In operation, a drill’s point does allthe cutting. It should come as no sur-prise, then, that the drill point is

where a majority of the tool’s cuttinggeometry is located.

The drill point is crammed with acomplex array of lands and edges,clearances and margins that all mustwork in unison in order to optimize adrilling operation. The first step in thisprocess is for users to learn what thedrill point’s geometric features are andhow they function.

Angle of AttackDrills are available with a variety of

point angles. The most common point,the standard 118°, balances the drill’sability to penetrate mild steels and con-trol the forces that arise when drillingthose materials. (For many years, mildsteel was the base material used for de-veloping drill designs.) However,today’s users tackle a variety of mate-rials, requiring them to apply morethan a one-size-fits-all point angle.

Softer metals, as well as many plas-tics, are easier to penetrate than hardermaterials. This improves tool perform-ance, due to the reduced forces encoun-tered when drilling. The downside,though, is the drill is also more proneto wander (or walk). The reason is be-cause when resistance to penetration islow, a drill tends to push instead ofshear material, causing it to walk.

A possible solution to the problem isto apply a drill with a steeper point

angle, such as 90°. It enters soft mate-rials more quickly and moves throughthem more efficiently than a 118°point.

A steeper point also has a longer cut-ting edge, or lip length, than a shallow-angle point. This means cutting forcesare distributed across a greater area ona steep point. Conversely, the largerarea means more of the drill contactsthe workpiece, increasing the torqueexerted on the tool.

Shallower point angles—say,140°—have proven more successful indifficult-to-drill materials than the118° point. The 140° cuts a thickerchip than a narrow point. Thick chipscarry more heat away from the workzone than thin chips.

Another benefit of the 140° point isit minimizes drill walking in hardermaterials. The shallow angle allows thecorners of the drill to contact the sur-face of the workpiece quicker; this al-lows the drill to stabilize much soonerin difficult-to-penetrate materials. Notonly does this shallow angle help onentry, it also allows the drill to exit ahole more quickly and efficiently,which, in turn reduces burr develop-ment on breakthrough.

Geometry LessonToo much clearance also causes

problems. A drill with an abundance ofclearance cuts freely, but, because ofthe excessive amount of material re-moved from behind the cutting edge, it

B Y J A S O N W E L L S

To theUnderstanding how a drill point’sgeometric features worktogether helps usersoptimize their drillingoperations.

Twist drill nomenclature.

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AUGUST 2005 / VOLUME 57 / NUMBER 8

is weaker. This can lead to edge chip-ping, cratering and catastrophic failure.

Clearance angles can be adjustedand optimized for specific materialsand applications. Angles vary accord-ing to tool diameter. Because small-di-ameter drills are much more fragilethan large-diameter tools, they requirea higher relief angle in order for thedrill to be free-cutting.

In addition to the actual clearanceangles, there also are three major typesof relief applied to drills:

■ Cam relief. This common grind isan axial-eccentric relief that runs fromthe cutting edge to the back of the land.A convex form, it follows the naturalshape of the point angle. With cam re-lief, more material is left behind thecutting edge, which creates a strongeredge. However, it does not give the

drill a satisfactory centering capabilitycompared to other forms of point re-lief, which can foster additional torqueand thrust.

■ Faceted relief. This is a flat grindthat forms a pyramid at the center ofthe tool. The relief is constant along thecutting lip; it provides the drill with anexcellent self-centering capability. Notreducing the amount of material in thecenter of the tool with this type of reliefmay result in excessive torque andthrust.

■ Helical relief. A much less com-mon relief than the first two, it is a spe-

cialized grind that features an “S”shape with a crown at the center. Al-though it reduces torque and thrust andforms a curled chip, some strength issacrificed with this design.

The Right BalanceAt the end of the web—the section

of the drill formed at the pinnacle of the drill point—is the chisel edge.The chisel edge, which connects thecutting lips, is ground at an angle andhandles more than half of the axialthrust that develops during drilling. Itslength (along with the thickness of theweb) plays a major role in determiningpenetration rates and controlling thelevel of thrust.

