driling

www.toolingandproduction.com Chapter 8/Tooling & Production 1

8.1 IntroductionDrilling is the process most commonly associatedwith producing machined holes. Although manyother processes contribute to the production ofholes, including boring, reaming, broaching, andinternal grinding, drilling accounts for the major-ity of holes produced in the machine shop. Thisis because drilling is a simple, quick, and eco-nomical method of hole production. The othermethods are used principally for more accurate,smoother, larger holes. They are often used aftera drill has already made the pilot hole.

Drilling is one of the most complex machiningprocesses. The chief characteristic that distin-guishes it from other machining operations is thecombined cutting and extrusion of metal at thechisel edge in the center of the drill. The highthrust force caused by the feeding motion firstextrudes metal under the chisel edge. Then ittends to shear under the action of a negative rakeangle tool. Drilling of a single hole is shown inFigure 8.1 and high production drilling of a platecomponent is shown in Figure 8.2.

Chapter 8Drills & DrillingOperations

Upcoming Chapters

Metal RemovalCutting-Tool Materials

Metal Removal MethodsMachinability of Metals

Single Point MachiningTurning Tools and Operations

Turning Methods and MachinesGrooving and Threading

Shaping and Planing

Hole Making ProcessesDrills and Drilling Operations

Drilling Methods and MachinesBoring Operations and Machines

Reaming and Tapping

Multi Point MachiningMilling Cutters and OperationsMilling Methods and Machines

Broaches and BroachingSaws and Sawing

Finishing ProcessesGrinding Wheels and OperationsGrinding Methods and Machines

Lapping and Honing

George Schneider, Jr. CMfgEProfessor Emeritus

Engineering TechnologyLawrence Technological University

Former ChairmanDetroit Chapter ONE Society of Manufacturing Engineers

Former PresidentInternational Excutive BoardSociety of Carbide & Tool Engineers

Lawrence Tech. Univ.: http://www.ltu.edu

Prentice Hall: http://www.prenhall.comFIGURE 8.2: Holes can be drilled individually as shown in Figure 8.1, or many holes can bedrilled at the same time as shown here. (Courtesy Sandvik Coromant Co.)

FIGURE 8.1: Drilling accounts for themajority of holes produced in industrytoday. (Courtesy Valenite Inc.)

http://www.engarena.com

http://www.engarena.com/

Chap. 8: Drills & Drilling Operations

2 Tooling & Production/Chapter 8 www.toolingandproduction.com

The cutting action along the lips ofthe drill is not unlike that in othermachining processes. Due to variablerake angle and inclination, however,there are differences in the cuttingaction at various radii on the cuttingedges. This is complicated by the con-straint of the whole chip on the chipflow at any single point along the lip.Still, the metal removing action is truecutting, and the problems of variablegeometry and constraint are present, butbecause it is such a small portion of thetotal drilling operation, it is not a distin-guishing characteristic of the process.Many of the drills discussed in thischapter are shown in Figures 8.3.

The machine settings used in drillingreveal some important features of thishole producing operation. Depth of cut,a fundamental dimension in other cut-ting processes, corresponds most close-ly to the drill radius. The undeformedchip width is equivalent to the length ofthe drill lip, which depends on the pointangle as well as the drill size. For agiven set-up, the undeformed chipwidth is constant in drilling. The feeddimension specified for drilling is thefeed per revolution of the spindle. Amore fundamental quantity is the feedper lip. For the common two-flute drill,it is half the feed per revolution. Theundeformed chip thickness differs fromthe feed per lip depending on the pointangle.

The spindle speed is constant for anyone operation, while the cutting speedvaries all along the cutting edge.Cutting speed is normally computed forthe outside diameter. At the center of

the chisel edge the cutting speed is zero;at any point on the lip it is proportionalto the radius of that point. This varia-tion in cutting speed along the cuttingedges is an important characteristic ofdrilling.

Once the drill engages the workpiece,the contact is continuous until the drillbreaks through the bottom of the part oris withdrawn from the hole. In thisrespect, drilling resembles turning andis unlike milling. Continuous cuttingmeans that steady forces and tempera-tures may be expected shortly after con-tact between the drill and the work-piece.

8.2 Drill NomenclatureThe most important type of drill is thetwist drill. The important nomenclaturelisted below and illustrated in Figure 8.4applies specifically to these tools.

Drill: A drill is an end-cutting toolfor producing holes. It has one or morecutting edges, and flutes to allow fluidsto enter and chips to be ejected. Thedrill is composed of a shank, body, and

point.Shank: The shank is the

part of the drill that is heldand driven. It may be straightor tapered. Smaller diameterdrills normally have straightshanks. Larger drills haveshanks ground with a taperand a tang to insure accuratealignment and positive drive.

Tang: The tang is a flat-tened portion at the end of theshank that fits into a drivingslot of the drill holder on thespindle of the machine.

Body: The body of thedrill extends from the shankto the point, and contains theflutes. During sharpening, itis the body of the drill that is

partially ground away.Point: The point is the cutting end of

the drill.Flutes: Flutes are grooves that are

cut or formed in the body of the drill toallow fluids to reach the point and chipsto reach the workpiece surface.Although straight flutes are used insome cases, they are normally helical.

