23.1 introduction

Chapter 23Chapter 23

Drilling and Related Hole- Drilling and Related Hole-Making Making

Processes Processes (Review) (Review)

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSpring, 2012Spring, 2012

23.1 Introduction23.1 IntroductionDrilling is most common single machining

operationDrilling makes up 25% of machiningMost drilling tools have two cutting edges,

or lips. Cutting action takes place inside the

workpieceThe only exit for chips is the hole that is

mostly filled by drillFriction between the margin and the hole

wall produces heat which is additional to that due to chip formation

Nomenclature and Geometry of Nomenclature and Geometry of a Drilla Drill

FIGURE 23-1 Nomenclature andgeometry of conventional twist drill.Shank style depends upon themethod used to hold the drill. Tangsor notches prevent slippage:(a) straight shank with tang,(b) tapered shank with tang,(c) straight shank with whistle notch,(d) straight shank with flat notch.

23.2 Fundamentals of the 23.2 Fundamentals of the Drilling ProcessDrilling ProcessA conventional two-flute drill, with drill of

diameter D, has two principal cutting edges rotating at an rpm rate of N and feeding axially.

The rpm of the drill is established by the selected cutting velocity or cutting speed with V in surface feet per minute and D in inches.

Conventional Drill GeometryConventional Drill Geometry

FIGURE 23-2 Conventionaldrill geometry viewed from thepoint showing how the rakeangle varies from the chisel edgeto the outer corner along the lip.The thrust force increases as theweb is approached.

23.2 Fundamentals of the Drilling 23.2 Fundamentals of the Drilling ProcessProcessFour actions take place a the drill tip

◦ 1. A small hole is formed by the web and chips are not cut here in the normal sense.

◦ 2. Chips are formed by the rotating lips.◦ 3. Chips are removed from the hole by the screw

action of the helical flutes.◦ 4. The drill is guided by lands or margins that rub

against the walls of the holeNew drill-point geometry and TiN coating

have resulted in improved hole accuracy, longer life, increased feed-rate capabilities.

US manufacturing companies consume 250 million twist drills per year.

23.2 Fundamentals of Drilling process23.2 Fundamentals of Drilling process

A conventional two-flute drill with diameter D, has two principal cutting edges rotating at rpm rate of Ns and feed fr.

Ns = (12v)/( D)

Where V – cutting speed at the outer cornerof the cutting lip (point X in Fig. 23 -2)in surface, feet per minute, D – diameter of drill in inch.

The depth of cut in drilling is a half of the feed rate, or t = fr/2 (see section A – A in Fig. 23-2), where is in inches per revolution.

23.2 Fundamentals of Drilling process23.2 Fundamentals of Drilling processThe length of cut in drilling equals the depth of the

hole, L, plus an allowance for approach and for the tip of drill, usually A = D/2.

In drilling, the speed and feed depend upon the material of workpiece, the cutting tool material, and the size of drill.

Table 23-1 gives some typical values for V and for carbide indexable insert drills.

The maximum velocity is at the extreme ends of the drill lips. The velocity is very small near the center of the chisel end of the drill.

Cutting time: Tm = (L + A)/(frNs)= (L +A)/fm

where fm is the feed rate inches per minute.

Material Removal RateMaterial Removal RateThe material removal rate (MRR) for drilling is:

Which reduces to or Where Tm is cutting time, fr is feed rate, and L is

depth of the hole.

/min


A cast iron plate is 2” thick, and needs 1” diameter hole drilled in it. An indexable insert drill has been selected. 1) Select cutting speed and feed; 2) the spindle Ns and feed rate (in/min.); 3) the maximum chip load (depth of cut); 4) MRR; and 5) the total motor horsepower (HPs = 0.33).

Solution:1)From table 23-1, select a cutting speed of 200

fpm, and a feed of 0.005 ipr.2)Ns = (12v)/( D) = (12 x 200)/ (3.14 x 1) =

764 rpm, pick 750 rpm.


2) Feed rate fm = fr x Ns = 0.005 x 750 = 3.75 in /min , and pick fm = 3.5 in/min.

3) the maximum chip load (depth of cut)t = fr/2 = 0.005/2 = 0.0025 in.

4) MRR = (/4) (D2) fm = (3.14/4) x 12 x 3.5 = 2.75 in3/min.

5) HP =HPs x MRR = 0.33 x 2.75 = 0.9(horsepower)

The value would typically represent 80% of the total motor horsepower needed, so in this case a horsepower motor greater than 1.5 or 2 would be sufficient. HP = 1.6 x 0.9 = 1.5 (horsepower)

23.3 Types of Drills23.3 Types of DrillsThe most common drills are twist drillsTwist drills have three parts

◦Body: consisting of two or more spiral grooves called flutes, separated by lands. Flutes serve as channels through which chips are withdrawn from hole and coolant gets to cutting edges.

