Turning and Chip-breaking Technology Part 5 pptx

ii Surface speed of the workpiece – in order to ob-tain full advantage of a grooving insert’s chip-form-ing abilities, the chip must be allowed to flow into the chip-former.. However,

wrap itself around either the tool, or workpiece, but

such a geometry is perfect for machining

alumin-ium, or non-ferrous materials

• Radial top rake (illustrated in Fig 4 middle and

to the left – three grooving insert sizes illustrated)

This radial top rake is designed to thin the chip

Such chip thinning, eliminates the need to

under-take finishing passes on the groove’s side walls

Fur-thermore, this type of grooving insert geometry

be-ing on-centre, enables axial turnbe-ing of diameters for

wide shallow grooves, or recesses

• Raised bumps on top rake (see Fig 27a – left)

This sophisticated grooving geometry is utilised for

materials where chip control is difficult, as it

pro-vides an ‘aggressive barrier’ to the curling chip The

raised bumps force the chip back onto itself, either

producing a tightly curled watch-spring chip, or

causes the chip to break

(ii) Surface speed of the workpiece – in order to

ob-tain full advantage of a grooving insert’s

chip-form-ing abilities, the chip must be allowed to flow into the

chip-former This chip-flow can be achieved by either

decreasing the workpiece’s surface speed, or increasing

the feed – more will be said on this shortly The former

technique of decreasing the surface speed, allows the

material to move slower across the top rake of the

cut-ting edge and as a result, has greater contact time to

engage the chip-former This slower workpiece speed,

has the benefit of increasing tool life, through lower

 A groove, or recess, can normally be considered as a

straight-walled recessed feature in a workpiece, as illustrated in Fig

40 Typical applications for grooves are to provide thread

re-lief – usually up to a shoulder – so that a mating nut and its

washer can be accurately seated , or for retaining O-rings As

the groove is produced in the workpiece, the tool shears away

the material in a radial manner, via X-axis tool motion The

chip formed with insert geometries having a flat top rake, will

have an identical width as the tool and can be employed to

‘size’ the component’s width feature However, this chip action

– using such a tool geometry, creates high levels of pressure

at the cutting edge as a result of the chip wall friction, which

tends to produce a poor machined surface texture on these

sidewalls Grooving with an advanced chip-former insert

ge-ometry, reduces the chip width and provides an efficient

cut-ting action, this results in decreasing the cutcut-ting edge pressure

somewhat Chip-formers offer longer tool life and improved

sidewall finishes with better chip control, than those top-rakes

that have not incorporated such insert chip-forming

geomet-ric features

tool/chip interface temperatures The negative factors

of such a machining strategy, are that the:

• Part cycle times are increased and as a result, any batch throughput will be lessened,

• As the cutting edge is in contact for a longer du-ration, more heat will be conducted into the tool, than into the chip, which could have a negative im-pact of inconsistent workpiece size control,

• Due to the lower workpiece surface speed, the ben-efits of the insert’s coating will be reduced, as such coating technology tends to operate more effec-tively at higher interface temperatures

(iii) Increasing the feedrate – by increasing the feed

allows it to engage the chip-former more effectively – this being the preferred technique for chip control A

heavier applied feedrate, produces a chip with a thicker

cross-section Further, a thicker chip engages the in-sert’s geometry with higher force, creating a greater tendency to break Hence, by holding a constant work-piece surface speed, allows the faster feedrate to reduce cycle times

Transversal, or Face Grooving

Transversal grooving geometry has a curved tear-shaped blade onto which, the insert is accurately lo-cated and positioned The transversal insert follows the 90° plunged feed into the rotating face of a work-piece These tools are categorised as either right-, or left-hand, with the style adopted depending upon whether the machine tool’s chuck rotates anti-clock-wise (i.e using a right-hand tool), or clockanti-clock-wise (i.e left-hand) The minimum radius of curvature for such transversal grooving tooling is normally about 12mm, with no limit necessary on the maximum radial curva-ture that can be machined For shallow face grooves, off-the-shelf tooling is available, but for deep angular face grooves they require specialised tools from the tooling manufacturers

