Smith G.T. Cutting Tool Technology: Industrial Handbook

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the power requirement for cutting. Fundamentally,

the product of the tangential force and the cutting

speed represent the power required for machining.

e s

pecic cutting force

is a unit expression for

the tangential cutting force, being closely related to

the material’s undeformed chip thickness and se-

lected feedrate,

•

Axial force (F

) – the magnitude of this force will

vary depending on the selected feedrate and the

chosen tool geometry and in particular, the ‘plan

pproach angle’ , or ‘entering angle’ , – more will

be said on this topic later. Its direction is from the

feeding of the tool, along the direction of workpiece

machining,

•

Radial force (F

) – is directed at right angles to the

tangential force from the cutting point. e ‘plan

approach angle’ and the size of the nose radius, will

inuence this force.

NB 

ese three component forces are signicantly

inuenced by the rake angle, with positive rakes

producing in general, lower cutting forces. e re-

ultant force, its magnitude and angle, will be af-

fected by all three component forces, in conjunc-

tion with the tool’s geometry and the workpiece

material to be cut.

Or thogonal Cutting Forces

In Fig. 19b the two-dimensional model for orthogonal

cutting is depicted, once again, for comparison to the

oblique cutting model, in a single-point turning op-

eration. For simplicity, if one assumes that the point of

the tool is innitely sharp and that the tool is at right

angles to the workpiece axis having no deection pres-

ent, then the two component forces are the tangential

force and axial force (i.e. previously mentioned above).

In this case, this tool geometry-workpiece congura-

tion, allows long slender bars to be turned, as there is

less likelihood of tool ‘push-o’ (i.e. as the radial force

8 In reality, the specic cutting force is a better indication of

the power requirement, as it is the force needed to actually

deform the material prior to any chip formation. It will vary

and is inuenced by the: undeformed chip thickness, feedrate,

and yield strength of the workpiece material. For example, if

the cutting conditions are kept the same and only the material

changed, then if a nickel-based alloy is machined, the initial

chip forming force (i.e. specic cutting force) will be more

than ten times greater than when cutting a pure aluminium

workpiece.

has been neutralised – as indicated by the fact that the

resultant force shows no X-axis oset). If any radial

force was present, this would create either a ‘candle-

tick eect’ , or ‘barrelling’ to the overall turned length.

In reality, there will always be some form of nose ra-

dius, or chamfer to the tool point, which will have

ome degree of ‘push-o’ , depending upon the size of

this incorporated nose feature – creating a ‘certain de-

gree’ of radial component force aect.

2.1.5 Plan Approach Angles

e manner in which the cutting edge contacts the

workpiece is termed the ‘plan approach angle’ (Fig.

20a), being composed of the entering and lead angles

for the selected tool geometry. In eect for single-

point turning operations, the tool’s orientation of its

plan approach, is the angle between the cutting edge

and feeding direction. When selecting a tool geometry

for turning specic workpiece feature – such as a 90°

shoulder – it is important as it will not only aect the

machined part geometry, but has an inuence on con-

sequent chip formation and the direction and magni-

tude of the component cutting forces, together with

the length of engagement of the cutting edge (see Fig.

20b). In single-point turning (Fig. 20b), the depth of

cut (D

)

, or ‘cutting depth’ is the dierence between

an un-cut and cut surface, this being half the dier-

ence in the un-cut and cut diameter (i.e. the diame-

ter is reduced by twice the D

in one pass along the

workpiece). is D

is always measured at 90° to the

tool’s feed direction, not the cutting edge. e manner

in which the cutting edge approaches the workpiece is

termed the ‘entering angle’ (i.e. plan approach angle),

this being the angle between the cutting edge and feed

direction (Fig. 20a – shown here in a cylindrical turn-

ing operation). Moreover, the plan approach angle

not only inuences the workpiece features that can be

produced with this cutting geometry, it also aects the

formation of chips and the magnitude of the compo-

nent forces (Fig. 20b).

e ‘entering angle’ aects the length of the cut-

ting edge engaged in-cut, normally varying from 45°

to 90°, as illustrated in the four cases of diering plan

approach angles shown in Fig. 20b. Here, in ‘case I’ an

9 In single-point turning operations, the depth of cut (D

) is

sometimes referred to by the term: ‘undeformed chip thick-

ness’.