Land: The land is the remainder ofthe outside of the drill body after theflutes are cut. The land is cut backsomewhat from the outside drill diame-ter in order to provide clearance.

Margin: The margin is a short por-tion of the land not cut away for clear-ance. It preserves the full drill diameter.

Web: The web is the central portionof the drill body that connects the lands.

Chisel Edge: The edge ground onthe tool point along the web is called thechisel edge. It connects the cutting lips.

Lips: The lips are the primary cut-ting edges of the drill. They extendfrom the chisel point to the periphery ofthe drill.

Axis: The axis of the drill is the cen-terline of the tool. It runs through the weband is perpendicular to the diameter.

Neck: Some drills are made with arelieved portion between the body andthe shank. This is called the drill neck.

In addition to the above terms thatdefine the various parts of the drill,there are a number of terms that apply tothe dimensions of the drill, includingthe important drill angles. Among theseterms are the following:

Length: Along with its outsidediameter, the axial length of a drill islisted when the drill size is given. Inaddition, shank length, flute length, andneck length are often used.(see Fig. 8.4)

Body Diameter Clearance: Theheight of the step from the margin to theland is called the body diameter clear-ance.

FIGURE 8.3: Many of the drills used in industry areshown here and described in this chapter. (CourtesyCleveland Twist Drill Greenfield Industries)

FIGURE 8.4: Nomenclature of a twist drill shown with taper and tang drives.

Taper shank

TangTang drive

Neck

Shankdiameter

AxisStraightshank

Land width

Point angle

Lip relief angle

Helix angle

Drill diameterClearance Diameter

Body Diameter Clearance

Chisel Edge Angle

Shank length

Overall length

Flutes

Flute length

MarginLip

WebChisel edge

Land

Body





Web Thickness: The web thicknessis the smallest dimension across theweb. It is measured at the point unlessotherwise noted. Web thickness willoften increase in going up the bodyaway from the point, and it may have tobe ground down during sharpening toreduce the size of the chisel edge. Thisprocess is called ‘web thinning’. Webthinning is shown in Figure 8.13.

Helix Angle: The angle that the lead-ing edge of the land makes with the drillaxis is called the helix angle. Drillswith various helix angles are availablefor different operational requirements.

Point Angle: The included anglebetween the drill lips is called the pointangle. It is varied for different work-piece materials.

Lip Relief Angle: Corresponding tothe usual relief angles found on othertools is the lip relief angle. It is mea-sured at the periphery.

Chisel Edge Angle: The chisel edgeangle is the angle between the lip andthe chisel edge, as seen from the end ofthe drill.

It is apparent from these partial listsof terms that many different drillgeometries are possible.

8.3 Classes of DrillsThere are different classes of drills fordifferent types of operations.Workpiece materials may also influencethe class of drill used, but it usuallydetermines the point geometry ratherthan the general type of drill best suited

for the job. It has already been notedthat the twist drill is the most importantclass. Within the general class of twistdrills there are a number of drill typesmade for different kinds of operations.Many of the special drills discussedbelow are shown in Figure 8.5.

High Helix Drills: This drill has ahigh helix angle, which improves cut-ting efficiency but weakens the drillbody. It is used for cutting softer metalsand other low strength materials.

Low Helix Drills: A lower than nor-mal helix angle is sometimes useful toprevent the tool from ‘running ahead’ or‘grabbing’ when drilling brass and sim-ilar materials.

Heavy-duty Drills: Drills subject tosevere stresses can be made stronger bysuch methods as increasing the webthickness.

Left Hand Drills: Standard twistdrills can be made as left hand tools.These are used in multiple drill headswhere the head design is simplified byallowing the spindle to rotate in differ-ent directions.

Straight Flute Drills: Straight flutedrills are an extreme case of low helixdrills. They are used for drilling brassand sheet metal.

Crankshaft Drills: Drills that areespecially designed for crankshaft workhave been found to be useful formachining deep holes in tough materi-als. They have a heavy web and helixangle that is somewhat higher than nor-mal. The heavy web prompted the use

of a specially notched chisel edge thathas proven useful on other jobs as well.The crankshaft drill is an example of aspecial drill that has found wider appli-cation than originally anticipated andhas become standard.

Extension Drills: The extensiondrill has a long, tempered shank toallow drilling in surfaces that are nor-mally inaccessible.

Extra-length Drills: For deep holes,the standard long drill may not suffice,and a longer bodied drill is required.

Step Drill: Two or more diametersmay be ground on a twist drill to pro-duce a hole with stepped diameters.

Subland Drill: The subland ormulti-cut drill does the same job as thestep drill. It has separate lands runningthe full body length for each diameter,whereas the step drill uses one land. Asubland drill looks like two drills twist-ed together.

Solid Carbide Drills: For drillingsmall holes in light alloys and non-metallic materials, solid carbide rodsmay be ground to standard drill geome-try. Light cuts without shock must betaken because carbide is quite brittle.

Carbide Tipped Drills: Carbide tipsmay be used on twist drills to make theedges more wear resistant at higherspeeds. Smaller helix angles and thick-er webs are often used to improve therigidity of these drills, which helps topreserve the carbide. Carbide tippeddrills are widely used for hard, abrasivenon-metallic materials such as masonry.

