Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 56. Special-purpose multi-functional tooling can be designed and manufactured to machine many part features

simultaneously

Drilling and Associated Technologies 111

Figure 57. The application of multi-spindle drilling heads to increase productive throughput.

112 Chapter 3

signicant economic savings can be made and their

initial capital outlay will have been worthwhile. How-

ever, such complex and expensive tooling used inap-

propriately can be counter-productive, so consider-

able thought and care should be made into any future

implementation of these tools.

3.1.7 Deep-Hole Drilling/

Gun-Drilling

Deep-Hole Drilling – an Introduction

Deep-hole drilling can be characterised by, high mate-

rial removal rates, having excellent: hole straightness,

dimensional tolerances and machined surface texture.

Deep-hole drilling applications are utilised across di-

verse industrial applications, including: aerospace,

nuclear power, oil and gas, as well as for steel and

chemical processing industries. ese industries place

a high demand on all aspects of drilled hole quality

and reliability, with components being very expensive,

any failures will have severe economic consequences.

e name Deep-hole drilling implies the machin-

ing of holes with a relatively long hole depth to its

diameter. Typically at the lower length-to-diameter

ratios they can be as short as x5 the diameter, con-

ersely at the other end of the scale, ‘ratios’ of > x100

the diameter can be successfully generated, with close

olerances and a surface texture approaching 0.1 µm

(Ra). ere are a considerable number of deep-drill-

ing production techniques, with each one having an

appropriate usage for a particular hole generation

method. A typical deep-drilling tooling assembly

is essentially ‘self-piloting’ ,

in that the cutting forces

generated are balanced, not with respect to the cut-

ting edges – as is the situation with Twist drills – but

invariably, by pads that are situated at 90° and 180° to

that of the cutting edge. ese pads rub against the

bore’s surface being generated and therefore support

the head, while burnishing

the surface. e machine

tools enabling these deep-drilled holes to be generated

can be expensive, along with the appropriate tooling,

but the production costs can be dramatically reduced,

by employing such a machining strategy. One of the

30 Burnishing will improve the surface nish and dimensional ac-

curacy, by plastically deforming the machined surface layers –

cusps – without removing any additional workpiece material.

major problems of utilising Deep-hole drilling, is chip

disposal, as the deeper the hole is drilled, the further

the chip must travel from the cutting edge to the hole’s

exit. is chip evacuation distance, can increase the

probability of chip-jamming, or binding in the ute

as the chip attempts to exit the deep-drilled hole.

Notwithstanding the problems associated with BUE,

which hinders the tool’s ability to break chips. Coolant

control and its operational usage is important in any

the Deep-hole drilling technique, as one of its main

functions is – apart from lubricating/cooling the cut-

ting edge and chip ushing – is to restrict frictional ef-

fects between the: drill, chip and hole wall. Moreover,

if friction builds-up due to poor coolant delivery, this

can result in higher torsional eects, which may cause

the drill to snap.

Gun-Drills

Gun-drills (i.e. see Fig. 58a), are normally utilised to

machine small, straight diameters to high tolerances

and having excellent nishes in a single operation.

Drilled hole sizes can range from as small as φ1

.5 mm

to φ7

5 mm in a single pass, with depths equating to

100 times the tool’s diameter

. e ‘drilling system’ is

a highly developed and ecient technique for produc-

ing deep holes in wide variety of workpiece materials,

ranging from: plastics, breglass, to high-strength ma-

terials such as Inconel. is tooling usually consists of

either a cemented carbide, or cemented carbide-tipped

drill head tted to a tube-shaped shank

. e former

solid carbide drill head version allows the tooling to

be reground as necessary, while the latter version is

normally employed for larger diameter hole drilling

operations. e drill head has two distinct designs,

ither having a ‘kidney-shaped’ , or a cylindrical hole

present, for the delivery of cutting uid, which pro-

vides:

•

Flow of cutting uid – to create the maximum ow

rate and chip-ushing,

31 ‘Special-purpose’ Gun-drills can be produced to generate

drilled holes up to φ150 mm having 200:1 length-to-diameter

ratios, at penetration rates of better than equivalent diameter

Twist drills.

32 Gun-drills would as a rule, have their drill head’s brazed – via

silver soldering – onto a tube-shaped shank, these in turn, are

also brazed onto a ‘driver’ (i.e. of various designs) of the re-

quired length for the successful drilling of long slender holes

in the workpiece.

