printer friendly version
Drills are used "new" only once. The majority of a drill’s life is spent in a reconditioned state. To realize the optimum hole making benefits throughout a drills life requires the user to recondition it before excessive wears occurs, and to do so one a regular basis. The four most common drill points are the conventional, Split-Point, notched, and helical (See Drill Dictionary on pp. 200-202 for point illustrations.) Each is reconditioned in a different way; each requires a specific machine setup to assure it is ground correctly. Four separate steps are involved in drill reconditioning: removing worn drill portions, regrinding the point surface, web thinning, and inspecting the drill. Worn Portions The outer corners are the first parts of a drill to wear (Figure 1A). Wear starts as a slight rounding, then proceeds along the drill’s margins. Wear causes a loss of size and a reverse taper of the drill body (Figure 18). Failure to remove all worn portions causes the drill to cut improperly and eventually break. There are two ways to remove worn portions. If wear is minimal, lightly regrinding the drill point surface will suffice. (Regularly recycled drills shouldn’t have to have more then 1/16” ground from their point surfaces.) For severe wear-when the drill body is worn back ¼” or more-cut away worn areas with an abrasive cutoff wheel, then restore the original point angle. Regrinding the Point Besides removing the worn portion of the drill, the surface of the point must be reground. These two conical surfaces intersect with the faces of the flutes to form the cutting lips. They also intersect with each other to form the chisel edge (figure 2). As with any other cutting tool, the surface behind the cutting lips (heel) must nit rub against the work piece during the drilling operation. To prevent rubbing, the cutting lips must be relieved which permits the chisel edge to penetrate. Without such relief, the drill would appear like Figure 3A. This drill could not penetrate material. It would only rub on the surface. Grinding clearance on the surface behind the cutting lips (Figure 3B) makes the heel lower then the cutting edge and permits the cutting lips to penetrate while drilling.  This clearance (lip relief) is measured at the periphery of the drill, across the margin portion of the land (figure 3C).		The amount of lip relief varies, depending on the material drilled. Relief is lower for drilling difficult-to-machine materials because lower relief increases lip edge strength. The degree of lip relief increase toward the center of the drill. The relive surfaces interest at the drill center and form the chisel-edge angle. There is a relationship between the chisel-edge angle and the lip-relief angle. The higher the lip-relief angle, the higher the chisel-edge angle. So, 8 to 10 degree lip relief = a 125 degree chisel-edge angle; 12 to 15 degree lip relief = a 135 degree chisel-edge angle; and when lip relief exceeds 15 degrees, chisel-edge angles may be as high are 145 degrees. These relationships apply to conventional, split, and notched points. The helical point has chisel-edge angles that are influenced by web thickness as well as lip-relief angles. The thicker the web, the lower the chisel-edge angle (Figure 4). Web Thinning For a drill to penetrate material properly, the web at the point must be a certain thickness. Most drills are manufactured so their webs increase in thickness from the drill point to the back of the flute. Removing worn portions from a drill shortens it. Accordingly, the drill point will have a thicker web at the point then it did prior to grinding, as well as a longer chisel edge (Figure 5). A drill’s chisel edge doesn’t actually cut, but wedges material out of the way. A longer chisel edge means additional power is needed to drive the drill. This generates extra heat, diminishes tool life, and could cause drill breakage. Reconditioning a drill necessitates thinning the heavier web so the chisel edge s restored to its correct length. The optimal web thickness for a particular-sized drill may vary slightly, depending on the workplace material. Drills for free-machining materials such as aluminum and mild steel have lighter webs then those designed for more difficult-to-machine materials like alloy steels. Unless unusual conditions prevail, re-pointed drills should be thinned to the specifications indicated in Table 1 on pg. 190. Web thinning should be performed with a grinder designed specifically for the task. A web-thinning grinder assures: a) equal stock is removed from each side of the web, providing good web centrality; b) uniform web shape; c) consistency from drill to drill.

Table 1. Web Thinning Specifications for Re-pointed Drills   Drill size in inches Web Thickness at Point   (% of Drill Diameter) over .0135 to .0350 30   .0320 to .0520 25   .0520 to .1875 20   .1875 to .2500 17   .2500 to .6250 15   .6250 to 1.3750 13   1.3750 to 2.3750 12   2.3750 and up 11 If a web-thinning grinder is unavailable, the operation can be performed by hand by an experienced operator. Whether by machine or by hand, the grinding wheel used should be soft enough to remove stock without burning the cutting edges. The distance web-thinning should extend up the flute is one-third to one-half the drill’s diameter. This assures that the thinning does not end abruptly and create a “chip pocket,” which prevents chips from flowing smoothly up the flutes. A chip pocket could eventually cause the drill to break. The next step is to check web centrality (Figure 6). A common way to web-thin conventional and notched points is with a radiused grinding sheel. The direction of the grind should be 3o degrees to the drill’s center-line, with a resulting rake angle equaling 8 to 12 degrees. The secondary cutting edge thatis created must be straight and at the 25-degree angle from the original cutting edge (Figure 7).		Figure 8 provides parameters for thinning the web of a split-point drill. Generally, point angles for split-points are 118 to 135 degrees. The focus here will be on the latter, because it is used most widely. When grinding the 135-degree split-point surface, the chisel-edge angle (Figure 9A) should measure 105 to 120 degrees. (See Table 2 for suggested lip-relief angles.) The two-notch/heel angles are ground Table 2. Suggested Lip Relief Angles at Periphery Drill General Hard, Tough Soft,Free- Diameters Purpose (degrees) Materials (degrees) Machining Materials (degrees) #97 to #81 28 22 30 #80 to #61 24 20 26 #60 to #41 21 18 24 #40 to #31 18 16 22 #30 to #1/4" 16 14 20 F to 11/32" 14 12 18 S to 1/2" 12 10 16 33/64" to 3/4" 10 8 14 49/64" and over 8 7 12 at a 30 to 40 degree angle (Figure 9B) so that they form a secondary chisel angle of approximately 130 degrees (Figure 9C) at the drill point center. The two notching angles should not exceed 40 degrees. A higher notch/heel angle will create a chip pocket. The notch grinds should also produce a 3 to 8 degree positive rake angel in the face of the notches; this improves the drill point’s cutting action. Helical point drills are the exception to the web-thinning rule. They require no web thinning because their design incorporates a heavy web and a point with an S-shaped chisel angle that acts as a continuous cutting edge. This positive cutting action centers the drill, reduces thrust, and improves chip formation; all of which enables the drill to cut close to the drill size.