Oil Hole Drills: Small holes throughthe lands, or small tubes in slots milledin the lands, can be used to force oilunder pressure to the tool point. Thesedrills are especially useful for drillingdeep holes in tough materials.

Flat Drills: Flat bars may be groundwith a conventional drill point at theend. This gives very large chip spaces,but no helix. Their major application isfor drilling railroad track.

Three and Four Fluted Drills:There are drills with three or four fluteswhich resemble standard twist drillsexcept that they have no chisel edge.They are used for enlarging holes thathave been previously drilled orpunched. These drills are used becausethey give better productivity, accuracy,and surface finish than a standard drillwould provide on the same job.

Drill and Countersink: A combina-tion drill and countersink is a useful tool

FIGURE 8.5: Special drills are used for some drilling operations.

(a) Jobber s drill

(b) Low-helix drill

(c) High-helix drill

(d) Straight-shank oil-hole drill

(e) Screw-machine drill

(f) Three-flute core drill

(g) Left-hand drill

(h) Straight-flute drill

(i) Step drill

(j) Subland drill





for machining ‘center holes’ on bars tobe turned or ground between centers.The end of this tool resembles a stan-dard drill. The countersink starts a shortdistance back on the body.

A double-ended combination drilland countersink, also called a centerdrill, is shown in Figure 8.6.

8.4 Related Drilling OperationsSeveral operations are related todrilling. In the following list, most ofthe operations follow drilling except forcentering and spotfacing which precededrilling. A hole must be made first bydrilling and then the hole is modified byone of the other operations. Some ofthese operations are described here andillustrated in Figure 8.7

Reaming: A reamer is used toenlarge a previously drilled hole, to pro-vide a higher tolerance and to improve

the surface finish of the hole.Tapping: A tap is used to provide

internal threads on a previously drilledhole.

Reaming and tapping are moreinvolved and complicated thancounterboring, countersinking,centering, and spot facing, andare therefore discussed inChapter 11.

Counterboring: Counterboringproduces a larger step in a hole toallow a bolt head to be seated

below the part surface.Countersinking: Countersinking is

similar to counterboring except that thestep is angular to allow flat-head screwsto be seated below the surface.

Counterboring tools are shown inFigure 8.8a, and a countersinking toolwith two machined holes is shown inFigure 8.8b.

Centering: Center drilling is usedfor accurately locating a hole to bedrilled afterwards.

Spotfacing: Spotfacing is used toprovide a flat-machined surface on apart.

8.5 Operating ConditionsThe varying conditions, under whichdrills are used, make it difficult togive set rules for speeds and feeds. Drillmanufacturers and a variety of referencetexts provide recommendations forproper speeds and feeds for drilling avariety of materials. General drillingspeeds and feeds will be discussed hereand some examples will be given.

Drilling Speed: Cutting speed maybe referred to as the rate that a point ona circumference of a drill will travel in 1minute. It is expressed in surface feetper minute (SFPM). Cutting speed isone of the most important factors thatdetermine the life of a drill. If the cut-ting speed is too slow, the drill mightchip or break. A cutting speed that istoo fast rapidly dulls the cutting lips.Cutting speeds depend on the followingseven variables:• The type of material being drilled.

The harder the material, the slowerthe cutting speed.

• The cutting tool material and diame-

FIGURE 8.6: A double-ended combination drilland countersink, also called a center drill.(Courtesy Morse Cutting Tools)

FIGURE 8.7: Related drilling operations: (a) reaming, (b) tapping, (c) counterboring,(d) countersinking, (e) centering, (f) spotfacing.

(a) (b)FIGURE 8.8: Counterboring tools (a) and countersinking operation (b) are shown here.(Courtesy The Weldon Tool Co.)

(a) (b) (c)

(d) (e) (f)





ter. The harder the cutting tool materi-al, the faster it can machine the materi-al. The larger the drill, the slower thedrill must revolve.• The types and use of cutting fluids

allow an increase in cutting speed.• The rigidity of the drill press.• The rigidity of the drill (the shorter

the drill, the better).• The rigidity of the work setup.• The quality of the hole to be drilled.

Each variable should be consideredprior to drilling a hole. Each variable isimportant, but the work material and itscutting speed are the most importantfactors. To calculate the revolutions perminute (RPM) rate of a drill, the diame-ter of the drill and the cutting speed ofthe material must be considered.

The formula normally used to calcu-late cutting speed is as follows:

SFPM = (Drill Circumference) x (RPM)

Where:SFPM = surface feet per minute, or

the distance traveled by a point on the drill periphery in feet each minute.

Drill Circumference = the distance around the drill periphery in feet.

RPM = revolutions per minute

In the case of a drill, the circumfer-ence is:Drill Circumference =

Pi/12 x (d) = .262 x d

Where:Drill Circumference = the distance

around the drill periphery in feet.Pi = is a constant of 3.1416d = the drill diameter in inches.

By substituting for the drill circum-ference, the cutting speed can now bewritten as:

SFPM = .262 x d x RPM

This formula can be used to deter-mine the cutting speed at the peripheryof any rotating drill.

