Smith G.T. Cutting Tool Technology: Industrial Handbook

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Conversely, for ‘Pilot’ hole drilling (Fig. 50b), these

mathematical formulae are modied in the following

manner:

F = C

(kg)

M = C

(kg mm)

Where:

a =

(mm),

eF and eM =

exponents indicating the D

’s inu-

ence.

ese axial force and torque formulae derived in the

work by Kaczmarek, are concerned with ‘so-called’ av-

erage twist drilling values. ese ‘averages’ are related

o drill diameters between 15 to 35 mm, having feed

ranges in the vicinity of 0.2 to 0.4 mm rev

–1

. erefore,

the entire axial force (F) and torque (M), comprises of

contributions of the lips, land and chisel point, in the

following manner:

•

Axial force (F) – lips (50%), land (10%) and chisel

point (40%),

•

Torque (M) – lips (80%), land (12%) and chisel

point (8%).

NB 

ese contributing factors to axial force and

torque are for drill depths that do not exceed 2.5d.

If the drilling force is signicantly increased, then this

has the eect of distorting the drill sha. Such distor-

tion, causes the drill’s cutting edge to move forward

into the workpiece material, in this manner it jointly

increases the D

and the drilling force. Correspond-

ingly, if the drilling force is reduced, the twist drill will

recover its shape, with the cutting edge moving back

from the workpiece, thus reducing both the D

and

cutting force. is stretching and compression of the

drill’s sha – somewhat like a spring

– is unique to

twist drilling, being an unstable element in the cut-

ting process. By way of comparison, most cutting tool

14 ‘Lengthening eect’ is associated with the twist drill’s sha

being twisted by the application of torque, with elastically

springs-back upon release of the drilling torque. Not only

will the twist drill ‘spring’ , but it can also ‘bend’ due to the

increased thrust loads produced by high penetration rates.

edges are normally deected away from increases in

the load.

A common form of failure of twist drills in opera-

tion is from shattering, with such catastrophic failure

being related to the dynamic nature of twist drilling.

By way of illustration, a φ4

.5 mm long-series twist drill

is capable of withstanding a torque of approximately

6 N

m before it catastrophically fails. Normally, the

torque for most drilling operations is around 1 Nm.

Temperatures in Twist Drilling

e accumulation of heat in the vicinity of cutting is

an important factor in the cutting process, with much

of the mechanical energy necessary for machining be-

ing converted into heat, then conducted into the chip,

workpiece and tool (Fig. 51). e consequential ther-

mal phenomena are important, as they can aect the:

•

Mode of deformation – elastic/plastic behaviour of

the chip,

•

Machined surface – for metals the ultimate metal-

lurgical state of the material,

•

Tool wear rate – which depends upon a number

of criteria, such as the tool’s coating, cutting data

employed, work-hardening ability of the workpiece

and coolant delivery and its eciency.

It is imperative to comprehend the factors that control

both heat generation and its dissipation, together with

the tool and work’s temperature distribution in and

near the cutting zone.

A drilling operation can be considered as a complex

machining process, with specic and unique charac-

teristics, not least of which, are the production of chips

when drilling. ese chips are in continuous contact

with the drill utes and the generated hole’s surface.

Hence, any minute changes in the drill’s geometry, can

cause enormous modications to the either the drill’s

wear rate and its predicted life. Heat generated whilst

rilling will be transformed by a range of ‘states’ , in-

cluding:

•

Conduction – through the chips, workpiece and

drill,

•

Convection and radiation – via the ‘air-spaces’

in the hole as the drill penetrates deeper into the

workpiece.

e drilling temperature during a prolonged operation

can approximate steady-state conditions, with the heat

generated whilst cutting when employing a new drill

Drilling and Associated Technologies 101

Figure 51. The drilling process and the asociated zones of heat generation whilst hole-making. [After:

Trigger and Chao, 1951]

102 Chapter 3

is associated with two distinct regions (i.e. see Fig. 51

– Section on X-X) at the:

•

Primary shear zone – where plas-

tic deformation occurs, this being the ma-

jor source of heat generation,

•

Secondary shear zone – from within the tool/chip

interface, where pronounced friction takes place.