◦Point: a wide variety of geometry are used, but typically have a cone angel of 118°, and a rake angle of 24°

◦Shank: a straight or tapered section where the drill is clamped.

Types of Twist DrillsTypes of Twist Drills

FIGURE 23-3 Types of twistdrills and shanks. Bottom to top:Straight-shank, three-flute coredrill; taper-shank; straight-shank;bit-shank; straight-shank,high-helix angle; straight-shank,straight-flute; taper-shank,subland drill.

Drill WalkingDrill WalkingStandard drills have a straight line chisel

point.This point caused drills to “walk” along

the surfaceThis effect is counter by using centering

techniques◦Center punches◦Pre-drilled guide holes for large holes

Specialized methods of grinding the point address walking

Specialized TipsSpecialized TipsSpecialized tips are used to produce

self centering holes where hole position is critical. ◦Helical tips◦Four-facet tips◦Racon◦Bickford◦Center core, or slot drills

Used in machining centers and high speed automatic NC systems where manual center punching is impractical

Drill Point Drill Point GeometryGeometry

FIGURE 23-4 As the drilladvances, it produces a thrustforce. Variations in the drill-pointgeometry are aimed at reducingthe thrust force.

Center Core DrillCenter Core Drill

FIGURE 23-5 Center core drills cangreatly reduce the thrust force.

Typical Causes of Drilling ProblemsTypical Causes of Drilling Problems

Depth-to-Diameter RatioDepth-to-Diameter RatioStandard drills typically are used to produce

holes with a depth to diameter ratio of 3:1Deeper holes result in drift of the tool

decreasing hole straightnessSpecialized drills called deep-hole drills or

gundrills are used for greater ratiosGundrills are single tipped tools with a coolant

channel delivering coolant to the tip and flushing chips to the surface

Ratios of 100:1 are possible with gundrills

Steps to High Accuracy Holes Steps to High Accuracy Holes with Conventional Drillswith Conventional Drills

FIGURE 23-10 To obtain ahole that is accurate as to sizeand aligned on center (located),this 4 step sequence ofoperations is usual.

Specialty DrillsSpecialty DrillsHole cutters: used for holes in sheet stockSubland drills: used for multi diameter holesSpade drills: used for holes over 1 inchIndexable drills: used for high speed

shallow holes in solid stockMicro drills (pivot drills): used for holes

0.02 to 0.0001 inch diameter where grain boundaries and inclusion produce non-uniform material properties

Subland DrillSubland Drill

FIGURE 23-11 Special purpose subland drill (above), and some of the operations possible with other combination drills (below).

23.4 Tool Holders for Drills23.4 Tool Holders for DrillsStraight-shank drills are typically

held in chucks◦Three-jaw jacobs chucks: used on manual drill

presses, require used of a key ◦Collet chuck: used with carbide tools where

high bearing thrust is used◦Quick change chucks: used were rapid change

is neededTapered shank drills held in mores taper

of the machine spindle

Correct Chucking of Carbide DrillsCorrect Chucking of Carbide Drills

FIGURE 23-17 Here are some suggestions for correct chucking of carbide drills.

23.5 Workholding for Drilling23.5 Workholding for DrillingFor prototype pieces, stock material is

held in simple clamping visesFor high production rates, custom jigs are

usedStock material is never to be held on the

work table by hand

23.6 Machine Tools for Drilling23.6 Machine Tools for DrillingDrilling can be performed on:

◦Lathes◦Vertical mills◦Horizontal mills◦Boring machines◦Machine centers

Specialized machines designed specifically for drilling called “drill presses”

Requirements of a Drill PressRequirements of a Drill PressDrill presses must have sufficient power

and thrust to perform cutDrill presses must be rigid enough to

prevent chatterDrill press consist of a base, a work

table, and a column that supports the powerhead and spindle

23.9 Reaming23.9 ReamingReams remove small amounts of

material to ensure exact hole size and improve hole surface finish

Reams are either hand operated or machined at slow speed

Ream types◦Shell reams◦Expansion reams◦Adjustable reams◦Tapered reams

SummarySummaryDrilling is the most common machining

operationDrilling can be performed on a number of

machine tools, drill presses are specialized machine tools for drilling only

Drills come in a wide variety of types and tip geometries depending upon production rate and accuracy needed

Hole geometries can be adjusted through the use of counterboring, countersinking and reaming

HW for Chapter 23HW for Chapter 23Review Questions:1, 2, 10, and 25 (page 653-654)

Problems (page 654):3, 5, 7, 9

23.1 introduction

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