Cutting Tool Technology Industrial Handbook by Graham T Smith_4 potx

Moreover, the top rake geometry of the cutting insert will significantly affect the chip forma-tion process, particularly when profile turning.. The machined surface texture generated b

is the strain-hardening tendency of the workpiece

material Therefore, the greater the

strain-harden-ing exponent, the higher will be its BUE formation

From experiments conducted on BUE formation, it

would seem that the higher the cutting speed, the

less is the tendency for BUE to form Whether this

lack of BUE formation at higher cutting data is the

consequence of increased strain rate, or the result

of higher interface temperatures is somewhat open

to debate However, it would seem that a paradox

exists, because as the speed increases, the

tempera-ture will also increase, but the BUE decreases The

propensity for BUE formation can be lessened by:

I Changing the geometry of the cutting edge –

by either increasing the tool’s rake angle, or

de-creasing the DOC, or both,

II Utilising a smaller cutting tip radius,

III Using an effective cutting fluid, or

IV Any combination of these factors.

• Discontinuous chips – consist of adjacent

work-piece chip segments that are usually either loosely

attached to each other, or totally fragment as they

are cut (Fig 29) The formation of discontinuous

chips usually occur under the following machining

conditions:

I Brittle workpiece materials – these materials

do not have the machining capability to

un-dergo the high shear strains,

II Hard particles and impurities – materials with

these in their matrix, will act as ‘stress-raisers’

and actively encourage chip breakage,

III Very high, or low cutting speeds – chip

veloc-ity at both ends of the cutting spectrum, will

result in lack of adherence/fragmentation of

the chip segments,

IV Low rake angles/large D OC ’s – either small top

rakes and heavy DOC’s will decrease the

adher-ence of the adjacent chip segments,

V Ineffective cutting fluid – poor lubricity,

com-bined with a meagre wetting ability, will

en-courage discontinuity of chip segments,

VI Inadequate machine tool stiffness – creating

vibrational tendencies and cutting instability,

leading to disruption of the machining

dynam-ics and loosening of chip segments

As mentioned in ‘Roman II’ above, the hard particles

and impurities tend to act as crack nucleation sites,

therefore creating discontinuous chips Large DOC’s

increase the probability that such defects occur in the cutting zone, thereby aiding discontinuous chip for-mation While, faster cutting speeds result in higher localised temperatures, causing greater ductility in the chip, lessening the tendency for the formation of dis-continuous chips If the magnitude of the compressive stresses in the both the primary and secondary shear zones significantly increase, the applied forces aid in discontinuous chip formation, this is because of the fact that the maximum shear strain will increase, due

to the presence of an increased compressive stress

NB Due to the nature of discontinuous chip

forma-tion, if the workpiece-tool-machine loop is not

suffi-ciently stiff, this will generate vibrational and chatter tendencies, which can result in an excessive tool wear regime, or machined component surface damage

• Segmented chips – are sometimes termed: in-, or

non-homogeneous chips, or serrated chips This

chip form has the characteristic saw-toothed pro-file which is noted by zones of low and high shear strain (Fig 30) These workpiece materials possess low thermal conductivity, as such, when machined their mechanical strength will drastically decrease with higher temperatures This continuous thermal cycle of both fracture and rewelding in a very nar-row region, creates the saw-toothed profile, being particular relevant for titanium and its alloys and certain stainless steel grades For example, to ex-plain what happens in realistic machining situation, the specimen Fig 30a is displayed, for an austenitic stainless steel quick-stop micrograph This micro-graph being the result of a less than continuous ma-chining process (1), utilising a 5° top rake-angled turning insert Here, variations in the cutting pro-cess have created fluctuations in the cutting forces, resulting in waviness of the machined surface (2) Prior to the material yielding, then the shearing process occurring, the workpiece material has de-formed against the cutting edge (3) To explain how changing the top rake angle influences the resul-tant chip formation for an identical stainless steel workpiece material, Fig 30b is shown Machining has now been undertaken with a 15° top rake, pro-moting a more continuous machining process than was apparent with the 5° tool (i.e illustrated in Fig 30a) This more efficient cutting process, results in smaller variations in the cutting forces (1 and 2) The chip is seen to flow over the rake face in a more

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

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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 The cutting data for machining the

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

180 m min– cutting speed, 0.3 mm rev– feedrate,

3 mm DOC

2.4 Tool Nose Radius

The 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 significantly affect the chip

forma-tion process, particularly when profile turning In Fig 31a, a spherically-shaped component is being ‘profile machined’ using a large nose-radiused turning insert Here, as the component nears its true geometric cur-vature, the cutting insert forces will fluctuate

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

– which is affected by the sharpness of the insert’s edge [Courtesy of Sandvik Coromant]

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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:

• Workpiece material composition – its heat

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.