NB 

e drill clearance surface temperature, is sig-

nicantly aected by the rake face interface tem-

perature.

e bulk rise in the drill’s temperature is multifarious,

due to the necessity to consider a range of factors, in-

cluding: heat ow distribution, the geometric shape of

the conducting bodies, together with any variation of

thermal properties of both the drill and workpiece ma-

terials with temperature changes. e generated heat

distribution when drilling depends upon the thermal

properties of the tool, workpiece and chip. erefore,

the thermal diusivity (K/ρc), will determine the rate

at which heat transfers through the material, while

also controlling the penetration depth of the surface

temperature. While the absorption coecient (Kρc),

determines the quantity of heat being absorbed by a

given mass of material. Drilling temperatures vary

considerably in the research work undertaken over

the years, being heavily inuenced by a wide range of

cutting-related parameters, making it extremely di-

cult to obtain meaningful comparisons of local tem-

peratures in a real-time drilling operation. For exam-

ple, the scatter of ‘bulk’ temperature values for say, a

φ6mm twist drill, can vary between approximately 200

to 380°C, under steady-state drilling conditions

, for

comparable workpiece materials, making it very dif-

cult to obtain meaningful drill life comparisons.

Coolant delivery is imperative when drilling and to

this end, through-the-nose coolant operation enables

the lubrication and cooling of the drill’s point (Fig. 52c

– illustrating the coolant holes behind the lips). is

ecient technique of ensuring that the coolant gets to

the action of drilling, gives better chip control, helping

15 Twist drill interface temperatures have been reported to be

over 870°C in the workpiece’s ‘plasticity region’ , which some-

what contradicts the ‘bulk’ temperatures, although in mitiga-

tion, it should be said that these very high temperatures at the

interface at somewhat localised.

to reduce machining temperatures signicantly and

aid drill penetration rates, while increasing tool life.

Coolant holes through-the-nose are not restricted to

twist drills, as Spade-

and Gun-drills

, together with

Indexable drills also oen incorporate this coolant de-

livery feature, to remove heat and lubricate the cutting

edges.

3.1.4 Indexable Drills

Indexable drills have some signicant advantages over

their twist drilling counterparts (i.e. a range of both

indexable and twist drills are depicted in Fig.52a).

ese indexable drills – allowing the cutting inserts to

be changed (see Fig. 52c), permit faster cutting speeds

and enable a wider range of workpiece materials to be

successfully drilled than when utilising conventional

twist drills. Normally, indexable drills are limited to

horter hole depths of around ‘4D’ , than equivalent

diameter twist drills.

Indexable drills must be set up with care and in the

correct relationship to the machine tool’s headstock/

spindle, ensuring that both the drill’s and the spindle’s

centrelines are coincident, otherwise over-, or under-

sized holes may be produced (see Fig. 53a – top). Yet

another problem that needs to be addressed when

employing these indexable drills, is termed ‘radial

runout’

, which aects the inserts centre height and

should be limited to <0.127 mm. One advantage of be-

ing able to manipulate the indexable drill’s axis, is that

it can be used to adjust the drilled hole’s diameter, by

parallel adjustment of the drill’s and spindle’s respec-

tive centrelines – this being very useful for controlling

16 Spade drills are twist drills (i.e. bodies are normally manu-

factured from either 1018, or 1020 low-carbon steel) with a

standard blade being inserted at the drill’s point, enabling

holes to be generated up to 8D deep. Blades are usually coated

micrograin HSS with high-cobalt content, or coated cemented

carbides, ranging from stub drills to extra-long lengths, with

either straight, or spiral utes.

17 Gun-, or Deep-hole drills will be mentioned in some detail later

n this chapter, but they allow considerable length-to-diameter

ratios to be drilled in workpieces, necessitating high-pressure

coolant delivery, with ecient chip-ushing capabilities.

18 Radial runout refers to misalignment in the radial direction,

which should be minimised, as it alters the position of the

drill’s cutting inserts.

Drilling and Associated Technologies 103

Figure 52. Short hole drilling. [Courtesy of Sandvik Coromant].

104 Chapter 3

drilled hole tolerances. On turning centres this can be

readily achieved by modifying the CNC cutting pro-

gram, to oset the drill with respect to the machine’s

centreline. Moreover, for turning operations employ-

ing drilled features, then the indexable insert’s top sur-

faces must remain parallel to the X-axis of the machine

tool. e arrangement of the inner and outer cutting

edges of an indexable insert drill relative to each other,

together with the drill’s position to the axis of rotation

are vital for perfect drilling operations

(i.e. see Fig.