If a relatively wide face groove requires machining

with respect to the insert’s width, then the key to

suc-cess here, is establishing where in the face to make the

first plunge This initial face plunge should be made within the range of the tool’s diameter, otherwise the tool will not have sufficient clearance and will ulti-mately break Successive plunges to enlarge the face groove should be made by radially moving the insert 0.9 times the insert’s width, for each additional plunge The rotational speed for face grooving is usually about 80% of the speed used for parting-off – soon to be

mentioned Feedrates are normally around 50% of

parting-off values, with the proviso that for material

which is subject to work-hardening, minimum feeds

are necessary

In transversal grooving operations, a unique chip

form occurs, because the chip is longer the further

away it is from the workpiece’s centre line of rotation

This results in the chip which no longer flows in a

straight line across the insert’s edge, instead it moves

at an angle Such a naturally curved chip is difficult to

exhaust from the face groove, particularly if it is

bro-ken Hence, no attempt should be made to break the

chip For deep and narrow grooves, the best solution

is to retract the tool at short intervals, to check that

the blade shows no signs of rubbing, this is to guard

against any likely breakage that might occur when

machining outside the blade’s range Due to the fact

that transversal grooving tooling is susceptible to

chat-ter, any excessive overhang of the tool should be

mi-nimised The chip should never be allowed to become

entangled within the transversal groove and should be

ejected speedily, otherwise the tool is likely to break

 Chatter is a form of self-excited vibration and such vibrations

are due to the interaction of the dynamics of the chip-removal

process, together with the structural dynamics of the machine

tool Such chatter, tends to be at very high amplitude, which

can result in either damage to the machine tool, or lead to

pre-mature tool failure Typically, chatter is initiated by a

distur-bance in the cutting zone, for several reasons, such as:

Lack of homogeneity – in the workpiece material (i.e

typi-cally a porous component, such as is found in a Powder

Metallurgy compact),

Workpiece surface condition (i.e typically a hard oxide scale

on a hot-rolled steel component, utilsing a shallow DOC),

Workpiece geometry (i.e if the component shape produces

either a variation in the DOC – for example, because of

un-even depth of casting material being machined, or light cuts

on interrupted shapes, such as hexagon, square, or

rectan-gular bar stock),

Frictional conditions (i.e tool/chip interface frictional

variations, whilst machining).

Regenerative chatter is a type of self-excited vibration,

result-ing from the tool cuttresult-ing a workpiece surface that has either

significant roughness, or more likely the result of surface

dis-turbances from the previous cut These disdis-turbances in the

workpiece surface, create fluctuations in the cutting forces,

with the tool being subjected to vibrations with this process

continuously repeating, hence the term ‘regenerative chatter’.

Self-excited vibrations can be alleviated by either

increas-ing the dynamic stiffness of the system, or by increasincreas-ing the

damping.

NB Dynamic stiffness can be defined as the ratio of the

am-plitude of the force to the vibrational amam-plitude

–

–

–

–

For any face grooving of workpiece material that is subject to a continuous chip formation, always use copious amounts of coolant and at high-pressure – if possible, to not only lubricate the cutting zone, but to aid in chip flushing from this groove

Parting-off

The parting-off process is normally considered to be a separate machining operation, but it simply consists of cutting a groove to centre of rotation of the workpiece,

to release it from the bar stock, or to ‘part-off’ to a pre-viously formed internal diameter (shown in Fig 40 for left-hand side operations) Essentially in a parting-off operation, two time-periods are worthy of mention, these are:

(i) At separation from the bar stock – a lower spindle

speed than was previously used on the workpiece, will prevent the ‘released part’ from hitting the machine and potentially damaging its surface Moreover, it al-lows an operator – if present – to hear the change in the lower spindle speed tone, as it is about to separate from the bar stock, avoiding the parting-off tool from getting ‘pinched’ between the stock and the

soon-to-be-released component Often, ‘Part-catchers’ are

utilised to reduce any surface damage to the falling component, once it has been parted-off

NB If the component to be parted-off is held in a

co-axial/sub-spindle, at component release, the additional spindle supports the workpiece and under these con-ditions, the parting-off operation is virtually identical

to that of found in a grooving cycle

(ii) Surface speed reduction – this effectively

oc-curs when the machine’s spindle attains its maximum speed For example, on a machine tool having a maxi-mum speed of 3,000 rpm, 90 m min– would only be achievable until the parting diameter has reached about 8.6 mm When parting to a smaller diameter than 8.6 mm, the surface speed would decrease at a

fixed spindle speed As the parting diameter reaches

5.8 mm the surface speed would be 55 m min–, or 60%

of the ideal, thus significantly increasing the chip load-ing as the tool approaches the workpiece’s centreline