Turning and Chip-breaking Technology 41

Figure 20. Insert approach angle geometry for turning operations.

42 Chapter 2

entering angle of 45° and lead angle of 45° is utilised,

giving rise to equal axial and radial component forces.

n ‘case II’ , the entering angle has changed to 75° and

lead angle is now 15°, these altered angles change the

component forces, with an increase in the axial force

hile reducing the radial force. In ‘case III’ , an or-

thogonal cutting action occurs, with only a 90° enter-

ing angle (i.e. the lead angle reduces to zero), showing

a large increase in the axial force component at the

expense of the radial force component which is now

zero

. In ‘case IV’ , an oblique cutting action has re-

turned (i.e. as in ‘cases I and II’), but here the entering

angle has changed to -15°, with the lead angle 75°, this

produces a large axial component force, but the radial

component force direction has now reversed. is last

tool plan approach angle geometry (i.e. ‘case IV’), is

similar to the geometry of a light turning and facing

tool, allowing cylindrical and facing operations to be

usefully undertaken – but the tool’s point is somewhat

weaker that the others, with the tool points becoming

of increased strength from right to le. erefore, in

‘

case I’ , for a given feedrate and constant D

, the cut

length/area is greater than the other ‘cases’ shown and

with this geometry, it enables the tool to be employed

or heavy roughing cuts. Returning to ‘case III’ , if this

tool is utilised for nish turning brittle-based work-

piece materials, then upon approaching the exit from a

cut, if the diameter is not supported by a larger shoul-

der diameter, then the axial component force /pressure,

will be likely to cause edge break-out (i.e. sometimes

termed ‘edge frittering’), below the machined surface

diameter at this corner (i.e. potentially scrapping the

machined part). In mitigation for this orthogonal cut-

ting tool geometry, if longer slender workpieces re-

quire cylindrical turning along their length, then with

the radial force component equating to zero, it does

not create signicant ‘push-o’ and allows the part to

be successfully machined

A single-point turning geometry is subject to very

complex interactions and, as one geometric feature is

modied such as changing the entering angle, or in-

10 In all of these cases, it is assumed – for simplicity – that there is

no nose radius/chamfer on the tool and it is innitely sharp.

11 I

n order to minimise the eects of the radial force component

when cylindrically turning long slender workpieces with ‘Case

I and II’ tool geometries, the use of a programmable steady,

or a ‘balanced turning operation’ (i.e. utilising twin separately

programmable turrets on a turning centre, with tools situ-

ated virtually opposite each other running parallel during the

turning operation – see Fig. 41), will reduce this ‘push-o’.

creasing the tool’s nose radius, this will inuence other

factors, which in turn could have a great impact on the:

type of machined surface nish produced, expected

tool life and the overall power consumption during the

operation. In fact, the main factors that inuence the

application of tooling for a specic turning operation

are:

Workpiece material – machinability, condition

(i.e. internal/external), mechanical and physical

properties, etc.,

II.

Workpiece design – shape, dimensions and ma-

chining allowance,

III.

Limitations – accuracy and precision require-

ments, surface texture/integrity, etc.,

IV.

Machine tool – type, power, its condition and

specications,

Stability – loop stiness/rigidity (i.e. from the

cutting edge to its foundations),

I. Set-up – tool accessibility, workpiece clamping

and toolholding, tool changing,

VII.

Tool programme – the correct/specied tool

and its tool osets, etc.,

VIII.

Performance – cutting data, anticipated tool-life

and economics,

IX. Quality – tool delivery system and service.

In order to gain an insight into the complex and im-

portant decisions that have to be made when select-

ing tooling for the optimum production of either part

batch sizes, or for continuous production runs, then

the following section has been incorporated.