Many drill producers grind away asmuch of this area as possible. The rea-son they do this is to reduce thrust andimprove penetration rates. The process,called web thinning, also aids in tool

life and hole quality.There are limits to

how thin the web can be,though. As an analogy,think about driving nailsinto a piece of softwood. Less force andenergy is needed todrive a finishing nailinto the wood than topound in a large framingnail. In hard wood, con-versely, the finishingnail is more susceptibleto bending than theframing nail. Similarly,a drill manufacturermust balance factorssuch as the workpiece

material, hole size and performance re-quirements when deciding how muchto thin a drill’s web.

Besides drill performance in a spe-cific material, users are concernedabout hole quality. A drill must pro-duce a straight, round hole while meet-ing the finished-hole-size tolerance.This starts with, and is heavily depend-ent on, applying a tool whose geomet-ric features are in balance.

The drill point’s geometry must en-sure that the pressure and load on eachcutting lip is equal and that chip forma-tion is balanced. This “symmetry” is

critical to a successful drilling applica-tion. An unsymmetrical drill will pro-duce out-of-round, out-of-toleranceholes; location accuracy and hole finishwill be unsatisfactory; and tool pres-sure will be excessive, thereby short-ening wear life.

Enhancing PerformanceA common misperception among

drill users is that little can be done to adrill point’s geometry to enhance toolperformance. That is patently untrue.Drill manufacturers continually findnew ways to improve their products.

Many material- and application-spe-cific drill-point configurations havebeen introduced in the past severalyears, spawning a whole group of high-performance drills.

A common feature of high-perform-ance carbide drills is the hone. This isa manufactured wear pattern ground onthe cutting lip of the drill. It addsstrength and protects the sharp edgestypically applied to drills. By varyingthe hone width and angle, the drill-maker can optimize performance for aspecific application. But, like other fea-tures, too much of a good thing can bebad. If the hone is too large, it cancause the drill to plow instead of shear.

Another feature that can be groundon a drill is a dub. Dubbing is the act ofplacing a flat grind across the flute facesfrom the outermost diameter to thechisel edge. This alters the rake angle ofthe tool to 0° or just slightly positive.Dubbing strengthens the cutting edgebut increases the plowing effect. A dubcan be useful in materials that tend to“grab” the drill or when additionalstrength is needed to prevent chipping.

to the point

The standard 118° included point anglebalances the drill’s ability to penetratemild steels and control the forces thatarise when drilling those materials.

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A common feature of high-performance carbide drills isthe hone. It is a manufactured wear pattern ground onthe cutting lip of the drill that adds strength and protects the sharp edges.

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Many drills have a straight cuttinglip, which gives the tool a strong neu-tral cutting edge and is less of a chal-lenge to produce. A curved cutting lipalso can be applied. A concave lip,wherein a hook is ground from thetool’s center to its outer corner, helpscurl the chip. However, this featuremay result in weak, sharp corners at theoutermost diameters.

A slightly convex shape, or roundededge, also can be added. With it, theouter corner falls away from the centerof the cutting lip. This creates a strongedge and offers greater corner protec-tion, but, again, it tends to make thedrill plow instead of shear and lessensthe holemaking tool’s self-centeringcapability.

Sharp corners can easily chip orbreak. Conversely, a dull corner cancause a drill to wander or produce inac-curate holes.

Adding certain geometric featurescan diminish or even eliminate theseproblems.

An example is the addition of eithera small radius or chamfer to corners.Either of these is recommended for

abrasive workpiece materials, becausethe frictive force that develops is dis-persed across the width of the chamferor radius instead of being localized atthe sharp intersection.

Plenty of other features can beadded to a drill point’s geometry, aswell as other sections of the tool, to en-hance performance. There are toomany to discuss here. But, hopefully,readers will use this article as a basefor learning more about tool geometryand as a jumping-off point to optimiz-ing their drilling operations. q

About the AuthorJason Wells is a journeyman toolmak-er who works as director of productdevelopment and marketing for a man-ufacturer of solid-carbide round tools.

to the point

A common misperceptionamong drill users is that littlecan be done to a drill point’sgeometry to enhance tool performance. That is patentlyuntrue.

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