PREPARED BY :

NORAZLINA BINTI AHMAD SARAIDEPARTMENT OF MECHANICAL ENGINEERING

Drill bits are cutting tools used to createcylindrical holes. Bits are held in a tool calleda drill, which rotates them and providestorque and axial force to create the hole.

Specialized bits are also available for

non cylindrical shaped holes.

DRILL - A rotary end cutting tool having oneor more cutting lips and having one or morehelical or straight flutes for the passage ofchips and the admission of a cutting fluid.

AXIS - The imaginary straight line whichforms the longitudinal center line of the drill.

BODY - The portion of the drill extendingfrom the shank or neck to the outer cornersof the cutting lips.

BODY DIAMETER CLEARANCE - That portion ofthe land that has been cut away so it will notrub against the walls of the hole.

CHISEL EDGE - The edge at the end of the webthat connects the cutting lips.

CHISEL EDGE ANGLE - The angle includedbetween the chisel edge and the cutting lip, asviewed from the end of the drill.

CLEARANCE DIAMETER - The diameter over thecut away portion of the drill lands.

DRILL DIAMETER - The diameter over the marginsof the drill measured at the point.

FLUTES - Helical or straight grooves cut or formedin the body of the drill to provide cutting lips, topermit removal of chips, and to allow cutting fluidto reach the cutting lips.

FLUTE LENGTH - The length from the outer cornersof the cutting lips to the extreme back end of theflutes. It includes the sweep of the tool used togenerate the flutes and, therefore, does notindicate the usable length of the flutes.

HELIX ANGLE - The angle made by the leading edgeof the land with a plane containing the axis of the drill.

LAND - The peripheral portion of the body betweenadjacent flutes.

LAND WIDTH - The distance in a transverse planebetween the leading edge and the heel of the landmeasured at a right angle to the leading edge.

LIPS - The cutting edges of a two flute drill extendingfrom the chisel edqe to the periphery. (CORE DRILLS)- The cutting edges extending from the bottom of thechamfer to the periphery.

LIP RELIEF ANGLE - The axial relief angle at the outercorner of the lip. It is measured by projection into aplane tangent to the periphery at the outer corner ofthe lip.

MARGIN - The cylindrical portion of the land whichis not cut away to provide clearance

NECK - The section of reduced diameter betweenthe body and the shank of a drill.

OVERALL LENGTH - The length from the extremeend of the shank to the outer corners of the cuttinglips. It does not include the conical shank end oftenused on straight shank drills, nor does it include theconical cutting point used on both straight and tapershank drills.

PERIPHERAL RAKE ANGLE - The angle between theleading edge of the land and an axial plane at thedrill point.

POINT - The cutting end of a drill, made up of theends of the lands and the web. In form itresembles a cone, but departs from a true cone tofurnish clearance behind the cutting lips.

POINT ANGLE - The angle included between thelips projected upon a plane parallel to the drillaxis and parallel to the cutting lips.

SHANK - The part of the drill by which it is heldand driven.

TANG - The flattened end of a taper shank,intended to fit into a driving slot in a socket.

TANG DRIVE - Two opposite parallel driving flatson the extreme end of a straight shank.

WEB -The central portion of the body that joinsthe lands. The extreme end of the web forms thechisel edge on a two-flute drill.

WEB THICKNESS - The thickness of the web at thepoint unless another specific location is indicated.

WEB THINNING - The operation of reducing theweb thickness at the point to reduce drillingthrust.

The speed of a drill is usually measured in terms of therate at which the outside or periphery of the tool movesin relation to the work being drilled.

The common term for this is Revolutions Per Minute orrpm.

The most economical speed for operating a drill willdepend on many variables, some of which are:

1. Composition and hardness of material2. Depth of hole3. Efficiency of cutting fluid4. Type and condition of drilling machine5. Quality of holes desired6. Difficulty of set-up

Most drill troubles arise from inaccurate pointing.Correct drill pointing will to a great extenteliminate drill breakage and inaccurate holes.