2.1.6 Cutting Toolholder/Insert

Selection

When deciding upon the correct selection of a tool-

holder/cutting insert for a given application, a range

of diverse factors must be considered, as indicated in

Fig. 21. As can be seen by the diagram (Fig. 21) and

associated text and captions, there are many other

variables that need to be considered prior to selection

of the optimum toolholder/insert. Generally, the xed

conditions cannot be modied, but by ‘juggling’ with

the variable conditions it is possible to accomplish the

best compromise toolholder/insert geometry, to opti-

mise these cutting conditions for the manufacture of

a specic workpiece and its intended production re-

quirements. Whenever toolholders and cutting inserts

are required for a specic manufacturing process, it

is important to view the tooling selection procedure

as a logical progression, in order to optimise the best

Turning and Chip-breaking Technology 43

Figure 21. The factors that must be considered prior to commencing a turning operation, when

utilising indexable inserts

44 Chapter 2

possible tools/inserts for the job in hand. Perhaps the

following selection strategy for a ‘start point’ in choice

and application of turning tools, can be undertaken

according by the following step-by-step approach:

tart Point → Edge clamping

system,

↓

Toolholder size

and type,

↓

Insert shape,

↓

Insert size,

↓

Nose radius,

↓

Insert type,

↓

Tool material,

↓

utting data →

inal Tool-

holder and

Insert Selection

Edge Clamping System

Initially, the tool holder clamping system should be

selected to provide optimum performance in dier-

ent applications over a wide range of workpiece geom-

etries. e type of machining operation and to a lesser

extent, the workpiece size determines tool holder se-

lection. For example, roughing-out operations on big

components will make considerably dierent demands,

to that of nishing passes on small components.

B Pin, clamp and lever are just three of the insert

clamping systems available – consultation with the

tool suppliers at this point might be benecial.

Toolholder Size and Type

Once the clamping system has been selected, the size

and type of toolholder must be determined, with its

selection being inuenced by: feed directions (i.e. see

Fig. 22 for turning insert shapes and feed directions),

size of cuts, workpiece and toolholder situated in the

machine for accessibility requirements. e work-

piece’s shape plays a decisive role if surface contouring

is necessary, this is particularly relevant for machining

part access, as a toolholder is dened by its: eective

entering and point angles

, together with the insert’s

shape (see Fig. 22).

Toolholders should be the largest possible size for

the turning centre’s tool turret, this requirement is vi-

tal, as it reduces the ‘tool overhang ratio’ – providing

rigidity and integrity to stabilise the insert’s cutting

edge.

NB A

ppendix 1a shows the ISO ‘Code Key’ – for Ex-

ternal Toolholders.

ppendix 1b shows the ISO ‘Code Key’ – for Solid

Boring Bars.

ppendix 1c shows the ISO ‘Code Key’ – for Car-

tridges.

Inser t Shape

e insert shape should be selected relative to the en-

tering angle needed for the tool’s accessibility, or ver-

satility. Here, the largest suitable point angle should

be chosen for strength and economy (see Fig. 23). In

Fig. 23, is illustrated a practical example of how chang-

ing only one variable – insert geometry (shape) – can

inuence an insert’s turning application. e shape

of an insert will determine its inherent weakness, or

strength, which is of particular relevance if rough-

turning operations are necessary. Furthermore, insert

shape will inuence whether it is prone to vibration, or

not and its predictable tool life. Hence, if one is con-

cerned about vibrations of either the tool, workpiece,

or both, then a weaker insert such as a light turning

and facing geometry with less cutting edge length ex-

posed in-cut, might be more suitable. Variable condi-

tions such as the selection of insert’s geometric shape

can aect other machining parameters and, this is

valid for other insert factors, so a compromise will al-

ways occur in any machining application.

12 Eective entering angles (κ

) must be carefully selected when

the operation involves proling, or copying. e maximum

proling angle (β) is recommended for each tool type – if

‘workpiece fouling’ is to be avoided.

NB κ

1 =

(for plunging into a surface), whereas κ

1 =

– β

(for

ramping-out of a surface), κ

1 =

(β = 0°) for cylindrical turn-

ing, Where: eective entering angle (κ

), entering angle (κ),

maximum in-copy angle (β). Always select the smallest enter-

ing angle that the part geometry will allow.