In order to alleviate the increasing tool loading, lower-ing the feedrate by about 50% until separation is just

about to occur, then finally dropping the surface speed

to almost zero at this point, reduces the tendency for a

‘pip’ to be present on the workpiece On a CNC driven spindle, it is not advisable for parting-off operations,

to utilise the ‘canned cycle’ such as the ‘constant surface

speed’  function

NB A more serious parting-off problem has been that

in order to eliminate the pip formed at the centre of the

‘released component’ , some tools have been ground

with the front edge angle of between 3° to 15° Such

a front edge geometry, can introduce an axial cutting

force component, leading to poor chip control, which

in turn, causes the tool to deflect This parting-off tool

deflection, can lead to the component’s face ‘dishing’ ,

creating a convex surface on one face and a concave

surface on the other – so this tool grinding strategy

should be avoided.

Today, parting-off inserts normally consist of two

main types with top rakes that are either of, negative,

or positive cutting edge chip-forming geometries The

negative-style of chip-formers are possibly the most

commonly utilised These inserts have a small

nega-tive land at the front edge which increases the insert’s

strength, giving protection in adverse cutting

condi-tions, such as when interrupted cutting is necessary

during a parting-off operation The land width – often

termed a ‘T-land’ , is relative to the breadth of the

part-ing-off tool This width of the insert’s land has a direct

correlation to the feedrate and its accompanying chip

formation The feedrate must be adequate to force the

workpiece material over the land and into the

chip-former

Notwithstanding the widespread usage of negative

parting-off tooling, positive-style insert geometries

have some distinct advantages The chief one being the

ability to narrow the chip at light feedrates, with

mini- ‘Constant surface speed’ CNC capability as its name implies,

allows the machine tool to maintain a constant surface speed

as the diameter is reduced The main problem with using

this ‘canned cycle’ , is that as the maximum spindle speed is

reached, the chip load will also increase This is not a

prob-lem, so long as the maximum speed has not occurred, such as

when parting-off a component with a large hole at its centre.

 Parting-off operations that employ a negative-style insert (i.e

with a land and accompanying chip-former), normally have

the feedrate determined in the following manner: by

multi-plying the width of the insert by a constant of 0.04 For

ex-ample, for a 4 mm wide tool, it is necessary to multiply the

insert’s width of 4 mm by 0.04 to obtain a feedrate of 0.16 mm

rev– This will give a ‘start-point’ for any parting-off

opera-tions, although it might be prudent to check this feedrate is

valid, from the tooling manufacturer’s recommendations

mal tool pressure If excessive tool pressure occurs,

this can promote work-hardening of the ‘transient surface’  of the workpiece These abilities are

impor-tant points when machining relatively low mechanical strength components, which might otherwise buckle

if machined with negative-style inserts when subse-quently parted-off

Positive cutting edge parting-off tooling having chip-formers, are ideal for applications on machine tools when either low fixed feedrates are utilised, or

if the workpiece material necessitates lower cutting speeds This positive-style of parting-off tooling, oper-ates efficiently when machining softer workpiece mate-rials, such as: aluminium-or, cooper-based alloys and many non-metallic materials, typically plastics Feed-rates can be very low with these positive-type part-ing tools, down to 0.0254 mm rev– with exceptional chip control and consistent tool life One major dis-advantage of using these positive tooling geometries

for parting-off, is that the tool is much weaker than its

equivalent negative geometry type

The concept of insert self-grip in its respective

tool-holder, was developed by the cutting tool manufac-turer Iscar tools in the early 1970’s and has now been adopted by many other tooling manufacturers (Fig 40 top left-hand side) These ‘self-grip’ tooling designs, rely on the rotation of the part and subsequent tool pressure to keep the ‘keyed and wedged’ insert seated

in its respective toolholder pocket Previously,

double-ended inserts termed ‘dogbones’ , were often used but

were limited to low DOC’s – due to the length of the secondary cutting edge, so have been somewhat over-shadowed by the ‘self-grip’ varieties of parting-off tooling

2.5.5 Chip Morphology

The Characterisation of Chip Forms (Appendix 2)

In the now withdrawn ISO 3685 Standard on Ma-chinability Testing Assessment, of some interest was the fact that this Standard had visually characterised

 Transient surfaces are those machined surfaces that will be

removed upon the next revolution of either the:

Workpiece (i.e in rotating part operations), or Cutter (i.e for rotating tooling – drilling, milling, reaming,

etc.).