Drill pointing should be varied depending uponthe materials to be drilled, but for general use,drills leave the factory sharpened to a 59° pointangle (118° included angle), 9° - 15° clearanceangle and with a chisel edge angle of 120° to135°.

CUTTING LIPS The cutting lips must be of equal length. Even

though the point angle is equal on both sides, if thecutting lips are not equal in length the chisel edgewill not be centered, and the drill will cut oversize.

A scale should be used to check the lengths of thecutting lips.

LIP CLEARANCE Sufficient clearance behind the cutting lips must be

provided so that the cutting edges can enter thework. Usually 9° to 15° lip clearance is sufficient foraverage work if the point is ground with the properangle and the proper clearance, but without theproper contour back of the cutting edges, the cuttinglip will be weakened.

RIGHTCorrectly ground lipshave the same angle tothe axis of the drill andare of equal length. Anaccurate hole will beproduced with this point.

WRONGUnequal lip lengthsproduce oversize holes.Unequal lip angles placemost of the cutting onone lip and point dullsfaster.

RIGHT

A point with proper lipclearance permits the drillto cut freely. The cuttinglip is sufficiently supportedto prevent excessivedulling or chipping.

WRONGToo much clearance (over15°) will cause cutting lipsto break down. Insufficientclearance (under 9°)requires excess feedpressure, causes drill tosplit up.

Drilling A drill bit enters the

workpiece axially and cuts ablind hole or a through holewith a diameter equal to that ofthe tool. A drill bit is a multi-point tool and typically has apointed end.

A twist drill is the mostcommonly used, but othertypes of drill bits, such as acentre drill, spot drill, or tapdrill can be used to start a holethat will be completed byanother operation.

Reaming A reamer enters the

workpiece axially andenlarges an existing holeto the diameter of thetool. A reamer is a multi-point tool that has manyflutes, which may bestraight or in a helix.

Reaming removes aminimal amount ofmaterial and is oftenperformed after drilling toobtain both a moreaccurate diameter and asmoother internal finish.

Tapping A tap enters the workpiece

axially and cuts internalthreads into an existing hole.The existing hole is typicallydrilled by the required tapdrill size that willaccommodate the desiredtap.

The tap is selected basedon the major diameter andpitch of the threaded hole.Threads may be cut to aspecified depth inside thehole (bottom tap) or thecomplete depth of a throughhole (through tap).

Boring

A boring tool enters theworkpiece axially and cutsalong the internal surface ofan existing hole to enlargethe diameter or obtain moreprecise dimensions.

The boring tool is a single-point cutting tool, whichcan be set to cut thedesired diameter by usingan adjustable boring head.

Counterboring

A counterbore tool enters theworkpiece axially and enlargesthe top portion of an existinghole to the diameter of thetool. Counterboring is oftenperformed after drilling toprovide space for the head of afastener, such as a bolt, to sitflush with the workpiecesurface. The counterboring toolhas a pilot on the end to guideit straight into the existinghole.

Countersinking

A countersink tool enters theworkpiece axially and enlarges thetop portion of an existing hole to acone-shaped opening.

Countersinking is often performedafter drilling to provide space forthe head of a fastener, such as ascrew, to sit flush with theworkpiece surface. Commonincluded angles for a countersinkinclude 60, 82, 90, 100, 118, and120 degrees.

Many different materials are used for or on drillbits, depending on the required application

SteelsLow carbon steel bits are used only in wood, as

they do not hold an edge well and requirefrequent sharpening

High carbon steel are an improvement on low-carbon steel due to the hardening andtempering capabilities of the material. Thesebits can be used on wood or metal, but losetheir temper, resulting in a soft cutting edge,if overheated.

High speed steel(HSS) is a form of tool steel; HSSbits are much more resistant to heat. They canbe used to drill metal and most other materialsat greater cutting speeds than carbon steel bits,and have largely replaced carbon steels incommercial applications.

Cobalt steel alloys are variations on high speedsteel which contain more cobalt. Their mainadvantage is that they hold their hardness atmuch higher temperatures, so they are used todrill stainless steel and other hard materials. Themain disadvantage of cobalt steels is that theyare more brittle than standard HSS.