For example: Given a .25 inch drill,what is the cutting speed (SFPM)drilling cast iron at 5000 RPM?

SFPM = .262 x d x RPMSFPM = .262 x .25 x 5000Answer = 327.5 or 327 SFPM

RPM can be calculated as follows: Given a .75 inch drill, what is the

RPM drilling low carbon steel at 400SFPM?

SFPM 400 400RPM = ________ = ________ = ____.262 x d .262 x .75 .1965

Answer = 2035.62 or 2036 RPM

Drilling Feed: Once the cuttingspeed has been selected for a particularworkpiece material and condition, theappropriate feed rate must be estab-lished. Drilling feed rates are selectedto maximize productivity while main-taining chip control. Feed in drillingoperations is expressed in inches perrevolution, or IPR, which is the distancethe drill moves in inches for each revo-lution of the drill. The feed may also beexpressed as the distance traveled by thedrill in a single minute, or IPM (inchesper minute), which is the product of theRPM and IPR of the drill. It can be cal-culated as follows:

IPM = IPR x RPM

Where:IPM = inches per minuteIPR = inches per revolutionRPM = revolutions per minute.

For example: To maintain a .015 IPRfeed rate on the .75 inch drill discussedabove, what would the IPM feed ratebe?

IPM = PR x RPMIPM = .015 x 2036Answer = 30.54 or 31 IPM

The selection of drilling speed(SFPM) and drilling feed (IPR) for var-ious materials to be machined oftenstarts with recommendations in the

form of application tables from manu-facturers or by consulting referencebooks.

8.5.1 Twist Drill WearDrills wear starts as soon as cuttingbegins and instead of progressing at aconstant rate, the wear accelerates con-tinuously. Wear starts at the sharp cor-ners of the cutting edges and, at thesame time, works its way along the cut-ting edges to the chisel edge and up thedrill margins. As wear progresses,clearance is reduced. The resulting rub-bing causes more heat, which in turncauses faster wear.

Wear lands behind the cutting edgesare not the best indicators of wear, sincethey depend on the lip relief angle. Thewear on the drill margins actually deter-mines the degree of wear and is notnearly as obvious as wear lands. Whenthe corners of the drill are rounded off,the drill has been damaged more than isreadily apparent. Quite possibly thedrill appeared to be working properlyeven while it was wearing. The marginscould be worn in a taper as far back asan inch from the point. To restore thetool to new condition, the worn areamust be removed. Because of theaccelerating nature of wear, the numberof holes per inch of drill can sometimesbe doubled by reducing, by 25 percent,the number of holes drilled per grind.

8.5.2 Drill Point GrindingIt has been estimated that about 90 per-cent of drilling troubles are due toimproper grinding of the drill point.Therefore, it is important that care betaken when resharpening drills. A gooddrill point will have: both lips at thesame angle to the axis of the drill; bothlips the same length; correct clearanceangle; and correct thickness of web.

(a) (b)FIGURE 8.9: The included lip angle varies between 90 and 135 degrees (a): two drillpoints are shown in (b). (Courtesy Cleveland Twist Drill Greenfield Industries)

C2

C

C

2





Lip Angle and Lip Length: Whengrinding the two cutting edges theyshould be equal in length and have thesame angle with the axis of the drill asshown in Figure 8.9a. Figure 8.9bshows two ground drill points.

For drilling hard or alloy steels, angleC (Fig. 8.9a) should be 135 degrees.For soft materials and for general pur-poses, angle C should be 118 degrees.For aluminum, angle C should be 90degrees.

If lips are not ground at the sameangle with the axis, the drill will be sub-jected to an abnormal strain, becauseonly one lip comes in contact with thework. This will result in unnecessarybreakage and also cause the drill to dullquickly. A drill so sharpened will drillan oversized hole. When the point isground with equal angles, but has lips ofdifferent lengths, a condition as shownin Figure 8.10a is produced.

A drill having cutting lips of differentangles, and of unequal lengths, will belaboring under the severe conditionsshown in Figure 8.10b.

Lip Clearance Angle: The clearanceangle, or ‘backing-off’ of the point, is

the next importantthing to consider.When drilling steelthis angle A (Fig.8.11a) should befrom 6 to 9 degrees.For soft cast ironand other soft mate-rials, angle A maybe increased to 12degrees (or even 15degrees in somecases)

This clearanceangle shouldincrease gradually

as the center of the drill isapproached. The amount ofclearance at the center of thedrill determines the chiselpoint angle B (Fig. 8.11b).

The correct com-bination of clearanceand chisel pointangles should be asfollows: When angleA is made to be 12degrees for softmaterials, angle Bshould be madeapproximately 135degrees; when angleA is 6 to 9 degreesfor harder materials,angle B should be115 to 125 degrees.

While insufficientclearance at the cen-ter is the cause ofdrills splitting up the web, too muchclearance at this point will cause thecutting edges to chip.

In order to maintain the necessaryaccuracy of point angles, lip lengths, lipclearance angle, and chisel edge angle,

the use of machine point grinding is rec-ommended. There are many commer-cial drill point grinders available today,which will make the accurate repointingof drills much easier. Tool and cuttergrinders such as the one shown inFigure 8.12 are often used.