53a). e cutting inserts positions by possible X-axis

adjustments, are critical for the: smooth running, re-

sultant cutting forces and, will inuence the drilled

hole’s alignment. Preferably, the cutting edges are ar-

ranged in such a manner that the inner- (SBI) and

outer-inserts (SBA) have identical cutting widths (Fig.

53a – bottom le). When new insert cutting edges are

utilised, this results in a balance of the cutting forces in

the Y-axis, guaranteeing drilled holes of accurate size

and surface texture without ‘

retraction striae’

. When

selecting the appropriate adjustment angles (χ

) and

(χ

), the lines of force via feed forces (F

and F

) will co-

incide with the drill’s axis in the centre of the clamping

sha (Fig. 53a – top). Hence, the clamping sha must

only transmit torque resulting from the cutting forces

and the bending moment of the resultant cutting force

which will be present. Typically, the outer insert’s re-

sultant cutting force F

(i.e. Fig. 53b) is comprised of

the following forces:

•

Remaining cutting force (∆F

) – generated through

greater wear rates at the periphery of the outer cut-

ting insert,

•

Passive force (F

) – generated by the corner radius

of the outer cutting insert.

With indexable drills, the chip ute is selected so that

the drill’s prole from the tip up to the chip ute, has

its runout twisted by between 65° to 85°. In the vicin-

ity of the chip ute the runout (i.e. the longest ‘lever

arm’ of the force), is where the maximum resistance

19 Some tooling manufacturers recommend that an indexable

drill’s inner insert is positioned slightly below the spindle’s cen-

treline, as this allows a small core of uncut material to pass

over the top surface of the insert and break o – being carried

away with the rest of the chips.

‘Retraction striae’ refers to the ‘trail-lines’ resulting from the

outer insert’s gouging, or ploughing the previously drilled sur-

face as it is withdrawn from the hole.

moment to the resultant cutting force F

is found.

e bending strength attained in this manner can be

greatly increased by employing round proled chip

utes. is rounded chip ute cross-section, does not

signicantly weaken the drill’s body and provides op-

timum chip-ow – even when drilling long-chipping

workpieces. A taper of the tool holder behind the in-

sert seats, prevents a ‘squeezing’ of the chips between

the drill and the drill-hole wall.

Due to the design of the indexable drill, the two

cutting inserts are subjected to very dissimilar stresses

when drilling. For example, the indexable drill (Fig.

53a – bottom), has the outer insert being subjected

to greater stress than its inner counterpart, typically

having both thermal and abrasive stresses, while the

inner insert must have high toughness characteris-

tics. Some cutting tool manufacturers recommend so-

called ‘

mixed-tipping’

of inserts, where a toughened

grade is used for the inner insert and a wear-resistant

grade for the outer insert. However, some discretion

should be used when utilising indexable drills with

mixed cutting inserts, so perhaps reference back to

the tooling manufacturer may be advisable if produc-

tion quantities are sucient in order to optimise this

potential ‘mixed grade strategy’. Typically, by exploit-

ng ‘mixed-tipping’ , for example when machining

free-cutting steel grades cutting speeds of up to 400 m

min

–1

are possible, whereas when drilling low-silicon

aluminium grades cutting speeds of 600 m min

–1

can

be achieved with tool lives of up to 45 min of cutting

time per edge.

Several design factors will inuence an indexable

drill’s performance, these include:

•

Sintered cutting insert chip-breakers – these will

improve chip control and enable high penetration

rates to be utilised,

•

Advanced ute design – allowing deeper chip gul-

lets, thus minimising chip-jamming tendencies,

•

Faster-, slower-, or straight-uted designs – with

wider ute proles reduce chip-binding and degra-

dation of the drilled hole surface, whilst also im-

proving penetration rates,

•

Cutting insert shape – utilising square (i.e. having

4 cutting edges), rectangular (i.e. with two edges),

or triangular inserts (i.e. having three edges) – the

21 ‘Mixed-tipping’ , refers to having dissimilar grades of inserts

for the outer and inner cutting edges, as they full dierent

mechanical working criteria whilst drilling.

Drilling and Associated Technologies 105

Figure 53. Indexable insert drills – insert position and ute geometry. [Courtesy of Kennametal Hertel].

latter being the most popular version for general-

purpose drilling operations.

NB D

ouble-sided cutting inserts are available, but

are mainly used for milling operations.

