each end. with a 17.9 mm (<%i in.) diam standard two-lip taper-shank twist drill

• The specially ground drill (Fig. 27) removed the excess stock from Ihe 17.9 mm CVm in.) diam counterbores and formed ihe sharp corners

• The counterbores were reamed lo 18.29/ 18.19 mm (0.720/0.716 in.) in diameter

These pans were drilled in a radial drill press, using a box jig and bushings lo ensure alignment of the cutting tools with the spindles. Processing details are given with Fig. 27.

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