Turning and Chip-breaking Technology Part 4 docx

The 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 a

Figure 31 The cutting insert’s tool nose radius when either profiling, or general turning, will modify both the profile and

diameter as flank wear occurs [Courtesy of Sandvik Coromant]

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ously as the insert progresses (i.e circular interpolates

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

the curved profile If the geometry of the tool was not

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

not be maintained, leading to significant variations in

chip formation If this lack of tool-work contact were

not to occur, then the machined profile would be

compromised and due to insufficient chip control, the

actual cut surface profile would not have a consistent

and accurate surface texture

The 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 The size of the tool

nose radius will have quite an effect on the surface

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

radius will create a different workpiece surface texture

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

large nose radius is selected for a lighter DOC, 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 This is because the ‘larger nose’ offers a smaller

plan approach angle, having the pressure of the cut

distributed across a longer cut length, creating an

en-hanced surface texture There are several

disadvan-tages to utilising a larger tool nose radius geometry,

these are that the:

• Chip formed becomes more difficult to bend and

effectively 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 flank wear, can significantly influence

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

 Flank wear is normally denoted by specific ‘zones’ – more will

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

in-sert wear ‘VB’ is shown in both Figs 31ci and cii

drift’ which could affect the process capability of the overall parts produced This flank wear ‘VB’ 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 finish turning The principle underlying a wiper insert for turning

op-erations, concerns the application of a modified ‘tool

nose radius’ (see Fig 32 – bottom left and right dia-grams) When a ‘standard’ tool nose geometry insert

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

peaks and valleys (i.e termed ‘cusps’) after 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 finishing

opera- Process capability denoted by ‘CP’ , is a measure of the quality

of the parts produced, which is normally found by the follow-ing simple relationship:

*CP = Drawing specification tolerance/6 σ

Where: σ = a statistical measure, termed the ‘standard devia-tion’ for the particular production process *CP values of <.0

denote low process capability, CP values of between .0 and

. are moderate process capability, CP values of >. are

termed as high process capability.

NB Today, process capabilities of .0 are often demanded for

high-quality machined parts for the automotive/aerospace sectors of industrial production, reducing likelihood of part scrappage.

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

radius geometry, positioned at regular intervals related to the

selected feedrate The cusp height (i.e the difference in height

between the peak and valley), will influence the machined surface texture of the component, in the following relation-ship:

Rmax = fn  × 250/rε (µm) Where: Rmax = maximum peak-to-valley height within the

sam-pling length fn = feedrate (m min–) rε = tool nose radius (mm).

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

Tools]

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tions With recent advancement in wiper geometry,

this has allowed them to be used at double the

previ-ous feedrates for semi-finishing/roughing operations,

without degrading the surface texture The 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 The 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

feedrates of 0.6 mm rev– at DOC’s of up to 4 mm For

example, with steel component hardnesses of 65HRc,

this often 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 finish, while increasing tool life, can be

considered as outstanding tool design

2.5 Chip-Breaking

Technology

2.5.1 Introduction to Chip-Breaking

The 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

 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, significantly 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

DOC’s, efficient 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 differ sig-nificantly Finally, for light finishing cuts (Fig 33c), chips are broken in a totally different manner to that of the previous examples Hence, with all of these differ-ing types of turndiffer-ing operations on workpieces, control

of the chip is vital, as it can drastically impair the over-all production rates and affect part quality, if not given due consideration

Chip formation is chiefly influenced by the follow-ing factors:

treat-ment (i.e if any), which affects 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 finishing 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 OC ) – 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

Note: Another important factor that can also play a

significant role in chip formation, is the application of

coolant and its supply velocity.

The shear angle has some effect on the contact length between workpiece and the rake face and, it is in this vicinity that cutting forces and machining-induced temperatures predominantly affect the cutting insert Moreover, the insert’s rake is significant, 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: DOC, feedrate, rake angle, together with the workpiece’s mechanical strength, noting that the chip starts forming in the primary deformation zone (see Fig 26) Thus, the chip is subsequently formed

by the bending force of the cutting action, effectively

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

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

[Courtesy of Sandvik Coromant]

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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 effect’ of preliminary chip formation which is

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

be-ing that any formation is related to the cuttbe-ing speed

A large insert rake angle normally means that there is

less tendency for chip curling through a larger radius,

but 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

NB: In this schematic representation: ‘h ’ represents

DOC 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 The manner to which

swarf affects machining operations depends upon the

operating conditions, but fundamentally there are

sev-eral requirements in any form of swarf control, these

are:

• The swarf must flow freely away from the cutting

zone, without impairing the cutting action’s

effi-ciency,

• Swarf must be of convenient size and shape to

fa-cilitate handling manually, or in swarf conveyors

(i.e if fitted), 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- 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 flow smoothly away from the cutting area, as with the latest chip-breakers fitted to today’s cutting inserts, chips can be readily broken and controlled0, 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 flow 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 The 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 specifications and this allows their properties to be ‘tailored’ to the specific 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 influence on the workpiece’s chip-breaking ability The 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-0 Today, many high-volume manufacturing companies have

re-alised the benefit 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.