– –

chip forms under eight headings, with several

varia-tions appearing in each groups (i.e see Appendix 2

for an extract showing these chip form classifications)

Although in the main, the chip forms were related to

turning, some of these chip morphologies could be

ex-trapolated to other manufacturing processes The chip

type that will be formed when any machining

opera-tion is undertaken is the product of many interrelated

factors, such as:

• Workpiece material characteristics – will the

mate-rial that forms the chip significantly work-harden?,

• Cutting tool geometry – changing, or modifying the

cutting insert geometries and its plan approach

angles will have a major influence on the type of

chip formed,

• Temperatures within the cutting zone – if high, or

low temperatures occur as the chip is formed, this

will have an impact on the type of chip formed,

• Machine tool/workpiece/cutting tool set-up – if

this ‘loop’ is not too rigid, then vibrations are likely

to be present, which will destabilise the cutting

process and affect the type and formation of chips

produced,

• Cutting data utilised – by modifying the cutting

data: feeds and speeds and DOC’s, with the insert

ge-ometry maintained, this can play a significant role

in the chip formed during machining operations

NB Chip formation has become a technology in

its own right, which has shown significant

devel-opment over the last few decades of machining

ap-plications

As has been previously mentioned, chip formation

should always be controlled, with the resultant chips

formed being broken into suitable shape formation,

such as ‘spirals and commas’ , as indicated by the

re-sultant chip morphology shown in Fig 35a

Uncon-trolled chip-steaming (i.e long continuous workpiece

strands), must be avoided, being a significant

risk-fac-tor to both the: machine tool’s operation and its CNC

setter/operator alike

 Chip-breaking envelopes (see Fig 34 middle right), are the

product of plotting both the feedrate and DOC on two axes,

with their relative size and position within the graphical area

being significantly affected by the cutting insert’s geometry

– as depicted by the three cutting insert geometric versions

shown by types: A, B and C (Fig 34).

For every cutting insert geometry, there is a recom-mended application area – termed its ‘chip-breaking envelope’ (i.e see footnote 38 below) – with regard to

its range of feedrates and DOC’s Within this ‘envelope’ , chips of acceptable form are produced by the

cut-ting insert’s geometry Conversely, any chips that are formed outside this ‘envelope’ are not acceptable,

be-cause they are either formed as unbroken strands, or are too thick and over-compressed When component profiling operations are necessary (Fig 31a), this nor-mally involves several machining-related parameters: variations in DOC’s, together with path vectoring of the feeds and as a result of this latter point, changes to the resultant chip’s path on the rake face These factors are important as they can modify the chip morphology when profiling operations include: recessed/undercut shoulders, tapers and partial arcs, facing and sliding operations with the same tool, together with many other combined profiled features All of these opera-tions make significant demands on the adaptability of the cutting insert’s geometry to efficiently break the chip

In general, the cutting insert’s chip formation prin-ciples are concerned with the chip-breaker’s ability to create a chip form that is neither not too tight a curl, nor too open

If chip curling is too tight for the specific

machin-ing application, the likely consequences are for a chip form creating:

• ‘Chip-streaming’ – producing long chip strands

that are undesirable, wrapping itself around the machined surface of the workpiece with work-hardened swarf and possibly degrading this ma-chined surface, or may become entangled around the various parts of the machine tool, which could impede its operation,

• Excessive heat generation – this can decrease tool

life, or be conducted into the machined part and consequently may affect specific part tolerances for the individual part, or could lead to modifications

in the statistical variability of a batch of parts,

 Statistical variability in component production can cause

variations from one part to another, as the standard deviation and mean changes, these important factors will be mentioned

later in the text.