Turning and Chip-breaking Technology 45

Figure 22. Tool paths in nish turning operations. [Courtesy of Sandvik Coromant].

46 Chapter 2

NB Appendix 1d shows the ISO ‘Code Key’ – for In-

dexable Inserts.

Inser t Size

An indexable insert size is directly related to the tool-

holder selected for the operation, with the entering

angle and insert shape having previously been estab-

lished. Only the matching-shaped insert can be tted

into the seat of a particular toolholder, as its shape

and size are predetermined by the seating dimensions.

In roughing-out operations, the largest cutting depth

for a given toolholder, will inuence the insert size.

For any insert, the eective cutting length has to be

determined (see Fig. 20b), as the entering angle will

inuence the size of the insert selected. If the eective

cutting edge length is less than the depth of cut (D

a larger insert should be chosen, or the D

should

be reduced. Sometimes in more demanding turning

operations, a thicker insert – of the same geometric

shape – gives extra reliability.

Figure 23. Selecting indexable inserts for turning operations. [Courtesy of Stellram].

Turning and Chip-breaking Technology 47

Nose Radius

Of particular relevance in any turning operation is the

insert’s tool nose radius (r

– see Fig. 17), as it is the key

factor with regard to:

•

inherent strength in roughing operations,

•

the resulting surface texture from nishing opera-

tions.

Further, the size of the nose radius aects vibrational

tendencies (see Fig. 23) and in certain instances, the

feedrates. e nose radius is the transition between the

major and minor cutting edges, which determines the

strength, or weakness of the point angle (see Figs. 16a

and 17), therefore it is an imperative factor to get right.

In general, roughing-out should be undertaken with

the largest possible nose radius, as it is the strongest

tool point (see Fig. 23). Further, a larger tool nose ra-

dius permits higher feedrates, although it is important

to monitor any possible vibrational tendencies. Later

in the relevant section, more will be said on the inu-

ence that the insert’s tool nose radius plays in the nal

machined surface texture, but it is worth mentioning

here that the feedrate for roughing operations should

be set to approximately half the size of the nose radius

utilised. e size of the nose radius has an aect on the

power consumed in turning in conjunction with the

material’s yield strength and chip-forming ability, par-

ticularly in rough-turning operations. e maximum

material removal rate (MMR) can be obtained by a

combination of high feedrate, together with a moder-

ate cutting speed, with other limiting factors, such as

depth of cut (D

), tool’s nose radius, under consider-

ation. Oen, the machine tool’s power (P) availability

n sometimes be a limiting factor when mmR is the

requirement and, in such circumstances the cutting

speed is usually lowered somewhat. For a given nose

radius and cutting insert geometry, the power can be

derived, to ensure that the machine tool will be able

cope with this pre-selected mmR, in the following

manner:

Machine tool’s power requirement (P):

P =

tangential force (F

) x cutting speed (V

)

P = F

× V

P = k

× A × V

∴ P = k

× f × a

× V

(kW)

Where:

f =

feed/rev (

mm/rev)

= depth of cut (mm)

Cutting speed (V

)

= πDN/1000 (m/min)

Where:

D =

workpiece diameter (mm)

N = workpiece rotational speed (rpm)

pecic cutting force (k

= F

/A (N/mm

)

Where:

A =

cutting area (mm

)

For example, for nishing operations, with the nose

radius in combination with the feedrate (i.e. pre-se-

lected), this will aect the surface texture and part ac-

curacy, in the following manner:

Machined surface texture (Rt):

(Rt, this parameter being: maximum prole height)

Rt =

/8 × r

x 1000 (µm)

Where:

= feedrate per revolution (mm/rev)

= nose radius (mm)

NB e surface texture parameter ‘Rt’ , can be con-

erted into other surface texture parameters – as nec-

essary.

By utilising either: larger turning insert tool nose ra-

dius, ‘wiper insert’ (yet to be discussed), a more posi-

tive plan approach angle, or in certain circumstances,

a higher cutting speed, the surface texture can be im-

proved. In general, the coordination of the tool’s nose

radius and the pre-selected feedrate in nishing op-

erations, indicates that the feed should be kept below

a certain level to achieve an acceptable machined sur-

face texture value.