Twist Drill Web Thinning: Thetapered web drill is the most commontype manufactured. The web thicknessincreases as this type of drill is resharp-ened. This requires an operation calledweb thinning to restore the tool’s origi-nal web thickness. Without the webthinning process, more thrust would berequired to drill, resulting in additionalgenerated heat and reduced tool life.Figure 8.13 illustrates a standard drillbefore and after the web thinningprocess. Thinning is accomplished witha radiused wheel and should be done sothe thinned section tapers gradually

(a) (b)

(a) (b)

A B

Roll type Dub type Notch type

Originalchisel edge

Chisel edgeafter drillhas been

shortened

FIGURE 8.10: Drill with equal lip angle but unequallip length (a), and drill with unequal lip angle andunequal lip length (b).

FIGURE 8.12: Tool and cutter grinders, are used toproperly sharpen drills and other cutting tools.(Courtesy K. O. Lee Co.)

FIGURE 8.13: Web thinning restores proper web thickness aftersharpening twist drills; three methods are shown.

FIGURE 8.11: Drill lip clearance angle (a) and drill chisel point angle (b).





from the point. This prevents a bluntwedge from being formed that would bedetrimental to chip flow. Thinning canbe done by hand, but since point cen-trality is important, thinning by machineis recommended.

8.6 Spade DrillsThe tool generally consists of a cuttingblade secured in a fluted holder (SeeFigure 8.14). Spade drills can machinemuch larger holes (up to 15 in. in diam-eter) than twist drills. Spade drills usu-ally are not available in diameterssmaller than 0.75 inch. The drillingdepth capacity of spade drills, withlength-to-diameter ratios over 100 to 1possible, far exceeds that of twist drills.At the same time, because of their muchgreater feed capability, the penetrationrates for spade drills exceed those oftwist drills by 60 to 100 percent.However, hole finish generally suffersbecause of this. Compared to twistdrills, spade drills are much more resis-tant to chatter under heavy feeds oncethey are fully engaged with the work-piece. Hole straightness is generallyimproved (with comparable size capa-bility) by using a spade drill. However,these advantages can only be gained byusing drilling machines of suitablecapability and power.

The spade drill is also a very eco-nomical drill due to its diameter flexi-bility. A single holder will accommo-date many blade diameters as shown in

Figure 8.14. Therefore, whena diameter change isrequired, only the bladeneeds to be purchased whichis far less expensive thanbuying an entire drill.

8.6.1 Spade Drill BladesThe design of spade drillblades varies with the manu-facturer and the intendedapplication. The most com-mon design is shown in Figure8.15. The locator length isground to a precision dimen-sion that, in conjunction withthe ground thickness of theblade, precisely locates the blade in itsholder. When the seating pads properlycontact the holder, the holes in the bladeand holder are aligned and the assemblycan be secured with a screw.

The blade itself as shown in Figure8.15, possesses all the cutting geometrynecessary. The point angle is normally130 degrees but may vary for specialapplications. In twist drill designs, thehelix angle generally determines thecutting rake angle but since spade drillshave no helix, the rake surface must beground into the blade at the cutting edgeangle that produces the proper webthickness. The cutting edge clearanceangle is a constant type of relief, gener-ally 6 to 8 degrees. After this clearanceis ground, the chip breakers are ground,about 0.025 inch deep, in the cutting

edge.These chip breakers are nec-

essary on spade drill blades andnot optional as with twist drills.These notches make the chipsnarrow enough to flush aroundthe holder. Depending on thefeed rate, the grooves can alsocause a rib to form in the chip.The rib stiffens the chip andcauses it to fracture or breakmore easily which results inshorter, more easily removedchips. Margins on the blade actas bearing surfaces once thetool is in a bushing or in thehole being drilled. The widthof the margins will vary from1/16 to 3/16 inches, dependingon the tool size. A slight backtaper of 0.004 to 0.006 inch isnormally provided and outsidediameter clearance angles aregenerally 10 degrees.

8.6.2 Spade Drill Blade HoldersThe blade holder makes up the majorpart of the spade drill. The blade hold-er is made of heat-treated alloy steel andis designed to hold a variety of blades ina certain size range as shown in Figure8.14. Two straight chip channels orflutes are provided for chip ejection.

The holder shank designs are avail-able in straight, Morse taper, and vari-ous other designs to fit the machinespindles. The holders are generally sup-plied with internal coolant passages toensure that coolant reaches the cuttingedges and to aid chip ejection.

When hole position is extremely crit-ical and requires the use of a startingbushing, holders with guide strips areavailable. These strips are ground to fitclosely with the starting bushing to sup-port the tool until it is fully engaged inthe workpiece. The strips may also beground to just below the drill diameterto support the tool in the hole when theset-up lacks rigidity.