• Increased built-up edge (BUE) formation – which

through ‘attrition wear’ 0 may cause the risk of

pre-mature cutting edge failure

When the chip curling is too open, this may result in

the following negative tendencies:

• Poor chip control – creating an inefficient

chip-breaking ability by the cutting insert,

• Chip hammering – breaking down the edge and

causing it to crumble and as a result creating the

likelihood of prematurely failing,

• Vibrational tendencies – affecting both the

ma-chined surface texture and shortening tool life

Chip formation and its resultant morphology, is not

only affected by the cutting data selected, but will be

influenced by the plan approach (i.e entering) angle

of the insert In most machining operations, they are

usually not of the orthogonal, but oblique cutting

in-sert orientation, so the affect is for the entering angle

to modify the chip formation process The insert’s

en-tering angle affects the chip formation by reducing the

chip thickness and having its width increased with a

smaller angle With oblique cutting geometry, the chip

formation is both ‘smoother and softer’ in operation as

the plan approach angle tends toward say, 10° to 60°,

furthermore, the chip flow direction will also

advanta-geously change with the spiral pitch increasing

As the nose radius is changed with different cutting

inserts, this has the effect of changing both the

direc-tion and shape of the chips produced This nose radius

geometry is a fundamental aspect in the development

of chips during the machining process – as depicted by

Fig 35b Here, an identical nose radius and feedrate

is utilised, but the difference being the DOC’s, with a

shallow DOC in Fig 35b (left), giving rise to a slow chip

helix, whereas in Fig 35b (right) the DOC is somewhat

deeper, creating a tighter chip helix which is

benefi-cial to enhanced chip-breaking ability Shallow cutting

depths produce ‘comma-shaped’ chip cross-sections,

0 Attrition wear is an unusual aspect of tool wear, in that it is

the result of high cutting forces, sterile surfaces, together with

chip/tool affinity, creating ‘ideal’ conditions for a pressure

welding situation Hence, the BUE develops, which builds-up

rapidly and is the ‘swept away’ by the chip flow streaming over

the top rake’s surface, taking with it minute atomic surface

lay-ers from the tool’s face This continuous renewal and

destruc-tion of the BUE, enhances crater wear formadestruc-tion, eventually

leading to premature cutting edge failure.

having a small angle when compared to the cutting edge Equally, a larger depth means that the nose ra-dius has somewhat less affect from its rara-dius and greater influence by the entering angle of the cutting edge, producing an outward directed spiral Feedrate also affects the width of the chip’s cross-section and its ensuing chip flow

Chip formation begins by the chip curving, this be-ing significantly affected by combinations of the cut-ting data employed, most notably: feedrate, DOC, rake angle, nose radius dimensions and workpiece condi-tion A relatively ‘square’ cross-sectional chip nor-mally indicates that an excessively hard chip compres-sion has occurred, whilst a wide and thin band-like chip formation is usually indicative of long ribbon-like chips producing unmanageable swarf If the chip curve

is tight helix, coupled to a thick chip cross-section, this means that the length of the chip/tool contact has increased, creating higher pressure and deformation

It should be noted that excessive chip cross-sectional thickness, has a debilitating effect on any machining process By careful use of CAD techniques coupled

to FEA to construct the insert’s cutting edge, comma-shaped chips are the likely product of any machining, providing that the appropriate cutting data has been selected In some machining operations, chip forma-tion can be superior using a slightly negative insert rake angle, thereby introducing harder chip compres-sion and self-breaking of the chip, particularly if utilis-ing small feeds Conversely, positive rakes can be give other important machining advantages, depending which chip form and cutting data would be the most advantageous to the part’s ensuing manufacture

Usu- Chip-flow is the result of a compound angle between the chip’s

side- and back-flow The chip’s side-flow being a measure of the flow over the tool face (i.e for a flat-faced tool), whilst back-flow establishes the amount of streaming into the chip-breaker groove Detailed analysis of chip side-flow (i.e via high-speed photography), has indicated that it is influenced

by a combination of groove dimensions and cutting data If the feedrate is increased, this results in a higher chip back-flow angle, promoting chip-streaming into the chip-breaker groove The ratio of feed-to-length of restricted contact has been shown to be an important parameter in the determina-tion of chip- back-flow Typically with low feedrates the cor-responding chip back-flow is going to be somewhat lessened, resulting in poor chip-breaker utilisation When the restricted contact between the chip and the tool is small – due to low feed – the chip-flow does not fully engage the chip-breaker and will as a result curve upward, with minimal ‘automatic’ chip-breaking effect.