Drilling and Associated Technologies 113

Figure 58. Deep-hole drilling operations, such as: (a) gundrilling, (b) double tube ejector drilling and (c) single tube ejector drill-

ing. [Courtesy of Sandvik Coromant]

114 Chapter 3

•

Minimal uid-ow disturbance – giving consis-

tent/regular ow-rate to drill-head,

•

Minimum of uid-turbulence – allowing chips to

be easily evacuated from the cutting region.

Typically, cemented carbide heads, have an external V-

shaped chip-ute which extends along the shank, the

angle of this chip-ute has been experimentally-deter-

mined to be 110°, providing the following advantages:

•

Optimum ute cross-section – allowing the most

rapid cutting uid return and chip transportation,

•

Facilitates an extra support pad – this is necessary

when drilling through crossing holes,

•

Provides optimal torsional strength – important

for workpieces having very long length-to-diameter

ratios,

•

Facilitates tool clamping – enabling the tool to be

held in a three-jaw chuck for convenient regrinding

on a suitable cutter-grinder.

Gun-Drill Failure

One of the main reasons for Gun-drills to fail in op-

eration, is through an excessive misalignment of the

drill bushing and this will be in relation to the drill’s

rotational axis (i.e. see Fig. 58a). is type of align-

ment failure mode is termed a ‘balk-crash’ – caus-

ing the tool to fracture into numerous pieces

. If the

drill is rotated rather than the workpiece, the stress is

re-applied to diering portions of the tip and, at the

weakest point, namely the drill’s corner, the tip will

most likely fracture in this region. A potential failure

mode is related to the Gun-drill’s length, which has its

rigidity decreased with increased length

. e shank

of a longer Gun-drill will not transmit a large amount

of bending force to the cutting tip – when misaligned

– however, the tip does not fracture, but instead, any

axis misalignment causes the shank to ex with each

revolution, a situation that is ideal for a fatigue fail-

33 ‘Balk-failure’ of Gun-drills is the result of the ‘brittle’ carbide

tip being unable to withstand the bending stresses created by

its unintentional axis misalignment.

34 Gun-drill ‘rigidity rule’: as the drill’s length increases, its ri-

gidity decreases by the ‘cube’ of the distance. For example, if

two identical Gun-drill diameters are employed for drilling

the same workpiece material, then if one drill is twice as long

as the other, then its rigidity will 8 times less rigid than its

counterpart (i.e. namely: 2

ure mode. Yet another Gun-drill failure situation may

arise if there is excessive clearance between the drill

bush and the drill’s tip. Under these circumstances, the

Gun-drill’s edge cuts a signicant volume of workpiece

material and, as this edge is not designed to cut – hav-

ing a zero clearance angle (i.e. created by the circular

margin at this edge) – the excessive cutting forces

cause the edge to prematurely fracture.

If insucient coolant ow occurs, this is also a typ-

ical factor in subsequent Gun-drill failure. is lack of

coolant causes the chips to pack in the V-ute, forming

a plug, which then creates excessive torque in the Gun-

drill and, this plug allows the tip to separate away from

the shank. Occasionally, end-users blame the Gun-drill

tooling manufacturer for poor brazing, if the tool’s tip

separates from the shank. However, when analysis of

the brazed fractured surfaces occurs, invariably, small

carbide particles are adhered to the shank, this being

evidence of the fact that the braze was stronger than

the tip, clearly demonstrating that the brazing was not

at fault.

In many circumstances, the Gun-drill tool manu-

facturer is blamed by the customer for its failure dur-

ing machining, but when investigated, it is usually

premature failure being the result of a poor tooling

installation and operation. One of the major causes of

Gun-drill failure, is via the coolant distribution sys-

tem, where inconsistent delivery of the uid can either

‘starve’ the Gun-drill’s cutting edge, or ‘over-ood’ the

system. One of the major factors contributing to this

over-/under-supply of coolant delivery, is due to the

fact that in the main, coolant pressure is being moni-

tored, rather than the measurement of coolant ow-

ate. If holes are Gun-drilled < φ4 m

m, then high-pres-

sure coolant ow-rate to the point is essential, but in

many cases of coolant systems tted to ‘standard’ ma-

chines, they are of relatively low-pressure delivery. Re-

cently, one machine tool manufacturer, has designed

and developed a coolant intensier pump coupled to

a special high-pressure union, which gives variable

pump pressures of over 200 bar, with special-purpose

couplings to overcome the problems of poor coolant

ow-rates to the cutting vicinity.