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 This chip occurrence causes the chip to be

inflexible, which can then subsequently be broken

There 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 left-hand

photograph), the latter two functions affecting the

chip cross-section, or chip thickness

 Chip thickness is influenced by the plan approach angle

utilised and the DOC, in association with the selected feedrate

The chip thickness is measured across the cutting edge,

per-pendicular to the cut (i.e along the main cutting edge) The

chip width and thickness are the dimensions that define 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, DOC

and the effective cutting depth The chip thickness is related to

the plan approach angle and this affects 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 The 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 This origi-nal form of chip-breaker geomtery was relatively in-efficient as the resultant force direction changed with the programmed tool path, this meant that the step would be approached by the chip from differing di-rections making chip-breaking less controlled Such chip-breakers were superseded in the 1970’s by built ‘wavy-shaped’ chip-breakers sintered into the in-sert’s top face (Fig 34 bottom left-hand photograph) Recent developments in designing chip-breaker geom-etries by computer-generated (i.e CAD) techniques, has shown a significant 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 deflect at a narrower angle, causing it to break off, either immediately, or just after the free end of the chip has hit either the tool’s flank or, the workpiece before the first 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 effect of gravity upon

it, with, or without any further collision

Chip flow direction will depend upon several fac-tors, such as the: chip-breaker profile, back rake and setting angles, nose radius, DOC and feedrate – these latter three factors require further discussion The relationship between the nose radius, DOC and feedrate will often change during vectored tool paths in any machining operation Even though the insert’s nose radius is preset, its influence on the chip direction differs for different DOC’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 different DOC’s and with a constant feedrate, the form of chip cross-section (i.e

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

edge preparations

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the ratio of chip width-to-thickness), will change and

this has a deleterious effect on the insert’s

chip-break-ing ability

2.5.4 Helical Chip Formation

Conventional Turning

For the general turning operations, such as sliding (i.e

Z-axis tool feeding) and facing (i.e X-axis tool path

motions), the chip is rolled into a helix, simply because

the chip edges are formed from different rotation radii

(Fig 34d) Here, the two edges of the chip consume

different quantities of workpiece material, creating

dif-fering edge lengths, coupled to the fact that a

varia-tion in cutting speed is present, these relavaria-tionships will

result in a helical chip formation The appearance of

the chip’s helix depends upon the workpiece’s diameter

and its metallurgical specification/condition, which

means the chip helices are extremely difficult to

quan-tify

Most common types of helical chip diameters are

determined either directly by the initial curvature

from its origin, or are the result of additional bending,

introduced by the chip-breaker For example, the

heli-cal chip type shown in Fig 34c (left), has its chip

seg-ments turned inwards, this being a desirable chip form

when not fully developed, that is prior to the first coil

being completed Whether, or not the chip is of this

form will already be determined even before it meets

the chip-breaker, this being the result of its

cross-sec-tion and the natural tendency to bend according to the

‘line of least resistance’ If the chips width is no larger

than its thickness, for example, the resistance to

bend-ing in the segment-stiffened thickness direction is

larger than in the width direction In this case, unless

this kind of chip is broken early, by colliding with

ei-ther part of the tool, or the workpiece whilst it is still

stiff and short – called ‘self-breaking’ – a helical chip

will be formed In this case, the barbed, or serrated

edge is turned outwards causing additional bending,

this being introduced by the chip-breaker For

exam-ple, the helical chip type shown in Fig 34c (right),

becomes difficult and awkward to control This

out-ward-curving helical chip also has weakened sections

in the serrations between the chip segments, but

ap-plied loads on it are readily absorbed by the spring

ac-tion of the chip This type of chip will break as it hits

the insert’s flank face (see Fig 27b) Only today’s very complex chip-breaker designs can reduce these out-ward-curling helical chips Although such chip helices produced by combinations of the feeds and DOC’s that result in the chip width being too small in relation to

its thickness must be avoided

Grooving and Recessing

In conventional turning operations, it is significantly easier to form a manageable chip, than for features re-quiring either grooving, or recessing The chip formed during plunge grooving counter-rotates in relation

to the workpiece, whereby it does not experience the same twisting force as chips produced by either Z-, or X-axes turning operations When grooving, ideally the chip resembles a ‘watch spring’ , indicating that the chip

is curling back onto itself and will ultimately break in several distinct ways: such as at the completion of the grooving cycle, or due to friction between the chip and its groove side walls – as the chip diameter becomes greater In grooving operations, three significant fac-tors affect chip control, these are:

(i) Insert geometry – applied to the rake face, can be

classified into distinct groupings:

• Radial-ground top rake (not shown), producing

the desired ‘watch-spring’ chip formation This grooving insert geometry will not thin the chip, therefore surface finish passes are necessary on both groove side walls

NB For long-chipping materials the chip-former

does not provide enough resistance to produce chip curling, hence, a straight flat chip occurs, that may

 One of the problems with this type of chip-breaking, is the

potential for secondary wear on the insert’s non-cutting zone

on the face, caused by the chip helix breaking locally against this face Such an occurrence happens when the chip helix at-tains such a diameter and pitch that its free-end continually strikes the non-cutting portion of the insert’s edge – termed

‘chip-hammering’ – causing the edge to be locally weakened

and to subsequently crumble.

NB Chip-hammering can be alleviated by slightly increasing

the helix diameter (i.e by somewhat modifying the cutting data) causing the chip to break against the tool’s flank – be-low the insert’s cutting edge, this being one of the previously employed and favoured chip-breaking mechanisms, as shown

in Fig 27b.

Figure 35 The chip-breaking envelopes related to cutting data and chip-curling behaviour [Courtesy of

Sandvik Coromant]

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