ally, for larger feedrates, a positive insert rake angle

might optimise the chip-curving tendency, by not

pro-ducing and excessively tight chip helix Chip curve, its

resultant chip flow direction, the chip helix and its

ac-companying shape are designed into each cutting edge

by the tooling manufacturers Tool companies ensure

that a controlled chip formation should result if they

are exploited within the recommended cutting data

ranges specified

In Fig 36a (left), effective chip-breaking

decision-making recommendations are shown on a flow-chart,

indicating how to obtain the desired chip-break-ing control In the chart shown in Fig 36a (right), the DOC’s indicate on the associated visual table the expected chip type showing that here types ‘C and D’ offer ‘good’ broken chips Such chip morphology charts as these from tooling manufacturers, attempt

to inform the user of the anticipated chip-breaking

if their recommendations are followed Whereas the flow-diagram illustrated in Fig 36b, indicates that

‘good chip control’ improved productivity will result,

if a manufacturing company adopts the machining

Figure 36 Chip-breaking control and chip morphology and its affect on productivity [Courtesy of Mitsubishi Carbide]

.

strategy high-lighted to the left-hand side On the

con-trary, ‘poor chip control’ with an attendant decrease in

productivity will occur, if the problems shown to the

right-hand side transpire

Chip morphology can indicate important aspects

of the overall cutting process, from the cutting edge’s

geometry and its design, through to work-hardening

ability of the workpiece Many other factors

concern-ing cuttconcern-ing edge’s mechanical/physical properties can

be high-lighted, these being important aids in

deter-mining a material’s machinability – which will be

dis-cussed in more depth later in the text

2.5.6 Chip-Breaker Wear

Any form of tool failure will depend upon a

combi-nation of different wear criteria, usually with one, or

more wear mechanisms playing a dominant role

Pre-viously, it was found that the workpiece surface texture

and the crater index act as appropriate tool failure

cri-teria, particularly for rough turning operations

More-over, tool life based upon these two factors,

approxi-mated the failure curve more exactly than either the

flank, or crater wear criterion

In cutting tool research activities, it has been found

that when machining with chip-breaker inserts, flank

wear (i.e notably VB) is not the most dominant factor

in tool failure In most cases, the ‘end-point’ of

use-ful tool life occurs through an alteration of the

chip-groove parameters, well before high values of flank

wear have been reached The two principal causes of

wear failure for chip-breaker inserts are:

• For recommended cutting data with a specific

in-sert, the design and positioning of chip-breakers/

grooves may promote ‘unfavourable’ chip-flow,

re-sulting in wear in the chip-breaker wall – causing

consequent tool failure,

• Alterations in the cutting data, particularly feedrate,

affects chip-flow, which in turn, generates various

wear patterns at the chip-breaker’s heel and edge

(see Fig 37)

In the schematic diagrams shown in Fig 37, are

il-lustrated the concentrated wear zones on the: back

wall (i.e heel), cutting edge, or on both positions for

a typical chip-breaker insert Under the machining

conditions for Fig 37a, the chip-groove utilisation

is very low, with the chip striking the heel directly

Thus, as machining continues, this results in abrasive

wear of the heel and ultimately this heel becomes flattened and chip-breaking is severely compromised Conversely, when the cutting data produces a wear zone concentrated at the insert’s edge (Fig 37b), then chip side-flow occurs and poor chip-breaking results, together with low tool life This accelerated tool wear, resulting from an extended tool/chip contact region over the primary rake face, promotes a rough surface texture to the machined part In the case of Fig 37c, these are ideal conditions for optimum chip-breaking action and a correspondingly excellent and predictable tool life, because the wear zones at both the heel and edge are relatively uniform in nature, illustrating virtually a perfect chip-forming/-breaking action

Higher tool/chip interface temperatures can result

as the heel wears, forming a crater at the bottom of the chip-breaker groove Combination wear – as shown in Fig 37c – generally results in significantly improved tool wear, in conjunction with more predictable tool life In the photographs of chip-breaker grooves shown for an uncoated and coated Cermet cutting insert ma-terial in Figs 38a and b respectively, the relative wear patterns can clearly be discerned In the case of Fig 38a – the uncoated insert – the predominant wear concentration is primarily at the edge, indicating that

the cutting data had not been optimised While in the

case of the coated Cermet insert of identical geometry (Fig 38b), the wear is uniform across the: edge, groove and heel This would seem to suggest that ideal cutting data had been utilised in its machining operation In both of these cases some flank wear has occurred, but

this would not render the chip-breaking ability when

subsequent machining invalid

NB A complex matrix occurs (i.e Fig 38c) with

Cer-mets, this ‘metallurgy’ can be ‘tailored’ to meet the needs of specific workpiece and machining require-ments

