Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 29. Discontinuous chip formation. [Courtesy of Sumitomo Electric Hardmetal Ltd.].

Turning and Chip-breaking Technology 61

consistent manner (3). It was found with this work-

piece material in an experimental cutting proce-

dure, that the tangential cutting force component,

was closer to the actual cutting edge than when

similar machining was undertaken on unalloyed

steel specimens.

NB 

e cutting data for machining the stain-

less steel specimens in Figs. 30 a and b, were:

80 m min

–1

cutting speed, 0.3 mm rev

–1

feedrate,

3 mm D

2.4 Tool Nose Radius

e insert’s nose radius has been previously mentioned

in Section 2.1.6, concerning: Cutting Tool holder/In-

sert Selection. Moreover, the top rake geometry of the

cutting insert will signicantly aect the chip forma-

tion process, particularly when prole turning. In Fig.

31a, a spherically-shaped component is being ‘prole

machined’ using a large nose-radiused turning insert.

Here, as the component nears its true geometric cur-

vature, the cutting insert forces will uctuate continu-

Figure 30. Segmented chip formation, resulting from machining stainless steel and the work-hardening zone

– which is aected by the sharpness of the insert’s edge. [Courtesy of Sandvik Coromant]

62 Chapter 2

Figure 31. The cutting insert’s tool nose radius when either proling, or general turning, will modify both the prole and

diameter as ank wear occurs. [Courtesy of Sandvik Coromant]

Turning and Chip-breaking Technology 63

ously as the insert progresses (i.e circular interpolates

with the X- and Z-axes of the machine tool) around

the curved prole. If the geometry of the tool was not

itself of round geometry, then the ‘point-contact’ could

not be maintained, leading to signicant variations in

chip formation. If this lack of tool-work contact were

not to occur, then the machined prole would be

compromised and due to insucient chip control, the

actual cut surface prole would not have a consistent

and accurate surface texture.

e machined surface texture generated by the pas-

sage of the cutting insert’s geometry, is to a large extent

the product of the relationship, between the nose ra-

dius and the feedrate and, to a lesser degree the cutting

speed and its tool wear pattern. e size of the tool

nose radius will have quite an eect on the surface tex-

ture produced, if the feedrate is set, then a small nose

radius will create a dierent workpiece surface texture

to that of a larger one (see Fig. 31b). Moreover, if a

large nose radius is selected for a lighter D

, or if the

feed is equal to the nose radius, then this larger nose

geometry will be superior to that of a smaller tool nose

radius. is is because the ‘larger nose’ oers a smaller

plan approach angle, having the pressure of the cut

distributed across a longer cut length, creating an en-

hanced surface texture. ere are several disadvan-

tages to utilising a larger tool nose radius geometry,

these are that the:

•

Chip formed becomes more dicult to bend and

eectively break,

•

Radial cutting forces are greater,

•

Power consumption increases,

•

Rigidity of the set-up is necessary – leading to pos-

sible vibrational tendencies on either weaker, or

unstable workpieces.

Tool wear (i.e. denoted by ‘∆’ in Figs. 31ci and cii) and

in particular ank wear

, can signicantly inuence

the resulting machined component dimensional accu-

racy (Fig. 31cii), which on a batch of components cut

with the same insert, will result in some level of ‘tool

25 Flank wear is normally denoted by specic ‘zones’ – more will

be said on this topic later – but, in this example, the tool’s in-

sert wear ‘V

’ is shown in both Figs. 31ci and cii.

dri’ which could aect the process capability

of the

overall parts produced. is ank wear ‘V

’ can be cal-

culated and utilised to determine the anticipated tool’s

life (ie, in-cut), this important factor in production

machining operational procedure, will be discussed in

due course.

Wiper blades (Fig. 32) are not a new insert geom-

etry concept, they have been used for face milling op-

erations for quiet a long time, but only in recent years

are they being utilised for component nish turning.

e principle underlying a wiper insert for turning op-

erations, concerns the application of a modied ‘tool

nose radius’ (see Fig. 32 – bottom le and right dia-

grams). When a ‘standard’ tool nose geometry insert

is used (i.e. Fig. 32 – bottom le), it creates a series of

peaks and valleys (i.e. termed ‘cusps’

) aer the pas-

sage of the ‘insert nose’ over the machined surface.

Conversely, a cutting insert with wiper blade geom-

etry (i.e. Fig. 32 – bottom right), has trailing radii that

blends – beyond the tangency point – with the tool

nose radius which remains in contact with the work-

piece, allowing it to wipe (i.e. smooth) the peaks, leav-

ing a superior machined surface texture.

