Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 38. Improved wear resistance obtained with an uncoated and coated cermet, when turning

ovako 825B steel, having the following cutting data: Cutting speed 250 m min

–1

, feed 0.2 mm rev

–1

1.0 mm and cut dry. [Courtesy of Sandvik Coromant]

Turning and Chip-breaking Technology 81

grooving, enabling the non-productive elements

the machining cycle to be minimised. In the original

multi-directional tooling concept, the top rake geom-

etry might include a three-dimensional chip-former,

comprising of an elevated central rib, with negative K-

lands on the edges. Such a top rake prole geometry

could be utilised for ecient chip-forming/-breaking

of the resultant chips. is tooling when utilised for

say, grooving operations, employed a chip-forming

geometry – this being extended to the cutting edge,

which both narrowed and curled the emerging chip

to the desired shape, thereby facilitating easy swarf

evacuation. A feature of this cutting insert concept,

was a form of eective chip management, extending

the insert’s life signicantly, thus equally ensuring that

adequate chip-ow and rapid swarf evacuation would

have taken place. When one of these multi-directional

tools was required to commence a side-turning opera-

tion, the axial force component

acting on the insert

caused it to elastically deect at the front region of the

toolholder. is tool deection enabled an ecient

feed motion along the workpiece to take place, be-

cause of the elastic behaviour of the toolholder created

a positive plan approach angle in combination with a

front clearance angle – see Fig. 39a and b (i.e. illus-

trating in this one of the latest ‘twisted geometry’ insert

multi-functional tooling geometries).

Any of today’s multi-functional tooling designs

(Figs. 39 and 40), allow a ‘some degree’ of elastic be-

haviour in the toolholder, enabling satisfactory tool

vectoring to occur, either to the right-, or le-hand

of the part feature being machined. ese multi-func-

42 Non-productive elements are any activity in the machining

cycle that is not ‘adding value’ to the operation, such as: tool-

changing either by the tool turret’s rotation, or by manually

changing tools, adjusting tool-osets (i.e. for either: tool wear

compensation, or for inputting new tool osets – into the ma-

chine tool’s CNC controller), for component loading/unload-

ing operations, measuring critical dimensional features – by

either touch-trigger probes, non-contact measurement, or

manual inspection with metrology equipment (i.e. microme-

ters, vernier calipers, etc.), plus any other additional ‘idle-time’

activities.

43 A

n Axial force component is the result of engaging the desired

feedrate, to produce features, such as: a diameter, taper, pro-

le, wide groove, chamfer, undercut, etc. – either positioned

externally/internally for the necessary production of the ma-

chined part.

tional tools are critically-designed so that for a specic

feedrate, the rate of elastic deection is both known

and is relatively small, being directly related to the ap-

plied axial force, in association with the selected D

’s.

At the tool-setting stage of the overall machining cycle,

compensation(s) are undertaken to allow for minute

changes in the machined diameter, due to the dynamic

elastic behaviour of one of these tools in-cut. For a

specic multi-functional tool supplied by the tooling

manufacturer, its actual tool compensation factor(s)

will be available from the manufacturer’s user-manual

for the product.

In-action these multi-functional tools (Fig. 39b),

can signicantly reduce the normal tooling inventory,

for example, on average such tools can replace three

conventional ones, with the twin benet of a major

cycle-time reduction (i.e. for the reasons previously

mentioned) of between 30 to 60% – depending upon

the complexity of features on the component being

machined. Some other important benets of using a

multi-functional tooling strategy are:

•

Surface quality and accuracy improvements – due

to the prole of the insert’s geometry, any ‘machined

usps’

, or feedmarks are reduced, providing excel-

lent machined surface texture and predictable di-

mensional control,

•

Turret utilisation improved – because fewer tools

are need in the turret pockets, hence ‘sister tooling’

can be adopted, thereby further improving any un-

tended operational performance,

•

Superior chip control – breaks the chips into man-

ageable swarf, thus minimising ‘

birds nests’

and

entanglements around components and lessens au-

tomatic part loading problems,

•

Improved insert strength – allows machining at sig-

nicantly greater D

’s to that of conventional in-

44 ‘Machined cusps’ the consequence of the insert’s nose geom-

etry coupled to the feedrate, these being superimposed onto

the machined surface, once the tool has passed over this sur-

face.

45 ‘Birds nests’ are the rotational entanglement and pile-up

of continuous chips at the bottom of both trough and blind

holes, this work-hardened swarf can cause avoidable damage

in the machined hole, furthermore, it can present problems in

coolant delivery for additional machining operations that may

be required.

82 Chapter 2

Figure 39. Multi-functional cutting insert geometry for ecient stock removal and increased part productivity. [Courtesy of Iscar

Tools]

Turning and Chip-breaking Technology 83

Figure 40. Multi-functional tooling for the machining of rotational features such as: turning, grooving and proling operations –

having excellent chip control. [Courtesy of Sandvik Coromant]

84 Chapter 2

serts, with improved insert security its toolholder

location,

•

CNC programming simplied – many tooling man-

ufacturer’s have specially-prepared soware to sig-

nicantly reduce CNC programming input times.

NB 

is latter point utilises CNC ‘canned cycles’ to

reduce program lengths.

In Appendix 3, a guide for ‘Trouble-shooting for turn-

ing operations’ are listed, with possible causes and rem-

edies to potential production problems.

In the following chapters, many other important

chip-forming production processes will be discussed,

with hole-making techniques such as drilling being

around 25% of all manufacturing techniques under-

taken by machining-related companies – these and

associated hole-production methods will be reviewed

next.

References

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Humphries, J.R. Energing Technologies and Recent Ad-

vances in Multi-functional Groove/Turn Systems. Int.