Inser t Type

e cutting insert type is for the most part determined

by the previously selected geometry – see Appendix 1d

for the selection of indexable inserts. In reality, vari-

ous cutting conditions and workpiece materials make

dierent demands on the insert’s cutting edge. For ex-

ample, when machining hardened steel parts, this will

be completely dierent from that to the machining of

aluminium components.

48 Chapter 2

Once the insert shape has been established in con-

nection with its plan approach angle together with the

nose radius dimension, this just leaves the type of ge-

ometry to be found. In this instance, the type of insert

geometry refers to the ‘working area’ (i.e. nominally

found by its depth of cut and feedrate – more will be

said concerning this topic later, when ‘chip-breaking

envelopes’ will be discussed). Additional factors can

inuence the type of cutting geometry choice, such

as: machine tool’s condition, its power, the stability of

the workpiece-tool-machine set-up, other factors that

could aect geometry selection include: whether con-

tinuous, or intermittent cutting occurs, any tendency

toward vibration while machining. Turning operations

an be separated into a number of ‘working areas’ , be-

ing based upon the removal of workpiece material and

the generation of accurate machined component di-

mensions, in combination with specic surface texture

requirements – as shown in Table 3.

When establishing an insert type, the feedrate

and depth of cut should be identied with one of the

‘working ranges’ (i.e. from Table 3), as the various in-

sert types to be chosen relate to this chart. It should

be borne in mind that the most suitable ‘working area’

selected, will vary, in combination with such factors as

the insert’s: size, shape and nose radius.

Tool Material

e penultimate evaluation to be made concerning

tooling decision-making is the choice of insert mate-

rial, or combination of materials that constitute the

cutter’s tool edge. Today, manufacturers of tooling

have a strategy for continuous improvement with varia-

tions in both tool matrices and coatings being consid-

erable. Not only are cutting tool material research and

development an on-going intensive activity, but their

application for wider ranges of machining applica-

tions are being considerably enhanced. A brief review

of just some of the current tool materials and coatings

have been previously mentioned in Section 1.2, with

the main range of cutting tool materials being: ce-

mented carbides, coated cemented carbides, ceramics,

cermets, cubic boron nitride, polycrystalline diamond

and monolithic (i.e. natural) diamond.

B A good ‘start-point’ for most machining opera-

tions, is to consider coated carbides initially, then if

these grades prove unsatisfactory, for whatever reason,

select one of the other materials – perhaps aer con-

sultation with a cutting tool manufacturer, or aer a

machinability testing procedure.

Cutting Data

Once all of the physical, metallurgical and geometrical

factors for the cutting tool have been established for

the machining operation, then it is necessary to set, or

calculate the cutting data – oen these criteria can be

found from tooling manufacturers recommendations

and cutting data tables. Certain variable factors such as

feedrate should have already been made, allowing the

cutting speed to be calculated, from the well-known

expression (below):

= πDN/1000 (m min

–1

)

Where:

= cutting speed (m min

–1

)

D =

Workpiece diameter (mm)

N = rotational speed (rpm)

13 In the case of drilling, reaming and tapping operations, it is

the diameter of the cutting tool that is used in the calculation.

For any other internal machining operations – such as in bor-

ing, it is the initial hole diameter that is employed in the cut-

ting speed calculation.

Table 3. Typical working areas for external turning opera-

tions

Type of machining

operation:

Feedrate (f): Depth of cut

Extreme nishing 0.05 to 0.15 0.25 to 2.0

Finishing 0.1 to 0.3 0.5 to 2.0

Light roughing 0.2 to 0.5 2.0 to 4.0

Roughing 0.4 to 1.0 4.0 to 10.0

Heavy roughing >1.0 6.0 to 20

Extremely heavy roughing >0.7 8 to 20

(mm) (mm)

[Courtesy of Sandvik (UK) Ltd]

Turning and Chip-breaking Technology 49

Once again, manufacturers data tables are oen useful

‘starting-points’ for estimating the initial cutting pa-

rameter information. Considerable care must be taken

if the material has either a high work-hardening ten-

dency, or intrinsic bulk (i.e. workpiece material) hard-

ness, as this can inuence the numerical data selected.