3.1.8 Double-Tube Ejector/Single-

Tube System Drills

Double-tube Ejector drills (i.e. oen just termed ‘Ejec-

tor Drills’), are designed around a twin tube system

Drilling and Associated Technologies 115

(i.e. see Fig. 58b – for the schematic and inset a photo

of the drill head). Here, the self-contained system (i.e.

not requiring specic sealing arrangements), of the

cutting uid, is externally pumped along the space be-

tween the inner and outer tubes. e major portion of

the cutting uid is fed forward to the drill head, while

the remainder is forced through a groove in the rear

section of the inner tube. A ‘negative pressure’ occurs

in the front portion of the inner tube, which causes the

cutting uid at the drill head to be sucked out through

the inner tube along with the chips. As is the case for

Gun-drilling coolant supply, it must be of sucient

pressure and volume, to overcome any likelihood of

‘starvation’.

e ‘ejector head’ of the drill comprises of: a con-

nector, outer and inner tubes, a collet and sealing

sleeve, together with a drill head. Disposable heads

with cemented carbide tips are utilised for diameters

ranging from 18.5 to 65 mm, normally supplied with

two types of cutting edge geometries, with the carbide

cutting tips precisely located on either side of the drill

head. e asymmetric design

of these ‘Ejector Drills’

has support pads provided, to absorb the radial cut-

ting forces and guide while supporting the tool as it

penetrates into the workpiece. At the commencement

of the deep-drilling operation, the drill bushing’s main

function

(i.e. shown in Fig. 58b), is to guide and sup-

port the drill at initial workpiece entry and until drill

penetration allows the support pads to bear on the

partially-drilled hole surface and thereupon remain-

ing in contact throughout the drilling operation.

Whilst deep-hole drilling, the drill and workpiece

centrelines must not deviate by > 0.02 mm, so any sub-

sequent drill bush wear needs to be carefully moni-

tored and controlled. It is usual practice to have a ro-

35 Asymmetric Drill Head design, refers to the fact that the

cutting inserts are not only radially, but are angularly oset.

erefore, they normally require two support pads to counter-

act and sustain the radial cutting forces generated while dril-

ling deep holes. By locating the cutting inserts on both sides

of the drill head, the greater percentage of radial forces are

negated at these pads.

36 Drill bushing tolerances between the drill and bush for both

the ‘Ejector’ and Single-tube Systems, require a t of ISO G6/

h6, equating to a minimum play of 0.006 mm. is drill bush

is usually manufactured from a hardened material (i.e. 60 to

62 HR

) such as cemented carbide, as it has a longer service

life, with bush wear normally limited to 0.03 mm.

tating workpiece and a stationary tool, with any centre

divergence resulting in bell-mouthing at the hole’s en-

trance and a wavy hole surface. Once the support pads

in the drill head have moved x5 their length down the

drilled hole, then any further waviness is negligible, as

they begin to press down on the hole’s curvature. Many

deep-drilled hole prole and tolerance abnormalities

result from centre divergence, which needs special at-

tention to minimise such eects.

Single-tube [Ejector] System drills (i.e. commonly

referred to and abbreviated as simply ‘SST’) are sche-

matically depicted in Fig. 58c. With this SST tooling

assembly, the cutting uid is pumped under pressure

between the drill and the hole wall (i.e. normally this

width of space is approximately 1 mm) and it exits

with chips through the inside of the drill tube (Fig.

58c). e quantity of cutting uid passing through the

drill is twice as great and with higher pressure, than

for an equivalent ‘Ejector’ tooling assembly. Hence, the

SST set-up provides improved chip-breaking and mi-

nimises any potential chip-jamming, even when vary-

ing chip lengths occur.

e drill head arrangement of cutting inserts will

vary from two, three, or more, depending on the drill’s

diameter, usually made of cemented carbide, oen as

brazed over-lapping tips, although disposable index-

able pocketed inserts with chip-breakers are oen

utilised for larger diameter holes. SST tools can be used

to drill small diameter holes, ranging from φ12.5 mm

upward, with 100:1 depth-to-diameter ratios. e SST

tooling system copes with dicult-to-machine work-

piece materials, such as Monel, Inconel and Hastel-

loy and other ‘exotic materials’. In actual production

machining trials, it has been found that SST tools can

produce deep-drilled holes up to 15 times faster than

is achievable by conventional Gun-drilling. is high

production output level gives an 80% improvement

in machining rates for this SST Deep-drilled hole

production output and, it has been shown in several

instances, to give a ‘Return on Investment’ (ROI)

about 6 months.