In the past, wiper insert geometries were only em-

ployed for surface improvement in nishing opera-

26 Process capability denoted by ‘C

’ , is a measure of the quality

of the parts produced, which is normally found by the follow-

ing simple relationship:

= Drawing specication tolerance/6 σ

Where: σ = a statistical measure, termed the ‘standard devia-

tion’ for the particular production process. *C

values of <1.0

denote low process capability, C

values of between 1.0 and

1.33 are moderate process capability, C

values of >1.33 are

termed as high process capability.

NB Today, process capabilities of 2.0 are oen demanded for

high-quality machined parts for the automotive/aerospace

sectors of industrial production, reducing likelihood of part

scrappage.

27 Cusps are the product of the partial geometry of the tool nose

radius geometry, positioned at regular intervals related to the

selected feedrate. e cusp height (i.e. the dierence in height

between the peak and valley), will inuence the machined

surface texture of the component, in the following relation-

ship:

max

= f

× 250/r

(µm)

Where: R

max

= maximum peak-to-valley height within the sam-

pling length. f

= feedrate (m min

–1

) r

= tool nose radius (mm).

64 Chapter 2

Figure 32. The application of wiper insert geometry on the resulting surface texture when ne turning. [Courtesy of Iscar

Tools]

Turning and Chip-breaking Technology 65

tions. With recent advancement in wiper

geometry,

this has allowed them to be used at double the previ-

ous feedrates for semi-nishing/roughing operations,

without degrading the surface texture. e wiper ge-

ometry being in contact with the workpiece’s surface

for longer than equivalent standard insert nose radius

tends to wipe – hence its name, or burnish the ma-

chined surface, producing a smoother surface texture.

Due to the fact that a ‘wiper’ has an extended edge, the

cutting forces are distributed across a longer tool/chip

contact region. e wiper portion of the insert, being

somewhat protected, enables these wiper inserts to in-

crease tool life by up to 20% more than when using

conventional tool nose geometries.

Wiper blades have their clearance lengths care-

fully designed, if they are too long, the insert gener-

ates too much heat, on the contrary, they need to be

long enough to cope with relatively large feeds, while

still smoothing over the surface cusps. Wipers with

positive turning insert geometries, they can cope with

eedrates of 0.6 mm rev

–1

at D

’s of up to 4 mm. For

example, with steel component hardnesses of 65HR

this oen negates the need for any successive precision

grinding operations. By designing wiper geometries

with the cutting edge and nose radii to improve ma-

chined surface nish, while increasing tool life, can be

considered as outstanding tool design.

2.5 Chip-Breaking

Technology

2.5.1 Introduction to Chip-Breaking

e technology of both chip-forming and chip-break-

ing has been one of the major areas of advancement

in recent years. A whole host of novel toolholders and

cutting inserts has been developed to enable the cut-

ting process to be under total chip control, allowing

some toolholder/inserts combinations to machine

multiple component features with just one tool, re-

moving at a ‘stroke’ the non-productive aspects of

28 Some tooling manufacturers have re-named wiper inserts as

high-feed inserts, as they have demonstrated in production

conditions to promote higher component output, without the

recourse to expensive capital outlay.

tool-changing and setting, signicantly increasing ma-

chine tool utilisation rates. Even when conventional

turning inserts are employed, for heavy roughing cuts

(Fig. 33a), where feedrates are high as are the large

’s, ecient control of the chip must be achieved. To

enable excellent control of chip-breaking with rough-

ing cuts (Fig. 33b), a similar overall insert geometry

is shown to that in the previous example, but here the

rake face embossed dimples/chip-breakers dier sig-

nicantly. Finally, for light nishing cuts (Fig. 33c),

chips are broken in a totally dierent manner to that of

the previous examples. Hence, with all of these dier-

ing types of turning operations on workpieces, control

of the chip is vital, as it can drastically impair the over-

all production rates and aect part quality, if not given

due consideration.

Chip formation is chiey inuenced by the follow-

ing factors:

•

Workpiece material composition – its heat treat-

ment (i.e. if any), which aects the chip’s strength,

•

Insert’s cutting geometry – rake and clearances, as

well as any chip-formers present, the geometry be-

ing associated with the work piece material,

•

Plan approach angle – depending upon whether

roughing, or nishing cuts are to be taken,

•

Nose radius – this being linked to the feedrate and

here, to a lesser extent, the surface texture require-

ments,

•

Undeformed chip thickness (i.e. D

) – this will af-

fect the chip curling aspect of the chip’s formation –

more will be said on this topic in the following sec-

tion.

ote: Another important factor that can also play a

signicant role in chip formation, is the application of

coolant and its supply velocity.

e shear angle has some eect on the contact length

between workpiece and the rake face and, it is in this

vicinity that cutting forces and machining-induced

temperatures predominantly aect the cutting insert.