Figure 41. By employing twin turrets on a mill/turn centre, ‘balanced turning’ of the component can remove large volumes of

stock at one pass. [Courtesy of DMG (UK) Ltd.]

Turning and Chip-breaking Technology 85

Conf. on Industrial Tooling, Shirley Press Ltd, 65–85,

Sept. 1979.

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chinist, 37–39, Aug. 1996.

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Inuencing Factor in Chip-breaking: An Experimen-

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(2), 145–154, 1995.

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Pub, Vol. 114, 393–399, Nov. 1992.

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2002.

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Chatter due to Multiple Regenerative Eect. J. of Engg.

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86 Chapter 2

Drilling and Associated

Technologies

‘In all things, success depends upon previous preparation

and without such preparation……there is sure to be failure.’

CONFUCIUS

(c550–c487BC)

[Analects]

3.1 Drilling Technology

3.1.1 Introduction to the Twist

Drill’s Development

Drilling operations are perhaps the most popular ma-

chining process being undertaken today, with their

origins being traced back to cutting tool develop-

ments in North America in the 19

century. In 1864

toward the latter part of the American Civil War, Ste-

ven Morse (i.e. later to design the signicant ‘Morse

taper’ – for accurate location of the ‘sleeved drills’

into their mating machine tool spindles) founded

the Morse Twist Drill and Machine Company in the

‘North’. Morse then proceeded to develop probably the

most important cutting tool advance to date, namely,

the ubiquitous twist drill. In Fig. 42, several of today’s

twist drills are illustrated along with just a small range

of ‘solid’ contemporary designs. Morse’s originally-de-

signed twist drill has changed very little over the last

150 years – since its conception. In comparison to the

somewhat cruder-designed contemporary drills of that

time, Morse stated: ‘e common drill scrapes metal to

be drilled, while mine cuts the metal and discharges the

chips and borings without clogging’. Morse’s statement

was at best, to some extent optimistic, whereas the

‘cold reality’ tells a dierent story, as a drill’s perfor-

mance is inuenced by a considerable number of fac-

tors, most of which are listed in Fig. 43.

3.1.2 Twist Drill Fundamentals

e basic construction of a conventional twist drill is

depicted in Fig. 44a. From this illustration two dis-

tinct cutting regions can be established: rstly, the

main cutting edge, or lips; secondly at the intersection

of the clearance and main cutting edge – termed the

chisel edge. In fact for a twist drill, the cutting process

can be equated to that of a le-hand oblique turning

tool, where the rake and clearance face geometries are

identical and the correlation between these two ma-

chining processes have been validated in the experi-

mental work by Witte in 1982. Both of these regions

remove material, with the cutting lips producing ef-

cient material removal, while the chisel edge’s con-

tribution is both inecient and is mainly responsible

for geometric errors in drilling, coupled to high thrust

loads.

e main cutting edges are accountable for a rela-

tively conventional chip formation, as shown in the

‘quick-stop’ photomicrograph in Fig. 44b. An oblique

cutting action occurs to the direction of motion, being

the result of an oset of the lips that are parallel to a

radial line – ahead of centre – which is approximately

equal to half the drill point’s web thickness and in-

creases toward the centre of the drill. is obliquity is

responsible for inducing chip ow in a direction nor-

mal to the lips in accordance with Stabler’s Law

. e

increasing chip ow obliquity can be seen in Fig. 45a,

by observing the ow lines emanating from the chip’s

interface along the lips and up the ute face. Such

an oblique cutting action serves to increase the twist

drill’s eective rake angle geometry. With the advent

of ‘Spherical trigonometric computer soware’ for ob-

taining direct three-dimensional calculations – previ-

ously described by Witte (1982) in two-dimensional

formulae for cutting edge performance – these calcu-

lations have been enhanced.

Under the chisel point, or web, the material re-

moval mechanism is quite complex. Near the bottom

of the utes where the radii intersect with the chisel

edge, the drill’s clearance surfaces form a cutting rake

surface that is highly negative in nature. As the centre

of the drill is approached, the drill’s action resembles

hat of a ‘blunt wedge-shaped indentor’ , as illustrated

in Fig. 45b. An indication of the inecient material

removal process is evident by the severe workpiece

deformation occurring under the chisel point, where

such deformed products must be ejected by the drill to

produce the hole. ese ‘products’ are extruded, then

wiped into the drill ute whereupon they intermingle

with the main cutting edge chips. is fact has been

substantiated by force and energy analysis, based on a

combination of cutting and extruding behaviour under

the chisel point, where agreement has been conrmed

with experimental torque and thrust measurements.

e chisel edge in a conventionally ground twist drill

has no ‘true’ point, which is one of the major sources

for a drilled hole’s dimensional inaccuracy.

1 Stabler’s Law – for oblique cutting, can be formulated, as be-

low:

Chip ow (cos η) = cos I (b

/b)

Where: I = inclination of cutting edge, b

= chip ow vector,

b = direction of cutting vector.

88 Chapter 3

e conventional twist drill chisel point geometry

can be seen in Fig. 46, together with associated no-

menclature for critical features and tolerance bound-

aries. From the relatively complex geometry and

dimensional characteristics shown in Fig. 46, the ob-

tainable accuracy of holes generated whilst drilling is

dependent upon grinding the drill to certain limits.

Any variations in geometry and dimensions, such as:

dissimilar lips and angles, chisel point not centralised,

and so on, have a profound eect on both the hole di-

Figure 42. A selection of just some of the many ‘solid’ and ‘through-spindle’ drilling varieties and ‘inserted-edge’

insert geometries currently available. [Courtesy of Seco Tools]

Drilling and Associated Technologies 89

Figure 43. The principal technical drill performance criteria and factors associated with drilling operations in this case for ex-

ample, on castings

90 Chapter 3