37 Return on Investment (ROI), for Deep-hole drilling operati-

ons (i.e. in % terms), is given (i.e. in simplistic terms) by the

following formula:

% ROI =

Cost of a -to- productivity gain

Total conversion cost

116 Chapter 3

3.1.9 Deep-Hole Drilling –

Cutting Forces and Power

In Deep-hole drilling operations, the underlying the-

ory for the calculation of cutting forces and for torque

are similar to that utilised for ‘conventional’ drilling

operations. e major dierence between the hole

production calculations for Deep-hole drilling to that

of ‘conventional hole-making’ techniques, lies in the

fact that support pads create a sizeable level of fric-

tional forces, that cannot be ignored. ese increased

frictional eect contributions – by the pads – to the

overall Deep-hole drilling cutting forces and torque

values are somewhat dicult to precisely establish,

however, an approximate formulae can be used to esti-

mate them, as follows:

Feed force (N):

+ F

pµ

= 0.65 × k

× a

× f × sinκ

Where:

= Feed force, or drilling pressure (N),

pµ

= Force and Frictional eects (N),

= Specic cutting force (N mm

–1

= Depth of cut (mm),

f = Feed per revolution (mm rev

–1

sinκ

= Entering angle (°).

Torque, or Moment (Nm):

+ M

� a

� f � D



(. − a

�D)

Where:

= Torque cutting (Nm),

= Torque and Frictional eects (Nm),

= Specic cutting force (N mm

–1

= Depth of cut (mm),

f = Feed per revolution (mm rev

–1

D = Hole diameter (mm).

Relatively high speeds are utilised for Deep-hole Drill-

ing operations, in order to achieve satisfactory chip-

breaking, this necessitates having a machine tool with

a reasonable power availability.

e underpinning theory for calculating the power

requirements, corresponds with that of ‘conventional’

drilling operations. However, the friction forces that are

present, due to the employment of support pads, gives

rise to a torque contribution (M

), which in turn pro-

duces an associated contribution ‘P

’ to the total Deep-

hole drilling power. erefore, in order to estimate the

machine tool’s power requirement (i.e. ‘P’ in kW ), an

allowance must be made for any power losses in the

machine tool. Hence, the gross power required can be

established by dividing the Deep-hole drilling power

(i.e. P

+ P

), by the machine tool’s eciency ‘η’. is

eciency indicates what percentage of the power sup-

plied by the machine tool, that can be utilised, while

Deep-hole drilling.

Power (kW):

+ P

) =

� a

� f � v

, 

(. − a

�D)

Where:

+ P

= Power contributions of: cutting and friction

respectively (kW),

= Cutting speed (m min

–1

∴P = P

+ P

/η

Where:

η = Machine tool eciency.

3.2 Boring Tool Technology –

Introduction

e technology of boring has shown some important

advances in recent years, from advanced chip-break-

ing control tooling (i.e. see Fig. 59, this photograph

illustrates just some of the boring cutting insert ge-

ometries that can be utilised), through to the ‘active

suppression of chatter’

– more will be mentioned on

the topic and reasons why chatter occurs and its sup-

pression later in the text. Probably the most popular

type of boring tooling is of the cantilever type (Fig.

59), although the popularity of either ‘twin-bore-’ , or

38 ‘Chatter’ , is one of the two basic types of vibration (i.e.

namely, ‘forced’ and ‘self-excited’) that may be present dur-

ing machining. In the main, chatter is a form of self-excita-

tion vibration.‘[It is]… due to the interaction of the dynamics

of the chip-removal process and the structural dynamics of the

machine tool. e excited vibrations are usually very high in

amplitude and cause damage to the machine tool, as well as

lead to premature tool failure’. [Aer: Kalpakjian, 1984].

Drilling and Associated Technologies 117

‘tri-bore-heads’ , with ‘micro-bore adjustment’ of the ei-

ther the individual inserts, or having a simultaneous

adjustment of all of the actual cutting inserts, is be-

coming quite common of late.

Boring operations invariably utilise cantilevered

(i.e. overhung) tooling, these in turn are somewhat

less rigid than tooling used for turning operations.