Moreover, the insert’s rake is signicant, in that as the

rake angle increases the contact length decreases, the

more positive the rake, the shorter the contact length.

Actual chip formation is primarily dependent upon

several factors: D

, feedrate, rake angle, together with

the workpiece’s mechanical strength, noting that the

chip starts forming in the primary deformation zone

(see Fig. 26). us, the chip is subsequently formed

by the bending force of the cutting action, eectively

‘

pivoting’ from the chip’s roughen ‘free top surface’ ,

66 Chapter 2

Figure 33. Turning cuts and associated insert geometries for forming and shearing of a chip.

[Courtesy of Sandvik Coromant]

Turning and Chip-breaking Technology 67

this being a somewhat shorter length than that of the

‘shiny’ underside at the tool/chip interface.

Many theories have been given for the actual ‘cause

and eect’ of preliminary chip formation which is

schematically illustrated Fig. 33d – ‘A’- one such, be-

ing that any formation is related to the cutting speed.

A large insert rake angle normally means that there is

less tendency for chip curling through a larger radius,

ut it will have lower cutting forces. In Fig. 33d – ‘B’ ,

is depicted a somewhat ‘idealised’ view of the actual

cutting process, which can be expressed via the simple

relationship of ‘λ’ and ∆X/∆Y.

B: In this schematic representation: ‘h

’ represents

and, ‘ϕ’ is the ‘shear plane angle’.

When utilising CNC machine tools and in particu-

lar turning centres, a major problem is the variety of

continuous chip forms created and the large quantity

and volume of swarf

produced. e manner to which

swarf aects machining operations depends upon the

operating conditions, but fundamentally there are sev-

eral requirements in any form of swarf control, these

are:

•

e swarf must ow freely away from the cutting

zone, without impairing the cutting action’s e-

ciency,

•

Swarf must be of convenient size and shape to fa-

cilitate handling manually, or in swarf conveyors

(i.e. if tted), together with any future large-volume

storage, then transportation and subsequent dis-

posal,

•

Any swarf should drop away into the machine’s

swarf tray, without snarling around, the workpiece,

tool, or interfering with other functions such as:

automatic tool-changing magazine/turret, in-situ

touch-trigger inspection probes, component load-

29 Individual chips when in any great volume are generally

termed swarf. It is important to be able to manage this swarf

volume and, satisfactory chip control can be determined by

‘Lang’s chip-packing ratio’ , this being denoted by the letter

‘R’ , in the following manner:

R = Chip volume (mm

)/Equivalent volume of uncut work-

piece material (mm

)

NB: ‘R’ ranges from values of 3-to-10, where an R-value of

4 gives satisfactory chip-breaking control, producing neatly

curled ‘6 and 9-shaped’ chips.

ing equipment, such as overhead gantries, or dedi-

cated robotic loading devices.

In terms of priority for these swarf control factors, pos-

sibly the most important one is that the swarf should

ow smoothly away from the cutting area, as with the

latest chip-breakers tted to today’s cutting inserts,

chips can be readily broken and controlled

, this will

be theme of the following section.

2.5.2 The Principles of Chip-Breaking

In machining, the cutting edge’s primary function is

to remove stock from the workpiece. Whether this

is achieved by forming a continuous chip, or by the

ow of elemental chips will depend upon several fac-

tors, including the properties of the workpiece mate-

rial, cutting data employed and coolant type and its

delivery. e terms ‘long-chipping’ and ‘short-chipping’

are utilised when considering the materials to be ma-

chined. Short-chipping materials such as most brasses

and cast irons, do not present a chip-breaking problem

for swarf disposal, so this section will concentrate on

the long-chipping workpiece materials, with particu-

lar focus on ‘steel family’ grades. Steels are produced

in a wide variety of specications and this allows

their properties to be ‘tailored’ to the specic indus-

trial applications. In addition, these steels methods of

primary processing, such as: casting, forging, rolling,

forming and sintering, together with the type of subse-

quent heat treatment, creates still further metallurgical

variations that may have an even greater inuence on

the workpiece’s chip-breaking ability. e workpiece’s

strength and hardness values describe the individual

material’s character to some extent, but it should be

borne in mind that it is the chip’s mechanical strength

that determines whether it can be broken with ease.

No absolute correlation exists between a steel com-

30 Today, many high-volume manufacturing companies have re-

alised the benet of the value of clean and briquetted swarf,

as opposed to oily scrap swarf, which sells at just ‘fractions’

of this value. At present, briquetted and cleaned aluminium

swarf can be sold for approaching £1,000/tonne, moreover,

the coolant/oil can be reclaimed, further driving down the

overall machining costs. For other non-ferrous ‘pure’ metals

and others, such as copper alloys and brasses, the economic

savings are even greater.