One major advantage that indexable drills have over

twist drills, is that they can be oset to produce dier-

ent hole diameters. is oset for turning centres can

e up to 3.8 mm, or 7.6 mm on a diameter

, which

in reality, amounts to a ‘ne-boring’ operation, giv-

22 On machining centres this oset is somewhat less, with the max-

imum radial oset being approximately 1 mm, or 2 mm on dia-

meter. Of note, is that when an indexable drill is oset, then the

maximum feedrate should be no greater than 0.15 mm rev

–1

106 Chapter 3

ing considerable scope in diametric sizing of holes.

ese drills should be selected for the minimum

overhang and need to have both the drill’s and ma-

chine tool’s centrelines p

arallel

to within 0.076 mm,

or better.

By utilising an indexable drill on a turning opera-

tion, they can be benecial when attempting to start

drilling angled, or uneven workpiece surfaces, as illus-

trated by some of the surface faces depicted in Fig. 54.

Indexable drills, employed for such non-at workpiece

faces, obviates the need for either a previous counter

boring, or spotfacing operation.

e major advantage of indexable drills over either

their HSS, or carbide twist drill counterparts, is their

ability to run at much higher rates. For example and,

by way of simplistic comparison, if a φ1

8 mm hole has

to be drilled in free-cutting stainless steel, then an:

•

HSS twist drill – can be run at 16.8 m min

–1

with

a 0.2 mm rev

–1

feed would produce 280 rpm at

57 mm min

–1

•

Solid carbide twist drill – can be run at 61 m min

–1

with a 0.1 mm rev

–1

feed would produce 1,019 rpm

at 103 mm min

–1

•

Indexable drill – can be run at 170 m min

–1

with

a 0.76 mm rev

–1

feed would produce 2,852 rpm at

217 mm min

–1

NB e indexable drill can b

e run up to 0.8 mm rev

–1

and if one were to attempt to utilise a twist drill with

the cutting data shown above, then it would either

burn-out, or catastrophically fail in the endeavour.

An important safety note when using these indexable

drills in turning operations, is that the high penetra-

tion rates, coupled to the exit of a through hole in the

part, creates a slug which is thrown clear from either

the chuck, or from the rear of the bore of the machine’s

headstock at tremendous force. If the machine tool is

not guarded appropriately, it could prove to be hazard-

ous to any operator in the local vicinity.

23 Axis oset/misalignment: for example, if there is a 1° mis-

alignment between these centrelines, then the centrelines will

be 0.43 mm apart at 25 mm from the point at which the cen-

trelines cross at the tool’s tip.

3.1.5 Counter-Boring/Trepanning

A counter-boring operation is an oen utilised tech-

nique for the enlargement of previously manufac-

tured holes, normally to provide them with accurate

dimensions and/or improved surface nish (i.e. see

Fig. 55a). Counter-bores are also produced to register

a larger-faced shouldered sha, or to sink a precision

cap-head bolt below the clamped part’s surface. In this

latter case, oen the previously-drilled hole is used

to align the bolt’s axis, by using a counter-boring tool

with a ‘pilot-bush’ of the same drilled diameter to act as

a guide to allow machining to a correct counter-bored

depth.

Counter-boring heads are employed to open-out

existing holes (Fig. 55a). e heads were in the past,

oen of a single insert design, but latterly, they are

produced with multiple inserts – particularly when the

working clearance is such, that it cannot cope with by

the single insert variety. e multi-insert counter-bor-

ing heads, can have their inserts nely diametrically-

adjusted by means of precision wedge and radial ad-

justing screws.

A trepanning operation is undertaken in one op-

eration, but instead of machining the complete hole,

only part of the hole is cut, leaving a core (Fig. 55b).

For large workpiece dimension machining, a trepan-

ning operation uses less power and axial pressure than

other equivalent manufacturing processes. However,

one major problem occurs with the trepanning opera-

tion, which is that the core produced as the trepanning

tool penetrates into the workpiece, becomes quite dif-

cult to handle (i.e. see Fig. 55b -lower diagram).

Trepanning heads are normally utilised on:

•

Large workpiece diameters – greater than 120 mm,

•

Limited machine tool power – alternatively, if it is

not prudent to switch to another machine tool and/

or lose part orientation and register

•

Core utilisation – large cores can be usefully

utilised as precision material stock.

24 Part orientation and register, refers to the initial setup, where

once the workpiece has been clamped and partially machined,

it cannot be satisfactorily removed then reset without a loss of

both accuracy and precision.