2.6 Multi-Functional Tooling

The concept of multi-functional tooling was

devel-oped from the mid-1980’s, when multi-directional tooling emerged This tooling allowed a series of

op-erations to be performed by a single tool, rather than many, typically allowing: side-turning, profiling and

Figure 37 Schematic representations of differing chip-breaking insert tool wear mechanisms – due to

altera-tions in the cutting data [Source: Jawahir et al., 1995]

.

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

ovako 825B steel, having the following cutting data: Cutting speed 250 m min–1, feed 0.2 mm rev–1,

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

.

grooving, enabling the non-productive elements in

the machining cycle to be minimised In the original

multi-directional tooling concept, the top rake

geom-etry might include a three-dimensional chip-former,

comprising of an elevated central rib, with negative

K-lands on the edges Such a top rake profile geometry

could be utilised for efficient chip-forming/-breaking

of the resultant chips This tooling when utilised for

say, grooving operations, employed a chip-forming

geometry – this being extended to the cutting edge,

which both narrowed and curled the emerging chip

to the desired shape, thereby facilitating easy swarf

evacuation A feature of this cutting insert concept,

was a form of effective chip management, extending

the insert’s life significantly, thus equally ensuring that

adequate chip-flow and rapid swarf evacuation would

have taken place When one of these multi-directional

tools was required to commence a side-turning

opera-tion, the axial force component acting on the insert

caused it to elastically deflect at the front region of the

toolholder This tool deflection enabled an efficient

feed motion along the workpiece to take place,

be-cause of the elastic behaviour of the toolholder created

a positive plan approach angle in combination with a

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

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

multi-functional tooling geometries)

Any of today’s multi-functional tooling designs

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

be-haviour in the toolholder, enabling satisfactory tool

vectoring to occur, either to the right-, or left-hand

of the part feature being machined These

multi-func- Non-productive elements are any activity in the machining

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

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

changing tools, adjusting tool-offsets (i.e for either: tool wear

compensation, or for inputting new tool offsets – into the

ma-chine tool’s CNC controller), for component

loading/unload-ing operations, measurloading/unload-ing critical dimensional features – by

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

manual inspection with metrology equipment (i.e

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

activities

 An Axial force component is the result of engaging the desired

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

pro-file, wide groove, chamfer, undercut, etc – either positioned

externally/internally for the necessary production of the

ma-chined part

tional tools are critically-designed so that for a specific

feedrate, the rate of elastic deflection is both known

and is relatively small, being directly related to the ap-plied axial force, in association with the selected DOC’s

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

compensation(s) are undertaken to allow for minute

changes in the machined diameter, due to the dynamic elastic behaviour of one of these tools in-cut For a specific multi-functional tool supplied by the tooling manufacturer, its actual tool compensation factor(s) will be available from the manufacturer’s user-manual for the product

In-action these multi-functional tools (Fig 39b), can significantly reduce the normal tooling inventory, for example, on average such tools can replace three conventional ones, with the twin benefit of a major cycle-time reduction (i.e for the reasons previously mentioned) of between 30 to 60% – depending upon the complexity of features on the component being machined Some other important benefits of using a multi-functional tooling strategy are:

• Surface quality and accuracy improvements – due

to the profile of the insert’s geometry, any ‘machined cusps’ , or feedmarks are reduced, providing excel-lent machined surface texture and predictable di-mensional control,

• Turret utilisation improved – because fewer tools

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

can be adopted, thereby further improving any un-tended operational performance,

• Superior chip control – breaks the chips into

man-ageable swarf, thus minimising ‘birds nests’  and entanglements around components and lessens au-tomatic part loading problems,

• Improved insert strength – allows machining at sig-nificantly greater DOC’s to that of conventional

in- ‘Machined cusps’ the consequence of the insert’s nose

geom-etry coupled to the feedrate, these being superimposed onto the machined surface, once the tool has passed over this sur-face.

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

of continuous chips at the bottom of both trough and blind holes, this work-hardened swarf can cause avoidable damage

in the machined hole, furthermore, it can present problems in coolant delivery for additional machining operations that may

be required.