8.6.3 Spade Drill Feeds and SpeedsThe cutting speed for spade drills isgenerally 20 percent less than for twistdrills. However, the spade drill feedcapacity can be twice that of twist drills.The manufacturers of spade drills andother reference book publishers provideexcellent recommendations for machin-ing rates in a large variety of metals.These published rates should generallybe observed. Spade drills work bestunder moderate speed and heavy feed.Feeding too lightly will result in eitherlong, stringy chips or chips reducedalmost to a powder. The drill cuttingedges will chip and burn because of theabsence of the thick, heat absorbing, C-shaped chips. Chips can possibly jam

FIGURE 8.14: Spade drills with various cuttingblades. (Courtesy Kennametal Inc.)

FIGURE 8.15: Spade drill cutting blades shows geometry specifications.

Seating pads

Rake surface

Chipbreakers

MarginBladethickness

Chiseledge

Web

Back taper Point angle O.D. Clearance

Locatorlength





and pack, which can break the tool orthe workpiece. If the machine cannotsupply the required thrust to maintainthe proper feed without severe deflec-tion, a change in tool or machine maybe necessary.

8.7 Indexable Carbide DrillsIndexable drilling has become so effi-cient and cost effective that in manycases it is less expensive to drill the holerather than to cast or forge it. Basically,the indexable drill is a two fluted, centercutting tool with indexable carbide

inserts. Indexable drills wereintroduced using square inserts(see Fig. 8.16). Shown in Figure8.17a are indexable drills usingthe more popular Trigon Insert(see Fig. 8.17b). In most casestwo inserts are used, but as sizeincreases, more inserts are addedwith as many as eight inserts invery large tools. Figure 8.18shows six inserts being used.

Indexable drills have the prob-lem of zero cutting speed at thecenter even though speeds canexceed 1000 SFPM at the outer-most inserts. Because speed gen-erally replaces feed to somedegree, thrust forces are usually25 to 30 percent of those requiredby conventional tools of the samesize. Indexable drills have ashank, body, and multi-edgedpoint. The shank designs gener-ally available are straight, taperedand number 50 V-flange.

The bodies have two flutes that arenormally straight but may be helical.Because no margins are present to pro-vide bearing support, the tools must relyon their inherent stiffness and on thebalance in the cutting forces to maintainaccurate hole size and straightness.Therefore, these tools are usually limit-ed to length-to-diameter ratios ofapproximately 4 to 1.

The drill point is made of pocketedcarbide inserts. These inserts are usual-ly specially designed. The cutting rakecan be negative, neutral, or positive,

depending on holder and insert design.Coated and uncoated carbide grades areavailable for drilling a wide variety ofwork materials. Drills are sometimescombined with indexable or replaceableinserts to perform more than one opera-tion, such as drilling, counterboring,and countersinking.

As shown in Figure 8.19a and Figure8.19b, body mounted insert tooling canperform multiple operations. Moreexamples will be shown and discussedin Chapter 10: Boring Operations andMachines.

The overall geometry of the cuttingedges is important tothe performance ofindexable drills. Asmentioned earlier,there are no support-ing margins to keepthese tools on line, sothe forces required tomove the cuttingedges through thework material must bebalanced to minimizetool deflection, partic-ularly on starting, andto maintain hole size.

While they areprincipally designedfor drilling, someindexable drills, asshown in Figure 8.20,can perform facing,and boring in lathe

FIGURE 8.18: Indexable drill using sixTrigon inserts for drilling large holes.(Courtesy Kennametal Inc.)FIGURE 8.16: Indexing drills were introduced

using square inserts; three sizes are shown here.(Courtesy Kennametal Inc.)

12¡

84¡156¡

(b)

FIGURE 8.17: (a) Indexable drills using Trigon inserts. (b) A Trigon insert and holder. (Courtesy Komet ofAmerica, Inc.)

(a)





applications. How well these tools per-form in these applications depends ontheir size, rigidity, and design.

8.7.1 Indexable Carbide Drill Operation

When used under the proper conditions,the performance of indexable drills isimpressive. However, the manufactur-er’s recommendations must be carefullyfollowed for successful applications.

Set-up accuracy and rigidity is mostimportant to tool life and performance.Chatter will destroy drilling inserts just asit destroys turning or milling inserts. Ifthe inserts fail when the tool is rotating inthe hole at high speed, the holder andworkpiece will be damaged. Even if lackof rigidity has only a minor effect on toollife, hole size and finish will be poor. Themachine must be powerful, rigid and

capable of high speed. Radial drill press-es do not generally meet the rigidityrequirements. Heavier lathes, horizontalboring mills, and N/C machining centersare usually suitable.

When installing the tool in themachine, the same good practice fol-lowed for other drill types should beobserved for indexable drills. Theshanks must be clean and free fromburrs to ensure good holding and tominimize runout. Runout in indexabledrilling is dramatically amplifiedbecause of the high operating speedsand high penetration rates.

When indexing the inserts is neces-sary, make sure that the pockets areclean and undamaged. A small speck ofdirt or chip, or a burr will cause stress inthe carbide insert and result in a micro-scopic crack, which in turn, will lead toearly insert failure.

8.7.2 Indexable Drill Feeds and Speeds

Indexable drills are very sensitive tomachining rates and work materials.The feed and speed ranges for variousmaterials, as recommended by somemanufacturers of these tools, can bevery broad and vague, but can be usedas starting points in determining exactfeed and speed rates. Choosing the cor-rect feed and speed rates, as well asselecting the proper insert style andgrade, requires some experimentation.Chip formation is a critical factor andmust be correct.