68 Chapter 2

ponent’s strength and the mechanical strength of the

chip, illustrating that a complex metallurgical and cut-

ting tool geometric relationship exists whilst machin-

ing occurs.

In particular for turning operations, a convention-

ally-turned chip is a rather frail product of serrated

appearance (see Figs 25 and 34a and b). In order to

promote good chip-breaking tendencies, thus enabling

short elements to be formed, it is necessary to encour-

age this basic character by causing these serrations to

be as deep as possible and the chip sections in between

to be rigid. is chip occurrence causes the chip to be

inexible, which can then subsequently be broken.

ere are several distinct ways in which chips can then

be broken, these include:

•

Self-breaking – this is when the chip’s mechanical

strength is not great enough to hold the chip seg-

ments together and they consequently break upon

exiting the machining region (Fig. 31a),

•

Chip collision with the workpiece – as the chip is

steered towards an obstacle such as the workpiece’s

surface this provides the breaking force (see Figs.

33 and 34b),

•

Chip is stopped by the tool – here the chip-curling

behaviour comes into play, this being a function of

the: tool’s nose radius geometry, depth of cut and

feedrate employed (see Fig. 34 bottom le-hand

photograph), the latter two functions aecting the

chip cross-section, or chip thickness

31 Chip thickness is inuenced by the plan approach angle

utilised and the D

, in association with the selected feedrate.

e chip thickness is measured across the cutting edge, per-

pendicular to the cut (i.e. along the main cutting edge). e

chip width and thickness are the dimensions that dene the

theoretical cut of the edge into the workpiece material. Hence,

the chip thickness will vary with the size of the plan approach

angle according to the relationships involving: feedrate, D

and the eective cutting depth. e chip thickness is related to

the plan approach angle and this aects the amount of pressure

bearing upon the cutting edge. Hence, the thinner the chip,

the smaller the distributed pressure along the edge and the less

power consumed, conversely, the thicker the chip, the greater

will be the machine tool’s power consumption. A thicker chip

is generally advantageous for an increased tool life, because of

the improved contact between the chip and its cutting edge.

Furthermore, if the plan approach angle is too small and chip

thickness is thin, this will reduce tool life, however, this can

be compensated for by increasing the feedrate, to produce a

thicker chip.

NB e helical formation of this chip-curling behav-

iour will shortly be mentioned, but prior to this, chip-

breakers/formers will be discussed.

2.5.3 Chip-Breakers and Chip-Formers

Chip-breakers have been utilised on turning tools for

many years, initially introduced in the 1940’s in the

form of an abutment, or step, situated behind the rake

face of the tool. Hence, with this type of early chip-

breaker, as the continuous chip moves across the rake

face it collides with this step and breaks. is origi-

nal form of chip-breaker geomtery was relatively in-

ecient as the resultant force direction changed with

the programmed tool path, this meant that the step

would be approached by the chip from diering di-

rections making chip-breaking less controlled. Such

chip-breakers were superseded in the 1970’s by in-

built ‘wavy-shaped’ chip-breakers sintered into the in-

sert’s top face (Fig 34 bottom le-hand photograph).

Recent developments in designing chip-breaker geom-

etries by computer-generated (i.e. CAD) techniques,

has shown a signicant step-forward in both chip-

former design enabling chip control and reduction in

frictional forces across the rake face at a range of cut-

ting data to be achieved. Such ‘automatic’ chip breaker

geometry forces the chip to deect at a narrower angle,

causing it to break o, either immediately, or just aer

the free end of the chip has hit either the tool’s ank or,

the workpiece before the rst coil has formed. If such

a collision does not take place, the result would be a

smaller diameter spiral chip and, it can be anticipated

that the chip would still break, but only when it be-

came slightly longer – this later chip breakage is due to

the increasing chip mass and the eect of gravity upon

it, with, or without any further collision.

Chip ow direction will depend upon several fac-

tors, such as the: chip-breaker prole, back rake and

setting angles, nose radius, D

and feedrate – these

latter three factors require further discussion. e

relationship between the nose radius, D

and feedrate

will oen change during vectored tool paths in any

machining operation. Even though the insert’s nose

radius is preset, its inuence on the chip direction

diers for dierent D

’s, depending on how much

corner rounding is represented by the total engaged

edge length (Fig. 34c). Further, the feedrate also af-

fects the chip thickness: at dierent D

’s and with a

constant feedrate, the form of chip cross-section (i.e.

Turning and Chip-breaking Technology 69

Figure 34. The principles of chip-breaking and chip-breaking envelopes for ‘coma-shaped swarf’ control and insert

edge preparations

70 Chapter 2