Drilling and Associated Technologies 107

Figure 54. A wide range of drilling/boring operations can be undertaken using indexable insert drills. [Courtesy of Seco Tools].

108 Chapter 3

Figure 55. Counterboring and trepanning. [Courtesy of Sandvik Coromant].

Drilling and Associated Technologies 109

3.1.6 Special-Purpose, or Customised

Drilling and Multi-Spindle

Drilling

Most of today’s Special-purpose, or Customised Drills

and Multi-spindle drills are normally designed and

manufactured to meet the following criteria:

•

Long production runs

– these enable the extra

cost of this purpose-designed and built tooling to

amortised

over the cost of the production period,

•

Short cycle times

– when time is the ‘essence of

importance’ in the production of the component,

but it is not necessarily related to the overall quan-

tity of the production run,

•

Tooling accuracy is reected in the part’s manufac-

ture – if for example the precision part must have

all its component features in accurate alignment,

or in a specic relationship to a particular datum

face, plane or point.

Special-purpose, or Customised tooling is normally

required if one, or several of the criteria mentioned

above are to be met. To have a tooling manufacturer

design special-purpose tooling to meet the production

demands of manufacture, is not undertaken lightly, as

for complex tooling, its: design, build and prove-out,

prior to use, could prove to be expensive. However,

many companies resort to this type of custom-built

tooling, because it is the only way that the product can

be manufactured economically. Oen, multiple fea-

tures are incorporated into just one tool, typically for

hole- and post-hole making operations, such as those

depicted in Fig. 56a. A relatively simple example of

this special-purpose tooling is illustrated in Fig. 56b,

where three production operations for the manufac-

ture of the female part of the pull-stud mechanism for

a milling cutter toolholder is depicted, namely, drilling,

25 Long production runs usually refers to some form of continu-

ous production , or large batch sizes, of > 5,000 components,

to make the cost of the Special-purpose tooling viable.

26 Amortisation, refers to the ‘pay-back’ of the tooling over the

‘life’ of the production of the parts produced.

27 Short cycle times, are considered to be the quickest time that the

part can be produced, under ‘standard’ machining conditions.

28 Datum – the term refers to origin of the measurements for

the particular component feature, which could be from a face,

plane, or point.

chamfering and counter-boring. In this case, not only

does the usage of special-purpose tooling here seem

the obvious solution, as it combines these individual

operations in one, it has the advantage of meeting all

three of the production criteria listed above, with the

added advantages of both using fewer tools and utilis-

ing less space in the tool magazine. Some special-pur-

pose tools are very complex in their design and quite

sophisticated in operation, but their supplementary

cost more than outweighs this by the production gains

oered by their consequent implementation.

ulti-spindle drilling

tooling is ideal when a series

of hole patterns are required on a component, such as

for specic congurations of: pitch circle diameters,

hole grid patterns, line of holes, or a combination of

these (i.e. see examples of specic patterns in Fig. 57

– top right). Hole pitch circle diameters can easily be

accommodated, for large and small pitch diameters on

the same tooling, Likewise, hole line and grid patterns

can be quite diverse, within the diametral area of the

‘cluster plate’ (i.e. see Fig. 57 – top le).

Multi-spindle drilling heads utilise a main drive

gear which is engaged with an idler and then onto the

drill spindle gear, this being attached to the individual

drill (i.e. see Fig. 57 – exploded view of a typical sys-

tem). e cluster plate orientates the individual drill

spindles and their rotational speeds can be margin-

ally increased, or decreased, by changing the driver-

to-driven gear ratio, moreover, their rotational direc-

tion can be changed by the introduction of another

idler into the gear train. erefore, if additional idlers

are present, to change the drill’s rotation, then the an

appropriate le-hand drill would be required here.

By purposefully modifying each drill’s rotational di-

rection, this has the advantage of minimising overall

torque eects on the multi-spindle drilling head, al-

lowing a large number of drills to be utilised for one

particular operation (i.e. see Fig. 57 – lower le-hand

photograph). An important point in utilising multi-

spindle drilling heads, is presetting their respective

drill lengths, so that they engage with the workpiece’s

surface at the correct height.

By the correct production application of both Spe-

cial-purpose and Multi-spindle drilling tooling, then

29 Multi-spindle tools, refer to more than one individual tool ro-

tating in its respective toolholder, enabling several holes to be

manufactured in just one operation.

110 Chapter 3