In general, soft low carbon steel callsfor high speed (650 SFPM or more),and low feed (0.004/0.006 IPR).Medium and high carbon steels, as wellas cast iron, usually react best to lowerspeed and higher feed. The exact speedand feed settings must be consistentwith machine and set-up conditions,hole size and finish requirements, andchip formation for the particular job.

8.8 TrepanningIn trepanning the cutting tool produces ahole by removing a disk shaped piecealso called slug or core, usually fromflat plates. A hole is produced withoutreducing all the material removed tochips, as is the case in drilling. Thetrepanning process can be used to makedisks up to 6 in. in diameter from flatsheet or plate. A trepanning tool alsocalled a “Rotabroach” with a core orslug is shown in Figure 8.21a and anend view of a Rotabroach is shown inFigure 8.21 b.

Trepanning can be done on lathes,drill presses, and milling machines, aswell as other machines using singlepoint or multi point tools. Figure 8.22shows a Rotabroach cutter machiningholes through both sides of a rectangu-lar tube on a vertical milling machine.

Rotabroach drills provide greater toollife because they have more teeth thanconventional drilling tools. Since moreteeth are engaged in the workpiece, thematerial cut per hole is distributed overa greater number of cutting edges.Each cutting edge cuts less material fora given hole. This extends tool life sig-nificantly.

Conventional drills must contendwith a dead center area that is prone tochip, thus reducing tool life. In the chis-el-edge region of a conventional drillthe cutting speed approaches zero. This

FIGURE 8.19: Body-mounted insert tool-ing can perform multiple operations.

(Courtesy Komet of America, Inc.)

FIGURE 8.20: In addition to drilling, indexable drills can perform boring and facingoperations.

(b)

To diameter Larger than Diameter

Boring Facing

(a)





is quite different from the speed at thedrill O.D. Likewise, thrust forces arehigh due to the point geometry.Rotabroach drills cut in the region fromthe slug O.D. to the drill O.D. Sinceonly a small kerf is machined, cuttingspeeds are not so different across theface of a tooth. This feature extendstool life and provides uniform machin-

a b i l i t y .Figure 8.23shows drillingholes with conventional drills and holebroaching drills.

8.8.1 Trepanning OperationsTrepanning is a roughing operation.Finishing work requires a secondaryoperation using reamers or boring barsto get a specified size and finish. Of themany types of hole-making operations,it competes with indexable carbide cut-ters and spade drilling.

Several types of tools are used to trepan.The most basic is a single or double pointcutter (Fig. 8.24). It orbits the spindle cen-terline cutting the periphery of the hole.Usually, a pilot drill centers the tool anddrives the orbiting cutter like a compassinscribing a circle on paper. Single/doublepoint trepanning tools are often adjustablewithin their working diameter. They areefficient and versatile, but do begin to haverigidity problems when cutting large holes- 6 1/2 inches in diameter is about the max-imum.

A hole saw is another tool thattrepans holes. It is metalcutting’s ver-sion of the familiar doorknob hole cut-ter used in wood. Hole saws have moreteeth and therefore cut faster than sin-gle, or double-point tools. Both hole

saws and single-pointtools curl up a chip inthe space, or gullet,between the teeth, andcarry it with them inthe cut.

Hole broachingtools are hybridtrepanners. (Fig. 8.21aand 8.21b) They com-bine spiral flutes like adrill with a broach-like progressive toolgeometry that splitsthe chip so it exits thecut along the flutes.With this design, the

larger number of cuttingedges and chip evacuation,combine to reduce the chipload per tooth so this drillcan cut at higher feed ratesthan trepanning tools andhole saws. Like the holesaw, a hole broaching toolhas a fixed diameter. Onesize fits one hole.

8.8.2 Cutting Tool Material Selection

M2 High Speed Steel (HSS)is the standard Rotabroach cutting toolmaterial. M2 has the broadest applica-tion range and is the most economicaltool material. It can be used on ferrousand non-ferrous materials and is gener-ally recommended for cutting materialsup to 275 BHN. M2 can be applied toharder materials, but tool life is dramat-ically decreased.

TiN coated M2 HSS Rotabroach drillsare for higher speeds, more endurance,harder materials or freer cutting action toreduce power consumption. The TiNcoating reduces friction and operates atcooler temperatures while presenting aharder cutting edge surface. Increasedcutting speeds of 15 to 25 % are recom-mended to obtain the benefits of this sur-face treatment. The reduction in frictionand resistance to edge build-up are keybenefits. The ability to run at higherspeeds at less power is helpful for appli-cations where the machine tool is slightlyunderpowered and TiN coated tools arerecommended for these applications.TiN coated tools are recommended forapplications on materials to 325 BHN.

Carbide cutting tool materials are

FIGURE 8.21: Trepanning tool also called Rotabroach withcore or slug. (Courtesy Hougen Manufacturing, Inc.)

FIGURE 8.23: Drilling holes with conventional drill and holebroaching drill. Surface speed increases with distance fromcenter.

FIGURE 8.24: Traditional trepanning toolorbits around a center drill.