Boring, in a similar manner to Deep-hole drilling and

Gun-drilling operations, has its rigidity decreased by

the ‘cube’ of the distance (i.e. its overhang), as the fol-

lowing equation predicts:

�

 � EI



+.M

)

Where:

= normal force acting on the ‘free end’ of the can-

tilever (i.e boring tool overhang),

*EI = exural stiness (i.e. I = cross-sectional moment

of Inertia) (Nm

= boring bar mass (kg),

L = length of cantilever (mm),

= Modulus of elasticity of the boring bar

(N mm

–2

* E, relates to the boring bar’s ‘Young’s modulus’.

Boring a hole will achieve several distinct production

criteria:

•

Enlargement of holes – a boring operation can en-

large either a single, or multiple series of diameters,

to be either concentric to its outside diameter (i.e.

O.D.), or machined eccentric

(i.e. oset) to the

O.D.,

•

Correction of hole abnormalities

– the boring

process does not follow the previously produced

39 ‘Eccentric machining’ of the bore of a component with respect

to its O.D., was in the past accurately achieved by ‘Button-bo-

ring’ – using ‘Toolmaker’s buttons’ (i.e. accurately ground and

hardened buttons of ‘known diameter’) that were precisely o-

set using gauge blocks (i.e ‘Slip-gauges’). is technique might

still be employed in some Toolrooms, but normally today, on

CNC-controlled slideways, a simple ‘CNC oset’ will achieve

the desired amount of bored eccentricity.

Correction of hole abnormalities, as Fig. 60 schematically il-

lustrates, how boring can correct for ‘helical wandering’ of the

drill as it had previously progressed through the workpiece.

e drill’s helical progression would cause undesirable hole

eccentricity, resulting from minute variations in its geometry,

hole’s contour, but generates its own path and will

therefore eliminate drill-induced hole errors by the

subsequent machining operation (i.e. see the sche-

matic representation shown in Fig. 60),

•

Improvement of surface texture – the boring tool

can impart a high quality machined surface texture

to the enlarged bored hole.

NB In this latter case, boring operations to previ-

ously drilled, or to any cored holes in castings, can be

adjusted to give exactly the desired machined surface

texture to the nal hole’s dimensions, by careful ad-

justment of the tool’s feedrate and the selection of an

appropriate boring tool cutting insert geometry.

3.2.1 Single-Point Boring Tooling

‘Traditional’ boring bars were manufactured as solid

one-piece tools, where the cutting edge was ground

to the desired geometry by the skilled setter/operator,

which meant that their useful life was to some extent

restricted. Later boring bar versions, utilised indexable

cutting inserts, or replaceable heads (Fig. 61). Boring

bars having replaceable heads are versatile, with the

same bar allowing dierent cutting head designs and

cutting inserts (Fig. 61a). Here, the insert is rigidly

clamped to the tool post, with replaceable ‘modular

tooling’ heads with the necessary mechanical coupling

to be utilised (i.e. Fig 61b), oering ‘qualied tooling’

dimensions.

necessitating correction by a boring operation. is ‘correc-

tion’ is necessary, because the drill’s centreline follows the

path indicated, ‘visiting’ the four quadrant points as it spirally

progresses through the part. Hence, hole eccentricity along

with harmonic departures from roundness can be excessive,

if the drill’s lip lengths and drill point angles are o-centre.

e cross-hatched circular regions represent the excess stock

material to be removed by the boring bar, where it corrects

these hole form errors, while machined surface texture is also

considerably improved.

41 ‘Qualied Tooling’ , refers to setting the tool’s osets, with all

the known dimensional data for that tool, allowing for ease of

tool presetting and ecient tool-changing – more will be said

on this subject later in the text.

118 Chapter 3

Figure 59. A selection of some tooling that can be employed for boring-out internal rotational features. [Courtesy

of Seco Tools]

Drilling and Associated Technologies 119

Figure 60. The harmonic and geometric corrections by a boring operation, to correct the previous helical drift, resulting from

the drill’s path through the workpiece

In the case of the boring bar’s mechanical interface

(i.e. coupling) example shown in Fig. 61a- top, the ser-

rated V-grooves across the interface along with the

four clamping screws provide an accurate and secure

tment for the replaceable head, with internal tension

adjustment via the interior mechanism illustrated.

120 Chapter 3