Moreover, the plan approach angle also has an eect

on the numerical value for the parameter, for example,

oblique machining allows a higher value than for or-

thogonal machining.

2.2 History of Machine Tool

Development and Some

Pioneers in Metal Cutting

2.2.1 Concise Historical Perspective

of the Development of Machine

Tools

Toward the end of the 1700’s, any high-quality machin-

ing at the time meant tolerances of 0.1mm being con-

idered as ‘ultra-precision’ , with this level of tolerance

having steadily improved from the beginning of the

Industrial Revolution. Pioneers in machine tool devel-

opment such as John Wilkinson (1774), developed the

rst boring machine, this being capable of generating

bored hole of 1270 mm in diameter, with a error of

about 1 mm. A contemporary of Wilkinson, namely

Henry Maudslay (1771–1831), invented many preci-

sion machine tools, but he was particularly noted for

the design and development of the rst engine lathe.

Slightly later, Sir Joseph Whitworth (1803–1887), de-

veloped the rst modern-day Vee-form screwthread

and nut (i.e. 55° included angle – ‘Whitworth thread’),

thereby enabling precision feed-motion to be achieved

via suitable gear trains on such machine tools. ese

early fundamental advances in machine design, al-

lowed others and in particular, Joseph R. Brown

(1852) to design the ‘dividing engine’. is newly-de-

veloped equipment, allowed precision engraving of

the hand dials on machine tool axes, enhancing them

with much better machinist’s judgment in both rotary

and linear control, in combination with consistent

repeatability by the skilled operative. Shortly aer

these developments, Eli Whitney produced the origi-

nal milling machine, which was rened still further

by the Cincinnati Screw and Tap Company in 1884.

is ‘Cincinnati machine’ was a direct forerunner of

today’s manual controlled knee-type milling machine

tools. Of particular note was the ergonomic grouping

of the controls centrally for a more ecient hand con-

trol by the skilled operator. At this time the machine

tool still utilised the Vee-form screw thread, with the

Acme-form (ie having the ability to take-up backlash)

still someway o development.

Steady development and renement of a range

of machine tools continued into the the rst half of

the 20

century until the next major ‘milestone’ oc-

curred. is signicant development was the ‘modern’

numerically-controlled (NC) machine. Around the late

1940’s, the ‘recirculating ballscrew’

was designed so

that it could take-up backlash in both directions of

rotation for machine tool axes. ese early ‘ballscrews’

were tted to a converted Cincinnati Milling Machine

Company’s ‘Hydro-Tel’ die-sinking machine tool,

at MIT (Massachusetts Institute of Technology).

is military research-funded project having been

commissioned by the United States Air force – who

required complex free-form aeronautical parts to be

automatically machined for the latest aircra. is

research was undertaken by MIT, in association with

‘Cincinnati’ and the Parsons Tool Company. e

binary-coded punched-paper tape, controlled the

simultaneous machine tool axes using alpha-numeri-

cal characters (ie the forerunner of today’s programs

using ‘G- and M-coded’ CNC controllers), through a

14 Who, when and where ‘recirculating ballscrew’ design and

development took place is open to some debate. As propo-

nents in the UK say it was Alfred Herbert and Sons, whereas

in the United States, the Parsons Tool Company are oen

quoted as the originators. However, what is not in question,

is that with its unique ‘Gothic’ arch’ (i.e. Ogival geometry),

having point contacts between the screw and the adjacent re-

circulating balls, allows the assembly to be pre-loaded in-situ,

thereby eliminating any appreciable backlash allowing accu-

rate control of these axes.

NB e previous Acme taper thread (i.e. 29° included angle)

tted to ‘conventional’ machine tools had an eciency of

no better that 40% – with backlash present, whereas today’s

hardened ‘ballscrews’ have eciencies of ~90%, coupled to

an impressive rigidity (~900 N µm

–1

) and minimal ‘stick-slip’ ,

therefore minimising the so-called ‘ballscrew wind-up’ due to

the action of torque-eects in combination with the cutting

forces.

50 Chapter 2