FIGURE 8.22: Rotabroach machining set-up on a milling machine. (CourtesyHougen Manufacturing, Inc.)

Velocity Approaches "Zero" at Center Point

Velocity of Cutting Edge (SFPM)

Kerf

Hole Broaching DrillConventional Drill

(a)

(b)





also available as a special option onRotabroach drills. Carbide offers cer-tain advantages over high-speed steel.Applications are limited and need to bediscussed with a manufacturer’s repre-sentative.

8.8.3 Rigidity and Hole Size Tolerance

Rotabroach drills were originallydesigned as roughing tools to competewith twist drills and provide similar holetolerances. Many users have successful-ly applied Rotabroach drills in semi-fin-ishing applications, reducing the numberof passes from two or more to just one. Arigid machine tool and set-up are requiredto produce holes to these specifications.Tolerances will vary with the applicationand are impossible to pin point.

Spindle rigidity or “tightness” andworkpiece rigidity are more crucial thanwith a twist drill. Even if a twist drill runsout slightly at first, the conical point tendsto center itself before the O.D. of the toolengages the workpiece. The higher thrustof a twist drill also tends to “ preload” thespindle and fixture. The trepanning cut-ter relies more on the rigidity of the sys-tem (workpiece, holder, and spindle). Ifexcessive spindle runout or, worse yet,spindle play exists, the cutter may chatteron entry. At best this will cause a bell-mouthed hole with poor finish, but it caneasily lead to drill breakage.

Hole tolerances are dependent onmuch more than the accuracy of any tooland its grind. The machine tool, work-piece, fixture, selection of speeds andfeeds, projection and type of applicationalso play an important part in determiningoverall results.

8.8.4 Chip ControlIn material such as aluminum, tool steelsand cast iron, proper selection of feedsand speeds usually causes the chips tobreak up and allows them to be flushedout of the cut by the cutting fluid. Inmany other materials, such as mild andalloy steels, the chips tend to be long andfrequently wrap themselves around thedrill to form a “bird’s nest”. In most man-ual operations this is an annoyance that isoutweighed by the other benefits of themethod. In automated operations, howev-er, the build-up of chips around the drillcannot be tolerated. Besides the obviousproblems that this can cause, the nest ofchips impedes the flow of additionalchips trying to escape from the flutes.

This in turncan cause theflutes to packand mayresult in drillbreakage.

There areseveral meth-ods that canbe used tobreak up thechips if thiscannot be accomplished by adjusting thefeeds and speeds. One method is to usean interrupted feed cycle. It is recom-mended that the drill not be retracted aswith a “peck” cycle, because chips maybecome lodged under the cutting edges.Instead employ an extremely short dwellapproximately every two revolutions.This will produce a chip that is usuallyshort enough not to wrap around the tool.A programmed dwell may not be neces-sary since some hesitation is probablyinherent between successive feed com-mands in an NC system

8.8.5 Advantages of Trepanning Tools

The twist drill has a center point, which isnot really a point at all - it’s the intersect-ing line where two cutting edge anglesmeet at the web of the drill. This point isthe so-called “dead zone” of a twist drill.

It’s called a dead zone because the sur-face speed of the cutting edges (a factorof revolutions per minute and diameter ofthe drill) approaches zero as the corre-sponding diameter nears zero. Slowersurface speed reduces cutting efficiencyand requires increased feed pressure forthe cutting edges to bite into the material.In effect, the center of the drill does notcut - it pushes its way through thematerial. The amount of thrustrequired to overcome the resis-tance of the workpiece often caus-es the stock to deform or dimplearound the hole, and creates a sec-ond problem - burrs or flakingaround the hole’s breakthroughside. As material at the bottom ofthe hole becomes thinner and thin-ner, if the feed is not eased off, thedrill will push through, typicallyleaving two jagged remnants ofstock attached.

Trepanning tools produceholes faster than more conven-tional tooling as shown in Figure8.25. From left to right are shown

a 1 1/2 inch hole drilled into a 2 inchthick 1018 steel plate with: a spade drill,with a twist drill, with an indexable car-bide drill, and with a Rotabroach. Withapproximately 50% to 80% fasterdrilling time, the cost per hole can besubstantially lower.

An indirect yet significant source ofsavings attributable to trepanning tool-ing is the solid slug it provides.Separating chips from coolant and oilsis increasingly called for by scraphaulers, In one application, while sig-nificant gains in productivity weremade with hole-broaching tools, thesavings in going from chips to a solidslug was enough to justify the change inprocess.

In Figure 8.26 the workpiece is a tubeholder for an industrial heat exchanger.When this workpiece is finished, betterthan 60 per cent of the plate has beenreduced to scrap.

Sixty percent of this heat exchangerplate was converted into chips by thesheer number of holes drilled. Besidesincreasing production, trepanning tool-ing’s solid core by-product increasedscrap value from $0.17 per pound ofchips, to $0.37 per pound for the coremetal.

FIGURE 8.25: Trepanning produces holes faster than more convention-al tooling. (Courtesy Hougen Manufacturing, Inc.)

FIGURE 8.26: Sixty percent of this heat exchang-er plate was converted to chips. (Courtesy HougenManufacturing, Inc.)



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